P-wave morphology and atrial activation in the domestic fowl.

The purpose of this study was to define the P-wave morphology in the open chest, anesthetized fowl, and to map the corresponding pattern of atrial epicardial activation. Using back lead electrocardiograms (ECG) recorded between the base of the neck and tail, seven P-wave morphologies were identified. Four of these morphologies were upright, two were inverted, and one was biphasic. Spontaneous conversion from one morphology to another was common. During the most common P-wave morphology, activation of the atrial epicardium was mapped by comparing the onset of up to 68 local electrograms with the onset of the P wave of the ECG. Pacemaker activity was found near the base of the left sinoatrial valve and preferential conduction was demonstrated caudally along the base of that valve and medially along the rostral border of the right atrium. Changes in P-wave morphology were shown to correspond with changing patterns of atrial activation. Spontaneous changes in P-wave morphology and epicardial activation indicate a labile pacemaker region in the bird and suggest a developmental stage of evolution similar to both their reptilian ancestors and to their mammalian counterparts.