GUT MUCOSAL ISCHEMIA DURING NORMOTHERMIC CARDIOPULMONARY BYPASS RESULTS FROM BLOOD-FLOW REDISTRIBUTION AND INCREASED OXYGEN-DEMAND

Impaired gut mucosal perfusion has been reported during cardiopulmonar y bypass. To better define the adequacy of gut blood flow and oxygenat ion during cardiopulmonary bypass, we measured overall gut blood flow and ileal mucosal flow and their relationship to mucosal pH, mesenteri c oxygen delivery and oxygen consumption in immature pigs (n = 8). Nor mothermic, noncross-clamped, right atrium-to-aorta cardiopulmonary byp ass was maintained at 100 ml/kg per minute for 120 minutes. Animals we re instrumented with an ultrasonic Doppler flow probe on the superior mesenteric artery, a mucosal laser Doppler flow probe in the ileum, an d pH tonometers in the stomach, ileum, and rectum. Radioactive microsp heres were injected before and at 5, 60, and 120 minutes of cardiopulm onary bypass for tissue blood how measurements. Overall gut blood flow significantly increased during cardiopulmonary bypass as evidenced by increases in superior mesenteric arterial flow to 134.1% +/- 8.0%, 13 7.1% +/- 7.5%, 130.3% +/- 11.2%, and 130.2% +/- 12.7% of baseline valu es at 30, 60, 90, and 120 minutes of bypass, respectively. Conversely, ileal mucosal blood flow significantly decreased to 53.6% +/- 6.4%, 4 9.5% +/- 6.8%, 58.9% +/- 11.6%, and 47.8% +/- 10.0% of baseline values , respectively. Blood flow measured with microspheres was significantl y increased to proximal portions of the gut, duodenum and jejunum, dur ing cardiopulmonary bypass, whereas blood flow to distal portions, ile um and colon, was unchanged. Gut mucosal pH decreased progressively du ring cardiopulmonary bypass and paralleled the decrease in ileal mucos al blood flow. Mesenteric oxygen delivery decreased significantly from 67.0 +/- 10.0 ml/min per square meter at baseline to 42.4 +/- 4.6, 44 .9 +/- 3.5, 46.0 +/- 3.6, and 42.9 +/- 3.9 ml/min per square meter at 30, 60, 90, and 120 minutes of bypass. Despite the decrease in mesente ric oxygen delivery, mesenteric oxygen consumption increased progressi vely from 10.8 +/- 1.4 ml/min per square meter at baseline to 13.4 +/- 1.2, 15.9 +/- 1.2, 16.7 +/- 1.4, and 16.6 +/- 1.54 ml/min per square meter, respectively. We conclude that gut mucosal ischemia during norm othermic cardiopulmonary bypass results from a combination of redistri bution of blood flow away from mucosa and an increased oxygen demand.