UCHSC Neuroanesthesia Handbook For Residents

INTRODUCTION

Welcome to Neuroanesthesia. We hope this month is an interesting and educational experience. This guide is for anesthesia residents as they prepare for anesthesia in neurosurgical patients. It outlines the curriculum for learning and allows a quick familiarization with some of the special considerations that apply to neurosurgical cases. These guides should not be used as a replacement for background reading, discussion with the anesthesia faculty and consultation with the surgeons, but should be helpful to supplement these.

During this 2 month long rotation, we recommend you follow the curriculum developed by the Society of Neurosurgical Anesthesia and Critical Care (SNACC). These are attached and under continual development. To accomplish these goals, residents will be asked to do the following:

1. Prior to the rotation: be familiar with the content of this introductory manual

Neuroanesthesia Reading List

The suggested readings below indicate the minimum reading expectations for two months of neuroanesthesia, which typically will comprise one month in the CA2 year and one month in the CA3 year. Additional reading is encouraged as well, and the faculty will be happy to make recommendations. Reading noted as “Miller” refers to Miller’s Anesthesia, Seventh Edition, RD Miller, editor, Elsevier, Philadelphia, 2009 or the electronic version that is available on-line through the Health Sciences Library under MDConsult. “Mongan” refers to Mongan PD, Sloan TB, Soriano S eds. A Practical Approach to Neuroanesthesia, Wolters Kluwer, 2013. The reading assignments in Miller intentionally contain fewer pages than those in Mongan, because each page contains more words. The Mongan book and the resident handbook will be provided by the department prior to the rotation through the residency coordinator.

REQUIRED READING:In order to facilitate a coordination of reading topics and discussion between residents and assigned staff the following chapters in Mongan will be assigned on a daily and weekly basis. Not every rotation will start on a Monday. Thus, the reading assignments will begin on the first day of the first week of the rotation according to the in the following table. For example if your rotation starts on a Thursday you will be responsible for Chapter 4 Routine Craniotomy. The other chapters will be covered in the fourth week of the rotation.

Mongan Chapter Week Day1 (Brain Metabolism and CBF) 1 Monday2 (Anesthesia effects on CBF) 1 Tuesday3 (Fluid Management) 1 Wednesday4 (Routine Craniotomy) 1 Thursday5 (Emergency Craniotomy) 2 Monday6 (Posterior Fossa) 2 Tuesday8 (Pituitary Gland) 2 Wednesday9 (Intracranial aneurysms) 2 Thursday10 (AVMs) 3 Monday12 (Spinal cord injury) 3 Tuesday13 (Intramedullary Spinal Cord Tumors) 3 Wednesday14 (Functional Neurosurgery) 3 Thursday23 (Stroke and Brain Protection) 4 Monday26 (Electrophysiology) 4 Tuesday27 (ICP) 4 Wednesday29 (TBI) 4 Thursday

RECOMMENDED READING:

Week 1. Cerebral Physiology, Ch 13 Anesthesia, 7 th edition, Ronald Miller Ed. Churchill Livingston, 2009

Week 2. Neurological Monitoring, Ch 46 Anesthesia, 7 th edition, Ronald Miller Ed. Churchill Livingston, 2009

Week 3/4. Neurosurgical Anesthesia, Ch 63 Anesthesia, 7th edition, Ronald Miller Ed. Churchill Livingston, 2009

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Table of Contents Introduction..............................................................................................................page 1

Curriculum .............................................................................................................page 4

Reading Material and References .........................................................................page 5

General Considerations .........................................................................................page 6

Emergency Craniotomy .........................................................................................page 8

Routine Craniotomy .............................................................................................page 11

Posterior Fossa/Tumor Surgery ...........................................................................page 15Sitting PositionAir Embolism

Transphenoidal Pituitary Resection .....................................................................page 20

Carotid Endarterectomy (CEA) ............................................................................page 21

Intracranial Vascular Surgery ...............................................................................page 24

Spine Surgery ......................................................................................................page 29

Parkinson’s Disease………………........................................................................page 33

Awake Craniotomy (for Excision of Seizure Foci) .................................................page 36

Neurophysiologic Monitoring ...............................................................................page 39

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Society of Neurosurgical Anesthesia and Critical Care Anesthesia Residency Curriculum

Goals

A. Administer anesthesia safely to patients with neurologic disease who are undergoing neurologic or non-neurologic surgery, diagnostic procedures requiring anesthesia, or non-surgical interventions requiring anesthesia.

B. Demonstrate knowledge of concepts of central nervous system (CNS) physiology and anatomy as they relate to neuroanesthesia.

C. Know the effect(s) of commonly used anesthetic agents and adjuvant agents. For example, opiates, isoflurane, and antihypertensitives, on patients with normal cerebral physiology as well as on patents with significant pathology such as brain tumor or subarachnoid hemorrhage.

D. Be aware of the anesthetic implications of the most common neurosurgical procedures, that is what is likely to happen during neurosurgery that will affect anesthetic management.

E. Know basic generators of the electroencephalogram, evoked potentials, and electromyogram as well as the significance of intraoperative changes.

F. Understand how concurrent non-neurological medical illnesses affect anesthesia during neurologic surgery.

Evaluation to Determine Goal Achievement

Preoperative discussion with the attending physician Attending evaluation of daily clinical performance in the operating room in the

following areas: 1. Preparation for case and ability to carry out plan discussed the night before 2. Recognition of intraoperative problems and communication with the attending; ability

to appropriately respond to changing clinical situation; clinical judgement 3. Technical skills of placing lines and positioning the patient 4. Application of basic and clinical science knowledge and skills to the neurosurgical

patient. Educational feedback, using an appropriate setting and a supportive critiquing

approach, is provided by the attending anesthesiologist At the conclusion of each neuroanesthesia rotation an overall performance

evaluation is made based an the above criteria. Performance on the In-Training Examination for the American Board of

Anesthesiologists (ABA) and/or departmentally held in-training examinations. The training staff review results from these sources and may adjust the curriculum accordingly.

Resources available to accomplish goals include but not limited to:

A. Neuroanesthesia grand rounds Page | 4

B. Human patient simulation C. Library resources (books, electronic textbooks, videotapes, network access, Internet) D. Core lecture program/case conferencesE. Problem-based learning program F. Mock oral examinations

Reference Reading Material (Chapters and Books)

Recommended Reading - Prior to the Rotation

UCH Neuro Manual

2. Required Reading - During the Rotation

Chapter 13: Patel PM, Drummond JC, Cerebral physiology and the effects of anesthetics and techniques, Miller, pp 305-340 (32 pages of text).

Chapter 46: Seubert CN, Mahla ME. Neurologic monitoring, Miller, pp 1477-1514 (34 pages of text).

Chapter 53: Drummond JC, Patel PM, Neurosurgical anesthesia, Miller, pp 2045-2088 (38 pages of text).

Second Month of Neuroanesthesia (Mongan)Chapter 5 (Emergency Craniotomy)Chapter 7 (Carotid Endarterectomy)Chapter 11 (Neurosurgical Procedures and Chronic Pain)Chapter 24 (Anesthetic-induced Neurotoxicity)Chapter 25 (Neurosurgery in the Pregnant Patient)Chapter 28 (Other Monitoring Techniques)Chapter 30 (intensive Care of Spinal Cord Injury)

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GENERAL CONSIDERATIONS

Objectives:

A. Review the medical history and physical examination of patients; define the type and severity of their major neurosurgical problem(s) as well as other medical problems that may affect anesthetic care; and know what information about nervous system function and pathology is important to the anesthesiologist.

B. Recognize both the adult and pediatric patient with poor elastance or increased intracranial pressure (ICP) (signs and symptoms).

C. Specifically, the resident should exhibit mastery of the following concepts: autoregulation of cerebral blood flow. cerebral blood flow in response to CO2 blood flow relationship to cerebral oxygen (CMR02) and glucose

(CMRglu) metabolic rates cerebrospinal fluid physiology (formation, reabsorption, flow, and

role in compensation for an intracranial mass); blood brain barrier composition and function

pathophysiology of ischemic brain injury (assists in understanding strategies for cerebral and spinal cord protection)

basic neuroanatomy (facilitates understanding of discussions with surgical colleagues about the proposed surgical procedure) including: blood supply to brain and spinal cord, organization of motor and sensory tracts in the brain and spinal cord, and ventricular system and normal flow of CSF

D. Know the effects of anesthetic and adjuvant drugs on autoregulation of cerebral blood flow, cerebral blood flow response to CO2, cerebral metabolism and its relationship to cerebral blood flow, and cerebrospinal fluid production/reabsorption. Important drug classes:

Hypnotic agents including barbiturates, benzodiazepines. etomidate, and propofol

Ketamine Nitrous oxide Potent inhalation agents (emphasize differences among these

agents) Opiates and antagonists Muscle relaxants - both depolarizing and nondepolarizing

Understand the indications, required techniques, and implications of the following special procedures sometimes used during neuroanesthesia: be aware of impact of each of these techniques on postoperative care of the patient.

1. Induced hypotension

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2. Induced hypertension 3. Moderate hypothermia 4. Pharmacologic cerebral protection a. Use an understanding of the pathophysiology of cerebral ischemia to understand

the rationale for currently used methods for cerebral protection.b. Be knowledgeable of current literature regarding effectiveness of pharmacologic

cerebral protection.c. Be aware of those pharmacologic techniques that have been demonstrated not

to be effective.

In general, the complexity of neurosurgical cases can vary widely between procedure and for each patient. Anesthesia plays an integral role in many of these procedures since the type of anesthetic, method of delivery and adjunctive management techniques can enormously influence the ability of the neurosurgeon to effectively conduct the procedure. Hence, planning of the anesthetic becomes extremely important.

First, consultation with the neurosurgeon preoperatively is very important. Small changes in the type and location of pathology can make major changes in their approach (e.g. position). Identifying how and in what position they will conduct the surgery is critical in planning the anesthetic. Secondly, it is important to ask about special intraoperative needs and monitoring (such as awake intubation, awake craniotomies, need for EMG monitoring, etc). Finally, a discussion should be made about postoperative plans and expectations so that a long term anesthetic plan can be made for the patient.

Prior to the surgery, the anesthesia team (resident-attending) should also plan their approach taking into account the above needs and the interaction of anesthetics. This should generally be done the night before and reviewed in the morning with any last minute changes.

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EMERGENCY CRANIOTOMY

Objectives:

Understand the indications for emergency craniotomy (usually as a result of traumatic brain injury):

Acute CNS physiology changes with intracranial hemorrhage

The natural history of intracranial hemorrhage, specifically regarding advantages of early versus delayed surgery

Know potential effects of intracranial hemorrhage on the function of other systems including the cardiovascular system, pulmonary system and renal system

Evaluate the neurologic status of the patients: C-spine, orientation, motor/sensory function and patients with subarachnoid hemorrhage and intracranial aneurysm by means of the Hunt-Hess grading system

Head injury: Know the pathophysiology and anesthetic implications of the following types of head injury:

Acute epidural hematoma

Acute subdural hematoma

Intracerebral hematoma

Chronic subdural hematoma

Diffuse axonal injury

The patient presenting for emergency craniotomy must be assumed to have both a full stomach and have raised ICP (which can delay gastric emptying).

Oxygen should be applied early, the head should be raised if possible and ensure that the blood pressure is adequate. A brief neurologic exam should rapidly be done to document major deficits. With trauma, the status of the cervical spine needs to be clarified as neck injury is common (10-20%) with head injury and it is important to know if a basilar skull fracture is present. Unfortunately 30% of unstable spines are missed by lateral cervical spine films so it may be prudent to consider the spine unstable with head and neck trauma (hence, use in-line stabilization during intubation).

Monitoring should be applied and ventilation assisted if necessary (avoid hypercarbia). Page | 8

An oral citrate preparation should be given if the patient is awake and cooperative. If the neck is not injured, induction is generally achieved with thiopental, propofol or etomidate, with some narcotic (fentanyl) and succinylcholine. If succinylcholine is not contraindicated, pretreatment with a nondepolarizing muscle blocker will prevent the ICP from increasing markedly as is typically seen with administering only succinylcholine. On induction, cricoid pressure should be used and ventilation supported if it is possible to do so without generating excessive pressure likely to place air in the stomach. Lidocaine (1-1.5 mg/kg) is often a useful adjunct to induction and intubation. The induction drug and dosage must be chosen based on cardiovascular stability, but lidocaine, some narcotic and sedative are almost always suitable. Alternate airway techniques should be used if neck or face trauma contraindicates direct laryngoscopy. A basilar skull fracture (suggested by intracranial air or CSF rhinorrhea) contraindicates the nasal route for intubation and NG tube placement.

Immediate methods to reduce ICP should be used in consultation with the neurosurgeon. Hyperventilation may not be appropriate with acute head trauma as reductions in cerebral blood flow may be deleterious. A reasonable anesthetic technique would involve a narcotic (fentanyl), muscle relaxant, air/oxygen. Low dose isoflurane (as tolerated) may be appropriate if brain swelling is not excessive. With very high ICP it is probably best to avoid nitrous oxide, ketamine or inhalational agents and only use an intravenous technique (TIVA). If intracranial air is seen on the CT, nitrous oxide must not be used. Drugs which cannot be reversed should be avoided in all but small doses as they may confuse the postoperative mental status. Fluid management is critical and if blood loss is moderate then replace with blood or colloid (preferably albumin) if needed. Hetastarch has an associated coagulopathy at higher doses so it is usually avoided.

Maintenance of adequate blood pressure is of paramount importance. If hypotension occurs, consider extracranial blood loss (e.g. long bone fractures, hidden abdominal bleeding, or intrathoracic/cardiac causes); hypotension is not common from intracranial injury (except with severe, lethal, injury to the pons).

Although an awake patient is desirable for evaluation of the post operative neurologic status, patients who were comatose or barely responsive preoperatively will likely be difficult to awaken and may best be taken to the ICU intubated and ventilated. Early consultation with the neurosurgeon about reasonable expectations for awakening and post operative plans should be made. In some cases, postoperative intubation is helpful for hyperventilation and protection of the airway. Further, severe head trauma may have accompanying pulmonary dysfunction (e.g. neurogenic pulmonary edema) necessitating aggressive pulmonary management.

Patients with severe head trauma (particularly involving tissue destruction) may develop DIC. Check DIC profile if suspicious. Review ECG for ischemia as this may appear as subendocardial ischemia which is rarely associated with myocardial dysfunction unless cardiac injury or infarction has occurred. However, when present it

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is usually a sign of severe intracranial problem.

Consultation with the intensive care unit team (including nurses) will be helpful to make a smooth transition to ICU.

ROUTINE CRANIOTOMYPage | 10

Objectives:There are several indications for craniotomy which include tumor resection, seizure foci resection, hydrocephalus, vascular abnormalities. This section focuses on indications that result in elevated ICP.

Understand the different presentations of a patient with hydrocephalus and choose an appropriate anesthetic regimen.

Know the differential diagnoses and treatment alternatives of intraoperative intracranial hypertension (“tight brain”). Understand the role of fluid management during neurosurgical procedures but especially in patients with high ICP. importance of volume of IV fluid infused versus osmolality of the plasma osmolality of commonly used IV fluids including normal saline solution,

plasmalyte, lactated Ringer’s solution, hydroxyethylstarch, protein solutions (e.g. 5% and 25% albumin)

Current concepts regarding use of colloid versus crystalloid solutions Mechanism of action and appropriate use of mannitol solutions during

neurosurgical procedures Appropriate use of non-osmotic diuretics during neurosurgery Appropriate use of glucose containing solutions during neurosurgery and

current concepts regarding effects of glucose on outcome following CNS ischemia

Know the basic differences between the following types of brain tumors; know the implications of size, speed of growth, vascularity, surrounding edema

Infratentorial -e.g. cerebellar astrocytoma or acoustic neuroma Supratentorial - e.g. meningioma, glioblastoma Pituitary tumors

Preoperatively, several considerations must be identified: 1) the location of the tumor/mass and 2) the surgical approach, notably the position required. Preoperative neurologic symptoms should be documented in the chart. In patients who have some level of altered mental status, premedication should generally be light (if at all). Drugs which depress respirations (eg. narcotics), thereby raising ICP or those that might further worsen mental status (thus causing a preoperative change in consciousness) should be used cautiously, if at all. Preoperative anti-seizure drugs and steroids (e.g. decadron) should generally be given if routinely taken by the patient. Stress dose steroids are not needed as decadron will likely be continued. Confer with neurosurgical team prior to administration.

Induction: When ICP may be elevated (intracranial pathology), induction should be accomplished by means to keep ICP low, 1) raising the head of bed 10-20o, 2)

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preoxygenation, 3) voluntary hyperventilation (if possible), 4) induction with pentothal, etomidate, or propofol supplemented with lidocaine, 5) pretreatment with a non depolarizing muscle relaxant if succinylcholine is to be used and, 6) neutral head position, (neutral in flexion, extension and rotation). Once a mask airway is established, hyperventilation should continue to achieve a PaCO2 ranging from 25-28 mm Hg.

For anesthetic maintenance use drugs and techniques which have a minimal tendency to increase ICP. These include IV anesthetics, narcotics, and muscle relaxants (avoid drugs that induce histamine release). Nitrous oxide, isoflurane, desflurane, and sevoflurane can likely be used in low to moderate dosages (1/2 to 1 MAC). During the procedure the following are helpful in reducing ICP and brain swelling:

elevation of head

neutral head position (<60o head rotation, neutral in flexion and extension)

hyperventilation (PaCO2 25-30 mm Hg)

oxygenation (PaO2 > 60 mm Hg)

low to normal CVP

low intrathoracic pressure (no peep)

good muscle relaxation (no cough or straining)

In addition, mannitol (.25-1.5 gm/kg) and furosemide (0.05-.1 mg/kg) can be extremely helpful and should be given after consultation with the neurosurgeon. Optimally, this is done during scalp incision such that the mannitol has at least 20-30 minutes before its effect is needed. Mannitol works by osmotically drawing tissue water out of the brain across regions of the brain with normal blood-brain barrier and excreting it via the kidneys (avoid use in patients with renal failure). If a dilute diuresis does not result, mannitol is likely not effective. Consider administering a small dose of furosemide (5-10 mg). Furosemide pulls water out of brain by affecting the Chloride-shift of the Na+/K+ pump and improves cerebral venous volume by causing a diuresis. Furosemide may facilitate the action of mannitol by initiating a diuresis.

Fluid management: General agreement exists that cerebral edema is worse if large amounts of fluid or if hypotonic fluids (e.g. D5W) are used. Note that lactated Ringers is slightly hypotonic (about 90cc free water per liter). The general agreement is to use fluids with normal sodium concentrations (normal saline or plasmalyte). Albumin 5% is acceptable as long as the sodium content is normal (however, albumin or synthetic colloids offer little value unless volume replacement is for rapid blood loss). Non-albumin colloids such as Hetastarch, may cause coagulation problems. Maintain

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normoglycemia and only administer glucose when glucose is low as hyperglycemia will promote ischemic injury. The neurosurgical management generally involves avoidance of hypervolemia, so avoid overhydration. If low urine output or hypotension occur, additional fluids may be required to maintain cerebral perfusion pressure.

Monitoring--Customary monitoring includes arterial line (transducer zeroed at ear level to reflect cerebral perfusion) and foley catheter in addition to usual monitors. A central venous catheter (CVP) may be helpful to manage fluid status (and will be required if air embolism is probable). The antecubital route is preferred as it is a relatively benign and effective method for placement of a CVP unless large bore CVP’s or PA catheters are used. A neuromuscular blockade monitor should be used as movement (especially patient in head pins) could be dangerous.

Emergence - Very little pain usually occurs during the intracranial portion of the surgery. Pain usually occurs at opening and closing. Thus, the level of anesthesia used for the intracranial portion of the procedure may need to be increased during closure. Further, hypertension commonly occurs during closing (not necessarily due to pain). In general, it is desirable to have the patient awake and extubated (if appropriate) to be examined neurologically shortly after the procedure. Consider methods to improve awakening time after long craniotomies such as conversion of isoflurane to less soluble anesthetics (desflurane, sevoflurane) and/or increased use of intravenous supplements (e.g. propofol). Uncomplicated craniotomy patients usually return to their preoperative neurological status (i.e. if the patient was not awake preop, they may not awaken post- op). Antihypertensives should be used if the BP is unusually high (systolic > 160-180 depending on the surgeons’ concerns) and pain relief is appropriate, and there is no evidence of hypoxia and/or hypercarbia. Extubation is highly desirable for neurological examination if appropriate. Patients must be observed carefully for ventilatory insufficiency and airway obstruction secondary to neurological or pharmacologic effects. The first hour is usually the most concerning time so patients are usually observed in the PACU despite going to the ICU. Often a CT scan is done immediately post-op after PACU stay.

Special problems - The most likely intraoperative problem will be brain swelling. If this occurs review the list above to ensure all is being done. Elimination of nitrous oxide and/ or reduction of inhalational agents may help. Intravenous thiopental (.5 - 1 mg/kg) may also help but may cause delayed awakening if large doses are given (administer slowly to avoid hypotension). The second most likely problem is low urine output (< 1/2 cc/kg/hr). Here the CVP becomes important; a low CVP probably indicates the need for a fluid bolus (100 - 200 cc) to allow adequate circulatory volume. If the CVP is not low, diuretics such as mannitol and furosemide may be preferred.

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The Semi-sitting (lounge chair) position with the head elevated for routine craniotomies

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POSTERIOR FOSSA SURGERY

Objectives: Know the preoperative and intraoperative considerations for patients undergoing skull base tumor resections including:

Meningiomas Glomus jugulare tumors Pituitary tumors

Know general principles of positioning the patient for neurologic surgery and the advantages and disadvantages of each position:

Lateral Prone 3/4 prone Supine- head neutral or head turned Sitting position (rarely used at this institutions)

Recognize the relative risks of different procedures and positions for air embolism

Understand appropriate changes in anesthetic management in response to air embolism

Detect and treat air embolism during neurosurgery Recognize precordial doppler sounds associated with air embolism;

recognize changes in end-tidal CO2, PA pressures, CVP and blood pressure consistent with air embolism

Patients having surgery in the posterior fossa present several different challenges including those of the sitting position and air embolism, which are discussed separately. Note that general considerations for tumors (elective craniotomy) also apply. Note that surgery in the posterior fossa can interact with brainstem structures causing physiologic changes. Thus, our monitoring can act as a guide to help the neurosurgeon. Sudden changes in pulse or blood pressure should be reported immediately to the surgeon. Stimulation of these brainstem centers have been associated with these changes:

gasps and respiratory arrhythmias (respiratory center) hypotension (Cardiovascular Center (C.V.) center, cranial nerve X) hypertension (C.V. center, cranial nerve V) bradycardia (C.V. center, dura, cranial nerve V, cranial nerve X) tachycardia (C.V. center) arrhythmias (C.V. center)

Bradycardia is seen most commonly and can also result from stimulation of the dura (via vagal innervation). Atropine, although helpful, is best avoided as it may mask detrimental surgical compromise of the brainstem structures. Occasionally atropine is

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needed and even external pacemakers have been used in asystole (plan pacemaker capability in patients with bradycardia preoperatively or with large tumors).

For patients undergoing posterior fossa surgery, postoperative considerations are very important. The small amount of space in the posterior fossa and the critical structures there give little margin for postoperative bleeding. Hypertension and severe coughing or straining can cause bleeding. A typical scenario is a patient who postoperatively has respiratory depression/unresponsive, is given naloxone, develops sudden hypertension, a posterior fossa bleed, and further neurological deterioration.

If the surgery involves damage to lower cranial nerves, the patient may have compro-mised airway protective reflexes post- peratively. In these patients, extubation should be carried out with a great deal of caution. Be aware, if damage to the respiratory center has occurred, the patient may have no respiratory drive. Fortunately, this is most uncommon.

A second major concern is cranial nerve injury in posterior fossa surgery. During surgery, the surgeons may wish to monitor cranial nerve function by EMG monitoring. This necessitates the omission of muscle relaxants so as to measure EMG or observe facial nerve function. The need for this is best identified pre-op and the induction relaxants allowed to dissipate at the appropriate time. For anesthesia with limited muscle relaxants, a moderate dose of inhalational agents (i.e. isoflurane) may be helpful to prevent patient movement. TIVA is also an option using adequate sedatives/narcotics.

For EMG recordings of the facial nerve, needle electrodes are placed in the facial muscles. The facial nerve is commonly of concern and the surgeon may stimulate it and ask for observation of ipsilateral eye (orbicularis oculi) or mouth (orbicularis oris) motion. It is important to differentiate jaw jerk and shoulder shrug from eye and mouth motion.

Prone position for posterior fossa surgerySITTING POSITION

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Although it is being used rarely these days, the sitting position is favored by many neurosurgeons because: 1) venous bleeding is reduced by gravity and 2) anatomy is oriented easily making surgery on the neck and brainstem easier to conduct. The disadvantages have prompted most surgeons to use the prone or lateral position for these cases.

From an anesthetic standpoint the disadvantages form the basis of concerns of management and are as follows:

Hypotension - often described as one of the most common problems, this is due to gravity reducing venous return. To minimize this: a) gradually adjust to the sitting position trying to keep the knees at heart level, b) use compression wraps (ace) or sequential compression boots on the legs c) use light anesthesia until in the sitting position d) correct severe fluid deficits prior to sitting and, e) place patient supine if hypotension persists in spite of all other measures.

Air embolism - see separate section; this is the classic problem for this position. Quadriparesis-a rare complication that appears to be due to excessive neck

flexion. Try to minimize flexion such that there is at least 1-2 finger breadths between jaw and sternum.

Massive tongue swelling - also rare, it appears to be reduced by not using an oral airway and avoiding excessive neck flexion

Injury from positioning a) make sure the horseshoe head holder or Mayfield tongs do not press on the forehead or nose - recheck when position is changed.b) avoid putting stretch on the face with tape that could injure the face - tape the circuit to the head holder to reduce strain on the endotracheal tube c) support the elbows to avoid the weight of the arms causing brachial plexus stretch d) pad and position the hands comfortably across the waiste) make sure the buttocks are well seated in the crotch of the table so there is no low back strain f) make sure hip flexion and knee flexion is adequate so as to reduce sciatic nerve stretch g) carefully pad the knees and legs to avoid pressure on the upright holder for the Maryfield h) pad the calves and heels to prevent pressure injury to the heels

Be careful to note how to manipulate the table if needed to position the patient head down for air embolism. Usually the whole bed needs to be moved (with trendelenburg) so that the bed moves as a unit. Just moving the back of the table down will often “hang” the patient by the head holder putting severe strain on the neck. Note position of Mayfield before sitting patient up.

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AIR EMBOLISM

In general, if the operative site is more than 5 cm above the atrium, air entrainment through the operative site can occur. Entrainment is very common through bone or large venous sinuses since these structures cannot collapse. Procedures recognized for their risk of air embolism include sitting position cases, transphenoidal surgery in the semi-sitting position, and some cases of lumbar laminectomy (when the abdomen hangs free lowering the CVP). In addition, some craniotomies in the supine or semi-sitting position may also have risks of air embolism (especially in children).

The most important aspect of management is detection of air embolism. The routine monitors that should be used for detection are (in order of sensitivity):

1. Precordial Doppler - the traditional sounds can warn of amounts of air that are usually less than a problem to the patient (unless they get to the left sided circulation). The Doppler should be placed on the chest using Doppler jelly at the edge of the left or right sternum in the 3-4, 4-5, or 5-6 intercostal spaces. Listen for optimal sounds of great vessel and heart motion with each heart beat. To confirm that the vascular structure being monitored is the vena cava, inject 10 cc of saline (shake the syringe to introduce “microbubbles”) in the CVP. Reposition if the sounds are not heard. Document this test of position on your record. 2. End tidal CO2 - generally considered next most useful, a sudden unexplained drop in ETCO2 may signal increasing dead space from pulmonary arterial obstruction from entrained air. Note that changes in ventilation (e.g. hyperventilation) or blood pressure (i.e. hypotension) can mimic this change. 3. CVP - a sudden, unexplained rise in CVP can signal pulmonary arterial obstruction from air. In addition blood withdrawal via the CVP can document air embolism by noting air bubbles (be sure the tubing connections are tight). Further aspiration via the CVP may remove a large amount of air if done promptly during the embolism. To be optimal, the CVP tip should be multiorificed and placed 2-3 cm above the atrio-vena caval junction. You should document how you confirmed this position of the catheter on your record (e.g. x-ray or electrocardiographic).

When using the ECG from the CVP catheter, connect an ECG lead to the catheter. The easiest is the chest lead for a V5 display. Be sure to remove air bubbles from the catheter. The best tracings are obtained if the catheter is filled with sodium bicarbonate (better electrical conductivity - equal to 3% saline). First adjust the catheter to get a large P wave to be sure you are near the heart and then adjust to a biphasic P (confirming position at the SA Node. Then pull back 1-2 cm.

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TRANSPHENOIDAL PITUITARY RESECTIONS

Most pituitary tumors are non-secretory or secrete prolactin. As such, only injury to normal pituitary by a mass effect causes endocrine dysfunction. Some tumors cause clinical changes, the most common is growth hormone secretion with acromegaly. A preoperative endocrine work up will reveal hormonal dysfunction that may require steroids or hormones replacement. A very high prolactin level may indicate invasion of the cavernous sinus by the tumor.

The growing mass causes the optic chasm to be compressed and produces the typical bitemporal hemianopsia. Usually ICP is not elevated unless the tumor is extremely large (usually a craniotomy will be needed for resection in this case). It is best to identify raised ICP preoperatively and treat those patients accordingly.

Induction for the procedure is normal, requiring only an intravenous line. A routine intubation is fine, unless the airway is a problem (such as with giantism). Most people prefer an oral RAE tube draped out of the middle or corner of the mouth to minimize the interference with the surgery. Avoid taping the tube such that the facial symmetry is distorted as a common postoperative complaint is a crooked nose. If sufficiently head up, air embolism is possible (via opening the cavernous sinus or other venous channels). In such a case, an arterial line, CVP, ETCO2 and precordial doppler should complement the basic monitoring regimen. As usual, the arterial transducer should be at head level.

Rarely, surgeons request a spinal drain for the procedure. It is usually placed be placed pre-induction. It is used for drainage of CSF after surgery if there is a dural leak, or for injection of air during surgery to highlight the tumor on fluoroscopy (avoid nitrous oxide after this injection) or to inject fluid to push the tumor down into the operative field (be very cautious about what you inject!). Valsalva maneuver can also push the tumor down by increasing CSF pressure.

Position: The patient will be semi-sitting (lounge chair) with the head elevated and pointed to the right (10o). Note that surgical access to the thigh may be needed for fat removal that is placed in the nasal cavity prior to closing.

Anesthetic Maintenance: No particular anesthetic type is superior as long as ICP is not elevated. Local anesthesia is usually applied to the nasal mucosa, so that a light anesthetic is often adequate.

Fluids - There are no unusual fluid requirements unless again ICP is raised. The urine output and CVP (if available) should serve as a guide to maintenance fluids. Keeping maintenance fluids to a reasonable and minimal amount will prevent excessive urine output that could be confused with the diuresis of diabetes insipidus (which is a common postoperative occurrence).

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CAROTID ENDARECTOMY (CEA)

Objectives:

Understand anesthetic concerns for patients undergoing extracranial neurovascular surgery (such as carotid endarterectomy).

Know cardiovascular concerns in the preoperative evaluation of these patients

Learn to evaluate the cerebral circulation completely (vertebral and carotid circulations) and know how patients with different types of extracranial neurovascular lesions may have different anesthetic consideration

Be familiar with both regional and general anesthetic techniques for carotid surgery as well as the advantages and disadvantages of each technique.

Be familiar with the different choices for neurologic monitoring during carotid surgery as well as the advantages and disadvantages of each technique including:

EEG Evoked potentials Transcranial Doppler Ultrasound Cerebral Oximetry Awake neurologic examination

CEA is performed by neurosurgeons, vascular surgeons and occasionally by other surgeons and represents a major anesthetic challenge. It must be remembered that the major operative risk is stroke, but the major mortality risk is myocardial infarction. Hence careful pre-op scrutiny of myocardial, carotid and cerebral vasculature must be conducted in addition to the usual evaluation. On very few occasions, a patient’s myocardial status may require a pulmonary artery catheter for optimal intraoperative management, as is the case in a combined carotid/CABG procedure. The discussion below will deal with the routine carotid where the myocardial risk is not considered high. Note, however, in a combined CEA-CABG that a fresh stroke with the CEA is uniformly fatal (to the brain) during by-pass so CEA management is critical.

Preoperatively, premedication should be tailored to the patient and is often useful. Give the patient’s cardiac and anti-hypertension medications, if appropriate. Intraoperative monitoring should include A line (due to wide blood pressure swings), ETCO2 (to adjust ventilation to normal PaCO2), EKG to detect ischemia (II and V5 or as best for the patient) and some form of cerebral monitoring for ischemia (such as EEG). For patients who may be labile, an IV access with “low dead space” should be available and an antihypertensive infusion (such as nitroprusside) and pressor (such as neosynephrine)

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hooked in, ready to use. Placement of central line may be indicated in some patients. Rarely, a jugular CVP is used as patients may be dependent on the non-operative carotid artery for cerebral perfusion. If used, don’t place it on the operative side; the subclavian route may be preferred in patients with bilateral carotid disease.

Some clinicians prefer to conduct CEA using regional block (deep or superficial cervical plexus block), but the majority are done here with general anesthesia. Awake CEA has the advantage of excellent neurological monitoring during the procedure. However, a superficial cervical plexus block may be helpful with postoperative pain.

Induction of general anesthesia should be chosen based on cardiovascular and myocardial reserves. Hence thiopental, propofol, etomidate, midazolam, fentanyl, etc. have been used. The goal should be adequate anesthesia without marked hypotension and without marked hypertension on intubation. Hence, usually some narcotic base is useful; inhalational anesthesia to provide afterload reduction to help control blood pressure, reduce cerebral metabolism and insure an asleep patient. Isoflurane is usually chosen because it is less myocardial depressive and most cerebral depressant.

Maintenance of anesthesia is best achieved with the following goals:

1) keep end-tidal CO2 near normal

2) keep blood pressure in the high normal range for this patient (i.e., blood pressure as measured in the OR by the same method.

3) try to keep a steady level of anesthesia if possible, if EEG is being recorded

4) avoid marked hypertension, especially after the carotid is open as intracranial hemorrhage can occur.

Note also that what may be needed to reduce myocardial demands may conflict with cerebral needs. But, when possible, keep the blood pressure in the high normal range to maintain cerebral perfusion and the heart rate normal (not increased) to reduce myocardial oxygen demands. Monitor for cerebral ischemia whenever possible. Critical times are:

1) positioning-as hyperextension of the neck can kink the vertebrobasilar artery that can be a major supply in bilateral carotid disease (you can check this preop!)

2) cross-clamping of the artery prior to opening the artery

3) kinking or obstruction of a shunt if placed

4) cross-clamping prior to closing the arteriostomy

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5) hypotensive episodes.

If signs of ischemia occur, alert the surgeon; if possible, raising the blood pressure may help (if myocardial risk or risk of intracerebral hemorrhage is not high).

Monitoring for ischemia during cross-clamping can be done by several methods. These include awake surgery, EEG and TCD (see separate sections), and stump pressure. The latter is done by connecting a sterile high pressure tubing to the arterial line transducer stopcock and recording the pressure in the carotid before and after the cross-clamping. MAP less than 25 mmHG is usually bad and above 50 mmHg signalling adequate intracranial collatterals.

Placement of a shunt is equivalent to a 70% stenosis, but carries risks of dislodging emboli, a false sense of security if the shunt clots or kinks, and usually increases the complexity and duration of the procedure.

At the conclusion of the procedure, be prepared to treat hypertension which usually occurs. Attempt to awaken in the O.R. so that neurologic exam can be done; good arm movement contralateral to the operated site is usually the most important first sign.

Note that the largest number of strokes are small ones due to emboli (air or plaque) and may not be detected (usually discovered as neuropsychiatric changes). We can often detect and correct major vascular ischemia from inadequate CBF during cross clamping. Although this is a minor numerical contribution to stroke, outcome is often devastating. Note about 30% of strokes are due to clots in the operative site and therefore have their onset immediately after surgery (such as recovery room) so that exploration of the surgical site may be emergent.

Note a hematoma in the surgical site can cause airway compression that is relieved by cutting open the dressing and opening the surgical site. A neck hematoma may also make intubation difficult if it is necessary to take the patient back to the OR for explora-tion of the surgical site for postoperative bleeding.

Carotid body injury may occur postoperatively, thus if a patient has had bilateral CEA’s, prolonged supplemental oxygen may be required. Cranial nerve injury may also occur following CEA: recurrent laryngeal nerve resulting in hoarseness, hypoglossal injury resulting in tongue deviation on protrusion, and marginal mandibular nerve injury resulting in drooping of the corner of the mouth.

INTRACRANIAL VASCULAR SURGERY

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Objectives:

Know the following concepts regarding aneurysmal subarachnoid hemorrhage:

Acute CNS physiology changes with subarachnoid hemorrhage

The natural history of subarachnoid hemorrhage, specifically regarding advantages of early versus delayed surgery

Know potential effects of subarachnoid hemorrhage on the function of other systems including the cardiovascular system, pulmonary system and renal system

Evaluate the neurologic status of the patient with subarachnoid hemorrhage and intracranial aneurysm by means of the Hunt-Hess grading system

Recognize preoperative vasospasm as well as identify patients at risk for developing vasospasm; be aware of proposed mechanisms for vasospasm as well as current accepted therapy (including calcium channel blockers, lazeroid compounds and hypertensive, hypervolemic hemodilution

Understand which patients are likely to require special techniques such as barbiturate protection, hypotension, induced hypertension, or temporary vessel occlusion

Know the natural history and treatment options for arteriovenous malformations. Be aware of the importance of preoperative blood flow through the AVM with respect to potential need for induced hypotension or other techniques to reduce “breakthrough” bleeding.

This section includes discussions on intracranial aneurysms and AVM. We will first address intracranial aneurysms. Intracranial aneurysms represent a challenge to both the anesthesiologist and the surgeon. A close working relationship is essential, especially with respect to blood pressure control and coordination at the time of clipping.

Philosophically speaking, the anesthesiologist has four major concerns. These are, 1) preventing rupture, 2) allowing exposure, 3) maximizing circulatory blood flow and 4) brain protection.

1) Since uncontrolled rupture of the aneurysm represents a very real threat to life, it is usually the focus of attention. The determinants of rupture are high blood

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pressure and low ICP, as such attention should be focused on these variables. In particular, hypertension presents the greatest threat. Therefore, carefully controlled blood pressure is essential. In past, deliberate hypotension was utilized (usually with sodium nitroprusside), but concerns about cerebral ischemia have given way to “controlled normotension”. Hence we need to discuss with the surgeons and agree on a blood pressure range for case management (this is usually the lower range of the preop normal as assessed by the same technique (cuff or a-line).

Since clearly the most dangerous problem is sudden hypertension, the availabi-lity of a rapid way to lower blood pressure is helpful. It is notable that sudden hypertension can occur with a) induction, b) intubation, c) Mayfield head holder placement (pins), d) skin incision, e) dural opening and f) aneurysm dissection and exposure. Because these occur before surgical control of bleeding is possible, anesthetic management is key. Some anesthesiologists prefer deep anesthesia to avoid sudden hypertension (but without hypotension). Some surgeons will utilize temporary clipping of the major artery feeding the aneurysm prior to clipping. In this case deliberate hypotension may not be desirable and, in fact, deliberate hypertension may be preferable. Means for short term elevation for blood pressure should be available (such as bolus neosynephrine).

Since sudden decreases in ICP can promote rupture, do not allow a CSF drain (if present) to be open unless the surgeons request it (usually not until the dura is opened) and keep track of the volume drained to avoid excessive withdrawal.

2) As usual with craniotomies, considerations apply to reduce brain congestion and facilitate the surgical exposure. Thus the principles listed for craniotomy and ICP control apply. Note that cerebral swelling can be a particular problem with these cases as blood from the SAH can cause excessive swelling and excellent exposure is needed to provide access to the base of the aneurysm. Since excessive hyperventilation can contribute to cerebral ischemia, hyperventilation is usually restricted (PaCO2 30-32). Sodium thiopental can assist in reducing cerebral swelling. Note (below) some surgeons may use a CSF drain to help with exposure.

3) Methods to facilitate microcirculatory blood flow are key to perfusion of the brain in the face of ischemia and vasospasm. Maintenance of adequate blood pressure, avoidance of excessive hypocarbia and adequate oxygenation have been discussed. Hypercarbia and cerebral vasodilators are avoided because they may “steal” blood from vessels that are spastic or have lost autoregulation. Mannitol may help by reducing blood viscosity as well as hemodilution with goal hematocrit of 30-32.

4) Brain protection may be key in protecting the brain from ischemia if temporary clipping, hypotension, or rupture occurs. Traditional methods have focused on

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cerebral metabolic depression. To assess this, many individuals utilize the EEG and look for burst suppression. Traditionally this is accomplished using isoflurane or thiopental. Thiopental has additional advantages over propofol of free radical scavenging. In cases involving hemorrhage (as a result of rupture), a growing appreciation of hypothermia suggest that it may be more valuable, even if only to moderate degrees (34-35o C). Hypothermia not only reduces cerebral metabolic rate, but also inhibits excitatory amino acid release. Since SAH may cause hyperthermia, cooling may be useful even if to lower the temperature to normal. Plan on cooling using a cold water blanket beneath the patient and a cold air blanket above (Bair Hugger). In patients with uncomplicated aneurysms, normothermia (36-37 35o C) is the goal.

The case management begins with the pre-op visit where the aneurysm location and patient neurological grade are identified. Location is important for position and usually position will be supine except for basilar artery aneurysms which will require special considerations (such as prone position). Neurological grades will identify the risk assessment - if the patient has major neurological deficits then one must be very careful as ischemia and cerebral injury have been prominent. Most patients are operated on in the supine position (beach chair) with the head turned so the surgical incision can be made on the temple. To avoid excessive neck rotation, consider rotating the body with a shoulder roll under one shoulder (semi lateral position).

Premedication is often light, but may be indicated if unnecessary anxiety causes hypertension. Prior to induction, an arterial line should be inserted to observe hyperten sion with induction. A CVP is very useful but can usually be inserted after induction. The surgeons may desire a S.G. catheter postop, but a long arm catheter is usually sufficient for the operative course. After induction the surgeons may wish the patient be placed lateral for a lumbar spinal drain placement; this should remain closed until the surgeon requests drainage to improve exposure by drainage of the lateral ventricule (usually at dural opening). Identify with the surgeon what blood pressure is desirable for the case. Be cautious of blood pressure rises, especially at induction, intubation, head holder placement, and incision. Judicious anesthetic choices (including narcotics on induction) can prevent rises; bolus thiopental can help stop a rise in progress. Mannitol and lasix will likely be used during craniotomy; consider fluids that are appropriate with any craniotomy. Cooling blankets will be beneficial; note body temperature will likely drift 0.5-1.0 degrees centigrade after cooling is stopped. Begin warming after the aneurysm is clipped.

During dissection be sure that you have at least one good IV for blood transfusion and blood readily available. Remember, rupture does not indicate a need to transfuse. The actual blood loss should be observed (recall the surgical field is magnified) and keep in mind that oxygen delivery is optimal at an Hct of 30-32. If possible, a bolus dose of thiopental one minute prior to clipping or temporary clip placement may afford extra protection. Be prepared to raise the blood pressure if a temporary clip is used; the

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surgeon will request it if needed.

During closure, discuss appropriate blood pressures for post-op care so you can begin to adjust for this at the conclusion of the case (~ 20% of patients have a second aneurysm).

Vasospasm remains a major cause of morbidity and death even after otherwise uneventful aneurysm clipping. It manifests by a new focal deficit or decreased level of consciousness. Attempts to prevent neurologic deficits from vasospasm by preoperative use of calcium-antagonists (nimodipine) may be indicated (discuss with surgical team). Once it develops, vasospasm is initially treated with volume therapy (CVP increased to 10-12 mm Hg or PCWP to 15-18 mm Hg). If this does not reverse the neurologic deficit, BP is elevated pharmacologically (systolic BP 160-180 mm Hg) if tolerated.

At the conclusion of the procedure, attempt to awaken the patient and extubate when possible and appropriate, so that neurological status can be observed. Rewarming (if applicable) may be key to awakening the patient.

AV MALFORMATIONS

AV malformation resection is similar to intracranial aneurysm management and most of those principles apply. However, there are a few important differences.

First, deliberate hypotension should not be used. The AVM acts as a low pressure bypass such that adjacent areas of the brain may receive inadequate cerebral blood flow at normal pressures. With deliberate hypotension, ischemia may occur such that the brain may be placed at risk. So in general, hypotension is undesirable.

A second difference is blood loss. Whereas blood loss with an aneurysm is generally not excessive (unless during a brief period if the aneurysm should rupture), the blood loss of an AVM resection may continue for a substantial period of time. Hence, having good venous access for transfusion, a central venous monitoring line to determine central volume and a watchful eye on urine output and blood loss is important. Fortu-nately, preoperative plugging of the AVM by neuroradiological methods has substan-tially reduced OR blood losses.

Third, once the AVM is resected, blood flow to the previously deprived cerebral tissue may now be increased. If that tissue was previously ischemic, the blood brain barrier may not be normal. Hence the brain will be more prone to cerebral edema and hypertensive cerebral hemorrhage (known as reperfusion breakthrough). Hence, post resection hypertension should be avoided as should excessive use of hypoosmotic fluids. Consultation with the neurosurgeon should allow identification of postoperative blood pressure goals.

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SPINE SURGERY

Objectives:

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Be able to differentiate between radiculopathy and myelopathy as causes of CNS generated pain and/or muscle weakness. Recognize which patients with spinal cord pathology may require special techniques such as awake intubation and positioning. 1) Intubate an awake patient such that coughing or movement are

minimal. 2) Master anesthesia for awake intubation, including but not limited to at

least one of the following techniques: appropriate use of drying and sedative/analgesic agents, topicalization techniques, superior laryngeal and glossopharyngeal nerve blocks and transtracheal injection of lidocaine.

Be able to describe the following different types of spinal operations as well as their anesthetic implications with respect to purpose, relevant anatomy, degree of complexity and anesthetic complications:

Anterior cervical discectomy and fusions, anterior cervical corpectomies, posterior cervical fusions, laminectomies, and foraminotomies

Laminectomies for excision of spinal cord tumors, both intramedullary and extramedullary

Thoracic and lumbar laminectomies and fusions (PLIF, ALIF, TLIF AxiaLIF), microdiscectomies and corpectomies.

Be familiar with anesthetic techniques that will facilitate neurologic monitoring techniques such as somatosensory evoked potentials, spontaneous and evoked EMG monitoring and motor evoked potentials.

Repair of traumatic cervical spine injuries (different approaches) and specifically be aware of anesthetic implications of acute spinal cord injury

Know how to evaluate patients with spinal cord injury at any level. Be aware of disruption in normal CNS, cardiovascular, and pulmonary physiologythat can be caused by acute spinal cord injury and what impact this disruption has on appropriate monitoring and anesthetic management.

Apply appropriate techniques for airway management in the patient with spinal cord injury.

Evaluate patients for suitability for extubation considering CNS and other organ system pathology induced by the trauma.

Know anesthetic considerations in the patient with chronic spinal cord injury undergoing neurologic or non-neurologic surgery including:

Autonomic hyperreflexia - what patients are at risk and how may this syndrome be prevented and treated

Know changes in neuromuscular function induced by chronic spinal cord injury

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and the impact these changes have on anesthetic management

The implications of spine surgery rest on the location (cervical, thoracic, lumbo-sacral), pathology (radiculopathy, stenosis, spinal cord injury, spinal deformity, tumor/metastasis) and position (lateral, prone, supine).

Location: Cervical surgery can be conducted prone or supine. For cervical procedures, preoperative consultation with the surgeon is important, not only because of position considerations, but also about intubation and induction. In addition to the usual airway/ intubation considerations, the surgeon may request an awake intubation with awake positioning. If these are requested, neurological function (moving arms and legs on command) should be documented after intubation and positioning.

Except for transabdominal, extraperitoneal approaches to the lumbar spine, (where the usual spine considerations apply), anterior cervical procedures are the only frequent supine procedures in the supine position. Here considerations mentioned above apply for anterior cervical procedures including arm placement at side. Also notable is that a surgeon’s arm may rest on the face causing pressure injury and kinking of the endotracheal tube.

If the surgical procedure on the cervical spine is anterior, the approach is usually via the right neck. Retractors are used to expose the spine. These retractors can cause kinking of the carotid artery. To monitor this, one should periodically feel the temporal artery pulsations - especially when the retractor is placed or adjusted. Be sure to place an OG/NG and/or esophageal stethoscope to help the surgeon identify the esophagus. Post surgically be aware of tracheal, laryngeal nerve and esophageal injury, particularly if the procedure is very high (C2-C3). Occasionally a trans-oral approach may be used for C1 odontoid procedures. In these cases a tracheostomy may be conducted initially. If vocalis EMG is requested for anterior cervical spine procedures, use the Zomed endotracheal tube and avoid muscle relaxants during the case.

In addition to the obvious prone considerations, the prone position has several potentially serious problems. First, caution must be made in positioning of the male genitals and female breasts (no pressure on nipples). Head position must be carefully done. For cervical and high thoracic procedures the head should be neutral on a foam block, horseshoe head holder or Mayfield tongs. Periodically check the face when prone, massage areas where pressure injury may occur (e.g. chin). Post operative visual loss (POVL) can occur via mechanisms that remain largely unknown.

When prone, the arms may be positioned up by the head on arm boards for low thoracic or lumbosacral procedures. However, for high thoracic or cervical prone procedures the arms must be positioned at the patient’s side. In this case lines (IV’s and arterial line) need to be anticipated and placed before positioning. Two IV’s may be desirable when arms at the side, in case one becomes dysfunctional during the case.

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The prone position has become known for some unexpected deaths due to air embolism. These appear to occur because the surgical field is sufficiently above the atrium to allow air entrainment. When prone, abdominal pressure may obstruct venous return via the vena cava and promote epidural venous bleeding reducing the chance of air embolism. Although this bleeding is undesirable, the opposite (the abdomen hanging free) can cause negative abdominal pressure due to gravity that enhances the gradient increasing the likelihood of air embolism. Hence, air embolism monitoring and precautions need to be taken in the prone position when positioned this way (when position or frames allow the abdomen to hang free).

Surgery on the lumbosacral spine is frequently done in the prone position. However, during ALIF or TLIF procedures the position is anterior or lateral respectively. The arms will be on arm boards up towards the head. Thoracic spine procedures can be conducted prone (arms to the side) or lateral through the chest (thoracotomy). If via a thoracotomy, a double lumen tube may be requested. Also, if lateral, the diaphragm may be surgically opened and closed. If this occurs, post-op ventilation may be necessary to reduce the strain on the diaphragmatic suture line. In addition, some patients after thoracic spine instrumentation surgery (whether prone or lateral) may not breath properly due to pain. Their breathing is usually dyscoordinate with shallow tidal volumes. If this is the case, postoperative ventilation may be needed usually the problem resolves in 12-24 hours.

Pathology: In general, radiculopathy procedures are on relatively healthy, young individuals; blood loss is often mild. Multilevel radiculopathies or laminectomies for spinal stenosis carry more blood loss (usually not excessive) and patients are often older, sicker and debilitated (if excessively debilitated succinylcholine may be contraindicated). Spinal cord injury is often associated with an unstable vertebral column that may necessitate awake intubation (instabilities above T4) and cautious positioning (such as awake positioning). If neurological injury is substantial (acute spinal cord injury) succinylcholine may be contraindicated (between one day and one year after injury). A variety of other acute and chronic medical considerations also apply.

Correction of spinal deformities, as well as some major spinal cord injury procedures (notably lumbo-thoracic) can be associated with substantial blood loss. Here good venous access, an arterial line and central venous access may be needed. Cell salvaging may be very helpful.

Some surgeons request deliberate hypotension during spinal procedures to reduce blood loss. If, however, the spinal cord is abnormal or injured, vascular auto regulation may be lost or hypoperfused (when injured) such that the cord is pressure sensitive. In these cases deliberate hypotension and hyper/hypocarbia may be detrimental. In general, use normal ventilation and blood pressure levels.

If the spinal procedure is for primary or metastatic tumor, blood loss can be massive Page | 31

(especially if metastatic renal cell carcinoma). For these procedures, preparations for the blood loss is essential. Often cell salvaging is not used because of the fear of reinfusing cancerous cells.

The anesthetic choice for spinal surgery is usually not critical. However, considerations should be given to choice of anesthetic when the following are required 1) awake intubation, 2) awake positioning, 3) prompt postoperative awakening and 4) monitoring with EMG, SSEP and MEP.

SSEP monitoring will require TIVA using a narcotic or ketamine based anesthetic and sedative and /or only low dose inhalational anesthetic. If motor evoked potentials are used the anesthetic will likely need to be totally intravenous and muscle relaxants may need to be avoided.

In general, it is highly desirable to have the patient sufficiently awake after spine surgery to examine neurological function as soon as possible (especially in the O.R.). This is because unexpected paralysis can often be reversed if the problem is corrected quickly by re-operation (usually 3-4 hours). Note that extubation is not needed, just sufficiently awake to follow commands. In general, delayed extubation is desirable with cervical surgery. This is because it is highly undesirable to emergently reintubate using direct laryngoscopy since the mechanical motion of the neck may damage the surgical procedure or not be possible if a halo brace has been placed.

The Prone Position for Cervical/Upper Thoracic Procedures DEEP BRAIN STIMULATOR PLACEMENT FOR PARKINSON’S

DISEASE

Objectives:

Understand the indications for electrode placement Keep in mind the issues involving providing anesthesia in a remote

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location such as the MRI suite Know the surgical complications associated with this procedure and how

anesthetic management affects them.

Without any medications except local anesthesia and propofol in MRI, this procedure involves deep brain stimulation via electrodes of generally, the thalamus, subthalamic nuclei or globus pallidus to interrupt the motor pathway involved in Parkinson’s movement. Sedation in MRI is needed to reduce motion artifact in order to get an adequate target location for the subsequent electrode placement. During electrode placement, active movement is needed as a marker to identify proper electrode placement. Since this movement is easily stopped with anesthetic agents, medications must be avoided except during the MRI.

This procedure is increasing in use because of effectiveness and relatively benign nature of the procedure. Candidates include patients who are not adequately treated by available medications or have unacceptable side effects on those medications.

Part I MRI

Some patients undergoing MRI require sedation. The anesthesiology personnel should accompany the patient into the MRI room to keep an eye on the patient. When entering the unit, be careful to not take metal objects in: they can be propelled into the unit and cause harm, or be damaged by the magnetic field (especially watches and credit cards). In the MRI, monitoring is available with a non-invasive BP and pulse oxygen saturation as well as visual inspection of breathing. Oxygen is supplied via the nasal cannula from stainless steel tanks in the room or larger cylinders outside the room.

The main anesthesia task in the MRI is to sedate the patient (propofol) to prevent movement of the head during the scan. Some body movement from the Parkinson’s can be tolerated because the head is somewhat restrained, but moderate motion will cause a blurred image preventing adequate acquisition of images. Sedation may also be needed to help the patient tolerate lying on the table for an hour or so or if claustrophobia is a problem. Propofol is typically administered by infusion. A special infusion pump is needed to work in the MRI area and it must be kept at a distance from the scanner. Infusion rates are usually 25-75 mcg/kg/min and need to be titrated to effect.

Part II Leksell Frame placement/ CT Scan

At our institution, the MRI is completed days in advance of the DBS electrode placement. The patient does get a CT scan on the morning of surgery with the Leksell frame placed by the surgical team. The anesthesia personnel are not involved in this part.

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Part III Electrode placement in the OR

The patient is returned to the OR and placed on the OR table. At this time monitors should be placed including A-line (for patients with hypertension) and foley catheter with sedation. The patient will then be attached to an immobile frame via the Leksell frame. Ensure that the patient is comfortable, pressure points should be padded prior to start of procedure. Oxygen should be supplied via nasal cannula and sedation provided while a burr hole is created. After the burr hole is placed on the skull, then a probe inserted and electrical stimulation done through the probes to confirm the location. Stimulated motor activity (including writing) or cessation of the tremor helps confirm location.

Preop and intra-op, avoid benzodiazepines (patients are typically elderly and sensitive to benzos) which can alter sensorium. In addition, keep patients normotensive with goal SBP< 140 to reduce the risk of intracranial hemotoma. To achieve this, use nicardipine, nitroprusside or hydralazine. Avoid beta- blockers as it can skew intra-op neurological exam findings. (Keep in mind that the patient may need lots of TLC as these can be long, uncomfortable cases).

In the unlikely event that intracerebral disaster occurs (such as blood vessel rupture), the major problem is airway management. Perhaps the fastest way to ventilate is by LMA, which could also serve as a track for fiberoptic intubation. Taking the patient out of the Leksell frame apparatus off may take several minutes.

Once the procedure completed patients are admitted for neurological observation.

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Patient in Leksell frame on OR bed

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AWAKE CRANIOTOMY FOR EXCISION OF SEIZURE FOCI

Objectives :Understand and anticipate preoperative and intraoperative anesthetic considerations for patients undergoing electrocorticography and be familiar with techniques for ‘awake’ craniotomies for patients undergoing resection of pathology near eloquent structures. Know the airway and sedation requirements for stereotactic neurosurgical procedures conducted with either general anesthesia or monitored anesthesia care

INDICATIONS FOR EPILEPSY SURGERY a. The identification of an isolated seizure focus or “epileptogenic zone” in

area of relatively dispensable brain. The planned resection must preserve motor and sensory function in the upper/lower extremity, language, and memory.

b. The seizures have proven unresponsive or “medically intractable” to an adequate trial of anticonvulsant therapy.

c. The seizures severely interfere with the quality of life and functional capacity of the patient.

d. Epilepsy surgery offers a reasonable opportunity to provide improved medical control or total elimination of seizures with improved quality of life and functional status for the patient.

ANESTHETIC GOALS To provide for the comfort and safety of the patient:

1) Establish rapport with the patients.2) Educate the patient with respect to the stages of the operation and the role of

the anesthesiologist as a member of the epilepsy team. 3) Maintain adequate spontaneous ventilation and stable hemodynamics during the

entire surgical procedure.4) Recognize and appropriately manage intraoperative seizures

Have propofol or thiopental readily available for bolus.

To facilitate the surgical procedure: 1) Optimize surgical exposure with mannitol, lasix, etc., as needed 2) The patient should be awake and participating during mapping and comfortable,

quiet, and still at other times.

To facilitate the intraoperative assessment during the resection:1) Use medications which do not alter the seizure threshold or interfere with the

intraoperative EEG (especially benzodiazepines).2) Tailor the anesthetic technique to facilitate a smooth and rapid emergence prior

to intraoperative testing. The patient should be awake, comfortable, and appropriate for testing.

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During craniotomy, the patient will require sedation. The typical anesthetic agents include propofol and remifentanil infusion as these have rapid onsets/offsets. The actual operating procedure is divided into six stages: Craniotomy, Corticography, Functional mapping, Cortical resection, Post resection EEG and Closure.

CraniotomyThe patient is usually positioned comfortably in the supine position (shoulder roll is sometime placed to achieve a semi-lateral position) on the operating table with the head in Mayfield pins. Pressure points should be padded. Monitors are placed and oxygen is administered with a nasal cannula with capnography monitoring. Propofol and Remifentanil infusions are titrated to an appropriate level of sedation. A foley catheter should be placed after adequate sedation is obtained. The sterile drapes should be positioned so as to allow full access to the patient’s face for airway management and adequate ventilation. In addition, this will decrease claustrophobia and facilitate intraoperative motor and speech mapping. Discontinue the propofol and narcotic infusions once the dura is opened and allow the patient to awaken.

Corticography EEG recording is performed directly from the cortical surface using interictal spiking to identify the seizure focus and delineate the extent of the planned resection. For this, the patient should be awake.

Functional mappingWith the patient should be awake and alert, motor and sensory cortex, language and memory areas will be identified. Seizures may occur and are usually self-limiting, however, may require treatment with iced saline to the brain or thiopental/propofol.

Cortical resection During this period, the surgeon may request that the patient remain awake. Resume the propofol and narcotics infusion after verifying with surgeon.

Post resection EEGThis is done to ensure that the seizure focus has been successfully resected. The patient should be awake. If the patient was sedated, coordinate with the surgical team to have the patient awake when needed.

ClosureThe patient will need to be sedated again as the dura/bone flap and scalp are closed.

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Basic Lateral Position for awake craniotomies

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NEUROPHYSIOLOGIC MONITORING

Objectives:

Know basic implications of changes induced by either anesthesia or surgery in the electroencephalogram (EEG)/processed EEG (pEEG) and evoked potentials.

Electroencephalogram Understand basic wave patterns and concepts of frequency and amplitude Understand typical changes in EEG patterns with induction of anesthesia

as well as changes associated with deepening of anesthesia Understand EEG changes associated with ischemia - mild, moderate and

severe Transcranial doppler (TCD) monitoring

Know theoretical basis for TCD monitoring Know indications and use for TCD monitoring Know limitations of TCD monitoring

Evoked potentials Have a general concept about the generators of brainstem auditory

evoked responses, somatosensory evoked responses, motor evoked potentials and electromyography.

Understand basic evoked potentials descriptors- latency and amplitude Know effects of general anesthetic agents (both inhalation agents and

intravenous agents) on evoked potentials Know the significance of non-pharmacologically induced changes in

evoked responses during surgery lntracranial pressure monitoring

Know indications and complications associated with ICP monitoring Understand advantages and disadvantages of commonly used ICP

monitoring techniques

EEG monitoring can be a useful supplement to surgery when seizures need to be identified (see “seizure surgery”), when the general state of cerebral metabolism needs monitoring or when cerebral ischemia can occur. EEG is a standard of care in many institutions for carotid endarterectomy. There are no special anesthesia requirements for EEG, but each agent may cause slightly different patterns. A constant level of anesthesia will better allow one see EEG changes representative of ischemia. Certain anesthetics need to be avoided if seizure monitoring is desired (especially inhalational and benzodiazepam).

The EEG is generated by synaptic potentials in the surface pyramidal cell layer of the cortex and hence only monitors the 20% of the brain on the cerebral surface. The EEG is monitored by applying surface electrodes; one electrode is used for all channels and is called a ground. This ground electrode can be placed anywhere on the head and is often placed somewhere on the midline.

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Each channel of the EEG uses 2 electrodes. The EEG machine simply amplifies the difference between these two electrodes. The activity seen represents the area of brain within 2-3 cm of each electrode.

Examining the raw, single channel EEG one can see the alpha rhythm of a lightly anesthetized individual. This is a rhythmical regular wave of 8-12 cycles per second. A faster, disorganized Beta (> 12 Hz) rhythm is seen when awakening. Slower theta waves (4-8 Hz) are seen with deep inhalational or moderate dose narcotic anesthe sia. Slow Delta (< 4 Hz) waves may be deep anesthesia, or may be ischemia if low amplitude. Deep anesthesia suppression can be seen as so called “burst suppression”. Here bursts of EEG activity are alternated by regions of inactivity. At this point near maximal metabolic suppression (40-50%) is produced by anesthesia. Deeper metabolic suppression requires hypothermia and is signalled by a flat line. Under special conditions “spike and wave” seizure activity may be seen representative of seizures.

Although one EEG channel is useful to examine the general state of the brain, 2 or more symmetrical channels are best used when examining for vascular ischemia. For example, when a specific vascular territory is at risk, one channel should be placed over the area at risk (such as the parietal cortex of the middle cerebral artery during a carotid endarterectomy) and a second channel placed in an area not at risk (such as the other side of the head). Thus when both channels change, the drop is a global event (e.g., deep anesthesia or hypotension) with a unilateral event signalling a vascular event. For CEA, a minimum of 2 channels are recommended with the electrodes placed over the left and right parietal cortices (the middle cerebral artery territory).

Note that deep anesthesia and ischemia are both shown on the EEG as slow waves and eventually flat EEG. Thus the EEG must be interpreted in the context of the rest of the surgery, physiology and anesthesia.

TRANSCRANIAL DOPPLER MONITORING (TCD)

TCD is discussed below because although not clinically relevant at this institution, it is used at some institution. The basic concept and its application are addressed below. The transcranial Doppler (TCD) uses the Doppler shift of sound waves reflected off moving red blood cells to measure blood flow velocity in the basal cerebral arteries (most commonly, the middle cerebral artery). The waveform resembles an arterial pulse waveform, quantified into systolic, mean and diastolic flow velocities and pulsatility index. When interpreting TCD values, an assumption is made that changes in blood flow velocity reflect changes in cortical CBF. However, this assumption is true only as long as the diameter of the conducting cerebral arteries does not change and the measurement angle of the Doppler probe (angle of isonation) remains constant. It is also important to note that technically satisfactory recordings cannot be obtained in all patients.

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A major use of TCD in neuroanesthesia has been during carotid endarterectomy (CEA) as an indicator of the adequacy of cerebral blood flow, and for detection of vasospasm following subarachnoid hemorrhage (SAH). During CEA, TCD is used to detect low CBF during the time of carotid cross clamping, which indicates the need for shunt placement. If a reduction in mean cerebral artery (MCA) velocity to 40% or less of pre-clamp values is seen, our neurosurgeons place a shunt. TCD is also useful to detect cerebral emboli (both particulate and air), which are most commonly seen at the time the carotid clamp is removed. One additional use of TCD during CEA is detection of cerebral hyperemia, which is associated with a large increase in MCA velocity (greater than 1 00%) after the carotid clamp is removed. This is a valuable indicator of the importance of avoiding emergence and postoperative hypertension, to prevent the development of postoperative headache, seizures and cerebral hemorrhage.

In the intensive care unit (ICU), the TCD is useful for detection and documentation of the severity of vasospam after SAH. As the diameter of the arterial lumen decreases with vasospasm, the velocity of blood flowing through the narrowed vessel must increase if flow is to be maintained. Vasospasm is diagnosed by mean MCA velocity of > 120 cm/ sec and a MCA/ICA ratio (Lindegaard Ratio) of > 3.

Although not part of the criteria for brain death, a characteristic TCD pattern is seen in patients who are clinically brain dead. Blood moves forward during systole, which is seen on the TCD as a short systolic inflow of blood. However, because blood flow has ceased in the microvascular bed, this is followed by flow reversal during diastole, associated with the exit of blood from the cranium.

EVOKED POTENTIALS

Evoked potentials (EP) can be used to examine specific neural tracts. To allow optimal EP monitoring, the anesthetic agents are important, but more importantly, the level of anesthesia must be constant so that changes noted on evoked potentials are not confused with the effect of changing anesthetic depth.

In general, evoked potentials are measured by placing recording electrodes so that small electrical “potentials” can be seen when they are “evoked” (elicited) by stimulating the nervous system. These electrical events are like the P, Q, R, S, T waves in the EKG and represent the electrical events that follow activation of a nervous system pathway. Unfortunately, differing from the EKG, these electrical signals are so small that several hundred to several thousand must be recorded and averaged so that background noise is removed. Hence a sophisticated recording device and a technician are required.

Brainstem Auditory Evoked Responses (BAER) or Auditory Brainstem Responses (ABR) are produced by stimulating the ear by sound “pips” or clicks using earphones. There are 5 recognizable waves in the first 10 milliseconds which represent the

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electrical response of the eighth cranial nerve, lower and upper brainstem. These are useful in ear surgery (where the cochlea is at risk) or in posterior fossa surgery on the brainstem (such as acoustic neuroma). Fortunately there are no anesthesia restrictions for this technique. It should be noted that the auditory cortex can be monitored by recording the potentials over the auditory sensory area (temporal-parietal cortex) after ear stimulation (midlatency auditory response). The anesthetic requirements are similar to the SSEP (i.e., somewhat restrictive).

SomatoSensory Evoked Potentials (SSEP) are produced by electrical signals placed over major peripheral nerves. Similar to a neuromuscular block stimulator these depolarize the nerve causing a muscle twitch (equivalent to tetanus) and initiate a series of recordable sensory signals traversing up the peripheral nerve, plexus (lumbar or brachial), spinal cord, brainstem and sensory cortex. Electrodes can be placed at various locations over these structures to record the signal. Neuroanatomically, the signals appear to correlate best with neural pathways of proprioception and vibration (joint sense). Median nerve, ulnar nerve, posterior tibial nerve and common peroneal nerve are most commonly used. However, because the cortical SSEP is produced by the sensory cortex, it can be used for some cortical surgery where the sensory cortex is placed at risk (such as vascular surgery). All inhalational agents (including N2O) have been shown to depress cortical SSEP with minimal effects on brainstem and more caudal potentials (e.g., peripheral nerve signals are very resistent). Hence if the cortical signals are desired (as they almost always are) a narcotic based anesthetic with supplemental low dose inhalational anesthetic is probably the best initial anesthetic. If signals are too small, then a total intravenous technique may be best. On rare occasions, the SSEP may not be recordable. Note that muscle relaxants do not interfere with sensory evoked potentials. However, it is useful with the SSEP to see the hand or foot twitch to verify proper nerve stimulation therefore moderate neuromuscular blockade is acceptable (TOF 2/4 or greater) unless contraindicated by the surgical procedure or some other form of monitoring such as electromyography (EMG). At this institution, EMG is almost always performed as part of neuromonitoring.

Visual Evoked Potentials (VEP) are recorded over the occipital cortex after stimulation of the eyes by light flashes. These are extremely sensitive to anesthesia. VEP has generally lost favor as a monitor. VEP are typically performed on patients with multiple sclerosis on an outpatient basis with no anesthesia personnel involvement.

Motor evoked potentials (MEP) differ from sensory evoked potentials in that they attempt to monitor motor tracts. MEP can be produced by transcranial stimulation from magnets (tcMMEP) with the potentials recorded over the spinal cord or by high voltage shocks (tcEMEP) through electrodes placed on the scalp with potentials recorded peripherally. An alternate to cranial stimulation is spinal cord stimulation via epidural electrodes (scEMEP) or by electrodes placed in or on the vertebral bodies (neurogenic MEP- nMEP). However, since the entire cord is stimulated, criticism is raised about monitoring not being pure motor tracts. scEMEP and nMEP are both very resistent to anesthetics and may require

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muscle relaxation to remove muscle interference. At this institution, tcEMEP is routinely used.

Electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction. It is usually achieved via intramuscular electrode placement. This technique allows for observation of neurotoxic discharges with nerve irritation. This technique is not sensitive to volatile anesthetics but is sensitive to neuromuscular blockade.

Anesthetic management

EEG only (moderately sensitive to volatile anesthetic insensitive to neuromuscular blockade)

Induction as usual Balanced Maintenance (≤1 MAC)with propofol, opioids and neuromuscular

blockade as needed

BAER only ( insensitive to volatile anesthetic or neuromuscular blockade) Induction as usual Maintenance as usual

SSEP only (sensitive to volatile anesthetic insensitive to neuromuscular blockade) Induction as usual Balanced Maintenance ( ½-1 MAC, ketamine, propofol and/or dexmedetomidine) Opioids and neuromuscular blockade as needed

MEP (very sensitive to volatile anesthetic & neuromuscular blockade) Induction as usual (neuromuscular blockade as needed for induction) TIVA – Opioids , propofol and ketamine. (Note:Use of dexmedetomidine not

recommended)

When EMG is used in combination with any of the techniques above, neuromuscular blockade should be avoided. Keep in mind that although some neuromonitoring techniques are insensitive to neuromuscular blockade, the procedure itself may preclude use of neuromuscular blocking agents.

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