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

This study explored how blood removal (phlebotomy) and subsequent reinfusion of the same blood (autologous transfusion) affected oxygen transportation during exercise in thoroughbred geldings. The results did not find significant impacts on exercise ability, oxygen delivery, heart rate, or fatigue times, despite evident changes in other metrics such as packed cell volume and stroke volume.

Study Design

• Six thoroughbred geldings underwent five treadmill exercise tests over four weeks. The exercise intensity was gradually increased with each test.
• The first exercise test was conducted six days before a phlebotomy procedure where 20ml/kg of the horse’s blood was removed.
• The next three exercise tests were carried out respectively on the 1st, 8th and 15th days after the blood removal.
• Six days after the final post-phlebotomy workout, the removed blood (which had been stored at 3°C in acid citrate dextrose) was reinfused into each horse. A final, fifth, exercise test was carried out 24 hours later.

Measurements and Outcomes

• During each exercise test, several measurements were taken, including: arterial and mixed venous blood gases, plasma lactate concentrations, heart rate and oxygen consumption (VO2), using an open flow system. Cardiac output was calculated using the direct Fick method.
• The phlebotomy and reinfusion procedures did have some noticeable effects. For example, there was a significant change in packed cell volume (PCV), lowest at the second post-phlebotomy test and highest following reinfusion.
• The researchers did not find a significant effect of blood removal or reinfusion on arterial blood gas values. However, the difference between arterial and venous oxygen content (also known as arteriovenous oxygen content difference) did change significantly post-phlebotomy and post-reinfusion.
• Neither phlebotomy nor reinfusion had a significant impact on heart rate. But stroke volume (the amount of blood pumped by the heart per beat) was higher during post-phlebotomy tests than in control or post-reinfusion tests.
• However, there was no significant impact on either submaximal or maximal oxygen consumption (VO2), nor any noticeable effect on the treadmill run time to fatigue. This shows that despite some haemodynamic adjustments, the procedures didn’t drastically alter oxygen delivery or exercise capacity in the horses.

Conclusions

• The researchers concluded that changes induced by phlebotomy or blood reinfusion didn’t hinder oxygen supply during exercise. Therefore, neither procedure showed evident benefits or detriments to exercise capacity in thoroughbred horses.
• However, they encouraged caution when interpreting their findings, as the changes in haemoglobin concentration were relatively small, and the small sample size could limit the statistical power of their experiment.