Increased Thiopental Sensitivity in Cardiac Patients

The average dose of thiopental necessary for induction was 3.4 mg/kg body weight in seven patients on long-term therapy with digoxin and a diuretic and 4.7 mg/kg body weight in seven matched control patients (P less than 0.05). The initial volumes of distribution (V1) did not differ significantly. The amounts of thiopental redistributed (removed from V1) during the induction were calculated as V1 X k12 X ct and V1 X k13 X ct, where k12 and k13 are the rate constants describing the drug transfer from V1 to V2 and V3 and ct the computer-calculated average thiopental concentration in V1 during the initial 40 s of the induction period. There was no evidence of a lower amount being removed during the induction in the cardiac patients. The intravascular drug concentration at the time of sleep was expressed as the average of the arterial and the venous sleep concentrations. In the cardiac patients the average of that value was 20.1 micrograms/ml and in the control patients 30.2 micrograms/ml (P less than 0.05). It is concluded that the lower dose requirement in the cardiac patients was caused at least in part by a higher cerebral sensitivity to the drug.