Isoflurane widely used inhalational Anaesthetic

Isoflurane :

    Difluromethyle ether isomer of Enflurane
    Ross Terrell – 1965 (Ohio) W.C. Stevens – 1971
     Before 1980 Isoflurane considered to produce carcinogenesis  

Physical properties:

    Clear colorless gas, 
    non-inflammable, 
    Slightly pungent smell 
    Stable does not react with metal and other substance 
    MAC is 1.15% in oxygen 
    MAC is 0.56% in nitrous oxide 
    Blood/gas solubility co-effecient 1.4 
    Oil/gas solubility coeff. = 98

Metabolism :

    0.17% of the absorbed dose is metabolized
    Metabolism take place in the liver by oxidation to difluromethanole and trifluro-acitic acid
     Difluromethanole further metabolized into formic acid and fluoride ions
    Because of minimal metabolism a small amount free fluoride serum concentration
    Free fluoride is neprotoxic     

Action on respiratory system :

    Dose dependent respiratory depression
    Decrease tidal volume 
    But increase respiratory rate in the absence of opioids
    Due to pungent smell it causes respiratory irritation thus make inhalational induction with Isoflurane is difficult
    Blunt the normal ventilatory response to hypoxia and hypercarbia  

Action on cardiovascular system:

    Slightly decrease myocardial contractility 
    Slightly decrease cardiac out put 
    Systematic hypotension is due to decrease systemic vascular resistance 
    Arrhythmias is uncommon
    Little sensitization of myocardium to catecholamine's
    In addition to dilating system arterioles isoflurane cause coronary vasodilatation produce coronary steel syndrome  
    Coronary steel syndrome “dilating of the normal arterioles offer low resistance to blood flow may reduce perfusion through stenosed neighboring vessels 
    Increased heart rate because of partial preservation of carotid baro-reflex  

Uterus and placenta :

    Produce uterine muscle relaxation
    Decrease placental blood flow which depend upon cardiac out put 
    Due to uterine relaxation increase the incidence of post partum hemorrhage  

Action on CNS :

    CNS depression 
    Low concentration of isoflurane does not causes any change in cerebral blood flow at normocapnia
    High concentration of isoflurane increase cerebral blood flow which increase intracranial pressure
    it high concentration of 2 MAC reduce cerebral metabolic oxygen requirement

Hepato-renal action:

    Total hepatic blood flow is reduce 
    Hepatic oxygen supply is better maintained with isoflurane than halothane 
    Liver function test is usually not effected 
    Isoflurane decrease renal blood flow
    Decrease glamorous filtration rate and urinary out put  

Action on muscle :

    Isoflurane dose dependent decrease of neuromuscular transmission (muscle relaxation)
    It potentiate the action of non-depolarizing muscle blocking agent 

Advantages:

    Rapid action
    Decreases blood loss
    Little risk of  PONV
    No hepatotoxicty, nephrotoxicity
    Useful in conditions with raised ICP
    No convulsive activity
    Negligible shivering post-op
    Rapid recovery 

Disadvantages:

    Breath-holding
    Respiratory depression
    Animal studies – Fetal asphyxia
    Trigger malignant hyperthermia 

Labels: Inhalational Anesthetics

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