Methohexitone provides rapid induction of anaesthesia (within one arm brain circulation time), but often (80%) causes pain on injection. It is less cumulative than thiopentone and before the introduction of propofol, it was used in the maintenance of short procedures by intermittent bolus. It still enjoys some popularity for ECT and may be useful in patients with hypersensitivity to propofol. Pain on injection may be less when reconstituted with saline. The rapid distribution half-life is greater than that of thiopentone (6 min) resulting in a slightly slower immediate recovery of wakefulness, but elimination is faster so that overall recovery is quicker.
Central Nervous System
Methohexitone causes cortical depression. It is a hypnotic and so provides anxiolysis, sedation and sleep. It is an anti-convulsant, although excitatory muscle movements occur in 20% of patients in a dose-related manner. The EEG is affected similarly to thiopentone but, in addition, epileptiform spikes may be present. Increasing the peak concentration by more rapid administration of the dose and the concomitant use of hyoscine or droperidol also increase the chance of excitatory movement. Opioids inhibit this movement. It has no analgesic activity. Cerebral blood flow and ICP are both reduced.
Methohexitone causes a dose-dependent reduction in vascular tone with a reduction in SVR, CVP and PAWP. This results in a decrease in mean arterial blood pressure that is less than with thiopentone as there is a greater increase in heart rate.
There is a dose-dependent reduction in both respiratory rate and tidal volume greater than that of thiopentone. The responses to CO2 and to hypoxia are both reduced.
Methohexitone reduces renal blood flow and increases ADH secretion resulting in a fall in urine output. Splanchnic vascular resistance is increased and intestinal activity reduced. Uterine tone is unaffected, but the drug readily crosses the placenta.
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