Day-to-day variability in the normal electrocardiogram.

Abstract

The ecg was recorded under carefully standardized conditions on 4 consecutive days in 36 men and 22 women aged 30 to 70 years. All were clinically free of heart disease and the ecg were individually normal. The tracings were examined for qualitative changes and measurements made of the daily variations in the P-R and QRS intervals, the levels of the S-T segments, and the amplitudes of the P, Q, R, S, and T waves. The limits of normal for the day-to-day variability of ecg were determined statistically for each of these features and tabulated. If these limits are exceeded in consecutive daily ecg recorded under standardized conditions, the series should be considered abnormal even though the tracings are individually normal.