Non-pharmacologic treatment of atrial fibrillation (AF) has evolved rapidly in the last decade and multiple technical approaches have been developed, starting from surgical atrial linear lesions to transcatheter ablation, leading to progress in AF physiopathology comprehension and to the understanding that a curative therapy for AF may exist, at least for a subgroup of patients.
The early transcatheter attempts were led by creating linear lesions in the atria in order to mimic the surgical Maze procedure. These experiences demonstrated that linear lesions ablation in the right atrium presented with a good safety profile but low curative efficacy; on the other hand left atrium ablation had a higher curative efficacy but a high complication rate (up to 10%).
The turn of the screw was represented by the pivotal report by Haissaguerre et al. on the trigger activity of pulmonary veins (PV) in paroxismal AF. The ablation strategy of triggering PV foci demonstrated a good acute success but high recurrence rate, with multiple ablation sessions required; moreover, it led to the discovery of PV stenosis as a severe complication. Many electrophysiology groups began to work on AF ablation and the results of preliminary reports were replicated by other authors with a wide spectrum of success rates. Limitations of trigger ablation technique are that the ablation of one PV foci does not prevent the appearance of other foci; moreover, atrial ectopic beats and AF induction are necessary during the procedure. Evolving knowledge led to other strategies in order to overcome the limitations previously described: on one hand PV ostial segmental disconnection (OSD) and on the other hand left atrial anatomical circumferential ablation (LACA).
OSD is best accomplished using a pre-shaped multi-electrode circumferential mapping catheter placed at the ostium of the PV; this catheter records fused electrograms composed of far-field atrial and near-field PV electrograms. At the same time an ablation catheter is used to deliver energy in order to obtain electrical disconnection of the PV guided by the circumferential electrical activation recorded. Early reports on OSD on paroxismal atrial fibrillation patients documented an outcome free from AF without drugs in 56-70% of patients, while results on persistent AF patients were less satisfactory.
In the same years, LACA emerged as an alternative to OSD; LACA is performed with a single catheter in the left atrium creating a circumferential ablation line in the atrial tissue around, but several millimetres away from, the PV ostia with the help of a 3-D colour-coded electro-anatomical reconstruction of the left atrium and PVs.
Which one of the two strategies is superior is still an open question. In one study comparing OSD and LACA in 80 patients, the six-month outcome was of 67% and 88% of asymptomatic patients, respectively, without drugs; no difference in complication rate was described. In this study LACA was associated with a posterior ablation line and with another block line at mitral isthmus - maybe the extensive ablation lines and the use of different catheter and power settings have influenced these results. Another study comparing the two strategies conducted on 100 patients reported a 54% versus 82% of asymptomatic patients after LACA and OSD, respectively; an explanation for these different results may be the outcome method evaluation: subjective in the former study and both subjective and objective using seven days Holter monitoring in the latter study. Moreover, LACA was performed during AF in the former study but during sinus rhythm in the other.
In patients treated with OSD, arrhythmia recurrences were observed to be linked to electrical conduction recurrence between one or more PVs and left atrium; the initial OSD technique has been progressively modified by addition of ablation lines as the left atrium roof line or the mitral isthmus line. In patients treated with LACA arrhythmia recurrences were related to both AF and left atrial macro-re-entry tachycardias due to incomplete lines favouring the substrate for re-entry.
Moreover, as the technical aspects of the two techniques are evolving in terms of kind of energy, type of catheters and ablation designs, the population of AF patients undergoing transcatheter procedures is also progressively wider. Initial reports of AF ablation in patients with chronic AF or with heart failure have been published. OSD associated or not with left atrium lines was performed in a group of patients with AF, heart failure and left ventricular dysfunction matched with a group of AF patients and no heart failure: approximately 70% of both groups were in sinus rhythm without drugs (78% and 84% of patients in sinus rhythm with drugs, respectively) and a significant better outcome was recorded in terms of left ventricular function and dimensions, symptoms and quality of life in this particular subgroup of patients with AF. Also, the early data regarding transcatheter ablation of chronic AF using LACA seem feasible, with a one-year outcome of 74% of success and a redo rate of 32% because of AF and atrial macro-re-entry tachycardias.
In order to overcome the limitations of LACA and OSD a combined strategy has been proposed: two circular mapping catheters are introduced in the left atrium and located at the ostium of ipsilateral PVs, a 3-D electro-anatomical map is created as for LACA and then a circumferential linear lesion is performed around the two ipsilateral PV several millimetres away from the ostium, until electrical disconnection is documented by the circular mapping catheters. Using this technical approach on paroxismal, persistent and permanent AF patients, freedom from AF has been obtained in 95% of patients, with 25-35% of patients undergoing the procedure again. Significantly, patients suffering from arrhythmia recurrence after this combined approach present mostly with PV-atrium conduction recurrence; another interesting observation is that the majority of PVs were isolated simultaneously, supporting the hypothesis of shared muscular fascicles between ipsilateral PVs.
In the wide panorama of catheter ablation of AF, where the different proposed strategies involve, in various degrees, PVs and left atrial antrum substrate, an alternative and novel approach was recently introduced: targeting left atrium substrate for AF ablation. Areas of atrial tissue displaying fractionated electrograms or very short cycle length electrograms thought to be a manifestation of slow conduction or functional conduction block were identified with a 3-D atrial map and targeted for AF ablation. With this approach ablation terminated AF in 95% of patients with paroxismal and chronic AF with an outcome of freedom from AF in 91% of patients at one-year follow-up and 4.9% of patients presenting with a major complication.
In order to understand whether the results reported from highly experienced and pioneering centres can be reproduced in the real world, a worldwide survey was published in 2005 referring to the period 1995-2002. The analysis of this survey has been conducted on 9,370 patients treated in 181 centres; it is interesting to underline that in this period the ablation strategies changed following the positive results of the literature reports of pioneering centres. Despite the technical approach used and in a heterogeneous population 76% of patients became asymptomatic (52% without drugs) after catheter ablation. It should be underlined that a clear correlation between the number of procedures performed per centre per year and the success rate was described, mostly as an expression of the learning curve. Moreover, the complication rate described in this survey was of 6% of treated patients; the rate of cardiac tamponade was of 1.2%, whereas significant PV stenosis occurred in 1.3% of patients, of whom half required interventional treatment. Atypical atrial flutter of new onset was reported in 3.7% of patients, mostly treated with 3-D guided compartmentalisation ablation. Periprocedural death was recorded in 0.05% of patients.
Regarding complications, a threatening one is the atrio-oesophageal fistula due to thermal injury of oesophagus muscular wall during ablation of the left atrial posterior wall. An open chapter is the one regarding thromboembolic complications linked to catheter ablation; in data from a single-centre experience on a population of 755 patients with paroxismal or chronic AF treated with oral anticoagulants at least three months after ablation, early embolic events were recorded within two weeks in 0.9% of patients, whereas late embolic events (six to 10 months after ablation) were reported in 0.2% of patients despite persistent anticoagulant therapy. It seems reasonable that prolonged anticoagulant therapy should be maintained in patients more than 65 years of age with previous embolic events and with more than two risk factors.
Catheter ablation of AF has evolved dramatically over the last few years. Several ablation approaches have been proposed and used in different subsets of AF patients; moreover, many insights into AF physiopathology have been reached through the results of ablation, leading to experimentation of different techniques. To date, there is no clear evidence that one ablation strategy is superior to another, but surely in the next future many new questions will be formulated and many answers found. Ôûá