THE techniques of open heart surgery represent a major advance in the treatment of certain cardiac conditions. Because these techniques impose great challenges to the body physiology in addition to the mechanical changes of the surgery per se, it is not unexpected that medical complications occur frequently in the postoperative period.1-6 A group of patients who had undergone open heart surgery were studied prospectively during the immediate postoperative period to assess the incidence and importance of these complications. Subjects and Methods During an 11-month period, 132 consecutive patients were studied who were admitted to Deborah Hospital for elective open-heart surgery (Table 1). Most of the younger patients had congenital heart disease (CHD) and most of the older patients had rheumatic heart disease (RHD). All patients had been admitted previously for diagnostic evaluation and, prior to their surgery, were observed for an additional week in the hospital. During this period