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Several cases of hypotension have been reported in patients who received angiotensin-converting enzyme inhibitors (ACEIs) before a surgical procedure, suggesting that interactions between ACEIs and anesthesia may be neither beneficial nor predictable. To determine if continuation of ACEI therapy until the morning of surgery leads to an unacceptable decrease in blood pressure on induction, we investigated 51 vascular surgical patients that were chronically treated for hypertension with either captopril or enalapril.After randomization, ACEI therapy was either continued until the morning of surgery or stopped at the time of the preanesthetic visit, at least 12 h (captopril) or 24 h (enalapril) before surgery. Each patient received a standardized anesthetic induction. If systolic blood pressure (monitored using a radial artery cannula) decreased to less than 90 mmHg in response to induction, ephedrine was administered.A marked decrease in plasma converting-enzyme activity was found in patients who received enalapril until the morning of the surgical procedure, and 100% of them required ephedrine after induction. In patients who received their usual dose of captopril on the morning of surgery, plasma converting-enzyme activity was reduced to a lesser extent (when compared with patients who received enalapril). Finally, in the patients in whom ACEI therapy, either enalapril or captopril, was stopped of the evening before surgery, the incidence of induction-induced hypotension was significantly less when enalapril or captopril therapy has been discontinued.These data indicate that in hypertensive patients chronically treated with ACEIs, maintenance of therapy until the day of surgery may increase the probability of hypotension at induction.