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Letters to the Editor will be published, if suitable, as space permits. They should not exceed 1,000 words (typed double-spaced) in length and may be subject to editing or abridgment. In their recent paper, Kilpatrick et al have used measured body surface potentials to calculate epicardial potentials by using an inverse solution. Particularly interesting, and apparently novel, is their use of their model to determine normal current density at the epicardial surface. A somewhat unanticipated finding is the presence of current leaving the heart superiorly in the case of inferior injury. Figure 1 of their paper shows a body surface isopotential map (BSPM) of a patient with an inferior infarction. This map is rather "dipolar" in appearance. Potentials are predominantly negative over the top half of the thorax (superiorly) and positive over the bottom half (inferiorly). Calculated isopotential epicardial maps for the same patient shown in Figure 2 depict two regions of highly positive potential: one on the superior aspect and one on the inferior aspect of the heart. A current density map exhibits similarly located regions of positive (outward) current. The authors postulate that the conducting pathway provided by the great vessels is responsible for the positive potential and current maxima on the upper surface of the heart, but they provide no further justification for their hypothesis. Note that the inverse solution for potential or current density on the epicardial surface involves only the body surface potentials and the nature of the volume conductor outside the heart. On the other hand, an explanation of epicardial currents involves the nature of cardiac sources and the volume conductor properties of the heart and torso. Two questions arise in our minds. Is the inverse solution plausible? Is the explanation for the epicardial currents plausible? From a consideration of currents in a volume conductor it is hard to see how current leaving the heart superiorly as shown in the figure will fail to produce a relative potential maximum somewhere on the superior body surface. Other investigators have shown a general similarity between body surface and epicardial potential distributions, a finding at variance with the one presented by Kilpatrick et al. Extensive examples of measured canine BSPMs and simultaneous epicardial recordings are given by Barr and Spach.' These maps all indicate that a region of positive heart potentials can be correlated in space to a region of thorax with positive potential. A similar result …

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