Individual inhalant anaesthetic agents

INDIVIDUAL INHALANT ANAESTHETIC AGENTS 

Halothane

  • Multihalogenated ethane – introduced in 1957
  • Rapid induction, recovery, minimal side effects and nonflammability
  • About 20 % metabolized - MAC 0.85% - not irritating to the mucous memb.
  • Depresses CNS in a dose related fashion – cerebral blood flow may increase → higher CSF pressure. Shivering found during recovery
  • Depression of cardiac output and bradycardia – due to vagus activity
  • May sensitize the heart to catecholamines → arrhythmia
  • Depresses Minimal neuromuscular blockade and analgesia
  • Minimal pathological changes in the liver. Hepatitis may be caused. Some metabolites may cause necrosis
  • Kidney unaffected but sometimes as a sequelae to blood flow changes
  • Poor muscle relaxation.
  • Malignant hyperthermia in pigs and man. Also in other sp.

Isoflurane

  • Popular agent.
  • MAC – for induction 3-5% and for maintenance – 1.5 – 2.5%
  • Flourinated ether. 1% metabolized into fluoride. But not toxic.
  • Lesser vasodilatation of the brain. Affects cardiac output less than halothane and has a wider margin of safety.
  • Dose dependent depression of the heart and respiration
  • Arrhythmias not reported
  • Renal function not affected
  • Better muscle relaxation than halothane
  • Not to be used in animals with a known susceptibility to malignant hyperthermia
  • Used in reptiles
  • Yet to replace halothane in popularity

Enflurane

  • Since 1963
  • cardiac depression, decrease cardiac output, decrease arterial BP as good as or higher than halothane
  • sensitization of the heart to catecholamines – very mild
  • 2-8% metabolized but may not cause fluoride nephrosis.
  • Hepatitis reported

Desflurane

  • Highly stable – stable at room temp. for > a year
  • Boils at close to room temp. and careful vaporization needed
  • Low solubility in blood → rapid induction
  • 0.2% metabolised
  • Not popular in humans because of its irritant nature
  • Tendency to cause malignant hyperthermia

Methoxyflurane

  • Stable compound
  • Less prone to cardiac depression, not irritating
  • Polyuric renal dysfunction due to fluoride metabolites
  • Hence withdrawn

Ether (diethyl ether)

  • Colourless volatile liquid. Stored in a cool place but not in refrigerator
  • Inflammable – High blood - gas solubility
  • Locally refrigerant, anaesthetic, rubefacient, skin antiseptic
  • Nausea and vomition during inhalation -- disadvantageous
  • Metabolites non toxic
  • CNS – slow and unpleasant induction—
  • Heart rate and BP decrease due to epinephrine release during induction
  • Respiration – Irritant to passage, bronchial and pulmonary complications seen
  • Transient depression of liver and kidney – causes metabolic acidosis
  • However ether has been in use for more than 100 years
  • Classical stages of anaesthesia can be produced 

Chloroform

  • Unpleasant odour – Powerful rapid inducer
  • Sensitizes myocardium to adrenaline → ventricular fibrillation → sudden death (20 – 30 times more potent than ether)
  • Toxicity to liver – necrosis, fatty degeneration
  • Kidney – anuria
  • Poisoning → acidosis, vomiting, acetonuria, albuminuria, pyrexia, icterus,
  • Discontinued

Nitrous oxide

  • Colourless gas – kept compressed at 40 atm.
  • N2O increases pulmonary ventilation – due to direct stimulation of respiration.
  • Increases  its own inhalation – (Concentration effect)
  • and also that of halothane or enflurane or oxygen when given together (second gas effect)
  • MAC > 100% (104 in man to 255 in cat) – weak anaesthetic-
  • But strongly analgesic
  • An anaesthetic adjuvant
Last modified: Monday, 18 June 2012, 6:44 AM