WPW EPICARDIAL QRS-T POTENTIAL DISTRIBUTIONS/Spach et al. 103 dioxide on bronchoconstriction after pulmonary artery occlusion. Am J Physiol214: 772-775, 1968 33. Severinghaus JW, Stupfel MJ: Alveolar dead space as an index of distribution of blood flow in pulmonary capillaries. J Appl Physiol 10:335-348, 1957 34. Severinghaus JW, Swenson EW, Finley TN, Lategola MT, Williams J: Unilateral hypoventilation produced in dogs by occluding one pulmonary artery. J Appl Physiol 16: 53-60, 1961 35.

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... n EW, Finley TN, Guzman SV: Unilateral hypoventilation in man during temporary occlusion of one pulmonary artery. J Clin Invest 40: 828-835, 1961 36. Carlens E, Hanson HE, Nordenstrom BEW: Temporary unilateral occlusion of the pulmonary artery; new method of determining separate lung function and of radiologic examinations. J Thorac Surg 22: 527-536, 1951 37. Folkow B, Pappenheimer JR: Components of the respiratory dead space and their variation with pressure breathing and with bronchoactive drugs. SUMMARY The WolfT-Parkinson-White (WPW) syndrome was experimentally mimicked by stimulating seven different ectopic (pre-excitation) sites in intact chimpanzees. The objective was to determine how to differentiate one ectopic site from another possible ectopic site close by. The approach used was to obtain a direct picture of total cardiac electrical activity in the form of epicardial potential distributions to understand the cardiac origin of the surface potentials throughout ventricular depolarization and repolarization. QRS-T wave body surface maps were interpreted by visually comparing them directly with the associated measured epicardial potential distributions and by quantitative comparison with those produced by adjacent ectopic sites. During early QRS (delta wave) all sites produced a body surface maximum within the same small area on the anterior chest; however, the position of the minimum was markedly different and was related spatially to the position of the ectopic site. The epicardial measurements showed that during early excitation there was a minimum of large magnitude at the ectopic site while the nearby maximum was of much lower magnitude. The body surface maxima and minima during QRS provided an easy way to distinguish between ectopic sites on one ventricle vs. the other, but between adjacent sites on the same ventricle there was frequently little change in the pattern of the QRS maximum and minimum. However, adjacent sites produced distinct changes in the distant low level potential areas. The combined analysis of QRS and T waves showed that subepicardial ectopic sites 2-3 cm apart produced detectable differences in the body surface distributions. Furthermore, the T wave patterns were as useful as or more useful than those during QRS for predicting the ectopic pre-excitation site. On the epicardium, the positions of the repolarization maximum and minimum were the same as those of the earliest and latest areas of ventricular excitation, a feature which resulted in a better indication of cardiac electrical events on the body surface during ST-T waves than during QRS. From the