‘Unblinding’ the cryoballoon

This editorial refers to ‘Verification of pulmonary vein isolation during single transseptal cryoballoon ablation: a comparison between the classical circular mapping catheter and the inner lumen mapping catheter’ by G.-B. Chierchia et al., on page 1708

Percutaneous pulmonary vein isolation, with point-by-point elliptical radiofrequency (RF) applications around the pulmonary vein ostia, remains the cornerstone of ablation for atrial fibrillation (AF).¹ The conventional procedure (point-by-point application) is highly dependent on the dexterity and experience of the operator. This implies that large-volume centres have higher success rates, with lower complication rates.²

Reconnection of pulmonary veins is still the Achilles’ heel of AF ablation. At least one of the veins is reconnected in 50% of the patients undergoing pulmonary vein isolation during a 6-month follow-up.³ Several limitations of the point-by-point technique can explain the high reconnection rate: non-contiguous lesions can cause tissue oedema, with acute temporary electrophysiological (EP) isolation. When oedema subsides, the vein conduction is unmasked. By this time, the patient is usually out of the EP lab. Longer waiting time, unmasking manoeuvres with drugs (e.g. adenosine, isoprenaline) or new catheter designs to ensure better tissue contact and more effective lesions (e.g. pressure monitoring) have been used, but its utility is still debatable.^4,5

The so-called ‘single-shot’ devices are expected to overcome some of these limitations. They are supposed to enable shorter learning curve, achieving better and safer outcomes, reducing procedure time, radiation exposure, and eliminate the need for three-dimensional mapping. If proved, these devices can allow the expansion of pulmonary vein isolation to a much higher number of centres and patients.⁶

Currently available cryoenergy techniques use a cryoballoon to occlude each of the pulmonary veins, theoretically allowing isolation with a single application and assuring continuous lesions (without oedema-related gaps). Distal applications are not always possible, especially when a common ostium is present, increasing the risk of pulmonary vein stenosis and phrenic nerve palsy.

In clinical practice, the results with the cryoballoon are not superior to the point-by-point ablation. A recently published systematic review of the cryoballoon trials has shown that in patients with paroxysmal AF, the 1-year freedom from recurrent AF without antiarrhythmic drugs was 73% with a 3-month blanking period and 60% with no blanking period.⁷ In many trials, these results could only be achieved with the additional use of an irrigated-tip RF catheter to close gaps in the cryolesion line. It has also been shown that the rate of complications with the use of the cryoballoon is higher than the expected: in the STOP-AF (Sustained Treatment Of Paroxysmal AF) trial phrenic nerve palsy occurred in 11.2%, pulmonary vein stenosis in 3.1%, and cardiac perforation in 1.2%. In regard to the risk of silent cerebral embolism or oesophageal thermal lesion, cryoenergy appears to be non-inferior to the irrigated-tip RF ablation catheter.^8,9 Different experiences among centres may also have impact on the complication and success rates.

The currently available cryoballoon system is completely anatomical (‘blind balloon’). To ensure pulmonary vein isolation, an additional circular pulmonary vein mapping catheter has to be used (identical to what is used in the conventional technique).

In this issue of the journal, Chierchia et al.¹⁰ compare, for the first time, the use of the ‘blind balloon’ (and a conventional circular mapping catheter) with the Achieve system. The Achieve circular mapping catheter is inserted in the inner lumen of the cryoballoon and placed distally to the catheter. It is used as a supporting guidewire and a circular mapping catheter. Nevertheless, it is important to recognize that the gold standard to assess pulmonary vein isolation is the use of a circular mapping catheter and this new guidewire catheter may not have a similar degree of spatial resolution to record pulmonary vein electrograms as the conventional circular catheter.

This prospective non-randomized trial has clearly shown three advantages of the Achieve system: there is no need to use an additional circular mapping catheter to assess pulmonary vein isolation, the radiation exposure is significantly reduced, as well as the duration of the procedure. One of the main limitations of the study is that it is not completely clear as to how patients with paroxysmal AF and a left atrial diameter <50 mm were assigned to the two treatment groups. It is also unclear if patients with the common left pulmonary vein ostia were excluded from the study.

One of the potential advantages of the Achieve system would be the real-time assessment of pulmonary vein isolation during cryoenergy application. It was possible only in half of the veins, because, according to the authors, the Achieve catheter had to be positioned deep in the vein in order to confer balloon support (the deeper in the vein it sits, the lower the likelihood of recording pulmonary vein potentials). In the veins where it was possible to record real-time pulmonary vein electrograms, time to isolation was related to acute reconnection (time >85 s to isolation was associated with a higher rate of reconnection). An interesting observation is that the difficulty in obtaining a stable position for the balloon in the right inferior pulmonary veins (as evaluated by the high number of incomplete occlusion) translates into longer time to obtain isolation and a higher incidence of acute reconnection.

As in other studies, the cryoballoon has not proved a truly ‘single-shot’ device. In 6% of patients, it was not possible to isolate the veins (mainly inferior veins and the anterior portion of the left superior pulmonary vein) without the use of an additional RF catheter.

The Achieve inner catheter is clearly a good and logical upgrade to the existing cryoballoon. It simplifies the procedure and improves electrophysiological information that can be obtained. Although it has not been apparent in this study, probably because it comes from a very experienced group, it is potentially safer by avoiding a second transseptal puncture or the exchange of catheters in the left atrial sheath. Subsequent clinical evaluation of these patients, with extended follow-up is needed to confirm the real value of Achieve cryoballoon.

Although cryoablation may appear the best among the ‘single-shot’ devices at present, the ‘competition’ with the new irrigated circular RF ablation catheters is about to start. A better knowledge of the AF substrate with tailored ablation lesions will be necessary to treat persistent and long-time persistent forms of the arrhythmia. Control of the atrial substrate is not possible with any of these ‘single-shot devices, which will be reserved to treat paroxysmal forms.

Conflict of interest: none declared.

References

Author notes