Differences between aortic and radial artery pressure associated with cardiopulmonary bypass

Previous investigators have identified an aortic-to-radial artery pressure gradient thought to develop during rewarming and discontinuation of cardiopulmonary bypass. The authors measured mean aortic and radial artery pressures before, during, and after cardiopulmonary bypass in 30 patients, to determine when the pressure gradient develops. The pressure gradient was also measured before and after intravenous injections of sodium nitroprusside (1 microgram/kg) and phenylephrine (7 micrograms/kg) to determine the effect of changes in systemic vascular resistance. A significant (P less than 0.05) pressure gradient (mean +/- SEM = 4.9 +/- 0.7 mmHg) developed upon initiation of cardiopulmonary bypass. This gradient did not change significantly during the middle of bypass (4.2 +/- 0.5 mmHg), with rewarming (4.8 +/- 0.7 mmHg), immediately prior to discontinuation of bypass (4.6 +/- 0.7), or 5 and 10 min following bypass (4.9 +/- 0.9 and 4.8 +/- 0.7 mmHg). Sodium nitroprusside significantly decreased systemic vascular resistance, by 15 +/- 2%, during the middle of bypass but did not affect the pressure gradient. Likewise, phenylephrine increased the systemic vascular resistance by 52 +/- 6% and 34 +/- 4% during the middle of bypass and rewarming, respectively, without affecting the pressure gradient. Although the exact mechanisms responsible for the pressure gradient remain unknown, these results suggest its etiology is associated with events occurring during initiation of cardiopulmonary bypass rather than with rewarming or discontinuation of cardiopulmonary bypass.