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Ann Thorac Surg 2003;76:853-859
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Mitral valve replacement in children: predictors of long-term outcome

^a Department of Pediatric Cardiology, University of Arkansas for Medical Sciences, Arkansas Children's Hospital, Little Rock, Arkansas USA
^b Department of Pediatric and Congenital Cardiac Surgery, University of Arkansas for Medical Sciences, Arkansas Children's Hospital, Little Rock, Arkansas USA
^c Department of Biostatistics, University of Arkansas for Medical Sciences, Arkansas Children's Hospital, Little Rock, Arkansas, USA

Accepted for publication March 25, 2003.

^* Address reprint requests to Dr Drummond-Webb, Pediatric Cardiovascular Surgery, Arkansas Children's Hospital, 800 Marshall St, Slot 677, Little Rock, AR 72202-3591, USA.
e-mail: drummond-webbjonathan{at}uams.edu

	Abstract

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 
BACKGROUND: Mitral valve replacement (MVR) in children has been associated with a high complication rate. We sought to assess predictors of outcomes in children undergoing MVR.

METHODS: A retrospective review of clinical, surgical, and echocardiographic records of patients undergoing MVR was performed. Between 1982 and 2000, 53 children underwent 76 MVR procedures at a median age of 5 years (range, 1 day to 18 years) and weight of 17 kg (range, 3 to 121 kg). Eighteen patients (34%) had more than one MVR. Previous cardiac surgery had been performed in 39 (74%), with 27 (51%) undergoing previous mitral repair. Patients were followed for 9.2 ± 4.8 (range, 2 to 20) years.

RESULTS: There were 14 patient deaths, with 6 patients dying within 30 days, and five transplants (36%). Ten-year freedom from reoperation was 66%. Long-term survivors were older at initial repair (7.0 vs 2.5 years, p = 0.02), with a lower incidence of residual cardiac lesions (3% vs 37%, p < 0.001) and a lower incidence of surgical procedures at the time of MVR (31% vs 63%, p = 0.04). Survivors had better left ventricular function preoperatively (ejection fraction, 68% vs 54%; p = 0.001) and placement of a prosthetic valve within 1 z-score of the echocardiographically measured mitral valve annulus (p = 0.02).

CONCLUSIONS: Adverse outcome after MVR is common, particularly in the young child undergoing palliative surgery or requiring additional surgical procedures. Preoperative assessment of mitral valve size and ventricular function is essential for risk stratification of these patients.

	Introduction

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 
Mitral valve (MV) abnormalities are noted in a variety of clinical situations in children, but are most commonly seen in the setting of a partial or complete atrioventricular septal defect, multiple left-sided obstructive lesions, or isolated congenital mitral valve dysplasia. Whereas techniques in mitral valve repair for both the regurgitant and stenotic valve have continued to develop [1–3], a number of children continue to require mitral valve replacement (MVR) during childhood.

The optimal timing of surgical intervention in the adult patient with mitral valve regurgitation remains imprecise and controversial [4]. Whereas the presence of New York Heart Association (NYHA) class II and III symptoms have been found to predict a higher postoperative mortality in this group of patients, several echocardiographic variables have been shown to correlate with a lower long-term survival. A preoperative ejection fraction (EF) less than 60% has been shown to be associated with a 19% lower long-term survival, and an EF less than 50% has been shown to be associated with a 40% reduction in long-term survival [5]. Not only are the indications for MVR very different in the child, but the comorbid lesions are also incomparable, making recommendations for the timing of MVR in the adult not applicable to the child.

In a recent multicentered review of MVR in children, Caldarone and associates noted that in addition to the anatomical diagnoses of atrioventricular septal defect and Shone's complex, an increased prosthetic mitral valve-to-body weight ratio was associated with higher early postoperative mortality [6].

Whereas the study raised the important issue of appropriate choice of valve size, the authors themselves recognize a significant study limitation in that predicted mitral valve annulus dimensions based on weight were used for comparison with prosthetic valve size as actual valve dimensions were not available.

The present study was undertaken as an attempt to define the historical outcome of a consecutive series of children undergoing MVR at a single, tertiary institution. We attempted to define clinical and echocardiographic features that may aid in identifying those patients at increased risk, particularly the relationship between the prosthetic mitral valve and actual MV dimensions.

	Patients and methods

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 
Patients
From an institutional review board–approved cardiac surgery database, we identified all consecutive patients undergoing prosthetic MVR with an age at initial MVR of less than 18 years. Patient diagnoses, age, weight, and body surface area at initial surgery, previous MVR, and presence of trisomy 21 were noted. The number of cardiac surgical procedures before MVR, the number of MVRs, the need for other surgical procedures at the time of MVR, and residual lesions after MVR were noted. Patients were followed until the time of death, transplant, or last clinic visit. Follow-up was comprehensive, with only 1 patient lost to follow-up.

Echocardiography
All patients underwent echocardiographic assessment immediately before surgical intervention, in the postoperative period, and at long-term follow-up. On the preoperative echocardiogram, the MV annulus was measured from hinge point to hinge point in the apical four-chamber view. The dimension was expressed as an absolute value, and as a z-score based on previously published normative data for body surface area [7]. Mitral stenosis and mitral insufficiency were noted to be present or absent based on color and pulsed-wave Doppler interrogation of the valve.

Associated lesions were noted. M-mode analysis was used for measurement of left atrial dimension, left ventricular end diastolic and end systolic dimensions, and calculation of left ventricular ejection fraction. Left atrial and ventricular dimensions were expressed as absolute values and as values indexed to body surface area. Left ventricular function was assessed using M-mode calculation of the EF from the parasternal short axis view on the preoperative, perioperative, and follow-up echocardiograms.

Prosthetic MV sizing
The MV dimension obtained from the preoperative echocardiogram was used to determine if prosthetic MV mismatch was present. The z-score of the patient's native MV was calculated based on the graphs of Daubeney and associates [7], and the z-score of the prosthetic MV was estimated in the same manner. If the difference between the echocardiographically determined MV z-score and prosthetic valve z-score was greater than or equal to 1, prosthetic valve mismatch was noted to be present. The group of patients receiving a mismatched valve was then subdivided into those patients in whom the prosthetic valve was disproportionately large and those in whom the valve was disproportionately small.

Statistical analysis
The characteristics of the study population are expressed as frequencies, medians with ranges, or means with standard deviations, as appropriate. Of interest were the clinical and echocardiographic characteristics of survivors versus treatment failures (patient death or transplant). Continuous variables were compared using a t test or a nonparametric Mann Whitney U test where the data were skewed. Categorical variables were compared using Pearson, ², or the Fisher's exact test, as needed. Time to the event of death or transplant was examined with Kaplan-Meier curves and differences between groups were compared by the Wilcoxon Rank test. A Cox proportional hazards model was used to explore the interrelated effects of risk factors on the time to event. A forward stepwise regression was used, and checked with a backward regression. Values of p less than 0.05 were considered to indicate statistical significance. SPSS software was used (SSPS Inc, Chicago, IL).

	Results

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 
Patient characteristics
Fifty-three children, aged 1 day through 18 years (mean, 6.4 years ± 5.3) at the time of initial MVR, were identified (Table 1). Thirty-five children underwent a single MVR, 13 underwent two MVRs, and five children underwent three MVRs. Twenty-six (49%) of the patients were boys. Mean weight was 22.7 ± 20.8 kg. Mean body surface area (BSA) was 0.78 ± 0.47 m². Previous cardiac surgery had been performed in 39 (74%), with 27 (51%) subjected to previous surgical mitral valve repair. Primary cardiac diagnoses included 15 patients with an atrioventricular septal defect (28%), 7 with congenital dysplastic MV (13%), 6 with parachute MV (11%), 6 with Shone's complex and mitral stenosis (11%), 9 with complex unbalanced ventricular morphology and mitral regurgitation (17%), 4 with bacterial endocarditis of an underlying normal MV (8%), 3 with isolated mitral valve prolapse (6%), 1 with hypertrophic cardiomyopathy (2%), 1 with Marfan's syndrome (2%), and 1 with anomalous left coronary artery (2%). Indications for initial MVR included mitral insufficiency in 36 patients, mitral stenosis in 10, and combined stenosis and regurgitation in 7.

Eleven patients were left with residual cardiac lesions, including residual ventricular septal defects in 2, left ventricular outflow tract obstruction in 3, right ventricular outflow tract obstruction in 2, a dilated aorta in the setting of Marfan's syndrome in 1, hypertrophic cardiomyopathy in 1, and single ventricle physiology in 3. The degree of outflow tract obstruction or ventricular septal defect shunting was not felt to require or be amenable to any further surgical intervention at the time of valve replacement.

Echocardiographic assessment
Preoperative echocardiography demonstrated a left atrial dimension of 32 ± 11 mm, with an indexed left atrial dimension of 57 ± 30 mm/m². Left ventricle end-diastolic dimension was 38 ± 13 mm, and 64 ± 31 mm/m² indexed to BSA. Left ventricle end-systolic dimension was 26 ± 9.5 mm, and 44 ± 25 mm/m² indexed to BSA. Mean MV annulus was 22 ± 7.3 mm (range, 8 to 35 mm), with a median z-score of -0.2 (-5.8 to +4.7). The median z-score for the prosthetic valves was +0.7 (-3.7 to +4.4). The median z-score difference (prosthetic MV z-score - actual MV z-score) was +0.8 (-1.5 to +8.1). Mean preoperative ejection fraction was 63% ± 15%. Post-MVR echocardiography at long-term follow-up revealed an average peak gradient across the prosthetic mitral valve of 17 ± 12 mm Hg in survivors. Ventricular function was reduced (EF < 50%) in seven out of 32 (22%) long-term survivors.

	Factors associated with survival after MVR

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 
Clinical predictors
The clinical characteristics of the study population are summarized in Table 3. For the purposes of this study, patients were treatment failures if they died or required heart transplant. When compared with treatment failures, long-term survivors were older at the time of initial MVR (7.0 vs 2.5 years, p = 0.02), were less likely to have had associated surgical procedures performed (31% vs 63%, p = 0.04), were less likely to have residual cardiac lesions (3% vs 37%, p < 0.001), and were not as likely to require permanent pacemaker implantation (15% vs 47%, p = 0.02).

View larger version (22K): [in this window] [in a new window]   Fig 1. Survival by age category. Solid bars = nonsurvivors; shaded bars = survivors.

Complications of endocarditis, thrombosis, and stroke were rare, occurring in 0, 6 (11%), and 1 (2%) patients, respectively, whereas the need for repeat mitral valve replacement was high, with 18 patients (34%) requiring two or more MVR and 5 (9%) patients requiring three or more MVR. Repeat MVR was well tolerated, with no early postoperative deaths in this group of patients.

Table 5 summarizes the characteristic of the treatment failures. Six patients died within 30 days after MVR, with a median survival time of 11.5 days (range, 0 to 19 days). Causes of early death included ventricular dysfunction, thrombosis of the prosthetic valve, and infection. Of the 47 surviving patients, there were eight late deaths. Median survival time in this group was 3.7 years (range, 147 days to 12.6 years). Late deaths occurred secondary to ventricular dysfunction with associated pulmonary hypertension or sudden cardiac arrest of uncertain etiology. Five patients received orthotopic heart transplants; median time post-MVR to transplant was 4.0 years (range, 2.0 to 12.7 years). Progressive left ventricular dysfunction with associated hemodynamic compromise was the basis for transplant in all patients. At late follow-up, 7 survivors (20%) had reduced cardiac function (EF < 50%), with 3 (8%) patients having at least NYHA II symptoms.

View larger version (17K): [in this window] [in a new window]   Fig 2. Kaplan-Meier curve demonstrating cumulative freedom from death or transplantation for patients receiving a well-matched prosthetic mitral valve (z-score difference < 1) versus a mismatched prosthetic mitral valve (z-score difference 1).

View larger version (18K): [in this window] [in a new window]   Fig 3. Kaplan-Meier curve demonstrating cumulative freedom from death or transplantation for patients with a preoperative EF of greater than 60% versus less than or equal to 60%. (EF = ejection fraction by echo before mitral valve replacement.)

View larger version (17K): [in this window] [in a new window]   Fig 4. Kaplan-Meier survival curve demonstrating freedom from death or transplantation for patients receiving a matched prosthetic mitral valve versus small mismatched prosthetic mitral valve (valve size < 90% predicted annulus) versus large mismatched (valve size > % predicted annulus) prosthetic mitral valve based on estimated mitral valve annulus size by body surface area (BSA).

Caldarone and associates noted that geometric disparity based on calculations of prosthetic valve size to patient body weight impacted on operative mortality [6]. Our data, measuring the difference between z-scores for the MV annulus by echocardiography and the prosthetic valve size, supports this conclusion. In addition to the possibility of contributing to left ventricle outflow tract obstruction, restriction of prosthetic valve mobility, and injury to the conduction system, prosthetic valve mismatch may lead to progressive left ventricular dysfunction. Attempts to "oversize" a MV prosthesis to avoid reoperation may further aggravate this problem.

Clearly, the intraoperative decision-making process can be divided into two broad categories: those patients who are too small (mitral annular dimension and BSA) to receive an appropriately sized prosthesis; and those patients who would intuitively be advantaged from inserting a larger MV prosthesis.

In those patients in whom a "no choice" situation exists, the need for the valve to fit the patient is critical for survival. However, the deliberate oversizing of the mitral prosthesis may be deleterious, and accepting more frequent reoperation, with a higher trans-mitral prosthetic gradient for a shorter period of time may be prudent. Avoiding oversizing may be the only modifiable choice in a situation with limited alternatives.

As has been reported elsewhere, this study confirmed a reduced preoperative EF to be a significant predictor of a poor outcome after MVR. Aggressive pre- and postoperative medical management of those patients identified to be at higher risk (EF < 60%) may theoretically improve outcomes, but this remains to be seen.

The results of this paper must be interpreted in the light of its limitations. We acknowledge the potential inaccuracies that may present in this retrospective analysis of a historical cohort. During this time, advancements have been made in cardiopulmonary bypass, myocardial protection, surgical techniques, prosthetic valve refinements, postoperative, and imaging techniques, which may have impacted on outcomes. Despite this, we were unable to identify differing outcomes in those patients repaired in the current surgical era (after 1990) and those repaired before 1990.

In conclusion, a mismatch of the dimensions of the MV annulus and the prosthetic MV is associated with a reduced survival. Routinely oversizing the prosthetic MV to allow for somatic growth, in an attempt to reduce the rate of reoperation, is not without risk and should be discouraged. Mitral valve replacement as a palliative procedure in patients with residual cardiac lesions is associated with particularly poor outcomes. Alternative therapy such as transplantation should perhaps be considered early on in those higher-risk cases.

	Acknowledgments

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 
We acknowledge the surgical contributions of Drs Stephen H. Van Devanter, James E Harrell, Richard G. Westerman; Carl W. Chipman, RN; and the cardiologists and nurses of the Arkansas Children's Hospital Cardiovascular Center in Little Rock, Arkansas.

	References

Top Abstract Introduction Patients and methods Results Factors associated with survival... Acknowledgments References

 

1. Aharon A.S., Laks H., Drinkwater D.C., et al. Early and late results of mitral valve repair in children. J Thorac Cardiovasc Surg 1994;107:1262-1270.[Abstract/Free Full Text]
2. Yoshimura N., Yamaguchi M., Oshima Y., et al. Surgery for mitral valve disease in the pediatric age group. J Thorac Cardiovasc Surg 1999;118:99-106.[Abstract/Free Full Text]
3. Poirier N.C., Williams W.G., Van Arsdell G.S., et al. A novel repair for patients with atrioventricular septal defect requiring reoperation for left atrioventricular valve regurgitation. Eur J Cardiothorac Surg 2000;18:54-61.[Abstract/Free Full Text]
4. Ling L.H., Enriquez-Sarano M., Seward J.B., et al. Early surgery in patients with mitral regurgitation due to flail leaflets. Circulation 1997;96:1819-1825.[Abstract/Free Full Text]
5. Enriquez-Sarano M., Tajik A.J., Schaff H.V., Orszulak T.A., Bailey K.R., Frye R.L. Echocardiographic prediction of survival after surgical correction of organic mitral regurgitation. Circulation 1994;90:830-837.[Abstract/Free Full Text]
6. Caldarone C.A., Raghuveer G., Hills C.B., et al. Long-term survival after mitral valve replacement in children aged < 5 years. Circulation 2001;104(Suppl I):I143-I147.
7. Daubeney P.E., Blackstone E.H., Weintraub R.G., Slavik Z., Scanlon J., Webber S.A. Relationship of the dimension of cardiac structures to body size: an echocardiographic study in normal infants and children. Cardiology in the Young 1999;9:402-410.[Medline]
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