Understanding the atrial fibrillation substrate: the case for repeat catheter ablation.

Radiofrequency catheter ablation has become a curative treatment for some patients with symptomatic drug refractory atrial fibrillation (AF). However, adequate selection of candidates for catheter ablation is important because of the potential, albeit infrequent, complications of the procedure (incidence reported in up to 4% of procedures), its costs, and the non-negligible rate of recurrence of the arrhythmia (up to 40%) even in experienced hands. In fact, one of the main limitations is our poor understanding of the real substrate underlying AF. Cardiac remodelling develops in response to myocardial contractile dysfunction to maintain stroke volume or to adapt to changes in overload. The atrium is not an exception, and atrial remodelling originates from the mechanical effects on the atrial wall, such as direct chronic volume overload (for instance, mitral regurgitation) or indirect chronic overload induced by elevated filling pressures (as is the case of dilated or hypertrophied left ventricles). The remodelled atrium finally acts as the necessary substrate for the initiation and perpetuation of AF. Remodelling includes the distortion of normal geometry, enlargementof thecavity, and substitution of the normal tissueby collagenor fibrotic tissue. Consequently, different parameters can be used as indicators of remodelling. With the aim of characterizing the atrial substrate, different approaches have been proposed to assess atrial remodelling, particularly to identify the best candidates for catheter ablation of AF. Several studies have described the relationship of atrial enlargement and the lower likelihood of successful arrhythmia suppression with catheter ablation. A study in 148 patients using M-mode and two-dimensional echocardiography has shown that an anteroposterior left atrium (LA) diameter .45 mm was an independent predictor of the absence of recurrent AF after ablation. Similarly, LA area ,34 mL/m predicted the absence of recurrent AF. Using computed tomography, LA volumes ,145 mL have also been related to successful ablation of AF. More recently, analysis of LA shape (sphericity index) has shown promising results in selection of patients with less advanced atrial disease who may be more suitable for catheter ablation. Analysis of LA functioneitherby three-dimensional ormyocardial deformation ultrasonic imaging has provided additional information on top of LA size for understanding remodelling and identifying adequate candidates for AF ablation. Impaired LA function is related to lower success of ablation, with higher sensitivity and specificity compared with the assessment of LA size, particularly in the setting of a mildly dilated LA, which is generally found in patients referred for ablation. Finally, direct quantification of myocardial fibrosis content in LA by delayed enhanced magnetic resonance has been related to success of catheter ablation in eliminating AF. The incidence of AF recurrence was 14% in patients with minimal enhancement, 43% in those with moderate enhancement, and 75% in those with extensive enhancement. Despite compelling progress in the identification of factors related to success of ablation after the first procedure, scarce data are available regarding LA characteristics in patients undergoing repeat procedures. Several studies have shown that LA size decreases after ablation without a significant impairment in LA function. A meta-analysis confirmed these findings, suggesting that the successful elimination of the arrhythmia by ablation induces reverse LA remodelling which in turn counteracts the potential negative effect of atrial scarring due to the ablation lines. These observations also highlight the unique atrial substrate in patients who have already undergone an ablation procedure for AF and need a repeat procedure; this substrate includes both changes in response to the ablation lines (scarring) and remodelling due to the arrhythmia and underlying atrial myocardial disease. In a previous study, LA size and LA function assessed by twoor three-dimensional echocardiography failed to predict AF recurrence after the second ablation procedure underscoring the fact that this could be due to a different atrial substrate. More recently, the potential of myocardial deformation imaging to identify best candidates for