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Ann Thorac Surg 2006;82:530-536
© 2006 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Prosthesis–Patient Mismatch is Not Clinically Relevant in Aortic Valve Replacement Using the Carpentier-Edwards Perimount Valve

^a Department of Cardiac Surgery, University Clinic Gasthuisberg, Leuven, Belgium
^b Department of Cardiology, University Clinic Gasthuisberg, Leuven, Belgium

Accepted for publication March 28, 2006.

^_* Address correspondence to Prof Dr Flameng, Cardiac Surgery, University Clinic Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium (Email: willem.flameng{at}med.kuleuven.be).

	Abstract

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BACKGROUND: Previous studies have shown that prosthesis–patient mismatch (PPM) results in higher early and late mortality after bioprosthetic aortic valve replacement. Careful selection of stented bioprostheses was recommended to avoid inadequate effective orifice area. We studied the incidence of PPM and its potential effects on clinical outcome in patients undergoing aortic valve replacement using the Carpentier-Edwards Perimount bioprosthesis.

METHODS: Independent predictors of early and late mortality and hospital readmission for cardiac reasons were defined in 506 patients (mean age, 73 years; range, 57 to 87 years) by multivariate analysis. Mean follow-up was 6.1 ± 4.8 years; maximum follow-up was 18.6 years.

RESULTS: The incidence of severe PPM (effective orifice area index < 0.65 cm²/m²) was 0.2% and of moderate PPM (effective orifice area index > 0.65 and < 0.85) was 20%. Multivariate analysis revealed that moderate PPM was not an independent predictor of early mortality, late mortality, or hospital readmission for cardiac reasons. Reduction of septal hypertrophy was similar in patients with and without moderate PPM.

CONCLUSIONS: The incidence of severe PPM is virtually nonexistent after aortic valve replacement using the Carpentier-Edwards Perimount valve. Moderate PPM is found in 20% of cases and is clinically irrelevant in this population.

	Introduction

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Prosthesis–patient mismatch (PPM) was defined by Rahimtoola [1] as being present when the effective orifice area (EAO) of the prosthesis being implanted is less than that of the individual's normal native valve. For prosthetic valves, except maybe stentless valves implanted as root replacement, the presence of the sewing ring, stents, or leaflet housing will always reduce the available orifice area and consequently induce some degree of PPM. The question, however, remains from which extent such a reduction in orifice area becomes critical and clinically relevant in terms of reduction of survival, inhibition of regression of hypertrophy, and association of cardiac morbidity. To define such a critical measure of PPM, EOA must be normalized for patient size, thereby producing a new value, the EOA index (EOAI). In the contemporary literature most studies use body surface area to do so. This, however, does not solve completely the problem of PPM quantification and determination of its critical degree. Data for EOA obtained in vitro can be used and projected on the patients, after correction for their body surface area to calculate PPM. Other clinical studies, however, prefer not to use in vitro EOA data, but the in vivo EOA values obtained from other clinical reports to project these values on the patients to calculate PPM.

This nonuniform mode of assessment of PPM makes it difficult to appreciate the results and induces considerable controversy about the topic [2, 3]. We believe that projected EOA values can be used, provided a close correlation is shown between several data sources for EOA, both in vitro and as echocardiographically obtained in vivo EOA values. In the present study, we made this comparison to calculate a reliable estimate of EOA for each valve size of the Perimount valve.

According to Pibarot and Dumesnil [3], the generally accepted criteria for PPM are an indexed EOA of less than 0.85 cm²/m² for moderate PPM and less than 0.60 cm²/m² for severe PPM. We used these criteria in the present study and calculated EOAI from both in vitro and in vivo obtained measurements of the EOA of the Carpentier-Edwards (CE) Perimount valves. The purpose of the study was to look for any association between PPM and in-hospital and late mortality, regression of left ventricular hypertrophy, and hospital readmission for cardiac reasons after aortic valve replacement using the CE Perimount valve.

	Patients and Methods

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Patient Population
The study population included 506 patients undergoing single aortic valve replacement between December 1985 and October 2003. All patients received a CE Perimount bioprosthesis. No annulus enlargements were performed. The ethics committee of the University Hospitals Leuven approved for the study. Preoperative and operative data are listed in Table 1.

Effective Orifice Area Index
The EOA index was calculated by means of echocardiographic-projected EOA values. This means that in 122 patients from this study the EOA was measured echographically before discharge from the hospital. A linear regression curve was fitted through the echographically measured EOA data and implanted valve size (Fig 1A). This linear regression curve is superimposed on a curve through the in vitro measured EOA values provided by the manufacturer and three other in vitro data sources (Fig 1B). In vitro data at different flow levels were included. Then, reported clinical EOA data were collected from nine different clinical papers studying the Perimount valve and a similar regression was made through it (Fig 1C). When EAO data were reported as means with a standard deviation, three values were added to the plot: the mean, the mean minus one standard deviation, and the mean plus one standard deviation. All three curves were almost identical with overlapping confidence limits (Fig 1D). For each valve size, the correspondent projected EOA value could be calculated using the mutual linear regression curve through all values. This way, the following fixed EOA values were calculated: 1.3, 1.5, 1.7, 2.0, 2.1, and 2.4 cm² for 19, 21, 23, 25, 27, and 29 mm valves, respectively. These values were indexed for body surface area from the individual patient. The criteria for PPM are an indexed EOA of less than 0.85 cm²/m² for moderate PPM and less than 0.65 cm²/m² for severe PPM.

View larger version (23K): [in this window] [in a new window]   Fig 1. Three separate linear regressions are shown. (A) Echocardiographic effective orifice area (EOA) determinations in 122 of our patients. (B) In vitro EOA data for the Carpentier-Edwards Perimount valve from four different data sources (Manufacturer; Marquez et al., J Heart Valve Dis 2001; Frater et al., Ann Thorac Surg 1992; Rao et al., Circulation 2000). (C) Collected echocardiographic EOA data originating from nine separate clinical studies (Cosgrove, Circulation 1985 (n = 81); Seitelberger, Eur J Cardiothorac Surg 2003 (n = 39); Salomon NW, Circulation 1991 (n = 50); Frater RW, Ann Thorac Surg 1992 (n = 719); Vitale N, Ann Thorac Surg 2003 (n = 94); Dellgren G, J Thorac Cardiovasc Surg 2002 (254); Banburry MK, Ann Thorac Surg 2002 (n = 267); Tasca G, Ann Thorac Surg 2003 (n = 88); Khan SS, Ann Thorac Surg 2000 (n = 41). (D) The three regression lines practically coincide completely as do the 95% confidence limits, resulting in a quite reliable estimate of a reference EOA value for every valve size in this valve.

	Results

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Incidence of Patient–Prosthesis Mismatch
Calculation of EOAI as described above resulted in a mean EAOI of 0.94 cm²/m² (range, 0.62 to 1.43 cm²/m²) for the whole population. Using the above-described criteria, 102 of 506 patients (20%) had a moderate PPM. Only 1 of 506 (0.2%) had a severe PPM (ie, 0.62 cm²/m²). Figure 2 shows the distribution of EOAI per valve size.

View larger version (10K): [in this window] [in a new window]   Fig 2. Distribution of effective orifice area index per valve size.

View larger version (11K): [in this window] [in a new window]   Fig 3. Global survival of all 506 patients and the 95% confidence limits. Patients at risk at appropriate time intervals (5, 10, and 15 years) are shown between brackets.

Figure 3 shows the overall survival of the whole population. When survival in patients without PPM (EOAI > 0.85 cm²/m²) was compared with survival in the subgroup of patients with PPM (EOAI < 0.85 cm²/m²), the univariate log-rank test revealed a significant difference, suggesting a reduced survival in the group having PPM (Fig 4). When only cardiac death was considered, similar survival curves could be constructed with similar statistics. These univariate data suggest that the groups carry certain determinants of late mortality to a different extent and suggest a possible effect of PPM on survival. However, in the multivariate setting neither EOAI nor PPM was withheld as a predictor for mortality. Given this observation, a separate analysis was performed to explore the independent determinants of PPM itself (ie, EOAI < 0.85). This analysis revealed that the patients having PPM differ from the patients not having PPM in a few important patient baseline variables: the PPM group has a significantly higher proportion of long CPB duration (>120 minutes; p = 0.02), more concomitant coronary artery bypass grafting (p = 0.07), and significantly more female patients (p = 0.01).

View larger version (19K): [in this window] [in a new window]   Fig 4. Survival of all patients, categorized for having patient–prosthesis mismatch (PPM) (effective orifice area index < 0.85) or not. Univariate log-rank testing gives a significant difference (p < 0.04), also illustrated by the difference in 95% confidence limits around both curves. Patients at risk for each subgroup at appropriate time intervals (5, 10, and 15 years) are shown between brackets.

Further analysis showed, at 14 years of follow-up with still 40 patients at risk, the following: freedom from thromboembolic events, 81.1% and freedom from endocarditis, 98.0%.

Regression of Left Ventricular Hypertrophy
In 78% of all patients specific data about septal wall thickness were obtained from late postoperative echocardiograms (1 to 2 years postoperatively). There was a significant reduction in septal thickness after valve replacement for the whole group: 1.43 ± 0.19 mm preoperatively versus 1.18 ± 0.16 mm postoperatively (p < 0.05). When patients with and without PPM were compared, a similar reduction in septal wall thickness was found.

	Comment

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Determination of Patient–Prosthesis Mismatch
The most logical approach to determine PPM would be to measure EOA in the individual patient after aortic valve replacement and to normalize this value for the body surface area of the patient. The pitfall is that EOA is a dynamic measure strongly dependent on the actual flow passing through the aortic valve. This explains why, when measuring EOA echographically in patients receiving the same nominal size of a given valve type, a large variation in data is obtained. The average values of EOA increase along with the nominal valve size [4, 5]. The nominal valve size normalizes for the patient size (as does body surface area) and represents the elephant or the mice according to Pibarot and Dumesnil [3]. The variation of EOA at a given valve size represents the variation in cardiac output induced by the mouse or the elephant itself. Therefore, Pibarot and Dumesnil [3] suggested the use of a fixed value of in vivo measured EOA, which they termed projected EOA. These reference values were obtained from various literature sources. Such projected EOA values have the advantage that they can be attributed in retrospect to patients who were never examined echocardiographically. In the present study, we calculated the mean value of all data collected for a given nominal valve size to obtain the in vivo reference value of the CE Perimount valve. This in vivo measured reference value was compared with the in vitro value that was obtained from the manufacturer. Such in vitro EOA values represent the mean values for a given nominal valve size of a series of EOA measurements at different flow rates within a physiologic range. Although in vitro measurements were said to be nonstandardized, nonreproducible, and unavailable [2], both values correspond well for every nominal valve size, allowing us to project them on the patients who were not studied echocardiographically. Above this, the regression lines through our own clinical data and the in vitro data correlated nicely with data collected from nine different clinical papers, reporting echographically based EOA data from the CE Perimount valve.

Incidence of Patient–Prosthesis Mismatch Using the Carpentier-Edwards Perimount Valve in Aortic Position
Using these projected values, we observed that only 1 of 506 patients (0.2%) had a severe mismatch, ie, an EOAI less than 0.65 cm²/m². Twenty percent of the patients had a moderate mismatch, ie, an EOAI between 0.65 cm²/m² and 0.85 cm²/m². These figures are definitely on the low side compared with those usually reported: values for moderate PPM of 20% to 52% were reported, and for severe PPM, between 2% and 11% [3, 6–9]. This can be related either to the choice of the reference EOA, which can be quite different among studies, to the wide variety of valve types included in these studies, or to the specific hemodynamic properties of the CE Perimount valve itself.

Clinical Relevance of Patient–Prosthesis Mismatch
Although considerable controversy still exists [3, 10], it is clear that PPM has some impact on postoperative outcome. The problem is that most studies refer to a mixed population of valve types having their own specific incidence and specific degree of PPM. It is shown that severe PPM is extremely rare [9] or negligible [11] in modern-type prostheses in contrast to older models. It was even suggested that the concept of PPM does not exist in stentless prostheses (like homografts, autografts, or xenografts) [6]. Although it can be expected that the negative impact on clinical outcome is related to the degree of PPM, it is not clear from the literature if so-called moderate PPM already results in a negative outcome. We studied a population of patients undergoing aortic valve replacement using only one type of prosthesis, ie, the CE Perimount valve, and found that severe PPM hardly exists when this valve is used. However, 20% of the patients showed moderate PPM. This allowed us to study the effect of moderate PPM on early and late mortality and morbidity in terms of hospital readmission for cardiac reasons. The answer was clear: moderate PPM has no clinical relevance. Univariate analysis suggested a slightly worse survival in patients with moderate PPM, but PPM was not withheld as an independent predictor in multivariate setting. A possible explanation for the univariate result can be the higher proportion of concomitant coronary artery bypass grafting, long CPB times, and female patients in the PPM group, as revealed by the analysis of baseline characteristics in both groups. The lack of a clear effect of PPM in this patient group is not surprising, because studies of native aortic valves show that aortic stenosis becomes associated with higher mortality and morbidity rates when the indexed EOA drops to less than 0.60 cm²/m² [12]. Furthermore, the multifactorial nature of reduced survival after aortic valve replacement may mask subtle individual components of it, such as small prosthesis–patient size. Also, elderly patients—as in our population—may not live long enough to manifest a survival decrement from small prosthesis–patient size [10].

Recent reports focused on the regression of left ventricular hypertrophy after aortic valve replacement and its relation to PPM [13, 14]. It is indeed possible that PPM, although moderate as in the presence of a CE Perimount valve, has no clinical impact but inhibits the reduction of hypertrophy after valve replacement. In our population, however, left ventricular hypertrophy diminished in patients with moderate PPM to the same extent as in patients without PPM.

In conclusion, the incidence of severe PPM is virtually nonexistent after aortic valve replacement using the CE Perimount valve. Moderate PPM is found in 20% of cases and is clinically irrelevant in the elderly patient.

Limitations
We were able to collect long-term echocardiographic data in only 78% of all patients. Of these echocardiograms, only 21% were performed in our own center; the remaining echocardiograms were performed in referring centers. Of all the collected echocardiographic data, only septal thickness was complete enough to use as a variable for ventricular hypertrophy. New York Heart Association functional class was not systematically recorded. Focus was placed on hospital readmission as a more objective and clinically relevant variable.

	Southern Thoracic Surgical Association: Fifty-Third Annual Meeting

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The Fifty-Third Annual Meeting of the Southern Thoracic Surgical Association (STSA) will be held November 9–11, 2006, in Tucson, Arizona.

Manuscripts accepted for the Resident Competition must be submitted to the STSA headquarters office no later than September 15, 2006. The Resident Award will be based on abstract, presentation, and manuscript.

Applications for membership should be completed be September 15, 2006, and forwarded to Chairman of Membership Committee, Southern Thoracic Surgical Association, 633 N Saint Clair St, Suite 2320, Chicago, IL 60611-3658.

	References

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1. Rahimtoola SH. The problem of valve prosthesis-patient mismatch Circ 1978;58:20-24.[Abstract/Free Full Text]
2. Gilinov A, Blackstone E, Rodriguez L. Prosthesis-patient sizemeasurement and clinical implications. J Thorac Cardiovasc Surg 2003;126:313-316.[Free Full Text]
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4. Flameng W, Vandeplas A, Narin K, Daenen W, Herijgers P, Herregods MC. Postoperative hemodynamics of two bileaflet heart valves in the aortic position J Heart Valve Dis 1997;6:269-273.[Medline]
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9. Hanayama N, Christakis GT, Mallidi HR. Patient prosthesis mismatch is rare after aortic valve replacementvalve size may be irrelevant. Ann Thorac Surg 2002;73:1822-1829.[Abstract/Free Full Text]
10. Blackstone EH, Cosgrove DM, Jamieson WR, Birkmeyer NJ, Lemmer JH, Miller DC. Prosthesis size and long-term survival after aortic valve replacement J Thorac Cardiovasc Surg 2003;126:783-796.[Abstract/Free Full Text]
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12. Bonow RO, Carabello BA, de Leon Jr AC. Guidelines for the management of patients with valvular heart disease. Executive summarya report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Management of Patients with Valvular Disease). Circulation 1998;98:1949-1984.[Free Full Text]
13. Tasca G, Brunelli F, Cirillo M, et al. Impact of the improvement of valve area achieved with aortic valve replacement on the regression of left ventricular hypertrophy in patients with pure aortic stenosis Ann Thorac Surg 2005;79:1291-1296.[Abstract/Free Full Text]
14. Ruel M, Rubens F, Masters R, et al. Late incidence and predictors of persistent or recurrent heart failure in patients with aortic prosthetic valves J Thorac Cardiovasc Surg 2004;127:149-159.[Abstract/Free Full Text]

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