8. canine and feline anesthesia

1Copyright © 2011, 2003, 2000, 1994 by Mosby, Inc., an affiliate of Elsevier Inc.

Canine and Feline Anesthesia

General Anesthesia Sedation Neuroleptanalgesia Local and Regional

Anesthesia

Chapter 8


Patient Preparation

Don’t take shortcuts Don’t skip steps Incomplete patient preparation can result in

life-threatening consequences


Selecting an Anesthetic Protocol

The list of anesthetics and adjuncts prescribed for a particular patient Includes calculated dosages, routes, and order of

administration Selected by the veterinarian-in-charge Calculate, check, and recheck drug doses, oxygen

flow rates, and fluid administration rates Takes into account minimum patient database,

patient physical status, and procedure Modified protocol for ill, pediatric, or otherwise

compromised animals


Minimize Adverse Effects of Anesthesia

Correct physiologic abnormalities prior to anesthesia Base the protocol on the results of the patient’s

minimum database Use a balanced protocol consisting of multiple agents Double-check all injectable drug doses prior to

administration Label all syringes with the patient name, drug name,

and drug concentration Administer no more than the minimum dose needed

to achieve the desired level of anesthesia Administer all IV agents “to effect” unless told

otherwise


General Anesthesia: IM Induction

Anesthetic agents are administered by intramuscular (IM) injection

Anesthetic depth gradually increases, peaks, and gradually decreases

After injection the anesthetist has little control over the anesthesia May administer more anesthetic if adequate depth

is not reached If a reversal drug is available for the anesthetic

agent, it can be administered if patient is too deep


Induction with an IM Agent or Combination


General Anesthesia: IV Injection and Ultra–short-acting Agent

Technique used for short procedures <10 minutes of anesthesia

Drugs: Propofol, methohexital, thiopental sodium, or

etomidate Drug is given to effect Anesthetic depth increases rapidly then

decreases gradually Anesthetist controls peak effect and can

increase depth by administering more anesthetic agent


Induction with an IV Injection of an Ultra–short-acting Agent to Effect


General Anesthesia: TIVA and Ultra–short-acting Agent

TIVA: Total intravenous anesthesia Patient is induced to effect; additional boluses

are administered every 3-5 minutes as needed to maintain surgical anesthesia Short-to-moderate length noninvasive procedures Propofol is the most commonly used agent Anesthetist can increase depth but can’t decrease

depth if excessive


Total Intravenous Anesthesia (TIVA) by IV Boluses of an Ultra–short-acting Agent


General Anesthesia: TIVA by CRI

Total intravenous anesthesia by constant rate infusion (CRI)

Patient is induced to effect Anesthesia is maintained by constantly

infusing small amounts of anesthesia via a syringe pump Slows down and moderates changes in depth as

seen with bolus administration


Total Intravenous Anesthesia by Constant Rate Infusion


General Anesthesia: Inhalant Agent

Not an injection technique Induction is faster than IM induction, but slower

than IV induction Anesthetist has control over depth of the

anesthesia; can make changes rapidly Delay between time dial setting is changed and

patient depth occurs Factors that affect delay time

Patient respiratory drive Agent used and carrier gas flow rate Type and volume of breathing circuit


Induction and Maintenance with an Inhalant Agent


General Anesthesia: IV Induction and Inhalant Maintenance

Most commonly used method of inducing and maintaining anesthesia in small animals

Dynamic elements of both IV and inhalant administration Rapid induction Good control over both increases and decreases

in anesthetic depth Rapid recovery


IV Induction and Maintenance with an Inhalant Agent


Equipment Preparation

Locate, check, and prepare all equipment needed for entire anesthetic period prior to induction Intubation equipment Syringes, needles, drugs, fluids required Equipment designed to prevent hypothermia Small animal anesthetic machine

• Semiclosed rebreathing system (≥2.5 to 3 kg patient weight)

• Non-rebreathing system (<2.5 to 3 kg patient weight) Crash cart with emergency drugs and equipment


Premedication or Sedation

Calms the patient and prepares the patient for anesthetic induction

Desired effects Sedation, cholinergic blockade, analgesia, muscle

relaxation Drugs

Tranquilizers, alpha2-agonists, opioids, dissociatives, anticholinergics


Premedication or Sedation (Cont’d)

After IM injection place the animal in a quiet yet accessible place Close observation until agent takes effect Stimulation or excitement may diminish the

beneficial effects Induction should follow immediately after

desired effects are reached


Anesthetic Induction

Patient loses consciousness and enters surgical anesthesia Take the patient from consciousness to stage III

anesthesia smoothly and rapidly Intubate when possible while animal is still light IV induction is most common and takes animals

through the excitement stage most rapidly Attempt to avoid the excitement/struggling stage,

which is seen more often with mask induction IM induction results in smooth, gradual CNS

depression with little apparent time spent in the excitement stage


IV Induction

Drugs used Mixture of equal volumes of ketamine and

diazepam or midazolam Propofol Neuroleptanalgesics Thiopental sodium Etomidate Various other combinations containing

dissociatives, tranquilizers, and opioids


IV Induction (Cont’d)

Administer IV to effect (unconsciousness) Don’t administer the entire calculated dose all at

once Allow for individual patient response to anesthetic


IV Induction (Cont’d)

Premedication drugs can affect the dose of general anesthetic required

Titration IV drugs given as a series of bolus injections and

discontinued when desired effect is reached IV induction produces up to 10-20 minutes of

anesthesia If more time is needed, anesthesia is maintained

with inhalation anesthetics or administration of propofol, methohexital, or etomidate by repeat boluses or CRI


Inhalation Induction

Anesthetic induction using a facemask or induction chamber

Drugs used: isoflurane and sevoflurane Low blood-gas solubility coefficient Results in rapid passage through stage II

anesthesia


Inhalation Induction (Cont’d)

Mask induction Use of a facemask to induce anesthesia Requires skillful restraint to prevent patient or

operator injury Don’t restrict chest excursions or the airway Fit the mask prior to induction Mask obscures muzzle and eyes normally used for

monitoring Need higher oxygen flow rates than with

endotracheal tube


Inhalation Induction: Facemask

Cautions Exposes personnel to waste anesthetic gas

• Need adequate room ventilation Patient struggling can lead to epinephrine release

• Use only on calm or sedated patients Longer induction period

• Avoid in patients with poor respiratory function Intubate immediately when possible

• To gain control of airway and ventilation Always keep airway open

• Don’t occlude nostrils or compress airway or chest


Inhalation Induction: Chamber

Placing patient in a closed chamber infused with anesthetic gas Patient is usually <5-7 kg body weight Used for small, aggressive patients

Examine chamber prior to use Tight-fitting lid with two gas ports


Inhalation Induction: Chamber (Cont’d)

Complications Stress, trauma, vomiting, airway blockage Hard to monitor patient Exposes personnel to waste anesthetic gas

• Attach scavenger Epinephrine release

• Predisposes patient to cardiac arrhythmias and hypotension


IM Induction

Neuroleptanalgesic combinations and a variety of combinations of tranquilizers, dissociatives, and opioids used to induce general anesthesia

Benefits Use in animals in which IV injections are difficult

• Young animals, aggressive animals, wild animals, captive animals in zoos

May need restraint equipment, blowpipe, or tranquilizing gun


IM Induction vs. IV Induction

The dose of a drug needed for IM induction is generally about twice the corresponding IV dose

IM induction takes longer to achieve high enough brain concentration to induce anesthesia

After peak effect of the IM drug is reached and the patient is still too light, an additional drug or inhalant agent must be administered to get the patient deep enough to intubate

IM induction results in a longer recovery period because of a longer metabolism time


Endotracheal Intubation

Endotracheal tube is placed in the patient’s airway after general anesthesia induction Conducts air or anesthetic gases directly from oral

cavity to trachea Bypasses the nasal passages and pharynx Can be connected to an anesthetic machine to

maintain anesthesia


Endotracheal Intubation (Cont’d)

Benefits Helps maintain an open airway

• Leave in place until the swallowing reflex returns More efficient delivery of anesthetic gas than

facemask• Decreased exposure of personnel to waste gas

With inflated cuff helps prevent aspiration of vomitus, blood, saliva

Reduces anatomic dead space• Improved efficiency of gas exchange

Ventilation can be supported manually or mechanically

• Especially useful for patients in cardiac or respiratory arrest


Feline Intubation


Equipment for Endotracheal Intubation

Three endotracheal tubes of slightly different diameters

Two-foot length of IV tubing or rolled gauze to secure tube

Gauze sponge to grasp tongue 12-mL syringe to inflate cuff Good light source Stylette for narrow diameter tubes Lidocaine injectable solution or gel to control

laryngospasm (cats) Laryngoscope with appropriate blade


Intubation Equipment


Selecting an Endotracheal Tube

Diameter Small enough to not cause trachea injury Large enough to provide a seal with inflated cuff

Length: minimize mechanical dead space Must reach the thoracic inlet Must not extend beyond the end of the muzzle

Patient Species, conformation, and breed

Preparation


Proper Endotracheal Tube Placement


Intubation Procedure

Know the anatomy of the throat Pharynx and larynx

Know the proper restraint and positioning techniques Don’t attempt intubation unless you can visualize

the larynx Have proper lighting Induce patient with IV anesthetic

Unconsciousness, no voluntary movement, no pedal reflex, sufficient muscle relaxation, no swallowing when tongue is pulled


Anatomy of the Pharynx


Intubation Procedure

Insert tube rapidly and correctly Place patient in lateral recumbency

Secure the tube and inflate the cuff Turn on the oxygen Attach the breathing circuit Turn on the anesthetic vaporizer Begin patient monitoring


Endotracheal Intubation in Small Animals


Checking for Proper Tube Placement

Revisualize larynx and confirm the tube is in the correct location

Watch reservoir bag as animal breathes Feel for air movement from the tube connector as

patient exhales Fogging of the tube during exhalation Unidirectional valve motion Palpate the neck Ability of patient to vocalize indicates misplaced tube Patient coughs during intubation Capnometer connection


Laryngospasm

Reflex closure of the glottis in response to contact with an object or substance Common in cats, swine, and small ruminants in

light plane of anesthesia Makes intubation very difficult; larynx is easily

damaged May lead to cyanosis or hypoxemia


Laryngospasm (Cont’d)

Prevention 2% injectable lidocaine or lidocaine gel Adequate depth of anesthesia Wait for glottis to open before intubating Don’t force the tube


Securing the Tube and Cuff Inflation

Tie the ET tube securely without compressing the tube

Cuff the tube Extend the patient’s head Have an assistant close the pop-off valve and

compress the reservoir bag Listen for gas leaks Inflate the cuff until the leaking just ceases at a

pressure of 20 cm H2O


Complications of Intubation

Vagus nerve stimulation Brachycephalic dogs or other breed deformities Overzealous intubation efforts Overinflation of cuff Obstructed endotracheal tube Waiting too long to remove the tube Improper cleaning and sanitizing between uses Tracheal and/or laryngeal irritation leading to

postsurgical cough


Maintenance of General Anesthesia

Inhalant agent Repeated boluses of ultrashort-acting agents Continuous rate infusion (CRI) Injectable and inhalant agents Intramuscular injections


Patient Positioning, Comfort, and Safety

Support the patient as it loses consciousness (especially the head)

Remove IV needle and syringe immediately after successful intubation

Lay patient in lateral recumbency immediately after intubation; then secure the tube and inflate the cuff

Ensure the endotracheal tube is inserted properly without bends or kinks


Patient Positioning, Comfort, and Safety (Cont’d)

Temporarily disconnect tube when turning the patient

Support anesthetic machine hoses so no drag is put on the endotracheal tube

Check position of hoses and tube during transfer and repositioning

Make sure reservoir bag is visible at all times Put animals in as normal a position as

possible on the surgery table


Patient Positioning, Comfort, and Safety (Cont’d)

Don’t use heavy drapes or instruments that will lie on the chest of small animals

Don’t overtighten leg restraints Place patient on a heat-retaining surface Place normal lung up if one lung is diseased Be cautious of tilting the surgery table Use artificial tears or other corneal lubricant


Anesthetic Recovery

The period between the time the anesthetic is discontinued and the time the patient is able to stand and walk without assistance

Influencing factors Length of anesthetic period Condition of patient Type of anesthetic administered and route of

administration Patient body temperature Patient breed


The Anesthetist’s Role in Recovery

Discontinue administration of anesthetic agents

Continually to monitor patient through the stages of recovery

Administer oxygen as necessary, especially to shivering patients Oxygen source placed close to the nostrils Elizabethan collar and cellophane cover Nasal catheter Oxygen cage

Administer reversal agents if available


The Anesthetist’s Role in Recovery (Cont’d)

Maintain patent airway and extubate when appropriate Prepare by deflating cuff and untying gauze Remove when the swallowing reflex returns (dogs,

cats) or when signs of impending arousal are present (voluntary limb, tail, or head movements)

Remove the tube in one slow, steady motion



Provide general nursing care Quiet handling, calm reassurance, attention to

patient comfort level Prior to consciousness remove all restraint ties

and make sure all accessory procedures are complete

Prior to consciousness remove all monitoring equipment, probes, cuffs, and electrodes

Be gentle when moving the patient Leave IV catheter in place until recovery is

complete



Provide general nursing care (Cont’d) Hasten recovery with gentle stimulation

(talking, rubbing, gently move ET tube) Turn every 10-15 minutes to prevent hypostatic

congestion Never leave patient unattended Gradually rewarm hypothermic patients



Provide adequate analgesia and other prescribed medications Analgesics should be administered before the

onset of pain Adequate analgesia

• Patient sleeps comfortably with minimal signs of discomfort

Dose adjustment or switching to a different analgesic may be necessary to control pain

Prepare patient for ongoing hospital care or prepare patient for release

8. canine and feline anesthesia

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