HALOTHANE

Chemistry - An inhalant general anesthetic agent, halothane occurs as a colorless, nonflammable, heavy liquid. It has a characteristic odor resembling chloroform and sweet, burning taste. Halothaneis slightly soluble in water and miscible with alcohol. At 20°C, halothane's specific gravity is 1.872-1.877 and vapor pressure is 243 mm Hg.

Storage, Stability, Compatibility

Store halothane below 40°C in a tight, light-resistant container. Halothane stability is maintained by the addition of thymol and ammonia. The thymol doesnot vaporize so it may accumulate in the vaporizer causing a yellow discoloration. Do not usediscolored solutions. Discolored vaporizer and wick may be cleaned with diethyl ether (all ethermust be removed before reuse).
In the presence of moisture, halothane vapor can react with aluminum, brass and lead (not copper).
Rubber and some plastics are soluble in halothane leading to their rapid deterioration.

Pharmacology - HALOTHANE

While the precise mechanism that inhalent anesthetics exert their general anesthetic effect is not precisely known, they may interfere with functioning of nerve cells in the brain by acting at the lipid matrix of the membrane. Some key pharmacologic effects noted withhalothane include: CNS depression, depression of body temperature regulating centers, increasedcerebral blood flow, respiratory depression (pronounced in ruminants), hypotension, vasodilatation, and myocardial depression.
Minimal Alveolar Concentration (MAC; %) in oxygen reported for halothane in various species:
Dog = 0.76; Cat = 0.82; Horse = 0.88; Human = 0.76. Several factors may alter MAC (acid/basestatus, temperature, other CNS depressants on board, age, ongoing acute disease, etc.).
Uses, Indications - Halothane remains a useful general anesthetic in veterinary medicine due to itsrelative safety, potency, controllability, non-flammability, and comparative low cost.

Pharmacokinetics - HALOTHANE

Halothane is rapidly absorbed through the lungs. About 12% of absorbeddrug is metabolized by the liver to trifluoroacetic acid (only small amounts), chlorine and bromineradicals which are excreted in the urine. The bulk of the absorbed drug is re-excreted by the lungsand eliminated with expired air. Halothane is distributed into milk.

Contraindications, Precautions, Reproductive Safety

Halothane is contraindicated in patientswith a history or predilection towards malignant hyperthermia or significant hepatotoxicity afterprevious halothane exposure (see Adverse Reactions below). It should be used with caution(benefits vs. risks) in patients with hepatic function impairment, cardiac arrhythmias, increased CSFor head injury, myasthenia gravis, or pheochromocytoma (cardiac arrhythmias due to catecholamines).
Some animal studies have shown that halothane may be teratogenic; use only when benefitsoutweigh potential risks.

Adverse Effects, Warnings

Hypotension may occur and is considered to be dosage related. A malignant hyperthermia-stress syndrome has been reported in pigs, horses, dogs and cats.
Halothane may cause cardiac depression and dysrhythmias. Halothane-induced hypotension may be treated by volume expansion and dobutamine. Lidocaine has been used to treat or prevent halothane-induced cardiac dysrhythmias.
In humans, jaundice and a postanesthetic fatal liver necrosis has been reported rarely. The incidence of this effect in veterinary species is not known. However, halothane should be considered contraindicated for future use if unexplained fever, jaundice or other symptoms associated with hepatotoxicity occur.

Drug Interactions

Acetaminophen is not recommended to be used for post-operative analgesiain animals who have received halothane anesthesia.
Because halothane sensitizes the myocardium to the effects of sympathomimetics, especiallycatecholamines, severe ventricular arrhythmias may result. Drugs included are: dopamine, epinephrine, norepinephrine, ephedrine, metaraminol, etc. If these drugs are needed, theyshould be used with caution and in significantly reduced dosages with intensive monitoring.
Non-depolarizing neuromuscular blocking agents, systemic aminoglycosides, systemiclincomycins should be used with caution with halogenated anesthetic agents as additive neuromuscular blockade may occur.
Reportedly, d-tubocurarine may cause significant hypotension if used with halothane.
Concomitant administration of succinylcholine with inhalation anesthetics (halothane, cyclopropane, nitrous oxide, diethyl ether) may induce increased incidences of cardiac effects (bradycardia, arrhythmias, sinus arrest and apnea) and in susceptible patients, malignant hyperthermia.
Laboratory Considerations - Halothane may transiently increase values of liver function tests.