The effect of pentobarbital anaesthesia upon the extracellular fluid volume in the dog,studied by continuous infusion and single injection methods

The effect of pentobarbital anaesthesia on the volume and ionic composition of the extracellular space was studied in adult male mongrel dogs with permanent catheters in aorta and pulmonary artery. The extracellular fluid volume (Q^ec) was determined with: a) methods based on equilibration of the indicator throughoutQ^ec by continuous infusion; b) methods based on the assumption that after a single injection of indicator the plasma indicator concentration equals extracellular indicator concentration as long as the log plasma indicator concentration-time curve is linear; c) a single injection method based on a closed flow system model with a single inflow and a single outflow orifice. The measurements were made before and 30 and 90 min after induction of anaesthesia. Thirty minutes after induction of anaesthesiaQ^ec as determined with the method sub a, had decreased by about 10% and remained so during the following 60 min. The values ofQ^ec as calculated by the method sub c fairly agreed withQ^ec as determined with the method sub a and also showed a decrease ofQ^ec during pentobarbital anaesthesia. The procedures sub b overestimatedQ^ec and yielded a seemingly higherQ^ec during anaesthesia, because the boundary conditions for these procedures do not apply. The haemoglobin concentration decreased by about 10% and the lactate concentration by about 50%. The phosphate concentration increased by about 25% while the other electrolyte concentrations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^–, HCO₃^–) did not change. A respiratory acidosis developed during the first 30 min and almost disappeared in the following 60 min. Possible explanations for the pentobarbital-induced concentration ofQ^ec are discussed.