Mitral valve surgery in neonates or infants presents a challenge for any pediatric cardiac surgeon. In such patients, it is essential not to rush surgical indications, as the larger the patient, the higher the likelihood of successful repair. Several limitations hinder a durable repair: anatomical variations, small annular size, dysmorphic valvular tissue, underdeveloped subvalvular apparatus, papillary muscle anomalies, and a hypoplastic left ventricle. Nevertheless, all possibilities of repair must be exhausted before considering valve replacement.

Mitral valve replacement should be a therapeutic decision of last resort. Small annular options are limited; biological options such as the Melody valve in the mitral position or the Ross II procedure are available but come with early structural degeneration. Conversely, mechanical prosthetic replacement presents the disadvantage of lifelong anticoagulation. Experience with any of these options is limited, and outcomes are suboptimal, with mortality rates of 11-36%, increasing to 52% when analyzing children under 2 years of age.

In today’s article, the Emory group describes their results with the 15-mm St. Jude mechanical mitral prosthesis. This is a single-center, retrospective study analyzing all cases of mitral valve replacement with this prosthesis. Over the 13 years analyzed (2009-2022), 16 mitral valve replacements were performed on 15 patients. The median age and weight were 6.2 months and 5.16 kg. Two-thirds of the patients had undergone prior mitral repair. The most common diagnosis in this cohort was atrioventricular (AV) canal defect with left AV valve lesions. Half of the cohort had a genetic anomaly. Ten prostheses were implanted in the supra-annular position. The median intubation time was 1.5 days, the median stay in the intensive care unit was 6 days, and the hospital stay was 17 days. Three patients experienced major postoperative complications, including diaphragm plication, cardiac arrest, and prosthetic re-replacement due to thrombosis. Another four patients required readmission within 30 days after discharge for anticoagulation imbalances, either below or above the therapeutic range. There was no in-hospital mortality, but four deaths occurred during follow-up (27%). Six patients required mitral prosthesis replacement after a median of 6.8 years, with one patient undergoing replacement 10 years after the initial implant. Currently, the series includes six patients with the 15-mm St. Jude mitral prosthesis, with a median follow-up of 4.7 years.

The authors conclude that their reported series demonstrates the lowest rate of adverse events and shows greater prosthetic durability compared to other groups. They emphasize the challenges in achieving and maintaining therapeutic anticoagulation ranges in these patients.

COMMENTARY:

One of the significant limitations of pediatric cardiac surgery is the lack of growth potential in the prostheses we implant, as well as the limited options for small sizes. In March 2018, following FDA approval, Abbott introduced one of the smallest commercially available mechanical prostheses, the 15-mm St. Jude Medical Masters HP. This prosthesis is approved for children under five years old and required a prospective, multicenter, single-arm, non-randomized study of only 20 patients (the HALO study) for approval. Despite the small number of patients, Emory’s cohort represents one of the largest reported experiences in the literature with the 15-mm St. Jude prosthesis.

The study’s main contribution compared to the HALO study or other reported cohorts is the 13 years of follow-up in a homogeneous cohort. It reveals the realities of opting for the mechanical prosthesis route: 20% major postoperative adverse events, one case of acute thrombosis requiring replacement, and another during follow-up successfully treated with fibrinolysis using rtPA (a management strategy scarcely documented in children). Six patients were readmitted due to poor warfarin control, adding to the four who died during follow-up. The study also provides hopeful data: no patients developed heart block, prosthetic replacements to larger sizes after a median of 6.8 years, and one patient maintained the 15-mm prosthesis for a decade before replacement. Two-thirds of the implants were performed in the supra-annular position to accommodate larger prostheses. This implantation position is not usually preferred by certain groups due to the risk of disc obstruction, circumflex artery compression, and post-capillary pulmonary hypertension resulting from reduced left atrial compliance.

Biological alternatives do not yield better outcomes, except for the advantage of avoiding anticoagulation. Options include the pulmonary autograft in the mitral position (Ross II), which is technically complex, rarely documented, and presents the significant drawback of early degeneration, making its use anecdotal. Another alternative is the implantation of a bovine jugular vein bioprosthesis (Melody). It is a viable solution for small mitral annuli (<15 mm), allowing subsequent balloon dilation as the patient grows. However, it also suffers from structural degeneration, with most requiring replacement within two years of implantation. Additionally, it carries a higher risk of paravalvular leakage, left ventricular outflow obstruction, and pulmonary vein obstruction. Consequently, the decision between mechanical and biological valve replacement is not straightforward and is primarily guided by the mitral annular size, which must be 8 mm or larger. A mechanical prosthesis can be forced into annuli up to 12 mm, relegating the biological option to smaller annuli.

This study has multiple limitations: a very small sample size, its retrospective nature, and the inclusion of different implantation techniques (annular and supra-annular). No data are available for patients who died during follow-up. The favorable outcomes compared to similar series may be due to institutional idiosyncrasies.

In conclusion, mitral valve replacement should be avoided whenever repair is possible, even if suboptimal. However, if no other option remains, one must fasten their seatbelt and prepare for the “Highway to hell.”

REFERENCE:

Mills M, John M, Tang R, Fundora MP, Keesari R, et al. Mitral Valve Replacement in Infants and Children: Experience Using a 15-mm Mechanical Valve. Ann Thorac Surg. 2023 Aug;116(2):322-329. doi: 10.1016/j.athoracsur.2023.04.035.