New Technologies for Catheter Ablation of Atrial Fibrillation
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- Gerstenfeld, E.P. Curr Treat Options Cardio Med (2011) 13: 393. doi:10.1007/s11936-011-0141-x
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The past decade of ablation for atrial fibrillation (AF) has seen the adaptation of catheters designed for “focal” tachycardias or single pathways to perform wide encirclement of the pulmonary veins (PV). During the next decade, technologies designed specifically for PV isolation will emerge. Each system has its unique attributes. The cryoballoon system offers rapid PV isolation and the promise of enhanced safety, whereas the success rate is likely to be similar to catheter-based approaches. Although preclinical studies do suggest a very low likelihood of left atria-esophageal fistula using this technology, concerns of phrenic nerve damage and a small incidence of PV stenosis need to be addressed. It is likely that use of the larger 28-mm balloon will mitigate these concerns. The cryoballoon is also the first balloon technology to be approved by the US Food and Drug Administration for clinical use, and this may gain the technology an early foothold in the AF ablation market. The laser balloon ablation system is a more time-consuming and technically demanding procedure, and the risk of thrombus formation if ablation is performed in stagnant blood is a concern. However, early studies suggest a high rate of persistent PV isolation, which hopefully will translate into high single-procedure efficacy. The Ablation Frontiers system is the only system currently being developed for more persistent forms of AF. This system offers a rapid approach to PV isolation and left atrial defragmentation. The early results do not demonstrate a success rate better than that described for catheter ablation; however, the results are difficult to compare to standard catheter ablation in this recalcitrant patient group without a prospective randomized study.
Catheter ablation of atrial fibrillation (AF) has advanced significantly since it was first described by Haissaguerre et al.  in 1998. Current American College of Cardiology/American Heart Association (ACC/AHA) guidelines now recommend catheter ablation as an acceptable option for symptomatic AF patients after breaking through a single antiarrhythmic drug, and ablation can be considered first-line therapy for paroxysmal AF in experienced centers [2•]. It is now well accepted that the premature atrial beats that initiate AF originate in sleeves of cardiac muscle that extend into the pulmonary veins (PV). Electrical isolation of these pulmonary veins can render patients free of AF, with success rates vastly superior to antiarrhythmic drug therapy in several randomized trials [3•, 4, 5, 6•, 7•]. The current approach uses standard electrode catheters, originally designed for ablation of focal arrhythmias, to ablate around the PVs in a “connect the dots” fashion. This approach has several limitations: 1) the procedure is complex and technically demanding; 2) gaps between lesions can lead to PV reconnection and AF recurrence; and 3) complications from the procedure, which include cardioembolic complications (stroke, myocardial infarction), mechanical complications (cardiac perforation), and ablative complications (PV stenosis, left atrial-esophageal fistula). Newer technologies designed specifically for PV isolation hold the promise of a simpler, more effective ablation procedure with fewer complications. These include cryoballoon ablation, endoscopic laser balloon ablation, and the Ablation Frontiers catheter system (Medtronic, Minneapolis, MN). Each system has unique attributes and limitations that will be discussed.
Cryoablation causes cell death by creating ice crystals that rupture the cell membrane. The cryoballoon system (Arctic Front; Medtronic, Minneapolis, MN) consists of a polyurethane balloon and a steerable 12-Fr delivery sheath. Ablation is performed by inflating the balloon inside the PV and filling the balloon with N2O (nitrous oxide), cooling the balloon to −80°C. Advantages of the cryoballoon include limited need for extensive mapping and fluoroscopy, simultaneous application of cryoenergy to the entire PV circumference, and stable contact. Although the position of the balloon inside the PV orifice is often more distal then the level of isolation performed with catheter ablation, the circumferential nature of the ablation without leaving “gaps” common in radiofrequency ablation may lead to more enduring PV isolation. However, concern exists that cryoenergy may not lead to as enduring cell death as radiofrequency ablation, and therefore PV reconnection may still occur.
- The cryoballoon catheter consists of a 10.5-Fr catheter with a distally mounted double balloon (Fig. 1). The console delivers liquid nitrogen to the inner balloon, which vaporizes as it contacts heat from the surrounding tissue. The outer balloon is pulled onto the inner balloon by vacuum suction, and acts as a safety device to prevent loss of the liquid nitrogen. A thermocouple is present on the central shaft near the proximal end of the balloon to detect the inner balloon temperature. A central lumen is present to deliver a wire for guiding the balloon and for injection of contrast. The balloon catheter is steerable with a 60-degree arc. The catheter is delivered via a 12-Fr steerable sheath that facilitates balloon entry into the PV.
Cryoballoon ablation technique
A transseptal puncture is first performed using a standard transseptal sheath. The transseptal puncture should be performed low and anterior, to allow access to the right inferior PV. The transseptal sheath is then exchanged over a wire for the cryoballoon sheath. A wire is then advanced out of the targeted PV, and the balloon is inflated in the left atrium and then advanced to the pulmonary vein ostium. It is important to first inflate the balloon in the left atrium rather than inside the PV. Although the latter approach may achieve better contact, the balloon may be positioned too distally inside the PV, which increases the risk of PV stenosis and phrenic nerve damage. Because the cryoballoon requires circumferential contact around the PV in order to allow adequate cryoablation, such contact should be confirmed prior to ablation. Intracardiac echocardiography has been used to exclude leaks around the balloon. However, the most effective method is injection of contrast through the central lumen of the catheter under fluoroscopy. This allows both localization of the balloon at the PV ostium/antrum, and confirmation of circumferential contact of the contrast remains opacifies the PV without any leaks. When circumferential contact is achieved, a 240-second cryoablation is then performed. After PV ablation, the PV can be interrogated with a circular mapping catheter to determine if PV isolation has occurred.
Several steps can enhance PV isolation. First, every effort should be made to obtain circumferential contact, rather than performing ablation when a leak is present. The most challenging PV for achieving circumferential PV contact is the right inferior PV because of the acute angle need from the transseptal puncture. If a leak is present at the bottom of the balloon, then occasionally cryoablation can be initiated, and once the superior aspect of the balloon has cryo-adhered to the superior aspect of the PV, the balloon can be withdrawn, causing the inferior edge to come into contact with the PV. Second, using a freeze-thaw-freeze cycle may help minimize PV reconnection. Third, if the PV has multiple proximal branches, engaging each branch with the wire and performing cryoablation from two different angles sequentially may help prevent gaps by delivering overlapping lesions. Two to three freezes are typically required per PV to achieve isolation.
For achieving proximal PV isolation, the larger 28-mm balloon should be used whenever feasible. Although the smaller 23-mm balloon may achieve circumferential contact with greater ease, the more distal deployment is more likely to lead to PV stenosis or phrenic nerve palsy. Recent data suggest that phrenic nerve palsy can be minimized using the 28-mm balloon, and that 98% of PVs can be isolated successfully using the larger balloon. During right PV cryoablation, the phrenic nerve should be paced continuously; loss of phrenic capture during ablation should prompt cessation of cryoablation. This is best accomplished using a separate catheter in the superior vena cava. Phrenic nerve paralysis is one of the most common complications of cryoballoon ablation, although persistent paralysis at 1 year is uncommon.
The recently completed Stop-AF trial randomized 160 patients with paroxysmal AF to cryoballoon isolation of the PV versus antiarrhythmic drug therapy for treating paroxysmal AF [7•]. Entry criteria included two episodes of AF over the past 2 months refractory to at least one antiarrhythmic drug. There was a 90-day blanking period after randomization, during which time a repeat ablation procedure or drug titration in that arm was allowed. The results overwhelmingly favored cryoablation, with 69.9% freedom from AF in the cryoablation group compared to 7.3% AF freedom in the antiarrhythmic group. There was 98% success in isolating PV using the cryoballoon and a 19% re-do rate in the cryoablation group during the blanking period. Complications included 1 (0.4%) stroke, 7 (3.1%) PV stenosis (2 symptomatic), and 29 (11.2%) patients with phrenic nerve palsy (4 persisting at 1 year). There were no cases of left atrial–esophageal fistula. This study clearly showed that cryoballoon PV isolation was feasible and superior to drug therapy in patients with paroxysmal AF who broke through an antiarrhythmic drug. However, although there were no occurrences of left-atrial esophageal fistula, other complications did occur, reinforcing the notion that new technologies will continue to be used with care and expertise.
Endoscopic laser balloon ablation system
- The endoscopic laser balloon ablation system (HeartLight; CardioFocus, Medford MA) allows visually guided PV isolation using infrared light energy. The advantage of the endoscopic system is that is allows direct visualization of the endocardial surface of the PV during ablation. The ablation system has three components: an outer compliant balloon, a lesion generator, and the endoscope. The balloon is made of a compliant material that has an adjustable size depending on the inflation pressure, with diameter that varies from 25 to 32 mm. The catheter shaft contains the lesion generator and endoscope (Fig. 2). The lesion generator produces a 30-degree arc of light energy and is powered by a 980-nm diode laser. The endoscope has a 115-degree field of view and allows direct visualization of the area of the balloon in contact with the left atrial endocardium; tissue appears white and blood appears red. Because patient discomfort can occur with laser ablation and high output pacing to monitor the phrenic nerve, we typically perform ablations under general anesthesia.
Laser balloon ablation technique
One should perform some type of left atrial imaging to identify the PV anatomy, typically either computed tomography or magnetic resonance angiography, prior to the procedure. The maximum balloon diameter is 32 mm, so isolation cannot be performed on a single PV > 32 mm in diameter. Most left common PVs can still be isolated more distally as individual PVs, but the trade-off of a more distal PV isolation should be considered. Finally, PVs with multiple proximal branches may be difficult to isolate with balloon techniques and a more standard catheter approach might be considered.
Left atrial access is first gained with a standard transseptal sheath; this sheath is then exchanged over a long wire for the custom 16-Fr outer diameter deflectable sheath. The deflated balloon can then advanced to the PV ostium under fluoroscopic guidance. The balloon diameter varies from 25 mm at low pressure (1 PSI) to 32 mm at highest inflation pressure (5 PSI).
PV isolation with the laser balloon is typically performed sequentially using a vein-by vein approach by “stitching” together the individual laser lesions. It is best to deliver contiguous lesions circumferentially around the PV with one balloon position, rather than trying to stitch together lesions in multiple balloon orientations. The endoscopic view allows a 270-degree view as seen down the shaft of the balloon. Approximately 90-degrees of the view is obscured by the shaft of the lesion generator, and while it is in place represents a “blind spot” in the field of view. Before each lesion is delivered, a green “aiming beam” is used to determine the exact location of each delivered lesion. The available ablation energy is projected perpendicular to the catheter shaft and ranges from 5.5 W to 14 W. Lesions delivered at the lowest power, 5.5 W, are delivered for 30 s; higher power lesions ranging from 7 W to 14 W are delivered for 20 s. Delivering laser energy into stagnant blood can lead to thrombus and should always be avoided. Because the human left atrium is quite pale, not all lesions are readily visible after they are delivered. The system contains software that allows tracking of laser lesions as they are delivered around the PV circumference. Typically 30% to 50% overlap of individual laser lesions is recommended. Once ablation has been performed over the 270-degree visualized radius, the entire catheter shaft can be rotated to visualize the remaining PV circumference and complete the circumferential ablation. As with any ablation technology, temperature monitoring of the esophagus is recommended during ablation. Energy is typically discontinued if a temperature rise >0.5°C occurs. During laser ablation of the septal side of the right PVs, pacing from the phrenic nerve using a catheter in the superior vena cava should be performed continuously to monitor for phrenic nerve damage. Any loss of phrenic nerve capture should prompt cessation of energy delivery. After delivering contiguous lesions around a pulmonary vein, the balloon is withdrawn and a circular mapping catheter can be used to judge PV isolation. If the PV remains connected, additional lesions can be delivered until isolation is achieved.
In a preliminary study, 30 patients with paroxysmal AF underwent laser balloon ablation . During the procedure, 105 of 116 PVs (91%) were successfully isolated. After a single procedure, the 12-month drug-free rate of freedom from AF was 60% (18 of 30 patients). There were no significant PV stenoses, but adverse events included one episode of cardiac tamponade, one stroke without residual defect, and one asymptomatic phrenic nerve palsy. There were no cases of left atrial–esophageal fistulae. The authors concluded that PV isolation using the laser balloon was feasible. A 100-patient feasibility trial was recently completed in the United States and is awaiting follow-up.
The advantages of the laser ablation system include direct visualization during ablation and a high rate of persistent PV isolation in preliminary studies. The negatives include a more distal level of PV isolation than can be performed with standard catheters , risk of thrombus formation if ablating in stagnant blood, and risk of phrenic nerve damage during right PV isolation. Although no cases of left atrial–esophageal fistulae have been reported, esophageal temperature rises do occur during posterior left atrial ablation; therefore this complication is not completely avoided using this technique.
Ablation Frontiers phased array catheter system
- As opposed to the cryoballoon and laser balloon systems, which are designed for ablation in patients with paroxysmal AF, the Ablation Frontiers system (Medtronic, Minneapolis, MN) is designed for ablation in patients with more persistent forms of AF. Because the success rate of PV isolation alone in persistent AF is suboptimal, many have advocated additional left atrial substrate ablation. This includes ablation of complex atrial fractionated electrograms (CAFE), which are most prevalent on the left atrial septum. The phased array ablation system is comprised of three catheters: 1) a circular PV ablation catheter (PVAC), 2) the multi-array septal catheter (MASC), and 3) the multi-array ablation catheter (MAAC) (Fig. 3). The PVAC catheter is a multi-polar circular catheter that is designed for both PV mapping and circumferential ablation. The catheter can ablate simultaneously through multiple electrodes or through one electrode at a time. The MASC catheter is designed to perform rapid mapping ablation of fractionated electrograms on the left atrial septum. The MAAC catheter is designed to perform rapid mapping and ablation of electrograms in the left atrial body. Together, the catheters allow pulmonary vein isolation and left atrial substrate ablation with much less effort than standard catheter ablation.
The PVAC catheter consists of a 9-Fr steerable shaft with a circular 25-mm diameter distal configuration containing ten electrodes with 3-mm interelectrode spacing mounted on a nitinol frame. The catheter has a central lumen through which a J-wire can be advanced to help maintain catheter stability at the PV ostium. The catheter can be advanced through a transseptal sheath and the distal end retracted to form the circular shape. A wire is then advanced out the distal end into the PV of interest, and the PVAC catheter is then advanced to the PV ostium. Bipolar electrograms can be recorded from the circular catheter before and after ablation. Ablation can be performed simultaneously among all electrodes, or between any two electrodes at a time. Each electrode has a thermocouple capable of maintaining temperature below a preset cutoff. A unique multi-channel duty cycled radio frequency generator (GENius; Medtronic, Minneapolis MN) needs to be used to deliver ablation energy. The generator can deliver ablative energy in either unipolar (between electrode and indifferent electrode on the body) or bipolar (between any two adjacent electrodes) mode. There are five pre-defined energy modes: 1) unipolar, 2) bipolar, 3) 4:1 bipolar (80% bipolar, 20% unipolar), 4) 2:1 bipolar, and 5) 1:1 bipolar. Energy is titrated to a set target temperature of 60°C with a maximum output of 8 to 10 W. Each lesion is delivered for 60 s, typically starting with a 4:1 bipolar to unipolar ratio to limit lesion depth, and then changing to a 2:1 ratio if PV isolation is not achieved. Because the electrodes do not cover the full 360-degree circumference of the PVAC catheter, after each lesion the catheter is rotated to cover the entire PV circumference.
The MASC is designed for septal CAFE ablation. It contains four electrodes on each of the three arms, for a total of six bipolar pairs of electrodes. The second arm contains two fluoroscopic markers for localization. The electrodes are mounted on a 9-Fr steerable catheter. The catheter is designed to be extended into a linear shape, advanced through the intra-atrial septum, and then expanded into its full shape and pulled back against the left atrial side of the septum for ablation. Electrograms can be recorded though any of the six bipolar pairs. Ablation can then be performed using the duty cycled radio frequency generator; after each 60-second lesion the catheter can be rotated to defragment the left atrial septum.
The MAAC catheter contains four arms, each with one pair of electrodes, and is used for mapping and ablation in the remainder of the left atrial body. This catheter can be used to ablate CFAEs on the posterior wall, anterior left atrium, and for ablating along the left atrial roof. In general, a 4:1 bipolar to unipolar ratio is used when ablating on the posterior wall, and a 1:1 ratio on the anterior wall or septum.
The Ablation Frontiers system has been studied in a European multi-center study . Fifty patients with long-standing persistent AF (>1 year) underwent PV isolation and left atrial defragmentation. All patients had to have persistent AF despite cardioversion and a trial of at least one antiarrhythmic medication. Patients with a left atrial diameter by transthoracic echocardiogram of >55 mm or left ventricular ejection fraction <40% were excluded. The entire ablation procedure lasted 155 ± 40 min (32 ± 12 min for PV isolation, 8 ± 3 min for septal ablation, 13 ± 5 min for left atrial body ablation). Only one patient converted to sinus rhythm during ablation; the remainder were cardioverted. Acute PV isolation and left atrial CAFE ablation were achieved in all patients. Due to recurrent AF, 50% (25 patients) underwent repeat ablation using the same approach. Two patients developed focal left atrial tachycardias requiring a third standard ablation procedure. One patient developed cardiac tamponade related to transseptal access and there was one ischemic neurologic event.
After 6 months, 27 of 50 (54%) patients were free of AF off antiarrhythmic drugs, and an additional eight patients (16%) were free of AF on antiarrhythmic drugs (n = 5 on amiodarone). At long-term follow-up (20 ± 4 months), 21 of 47 (45%) patients were free of AF off antiarrhythmic drugs and an additional seven patients (15%) were free of AF on antiarrhythmic drugs.
The advantages of the Ablation Frontiers system include a short procedure time (155 min) in a complex patient group. The negatives include a more distal level of PV isolation than can be performed with standard catheters and the lack of irrigated ablation. Non-irrigated linear ablation has often been limited by thrombus formation in pre-clinical studies using other technologies.
E.P. Gerstenfeld has received honoraria from Medtronic Inc. and research grants from CardioFocus and Medtronic Inc…