1.6 Halothane: 

It is a colorless liquid with a relatively pleasant smell. Halothane is susceptible to decomposition. For this reason, it is stored in amber-colored bottles and thymol 0.01% is added as preservative. Halothane can be used safely with soda lime It has a relatively low blood gas solubility so induction is rapid though slower than enflurane, isoflurane and sevoflurane. Halothane is mainly excreted by the lungs with only about 20% metabolized in the liver and excreted in the urine. 

1.6.1 Uses of halothane: 

Used as an induction agent and maintenance of anesthesia with oxygen, muscle relaxants, opioids or alone 

• In children, this is often more acceptable than an injection. Halothane is the agent of choice for inhalational induction in pediatric anesthesia. 
• In adults with poor veins or when IV agents are contraindicated. 
• In adults where airway obstruction is a possibility. 

Inhalational induction with halothane: The mask is placed closely over the patient's face. The vapor strength is increased gradually from 0.5% to 4% (if the patient is a fit adult) or 0.5% to 2% (in ill or frail adults or children). Once the eyelash reflex has disappeared, the mask is placed on the patient's face and the concentration of halothane decreased to 2% and adjusted as required. 

Advantages of halothane 

• Gives a rapid smooth induction. 
• Is relatively non-irritant to the respiratory tract. 
• Is a bronchodilator. 
• Is a vasodilator – this may aid in vein puncture in patients with difficult veins. 
• Gives relatively rapid recovery. 

Disadvantages of halothane 

• Halothane is very potent and the delivered concentration should be known at all times. 
• Poor analgesic properties. 
• Causes cardiac arrhythmias especially when adrenaline is used. 
• Causes hypotension. 
• Uterine relaxation resulting in hemorrhage in obstetric patients. 
• Shivering in the post-operative period. 
• There is the possibility of liver damage especially after repeated administration. 

1.6.2 Effects on organ systems and side effects 

Cardio vascular system: It is a potent myocardial depressant and these effects are augmented in the presence of beta-blockers. Hypotension and bradycardia occur and which is augmented when thiopentone are given together. Halothane sensitizes the heart to the arrhythmia effects of epinephrine. Intravenous and locally infiltrated adrenergic agents should be used cautiously during inhalation anesthesia, especially with halothane. 

The following schedule has been found to be relatively safe during inhalation anesthesia. 

• Epinephrine concentrations no greater than 1:100,000–1:200,000 (1:200,000 = 5 μg/mL). 
• Adult dose should be no greater than 10 mL of 1:100,000 or 20 mL of 1:200,000 within 10 minutes. 
• Total dose should not exceed 30 mL of 1:100,000 (60 mL of 1:200,000) within 1 hour. 

Other factors increasing the risk of arrhythmias with halothane: 

• Carbon dioxide retention 
• The injection of atropine 
• Very light anesthesia resulting in sensory stimulation. 

Respiratory system: Halothane typically causes rapid, shallow breathing. The increased respiratory rate is not enough to counter the decreased tidal volume, so alveolar ventilation drops and resting PaCO2 is elevated. Apneic threshold, the highest PaCO2 at which a patient remains apneic, also rises. The elevated carbon dioxide does not provoke a compensatory increase in ventilation, because halothane causes a concentration-dependent inhibition of the ventilatory response to carbon dioxide.

Halothane is considered a potent bronchodilator, as it often reverses asthma-induced bronchospasm. In fact, halothane may be the best bronchodilator among the currently available volatile anesthetics. Halothane also depresses clearance of mucus from the respiratory tract, promoting postoperative hypoxia and atelectasis. 

Nervous system: by dilating cerebral vessels, halothane lowers cerebral vascular resistance and increases cerebral blood flow. Autoregulation, the maintenance of constant cerebral blood flow during changes in arterial blood pressure, is blunted. Concomitant rises in intracranial pressure can be prevented by establishing hyperventilation prior to administration of halothane 

Neuromuscular: Halothane relaxes skeletal muscle and potentiates nondepolarizing neuromuscular-blocking agents. Like the other potent volatile anesthetics, it is a triggering agent of malignant hyperthermia, a syndrome characterized by severe muscle contraction, rapid development of hyperthermia, and a massive increase in metabolic rate in genetically susceptible patients. This syndrome frequently is fatal and is treated by immediate discontinuation of the anesthetic and administration of Dantrolene. 

Drugs which are safe for malignant hyperthermia are antibiotic antihistamines, barbiturates, benzodiazepines, droperidol, ketamine, local anesthetics nitrous oxide, nondepolarizing neuromuscular blockers, opoids, propofol, propranolol & Vasoactive drugs. Drugs which are unsafe are succinylcholine and all inhalation agents (except nitrous oxide). If Malignant Hyperthermia occurs intra operatively use the following treatment protocol 

• Etiologic Treatment: Dantrolene (2-3 mg/kg IV) as an initial bolus, followed with repeat doses every 5–10 minutes until symptoms are controlled (rarely need total dose > 10 mg/kg) Prevent recrudescence (dantrolene 1 mg/kg IV every 6 hours for 72 hours) 
• Symptomatic Treatment 
• Immediately discontinue inhaled anesthetics and conclude surgery as soon as possible 
• Hyperventilate the lungs with 100% oxygen 
• Initiate active cooling (iced saline 15 mL/kg IV every 10 minutes, gastric and bladder lavage with iced saline, surface cooling) 
• Correct metabolic acidosis (sodium bicarbonate 1–2 mEq/kg IV based on arterial pH) 
• Maintain urine output (hydration, mannitol 0.25 g/kg IV, furosemide 1 mg/kg IV) 
• Treat cardiac dysrhythmias (procainamide 15 mg/kg IV) 
• Monitor in an intensive care unit (urine output, arterial blood gases, pH, electrolytes 

Uterus: Uterine smooth muscle is relaxed by halothane. This is a useful property for manipulation of the fetus (version) in the prenatal period and for delivery of retained placenta post natal. Halothane, however, does inhibit uterine contractions during parturition, prolonging labor and increasing blood loss. Halothane therefore is not used as an analgesic or anesthetic for labor and vaginal delivery. 

Renal: Halothane reduces renal blood flow, glomerular filtration rate, and urinary output. Part of this decrease can be explained by a fall in arterial blood pressure and cardiac output. 

Liver: Halothane causes hepatic blood flow to decrease in proportion to the depression of cardiac output. 

Postoperative shivering: is a common post operatively, treat this by 

• Reassuring the patient 
• Giving oxygen 
• Keeping the patient warm 
• Pethedine 25–50 mgs IV may help 

1.6.3 Contraindication and drug interaction 

Contraindications: It is prudent to withhold halothane from patients with unexplained liver dysfunction following previous exposure. Halothane should be used with great caution in patients with intracranial tumor. Hypovolemic patients and some patients with severe cardiac disease (aortic stenosis) may not tolerate halothane. 

Drug interactions: The myocardial depression seen with halothane is exacerbated by β- adrenergic-blocking agents (e.g., propranolol) and calcium channel-blocking agents (eg, Verapamil). The combination of halothane with aminophylline and adrenaline has resulted in serious ventricular arrhythmias.