Hypotension after the release of aortic cross clamp in patients undergoing open heart surgery

Abstract

One hundred consecutive adult patients under-going various elective open heart surgical procedures were included in this prospective study. An indwelling radial arterial cannula was used to measure mean arterial pressure (MAP). Systemic vascular resistance (SVR) during bypass was calculated using the formula SVR=MAP×80/pump flow dynes-sec-cm−5. Patients in whom vasodilators were used during cardiopulmonary bypass were excluded. Measurements were made just before the release of aortic cross clamp when the pump flows were normal; and 1,3,5 and 10 minutes following the cross clamp release. There were 60 males and 40 females with a mean age of 29.4±13.9 years and mean weight of 46±13 Kg. The MAP fell from 65±14 to 47±15 mm Hg (p<0.00001) and the SVR fell from 1699±511 to 1163±365 dynes-sec-cm−5 (p<0.00001) one minute after the release of aortic cross clamp. There was some recovery during the subsequent period, but the change continued to remain statistically significant upto 10 minutes after the release of aortic cross clamp. MAP and SVR decreased in all except 9 and 3 patients respectively at 1 minute. The mean temperature at the time of release of aortic cross clamp was 33.7±2.7°C. There was a poor correlation between the temperature and duration of bypass and extent of decrease in MAP and/or SVR. We conclude that there is a significant decrease in MAP (27%) and SVR (31%) after the release of aortic cross clamp and it persists for 10 minutes.