Studies have shown that horizontal or downsloping ST-segment depression of 1 mm or greater on electrocardiogram (ECG) in response to a treadmill exercise test is a predictor of future coronary events in healthy asymptomatic patients. Although lesser ST-segment shifts defined by horizontal depression less than 1 mm or slowly upsloping ST segments are significant in symptomatic patients, the significance of these results in asymptomatic patients is unknown. Rywik and colleagues performed treadmill tests on 1,083 healthy volunteers from the Baltimore Longitudinal Study of Aging between 1978 and 1993 to determine if lesser ST-segment shifts were indicators for later development of coronary events, such as angina pectoris, myocardial infarction, and sudden cardiac death.
Subjects were included in the study if they were free of major illness according to history, physical examination, and resting ECG. Exercise-induced ECG changes were assessed by a single observer using the Minnesota Code criteria. Subjects were excluded if they had exercise-induced angina pectoris, pathologic Q waves on any ECG, known valvular heart disease, present use of cardiac glycosides or antiarrhythmic drugs, previous heart failure, or if they were unable to achieve the target heart rate.
A total of 472 participants demonstrated exercise-induced ST-segment changes, including 213 patients who developed ischemic ST depression, 75 patients who demonstrated ST depression of 0.5 to 0.9 mm, 124 patients who presented with slowly upsloping ST-segment changes, and 60 patients who experienced worsening of pre-exercise ST depression to at least 1 mm. Six-hundred and eleven patients did not show any changes. Baseline characteristics, including age, sex, body mass index, cholesterol levels, smoking status, presence of hypertension, presence of diabetes, and presence of family history of coronary heart disease, were compared among the groups.
During a median follow-up of 7.9 years, 76 patients developed coronary events. The incidence of coronary events was highest in the group with ischemic ST-segment depression (15 percent) and lowest in the group with no ST changes (3 percent). The authors constructed a Cox proportional hazards model that included the baseline characteristic variables that were significant on univariate analysis, the exercise test variables, and the ST-segment changes to determine the independent predictors of future coronary events. As expected, older age, male gender, higher cholesterol levels, and shorter exercise duration were strongly predictive of future coronary events.
The authors conclude that their findings support previous studies in that minor pre-exercise ST-segment changes that intensify to at least 1 mm of flat or downsloping ST depression with exercise have the same prognostic significance as classic ischemic response. They also found that the lesser degrees of ST changes did not significantly affect prognosis.