Discussion
Prior studies investigating risk factors for the development of AF after atrial flutter ablation have yielded inconsistent results,1-2, 4-6,15-16 and effective risk stratification for development of AF after AFL ablation remains an important, unmet clinical need. Our primary findings are as follows: 1) An overall incidence of AF after CTI AFL ablation of 40% in 114 consecutive patients at three years follow-up with routine clinical care 2) LAVI was the only independent predictor of AF after CTI AFL ablation and 3) LAVI > 30 ml/m2 identified patients significantly more likely to develop AF with a hazard ratio of 2.25, with similar risk of incident AF observed with LAVI 30-40 ml/m2 compared to LAVI ≥ 40 ml/m2.
While incidence of AF after typical AFL ablation in patients with no known history of AF at three years follow-up has been reported to be up to 82%,5 the 40% incidence observed in the present analysis is consistent with that of several prior studies in which incidence of AF after typical AFL ablation was observed to be 25-50% at three years follow-up.1,6 Routine, ambulatory arrhythmia monitoring was performed in ~40% of analyzed patients, and monitoring intensity was similar between patients who developed AF and those who did not develop AF.
Chen et al. investigated predictors of incident AF, including HATCH score, in 216 patients after CTI AFL ablation and found that patients with a HATCH score ≥ 2, and those with increased LA diameter were significantly more likely to develop incident AF. 7The area under the receiver operator curve for HATCH score as a predictor of incident AF after AFL ablation was 0.7. The authors postulated that the HATCH score likely represented those patients with enlarged and remodeled left atriums, however LAVI was not evaluated in that study.7 The significance of the HATCH score was subsequently investigated by Garcia-Seara et al. in 408 patients who underwent typical AFL ablation and it was found that neither a HATCH> 2, nor a HATCH > 3 were significant predictors of incidence of AF after typical AFL ablation.8 They did find LA diameter to be significantly associated with incidence of AF, with degree of enlargement correlating with risk of incident AF, however LAVI was not investigated in this study either. Neither HATCH score, nor any other clinical or electrophysiologic parameter have been established as reliable predictors of AF after AFL ablation. Our data is largely consistent with these prior studies suggesting modest utility of HATCH score and LA diameter as predictors of incident AF after typical AFL ablation.
Studies investigating incidence of AF after AFL ablation have not consistently included LAVI as a variable of interest. Limitations of LA size assessment by LA diameter are well recognized.17Left atrial volume index is calculated via the biplane disk summation technique, which incorporates fewer geometric assumptions than the area-length methods and thus is perceived to be more accurate. Body surface area is also known to largely impact left atrial size, therefore indexing the calculated left atrial volume to body surface area also allows for more accurate interpretation of left atrial volume measurement. In the only prior study that we are aware of that assessed LAVI as a predictor of AF after typical AFL ablation, Lee et al. found a LAVI of 42.6 ml/m2 to be predictive of AF after AFL ablation with a 69% sensitivity and 69.8% specificity.16 The LAVI cutoff proposed by Lee, et al. is considerably greater than our proposed cutoff of 30 ml/m2, and may have been related to greater prevalence of structural heart disease and other comorbidities in their study. Given the markedly higher LAVI cutoff found in Lee et al., sensitivity analysis in our cohort comparing incidence of AF after CTI AFL ablation in patients with LAVI 30-40 ml/m2 to that of patients with LAVI >40 ml/m2 showed a similarly elevated risk of incident AF in both groups.
Discordance of prognostic significance between LA diameter and LA volume for development of AF was previously shown by Abecasis, et al. in patients who have undergone PVI and CTI ablation for drug resistant AF.18 In this study, LA volume derived from CT scan was a significant predictor of arrhythmia recurrence, however echocardiographic parameters including LA diameter did not have significant predictive value.18 LAVI determined by echocardiography was recently found to be significantly associated with incidence of cardioembolic stroke and incident AF in patients with prior cryptogenic stroke.19 Cryptogenic stroke and typical AFL are similar conditions in that both are strongly associated with coexistent AF, and identification of coexistent AF has significant clinical implications including consideration of anticoagulation. The consistency of association between elevated LAVI and incident AF across study cohorts and disease states provides greater credibility for the potential utility of LAVI for risk-stratification.
Three randomized clinical trials have evaluated prophylactic PVI in patients with typical AFL and no prior history of AF, each of which has yielded results favoring combined PVI and CTI AFL ablation.4-6 These studies of relatively unselected patients undergoing typical AFL ablation found that CTI plus prophylactic PVI ablation resulted in absolute risk reductions for incident AF of 10-28% compared to CTI ablation alone.10 The benefit of prophylactic PVI would be expected to be greatest in patients at greatest risk for development of AF. The established benefit of prophylactic PVI may be substantially greater than previously demonstrated in patients with LAVI of >30 ml/m2, particularly with use of improved ablation techniques for PVI.20
There were several limitations to this study. Although patients’ medical records including all available ECG documentation were carefully reviewed to exclude the presence of AF before ablation, minimally symptomatic AF may have been present and undiagnosed. Similarly, the frequency of post-ablation AF may be underestimated due to the absence of longitudinal arrhythmia monitoring in all patients. LAVI was also collected directly from echo reports instead of re-calculated, thus the possibility of echocardiographer variability in measuring LAVI is present. Additionally, this was a predominantly male patient population and therefore these results cannot be generalized to females. Finally, there are limitations to this study that are inherent to its retrospective nature.