Time course of increased haemodilution in hypotension induced by extradural anaesthesia

Volume loading with crystalloid solution results in more pronounced haemodilution in patients who develop arterial hypotension during induction of extradural anaesthesia than in those who remain normotensive. The aim of this study was to describe the time course of this increase in haemodilution. Heart rate, systolic arterial pressure and blood haemoglobin concentration were measured every 3 min during the onset of extradural anaesthesia in 22 elderly men undergoing short urological operations. Fluid therapy consisted of 15 ml/kg body weight of Ringer's acetate solution. Patients with a decrease in systolic pressure of > 25% retained 50% (SD 12%) of the infused fluid in the circulation, while the others retained 36 (8%) (P < 0.002). In both groups, arterial hypotension was followed by increased haemodilution after a delay of as much as 15 min. This suggests that, despite volume loading, there is relative hypovolaemia throughout the development of hypotension.