An analysis of activation in human atria.

The P wave and Psi-loop were derived from coplanar outlines of the contour of seven human atria, including left atrial enlargement, in which it was assumed that the process of depolarization spread in a radial fashion from the sinoatrial node. The "derived" planar projections of the P waves of the standard leads and Psti-loops were similar in relative amplitude and general contour to those recorded electrocardiographicallyv and vectorcardiographically prior to death. INCE an organized conduction system has not been demonstrated in the atria and because of the thinness of atrial muscle, the excitation wave is usually considered to spread through these structures in a simple radial fashion. Such an order of activation is less complex than that occurring in the thicker ventricles with their intricate conduction system and may, therefore, be susceptible to simpler methods of analysis. If a radial spread of activation is considered to occur and if certain assumptions regarding the anatomy of the atria are made, an approximate analysis of the electrocardiographic and vector-cardiographic manifestations of atrial excita-tion becomes feasible. METHODS AND MATERIALS In the analysis to be presented, the atria were considered as two surfaces. One of these consisted of those portions of the free walls of the atria which were visible on the external surface of the heart (free external atrial walls). The remainder of the atria, which included the interatrial septum and those portions of the free walls which were not visible on the external surface of the heart (free internal atrial walls), were considered to form the second surface. Activation was assumed to begin on the free external atrial wall at the anterior margin of the junction of the superior vena cava and the free external wall of the right atrium. This point represents the approximate location of the superior portion or head of the sinoatrial node as described by Keith and Flack.1 It was further assumed that the wave spread in all directions at uniform speed. This 97 assumption is supported by the studies of Lewis2 and those of Puech and coworkers.3 The latter group interpret their data to indicate that, with the exception of the taenia terminalis region, conduction speed in dog atria was virtually constant. Since the atria were divided into two sections for this analysis, it was necessary to locate the point of origin of the impulse relative to each of these sections as illustrated diagrammatically in figure 1. …