Simultaneous multisite mapping of the right and the left atrial septum in the canine intact beating heart

Background-The spread of activation between the right atrium (RA) and left atrium (LA), particularly along the right and left aspects of the interatri al septum, is not clear. Methods and Results-Basket-shaped catheters carrying 64 electrodes were dep loyed into both the RA and LA of 10 dogs. Position and orientation of the b askets were determined by fluoroscopy and echocardiography. Basket unipolar electrograms were simultaneously recorded in each dog during sinus rhythm, right ventricular pacing, and pacing of the right septum through the baske t in the superior and inferior regions. Isochrone maps depicting all aspect s of the atria, including the septum, were compared. During sinus rhythm an d superior right septal pacing, wave fronts propagated predominantly from s uperior to inferior regions on both the right and left septum. However, act ivation of the left septum was delayed compared with the right septum. Duri ng right ventricular pacing and inferior right septal pacing, activation of the septum was discordant; 1 wave front propagated rapidly on the right se ptum from inferior to superior regions, whereas 2 opposing wave fronts orig inated on the left septum in both the superior and inferior regions. The le ft septum was activated predominantly by the superior wave front. Activatio n of the left septum was completed in a significantly shorter time during p acing of the right septum in the inferior region compared with the superior region. Conclusions-In dogs, activation of the right and left aspects of the intera trial septum is discordant. Electrical connections are present between the RA and LA in regions superior as well as inferior to the septum.