BACKGROUND: Arterial pressure measured in a peripheral artery sometimes underestimates central arterial pressure after discontinuation of cardiopulmonary bypass (CPB) for reasons that are still unclear. Therefore, the sideport of the aortic cannula is often used for measurement of central pressure because it is accessible and is near the ascending aorta. This study was designed to assess whether blood pressure measured at the sideport of the aortic cannula mirrors blood pressure measured within the ascending aorta. METHODS: Blood pressure was compared in 21 patients 5 minute after discontinuation of CPB at two sites: at the tip of the 23-gauge needle that points distally at a 25degrees to 30degrees angle to the blood flow within the ascending aorta and at the sideport of the aortic cannula. The effects on the sideport-to-aorta pressure differences and wave contour were tested by placing a clamp at the proximal and distal end of the arterial line (aortic cannula sideport-oxygenator tube) of the CPB circuit. RESULTS: There were no statistical differences between systolic, diastolic and mean blood pressure measured at the sideport and ascending aorta when the clamp was applied to the proximal end. The average sideport-to-aorta systolic pressure differences (0.3+/-3.1 mmHg ranged from -5 to 6 mmHg), diastolic pressure differences (-0.9+/-2.1 mmHg ranged from -4 to 4 mmHg) and mean pressure differences (-0.5+/-1.0 mmHg ranged from -2 to 2 mmHg) were not significantly different. No statistical differences were noted between pressures measured at the two sites when the clamp was applied to the distal end of the arterial line. CONCLUSIONS: It is concluded that the pressure measured at the sideport of the aortic cannula reflects the pressure in the ascending aorta after discontinuation of CPB.