Effects of heart position on the body-surface electrocardiogram

Previous studies have examined the influence of body position, respiration, and habitus on body surface potentials. However, the authors could only es timate the sources of the effects they documented. Among the proposed origi n of changes in body surface potentials from those studies were the positio n of the heart, alterations in autonomic tone, differences in ventricular b lood volume, and variations in torso resistivity. The goal of this study wa s to investigate specifically the role of geometric factors in altering bod y surface potentials and the electrocardiogram. For this, we used experimen ts with an isolated, perfused dog heart suspended in a realistically shaped electrolytic torso tank. The experimental preparation allowed us to measur e epicardial and tank surface potentials simultaneously, and then reconstru ct the geometry of both surfaces. Our results mimicked some of the features described by previous investigators. However, our results also showed diff erences that included considerably larger changes in the peak QRS and T-wav e amplitudes with heart movement than those reported in human studies. We d etected smaller values of root-mean-squared variability from heart movement s than those reported in a human study comparing body surface potentials du ring change in inspiration and body position. There was better agreement wi th relative variability, which in these studies ranged from 0.11 to 0.42, a greeing well with an estimate from human studies of 0.40. Our results sugge st that the isolated heart/torso tank preparation is a valuable tool for in vestigating the effects of geometric variation. Furthermore, the geometric position of the heart appears to be a large source of variation in body sur face potentials. The size of these variations easily exceeded thresholds us ed to distinguish pathologic conditions and thus such variations could have important implications on the interpretation of the standard electrocardio gram.