The Brody effect and change of volume of the heart

A simple model study2 is used to compare the relative contributions to the surface ECG of the "Brody effect"1 and positional changes of the heart when the heart volume changes. In certain studies, such as exercise, it is the change in heart volume and its effect on the surface ECG which is of interest. This paper suggests that when the heart volume changes the extent to which it is accompanied by positional change will strongly influence the surface voltage change; the surface voltage may in fact increase or decrease with volume increase. This may explain some of the controversy surrounding the influence of volume change on the surface signal.