Poor medullary oxygenation is implicated in the evolution of acute kidney injury. The authors sought to determine if increasing systemic flow and mean arterial pressure could improve urine oxygen tension (PuO₂) measured in the bladder, a surrogate of kidney medullary oxygenation, in patients undergoing on-pump cardiac surgery.

Randomized crossover study.

University-affiliated hospital.

Twenty adult patients undergoing cardiopulmonary bypass (CPB) with expected cross-clamp time of >60 minutes and estimated glomerular filtration rate of >30 mL/min/1.73m².

Sequential 20-minute periods of 2 interventions: Intervention H (“High”) or Intervention N (“Normal”). The order of interventions was determined by randomization. Intervention H: targeted CPB flow 3.0 L/min/m² and mean arterial pressure (MAP) 80 mmHg. Intervention N: targeted CPB flow 2.4 L/min/m² and MAP 65 mmHg.

PuO₂ was measured by an oxygen sensor introduced into the bladder via a urinary catheter. Clear separation was achieved in CPB flow and MAP between intervention periods (p < 0.001 for group-time interaction). PuO₂ during Intervention H was higher than during Intervention N (p < 0.001 for group-time interaction). After 17 minutes, PuO₂ was statistically higher in Intervention H at each time point. There were no differences in markers of hemolysis between interventions.

PuO₂ was higher when systemic flow and MAP were increased during CPB. These findings suggest that PuO₂ is responsive to changes in hemodynamics and that higher flow and pressure may improve medullary oxygenation.