As mechanisms underlying AF have been determined, strategies to treat AF with catheter ablation have evolved in parallel. Early ablative procedures consisted of linear lesion sets in the right atrium, left atrium, or both and were delivered with the goal of interrupting reentrant circuits.
29-32 These strategies, designed to alter the substrate by parsing left and right atrial tissue into regions small enough to be incapable of supporting sustained AF, were of limited efficacy. The linear lesions applied to modifythe substrate with this strategy were long and complex, with complete lines of block difficult to deliver. With Haisseguerre’s seminal report that AF is triggered by PV foci,
33 ablative strategies targeting triggeres located in PVs were adopted. Initially, focal ablation of the PV foci themselves was performed,
33-35 but focal ablation gave way to segmental
36 and eventually to circumferential lesions designed to isolate, rather than eliminate, PV targets.
37-40 Most procedures today combine some combination of circumferential and segmental isolation, with the goal of isolating each of the pulmonary veins.
41 In patients with persistent AF, some operators deliver additional lesion sets to further divide atrial tissue, eliminate rotors, or destroy ganglia. Catheter ablation for AF has provided unique opportunities to investigate in situ atrial fibrosis, with the aim of understanding the mechanisms of AF induction and maintenance more completely, as well as determining whether there exists a relationship between AF scar burden and patient prognosis.The Michigan group performed an analysis of bipolar atrial electrograms in 47 patients undergoing PVI for persistent AF.
42 Patients in the cohort had LA diameters of 46+/-5 mm, and had been in persistent AF for 2+/-1 years prior to ablation. The authors found that atrial fibrosis correlated strongly with patient age, and that age (and associated fibrosis) correlated inversely with AF cycle length. These findings mirrored investigational and ex vivo studies (referred to above) correlating age with atrial fibrosis. Natale and colleagues investigated LA scar, as assessed during catheter ablation, in a much larger series of patients (700) undergoing initial PVI.
43 Patients included in the study underwent mapping of LA scar using a decapolar lasso catheter, with scar defined as absence of electrograms on all 10 catheter poles in three distinct lasso positions. LA scar was noted to be present in 42 patients (6%). There was apparent correlation between scar and LA size, low EF, and increased plasma C-reactive peptide levels. The incidence of AF recurrence was significantly higher (57%) in patients with LA scar than in patients without scar (19%). LA scar was the only predictor of AF recurrence on multivariate analysis. In patients with LA scar who underwent CARTO electroanatomical mapping, scar area averaged 21 +/- 11% of LA surface area, and was associated with large regions of low voltage electrograms. The authors postulated that scarring likely contributes to AF etiology by providing abnormal patterns of LA conduction that allow for triggers of AF (from the PVs or elsewhere) to successfully initiate fibrillation, and that myopathic, diseased tissue itself may provide triggered beats. Olgin and colleagues studied regional patterns of LA fibrosis in patients with AF versus those with focal atrial tachycardia.
44 They found that in both patient populations, the anterior LA and appendage had highest bipolar voltage electrogram amplitudes, and that scar (as assessed by voltage amplitude) was most often found in the posterior and septal LA.In patients with AF, there was greater regional variability in voltage amplitude, perhaps suggesting increased heterogeneity of conduction. Similar to the findings of Morady and colleagues, the authors also found that low voltage and scar correlated strongly with patient age In summary, there have been a number of systematic analyses of LA scar, as defined by low-voltage bipolar atrial electrograms, in patients undergoing catheter ablation for AF. Generally these studies support data from ex vivo studies or experimental models that LA scar burden correlates with age, left atrial size, and with reduced cardiac function. LA scar burden appears to be a negative prognostic indicator of long-term freedom from AF after initial PVI. While this information is clinically helpful there are obvious disadvantages to catheter-based scar mapping of the LA: such maps are by definition invasive, laborious, and must be performed during the procedure (rather than informing decisions about whether to proceed with AF ablation at all).For these and other reasons, a number of centers have developed imaging techniques – predominantly MRI-based – to analyze LA scar in pre- and post-ablation patients.