Comparison of the anaesthetic and haemodynamic effects of chlormethiazole and thiopentone

Two groups of eight women (60-85 yr) undergoing gynaecological operations of 50 to 130 min duration were compared. Anaesthesia was induced with either thiopentone (mean 4.5 mg kg-1) or chlormethiazole (mean 6.0 mg kg-1) and maintained with nitrous oxide and pethidine in combination with the drug used for the induction. The hourly maintenance dose and the plasma concentration determined at equilibrium were greater for chlormethiazole (means 4.7 mg kg-1 h-1 and 27 mumol litre-1) compared with 1.3 mg kg-1 h-1 (P less than 0.01) and 16 mumol litre-1 respectively for thiopentone (P less than 0.02). Impedance cardiography showed that cardiac output was decreased by 30-40% in the thiopentone group (P less than 0.01), whereas no significant change was observed in the chlormethiazole group. Chlormethiazole anaesthesia was followed by a significant increase (P less than 0.02) in stroke volume. No correlations were found between the plasma concentrations and changes in the haemodynamic indices for either of the drugs.