The human atrial pacemaker complex

Electrocardiographers have frequently observed and commented upon the different morphologies of the P wave. This in spite of the fact that the gain and speed of the standard ECG were designed primarily to emphasize the higher-voltage QRS complex. Variations in the morphology of the P wave not apparent in the standard electrocardiogram are conspicuous in high-gain, high-speed tracings. Different P wave configurations, as well as cycle-to-cycle changes in P morphology with sinus arrhythmia, multifocal atrial rhythm, and tachycardia, ectopic left atrial rhythms, and tachycardias, are well described. Not commonly appreciated is progressive change in P morphology associated with progressively increasing heart rates during standard exercise stress tests. In addition, the tall, peaked, vertically oriented P wave associated with sinus tachycardia has frequently been commented upon. The first systematic study of the P wave was undertaken by Brody et al., in 1967.1 These investigators observed that no single P wave morphology existed for a normal individual. They determined that the P wave could not be signal averaged and thereby improve on the signal-to-noise ratio if one assumed a single basic morphology. They reported three or more different P waves in a single normal subject. These changes could not be related to differences in respiratory excursion and therefore different heart-lead interrelationships or to other extracardiac factors. They speculated that these different patterns might represent differences in the