A young, healthy female presented to her general practitioner with a three-month history of daily “skipped beats.” A 12-lead ECG taken during her visit demonstrated an irregular rhythm (not shown). Closer inspection revealed normal sinus rhythm with several isolated premature atrial contractions (PACs). The patient reported experiencing the “skipped beats” at the time of the ECG.
She was referred to an electrophysiologist and commenced on Verapamil, but her symptoms persisted. A 24-hour Holter monitor revealed a PAC burden of ~7% despite medical therapy. She was therefore referred for consideration of curative radiofrequency ablation. Figure 1 shows her presenting rhythm, including a clinical PAC.
Figure 1: 12-lead ECG demonstrating the clinical PAC. The P-wave is inferiorly directed with a positive/negative morphology in V1. The morphology is similar to the sinus P wave, albeit more peaked inferiorly, suggesting a high crista terminalis focus.
Ectopic foci are common targets in the EP lab—most often PVCs, but the principles are similar for PACs. The aim is to use mapping to identify the site of origin (SOO) and perform ablation if safe to do so.
The first step is obtaining an accurate 12-lead ECG and assessing the P-wave morphology. Fortunately, the clinical PAC was not obscured by the preceding T wave. With a clear ectopic P wave available, a P-wave morphology algorithm can be used to estimate the SOO. It is important to remember that no algorithm is perfectly accurate; factors such as electrode position, patient anatomy, or scar can all reduce reliability.
One commonly used algorithm is from Peter Kistler’s 2005 paper analysing P-wave morphology in 130 atrial tachycardias. A positive/negative P wave in V1 with an inferior axis suggests a high crista terminalis origin. Figure 2 demonstrates this algorithm.
Figure 2: Kistler et al. (2005) P-wave morphology algorithm indicating a likely high crista terminalis location (marked red).
Prior to mapping a baseline EP study was performed which demonstrated no evidence of dual AV node or accessory pathway physiology further confirming the PACs as the cause of the patients’ symptoms. Using a high-density mapping catheter (Abbott HD Grid), the PAC was quickly mapped to the lateral SVC. Figure 3 shows the earliest activation occurring 80ms prior to the surface P wave. The 3D LAT map confirmed a discrete focal region high in the SVC (shown in white) with centrifugal spread.
RF ablation is generally safe and effective as it can be titrated but collateral damage to adjacent structures can occur. RF ablation at this location poses risk to the right phrenic nerve, which courses along the lateral RA/SVC border. High-output pacing was therefore used for phrenic nerve mapping, which demonstrated phrenic nerve capture directly over the site of preferred ablation, illustrated by the blue markers in Figure 3.
Given that ablation at the SOO carried a significant risk of phrenic nerve injury, the question became: is focal ablation worth the risk, or could a safer strategy achieve the same outcome?
Figure 3: Left: HD Grid electrogram leading the surface P wave by 80 ms in the lateral SVC. Right: 3D LAT map showing the discrete early activation zone (white) with surrounding centrifugal spread. Blue markers indicate phrenic nerve capture sites.
A safer alternative that was decided was to simply isolate the entire SVC rather than ablate the focal source directly. SVC isolation is not a novel technique and is sometimes used for non-pulmonary vein triggers in atrial fibrillation.
A sinus node map was created to guide lesion placement and minimise risk to the sinus node. The sinus node breakout is shown as the black triangle in Figure 3. Using both the phrenic nerve map and sinus node breakout, a proposed circumferential ablation line was drawn around the SVC.
Linear irrigated RF lesions at 35 W were delivered without complication. The final lesion set is shown in Figure 4. Although some ablations were delivered close to the sinus node breakout region, no sinus node injury was observed.
Figure 4: Right lateral and AP views showing the circumferential SVC lesion set. Ablation near the sinus node breakout region was performed safely without complications.
After circumferential ablation of the SVC, the RF catheter was positioned above the line to assess for SVC block. Figure 5 demonstrates complete electrical dissociation of the SVC from the right atrium.
How do we know the SVC was isolated from this picture?
The PACs fortunately continued to fire frequently post-ablation, and the strip shows two ectopic beats failing to exit the SVC. They remain completely dissociated, while the underlying sinus rhythm continues uninterrupted. Each sinus beat also blocks into the SVC region where the ablation catheter was positioned confirming bi-directional block. Clinically, the patient reported feeling her “skipped beats” throughout the case—right up until SVC isolation was achieved. After isolation, although the PACs continued to fire, she no longer perceived any symptoms.
Figure 5: Post-ablation sinus rhythm with the ablation catheter positioned within the SVC. Frequent dissociated PACs are seen that fail to conduct to the atria.
This case demonstrates a safe and effective alternative approach to focal ablation of PACs or atrial tachycardias arising from the SVC, particularly when phrenic nerve injury risk is high. The patient left the EP lab in sinus rhythm, symptom-free, and with no complications.