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Figure 6-1: Explanation

Figure 6–1A

The patient had electrophysiologic assessment for WPW and recurrent SVT. AVRT over a posteroseptal AP was found with earliest atrial activation during tachycardia at the proximal coronary sinus (PCS) electrode. Ablation adjacent to the CS os was performed and posteroseptal preexcitation was abolished with no inducible tachycardia. Fifteen minutes later, SVT was again induced and a PVC was introduced into the cycle as in the figure. The ablation catheter is near the previous site and the coronary sinus catheter has not moved. Where would one ablate now?

The PVC programmed into the cardiac cycle is relatively late coupled with minimal fusion and yet preexcites the next atrial cycle, so it is clearly AVRT. Earliest atrial activation is still near the CS os as identified on the ABL atrial electrogram, but now the atrial activation in the coronary sinus has shifted (long arrow) from distal to proximal, although the far-field PCS EGM is relatively earlier (short arrow). One may speculate that there is a now a left pathway but the atrial activation at the CS os (ABL catheter) is still very early. Ablation at this site terminated tachycardia and this was no longer inducible. The initial ablation at the CS os region likely resulted in interatrial block over the CS interatrial connection. Although AVRT was still occurring over a posteroseptal accessory pathway, there was a change in atrial activation of the LA resulting in a distal to proximal CS activation in spite of the origin of preexcitation near the CS os. This is infrequently seen with minimal ablation in the CS os region but is an interpretative issue that has consequences for successful ablation.

Figure 6-2: Explanation

Figure 6–2A

The patient was undergoing PVI ablation and the left upper PV had just been completed. The CS electrograms are shown in lower part and the circular mapping catheter electrodes are shown in the middle of the tracing. Pacing is attempted from the circular mapping catheter within the left pulmonary vein. How would one interpret the observations?

The first observation is that there is some local activation after each pacing spike (asterisk) so that the spike is capturing the PV. The second is that the underlying rhythm is sinus (CL 890 milliseconds) and is dissociated from the paced activity inside the PV. The potentials in the pulmonary vein are "far field" coming from the left atrial appendage and this characteristically shows the biggest far-field potentials in the anterior part of the circumferential mapping catheter (i.e., 13–14, 11–12, 9–10, 7–8). Thus, "exit block" ...

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