Halothane a volatile liquid with sweet odour, non-irritant and noninflammable. Solubility in blood is moderate induction is reasonably quick and pleasant. Halothane, sold under the brand name Fluothane among others, is a general anaesthetic. It can be used to induce or maintain anaesthesia. One of its benefits is that it does not increase the production of saliva, which can be particularly useful in those who are difficult to intubate.
Ether and chloroform were rapidly replaced by halothane upon its introduction in 1956. Halothane is associated with a lower risk of nausea and vomiting than fluorinated methyl ethyl ether agents. Due to its favorable side effect profile, halothane became the standard of practice, used in almost every operating room and for the comparison of other inhalational anesthetics as they came to the market.
Halothane is a potent anaesthetic; therefore precise control of administered concentration is essential. For induction 2-4% and for maintenance 0.5-1% is delivered by the use of a special vapourizer. It is not a good analgesic or muscle relaxant, but it potentiates competitive neuromuscular blockers.
Halothane causes direct depression of myocardial contractility by reducing intracellular Ca2+ concentration. Moreover, sympathetic activity fails to increase reflexly. Cardiac output is reduced with deepening anaesthesia. Blood pressure starts falling early and parallels the depth. A 20-30 mm Hg drop in blood pressure is common. Many vascular beds dilate but total peripheral resistance is not significantly reduced. Heart rate is reduced by vagal stimulation, direct depression of SA nodal automaticity and absence of baroreceptor activation even when blood pressure falls. It tends to sensitize the heart to the arrhythmogenic action of Adr. The electrophysiological effects are conducive to reentry-tachyarrhythmias occur occasionally, particularly if sympathetic stimulation occurs due to inadequate anaesthesia.
Halothane causes relatively greater depression of respiration; breathing is shallow and rapid partial pressure of CO, in blood rises if respiration is not assisted. Cerebral blood flow increases. Ventilatory support with added oxygen is frequently required. It tends to accentuate perfusion ventilation mismatch in the lungs by causing vasodilatation in hypoxic alveoli.
Pharyngeal and laryngeal reflexes are abolished early and coughing is suppressed while bronchi dilates. As such, halothane is the preferred anaesthetic for asthmatics. It inhibits intestinal and uterine contractions. This property is utilized for facilitating external or internal versions during late pregnancy. However, its use during labour can prolong delivery and increase postpartal blood loss.
Urine formation is decreased during halothane anaesthesia primarily due to low g.f.r. as a result of the fall in blood pressure. Hepatitis occurs in rare susceptible individuals especially after repeated use and in those with familial predisposition. A metabolite of halothane is probably involved which is believed to cause chemical or immunological injury. Halothane toxicity is less frequent in children.
A genetically determined reaction malignant hyperthermia occurs rarely. Many susceptible subjects have an abnormal RyR1 (Ryanodine receptor) calcium channel at the sarcoplasmic reticulum of skeletal muscles. This channel is triggered by halothane to release massive amounts of Ca intracellularly causing persistent muscle contraction and increased heat production. Succinylcholine accentuates the condition. Rapid external cooling, bicarbonate infusion, 100% O, inhalation and intravenous dantrolene are used to treat malignant hyperthermia.
About 20% of halothane that enters blood is metabolized in the liver, the rest is exhaled out. Elimination may continue for 24-48 hours after prolonged administration due to accumulation in fatty and other tissues. Recovery from halothane anaesthesia is smooth and reasonably quick; shivering may occur but nausea and vomiting are rare. Psychomotor performance and mental ability remain depressed for several hours after regaining consciousness.
Halothane is a frequently used anaesthetic in developing countries, because it is relatively cheap and nonirritant, noninflammable, pleasant with relatively rapid action. It is particularly suitable for use in children, both for induction as well as maintenance. In adults, it is mainly used as a maintenance anaesthetic after i.v. induction. However, in affluent countries it has been largely replaced by the newer agents which are costlier. Its deficiencies in terms of poor analgesia and muscle relaxation are compensated by concomitant use of N,O or opioids and neuromuscular blockers.
