Background and objective: The study investigated the effect of acute normovolaemic haemodilution on haemodynamics, blood flow and oxygen transport variables with regard to skeletal muscle tissue oxygenation in a canine model.

Methods: Twenty foxhounds were anaesthetized, mechanically ventilated with 30% oxygen in air and underwent first-step normovolaemic haemodilution with Ringer's lactate solution to haematocrit (Hct) 30 and 25% and second-step acute normovolaemic haemodilution with 6% Hetastarch 70 000/0.5 to Hcts of 20, 15 and 10%. Catheters were inserted into femoral arteries and veins and into the pulmonary artery for measurements of haemodynamics, temperature, and sampling of arterial and mixed-venous blood. A flow probe was placed around the left femoral artery. Skeletal muscle tissue oxygen tension (tPO2) was measured in the gastrocnemius muscle using a stepwise driven polarographic needle probe creating histograms from 200 single tPO2 measurements.

Results: Until a Hct of 25% was reached, the heart rate, mean arterial pressure, global and muscular oxygen delivery and consumption remained constant, while the cardiac index and oxygen extraction ratio were significantly increased when compared with baseline. The median tPO2 was significantly decreased at Hcts 15 and 10%, despite increased cardiac index and regional blood flow. The ratio of tPO2 and cardiac index as a marker for efficiency of acute normovolaemic haemodilution started to decline beyond Hcts of 25% (change of slope).

Conclusions: In acute normovolaemic haemodilution to the level of Hct of 25%, the ratio between tPO2 and cardiac index decreases in the healthy dog, indicating an uneconomic relation at the threshold of Hct of 25%.