Halothane

Halothane is a potent halogenated volatile anesthetic formerly used widely in veterinary general anesthesia. It is delivered as vapor to produce a controllable depth of anesthesia in dogs, cats, horses and other species. Its relatively high blood and tissue solubility contributes to slower changes in anesthetic depth and recovery than with less soluble inhalants.

Group

Inhalational anaesthetic

Mechanism of Action

Halothane produces reversible depression of central nervous system activity through effects on multiple neuronal ion channels and synaptic pathways. Enhancement of inhibitory signaling, including GABA-A and glycine-receptor activity, and reduction of excitatory transmission contribute to unconsciousness and immobility. The full mechanism is distributed across brain and spinal-cord targets rather than a single receptor. Clinical depth follows the effective concentration reaching these tissues.

Dose Rates

Dogs

Use / stage	Dose	Route	Instructions
Anaesthetic induction — Initial	Reference only	INHALATION	Inspired concentration approximately 3%, titrated to effect. Percent vapour concentration is not a body-weight dose.
Anaesthetic maintenance — Maintenance	Reference only	INHALATION	Inspired concentration 0.5-1.5%, titrated to depth.
Alternative anaesthetic concentration	Reference only	INHALATION	0.5-3.5% inhaled, titrated to depth and fresh-gas flow.

Cats

Use / stage	Dose	Route	Instructions
Anaesthetic induction — Initial	Reference only	INHALATION	Inspired concentration approximately 3%, titrated to effect. Percent vapour concentration is not a body-weight dose.
Anaesthetic maintenance — Maintenance	Reference only	INHALATION	Inspired concentration 0.5-1.5%, titrated to depth.
Alternative anaesthetic concentration	Reference only	INHALATION	0.5-3.5% inhaled, titrated to depth and fresh-gas flow.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Anaesthetic requirement	Reference only	INHALATION		Large-animal MAC approximately 1%; MAC is a potency measure rather than a fixed vaporizer prescription.
Draft-horse anaesthesia	Reference only	INHALATION		After induction and intubation, initial 4-5% in an oxygen-enriched semi-closed circuit; reduce promptly to effect. Draft-horse maintenance described at 2.5-3%, using the lowest effective concentration.

Mice

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Non-rebreathing circuit: induction 2-4%, maintenance 0.25-2%; adjust to ventilation and anaesthetic depth.

Rats

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Non-rebreathing circuit: induction 2-4%, maintenance 0.25-2%; adjust to ventilation and anaesthetic depth.

Gerbils

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Non-rebreathing circuit: induction 2-4%, maintenance 0.25-2%; adjust to ventilation and anaesthetic depth.

Hamsters

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Non-rebreathing circuit: induction 2-4%, maintenance 0.25-2%; adjust to ventilation and anaesthetic depth.

Guinea pigs

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Non-rebreathing circuit: induction 2-4%, maintenance 0.25-2%; adjust to ventilation and anaesthetic depth.

Chinchillas

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Non-rebreathing circuit: induction 2-4%, maintenance 0.25-2%; adjust to ventilation and anaesthetic depth.

Birds

Use / stage	Dose	Route	Schedule	Instructions
General anaesthesia	Reference only	INHALATION		Small birds: induction around 2%; large birds 2.5-3%. Maintenance 0.5-1.5%. Watch for bradycardia, hypotension and hypothermia.

Indications

Inhalational induction of general anesthesia in selected veterinary patients where halothane is used.
Maintenance of general anesthesia during surgical or diagnostic procedures in dogs, cats, horses and other veterinary species.

Contraindications

Cardiovascular disease requires particular caution.
Raised intracranial pressure and catecholamine-associated arrhythmia risk require careful assessment.
Previous malignant hyperthermia or serious hepatotoxicity after halothane exposure.

Interactions

Other anaesthetics lower inhaled requirements.
Catecholamines increase ventricular arrhythmia risk.
Succinylcholine may trigger malignant hyperthermia; aminoglycosides, lincosamides and neuromuscular blockers may increase paralysis. Avoid postoperative paracetamol after halothane exposure.

Adverse effects

Dose-dependent myocardial and respiratory depression
Hypotension, arrhythmias and increased intracranial pressure

Toxicity

Excess inhaled concentration can cause severe cardiorespiratory depression. Reduce or stop delivery and support oxygenation, ventilation and circulation.

Monitoring and Administration

Use a calibrated vaporizer, oxygen delivery and waste-gas scavenging.

Monitor anaesthetic depth, ventilation, heart rate and rhythm, blood pressure and temperature.

Keep the volatile liquid tightly sealed. MAC values are approximately 1.04% in cats, 0.87% in dogs and 0.88% in horses.

Halothane is supplied as volatile liquid in a 250 mL bottle.

Store below 40°C in a light-resistant container. Discard discoloured liquid; thymol can accumulate in the vaporizer. Moist halothane can react with certain metals and damage rubber or plastics.