LOCAL ANESTHETICS

AND REGIONAL

ANESTHESIA

Dr Walid Zuabi

FCA RCSI

Local Anesthetics- History

◼ 1860 - cocaine isolated from erythroxylum coca.

◼ Koller - 1884 uses cocaine for topical anaesthesia.

◼ Halsted - 1885 performs peripheral nerve block with LA.

◼ Bier - 1899 first spinal anesthetic .

Local Anesthetics - Definition

A substance which reversibly inhibits nerve conduction when applied directly to tissues at non-toxic concentrations.

◼ Local anesthetics block generation, propagation, and oscillations of electrical impulses in electrically excitable tissue.

By: acting on Na channels.

PHARMACOLOGY AND

PHARMACODYNAMICSClinically used local anesthetics consist of lipid-

soluble, substituted benzene ring linked to amine group via alkyl chain containing an amide or ester linkage.

• Type of linkage separates local anesthetics into either aminoamides (metabolized in liver) or aminoesters (metabolized in liver or by plasma cholinesterase).

PHARMACOKINETICSSystemic toxicity depends on blood levels of LA:

• End organs of main concern for toxicity are CV and CNS.

• Determinants of systemic absorption:

» Site of injection (intercostal > caudal > brachial plexus > sciatic/femoral)

» Dose.

» Physiochemical properties (lipid solubility, protein binding)

» Addition of epinephrine

Local anesthetics - Mechanism

Limit influx of sodium, thereby limiting

propagation of the action potential.

Local Anesthetics - Classes

Esters

Local anesthetics - Classes

Esters

Cocaine

Chloroprocaine

Procaine

Tetracaine

Amides

Bupivacaine

Lidocaine

Ropivacaine

Etidocaine

Mepivacaine

Bupivacaine

Amide

• Infiltration: use 0.25%, fast onset, 2- to 8-hr

duration, max dose 175 mg (225 mg with

epinephrine)

• Peripheral nerve block: use 0.25-0.5%, slow

onset, 4- to 12-hr duration, max dose 175 mg

(225 mg with epinephrine

Bupivacaine

Epidural anesthesia: Use 0.5-0.75%, moderate

onset, 2- to 5-hr duration, max dose 175 mg

(225 mg with epinephrine)

• Spinal anesthesia: Use 0.5-0.75%, fast onset, 1-

to 4-hr duration, max dose 20 mg

◼ levo (-) bupivacaine less cardiotoxic than

racemic bupivacaine, same profile.

Lidocaine

Amide

• Infiltration: use 0.5-1%, fast onset, 2- to 8-hr

duration, max dose 300 mg (500 mg with

epinephrine)

• Peripheral nerve block: use 1.0-1.5%, fast onset,

1- to 3-hr duration, max dose 300 mg (500 mg

with epinephrine)

Lidocaine

Epidural anesthesia: use 1.5-2%, fast onset, 1- to 2-hr duration, max dose 300 mg (500 mg with epinephrine)

• Spinal anesthesia: use 1.5-2%, fast onset, 0.5- to 1-hr duration, max dose 100 mg

• Topical anesthesia: use 4%, fast onset, 0.5- to 1-hr duration, max dose 300 mg

• IV regional: Use 0.25-0.5%, fast onset, 0.5-1 hr duration, max dose 300 mg

Ropivacaine

Amide

• Infiltration: use 0.2-0.5%, fast onset, 2- to 6-hr duration, max dose 200 mg

• Peripheral nerve block: use 0.5-1.0%, slow onset, 5- to 8-hr duration, max dose 250 mg

• Epidural anesthesia: Use 0.5-1.0%, moderate onset, 2- to 6-hr duration, max dose 200 mg

• less cardiac toxicity than bupivacaine

Local anesthetics - FormulationBiologically active substances are frequently administered as very dilute solutions which can be expressed as parts of active drug per

100 parts of solution (grams percent)

Ex.: 2% solution =

_2 grams__ = _2000 mg_ = __20 mg__

100 cc’s 100 cc’s 1 cc

Local Anesthetics - Allergy

◼ True allergy is very rare.

◼ Most reactions are from ester class - ester hydrolysis (normal metabolism) leads to formation of PABA - like compounds.

◼ Patient reports of “allergy” are frequently due to previous intravascular injections.

Local Anesthetics - Toxicity

Tissue toxicity - Rare

◼ Can occur if administered in high concentrations (greater than those used clinically)

◼ Usually related to preservatives added to solution

Systemic toxicity - Rare

◼ Related to blood level of drug secondary to absorption from site of injection.

◼ Range from lightheadedness, tinnitus to seizures and CNS/cardiovascular collapse

Local anesthetics - Duration

◼ Determined by rate of elimination of agent from the site injected

◼ Factors include lipid solubility, dose given, blood flow at site, addition of vasoconstrictors (does not reliably prolong all agents)

◼ Some techniques allow multiple injections over time to increase duration, e.g. epidural catheter

Local anesthetics - vasoconstrictors

Ratios

Epinephrine is added to local anesthetics in extremely

dilute concentrations, best expressed as a ratio of

grams of drug:total cc’s of solution. Expressed

numerically, a 1:1000 preparation of epinephrine

would be

1 gram epi

1000 cc’s solution

1000 mg epi

1000cc’s solution=

1 mg epi

1 cc=

Local anesthetics - vasoconstrictors

Therefore, a 1 : 200,000 solution of epinephrine would be

1 gram epi

200,000 cc’s solution=

1000 mg epi

200,000 cc’s solution

or

5 mcg epi

1 cc solution

Local anesthetics - vasoconstrictors

Vasoconstrictors should not be used in the following locations

◼ Fingers

◼ Toes

◼ Nose

◼ Ear lobes

◼ Penis

REGIONAL ANESTHESIA

Regional anesthesia - Definition

Rendering a specific area of the body, e.g. foot, arm, lower extremities,

insensate to stimulus of surgery or other instrumentation

Regional anesthesia - Uses

◼ Provide anesthesia for a surgical procedure

◼ Provide analgesia post-operatively or during labor and delivery

◼ Diagnosis or therapy for patients with chronic pain syndromes

Regional anesthesia - types

◼ Topical

◼ Local/Field

◼ Intravenous block (“Bier” block)

◼ Peripheral (named) nerve, e.g. radial n.

◼ Plexus - brachial, lumbar

◼ Central neuraxial - epidural, spinal

Topical Anesthesia

◼ Application of local anesthetic to mucous membrane - cornea, nasal/oral mucosa

◼ Uses :

• awake oral, nasal intubation, superficial surgical procedure

◼ Advantages :

• technically easy

• minimal equipment

◼ Disadvantages :

• potential for large doses leading to toxicity

Local/Field Anesthesia

◼ Application of LA subcutaneously to anesthetize distal nerve endings

◼ Uses:

• Suturing, minor superficial surgery, line placement, more extensive surgery with sedation

◼ Advantages:

• minimal equipment, technically easy, rapid onset

◼ Disadvantages:

• potential for toxicity if large field

IV Block - “Bier” block

◼ Injection of local anesthetic intravenously for anesthesia of an extremity

◼ Uses

• any surgical procedure on an extremity

◼ Advantages:

• technically simple, minimal equipment, rapid onset

◼ Disadvantages:

• duration limited by tolerance of tourniquet pain, toxicity

Peripheral nerve block

◼ Injecting local anesthetic near the course of a named nerve

◼ Uses:

• Surgical procedures in the distribution of the blocked nerve

◼ Advantages:

• relatively small dose of local anesthetic to cover large area; rapid onset

◼ Disadvantages:

• technical complexity, neuropathy

Plexus Blockade

◼ Injection of local anesthetic adjacent to a plexus, e.g cervical, brachial or lumbar plexus

◼ Uses :• surgical anesthesia or post-operative analgesia in

the distribution of the plexus

◼ Advantages:• large area of anesthesia with relatively large dose

of agent

◼ Disadvantages:• technically complex, potential for toxicity and

neuropathy.

Central neuraxial blockade - “Spinal”

◼ Injection of local anesthetic into CSF

◼ Uses:

• profound anesthesia of lower abdomen and extremities

◼ Advantages:

• technically easy (LP technique), high success rate, rapid onset

◼ Disadvantages:

• “high spinal”, hypotension due to sympathetic block, post dural puncture headache.

Central Neuraxial Blockade -

“epidural”

◼ Injection of local anesthetic in to the epidural space at any level of the spinal column

◼ Uses:

• Anesthesia/analgesia of the thorax, abdomen, lower extremities

◼ Advantages:

• Controlled onset of blockade, long duration when catheter is placed, post-operative analgesia.

◼ Disadvantages:

• Technically complex, toxicity, “spinal headache”

TOXICITY OF LOCAL ANESTHETICS

Rx of systemic toxicity is primarily supportive:

» Stop injection

» Administer oxygen

» Support ventilation

» Tracheal intubation and control ventilation if necessary

» Suppress seizure activity (thiopental, midazolam, propofol)