Heart rate adjustment of ST depression in patients with coronary disease and negative standard exercise tests

Heart rate (HR) adjustment of ST depression (STD) has been shown to correctly classify exercise test findings in up to 85% of normal subjects and patients with "equivocal" electrocardiographic (ECG) responses (> or =100 microV upsloping STD), but the performance of these methods in patients with truly negative ECG responses (<100 microV STD) has not been examined in detail. We reviewed negative standard exercise ECGs in 54 men and women (mean age 61 years) with coronary disease, comprising 16% of consecutive treadmill tests that were performed in 337 patients with angiographic coronary artery disease or stable angina. Mean STD was only 63 +/- 21 microV (0.63 mm) in these negative tests. Despite these subthreshold values for STD, the ST/HR index was abnormal (> or =1.6 microV/bpm) in 27 of 54 patients (50%) when STD was adjusted for the change in HR during exercise. Compared with patients with normal values for HR-adjusted STD, patients with an abnormal ST/HR index were slightly older (64 vs. 58 years, P < 0.05) and demonstrated a trend toward lower exercise duration (10.0 vs. 11.8 min). An abnormal ST/HR index was associated with greater subthreshold STD (73 vs. 53 microV, P < 0.0005) and smaller HR change (35 vs. 56 bpm, P < 0.0001) with exercise. Among the 27 patients with a normal ST/HR index by simple HR adjustment, 11 (44%) had abnormal ST/HR slopes (> or =2.4 microV/bpm) by the more complex linear regression method. Therefore, HR adjustment of STD contributes to the improved sensitivity of the exercise ECG by correct classification of some patients with truly negative standard tests. The magnitude of subthreshold STD and the extent of HR change with exercise both contribute to improved test performance. The increased sensitivity afforded by HR adjustment of STD highlights the importance of the precise measurement of subthreshold STD that is afforded by computerized ECG during exercise testing.