Abstract
Cryoballoon ablation is a safe and predictable toolset to generate a reliable large-area pulmonary vein antral modification for the treatment atrial fibrillation. Its reproducibility amongst a wide range of operators have made it an easier technology to adopt with a shorter learning curve and consistent outcome. Understanding of the science of cryoballoon technology has been an ongoing discovery process since its launch, and its best-practice continues to be improved. To improve outcome and to minimize potential complications, technical and dosing recommendations are discussed in detail—access technique, transseptal cryoballoon maneuvers, dosing, and avoidance of complications.
Keywords
atrial fibrillation, cryo dosing, cryoablation, cryoballoon, pulmonary vein isolation
Key Points
Mapping
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Pulmonary vein antral positioning of the cryoballoon with proximal-seal method
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Pulmonary vein potential recording for time-to-isolation with Achieve Mapping Catheter
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Postablation voltage map (optional)
Ablation Targets
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Pulmonary vein antral modification and pulmonary vein isolation
Special Equipment
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Cryoballoon ablation system: Flexcath Advance Cryoballoon
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Circular Achieve mapping catheter
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Optional intracardiac echocardiogram for transseptal
Sources of Difficulty
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Common ostium will need segmental ablation for durable isolation
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Narrow and ovoid antrum with poor occlusion will need segmental ablation
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Phrenic nerve and esophageal proximity
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Cryoballoon dosing with time-to-effect monitoring
Introduction
Electrical isolation of the pulmonary veins (PVs) via antral modification has been established as the foundation of treatment for both paroxysmal and persistent atrial fibrillation (AF). Traditional focal ablation used to isolate the PVs is highly technically dependent as it requires maintenance of stable, point-by-point catheter–tissue contact, is associated with a prolonged learning curve, and has a known complication profile that may be difficult to control. Unlike traditional focal ablation, cryoballoon ablation achieves wide antral PV modification with predictable, homogeneous, and large-area contact. Cryoballoon ablation has been demonstrated to be both effective for AF therapy and reproducible among a vast group of operators. The unique, histologic pattern of the cryolesion created by the cryoballoon has also been demonstrated to be advantageous; evidence suggests it preserves tissue architecture and therefore may reduce the incidence of complications such as thromboembolic phenomena.
Low rates of complications such as pulmonary vein stenosis, phrenic nerve injury, atrial-esophageal (AE) fistula, stroke, and even death have been reported among operators of varying experience with a total of over 310,000 cases worldwide to date. The complications associated with cryoballoon ablation are dependent on dosing and technical acumen. Availability of progressive generations of the cryoballoon and mapping tools along with refined technique and dosing norms have improved both success and complication rates of cryoballoon ablation.
Preprocedural Considerations
Patients undergoing cryoballoon ablation have several considerations to make in their peri-procedural medical management of AF. Antiarrhythmic medications should be stopped at five half-lives before the procedure to avoid masking potential AF triggers. Anticoagulation management is determined on a patient-by-patient basis to balance the risks and benefits to each individual. In general, anticoagulation should be continued for patients with a CHADSVASC score of 2 or above, and patients at a high risk of thromboembolic events should have preprocedural anticoagulation longer than 30 days. A preprocedural evaluation with either transesophageal echocardiogram or intracardiac ultrasound to rule out left atrial thrombus is a prudent precaution in AF patients undergoing catheter ablation.
Either cardiac magnetic resonance imaging or computed tomography scan imaging of the left atria has been routine in many centers; however, cryoballoon can be amenable to variable PV anatomies and can be successfully used without preprocedural imaging if the operator is comfortable with the anatomy. The requirement for additional focal touch-ups following cryoballoon ablation has been minimal since the introduction of the second generation cryoballoon, Arctic Front Advance.
Cryoballoon Ablation Procedure
Cryoballoon ablation tools and familiarity of its operations should be reviewed by all supportive staff before the cases. Itemization of required and commonly used tools is summarized in Table 15.1 . 
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Sedation and Anesthesia Considerations
The minimally invasive nature of cryoablation allows for minimal procedural related patient discomfort. Whereas most centers in United States perform the procedure under general anesthesia, most Asian and European centers perform the full procedure with minimal or no sedation. Patients without deep or general anesthesia have been noted to more frequently express a vaso-vagal response after the left superior PV thawing phase. Because 3-dimensional mapping is also optional during cryoballoon procedures, potential patient motion throughout the procedure that would otherwise compromise positional references is less of a concern and is not a motivating factor to use general anesthesia. Further, Foley catheterization is not necessary because of the relatively short procedure time and lack of saline irrigation compared with that of radiofrequency ablation. If general anesthesia is used, it is critical to note that paralytics should be avoided during the procedure to allow for phrenic nerve pacing during right-sided PV ablation.
Access
Cryoballoon access through the right femoral vein is typically preferred because of its proximity to the operator. Cryoballoon access is obtained via the Flexcath Advance, which has a 12 F inner diameter and a 14.5 F outer diameter. There is a noticeable step up between the dilator and the sheath caused by the reinforced sheath tip, which allows balloon retraction without deformation. This may be a point of hang-up in introducing the balloon through the femoral access and transseptal passage. Therefore sheath-exchange posttransseptal access dilatation with a 14 F short dilator or sheath is required. At least one additional femoral vein access is necessary for phrenic nerve monitoring with pacing, as well as backup pacing in the right ventricle in the event of a vagal response. Access for intracardiac ultrasound may be placed first for preoperative assessment. Therapeutic anticoagulation is recommended after femoral vein access and pretransseptal access via a heparin bolus with a target activated clotting time of 350 to 400 seconds or according to the operator’s convention similar to that of the radiofrequency ablation.
Transseptal Access
Careful placement of the transseptal access point maximizes the mechanical advantage of the catheter. A low and anterior transseptal puncture at the lower limbus of the septum is optimal ( Fig. 15.1 ). In addition to the mechanical advantage, the tissue of the intraatrial septum is more fibrous at the thinner portion versus the limbus region, which is more muscular. This difference in tissue architecture eases the ability to puncture through the septum even though it is a slightly thicker portion of interatrial septum. Tissue characteristics at this location likely contribute to healing and may diminish the development of iatrogenic atrial septal defect. Initial transseptal accesses can typically be obtained with SL1 as it is used to direct toward the left superior PV. A Mullins sheath along with needle reshaping with a larger curve are advantageous to direct toward the more anterior and inferior portions of the septum.
Exchange of the Flexcath Advance is accomplished with an extra-stiff guidewire, preferably stationed at the left superior PV. However, if a significant amount of resistance is encountered during the passage of the Flexcath Advance, repositioning the guidewire with a deflection of the sheath toward the right superior PV may be allow for a straighter passage; this maneuver is useful for more difficult septal punctures caused by a thick, fibrotic, or previously accessed septum. Predilatation of the septum using just the dilator from the Flexcath Advance may reduce resistance during the initial passage of the sheath. In addition, twisting or rotational motion will aid in the passage of the step-up portion between the dilator and the sheath. Postinsertion of the Flexcath Advance sheath, the dilator should be removed slowly. To fully flush the sheath, the side-arm is opened while the valve is covered with the left thumb. Vigorous tapping of the handle will help release small, trapped air at the level of the side-arm and the valve while it is open. The side-arm flow rate should be kept low (less than 5 mL per minute) to avoid the Venturi-effect at the valve during the insertion of the cryoballoon.
The cryoballoon is prepared per the manufacturer’s recommendation, immersing the balloon folds underwater and sliding the introducer over the balloon folds to remove luminal fold air. The Achieve mapping wire should be used to monitor time-to-isolation (TTI), which enables appropriate dosing of cryoballoon ablation time and will be discussed in detail in the dosing section. As the cryoballoon is advanced into the sheath, care should be taken to avoid the introduction of air and bending of the cryoballoon. The Achieve should lead the advancement of the cryoballoon. To aid in localization, the end of the sheath is indicated by an initial white marker-band on the cryoballoon catheter. Exposure of the second white marker-band denotes when the full cryoballoon is extended beyond the sheath.
Cryoballoon Maneuvers
Cryoballoon procedural techniques are primarily governed by safety and secondarily to maximize success. This section discusses maneuvers generally and is followed by cryoballoon dosing details and means to minimize procedural complications. It should be noted from the onset that in its present form, the cryoballoon is noncompliant and comes in both a 23-mm and 28-mm round cross-section. Most operators will find that the second generation cryoballoon, Arctic Front Advance, 28-mm size works well for nearly all patients. Most PVs may be engaged with the cryoballoon for full-occlusion, and a wide area of PV antrum can be modified at once. However, because of the noncompliant nature of the cryoballoon and the more ovoid shape of the PV antrum, a mismatch of contact area is possible. Some of the PVs are simply unable to be engaged in a single-ablation manner. Indeed, the more ovoid a PV cross-section, the more likely it is necessary to perform segmental isolation by positioning one ablation toward the superior arc of the PV, and an additional ablation directed toward the inferior antrum, as shown in Fig. 15.2 . The resultant antral modification has a closer resemblance to the typically intended wide area circumferential ablation around each of the PV antrum than a single ablation approach.
In general, cryoballoon ablation differs from that of typical catheter ablation in that most of the maneuvers depend on the sheath to define the angle of engagement rather than the cryoballoon itself. The maneuvers required to access each individual pulmonary vein are divided into ten steps. (1) Access the PV using Achieve (ideally the lower branch of the PV for the inferior PV); (2) inflate the cryoballoon within the left atria to avoid PV injury; (3) engage the PV antrum and attempt to maximize cryoballoon-antrum contact. Sheath angulation and direction should be optimized to allow coaxial mechanical support. Orthogonal fluoroscopy view can aid in the assessment of sheath coaxial alignment; (4) reposition the Achieve catheter to ensure PV signal monitoring to optimize dosing and to predict ablation success. The retro-deflection of the Achieve is likely required for optimal PV signal recording; (5) contrast injection or intracardiac echocardiogram (ICE) evaluation to minimize leak/optimize cryoballoon-PV engagement. It is important to note that not all PVs can be occluded because of an ovoid cross-sectional shape of the PV; (6) if the PV is completely occluded, as determined by contrast injection, relax forward motion and if possible pull-back the cryoballoon until contrast leak is seen around the true antrum. This step is critical and will ensure the cryoballoon is not deeply seated inside the PV ( Fig. 15.3 ); (7) initiate ablation for approximately 3 seconds to allow the cryoballoon to enlarge and increase in pressure before advancing the balloon toward the antrum for optimal ostial engagement; (8) some operators may find it useful to check for leaks again during this final engagement with ICE or additional contrast injection within the first 10 seconds of ablation before the lumen is frozen; (9) pacing of the phrenic nerve should be performed during both right superior PV and right inferior PV ablation. The right phrenic nerve is reliably located at the junction of the superior vena cava and the right subclavian vein. The general techniques for the cryoballoon maneuver are summarized in Table 15.2 .
Steps in Pulmonary Vein Isolation Using Cryoballoon
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Although inflation of the cryoballoon within the PV is not recommended, the inflation pressure without ablation is low and is unlikely to cause injury. However, ablation within the PV should never be performed; cryoballoon ablation within the PV is the likely cause of PV stenosis with mechanical trauma and increases risk of collateral tissue damage such as lung and phrenic injury. Avoidance of deep seating the cryoballoon is particularly important in the right superior PV because of proximity to the phrenic nerve. Risk of phrenic nerve injury can be reduced by diligent placement of the cryoballoon at the antrum of the PVs with the maneuver described earlier.
Maneuvering the cryoballoon after ablation initiation should be avoided to prevent tissue injury. It is important to understand the limitations of maneuvers such as the “pull-down” method. In this method, the superior part of the cryoballoon is allowed to freeze for the first 60 seconds, which is followed by pulling down the cryoballoon to engage or seal off the inferior part of the PV. Although this maneuver often seems successful, one must understand the inherent risk of mechanical trauma that damages the PV. Further, in this method a layer of ice forms on the surface of the cryoballoon and acts as an insulator when the cryoballoon is pulled down preventing an effective ablation along the inferior PV. The inferior portion of the PV ablated during the pull-down will often be acutely isolated or injured, but it is often noted to recover upon follow-up. The appropriate way to ablate a PV antrum without a complete seal is to perform segmental ablation with the cryoballoon at two different angles: a superior ostium engaged ablation followed by an inferior ostium engaged ablation. The segmental approach is described in more detail in a later section. After initial cryoablation, the thawing process can be slow. The operator should not move the balloon catheter until the catheter temperature reading reaches 35ºC to avoid tissue trauma with residually frozen tissue.
Phrenic Nerve Preservation
Right phrenic nerve injury has been one of the most commonly reported complications in cryoballoon ablation, and it is caused by either ablation near the phrenic nerve deeper inside the PV or not monitoring the status of the phrenic nerve diligently. Most experienced operators have reported significantly lower incidences of phrenic nerve injury when using the proximal-seal method described earlier, and simply by increasing the distance between the cryoballoon to the phrenic nerve, injury is much less likely to occur. However, because of the proximity of the phrenic nerve, monitoring diligently is required regardless of the balloon position. Immediate termination of the cryoablation will ensure a more rapid return of the phrenic nerve function when a reduction in output is detected.
Various methods of phrenic monitoring have been studied. It is most important to actively pace the right phrenic nerve during the right superior PV and right inferior PV ablation. If paralytic was administered during the anesthesia induction, sufficient time is needed to allow the recovery of the phrenic nerve before the ablation of right-sided PV for adequate monitoring. Consistent and stable phrenic capture for pacing is often found at the junction of the superior vena cava to the right subclavian. Pacing cycle length of 1000 to 2000 ms is recommended depending on the signal stability and respiratory variation during the capture. Direct palpation by placing the left hand on the patient’s abdomen can detect cyclical changes in the strength of the phrenic capture. Typically, phrenic nerve paralysis progresses rapidly during cryoballoon ablation, and rapid detection of the decrement in diaphragm contraction is critical for immediate return of phrenic nerve function.
Other methods of phrenic nerve monitoring have been described, from intracardiac echocardiogram to monitor hepatic/diaphragmatic contraction, to fetal ultrasound for an audible warning. One of the methods used to increase the sensitivity of early phrenic nerve injury detection is the use of compound motor action potential, and use of this technique has been reported to decrease the incidence of phrenic nerve injury to less than 1.5%.
When phrenic nerve injury is suspected, immediate termination of the ablation should be performed. Some operators have suggested rapid termination of the freeze by using a “double stop” technique for immediate cryoballoon deflation. This technique has been considered safe without adverse events. Reevaluation of the cryoballoon position should be done to ensure that a proximal-seal technique is performed to increase the distance between the balloon and the phrenic nerve at repeat ablation. If phrenic nerve recovery is not seen, repeat ablation should not be performed. Postprocedure inspiratory and expiratory chest X-ray should be performed to establish the baseline of the phrenic nerve injury. A summary of difficulties that may be encountered during the procedure and its possible solution is listed in Table 15.3 .
| Problem | Causes | Solution |
|---|---|---|
| Inability to record TTI | Achieve mapping catheter deep inside the PV | Manipulation and pull-back of the Achieve Mapping toward front of the balloon or repositioning of the cryoballoon to allow the Achieve recording catheter |
| Inability to occlude PV | Mismatch of ovoid antrum to the round cryoballoon | Segmental isolation of the PV antrum instead of single freeze isolation |
| Long TTI | Poor or partial cryoballoon-PV contact | Assess area of poorer contact and perform segmental approach ablation to cover circumferentially |
| Recurrent AF | Extra PV triggers | Need for repeat mapping and ablation of extra-PV AF causes |
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