Halothane is a potent inhalational anaesthetic. It is a clear,
colourless liquid. It is a poor analgesic, but when co-administered
with nitrous oxide and oxygen, it is effective and convenient.
It is inexpensive and used world-wide, although only
infrequently in the UK. Although apparently simple to use, its
therapeutic index is relatively low and overdose is easily produced.
Warning signs of overdose are bradycardia, hypotension
and tachypnoea. Halothane produces moderate muscular
relaxation, but this is rarely sufficient for major abdominal
surgery. It potentiates most non-depolarizing muscle relaxants,
as do other volatile anaesthetics.
Adverse effects
• Cardiovascular:
• ventricular dysrhythmias;
• bradycardia mediated by the vagus;
• hypotension;
• cerebral blood flow is increased, which contraindicates
its use where reduction of intracranial pressure is
desired (e.g. head injury, intracranial tumours).
• Respiratory: respiratory depression commonly occurs,
resulting in decreased alveolar ventilation due to a
reduction in tidal volume, although the rate of breathing
increases.
• Hepatic. There are two types of hepatic dysfunction
following halothane anaesthesia: mild, transient
subclinical hepatitis due to the reaction of halothane with
hepatic macromolecules, and (very rare) massive hepatic
necrosis due to formation of a hapten–protein complex
and with a mortality of 30–70%. Patients most at risk are
middle-aged, obese women who have previously (within
the last 28 days) had halothane anaesthesia. Halothane
anaesthesia is contraindicated in those who have had
jaundice or unexplained pyrexia following halothane
anaesthesia, and repeat exposure is not advised within
three months.
• Uterus: halothane can cause uterine atony and postpartum
haemorrhage.
