Lead specificity of the maximum ST/heart rate slope response

Computer-based implementation of the ST-segment/heart rate slope

The ST-segment/heart rate (ST/HR) slope has markedly improved the accuracy of the exercise electrocardiogram for the identification and quantification of coronary artery disease. However, clinical use of this technique has been limited by time-consuming manual ST-segment measurements and calculator-based linear regression analyses after testing. To assess the accuracy of on-line computerized measurement of ST-segment depression and recently implemented ST/HR slope algorithms, 50 patients exercised with a Marquette Electronics CASE 12 system and 50 patients exercised with a Quinton Q5000 system were evaluated. Computerized ST-segment measurements, to the nearest 10 microV, were compared with averaged manual measurements from the raw signal, to the nearest 25 microV, at peak exercise in leads II and CM5. The CASE 12 and Q5000 algorithms for selection of data and calculation of the maximal ST/HR slope were compared with the ST/HR slope calculated from end-stage data points according to a standard off-line procedure. Manual ST-segment measurements correlated highly with CASE 12 and Q5000 determinations (r = 0.996 to 0.998). The slopes of their regression lines approached unity (range 0.98 to 1.02), and the standard errors of the estimate ranged from 12 to 15 microV. Each on-line program selected data points and determined ST/HR slopes in complete accordance with standard calculations. It is concluded that computer-based measurement of the ST/HR slope can be performed with a high degree of accuracy.

Heart rate adjustment of ST segment depression for improved detection of coronary artery disease

Normal values for heart rate-adjusted indexes of ST segment depression during treadmill exercise electrocardiography (the ST segment/heart rate slope and the delta ST segment/heart rate index) were derived from evaluation of 150 subjects with a low likelihood of coronary artery disease, including 100 normal subjects and 50 subjects with nonanginal chest pain. Partitions chosen by the method of percentile estimation to include 95% of normal subjects remained highly specific in subjects with nonanginal pain syndromes. Sensitivities of the derived partitions for detection of myocardial ischemia were tested in an additional 150 patients with a high likelihood of coronary disease, including 100 patients with angiographically demonstrated coronary obstruction and 50 patients with stable angina. In contrast to the 68% (102 of 150 subjects) sensitivity of standard exercise electrocardiographic criteria for the detection of disease in this population, the sensitivity of an ST segment/heart rate slope partition of 2.4 muV/beats/min was 95% (142 of 150 subjects, p less than 0.001), and the sensitivity of a delta ST segment/heart rate index partition of 1.6 muV/beats/min was 91% (137 of 150 subjects, p less than 0.001). Analysis of receiver-operating curves confirmed the superior performance of the heart rate-adjusted indexes throughout a wide range of test specificities. These findings suggest that heart rate adjustment of ST segment depression can markedly improve the clinical usefulness of the treadmill exercise electrocardiogram.