Surgical treatment of mitral valve disease in the presence of MAC presents a significantly increased risk of cardiovascular and all-cause mortality. Until recent years, surgical options were rarely offered to these patients due to their prohibitively high-risk profile. However, over the past decade, various surgical and transcatheter techniques involving balloon-expandable valves (BEV) have been described to treat mitral disease complicated by severe MAC. Nevertheless, reported case series remain limited, and outcomes still correlate with high operative morbidity and mortality.

This article presents the experience of two hospitals in Virginia, USA, with the open surgical implantation of a BEV in the mitral annular position through a transatrial approach (BEV-in-MAC transatrial) as an alternative strategy for patients with severe MAC. The BEV implantation was performed with direct visualization via the left atrium, using either a median sternotomy or a minimally invasive approach. The midportion of the anterior leaflet was excised, and, in cases with a high risk for left ventricular outflow tract (LVOT) obstruction, a septal myectomy was performed via a transatrial approach. The primary endpoint was technical success as defined by the Mitral Valve Academic Research Consortium, with secondary outcomes including 30-day and 1-year mortality. Between October 2015 and October 2020, a total of 51 patients from these institutions underwent BEV-in-MAC implantation (mean age: 73.9 years; 60.8% female; predicted mortality risk by STS score: 6.8%). Technical success was achieved in 94.1% (48/51) of cases. Thirty-day and 1-year mortality were 13.7% (7/51) and 33.3% (15/45), respectively. Stroke rates were 3.9% (2/51) at 30 days and 4.4% (2/45) at one year.

Surgical implantation of a BEV in the mitral position offers a treatment option for patients with mitral valve disease complicated by severe MAC who face elevated risks with conventional open surgery and/or a high risk of LVOT obstruction with percutaneous treatment.

COMMENTARY:

The primary results of this study can be summarized as follows: (1) BEV-in-MAC transatrial can be performed with a high rate of technical success (94%) and low incidence of LVOT obstruction and paravalvular leakage; (2) thirty-day mortality was 13.7%, and one-year mortality was 33.3%. This study represents the most extensive published series on BEV-in-MAC to date and positions this technique as a valuable alternative to conventional surgical and purely percutaneous treatment, particularly in cases with a high risk of LVOT obstruction and embolization.

With the traditional surgical approach, options were limited to modifications of valve implantation, such as heterotopic and orthotopic implantation. In the heterotopic approach, the mitral prosthesis was anchored to non-calcified tissue, often the left atrium, with ventricularization of part of the structure, which commonly involved the left atrial appendage, requiring ligation. Orthotopic implantation within the native annulus demanded limited resection of calcified tissue when feasible or complete decalcification of the mitral annulus with or without sealing through pericardial patch addition. Ensuring a watertight suture line was imperative in both approaches. These techniques have been associated with a high percentage of atrioventricular groove disruption, circumflex artery injury, and extended ischemic times, ultimately contributing to a mortality rate between 9% and 24%. Conventional surgery in these patients is also associated with smaller prosthetic sizes, frequently mechanical, and a higher risk of paravalvular leakage.

From the pioneering work of Guerrero et al. on the first percutaneous prostheses implanted in MAC (valve-in-MAC via transeptal or transapical approaches), it became clear that one of the major limitations was the difficulty in achieving a stable seal without paravalvular leakage, alongside limitations related to the annular sizes that could be treated. Other notable drawbacks included the high likelihood of LVOT obstruction without the possibility of anterior mitral leaflet resection, and the risk of prosthesis migration. One-year mortality with this approach hovers around 50%, with LVOT obstruction emerging as the most potent independent predictor of poor prognosis.

The transatrial BEV-in-MAC approach described in this article offers several potential advantages compared to conventional surgery or transeptal or transapical valve-in-MAC approaches:

• It avoids the need for annular debridement, reducing the risk of groove rupture and circumflex artery damage.
• Direct visualization of the prosthesis deployment and manual stabilization verification, along with additional sutures in atrial tissue or calcified leaflets, anchor the struts, reducing the risk of prosthesis migration and paravalvular leaks. Some authors suggest adding Teflon bands around the prosthesis’s intended location to fill irregular spaces and secure the prosthesis upon expansion, thereby improving anchoring and peri-prosthetic sealing.
• LVOT obstruction is prevented in two ways: by excising the anterior leaflet in all cases and performing a prophylactic septal myectomy in patients with a predicted small LVOT area.
• The transatrial approach permits concomitant surgeries, such as aortic or tricuspid valve procedures, coronary surgery, or other interventions.

Regarding the results of this procedure, the mean cross-clamp time exceeded two hours (128 minutes), likely because more than half of the patients underwent concomitant surgery, which could have contributed to the relatively high 30-day (13.7%) and one-year (33.3%) mortality rates. These outcomes are consistent with those reported by other groups performing the same intervention. Furthermore, these results are slightly better than those reported with transapical or transeptal valve-in-MAC techniques, which yield a 30-day mortality of 20% and a one-year mortality of 40%. Consequently, no technique currently offers low morbidity, and none appears to be clearly superior; therefore, the choice should be individualized based on patient characteristics and surgeon preference.

The main limitations of this study arise from its retrospective nature. Additionally, the absence of a control group limits our ability to make comparisons, and the sample size is small. Caution is needed in interpreting one-year outcomes due to the high rate of missing echocardiographic data during follow-up. Awaiting the results of the ongoing SITRAL (Surgical Implantation of Transcatheter Valve in Native Mitral Annular Calcification) study is essential to gain further insights into cases where a hybrid approach, as described here, could be most beneficial.

Lastly, the importance of preoperative imaging studies and meticulous surgical technique for the success of this procedure cannot be overstated. Both aspects are detailed within this study’s methodology. Preoperative assessment requires at least a transthoracic and transesophageal echocardiogram and, crucially, a 4D computed tomography for mitral valve reconstruction, LVOT size estimation, and optimal surgical planning. Consequently, transcatheter valve-in-MAC generally necessitates near-circumferential annular calcification (at least three-quarters of its circumference), while the transatrial approach can address less extensive calcification (often confined to the posterior leaflet, two-thirds of the circumference). This factor, along with others such as LVOT obstruction risk, makes imaging the cornerstone of approach allocation.

REFERENCE:

Smith RL, Hamandi M, Ailawadi G, George TJ, Mack MJ, DiMaio JM, et al.; BEV-in-MAC Collaborative. Surgical implantation of balloon-expandable heart valves for the treatment of mitral annular calcification. J Thorac Cardiovasc Surg. 2023 Jul;166(1):62-70. doi: 10.1016/j.jtcvs.2021.08.047.