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550 SecTioN iii Neurology aNd Special SeNSeS ` neurology—PhArmACology Neurology aNd Special SeNSeS ` neurology—PhArmACology
Inhaled anesthetics Desflurane, halothane, enflurane, isoflurane, sevoflurane, methoxyflurane, N O.
2
meChAnism Mechanism unknown.
eFFeCts Myocardial depression, respiratory depression, postoperative nausea/vomiting, cerebral blood flow,
cerebral metabolic demand.
AdVerse eFFeCts Hepatotoxicity (halothane), nephrotoxicity (methoxyflurane), proconvulsant (enflurane,
epileptogenic), expansion of trapped gas in a body cavity (N O).
2
Malignant hyperthermia—rare, life-threatening condition in which inhaled anesthetics or
succinylcholine induce severe muscle contractions and hyperthermia. Susceptibility is often
inherited as autosomal dominant with variable penetrance. Mutations in voltage-sensitive
ryanodine receptor (RYR1 gene) cause Ca release from sarcoplasmic reticulum.
2+
Treatment: dantrolene (a ryanodine receptor antagonist).
Intravenous anesthetics
Agent meChAnism AnesthesiA use notes
Thiopental Facilitates GABA (barbiturate) Anesthesia induction, short cerebral blood flow. High lipid
A
surgical procedures solubility
Effect terminated by rapid
redistribution into tissue, fat
Midazolam Facilitates GABA Procedural sedation (eg, May cause severe postoperative
A
(benzodiazepine) endoscopy), anesthesia respiratory depression, BP,
induction anterograde amnesia
Propofol Potentiates GABA Rapid anesthesia induction, May cause respiratory
A
short procedures, ICU depression, hypotension
sedation
Ketamine NMDA receptor antagonist Dissociative anesthesia cerebral blood flow
Sympathomimetic Emergence reaction possible
with disorientation,
hallucination, vivid dreams
Local anesthetics Esters—procaine, tetracaine, benzocaine, chloroprocaine.
Amides—lIdocaIne, mepIvacaIne, bupIvacaIne, ropIvacaIne (amIdes have 2 I’s in name).
+
meChAnism Block Na channels by binding to specific receptors on inner portion of channel. Most effective in
rapidly firing neurons. 3° amine local anesthetics penetrate membrane in uncharged form, then
bind to ion channels as charged form.
Can be given with vasoconstrictors (usually epinephrine) to enhance local action— bleeding,
anesthesia by systemic concentration.
In infected (acidic) tissue, alkaline anesthetics are charged and cannot penetrate membrane
effectively need more anesthetic.
Order of nerve blockade: small-diameter fibers > large diameter. Myelinated fibers > unmyelinated
fibers. Overall, size factor predominates over myelination such that small myelinated fibers
> small unmyelinated fibers > large myelinated fibers > large unmyelinated fibers.
Order of loss: (1) pain, (2) temperature, (3) touch, (4) pressure.
CliniCAl use Minor surgical procedures, spinal anesthesia. If allergic to esters, give amides.
AdVerse eFFeCts CNS excitation, severe cardiovascular toxicity (bupivacaine), hypertension, hypotension,
arrhythmias (cocaine), methemoglobinemia (benzocaine).
FAS1_2019_12-Neurol.indd 550 11/8/19 7:40 AM
