In pediatric patients with mitral valve disease requiring surgery, repair is the preferred treatment. This approach allows time to delay valve replacement, a procedure associated with higher complexity, complication rates, and mortality in this age group compared to adults. The size of the mitral annulus remains a significant limitation. However, not all cases are suitable for repair. 

This article reports the outcomes of the HALO trial conducted in children under five years of age using the 15-mm St. Jude Medical Masters HP® mechanical prosthesis from Abbott®. 

A total of 23 patients from 15 healthcare centers in the United States underwent mitral valve replacement (MVR) between May 2015 and March 2017. Clinical, surgical, echocardiographic, and postoperative variables were analyzed. 

The mean age was 7.8 months (range: 2 weeks to 27.4 months), and the mean weight was 5.5 kg (range: 1.9 to 10.9 kg). Diagnoses included mitral stenosis (n=10), mitral regurgitation (n=6), and combined lesions (n=7). Underlying cardiac conditions included atrioventricular septal defects in 10 patients (7 complete, 1 partial, and 2 transitional) and congenital mitral valve disease in 13 patients. 

Six patients (30%) underwent correction of cardiac anomalies unrelated to mitral valve disease during the same procedure, including pulmonary vein stenosis surgery, pacemaker implantation in one patient with preoperative second-degree atrioventricular (AV) block, ventricular septal defect closure in three patients, and aortic arch repair in two patients. The mean durations of cardiopulmonary bypass and aortic cross-clamping were 154.7 (range: 65.0-273.0) and 90.0 (range: 42.0-190.0) minutes, respectively. 

Mitral valve replacement was performed in all patients: 43.5% (n=10) in the annular position and 56.5% (n=13) in the supra-annular position. Three patients required ECMO support upon weaning from cardiopulmonary bypass; all were successfully decannulated, though one died 45 days later due to severe biventricular dysfunction. 

Six patients died during follow-up. Survival rates were 91.3% at 30 days, and 71.0% at 12 months and 5 years. Procedure-related mortality was 0%. Four patients required pacemaker implantation due to postoperative AV block (two with annular prostheses and two with supra-annular prostheses). 

Adverse events during the five-year follow-up included endocarditis (n=1), anticoagulant-related bleeding (n=5), thromboembolic events (n=1), prosthetic dysfunction (n=1), prosthetic thrombosis (n=4), and prosthetic valve replacement (n=13). All events occurred within 12 months of surgery. 

For mitral prosthetic thrombosis, three of four affected patients required surgical valve replacement. Two received low-molecular-weight heparin (LMWH) anticoagulation, and one had a factor V Leiden deficiency. 

Freedom from bleeding was 85.4% at 30 days and 74.1% at 5 years. None of the patients experienced severe hemorrhage. Thirteen patients underwent valve replacement surgery (9 for patient-prosthesis mismatch, 3 for thrombosis, and 1 for prosthetic stenosis). The median time to replacement for mismatch was 31 months compared to 21 days for thrombosis-related replacement. 

The mean transvalvular gradient was 7.3 mmHg at one year, 10.4 mmHg at two years, and 14.6 mmHg at three years. Echocardiographic analysis at five years in four patients showed no or mild regurgitation in all cases. 

COMMENTARY: 

Mitral valve disease poses a significant surgical challenge in pediatric patients, particularly those of younger age and smaller weight. Whenever possible, repair should be the first-line approach, though this is not feasible for all patients. 

Three critical considerations emerge in pediatric MVR: 

• High procedural mortality: Mortality rates for MVR in this age group range from 10-36% in published series. In the study by Ibezim et al., which included 441 pediatric patients, mortality was 11.1%, with age under two years being the main prognostic factor. In the HALO trial, which included younger patients, 31-day and 5-year survival rates were 91.3% and 71.0%, respectively, with no procedure-related mortality. 

• Chronic anticoagulation: Freedom from bleeding in the HALO trial was 74.1% at five years, a higher incidence compared to other series, likely due to the younger patient population. Regarding prosthetic thrombosis, three of four cases required surgical replacement. Two patients were on LMWH, and one had a factor V Leiden deficiency. A notable limitation highlighted by the authors is the absence of standardized anticoagulation guidelines for pediatric patients. 

• Prosthesis replacement due to growth: Prostheses have fixed diameters, necessitating reoperation as children grow. In this study, nine patients required replacement for patient-prosthesis mismatch (median: 35 months post-surgery). 

The study validates the 15-mm St. Jude Medical Masters HP® prosthesis as an effective option for children under five years ineligible for mitral valve repair, offering context to surgeons, cardiologists, and families confronting this situation. 

The authors advocate avoiding LMWH based on outcomes and emphasize the need for standardized anticoagulation guidelines. Despite the study’s limitations, such as small sample size and absence of a control group, the low rate of severe hemorrhagic events and prosthetic thrombosis emphasizes the prosthesis’s utility. Further, the authors note the need for more extensive long-term studies to refine management in this demographic. 

Alternative options like the Melody® jugular vein prosthesis offer advantages such as potential expansion through percutaneous dilation and no anticoagulation requirement. However, issues like left ventricular outflow tract obstruction and perivalvular leaks remain concerns. 

One innovative technique, the “chimney technique,” developed at Hospital Infantil La Paz, uses a Dacron conduit segment for supra-annular mitral valve positioning and enlarges the left atrium with a heterologous pericardial patch to prevent pulmonary vein return issues. 

At our center, we consider the Melody® prosthesis as the second-line option for small mitral annuli. Our series includes six patients, with satisfactory initial outcomes. However, limited durability (median: 3 years) remains a significant limitation. In two cases with a 13-mm annulus, the chimney technique achieved a prosthesis duration of two years. 

Regarding experience with the 15-mm St. Jude Medical Masters HP® mechanical prosthesis, we implanted it in one patient who experienced two episodes of thrombosis. The first resolved with fibrinolysis; after the second, the valve was replaced with a Melody® prosthesis. 

In summary, significant challenges persist in the management of pediatric patients ineligible for mitral valve repair. The ideal valvular substitute remains elusive for this age group, with all options linked to high morbidity, mortality, and limited durability. More studies are needed to evaluate long-term outcomes and refine treatment strategies. 

REFERENCE: 

Shaw FR, Kogon B, Chen J, Mitchell MB, Fraser C, Kanter K. Mitral Valve Replacement in Infants and Children: Five-Year Outcomes of the HALO Clinical Trial. Ann Thorac Surg. 2024 Aug;118(2):449-457. doi: 10.1016/j.athoracsur.2024.04.025.