Cryoballoon catheter ablation offers an alternative to radiofrequency ablation for the treatment of paroxysmal atrial fibrillation (PAF). The goal of ablation for PAF is achievement of pulmonary vein isolation (PVI). With radiofrequency catheter ablation, point-by-point application around the PV antrum is required and is commonly guided by three-dimensional mapping tools. In contrast, the cryoballoon ablation technique is centered on achievement of pulmonary vein isolation with minimal (typically 2-4) applications of cryothermal energy to the antrum of the pulmonary vein. Since approval in the U.S. in 2010 (the experience in Europe dates to 2005), many EP laboratories are adopting this technique, including some that are new to the realm of AF ablation. Missouri Baptist Medical Center was recently the first to perform a cryoballoon ablation in our region, and this report highlights our experience with the first case and lessons helpful for others embarking on this new technique.
A 53-year-old man was referred with recurrent, symptomatic episodes of PAF manifested by palpitations and dyspnea. He had episodes lasting up to twelve hours several times per week despite therapy with sotalol. Amiodarone had been initiated, and the frequency of palpitations improved, but the concern for side effects and documented recurrences of PAF prompted referral to our Arrhythmia Center. The patient had a CHADS2 score = 0, normal heart by echo (with left atrial size of 4 cm), and a past medical history significant only for hypothyroidism and remote atrial flutter treated successfully with ablation of the cavotricuspid isthmus. The patient elected to pursue catheter ablation.
Cryoballoon Ablation: Technical Considerations
The Arctic Front® cryoballoon system is manufactured by Medtronic and consists of either a 23 mm or 28 mm balloon mounted on a 10.5F dual lumen, deflectable catheter. The Arctic Front catheter is delivered to the left atrium via a steerable, 12F transseptal sheath (Medtronic FlexCath®). The Arctic Front catheter is advanced over a guidewire (or a 3F Medtronic Achieve™ catheter — see below) that has been positioned in the target PV. Balloon inflation in the left atrium is followed by advancement of the balloon into the PV antral region to occlude the vein. The refrigerant N20 is delivered into the balloon, resulting in a cooling temperature of up to approximately -80 C, and a four-minute application creates a circumferential extra-ostial ablation lesion. PVI can be achieved with one lesion, but more may be required, and a minimum of two lesions per vein is recommended.
The crucial technical task of cryoballoon ablation is achievement of PV occlusion. The inner lumen permits contrast injections distal to the balloon, and PV angiography is important to assess for the adequacy of the “seal.” The best indicator of occlusion (and visual cue to be alert to) is contrast trapped in the vein. If the vein is not occluded as evidenced by contrast “leak” (Figure 1), then manipulation of the guidewire, sheath, or catheter is required to improve the orientation of the apparatus and occlude the PV (Figure 2). Of note, care should be taken not to inflate the balloon in the vein or advance the balloon into the tubular portion of the vein to avoid complications.
Despite these attempts, complete occlusion may not be attained, in which case cryo application may still be delivered as balloon expansion may result in better contact during the freeze. Also, multiple applications can be given with different balloon orientations to produce overlapping antral lesions and sequentially complete the encirclement.
Other techniques to assess for PV occlusion other than contrast venography can be considered, such as pressure waveform monitoring from the distal lumen of the Arctic Front, or Doppler interrogation of the PV with intracardiac echo.
Assessing for Isolation
There are two common methods to assess for PVI with the cryoballoon technique. There is most experience with use of a circumferential multipolar mapping catheter (e.g., Biosense Webster’s Lasso) placed via a second transseptal sheath. The Lasso is placed in each PV prior to cryoablation, and then again following the second application to verify PVI. This is similar to what is done during radiofrequency ablation. An alternative approach is use of the Medtronic Achieve catheter, designed for use with the Arctic Front system. The Achieve is a 3.3F circular mapping catheter that can be used in place of the guidewire in the Arctic Front catheter. The cryoballoon catheter can be advanced to the PV over the Achieve as if over a guidewire, but the Achieve is also capable of pacing and recording similar to a Lasso. Pulmonary vein potentials are recorded before balloon inflation (Figure 3 and 4), and isolation is confirmed after completion of ablation (Figure 5). The key advantage of the Achieve is the obviation for a second transseptal puncture and sheath. In contrast to the Lasso, however, the Achieve catheter has only eight electrodes. It is available in 15 mm or 20 mm fixed diameter loops. These characteristics result in decreased resolution and utility in assessing for PV potentials in comparison to the Lasso.
Avoidance of Complications
The most common complication of cryoballoon catheter ablation for PAF is phrenic nerve injury (PNI), which had an incidence in the STOP-AF trial of 11.2%. Careful monitoring of diaphragmatic excursion is mandatory during ablation at the right pulmonary veins. This is performed by pacing the phrenic nerve (10 mA with a multipolar catheter in the SVC, above the level of the right superior pulmonary vein, at an interval of 1500 ms) and manual palpation of the strength of diaphragmatic excursion during the freeze. The ablation application should be immediately terminated and the balloon deflated if diminution or cessation of the diaphragmatic contraction is observed.
Other tools to assess for phrenic nerve palsy have been used, such as an intracardiac echo catheter placed in a hepatic vein to visualize diaphragmatic excursion, use of a fetal heart rate monitor over the diaphragm to provide auditory confirmation, and myopotential recordings on the EP recording system via repositioned surface EKG leads.
In addition, the risk for PNI can be decreased by using the larger 28 mm balloon and ensuring placement of the balloon in the antral region and not deep into the PV (it is rare to use the 23 mm balloon for any case).
Pulmonary vein stenosis is less common (3%) and is also prevented by ensuring ablation in the antral region outside the tubular portion of the PV. Displacement of the balloon deep into the PV is indicated by a cylindrical appearance of the inflated balloon on fluoroscopy.
Pre-Procedure Planning Tips
The importance of pre-procedural planning cannot be overemphasized. The day prior to the procedure, I met with the entire EP lab staff to review the procedure, outline the sequence of events to occur, and demonstrate the operation of the balloon catheters, sheath, and console. The presence of the participating Medtronic representative was particularly helpful as well. In addition, a meeting with our anesthesiologist prior to the procedure was conducted, with particular emphasis on the need to avoid paralytic agents prior to the time of right PV ablation to permit monitoring for PNI. Finally, a patient education video was provided to the patient for review prior to the procedure.
Several specific details should be reviewed during these planning sessions:
- The imaging equipment settings should be configured for adequate PV angiography. The X-ray equipment in most EP labs is set for low fluoroscopy and cine doses, which may potentially result in inadequate resolution for PV angiography
- If the Achieve catheter is used, configure the recording system before the case to record from the eight electrodes. We elected to use seven overlapping bipoles. (In addition, the ablation electrograms can be removed from the recording screen, as no recordings are made with the Arctic Front ablation balloon.)
- Review the location of the radiographic markers on the FlexCath sheath and Arctic Front catheter, and the relative location of each as it relates to balloon inflation and catheter withdrawal.
- Have the appropriate guidewires, hemostatic valves, and stopcocks available. Some of these tools are ubiquitous in cardiac catheterization labs, but may be less familiar in the EP lab. A catheter manifold for contrast and saline flush management is particularly helpful.
- The plan for diaphragmatic monitoring should be reviewed, and the importance of these steps should be understood by every member of the team including anesthesiology (remember to avoid paralytic agents). Know how to pace the phrenic nerve and consider practicing the maneuver before the case.
- Provide the patient with the cryoballoon educational video.
The key “decision points” for the cryoballoon procedure are summarized as follows (with our selections for our first case indicated as well):
- What method of monitoring for phrenic nerve injury will be used? We decided to use both manual palpation and a fetal heart rate monitor, but during the case abandoned this as it added an additional, unfamiliar variable.
- How will balloon occlusion of the PVs be assessed? We chose the standard approach of PV angiography.
- How will PV isolation be assessed? We elected to use a single transseptal and a 15 mm Achieve catheter. However, we found that the recording of pulmonary vein potentials was not as robust as with a Lasso, and the Achieve did not sufficiently contact the walls of the larger PVs. Use of a 20 mm Achieve (or a second transseptal and Lasso) may be a better option.
The patient presented in sinus rhythm and without prior anticoagulation therapy other than aspirin (CHADS2 = 0). The procedure was performed under general anesthesia. A single transseptal puncture was performed and an SL0 sheath was placed and subsequently exchanged for the FlexCath. We performed the procedure without pre-procedure imaging (CT/MRI), intracardiac echo, or three-dimensional mapping. All four PVs were identified and visualized with selective pulmonary angiography using a 5F multipurpose catheter and hand injections via the manifold. The images were stored for reference. The Arctic Front was advanced over the Achieve catheter and baseline PV recordings were made prior to ablation of each vein. Manipulation was required to achieve adequate “seal” and PV occlusion was achieved. See Figures 1-5 for an example in the RSPV. The LSPV required three applications (four minutes each) and the other PVs received two applications. Entrance and exit block were demonstrated for all PVs except the RIPV (the Achieve could not be adequately engaged in the RIPV to assess for PV potentials, and empiric ablation was performed). Protamine was given to reverse the acute anticoagulation, and dabigatran was administered for three months post procedure. Amiodarone was discontinued after the procedure. At three-month follow up, the patient has had no recurrent palpitations and no documented AF. These results are encouraging, and long-term follow up is scheduled to assess the outcome of cryoballoon therapy.
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