[Thiopental distribution in the blood of surgical patients]

The influence of hematocrit, of the concentration of plasma proteins, and of CO2 pressure on the distribution of thiopental in the blood was studied in vitro in 30 venous blood samples. Thiopental was bound to plasma proteins to 80.3-89.2%, mean 85.0%, at a plasma protein concentration of 60 g/l and a pH of 7.37. Considerable changes in the plasma protein concentration had a considerable influence on the binding rate. By halving the protein concentration the free fraction was almost doubled. Increasing the protein concentration to 90 g/l led to binding rates of around 88-89%. When the pCO2 was increased from 40 to 60 torr protein binding decreased slightly, indicating that this barbiturate is mainly bound in its anionic form. At a hematocrit of just under 40 vol.%, 14-34% of the thiopental was absorbed by the erythrocytes. Changes in the plasma protein binding rate influenced the thiopental proportion of erythrocytes, as changes in the hematocrit influenced the plasma thiopental proportion. The concentration of liposoluble thiopental acid physically dissolved in the plasma fluid, just under 8% of the thiopental in the blood, changed relatively little on variation of the hematocrit and the CO2 pressure, thanks to the compensatory function of the plasma proteins and erythrocytes. Since most of the thiopental in the blood is bound to plasma proteins, changes in the protein concentration had the greatest effect on the proportion of thiopental acid of the barbiturate content of the blood.