Effects on the Canine P Wave of Discrete Lesions in the Specialized Atrial Tracts

The role of the specialized atrial tracts in determining the polarity, morphology, and duration of normal and ectopic P waves in the canine heart was studied by producing discrete surgical lesions in selected portions of these tracts. Effects of these lesions were correlated with changes in the polarity, morphology, and duration of the P waves and with conduction time to selected atrial sites when the atria were paced from the sinoatrial (SA) node, the left atrial portion of Bachmannʼs bundle, the low interatrial septum near the atrioventricular (AV) node, and the posterior-inferior left atrium. A lesion in the anterior internodal tract where it leaves the head of the SA node prolonged conduction time between the SA and AV nodes and significantly increased P wave duration when the atria were paced from the SA node and low interatrial septum sites. A lesion in the branch of the anterior internodal tract running through Bachmannʼs bundle significantly changed P wave polarity, morphology, and duration when the atria were paced from the SA node, Bachmannʼs bundle, and low interatrial septum sites. A lesion in the posterior internodal tract failed to change P wave polarity, morphology, or duration. When the atria were paced from the posterior-inferior left atrium, the lesions had no significant effect on P-wave polarity, morphology, or duration. It is concluded that specialized atrial tracts play a functionally important role in the sequence of atrial activation during many normal and ectopic rhythms.