Halothane
Halothane
physical properties
effects on organ system
biotransformation and toxicity
contraindications
drug interactions
physical properties
-MAC value: 0.75 %
-vapor pressure: 243 mmHg at 20 degree celcius
-halogenated alkane
-nonflammable and nonexplosive agent due to the carbon-flouride bonds
-amber-colored bottles which contain thymol preservative slow the spontaneous oxidative decomposition
-least expensive volatile anesthetic
Central Nevous System
-decreased CMR02
-increased CBF
-increased ICP
-altered/blunted cerebral autoregulation
Cardiovascular System
-dose dependant reduction in mean arterial blood pressure
-direct myocardial depressant by interference of the sodium-calcium exchange and utilization of intracellular calcium
-increases in right artrial pressure (CVP)
-coronary arterial blood flow decreases secondary to decrease in mean arterial pressure
-coronary arterial blood flow decreased despite halothane induced coronary artery vasodilation
-generally well maintained perfusion to heart due to balance of decreased oxygen demand with decreased blood supply to the heart
-inhibits normal baroreceptor reflex tachycardia in response to decreased mean arterial pressure
-inhibited baroreceptor reflex may lead to junctional rhythms and bradycardia
-sensitizes myocardium to epinephrine-induced dysrhythmias, therefore avoid epinephrine dosages > 1.5 ug/kg
Respiratory System
-increases respiratory rate
-decreases tidal volume
-resultant decreased alveolar ventilation
-elevated resting PaC02
-elevated apneic threshold
-blunted response to elevated PaC02
-potent vasodilator
-may reverse asthma-induced bronchoconstriction
-may be the most potent bronchodilator amongst the current volatile anesthetic agents
-depresses respiratory mucociliary clearance which may promote postoperative atelectasis and hypoxia
Hepatic System
-decreases hepatic blod flow proportional to decreases in cardiac output
-may impair hepatic metabolism and clearance of some drugs (ex. fentanyl, phenytoin, and verapamil)
-may cause hepatic cellular dysfunction evidenced by minor elevation of liver transaminases
Renal System
-decreases renal blood flow
-decreases GFR
-decreases urinary output
-filtration fraction is increased due to a greater decrease of renal blood flow compared to the decrease in GFR
Neuromuscular
-potential triggering agent for malignant hyperthermia
-prolongs muscle relaxation from nondepolarizing muscle relaxants (NDMR)
Biotransformation and toxicity
-halothane oxidized by the liver cyto p450 (2EI) to trifluoroacetic acid metabolite which can be inhibited by disulfiram
-presense of ischemia may lead to reductive hepatotoxic metabolites of halothane which covalently bind to macromolecules
Factors which may be associated with postoperative hepatic dysfunction:
-viral hepatitis
-impaired perfusion to the liver
-preexisting liver disease
-hepatocellular hypoxia
-sepsis
-hemolysis
-drug induced hepatitis
Halothane hepatitis associations:
-rare: 1 in 35,000 cases
-multiple, short interval anesthetic exposure
-middle-aged obese females
-familial history of halothane toxicity
contraindications
-patients with pre-existing unexplained liver dysfunction
-possibly in patients with intracranial mass lesions due to potential of increasing ICP
-patients with pheochromocytoma due to possible association of epinephrine induced dysrythmias
-cardiac patients who cannot tolerate direct cardiodepressant effects of halothane
drug interactions
-beta blockers and calcium channel blockers exacerbate direct cardiodepressant effects of halothane
-possible arterial pressure fluctuation and dysrhythmia effects with tricyclic antidepressants and monoamine oxidase inhibitors
-possible severe ventricular dysrhythmias with combination of halothane and aminophylline
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