Effects of phlebotomy and autologous blood transfusion on oxygen transport in the racehorse.

Because the haemoglobin concentration Hb[ during exercise may be at the upper limit of blood viscosity for effective oxygen delivery, we hypothesised that administration of blood would not further enhance oxygen delivery or exercise capacity. Six Thoroughbred geldings were used in 5 incremental treadmill exercise tests over a period of 4 weeks. The first test was performed 6 days prior to phlebotomy, which involved the removal of 20 ml/kg bwt of venous blood. Exercise tests were performed at 1, 8 and 15 days after blood removal. Six days after the 15 day post phlebotomy exercise test, blood from each horse was reinfused and the final test performed 24 h after blood reinfusion. During the 3 weeks following blood collection, the blood, collected into acid citrate dextrose, was stored at 3 degrees C. Each exercise test involved measurements of arterial and mixed venous blood gases, plasma lactate concentrations, heart rate and VO2 using an open flow system. Cardiac output was measured by direct Fick. The removal and reinfusion of blood had significant effects on packed cell volume (PCV) with the lowest PCV value during exercise of 0.57 +/- 0.04 l/l being recorded in the second post phlebotomy test and the highest value of 0.67 +/- 0.04 l/l found after blood reinfusion. There were no significant effects of phlebotomy or blood reinfusion on arterial blood gas values. However, arteriovenous oxygen content difference was significantly altered by phlebotomy and reinfusion. The maximal values during the 3 tests following phlebotomy were significantly lower (P < 0.05) by about 7% than those before phlebotomy and after blood reinfusion, mirroring the alterations in Hb[. There was no significant effect of phlebotomy or blood reinfusion on heart rate but stroke volume was significantly higher (P < 0.01) during the 3 post phlebotomy tests than for the control or blood reinfusion tests. There was no significant effect of phlebotomy or blood reinfusion on either the submaximal or maximal values for VO2. No significant effect of blood removal or reinfusion was found on the treadmill run time to fatigue. We concluded that haemodynamic adjustments following phlebotomy or blood reinfusion maintained oxygen delivery during exercise. No detrimental effects on exercise capacity were found from phlebotomy nor beneficial effects from autologous blood transfusion. However, it should be noted that the extent of change induced in Hb[ was relatively small and with the number of horses in the study, the experimental power was not great.