Transient asymptomatic S-T segment depression during daily activity.

The significance of asymptomatic episodes of ischemic type S-T segment depression was studied in 20 patients with coronary heart disease. Continuous 10 hour electrocardiographic recordings accompanied by detailed daily diaries of activity and symptoms were obtained periodically during a mean time of 16 months. All patients had ischemic type S-T depression associated with angina pectoris during treadmill exercise. Measurements of heart rate, S-T depression and exercise level at the onset of angina obtained during repeated controlled exercise tests at the start of each study period were compared with the measurements recorded during daily activity. After 2,826 hours of recording, 411 transient epidsodes of ischemic type S-T depression were noted during usual daily activity. Only 101 (25 percent) of these episodes were associated with angina. The remaining episodes were unrelated to other symptoms or to posture. All occurred at heart rates significantly lower than those observed at the onset of angina during exercise testing. Of these episodes of asymptomatic S-T depression, 72 percent occurred only at rest or during very light activity such as slow walking or sitting. Nitroglycerin administered hourly significantly reduced the frequency of these episodes, thus supporting the concept that they represent painless ischemia. Because the episodes of asymptomatic ischemic type S-T depression occurred more frequently than angina during usual daily activity and were evident at heart rates and activity levels well below those expected to evoke ischemia, they may be caused by factors other than those that cause angina.