Impact of Cardiopulmonary Bypass Flow on Renal Oxygenation in Patients Undergoing Cardiac Operations

Cardiac surgery with cardiopulmonary bypass (CPB) is associated with acute kidney injury, and the risk increases with low oxygen delivery during CPB. We hypothesized that renal oxygenation could be improved at higher than normal CPB flow rates.

After ethical approval and informed consent, 17 patients with normal serum creatinine undergoing normothermic CPB were included and received pulmonary artery and renal vein catheters after anesthesia induction for measurements of systemic and renal variables. Renal oxygen extraction, a direct measure of the renal oxygen delivery/renal oxygen consumption ratio, and renal filtration fraction were measured, the latter by renal extraction of ⁵¹chromium-ethylenediaminetetraacetic acid. After start of CPB and aortic cross-clamp, the pump flow rate was randomly varied between 2.4, 2.7, and 3.0 L · min^-1 · m^-2, and measurements were made after 10 minutes at each flow rate.

Renal oxygen extraction increased by 30% at a flow rate of 2.4 L · min^-1 · m^-2 versus pre-CPB (p < 0.05). At a flow rate of 2.7 and 3.0 L · min^-1 · m^-2, Renal oxygen extraction was 12% (p < 0.05) and 23% (p < 0.01) lower, respectively, compared with 2.4 L · min^-1 · m^-2. This corresponds to a 14% and 30% improvement, respectively, of the renal oxygen supply/demand relationship. Filtration fraction was not affected by changes in flow rate, indicating that the glomerular filtration rate increased in proportion to the increase in renal perfusion.

The impaired renal oxygenation seen during CPB is ameliorated by an increase in CPB flow rate. Thus, one way to protect the kidneys during CPB could be to use a higher flow rate than the one traditionally used.