The role of increased myocardial oxygen demand in the pathophysiology of myocardial ischemia occurring during daily activities was evaluated in 50 patients with coronary artery disease and exercise-induced ST segment depression. Each patient underwent ambulatory electrocardiographic (ECG) monitoring for ST segment shifts during normal daily activities and symptom-limited bicycle exercise testing with continuous ECG monitoring. All 50 patients had ST depression greater-than-or-equal-to 0.1 mV during exercise. A total of 241 episodes of ST depression were noted in the ambulatory setting in 31 patients; only 6% of these were accompanied by angina pectoris. Significant (0.1 mV) ST depression during ambulatory monitoring was preceded by a mean increase in heart rate of 27 +/- 12 beats/min. Patients with ischemia during daily activities developed ST depression earlier during exercise (7.9 +/- 4.4 vs. 14.2 +/- 6.4 min, p < 0.001) and tended to have significant ECG changes at a lower exercise heart rate and rate-pressure product than did those without ST depression during ambulatory monitoring. In the 31 patients with ischemia during daily activities, the mean heart rate associated with ST depression in the ambulatory setting was closely correlated with the heart rate precipitating ECG changes during exercise testing (r = 0.74, p < 0.001). An important role for increased oxygen demand in the pathophysiology of ischemia during daily activities was supported by the observations that 1) most episodes of ischemia during daily activities were associated with significant increases in heart rate; 2) patients with ischemia during daily activities developed ST depression earlier during exercise and tended to develop ECG changes at a lower exercise heart rate and rate-pressure product than did those without ST shifts in the ambulatory setting; and 3) patients with a relatively high exercise ischemic threshold developed ST depression in association with high heart rates during daily activities.
