Influence of blood haemoglobin concentration on renal haemodynamics and oxygenation during experimental cardiopulmonary bypass in sheep

Aim: Blood transfusion may improve renal oxygenation during cardiopulmonary bypass (CPB). In an ovine model of experimental CPB, we tested whether increasing blood haemoglobin concentration [Hb] from ~7 g dL⁻¹ to ~9 g dL⁻¹ improves renal tissue oxygenation. Methods: Ten sheep were studied while conscious, under stable isoflurane anaesthesia, and during 3 hours of CPB. In a randomized cross-over design, 5 sheep commenced bypass at a high target [Hb], achieved by adding 600 mL donor blood to the priming solution. After 90 minutes of CPB, PlasmaLyte^® was added to the blood reservoir to achieve low target [Hb]. For the other 5 sheep, no blood was added to the prime, but after 90 minutes of CPB, 800-900 mL of donor blood was given to achieve a high target [Hb]. Results: Overall, CPB was associated with marked reductions in renal oxygen delivery (−50 ± 12%, mean ± 95% confidence interval) and medullary tissue oxygen tension (PO₂, −54 ± 29%). Renal fractional oxygen extraction was 17 ± 10% less during CPB at high [Hb] than low [Hb] (P =.04). Nevertheless, no increase in tissue PO₂ in either the renal medulla (0 ± 6 mmHg change, P >.99) or cortex (−19 ± 13 mmHg change, P =.08) was detected with high [Hb]. Conclusions: In experimental CPB blood transfusion to increase Hb concentration from ~7 g dL⁻¹ to ~9 g dL⁻¹ did not improve renal cortical or medullary tissue PO₂ even though it decreased whole kidney oxygen extraction.