Perioperative myocardial ischemia in patients undergoing noncardiac surgery--I: Incidence and severity during the 4 day perioperative period. The Study of Perioperative Ischemia (SPI) Research Group. To determine the incidence and characteristics of perioperative myocardial ischemia, the electrocardiographic (ECG) changes consistent with ischemia during the 4 day perioperative period were documented and characterized in 100 patients with or at risk for coronary artery disease undergoing noncardiac surgery. Using continuous two channel ECG monitoring (leads CC5 and CM5), the frequency and severity of ECG ischemic episodes defined by ST segment depression greater than or equal to 1 mm or elevation greater than or equal to 2 mm during the preoperative (up to 2 days), intraoperative and early postoperative (first 2 days) periods were compared. Preoperatively, 28 patients (28%) exhibited 105 episodes of ischemia; intraoperatively, 27 patients exhibited 39 episodes and postoperatively, 42 patients exhibited 187 episodes. There was no difference between the pre- and intraoperative episode characteristics. However, postoperative ischemic episodes were the most severe. The mean ST change was 1.5, 2 and 2.6 mm for pre-, intra- and postoperative episodes, respectively (p less than 0.0001 postoperative versus pre- or intraoperative); duration of ischemic episodes was 69, 45 and 207 min, respectively (p less than 0.005 postoperative versus preoperative, p less than 0.001 versus intraoperative) and area under the ST curve was 88, 74 and 383 mm.min (p less than 0.009 postoperative versus preoperative, p less than 0.005 versus intraoperative). Ninety-four percent of all postoperative ischemic episodes were silent; 80% of all episodes occurred without acute change (+/- 20% of control) in heart rate and 77% of intraoperative episodes occurred without acute change in blood pressure. 