1. Clinical Overview
Halothane is a volatile, non-flammable, halogenated hydrocarbon general inhalation anesthetic. It is a potent agent providing rapid induction and recovery, with good muscle relaxation and bronchodilation. Its use has declined significantly in developed nations due to the risk of severe hepatotoxicity (halothane hepatitis), but it remains in use in certain cost-sensitive and resource-limited settings in India, primarily for pediatric induction and in veterinary practice. It is typically administered via a calibrated vaporizer in a mixture of oxygen or nitrous oxide-oxygen.
| Onset | Duration | Bioavailability |
|---|---|---|
| Within 2-5 minutes of inhalation | Duration is directly dependent on the length of administration; effects dissipate rapidly (within 5-15 minutes) after cessation. | Approximately 60-80% of inhaled dose is systemically absorbed via the alveoli. |
2. Mechanism of Action
Halothane's precise mechanism is multifactorial. It potentiates inhibitory neurotransmission (GABA-A, glycine receptors) and inhibits excitatory neurotransmission (NMDA glutamate receptors). It activates potassium channels (TREK-1) and inhibits neuronal sodium channels. The overall effect is a dose-dependent, reversible depression of the central nervous system, leading to loss of consciousness, amnesia, analgesia, and muscle relaxation.
3. Indications & Uses
- Induction and maintenance of general anesthesia
- Provision of controlled hypotension during surgery
4. Dosage & Administration
Adult Dosage: Induction: 2-4% vapor concentration in oxygen/nitrous oxide-oxygen. Maintenance: 0.5-2.0% vapor concentration. Dose is titrated to clinical effect (BP, HR, depth of anesthesia).
Administration: Administered ONLY via a calibrated, temperature-compensated, specifically designed halothane vaporizer. Must be used with adequate oxygen flow (minimum 30%). Never use with epinephrine-containing local anesthetics in doses >100 mcg/10 minutes due to risk of ventricular arrhythmias. Protect from light. Do not use if discolored (additives include thymol as stabilizer; yellowing indicates decomposition).
5. Side Effects
Common side effects may include:
- Postoperative nausea and vomiting (PONV)
- Shivering
- Transient hypotension
- Bradycardia
- Respiratory depression
- Mild, transient liver enzyme elevation
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Aminophylline, Theophylline | Increased risk of cardiac arrhythmias | Major |
| Epinephrine, Norepinephrine | Markedly increased risk of severe ventricular arrhythmias; dose must be limited | Major |
| Non-Depolarizing Muscle Relaxants (e.g., Atracurium, Vecuronium) | Potentiation of neuromuscular blockade, prolonged apnea | Moderate |
| Opioids (e.g., Fentanyl, Morphine) | Additive respiratory and cardiovascular depression | Moderate |
| Beta-blockers (e.g., Propranolol) | Additive bradycardia and myocardial depression | Moderate |
| Rifampicin, Isoniazid | Induction of CYP2E1 may increase production of hepatotoxic metabolites | Moderate |
| MAO Inhibitors (e.g., Phenelzine) | Unpredictable BP responses; risk of hyperpyrexia | Major |
7. Patient Counselling
- DO fast as instructed by your anesthesiologist prior to surgery.
- DO inform your doctor of ALL medications, supplements, and herbal products you are taking.
- DO disclose any personal or family history of problems with anesthesia (especially high fever, jaundice, muscle rigidity).
- DO NOT drive, operate machinery, or sign legal documents for at least 24 hours post-procedure.
- DO NOT consume alcohol for at least 24 hours after anesthesia.
8. Toxicology & Storage
Overdose: Profound cardiovascular depression (severe hypotension, bradycardia, asystole), respiratory arrest, deep coma, loss of brainstem reflexes, hypothermia, and ultimately cardiac arrest.
Storage: Store in tightly closed, amber-colored bottles, protected from light and moisture at a temperature not exceeding 25°C. Do not refrigerate. Keep away from open flame. The liquid contains 0.01% thymol as a stabilizer. Discard if the liquid is discolored (yellow or brown) or contains visible particulate matter. Keep out of reach of children.