Halothane
Halothane is a classic **halogenated inhalant general anesthetic** that was historically a mainstay in veterinary medicine due to its potency, controllability, and non-flammability. While it paved the way for modern veterinary anesthesia, it is **largely obsolete and no longer commercially available** in many countries. It has been widely replaced by newer agents (such as **isoflurane**, **sevoflurane**, and **desflurane**) which offer superior safety profiles, particularly regarding cardiovascular depression and hepatotoxicity. **Clinical Pearls:** * Halothane has a higher blood-gas partition coefficient compared to modern inhalants, leading to slower induction and recovery times. * It is uniquely known for sensitizing the myocardium to catecholamines, significantly increasing the risk of ventricular arrhythmias. * Approximately 12% of the drug undergoes hepatic metabolism (unlike isoflurane which is <1%), contributing to its potential for immune-mediated hepatotoxicity (halothane hepatitis).
Mechanism: The exact mechanism of inhalant anesthetics remains incompletely understood, but involves multiple central nervous system targets: * **Lipid Matrix Interaction:** Historically explained by the Meyer-Overton rule, halothane interacts with the lipid bilayer of neuronal membranes, disrupting normal ion channel function. * **Receptor Modulation:** Modern evidence suggests halothane acts via **positive allosteric modulation of inhibitory receptors** (such as **GABA_A** and **glycine** receptors) โ enhancing CNS depression. * **Excitatory Inhibition:** It simultaneously inhibits excitatory receptors (such as **NMDA** and **nicotinic acetylcholine** receptors) โ dampening neuronal excitability. **Systemic Effects:** * Profound CNS depression and blunting of thermoregulatory centers. * Increased cerebral blood flow. * Dose-dependent respiratory depression (especially pronounced in ruminants). * Significant myocardial depression, vasodilation, and hypotension.
Dosing by species
- General Anesthesia ยท 3% (induction); 0.5-1.5% (maintenance) ยท Inhalation ยท Continuous ยท As needed for procedure ยท Concentrations are dependent upon fresh gas flow rate.
- General Anesthesia ยท 0.5-3.5%, inhaled ยท Inhalation ยท Continuous ยท As needed for procedure
- Induction of anaesthesia (unpremedicated) ยท 3-4% concentration ยท Inhalational ยท Continuous ยท Until induced ยท Delivered via calibrated vaporizer
- Maintenance of anaesthesia ยท 0.8-1.0% (MAC) ยท Inhalational ยท Continuous ยท Duration of procedure ยท Adjust according to individual requirement and concurrent drugs
- General Anesthesia (Draft Horses) ยท 4%-5% initially, maintenance 2.5%-3% ยท Inhalation ยท Continuous ยท As needed for procedure ยท Administered in an oxygen-enriched semi-closed large animal circle system. Reduce as indicated by physical monitoring.
- General Anesthesia ยท 2% (small birds) or 2.5%-3% (large birds) for induction; 0.5%-1.5% for maintenance ยท Inhalation ยท Continuous ยท As needed for procedure ยท Induction usually occurs in 2-4 minutes. Recovery takes 3-5 minutes. Bradycardia, hypotension, and hypothermia may occur but rapidly resolve upon discontinuation.
- General Anesthesia (Mice, Rats, Gerbils, Hamsters, Guinea pigs, Chinchillas) ยท Induction: 2%-4%, maintenance: 0.25%-2% ยท Inhalation ยท Continuous ยท As needed for procedure ยท Using a non-rebreathing system.
Routes of administration
Contraindications
- History or predilection towards malignant hyperthermia
- Significant hepatotoxicity or unexplained fever/jaundice after previous halothane exposure
- Liver disease
- Patients with a history of malignant hyperthermia
Adverse effects
- Dose-related hypotension
- Cardiac depression and decreased cardiac output
- Cardiac dysrhythmias (sensitizes myocardium to catecholamines)
- Malignant hyperthermia-stress syndrome (reported in pigs, horses, dogs, cats)
- Hepatotoxicity (jaundice, fatal liver necrosis)
- Respiratory depression
- Hypothermia
- Dose-dependent hypotension
- Bradycardia
- Decreased myocardial contractility
- Reduced hepatic blood flow
- Ventricular arrhythmias
- Neonatal depression (crosses placental barrier)
Drug interactions
- Acetaminophen ยท Not recommended for post-operative analgesia in patients that have received halothane anesthesia due to increased risk of hepatotoxicity.
- Acepromazine ยท Can decrease requirements of halothane by up to 40%.
- Aminoglycosides ยท Use with caution; additive neuromuscular blockade may occur.
- Lincosamides ยท Use with caution; additive neuromuscular blockade may occur.
- Non-depolarizing neuromuscular blocking agents ยท Additive neuromuscular blockade may occur.
- Succinylcholine ยท May induce increased incidences of cardiac effects (bradycardia, arrhythmias, sinus arrest, apnea) and malignant hyperthermia in susceptible patients.
- Sympathomimetics (dopamine, epinephrine, norepinephrine, ephedrine, metaraminol) ยท Halothane sensitizes the myocardium to catecholamines; severe ventricular arrhythmias may result. Use with extreme caution at reduced doses.
- d-Tubocurarine ยท May cause significant hypotension if used with halothane.
- Sedatives ยท Reduces the concentration of halothane required to achieve surgical anaesthesia ยท moderate
- Opioid agonists ยท Reduces the concentration of halothane required to achieve surgical anaesthesia ยท moderate
- Nitrous oxide (N2O) ยท Reduces the concentration of halothane required to achieve surgical anaesthesia ยท moderate
Monitoring
- Respiratory and ventilatory status
- Cardiac rate and rhythm (ECG is highly recommended due to arrhythmogenic potential)
- Blood pressure (especially in 'at risk' patients)
- Level of anesthetic depth
- Body temperature (monitor for hypothermia or malignant hyperthermia)
- Heart rate and rhythm (ECG)
- Blood pressure
- Respiratory rate and depth
- Capnography (ETCO2)
- Anaesthetic depth (jaw tone, palpebral reflex)
- Body temperature
Overdose
Overdosage of halothane results in profound **cardiopulmonary depression**, characterized by severe hypotension, bradycardia, arrhythmias, and apnea. **Treatment:** 1. Immediately turn off the anesthetic vaporizer. 2. Flush the breathing circuit with 100% oxygen. 3. Institute positive pressure ventilation. 4. Provide cardiovascular support (e.g., IV fluid volume expansion, positive inotropes like dobutamine, and anticholinergics for bradycardia). 5. Lidocaine may be required to treat halothane-induced ventricular dysrhythmias.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.