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8 General anaesthetic agentselectronic Tec 6 vaporizer that heats desflurane to 39◦Cat2atmospheres. Its low
blood: gas partition coefficient (0.42) ensures a rapid onset and offset, but high con-
centrations are required due to its MAC of 6.6%.Effects
Respiratory – desflurane shows similar respiratory effects to the other agents, being
more potent than halothane but less potent than isoflurane and enflurane. PaCO 2
rises and minute ventilation falls with increasing concentrations. Desflurane has
apungent odour that causes coughing and breath-holding. It is not suitable for
induction of anaesthesia.
Cardiovascular – these may be thought of as similar to isoflurane. However, in
patients with ischaemic heart disease particular care is required as concentrations
above 1 MAC may produce cardiovascular stimulation (tachycardia and hyperten-
sion). It does not sensitize the heart to catecholamines. Vascular resistance to both
cerebral and coronary circulations is decreased.Metabolism
Only 0.02% is metabolized and so its potential to produce toxic effects is minimal.Xenon
Xenon (Xe) is an inert, odourless gas with no occupational or environmental hazards
and makes up 0.0000087% of the atmosphere. It has a MAC = 71% and a very low
blood:gas partition coefficient (0.14). Consequently, its onset and offset of action are
faster than both desflurane and N 2 O.Manufacture
Xenon is produced by the fractional distillation of air, at about 2000 times the cost
of producing N 2 O.Effects
Respiratory – in contrast to other inhaled anaesthetic agents xenon slows the respi-
ratory rate, while the tidal volume is increased so that the minute volume remains
constant. Compared with N 2 O,xenon has a higher density (×3) and viscosity (×1.5),
which might be expected to increase airway resistance when used in high concen-
trations. However, its clinical significance is probably minimal. Despite its use at
high concentrations it does not appear to result in diffusion hypoxia in a manner
similar to that seen with N 2 O.
Cardiovascular – xenon does not alter myocardial contractility but may result in a
small decrease in heart rate.
Central nervous system – xenon may be used to enhance CT images of the brain
while^133 xenon may be used to measure cerebral blood flow. However, in humans