The aim of this study was to determine the relationship between splanchnic perfusion and oxygen consumption, and flow types in cardiopulmonary bypass (CPB), by measuring gastric intramucosal pH. Twenty patients undergoing elective open-heart surgery were prospectively randomized to receive either pulsatile or nonpulsatile flow during CPB. Gastric intramucosal pH was measured using gastric tonometry. A flowmeter was used to measure the inferior caval vein flow. A catheter was inserted through the femoral vein to sample blood from the iliac vein. Systemic vascular resistance index, gastric intramucosal pH, inferior caval vein flow and arterial, inferior vena caval and iliac venous blood gases were recorded at different times. Gastric intramucosal pH decreased in all patients; only in the nonpulsatile group was this decrease statistically significant. After 45 min of CPB, the pH was 7.37 +/- 0.03 compared with the prebypass value of 7.48 +/- 0.04 (p = 0.00016). After weaning from CPB, the pH was 7.358 +/- 0.02 compared with the prebypass value (p = 0.000037). At 2 h post-operatively the pH was 7.416 +/- 0.025 (p = 0.02). Systemic vascular resistance index rose in all patients during bypass in both groups. These changes did not have any statistical significances and after weaning from bypass returned to prebypass levels. We conclude that nonpulsatile flow in CPB is associated with reduced gastric intramucosal pH and the measurement of intramucosal pH during open-heart surgery provides important information about splanchnic perfusion.