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Entrance and exit block may not be as good at gauging the procedural success of catheter-based radiofrequency (RF) ablation for atrial fibrillation (afib) as thought, perhaps explaining why so many cases recur, a small single-center study suggested.

The continuous electrical conduction barrier aimed for with RF ablation lesions was often not present when there was an entrance or exit block, , of Brigham and Women's Hospital, MA, and colleagues found.

Among 30 afib patients getting pulmonary vein isolation (PVI), 62% of their pulmonary vein pairs showed entrance block of electrical impulses from the atria to the ventricles early, when the ablation scar was incomplete and still had visible gaps measuring more than 10 mm long.

of the anatomical barrier to the pulmonary vein, the researchers reported in JACC: Clinical Electrophysiology.

Their study exposed the "pitfall" of using entrance and exit block as endpoints for an afib ablation procedure, they concluded, as "recurrences are commonly associated with PV conduction recovery."

"These observations unequivocally suggest that presence of acute PV entrance and exit block is by itself an insufficient endpoint of catheter ablation," agreed André d'Avila, MD, PhD, of Brazil's Instituto de Pesquisa em Arritmia Cardiaca, and , of Dignity Health Heart and Vascular Institute in Sacramento, Calif., in an accompanying editorial.

"As PV reconnection seems to by and large underlie the mechanism of AF following catheter ablation," the findings "are entirely consistent with the clinical experience and the published literature on the long-term outcomes of PV isolation and AF ablation," they added. The common failure to prevent PV reconnection after ablation has been documented in studies such as GAP-AF and EFFICAS I.

The researchers found that a longer total time spent on RF was associated with entrance blocks that occurred after complete ablation. RF ablation lasted a mean of 18.6 minutes for patients with early entrance blocks, compared to 22.7 minutes for those who showed conduction blocks after formation of the full ablation line (P=0.015).

Additionally, the investigators discovered that the gaps in the incomplete RF ablation lines at the time of early entrance block measured a mean of 17 mm for the left PV pairs and 20 mm for the right PV pairs (P=0.198).

How entrance block can develop while there are large gaps in an RF ablation line remains unclear. Conduction blocks may occur after incomplete ablation due to "tissue stunning, edema- or injury- wavefronts," Michaud and colleagues suggested, all of which can "alter [the] electrical excitability of PV tissue" before dissipating. They wrote, however, that they could not explain "why certain PVs appear to be more vulnerable to potentially transient injury."

Besides tissue stunning, "alternative explanations for the observed phenomenon include PV ischemia or infarct, which may cause permanent unexcitability," they added.

The findings suggested that RF ablation might be improved by "novel technologies such as force sensing" -- directly measuring the contact force of the ablation catheter, according to d'Avila and Aryana. Also,"pacing along the PV isolation line to ensure non-excitability" may also be necessary "to maximize the long-term procedural efficacy" of RF ablation, they added.

Adenosine might be a helpful pharmacological agent for the discovery of dormant conduction, the editorialists noted. However, the UNDER-ATP trial recently showed that adenosine aided clinicians to retarget RF ablations but failed to prevent atrial arrhythmias in the long-term, they acknowledged. This finding challenged the data from , a smaller trial, which showed that adenosine-guided retargeting successfully reduced the long-term risk of future atrial arrhythmias by 27%.

While the debate over the benefits of adenosine continues, the latest study nonetheless makes it clear that clinicians need new ways to gauge their RF ablation success. "Conduction block alone should not be used as a steadfast indicator of durable PV isolation," concluded d'Avila and Aryana.