Functional mitral regurgitation (MR) refers to regurgitation occurring in the setting of a structurally normal valve as a consequence of imbalance between tethering and closing forces, most commonly in the context of ventricular remodeling and dilation of ischemic origin. More recently, atrial functional mitral regurgitation (AFMR or ASMR) has been described as a separate category, characterized by preserved left ventricular ejection fraction and driven primarily by atrial dilation and mitral annular remodeling secondary to atrial fibrillation and/or heart failure with preserved ejection fraction (HFpEF).

Patients with functional MR have been reported to experience high mortality rates even after surgical intervention, approaching 20% at 2 years in some series. Whether this newly recognized atrial phenotype identifies a subgroup with more favorable surgical outcomes remains uncertain, and this question represents the central objective of the present study.

This was a single-center retrospective study including adult patients who underwent elective mitral surgery for moderate or severe functional MR between 2000 and 2022, excluding those with concomitant aortic surgery or prior mitral interventions. AFMR was defined by preserved ejection fraction (>50%) with atrial fibrillation and left atrial enlargement, whereas VFMR was characterized by reduced ejection fraction (<50%). Primary endpoints were overall survival and freedom from surgical failure, defined as recurrent moderate or greater MR, later expanded to include mitral reintervention within a composite endpoint. Time-to-event analyses were performed using Kaplan–Meier estimates and Cox models accounting for death as a competing risk.

Among more than 4000 patients undergoing mitral surgery during the study period, 375 met inclusion criteria and were classified as AFMR or VFMR. Patients with AFMR were older, more frequently female, and had a higher prevalence of atrial fibrillation, with lower diabetic burden and lower STS-PROM risk. Echocardiographically, they exhibited smaller ventricular dimensions, preserved ejection fraction, and greater atrial enlargement. Propensity matching improved covariate balance, although residual differences persisted in atrial fibrillation prevalence and ventricular parameters. Surgically, tricuspid repair and concomitant atrial fibrillation ablation were more common in AFMR, whereas concomitant coronary surgery predominated in VFMR. Rates of mitral valve repair were similar between groups.

Operative mortality was significantly lower in patients with atrial functional MR (3.3% vs 13%; p = .03). Long-term survival was overall comparable between groups. Although one adjusted analysis suggested a potential advantage for AFMR, this finding was not consistent across statistical models. No significant differences were observed in recurrent MR or overall surgical failure. In multivariable analysis, age and comorbidity, particularly renal and pulmonary disease, were the main predictors of mortality. Ventricular MR etiology and concomitant atrial fibrillation ablation were not independently associated with outcomes. Exploratory subgroup analyses were inconclusive.

COMMENTARY:

The most relevant findings of this study are the significantly lower operative mortality observed in atrial functional MR and the suggestion of improved long-term survival compared with ventricular functional MR, with similar durability of mitral valve surgery in both phenotypes.

Nevertheless, several limitations warrant careful consideration. The retrospective classification of atrial versus ventricular functional MR is inherently susceptible to bias. The absence of a non-surgical control group, together with the long inclusion period spanning more than two decades, introduces potential variability in medical management and surgical techniques over time. Follow-up echocardiographic data were unavailable in a substantial proportion of patients, limiting accurate assessment of recurrent MR. Additionally, the mechanisms underlying atrial fibrillation recurrence were not systematically characterized, and data regarding late ablation procedures during follow-up were lacking.

The observed difference in long-term survival was not consistent across all propensity adjustment strategies. While significance was reached in the matched cohort, it was not confirmed with IPTW analysis, raising the possibility that residual baseline imbalance and limited overlap between groups may account for part of the observed effect. Moreover, emphasis on non-significant trends, such as freedom from reoperation or sensitivity analyses excluding coronary surgery, should be interpreted cautiously.

Among the strengths of the study is its focus on a clinically relevant and evolving concept. Median survival reached 9.2 years in AFMR and 7.7 years in VFMR, markedly longer than previously reported in historical cohorts. These findings reinforce the potential prognostic benefit of surgical management in functional MR compared with medical therapy alone. Particularly in AFMR, the data support the concept that this phenotype may represent a subgroup with more favorable surgical outcomes than ventricular functional MR, a finding of importance given the comparatively less robust results reported with edge-to-edge therapies in this population.

Although exploratory, the trend toward worse outcomes in the subgroup with concomitant atrial fibrillation and HFpEF suggests that surgery may be undertaken at a stage when established myocardial fibrosis limits the impact of intervention. Concomitant surgical ablation did not confer a survival benefit in this study. Given previous data suggesting reductions in atrial fibrillation recurrence and MR progression with ablation, timing of intervention rather than technique may be a critical determinant of outcome in this etiologically atrial disease.

In contrast to American guidelines, the current ESC/EACTS valvular heart disease guidelines distinguish between atrial and ventricular secondary MR and provide differentiated recommendations (Class IIa, Level B for surgery in AFMR vs Class IIb, Level C in VFMR). The present findings lend support to this conceptual separation and underscore the need for further investigation to refine patient selection and strengthen the evidence base for tailored management strategies.

REFERENCE:

Pyeatte SR, Rahimi M, Braasch MC, Marghitu T, Damiano M, He J, Brescia AA, et al. Outcomes of Mitral Valve Surgery for Atrial Functional Mitral Regurgitation vs Ventricular Functional Mitral Regurgitation. Eur J Cardiothorac Surg. 2025 Oct 2;67(10):ezaf319. doi: 10.1093/ejcts/ezaf319.