Halothane

Introduction

Halothane was one of the most widely used inhalation agents in veterinary patients. However, it has now been overtaken by other agents such as isoflurane. It is most commonly used to maintain anaesthesia after induction with an injectable agent, although it has also been used to induce anaesthesia when injection is not possible e.g. poor intravenous access.

Pharmacokinetics

At room temperature, halothane is a liquid and so requires a vaporiser before it reaches the patient. It needs to be stored in a darkened bottle due to ultraviolet degradation. It contains a preservative, Thymol, which can accumulate within the vaporiser The blood:gas partition coefficient is moderately low meaning that it is relatively insoluble in blood. This results in relatively rapid induction, recovery and depth change. The MAC for halothane is approximately 0.9% in dogs and 1.1% in cats, meaning it is highly potent. Halothane undergoes a degree of hepatic metabolism by the cytochrome P450 system found in hepatocytes.

Adverse Effects

Central Nervous System

• Causes dose dependent depression but poor analgesic effect.
• Cause cerebral vasodilation but increases intracranial pressure (ICP) therefore should not be used in patients with an increased ICP.

Cardiovascular System

• Decreased cardiac output due to myocardial contraction depression.
• Decreased arterial blood pressure.
• Sensitizes the myocardium to catecholamines so may cause cardiac arrhythmias.

Respiratory System

• Beneficial in patients with increased airway resistance due to bronchodilatory effect.

Other Systems

• Blood flow through the hepatic artery and portal vein are both reduced, which may result in hepatic damage, although unlikely to see clinical signs.
• Reduced renal blood flow and therefore a reduction in glomerular filtration rate.
• Of all the inhalation agents, halothane is the most likely to cause malignant hyperthermia.

Contraindications

Halothane should be avoided in patients with increased ICP, cardiac dysfunction, hepatic disease and susceptibility to malignant hyperthermia.