Halothane

Halothane:

• Halothane was synthesized in 1951 and introduce in clinical practice in 1956
• It is decomposed by light that’s why stored in amber-coloured bottle 
• It have 0.01% of thymol as a preservative 
• It corrodes with metal in vaporizers and breathing circuits 

Physical properties :

• Heavy 
• Colorless liquid  
• sweet smelling liquid(pleasant smell)
• Oil/gas solubility coeff. = 224
• Blood/gas solubility coeff. = 2.5
• MAC in oxygen =1.1% in neonate  0.95 % in infant 
• 0.9% in 1-2 year 
• 0.7% in adult 
• 0.65% in old age 
• 0.3% with nitrous oxide 

Metabolism :

• Approximately 20% is metabolized in the liver by oxidative pathway 
• The end product is excreted in the urine
• The main metabolites is bromine, chlorine, trifluro-acitic acid and trifluoro-acethylethenole amide  
• A small proportion of halothane may under go reductive metabolism particularly in hypoxemia   

Action on respiratory system :

• Halothane is non irritant and pleasant to breathe during induction 
• It loss pharyngeal and laryngeal reflexes and inhibition of salivary and bronchial secretion
• Halothane increase ventilatory rate and decrease tidal volume thus resting paco2 is elevated with the depth of anesthesia 
• Halothane decrease macuciliary function this may contribute to post-operative sputum retention, hypoxia  and atelectasis.

• It produce bronchial smooth muscle relaxation and reduce airway resistance 
• Halothane is considered a potent bronchodilator as it often reverse asthma induce Bronchospasam 
• Hypoxic  drive is severely depressed by even low concentration of halothane 0.1 MAC 

Action on cardiovascular :

• Halothane is a potent depressant of myocardial contractility and myocardial metabolic activity (advantages in coronary disease) 
• Decrease cardiac out put due to decrease cardiac contractility and effect on peripheral resistance
• Decrease in arterial pressure  
• Hypotensive effect also associated with reduction in heart rate (Atropine) 
• Arrhythmias are very common during halothane anesthesia which is due to increased myocardial excitability in the presence of Hypercapnia and hypoxemia 
• Halothane also increase cardiac sensitivity to the arrhythmic action of catecholamine
• During local infiltration with local anesthetic solution containing adrenaline ventricular exrasystole, sinus trachycardia has been observed and cardiac arrest has been reported    

Action on CNS:

• Halothane dilating cerebral blood vessels, lower vascular resistance increase CBF 
• Also increase ICP 
• Rise ICP can be prevented by hyperventilation prior to administration of halothane
• It does not causes seizers activity 
• Decrease cerebral activity  

Action on GIT,Hepatic system :

• GI motility is inhabited post operative nausea and vomiting is very  common with halothane 
•  halothane causes hepatic blood flow to decrease  cardiac out put 
• Hepatic artery vasospasm has been reported 
• The metabolism and Clarence of some drug seem to impaired with halothane 
• Halothane cause hepatic cellular dysfunction and derangement of liver function test  
• Halothane is considered is  hepatotoxic  
• Halothane induce jaundice (repeated exposure within three month)      

Action on renal :

• Decrease renal blood flow 
• Decrease glomerular filtration rate
• Decrease urinary out put 
• Postoperative hydration limits these changes   

Action on neuromuscular :

• Halothane relaxes skeletal muscle and potentiate non-depolraizing   neuromuscular blocking agent 
• It is triggering agent of malignant hyperthermia 
• Post operative shivering is common with halothane this increase oxygen requirement 

 
Action On The Uterus and Placenta :

• Halothane relaxes uterine muscle relaxation and may causes post-partum hemorrhage 
• Increased blood loss in cesarean section
• Blood loss is more than other newer agent 
• High concentration may causes uterine atony 

Advantages:

• Highly potent, non-irritating
• Good relaxation at low doses
• Decreases BP = decreases blood loss
• Smooth induction 
• Minimal stimulation of salivary and bronchial secretion
• Bronchodilatation  

Disadvantages:

• Halothane hepatotoxicity
• Malignant hyperthermia
• Arrhythmias
• Headache
• Shivering
• Slow recovery