Body surface distribution of exercise-induced QRS changes in normal subjects.

Previously reported clinical studies have suggested an important diagnostic role for exercise-induced QRS changes. To explore the spatial features of such alterations, anterior thoracic isopotential distributions were examined at rest and at 85 percent of maximal heart rate in 30 normal male volunteers. Results demonstrated that exercise caused a reduction in positive forces and an augmentation of negative voltages at most anterior thoracic sites. However, the locations of map extrema were not significantly altered. Difference maps, computed by subtracting time-aligned distributions at rest and during exercise, suggested that exercise generated an electrical field characterized by a central anterior minimum bordered superiorly by positive potentials. The minimum reached a peak magnitude in the middle portion of the QRS pattern, then declined in intensity. Although difference patterns during inscription of the QRS complex were similar to those computed during the P-R and S-T segments, time-intensity curves were discontinuous with marked and abrupt early increases and late decreases in strength in the QRS complex. These results suggest that two commonly considered mechanisms (effects of changes in intracavitary blood mass and of atrial repolarization) cannot be solely responsible for the observed effects.