- Induction of anaesthesia (1 - 2mg/kg) for short procedures due to short duration of action e.g.:
- ECT
- Short surgeries
- Pre-hospital induction
Methohexital
Methohexital
- Methohexital is formulated as a white powder containing:
- The sodium salt of methohexital
- 6% Na2CO3 by weight
- The addition of water creates an alkaline (pH 11) racemic mixture of 1% methohexital solution, of pKa 7.9
- The shelf-life is 6 weeks at room temperature (less if dextrose or saline is used)
- The solution is bacteriostatic
Absorption
- With a pKa of 7.9, it is 76% unionised at pH 7.4 (vs. 60% thiopental)
- Methohexital has a faster onset than thiopental due to:
- Greater unionised fraction at physiological pH
- Reduced protein binding therefore more free drug
Distribution
- It is only 60% protein bound (vs. 80% thiopental)
- It has a volume of distribution of 2L/kg
Metabolism
- High hepatic clearance
- Metabolised to hydroxymethohexitone, which has minimal hypnotic activity
Excretion
- Clearance 11ml/kg/min
- Half life of 3-5hrs
Respiratory
- Less respiratory depression than other agents but greater degree of laryngospasm
Cardiovascular
- Treater compensatory tachycardia than thiopental so MAP falls less
Neurological
- Excitatory phase prior to loss of consciousness
- Can precipitate seizures in those with epilepsy
Renal
- Similar renal effects to thiopentone
Other
- Can cause pain on injection and similar effects to thiopental following intra-arterial injection
- Doesn't cause histamine release