Day - to - Day Variation of the Normal Orthogonal Electrocardiogram

Day-to-day variation of the corrected orthogonial electrocardiogram was investigated in 20 normal subjects. Ten recordings were made in each individual witlh chest electrode positions left unmarked first, after which 10 subjects underwent another series of 10 consecutive daily recordings with marked electrode locations. Mean and maximal day-to-day variations of durations and amplitudes of different deflections of scalar leads as well as variations of directions and magnitudes of several QRS and T spatial vectors have been studied using computer technics for measurement and analysis. Repeat variability in the unmarked recordings was relatively large. For example, the maximum (96%) variability in QRS spatial maximum and in R-wave amplitudes in leads X and Z were, respectively, 0.50, 0.61, and 0.35 mv. Marking of the chest did reduce variability of amplitude and angular measurements by approximately 25%, but even then a substantial variation from day-to-day remained. Relative chaniges in T-xwave amplitude and direction were greater than those of the QRS complex. The results presented can be used as standards to assess ECG changes observed in serial electrocardiography.