It should be noted that these data come from use of the first-generation technologies. An observed higher rate of silent cerebral infarcts on magnetic resonance imaging with this technology 17 led to initial withdrawal of the PVAC catheter and after technological improvements it has been re-introduced into the clinical arena However, the newer data with the second-generation catheter containing 9 gold electrodes are not adequate as yet to definitely document its comparative efficacy and safety with the other technologies 18 , There ensued the development of another type of irrigated circular ablation catheter nMARQ catheter, Biosense Webster Inc, Diamond Bar, CA, USA , with good initial results 20 , 21 , however, due to observed fatal complications from atrio-esophageal fistula, this catheter was recalled from the market in Attention has been consequently shifted to the cryoballoon catheter technology.
Cryoenergy applied via a balloon catheter introduced into the left atrium via a single transseptal puncture has emerged as a more practical, versatile and effective technique over the recent years, aspiring to become the dominant ablation method promising to facilitate PVI, decrease the complication rate, shorten the duration of the procedure and probably enhance the success rate of ablation 6.
It is a single-shot technique as the balloon catheter also accommodates a small circular mapping catheter introduced via the balloon catheter that can map the PVs and also confirm or even provide real-time monitoring of successful ablation with its newer generation versions. In its newer version second generation , the balloon catheter appears to be able to produce more effective and durable transmural lesions around the PV antrum with shorter, 3- vs.
Newer data provide guidance to a more practical and effective strategy while applying this freezing technique The authors of this study suggest that when time to PVI exceeds 43 sec, this denotes an insufficient freeze, which should be terminated early and a new balloon position should be sought for a more effective application. Hence, the need to have improved technology to be able to continuously monitor real-time PV potential recordings during cryothermic energy application is crucial. With current technology, this may not be feasible at all times, as the recording circular catheter cannot always be kept during freezing in proximal positions required for this kind of monitoring.
Newer technology with shorter tipped third generation balloons may indeed render this possible 25 ; however, further technical details for more versatile and effective balloon catheters may have to be worked out before these balloons become more widely available. Different sites of transseptal puncture have no influence in grades of PV occlusion, rates of PVI, mid-term outcome and rates of complications during cryoablation 6. Imaging techniques, such as intracardiac echocardiography, guiding exact balloon placement may predict acute ablation success and prevent acute narrowing of PV ostia.
Total duration of cryoenergy application has not been shown to affect procedural success, with similar success rates obtained with 3- and 4-minute applications Thus, shorter time of freezing application can expedite the procedure and lead to lower radiation exposure without compromising procedure efficacy.
Cryoablation in persistent atrial fibrillation – a critical appraisal
All these parameters have also been shown to predict favorable clinical outcomes. The most feared complication relates to cardiac tamponade which can necessitate surgical intervention and can rarely lead to fatal outcome. Another dreaded complication, the development of an atrio-esophageal fistula, which is usually fatal, has also been observed more rarely with the cryoablation approach compared to the RF technique The higher rate of phrenic nerve injury with PVI of the right PVs using cryoenergy appears to be manageable with monitoring of the phrenic nerve function by applying phrenic nerve pacing via a catheter positioned at the right superior vena cava.
Furthermore, the reversibility of this injury over time in the majority of patients is very encouraging An important aspect of AF ablation relates to operator proficiency and dexterity in performing transseptal catheterization safely Then, careful and gentle handling and maneuvering the ablation tools inside the left atrium and the PVs is of utmost importance to avoid further complications. Single-shot techniques compare well with the conventional point-by-point RF ablation approach, although the latter has recently received a boost with the advent of contact force monitoring during ablation, albeit without managing to surpass cryoablation in terms of procedure duration, which remains shorter with the second generation cryoballoon Of the two single-shot techniques, cryoballoon ablation has taken a lead mostly due to technical issues that have emerged with the circular catheters, and also the difference in energy source that might be providing an edge over conventional RF energy 16 , 34 , 36 - According to our own experience with these two techniques Figures 1 , 2 , Table 1 , the circular catheter was employed during the early years of its availability and replaced the conventional irrigated catheter in our laboratory, while the cryoballoon was adopted later when the circular catheter was withdrawn due to reports of silent cerebral infarcts attributed to technical issues.
The development of char and coagulum posed a risk to lead to thromboembolic complications, while steam pop could result in cardiac perforation. In attempts to improve catheter safety, ablation was most commonly performed in a temperature control mode, which allowed for a reduction in power output to maintain a maximum allowed electrode temperature.
In addition, several factors fundamental to lesion formation could not be assessed or modified with early-generation catheters, such as convective heat loss, the above-mentioned coagulum formation which would further limit power delivery , and catheter-tissue contact force. As a result, ablation time was often the only parameter under direct and measurable control of the electrophysiologist. Such was often insufficient to secure an optimally successful procedure, as biophysical studies of radiofrequency RF ablation have demonstrated that lesion formation is nonlinearly dependent on four critical factors: ablation time, power, catheter myocardial contact force, and convective heat loss.
The development of externally irrigated ablation catheters was a significant advancement and obviated several of the deficiencies inherent to standard, non-irrigated catheters. Initial irrigation was predominantly accomplished via irrigation holes within the distal aspect of the ablation electrode.
Despite affecting only this segment of the electrode, there was significantly improved cooling at the electrode—tissue interface, where overheating is most common. This reduction in catheter overheating led to improved power delivery, as well as reduced char and coagulum formation at the distal electrode. Limitations, however, remained with first-generation externally irrigated catheters. To achieve adequate electrode cooling, these catheters required substantial fluid delivery, a stipulation often leading to clinical volume overload states following long procedures.
Additionally, first-generation designs cooled only the distal portion of the ablation electrode, leaving them vulnerable to char formation at the proximal portion. Second-generation externally irrigated ablation catheter designs improved on both these deficiencies by increasing the number of irrigation holes and spreading them more completely over the entire electrode surface area.
This technology decreased the risk of char formation at the proximal portion of the electrode and allowed for a reduction in the needed amount of fluid delivery, due to improved electrode cooling. Despite the improved design, the measurement of contact force an important biophysical measure was not addressed and, without this knowledge, lesion formation was neither consistent nor reproducibly achieved.
Surrogate methods to assess lesion formation, such as impedance drop and electrogram elimination, were therefore commonly employed and were found to be helpful. This shortcoming led to the next significant advance in catheter design: the advent of contact force-sensing catheters allowed for the direct measurement of catheter-myocardial contact and directionality.
More recently, this important development has translated to an improved success rate for ablation of AF, 34 , 35 likely due to an improvement in lesion formation. In spite of this improvement, however, lesion formation continues to be inadequate in certain circumstances, and there remains an unmet need for accurate lesion measurement or software-based prediction methods.
Evidence from the chronic assessment of PVI has demonstrated that even with the use of contact force catheters, frequent pulmonary vein recovery still occurs. This absence has led to interest in software algorithm methods to predict or estimate lesion formation.
These include nonlinear algorithms that utilize contact force, ablation time, and power settings. Perhaps the use of such technology may allow for the accurate prediction of lesion formation, and could offer a consistent and operator-independent method to improve procedural success rates. The final element affecting lesion formation is convective head loss and its effect on ablation-induced tissue temperature. Current-generation catheters are unable to monitor for heat loss or to measure tissue temperature in any form.
A surrogate for convective heat loss may be the accurate measurement of myocardial tissue temperature during RF ablation. The next step in catheter development is likely to focus on such measurements.
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An alternative method to achieve lesion formation is ablation based on tissue cooling, or cryoablation, which leads to myocardial cell death due to both a direct cryothermal affect and microvascular destruction. Originally used for the ablation of the atrioventricular node slow pathway for atrioventricular nodal reentrant tachycardia, this technology was later deployed for PVI for AF. Cryoablation was deemed less likely than RF ablation to lead to atrioesophageal or esophageopericardial fistula formation, although these conditions have since been described in case reports.
The use of 4- or 5-mm electrode cryoablation catheters for PVI was adopted at several centers that were attempting to minimize fistula formation. Ultimately, however, this method proved too time-consuming for incorporation as standard use, and the efficacy for PVI achieved was not optimal in comparison with that of externally irrigated RF ablation. Next-generation cryoablation for PVI was achieved through the development of balloon-based technology during which the balloon was positioned at the pulmonary vein ostium and PVI was performed in a single step.
This led to an improved procedure efficiency rate in certain centers.
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Freedom from AF was observed in Optimal use of either cryoablation or RF ablation is important to achieve a higher procedural success rate. Alternative methods for achieving PVI have been developed, but are investigational in nature and are not currently FDA approved. These include balloon-based technologies that incorporate laser or RF energy or circular multielectrode ablation catheters.
Diaphragm paralysis was observed in 3. In regards to circular multielectrode catheters, there were safety concerns regarding non-irrigated catheters, as an analysis that compared the performance of either external irrigated RF ablation or cryoballoon demonstrated a 1. Importantly, despite the early mixed success rates for these emerging technologies, these studies typically represented first- or early-generation devices that had likely not reached full maturity or efficacy.
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The goal of performing longer or larger contiguous lesions with a single ablation procedure remains attractive, especially for its potential to improve procedural efficiency. Another method to improve outcomes with ablation has focused on electrical isolation of the left atrial appendage LAA. There has been interest particularly involving this subject and longstanding persistent AF, where the LAA may be a source of AF rotors or focal drivers.
Further studies will be needed to assess whether these results are widely applicable, and whether the potential increased risk for appendage thrombus formation following isolation is acceptable. This result, however, could not be replicated in other randomized studies. Therefore, currently recommendations are to proceed with SVC isolation only when tachycardia, frequent ectopy, or AF is documented to originate from this structure. The intrinsic cardiac autonomic system located in the ganglionated plexuses GP has also been shown to participate in the initiation and maintenance of AF.
The GP are located near the left superior and inferior pulmonary veins, the right superior and inferior pulmonary veins, and the Marshall tract. In a randomized study of paroxysmal AF, pulmonary vein isolation alone or in combination with GP ablation was performed.
Catheter Ablation of Atrial Fibrillation: A Review of the Current Status and Future Directions
Current ablation techniques focus on pulmonary vein antrum isolation, which encompasses the ablation of the GP locations inadvertently. This additional GP ablation may account for why antral ablation has a higher success rate than ostial PVI. As a result of the wider ablations performed, most centers no longer ablate the GP as a separate step in the procedure.
PVI remains the cornerstone in the ablation of both paroxysmal and persistent AF.
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PVI alone in persistent AF has not proven as successful. For this reason, intensive research has been performed for methods to map non-pulmonary vein drivers.
Ablation of these additional sites holds promise to improve ablation in persistent AF, as well as to identify the subset of paroxysmal patients with non-pulmonary vein triggers in whom additional ablation may be helpful. Prior studies have indicated the importance of several non-pulmonary vein triggers arising from locations such as the SVC, the vein of Marshall, the coronary sinus, the crista terminalis, and the posterior left atrium. Basic electrophysiology research has demonstrated the presence of a variety of non-pulmonary vein AF driver types, including microreentry, spiral rotors, and focal triggers, among others.
The translation of these mechanistic findings into an effective ablation strategy that improves freedom from AF has been challenging. This study did not determine the occurrence of any significant improvement in freedom from AF with the addition of ablation beyond PVI.