These are the vapours which in clinically useful concentrations help to maintain anaesthesia and decrease awareness. In Britain they are generally added to the fresh gas flow by passing a fraction of the carrier gas (N2O/O2 or oxygen enriched air) through a plenum vaporizer.
Halothane
This gas has little analgesic effect. It decreases cardiac output (vagal tone รข†‘, leading to bradycardia, vasodilation, and hypotension). It sensitizes the myocardium to catecholamines (beware in patients with arrhythmias; surgical infiltration with local anaesthetic and adrenaline/ epinephrine). Halothane has been replaced by safer inhalational agents, due to the rare but serious complication of postoperative hepatitis.
Isoflurane
This is a halogenated ether. Theoretically induction should be quick, but isoflurane is irritant, so coughing, laryngospasm, or breath- holding may complicate the onset of anaesthesia.
Sevoflurane
This is a halogenated ether which is well-tolerated. It is the agent of choice for inhalation induction of general anaesthesia with low blood:gas solubility.
Desflurane
is another halogenated ether with a pungent smell, rapid onset of anaesthesia, and quick recovery. Meta-analyses of trials in which the duration of anaesthesia was h indicate that patients receiving either desflurane or sevoflurane did not have significant differences in time to be discharged from the recovery unit, or in nausea/vomiting frequency. Patients receiving desflurane followed commands, were extubated, and were oriented 21 minute earlier than those receiving sevoflurane.1 But this may not be significant clinically. Also, desflurane needs a special vaporizer.
Stopping inhalation reverses all the above effects except for hepatitis resulting from drug metabolism.
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