Atrial T (Ta) loop in dogs with or without atrial injury

In 46 dogs with experimentally produced complete A-V block, the P and the Ta waves before and after the atrial injury were recorded in scalar orthogonal ECG leads at high speed and high amplification. The P and the Ta loops were drawn by hand from the scalar ECG'S. In the dogs without atrial injury, the maximum Ta vector was oriented to the right, superiorly and anteriorly. The P-T angle was close to 180 degrees in each of the three planes. The magnitude of the maximum Ta vector was nearly proportionate to that of the maximum P vector in each plane. The Ta loop was a smooth, elongated ellipse in configuration and showed clockwise rotation in all planes, as did the P loop. The spatial atrial gradient obtained from the scalar ECG was small. In the dogs with atrial injury, the Ta loop changed in direction, configuration, and inscription direction. The P-Ta angle and the relative magnitude of the maximum Ta vector to that of the maximum P vector changed markedly. The change of the P loop remained minimum. The maximum Ta vector and spatial atrial gradient were oriented toward the atrial injury site. The magnitude of the spatial atrial gradient was extremely large after injury. These findings were thought to be important in suggesting the presence of atrial injury and its localization.