Volume kinetics of Ringer's solution and dextran 3% during induction of spinal anaesthesia for caesarean section

To study how the body handles fluid given intravenously during the onset of spinal anaesthesia in women scheduled for Caesarean section.

The effect of spinal anaesthesia on the volume kinetics of a constant-rate infusion of 25 ml.kg-1 of Ringer's solution (n = 11) and 10 ml.kg-1 of dextran 3% 60 (n = 8) was studied before elective Caesarean section. Measurements of the blood haemoglobin concentration and urine excretion served as input variables in calculations of the size(s) of the body fluid spaces expanded by the infused fluid. The blood glucose level was also monitored.

When a one-volume kinetic model were fitted to the data, spinal anaesthesia reduced the size of the expanded body fluid space by 30% (Ringer's) and 58% (dextran) (P < 0.02) When a two-volume model was statistically justified, anaesthesia reduced the rate of fluid equilibration between the two expanded body fluid spaces by 47% and 19%, respectively (P < 0.04) The baseline volume for the primary (central) fluid space was smaller than the expected plasma volume; 1.5 l for Ringer's solution and 0.9 l for dextran. Only small changes in the blood glucose concentration were found.

The onset of spinal anaesthesia induces acute changes in the body's handling of infused fluid that can be described by volume kinetic analysis.