Halothane is a widely used inhalational agent.
Physical Properties of Halothane :
- It is colorless liquid, volatile anesthetic.
- Pleasant to smell so excellent for induction in children.
- Stored in amber colored bottles and contains thymol 0.01% as preservative (to prevent decomposition by light).
- Non inflammable, non explosive.
- Nonirritant, so induction is very smooth. Boiling point of Halothane is 50°C.
- Halothane has highest fat/blood coefficient (51) can get deposited in adipose tissue after prolonged exposure.
Anesthetic Properties of Halothane :
- It is potent anesthetic (MAC = 0.74).
- Blood gas coefficient of 2.4 makes it agent with moderate induction and recovery time.
- Not a good analgesic.
- Muscle relaxation is moderate.
Metabolism of Halothane :
Halothane undergoes extensive metabolism (both oxidation and reduction).
The metabolic products of Halothane are:
- Trifluoroacetic acid.
- Chloride (Cl ).
- Bromide (Br ).
- Fluoride (F ) produced only under anaerobic conditions.
Systemic Effects of Halothane :
Halothane causes significant
- Decrease in cardiac output which is because of direct depression of myocardium and bradycardia (13 blocking action).
- Blood pressure is decreased by direct action on smooth muscle of blood vessels as well as decreased central sympathetic tone.
- It sensitizes the heart to adrenaline (exogenous to heart) producing severe ventricular arrhythmias. So maximum permissible dose of adrenaline with halothane for local ischemia is 1.5 p.g/kg or not more than 30 mI/hr of 1 in 1,00,000 solution.
- Blunts the baroreceptor reflex.
So it can be concluded that halothane should not be used in cardiac patients.
- It decreases tidal volume and increases frequency initially but at higher dose frequency also decreases causing significant decrease in minute volume.
- Both hypercarbic and hypoxic reflexes are blunted.
- Laryngeal and pharyngeal reflexes are depressed.
- It is a very potent bronchodilator and this bronchodilatation is because of inhibition of reflex pathways of bronchoconstriction. That is why halothane is inhalational agent of choice for asthmatics.
Central nervous system:
There is marked increase in intracranial tension with halothane.
Both GFR and urinary flow is decreased which is because of decrease in cardiac output.
Halothane produces moderate muscle relaxation (decreases the dose of muscle relaxants by 20 to 30%).
Like other inhalational agents it can cause uterine atony and postpartum haemorrhage so should not be used in obstetrics for cesarean section and abortions. But this property makes it appropriate agent for external versions and manual removal of placenta.
Post operative shivering (halothane shakes) and hypothermia is maximum with halothane among inhalational agents.
Contraindications for its use:
- History of previous halothane hepatitis.
- Patients with intracranial lesions and head injury.
- Severe cardiac disease like aortic stenosis (as it causes decrease in cardiac output).
- Pheochromocytoma (as it sensitizes myocardium to adrenaline).
1. Beta blockers and calcium channel blockers can produce severe depression of cardiac function with halothane.
2. Aminophylline can produce serious ventricular arrhythmias with halothane.
3. All cytochrome P450 enzyme inducers enhance its metabolism.
4. Action of nondepolarizing muscle relaxants is potentiated.