Activation sequence during unifocal and bifocal atrial pacing: new observations and insights in an acute canine model

Background/significance: Alternate single- and dual-site atrial pacing for atrial resynchronization has been suggested to impact atrial fibrillation and diastolic heart failure that have been attributed in part to right atrial appendage (RAA) pacing, but clinical studies have not shown superiority of one approach. Purpose: We studied atrial electrical activation sequence during unifocal and bifocal pacing to assess the atrial activation sequence during various pacing approaches. Methods: Eight mongrel dogs (29.4±1.4 kg) were studied. Surface ECG leads I, II, V1, proximal, middle and distal coronary sinus recordings from a multielectrode catheter, and intracardiac recordings from bipolar plunge wires placed over the surface of the surgically accessed left atrium (LA) were monitored. Bipolar pacing was performed at 30-50 ms less than intrinsic sinus cycle length. Unifocal pacing was performed from the sinus node (SN), RAA, intra-atrial septal aspect of Bachmann’s bundle (BBR), coronary sinus ostium (CSO) and perimitral low lateral left atrium (LLL). Bifocal simultaneous pacing was performed from RAA+BBR, RAA+CSO and RAA+LLL. Measured and derived intervals included activation time at non-paced sites in each atrium, intra-atrial activation time, global activation time, activation dispersion, activation dispersion index. Results: RSinus node pacing homogenously activated the right atrium (RA); the left atrium (LA) was activated by anterior and posterior wavefronts simultaneously propagating septally to laterally and meeting at the lateral LA. RAA pacing reversed intra-RA activation and was associated with conduction delay and intra-atrial activation dispersion. Both RAA pacing as well as unifocal septal pacing from either the coronary sinus ostium or Bachmann’s bundle (BBR) resulted in dispersion of septal activation, which then promoted dispersion of LA activation concordant with degree and pattern of septal activation. Lateral LA sites were activated by the first wavefront that arrived, whether anterior or posterior. Bifocal RAA+BBR pacing: 1) was associated with homogeneous RA activation, 2) preserved global atrial activation order, 3) did not produce LA activation dispersion, and 4) resembled the activation sequence of sinus node pacing. Conclusion: Potentially deleterious effects of RAA pacing may arise from biatrial conduction delay and activation dispersion; these effects also may be seen with unifocal septal pacing; and such effects can be avoided by simultaneous bifocal pacing from RAA and BBR