Ischemic threshold during two exercise testing protocols and during ambulatory electrocardiographic monitoring

The aim of this study was to examine the dependence of the ischemic threshold during exercise testing on the exercise protocol employed and to determine the relation between the ischemic thresholds observed during exercise and during daily activity.

The ischemic threshold (heart rate at 1-mm ST segment depression) during daily activity has been reported to be lower than that observed during exercise testing. Recent reports have hypothesized that this difference is probably dependent on the exercise protocol employed.

Twenty-two patients with known coronary artery disease, not receiving antianginal medications, were evaluated by repeated exercise testing according to the Bruce and the modified Davidson protocols and by 48-h ambulatory electrocardiographic monitoring.

Although the heart rate at 1-mm ST segment depression was somewhat lower with the Davidson than with the Bruce protocol (112 +/- 14 vs. 115 +/- 14 beats/min), the rate-pressure product at 1-mm ST segment depression was similar during 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 that observed during both exercise protocols (p < 0.001 for both comparisons).

Exercise-induced ischemia occurs at a relatively fixed threshold that is mainly dependent on myocardial oxygen demand and is independent of the exercise protocol employed. Ischemia on ambulatory monitoring, however, occurs at a much more variable threshold that is commonly lower than that observed during exercise and is therefore dependent on other factors in addition to increased demand.