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European Heart Journal Advance Access originally published online on September 27, 2007
European Heart Journal 2007 28(21):2620-2626; doi:10.1093/eurheartj/ehm393
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2007. For permissions please email: journals.permissions@oxfordjournals.org

Low-gradient aortic stenosis: impact of prosthesis-patient mismatch on survival

Jean-Luc Monin1,*, Mehran Monchi3, Matthias E.W. Kirsch2, Hélène Petit-Eisenmann4, Serge Baleynaud5, Christophe Chauvel6, Damien Metz7, Catherine Adams8, Jean-Paul Quere9, Pascal Gueret1 and Christophe Tribouilloy9

1 Department of Cardiology, Assistance Publique-Hôpitaux de Paris, Henri Mondor Hospital, 51 avenue De Lattre de Tassigny, 94010 Créteil, France
2 Department of Cardiac Surgery, Assistance Publique-Hôpitaux de Paris, Henri Mondor Hospital, Créteil, France
3 Department of Intensive Care Medicine, Institut Jacques Cartier, Massy, France
4 Department of Cardiac Surgery, University Hospital, Strasbourg, France
5 General Hospital, Lorient, France
6 Clinique Saint-Augustin, Bordeaux, France
7 University Hospital, Reims, France
8 General Hospital, Argenteuil, France
9 University Hospital, Amiens, France

Received 2 May 2007; revised 19 July 2007; accepted 23 August 2007; online publish-ahead-of-print 27 September 2007.

* Corresponding author. Tel: +33 1 49 81 23 30; fax: +33 1 49 81 28 83. E-mail address: jean-luc.monin{at}hmn.aphp.fr


   Abstract
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 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusions
 Funding
 References
 
Aims: To assess the prognostic impact of prosthesis-patient mismatch (PPM) in a large consecutive series of patients operated for low-gradient aortic stenosis (AS).

Methods and results: Outcomes after surgery for low-gradient AS were prospectively assessed in 152 consecutive patients from seven institutions. There were 113 men (74%); mean age was 72 years (64–76); valve area, 0.7 cm2 (0.6–0.8); left ventricular (LV) ejection fraction 0.31 (0.25–0.37) and baseline mean transaortic pressure gradient (MPG), 30 mmHg (25–35) Among 139 patients with available prosthetic valve effective orifice area (EOA), PPM (defined by an indexed EOA ≤ 0.85 cm2/m2) was present in 79 patients (57%) and had no significant impact on post-operative mortality. Independent predictors of overall mortality were LV contractile reserve [hazard ratio (HR) 0.52; 95% confidence interval (CI) 0.35–0.78; P = 0.002], associated coronary artery bypass grafting (HR 1.87; 95% CI 1.24–2.82; P =0.003), baseline MPG (per 1 mmHg decrease to 10 mmHg; HR 1.03; 95% CI 1.01–1.06; P = 0.021), previous cancer (HR 2.13; 95% CI 1.05–4.29; P = 0.037), and logistic EuroSCORE (per 1% increase; HR 1.02; 95% CI 1.01–1.04; P = 0.040).

Conclusion: In this large multicentre series of patients with low-gradient AS, we found that PPM (moderate in most cases) had no influence on post-operative mortality. Therefore, the performance of more complex interventions in order to avoid moderate PPM may not be justified in the setting of low-gradient AS, because their higher risk probably outweighs the expected benefit.

Key Words: Valves • Stenosis • Surgery • Prognosis


   Introduction
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 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusions
 Funding
 References
 
Surgical valve replacement is currently the treatment of choice for most patients with severe aortic stenosis (AS) and severe left ventricular (LV) dysfunction resulting in low transaortic pressure gradient, a condition referred to as low-gradient AS.1 In this setting, operative risk stratification and evaluation of post-operative outcome remain challenging issues that have been addressed in a limited number of studies.2–7 Valve prosthesis-patient mismatch (PPM) has been described when the implanted prosthetic valve is of insufficient size considering the patient’s body surface area.8 Several studies have reported that PPM in the aortic position results in lower post-operative survival.9–15 The impact of PPM on post-operative mortality depends on the severity of mismatch9,12 and the degree of pre-operative LV dysfunction;9,14 thus, PPM should theoretically have a major impact on low-gradient AS. Of note, in the only study addressing this issue, LV contractile reserve was not assessed.7 Conversely, we have reported that LV contractile reserve assessed by dobutamine haemodynamics was a strong predictor of post-operative outcome in low-gradient AS, but the influence of PPM was not evaluated in our study.3 Therefore, the aim of the present study is to assess the prognostic impact of PPM in a large consecutive series of patients operated for low-gradient AS with pre-operative assessment of LV contractile reserve.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusions
 Funding
 References
 
Patient population
From April 1994 to March 2005, 211 consecutive patients were evaluated at seven medical centres in France for low-gradient AS and follow-up data were prospectively collected at each centre. Inclusion criteria were the presence of severe symptomatic AS (valve area ≤ 1 cm2 or indexed valve area ≤ 0.6 cm2/m2) with low pressure gradient [mean transaortic pressure gradient (MPG) <40 mmHg] and low LV ejection fraction (LVEF ≤ 0.40)1 or low cardiac index (<3.0 L/min/m2).3 Exclusion criteria were severe extra cardiac comorbidities (life expectancy shorter than 1 year), more than mild aortic or mitral regurgitation, and atrial fibrillation. The 152 patients who underwent aortic valve replacement form the population of this study. Ninety-five patients from this series were included in a previous report3; enhanced data from these 95 patients (including extended follow-up, logistic EuroSCORE and surgical data) were included in the present series. This study was approved by local institutional review boards and informed consent was obtained from the patients before all procedures.

Pre-operative evaluation
All 152 patients underwent a comprehensive Doppler echocardiographic study at baseline and a Dobutamine Doppler haemodynamic study using commercially available ultrasound systems. Transaortic gradients were calculated using the simplified Bernoulli equation. Aortic valve area was calculated by the continuity equation. LVEF was calculated according to biplane Simpson’s rule or visually estimated. After baseline measurements, dobutamine was infused at 5 µg/kg/min, titrated upward in steps of 2.5 µg/kg/min every 5 min up to a maximal dose of 20 µg/kg/min. The infusion was stopped when the maximal dose or heart rate acceleration more than +10 b.p.m. was reached. Left ventricular contractile reserve was defined as a dobutamine increase in stroke volume of ≥20% compared with baseline value.3 Pre-operative coronary angiography was performed in all cases.

Logistic EuroSCORE
The operative risk (expressed as a percentage) was evaluated retrospectively for each patient by the logistic EuroSCORE (http:www.euroscore.org). Integrated risk factors in the EuroSCORE are patient-related, cardiac factors, or operation-related. Patient-related factors are age over 60, female sex, chronic pulmonary disease, extra-cardiac arteriopathy, neurological dysfunction, previous cardiac surgery, renal failure, active endocarditis, and critical pre-operative state. Cardiac factors are unstable angina, reduced LVEF, recent (<90 days) myocardial infarction, and pulmonary systolic pressure >60 mmHg. Operation-related factors are emergency, other than isolated coronary surgery, thoracic aorta surgery, and surgery for post-infarct septal rupture.16

Surgical data and prosthesis-patient size
The following data were obtained from the Cardiac Surgery Departments: model and size of the inserted aortic valve prosthesis, associated coronary artery bypass procedure, duration of cardiopulmonary bypass, and aortic cross-clamp time. The effective orifice area (EOA) of the inserted valve prosthesis was available for 139 patients; it was taken from the in vivo reference values used by Blais et al.,9 except for the following prosthesis: Medtronic Hall,17 ATS,18 Sorin Bicarbon,19 Labcor,20 and the 19 mm Medtronic Mosaic.21 The indexed EOA of the inserted prosthesis for each patient was derived from the reference value divided by the patient’s body surface area (m2). According to current definitions, PPM was defined by an indexed EOA ≤ 0.85 cm2/m2; moderate PPM by an indexed EOA > 0.65 and ≤ 0.85 cm2/m2, and severe PPM by an indexed EOA < 0.65 cm2/m2 9.

Endpoints and follow-up
The study endpoint was all-cause mortality, including in-hospital mortality. Cardiovascular deaths were assessed as a secondary endpoint. Peri-operative and sudden deaths were classified as cardiovascular deaths. Patients were followed up by their referring physician. Investigators from each participating centre prospectively collected follow-up data on a yearly basis and patient’s data were censored in the case of death. Last follow-up data were collected by the investigators between December 2005 and March 2006. Follow-up was completed in all patients at a median interval of 44 months (range 24–67 months) by direct patient examination or telephone interview.

Statistical analysis and survival modelling
Continuous data are presented as median values and corresponding 25th and 75th percentiles. Survival models analyse time-to-event data to identify predictors of outcome. The most common approach, described by Cox assumes constant hazard ratios (HRs) throughout a subject’s time course. This assumption is known as the ‘proportional hazards’ assumption. However, this assumption probably does not hold in cardiac surgery, because the HR for death will likely decrease after the peri-operative period. Gray’s piecewise-constant time-varying coefficients survival model22 is a new approach that directly estimates how the hazards from individual risk factors change over time and therefore may be better suited for modelling survival after surgery. We constructed single-variable models using Gray’s extension of the Cox proportional hazards model, which allows the regression coefficients to change over different time intervals.22 We fit these models using five intervals: the peri-operative period (first month) and four subsequent time intervals, each containing approximately equal numbers of events (deaths), as per Gray’s recommended approach.22 The duration of each interval was determined automatically, based on when the events occur. Gray’s model has its own test of proportionality and each single variable was tested for non-proportionality, with a P < 0.10 assuming non-proportionality. On the basis of the results of these single-variable models, we constructed a multivariable model using stepwise forward selection with a significance level of 0.10. For each factor, a specific HR for each of five time intervals was found. For proportional variables, the mean of these values of HR were reported with the mean of 95% confidence interval (CI) limits. For non-proportional variables, the range from minimum to maximum HR is reported with a graph showing the variation of the HR with time. The overall significance of effect of non-proportional risk factors on survival is given by the P-value. The effect of PPM on survival was analysed using two approaches: first with the indexed EOA entered as a continuous variable and then with PPM entered as a dichotomous variable depending on its absence (indexed EOA > 0.85 cm2/m2) or presence (indexed EOA ≤ 0.85 cm2/m2). These methods were applied first for global mortality and then for cardiovascular mortality. Two-tailed P-values less than 0.05 were considered statistically significant. We used STATA 9.2 (Stata Corporation, College Station, TX, USA) and the R Project for Statistical Computing (release 2.3.1) software packages for statistical analysis. Gray’s program, written for R, is available from the author’s website (http://biowww.dfci.harvard.edu/~gray/) and is compatible with the 2.3.1 release of R.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusions
 Funding
 References
 
Patient’s characteristics and operative data
Baseline clinical, haemodynamic, and surgical data for the 139 patients with available EOA are detailed in Table 1. All patients underwent surgery at a median interval of 32 days (15–50) after initial evaluation. The numbers, types, and sizes of the inserted prostheses are outlined in Table 2. The inserted prosthetic valves were fairly balanced between mechanical and bioprosthetic valves. Furthermore, most of the bioprosthetic valves were stented prostheses (n = 60/71, 85%); stentless valves were implanted in only 11/71 patients (15%).


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  		Table 1 Baseline clinical, haemodynamic, and surgical data for the whole group with available indexed effective orifice area and according to presence or absence of prosthesis-patient mismatch

 

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  		Table 2 Types and sizes of prosthetic valves implanted in the 139 patients with available indexed effective orifice area

 
Prevalence of prosthesis-patient mismatch
PPM was present in 79 patients (57%); it was moderate in most cases (n= 72 patients, 52%) and severe in seven patients (5%). Therefore, considering the small number of severe PPM, patients with moderate and severe PPM were grouped for analysis. Patients with PPM were significantly older, with a higher prevalence of coronary artery disease (CAD) and bioprosthetic valves, higher logistic EuroSCORE, and a trend towards a higher prevalence of female gender (Table 1). Otherwise, there was no significant difference in body mass index, NYHA functional class, prevalence of LV contractile reserve, or extra-cardiac comorbidities between the two groups (Table 1).

Post-operative outcomes
Eighteen patients died within 30 days after surgery or before hospital discharge, thus operative mortality was 12%. Forty-six additional patients died after the post-operative period at a mean interval of 31 post-operative months (15–57). Among these latter patients, 18 patients died from cardiac causes (congestive heart failure, n= 11; sudden death, n 6; endocarditis, n= 1), 10 from a vascular disease (stroke, n= 8; peripheral vascular disease, n= 2), and 18 from non-cardiovascular causes (infection, n= 7; cancer, n= 4, or miscellaneous non-cardiac causes, n= 7). Survival for the whole group was 82% (95% CI 74–86) and 60% (95% CI 51–68) at 1 and 5 years, respectively.

Predictors of post-operative mortality
Using Gray’s piecewise-constant time-varying coefficients survival analysis, univariable predictors of all-cause mortality were age, LV contractile reserve, EuroSCORE, associated CAD, previous cancer, cardiopulmonary bypass and aortic cross-clamp time, associated coronary artery bypass grafting (CABG), dobutamine LVEF, and baseline and peak MPGs (Table 3). Neither label valve size, EOA or indexed EOA as continuous variables nor moderate–severe PPM as dichotomous variable was predictive of all-cause mortality by univariable analysis (Table 3, Figure 1). Multivariable analysis revealed five independent predictors of all-cause mortality: LV contractile reserve (Figure 2), associated CABG, MPG at baseline, previous cancer and logistic EuroSCORE (Table 4). Of note, the impact of LV contractile reserve on mortality was not constant over time, being important during the early peri-operative period with a subsequent progressive decrease up to 10 months after surgery (Figure 3). With the same methods, we found only 3 independent predictors of cardiovascular mortality: LV contractile reserve, associated CABG and baseline MPG (Table 4).


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  		Table 3 Univariable analysis of risk factors associated with all-cause mortality

 

Figure 1
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  		Figure 1 The Kaplan–Meier survival estimates according to the absence of prosthesis-patient mismatch (solid line) or its presence (dashed line).

 

Figure 2
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  		Figure 2 The Kaplan–Meier survival estimates according to the presence (CR+, solid line) or absence (CR–, dashed line) of left ventricular contractile reserve.

 

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  		Table 4 Independent predictors of outcome (multivariable Gray’s piecewise-constant time-varying coefficients survival analysis)

 

Figure 3
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  		Figure 3 Non-proportionality of the effect of left ventricular contractile reserve on mortality according to Gray’s model. The solid line represents the evolution of the hazard ratio for mortality over time and the dashed lines the 95% confidence intervals.

 

   Discussion
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 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusions
 Funding
 References
 
The insertion of a small-size prosthesis is an issue of concern in patients with AS, because the incomplete relief of left ventricular obstruction might have the same consequences as native AS, namely high transaortic pressure gradients and increased LV afterload leading to the lack of regression of LV hypertrophy and persistent symptoms.23 However, the impact of PPM on survival remains controversial: early post-operative mortality was influenced by PPM in some studies9–11,13 but not in others.24 Conversely, long-term survival was influenced by PPM in some studies,12,15 but not in others.10,11,13,24 In previously published series, cutoff values for severe PPM vary widely, ranging from 0.60 cm2/m2(by Hanayama et al.11) to 1.1 cm2/m2 (by Blackstone et al.10). Furthermore, manufacturer’s labelled size derived from in vitro measurements has been mostly considered for calculation of patient-prosthesis size,10,11,23 the limitation of which is well described.25 Therefore, in the present study, we used in vivo reference values for EOA as proposed by Blais et al.9 who were the first to demonstrate a strong impact of PPM on post-operative mortality.

Moderate vs. severe prosthesis-patient mismatch
In the present study, PPM had no influence on post-operative mortality (including early and long-term outcome), in contrast to three recent studies who found a strong impact of PPM on early post-operative9 or long-term survival.14,15 One could argue that this result is due to the low prevalence of severe PPM in our population. In fact, considering the small number of severe PPM in our series, patients with moderate and severe PPM were grouped for analysis and defined by an indexed EOA ≤ 0.85 cm2/m2, as it was done in previous studies.9,14 Of note, the prevalence of PPM in our series was 57%, that is even more than the 38–47% prevalence reported in the latter series, in which moderate PPM was also the most frequent occurrence.9,14,15 Furthermore, we did not find any relationship between post-operative mortality and indexed EOA entered as a continuous variable, this point being against a proportional effect of PPM according to its degree of severity in our population. Therefore, we assume that our results are mainly valid for moderate PPM, as it was the case in previous studies.9,14,15 However, our conflicting results cannot be explained by a difference in definition or prevalence of PPM when compared with these studies.9,14,15

Possible interaction between prosthesis-patient mismatch and other risk factors
Ruel et al.14 found that late survival after aortic valve surgery was influenced by PPM in the case of pre-operative LV dysfunction, defined by an EF < 50%. In addition, Blais et al.9 insisted on the dramatic increase in early post-operative mortality due to the combination of moderate PPM and depressed LV function; therefore, moderate to severe PPM should theoretically have a major impact on outcome in the setting of severe LV dysfunction, as it is the case for most of our patients. A close look at the data from the two latter studies reveals that in both series, patients with PPM were significantly older, with a higher proportion of women and a higher prevalence of CAD, when compared with the control group without PPM.9,14 In the present series, patients with PPM were also significantly older, with a higher prevalence of CAD, higher logistic EuroSCORE, and a trend towards a higher proportion of females. Furthermore, the series of Blais et al.9 included patients with infectious endocarditis or emergent operation, both items turning out to be independent predictors of mortality. Therefore, despite multivariate analysis, we submit that an interaction between PPM and other strong risk factors (namely age, female gender, LV dysfunction, associated CAD, endocarditis, and emergency operation) remains possible in the two latter studies.26,27 This is in contrast to our series where LV dysfunction is present (by definition) in all patients, and other important risk factors (emergency operation, endocarditis) were excluded. These significant differences in patient’s characteristics might account for the conflicting results between our series and the two latter studies.9,14

Low-gradient aortic stenosis and prosthesis-patient mismatch
The issue of PPM in low-gradient AS was specifically addressed in one recent single-centre study, in which PPM was found to be an independent predictor of recurrent episodes of heart failure and impaired LV mass regression.7 Interestingly, there was a non-significant trend towards increased late mortality in this study (P = 0.084). Of note, neither peri-operative mortality nor LV contractile reserve was assessed in this study; thus, our results are not in contradiction with this latter series.7

Effect of left ventricular contractile reserve on mortality
The present study confirms the major prognostic impact of LV contractile reserve on post-operative outcome in low-gradient AS, as previously reported.3,4 Furthermore, by using Gray’s survival model,22 we were able to demonstrate that the effect of LV contractile reserve on mortality varies over time, with a high impact during the early post-operative period and a progressive decrease until 10 months after the operation. In some studies, PPM had a significant impact on early post-operative survival, but not on long-term outcome.10,11 Added to this, others have demonstrated the deleterious combination of moderate PPM and systolic LV dysfunction on early mortality.9 Taken together, these data suggest that the greatest impact of PPM on mortality should be during the early post-operative period, when the LV is the most vulnerable.9 Therefore, LV contractile reserve and PPM may have most of their impact during the early post-operative period. Another possible explanation to our results is that the strong impact of LV contractile reserve on post-operative mortality in a homogenous population of patients with low-gradient AS overshadows the effect of PPM, due to this homogeneity for severe LV dysfunction. This paradox might explain in part the lack of impact of PPM in our population.

Other independent predictors of survival
Associated CABG is universally associated with an increased operative risk when compared with isolated valve surgery.28,29 This parameter was not predictive of outcome in our previous report,3 in contrast to the present study. We assume that the larger study population and longer follow-up duration (44 vs. 14 months previously) might explain this result. For the same reason, previous cancer was also predictive of all-cause mortality in the present series, as it is the case for most long-term follow-up studies in western populations.30,31 We also found that logistic EuroSCORE was an independent predictor of all-cause mortality in the case of low-gradient AS, as it is the case in large European and North American cohorts of patients undergoing coronary or valve surgery.32 Of note, cardiovascular mortality was not independently predicted by the EuroSCORE, which evaluates global post-operative outcome given that seven of the 17 variables incorporated in the EuroSCORE are not cardiac-related.

Clinical implications
The performance of aortic root enlargement in order to prevent PPM is at the risk of increased operative mortality.33 Therefore, this technique may not be recommended in the case of low-gradient AS, given the already high operative risk and the lack of impact of moderate PPM on survival in these patients. Furthermore, it has been argued that even moderate AS (pseudo-severe AS) might have a detrimental impact on outcome in the setting of low-gradient AS, due to severe LV dysfunction, and that these patients would eventually benefit from valve replacement. This hypothesis is not supported by our results, since moderate PPM can be paralleled with pseudo-severe AS. Therefore, our data support the fact that the assessment of real AS severity by low-dose dobutamine is of clinical interest in the setting of low-gradient AS.

Study limitations
Although most clinical and haemodynamic patient’s data were prospectively collected, the present study is mainly retrospective and observational in design. However, the general consensus is that randomized studies in the setting of low-gradient AS are neither ethical nor feasible34; therefore, most previous studies in this subset were also retrospective.2,5,35–37 We are also aware of the relatively small sample size of our study, but to the best of our knowledge, we report here the largest consecutive series of patients operated on for low-gradient AS. The number of patients with severe PPM was not sufficient in our population to allow a separate analysis; thus we cannot draw any meaningful conclusion with regard to severe PPM in this series and our conclusions are mainly valid for moderate PPM.


   Conclusions
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
Funding
 References
 
We report post-operative outcomes in a large multicentre consecutive series of patients who underwent aortic valve replacement for low-gradient AS. In these patients, we found that PPM (moderate in most cases) has no influence on early post-operative mortality, as well as on long-term post-operative outcome. In contrast, our results confirm that overall post-operative mortality in this setting is mainly influenced by LV contractile reserve, associated CABG, baseline MPG, previous cancer, and the logistic EuroSCORE. Therefore, the performance of more complex interventions in order to avoid moderate PPM may not be justified in the setting of low-gradient AS, because their higher risk probably outweighs the expected benefit.

Conflict of interest: none declared.


   Funding
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
 Funding
References
 
No financial support was received for this study.


   References
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 Results
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 References
 

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