Halothane

Halothane is a volatile anaesthetic which is suitable for the induction and maintenance of anaesthesia for all types of surgery and in patients of all ages.

When inhaled, Halothane is absorbed through the alveoli into the bloodstream. In the bloodstream, Halothane circulates through the body to the principal site of action, the brain. Here Halothane causes a progressive depression of the central nervous system, beginning with the higher centers (cerebral cortex) and spreading to the vital centers in the medulla. This depression is reversible. However, its mode of action, like all anaesthetic agents, is unknown.

Halothane has a relatively low solubility in blood and therefore alveoli/blood concentrations equilibrate rapidly. The triexponential decline in Halothane blood concentrations following the end of administration is thought to represent distribution into three compartments; the vessel rich group (brain/heart/liver), the musculature and adipose tissue. Approximately 80% of the inhaled Halothane is eliminated unchanged by the lungs. The remaining 20% is metabolized in the liver by oxidative and under hypoxic conditions, reductive pathways. The main metabolites are trifluoroacetic acid, bromide and chloride salts (via the oxidative pathway) and fluoride salts (via the reductive pathway). The concentrations of metabolites peak 24 hours post-operatively and are eliminated by renal excretion during the following week.

A number of anaesthetic vaporisers specially designed for use with Halothane are available. Open, semi-open, semi-closed and closed circuit systems have all been used with good results.

For induction of anaesthesia:

• Adult: A concentration of 2-4% Halothane in Oxygen or Nitrous Oxide may be used.
• Children: A concentration of 1.5-2% Halothane in Oxygen or Nitrous Oxide is used.

For maintenance of anaesthesia:

• Adults and children: A concentration of 0.5-2% is usually required for maintenance of anaesthesia. The lower concentration is usually most suitable for elderly patients.

Increased risk of ventricular dysrhythmias with epinephrine. Increased risk of malignant hyperthermia with suxamethonium. Prolonged recovery from anaesth with concurrent use of ketamine for induction. May potentiate response to non-depolarising muscle relaxants, hypotensive agents (e.g. hexamethonium bromide, trimetaphan camsilate).

Halothane can induce liver damage; however, the incidence of severe liver damage (jaundice, which may lead to hepatic failure as a consequence of massive hepatic cell necrosis) is unknown. The risk of developing hepatic failure appears to be increased by repeated exposure. Although short intervals of time between exposures are likely to increase the risk of hepatotoxicity, even long intervals between exposures may
not eliminate the risks, since some patients have developed severe reactions following Halothane given many years after the previous exposures. On the information which is available at the present time, it is advised that the following
precautions be taken

• A careful anaesthetic history should be taken prior to use, to determine previous exposure and previous reactions following Halothane anaesthesia.
• Repeated exposure to Halothane within a period of at least 3 months should be avoided unless there are overriding clinical circumstances.
• History of unexplained jaundice and pyrexia in a patient following exposure to
• Halothane is a contraindication to its future use in that patient unless absolutely essential.
• Patients should be informed if they have developed a reaction possibly related to Halothane anaesthesia; such patients should be provided with a medical alert card stating the problem.

Post-op nausea, vomiting, and shivering; resp depression, hypotension, skeletal muscle relaxation, bradycardia.

Although the data from experimental investigations in animals cannot be directly related to man, it would be prudent to avoid general anaesthesia with inhalation agents during early pregnancy, except where such use is essential. There are no well controlled studies with Halothane in lactating women. Halothane has been detected in breast milk of lactating women, but the effect of Halothane on breast feed neonates has not been established. However, Halothane has been in wide use for over 30 years without apparent ill consequence

Caution should be exercised during administration of adrenaline to patients anaesthetised with Halothane as dysrhythmias may be precipitated. For this reason the dose of adrenaline should be restricted and beta-receptor antagonists administered if necessary. Ensure adequate room ventilation when Halothane is being used. Keep the concentration of Halothane in air as low as possible.

Effect on ability to drive or operate machinery: Patients should be advised that performance at skilled tasks, such as driving and operating machinery, may be impaired for some time after general anaesthesia.

Accidental ingestion: Cases of ingestion must be treated symptomatically.

As Halothane causes relaxation of the uterine muscle; it is advisable that anaesthesia should be maintained in the lightest plane possible during obstetric operations. The use of moderate hyperventilation during neurosurgery is recommended to counteract the rise in cerebrospinal fluid pressure which may occur with Halothane Malignant hyperpyrexia has been reported in some patients receiving Halothane. This syndrome occurs with other anaesthetic agents and may respond to intravenous dantrolene sodium.

During the induction of Halothane anaesthesia, a moderate fall in blood pressure commonly occurs. The pressure tends to rise when the vapour concentration is reduced to maintenance levels, but it usually remains steady below the pre-operative level. This hypotensive effect is useful in providing a clear operating field and a reduction in haemorrhage. However, if necessary, intravenous dose of methoxamine (5 mg are usually adequate) can be given to counteract the fall in blood pressure.

Cardiac arrhythmias have been reported during anaesthesia. Halothane augments the action of non-depolarising muscle relaxants. Caution should be exercised during administration of adrenaline to patients anaesthetised with Halothane as dysrhythmias may be precipitated. For this reason the dose of adrenaline should be restricted and beta-receptor antagonists administered if necessary. Ensure adequate room ventilation when Halothane is being used. Keep the concentration of Halothane in air as low as possible.

Bottles of Halothane must be securely closed and stored in a cool dry place, protected from light. Halothane must be kept in the original container until immediately prior to its use. Whilst in the liquid phase, Halothane must not be diluted or contaminated; however, in the vapour phase it may be administered together with Oxygen or a mixture of Nitrous Oxide and Oxygen.