ISCHEMIC THRESHOLD DURING 2 EXERCISE TESTING PROTOCOLS AND DURING AMBULATORY ELECTROCARDIOGRAPHIC MONITORING

Objectives. The aim of this study was to examine the dependence of the ischemic threshold during exercise testing on the exercise protocol e mployed and to determine the relation between the ischemic thresholds observed during exercise and during daily activity. Background. The is chemic threshold (heart rate at 1-mm ST segment depression) during dai ly activity has been reported to be lower than that observed during ex ercise testing. Recent reports have hypothesized that this difference is probably dependent on the exercise protocol employed. Methods. Twen ty-two patients with known coronary artery disease, not receiving anti anginal medications, were evaluated by repeated exercise testing accor ding to the Bruce and the modified Davidson protocols and by 48-h ambu latory electrocardiographic monitoring. Results. Although the heart ra te at 1-mm ST segment depression was somewhat lower with the Davidson than with the Bruce protocol (112 +/- 14 vs. 115 +/- 14 beats/min), th e rate-pressure product at 1-mm ST segment depression was similar duri ng the two protocols (16,900 +/- 4,000 vs. 17,700 +/- 3,600). The mean heart rate (100 + 12 beats/min) at 1-mm ST segment depression during ambulatory ischemic episodes (n = 137) was significantly lower than th at observed during both exercise protocols (p < 0.001 for both compari sons). Conclusions. Exercise-induced ischemia occurs at a relatively f ixed threshold that is mainly dependent on myocardial oxygen demand an d is independent of the exercise protocol employed. Ischemia on ambula tory monitoring, however, occurs at a much more variable threshold tha t is commonly lower than that observed during exercise and is therefor e dependent on other factors in addition to increased demand.