Earlier Activation of the Distal than the Proximal Site of the Coronary Sinus May Represent Retrograde Conduction Through AV Node: Significance of Recording of Far Distal Coronary Sinus

During retrograde conduction through an accessory pathway (AP) or the atrioventricular (AV) node, earlier activation of the distal recording site than a more proximal site of the coronary sinus (CS) generally indicates retrograde conduction via a distally located AP. Thus, after successful ablation of a left-sided AP, if the distal CS recording site is activated earlier than a more proximal site retrogradely, it is considered to suggest—in the absence of His-bundle recording or more frequently in the setting of poor recording of the low septal right atrial electrogram—a conduction via a second AP (located more distally), and not conduction via the AV node. Yet, we hypothesized that retrograde conduction through the AV node may activate the far distal site of the CS (CSD) earlier than a more proximal site, as the anterior atrial wavefront, coming retrogradely from the AV node and traveling along the anterior mitral annulus, could reach the CSD earlier than a more proximal site. To test this we studied 18 patients with intact retrograde conduction via the AV node, but without evidence of an AP. The CSD was recorded by means of a quadripolar catheter (interelectrode distance of 2–5 mm); retrograde activation sequence at the distal (CSD1–2) versus proximal (CSD3–4) bipolar recording site was determined during ventricular stimulation. In 12 of 18 patients the CSD1–2 recording site was activated 5–10 ms earlier than the CSD3–4 recording site, in 3 of 18 patients the CSD1–2 site was activated 5 ms later than the CSD3–4 site; in the remaining 3 patients both recording sites were depolarized simultaneously. The results indicate that the CSD was often depolarized earlier than a more proximal site by impulses that conducted to the atria retrogradely via the AV node while the quadripolar recording catheter was placed at the CSD. This observation, al- though not well documented previously, suggests that the sequence of retrograde atrial activation in the CS should be studied carefully in consideration of the actual location of the mapping catheter in order to correctly diagnose the presence or absence of conduction via an AP.