Extracellular fluid volume and composition changes during sodium pentobarbitone anaesthesia in the dog.

Serial measurements of extracellular fluid (ECF), and plasma volumes were evaluated in dogs before and during general anaesthesia with sodium pentobarbitone and under controlled conditions of arterial pH, pO2, pCO2, and blood pressure. Sodium pentobarbitone anaesthesia caused an early, significant rise in ECF volume with a fall in haematocrit, plasma protein, and plasma potassium concentrations. Plasma osmolality and sodium concentrations were unchanged. The lack of change in ECF sodium concentration suggests that the total ECF sodium content increased in parallel with the expansion of this compartment. Sodium bound to macromolecules in the interstitial space or to bone is suggested as a possible source of sodium ions. It is unlikely that intracellular sodium stores contribute to a significant extent in these changes. During prolonged anaesthesia plasma volume progressively increased while total ECF volume returned towards control values. This work clarifies previous observations and suggests that major fluid movements occur during sodium pentobarbitone anaesthesia primarily associated with altered cell membrane properties and generalised haemodynamic changes.