THIOPENTONE AND ETOMIDATE CONCENTRATIONS IN MATERNAL AND UMBILICAL PLASMA, AND IN COLOSTRUM

We have measured concentrations of etomidate and thiopentone in maternal plasma, umbilical venous plasma and colostrum after induction of anaesthesia in 40 patients undergoing Caesarean section. Mean plasma etomidate concentration declined rapidly (1242.0 ng ml-1 at 5 min, 434. 0 ng ml-1 at 15 min, 64.2 ng ml-1 at 30 min, 7.0 ng ml-1 at 60 min and undetectable 2 h after the injection). Mean plasma concentrations of thiopentone declined more slowly (6.09 mug ml-1 at 5 min, 2.64 mug ml-1 at 2 h, 1.35 mug ml-1 at 4 h, 0.86 mug ml-1 at 9h and 0.59 mug ml-1 at 12 h). Mean umbilical venous thiopentone concentration was 4.72 mug ml-1, whereas the thiopentone concentration in the maternal sample at 5 min was 6.09 mug ml-1, giving an umbilical:maternal vein ratio of 1:1.3. Mean umbilical etomidate concentration was 51.7 ng ml-1 and the corresponding maternal vein sample (5 min) was 1242.0 ng ml-1 (P<0.001), giving an umbilical:maternal vein ratio of 1:24. Mean concentrations of thiopentone in colostrum were 1.98 mug ml-1 at 30 min, 0.91 mug ml-1 at 4 h and 0.59 mug ml-1 at 9 h, colostrum:plasma ratios at 4 h and 9h being 0.67 and 0.68, respectively. Mean concentrations of etomidate in colostrum were 79.3 ng ml-1 at 30 min and 16.3 ng ml-1 at 2h, being undetectable at 4 h. The colostrum:plasma etomidate concentration ratio was 1.2 at 30 min. We conclude that, although plasma and colostrum concentrations of thiopentone and etomidate declined rapidly, the decrease was faster with etomidate.