Methohexitone

The most commonly used barbiturates are thiopentone [5-ethyl-5-(l-methylbutyl)-2-thiobarbituric acid], methohexitone [l-methyl-5-allyl-5-(l-methyl-2-pentynyl) barbituric acid], and thiamylal [5-allyl-5-(l-methylbutyl)-2-thio-barbituric acid]. Thiamylal is slightly more Table 4.3 Incidence of life-threatening hypersensitivity reactions to intravenous anaesthetic agents... 

Methohexitone (Figure 4.3) is a water-soluble, methylated barbiturate with a pKa of 7.9. It is commercially available as a white powder, combined with 5% anhydrous sodium carbonate. The resultant 1% solution has a pH of 11.1 and although stable, when refrigerated for 6 weeks, should be discarded after 24 hours, as it does not contain an antibacterial preservative. 

The uptake, redistribution and protein binding of methohexitone are somewhat similar to that of thiopentone. Although methohexitone is less lipid soluble than thiopentone, a greater proportion (75%) is non-ionised at body pH and therefore available for pharmacological effect. Hepatic clearance (11 mL-kg-l-min-1) is higher for methohexitone than for thiopentone and the elimination half-life considerably shorter ( 4 hours). While cumulation is less likely to occur with repeated doses, prolongation of anaesthetic effect has been demonstrated when methohexitone was infused for longer than 60 minutes. 

As methohexitone is approximately 2.7 times more potent than thiopentone, a dose of 1.5 mg-kg-1 is equivalent to 4 mg-kg-1 of thiopentone in adults. As with thiopentone, the induction dose should be reduced in the elderly, in American Society of Anesthesiologists (ASA) grade III or IV patients, and in early pregnancy. 

Onset of anaesthesia is as rapid though not as smooth as after thiopentone administration, but recovery is more rapid with methohexitone and occurs within 2-5 minutes through redistribution. Drowsiness persists for much longer as the drug is slowly metabolised in the liver. While methohexitone has been reported to cause occasional EEG seizure-like activity in epileptics the drug also possesses anticonvulsant properties. 

The cardiovascular effects of methohexitone are similar to those of thiopentone except for a more marked tachycardia and less hypotension. 

Respiratory depression and duration of apnoea are of a lesser magnitude with methohexitone than with thiopentone. 

Pain on injection is more common (8-20%) than with thiopentone, especially if a small vein is used for administration but can be reduced by flushing the vein with lidocaine (lignocaine) prior to methohexitone administration. 

Epileptiform EEG activity has been described in epileptic subjects. Tissue damage after perivenous extravasation is uncommon. Intra-arterial injection of 1% methohexitone may cause gangrene but the risk is much less than that with 2.5% thiopentone. Allergic reactions occur but are uncommon. Indications... 

Excitatory phenomena. More common than with thiopentone but iess than with methohexitone. 

Onset of unconsciousness is rapid but it is associated with a high incidence of excitatory effects, comparable to methohexitone, but these can be reduced by prior administration of an opioid. Analogous to the barbiturates, etomidate decreases CMR02, CBF and ICP but the haemodynamic stability of the drug will maintain CPP. Its well-known inhibition of adrenocortical function limits its clinical usefulness for long-term control of elevated ICP. While etomidate can produce convulsion-like EEC potentials in the absence of apparent convulsions, it has been used to terminate status epilepticus. 

Equipotent doses of etomidate and methohexitone produce a similar shift in the carbon dioxide-response curve but at any given carbon dioxide tension ventilation is greater after etomidate than after the barbiturate. 

Mepyramine (pyrilamine) Methanthelinium bromide (methantheline bromide) Metharbitone (metharbital) Methylergometrine (methylergonovine) Methohexitone (methohexital)... 

Tablets Methohexital (Methohexitone) Bulk HPLC TLC phase Mix powder with mobile phase Silica dihydrogen phosphate (6.8 g/1, pH 4.5) S, = CHCfacetone...

Smith and Sanagi also studied the SFC of barbiturates (barbitone, butobarbitone, amylobarbitone, pentobarbitone, talbutal, quinalbarbitone, methohexitone, phenobarbitone, and heptabarbitone) using a packed polystyrene-divinylbenzene or octadecylsilane column with a methanol-modi-... 

Fentanyl Care should be taken to avoid skin contact and inhalation of fentanyl citrate to prevent adverse effects. Incompatibility has been reported with other drugs such as thiopentone sodium, methohexitone sodium, and fluorouracil.54 This drug should not be administered along with alkaline drugs. 

Mannitol should neither be mixed with whole blood for transfusion nor be administered through the same sets by which blood is being infused. Visual incompatibilities are found when methyldopate hydrochloride is mixed with methohexitone sodium, amphotericin, tetracyclines, or sulfadiazine.119... 

Atanassoff PG, Alon E, Pasch T. Recovery after propofol, midazolam, and methohexitone as an adjunct to epidural anaesthesia for lower abdominal surgery. Eur J Anaesthesiol 1993 10(4) 313-8. 

Mobile Phase. Chloroform acetone (4 1). Reference Compounds. Methohexitone Rf 73, quinalbarbitone Rf 55, clonazepam Rf 35,... 

The barbiturates can be metabolised by five major routes (1) hydroxylation of alkyl and aryl substituents at (e.g. pentobarbitone and phenobarb-itone) (2) A-dealkylation at the -position (e.g. methohexitone) (3) oxidation of alkyl groups at to give carboxylic acids (e.g. thiopentone) ... 

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