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Ann Thorac Surg 1999;68:1657-1660
© 1999 The Society of Thoracic Surgeons

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Original Articles

Patient–prosthesis mismatch is negligible with modern small-size aortic valve prostheses

^a Bristol Heart Institute and Research and Development Support Unit, University of Bristol, Bristol, England, UK

Address reprint requests to Dr Izzat, Cardiovascular Surgical Center, Damascus University, PO Box 33831, Damascus, Syria
e-mail: izzat{at}cyberia.net.lb

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. Concern has been raised about residual significant gradients when small aortic prostheses are used, particularly in patients with large body surface areas. We studied the performance of six types of small aortic prostheses using dobutamine stress echocardiography.

Methods. Sixty-three patients (mean age, 67 ± 7 years) who had undergone aortic valve replacement 17 ± 6 months previously were studied. Two bileaflet mechanical prostheses (St. Jude Medical and CarboMedics: sizes, 19 mm and 21 mm) and two biological prostheses (Medtronic Intact and St. Jude BioImplant: size, 21 mm) were evaluated. A graded infusion of dobutamine was given and Doppler studies of valve performance were carried out.

Results. All prostheses except one biological valve had acceptable hemodynamic performance under stress. Using regression modeling, gradient at rest was the only variable found to predict gradient under stress (p < 0.001). Moreover, the most important predictor of gradient at rest was valve design, which accounted for 72% of the variance (p < 0.001). This relationship was independent of valve size (19 mm or 21 mm) or material (ie, mechanical or biological). Body surface area accounted for 4% of the variance in gradient only.

Conclusions. The main predictor of transprosthetic gradient is the inherent characteristics of each particular prosthesis, with relatively insignificant contribution from variations in body surface area. Patient–prosthesis mismatch is not a problem of clinical significance when certain modern valve prostheses are used.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
The hemodynamic performance of currently available aortic valve prostheses remains inferior to those of the native aortic valve. With the exception of homografts, and probably stentless valves, all current prosthetic valve designs produce measurable transprosthetic gradients that, potentially, could place persistent additional demands on the left ventricle, and may hinder or delay the regression of left ventricular hypertrophy. This is said to occur more frequently when the size of the implanted prosthesis is limited by the presence of a small aortic annulus, particularly in a patient with a large body surface area (BSA), when there is a mismatch between prosthesis and patient, as defined by Rahimtoola [1].

Recently, we described the use of dobutamine stress echocardiography for the in vivo assessment of the hemodynamic performance of aortic valve prostheses. Subsequently, we conducted individual studies of several small-sized aortic prostheses, aiming to describe the distribution of transprosthetic gradients under stress and to explore factors that might influence these gradients [2–6]. We were able to demonstrate that gradients across small-sized aortic prostheses were often acceptable from a clinical viewpoint, and that the only important predictor of gradient under stress is gradient at rest, with no evidence that BSA is an important predictor. These studies, however, were limited by the small sample sizes, which was dictated by the infrequent use of small-sized aortic prostheses. Having by now studied six different types of valve prostheses, the large overall sample size presents us with the opportunity to investigate the relationship between patient characteristics and transprosthetic gradients at rest and under stress, with greater power for factors that hold true irrespective of the valve type. It is also possible to compare several valve types directly after adjusting for possible differences between patient groups.

The present analysis, therefore, aims to address the following key questions, are valve type, BSA, or other patient characteristics related to: (1) transprosthetic gradient at rest and hence under stress, or (2) to transprosthetic gradient under stress after adjusting for transprosthetic gradient at rest? This analysis is the subject of the present report.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Sixty-three patients who had undergone isolated aortic valve replacement with small size (19 or 21 mm) prostheses were studied using dobutamine stress echocardiography. Two bileaflet mechanical prostheses (St. Jude Medical [St. Jude Medical, Inc, St. Paul, MN] and CarboMedics [CarboMedics, Inc, Austin, TX]: sizes, 19 mm and 21 mm) and two biological prostheses (Medtronic Intact [Medtronic, Inc, Minneapolis, MN] and St. Jude BioImplant: size, 21 mm) were evaluated.

Dobutamine stress protocol and Doppler calculations have been described previously [2, 3]. Briefly, a graded infusion of dobutamine was administered at increments of 5, 10, and 20 µg · kg^-1 · min^-1 at 15-minute intervals, during which serial cardiac output measurements and Doppler studies of valve performance were obtained.

Using an Aloka SSD-830 ultrasound system and a 2.5-MHZ transducer (Aloka, Tokyo, Japan), cardiac output was calculated from pulsed-wave Doppler flow velocity in the left ventricular outflow tract, the modified Bernoulli equation was used to calculate mean transprosthetic gradient from continuous-wave Doppler, and prosthetic valve effective orifice area was calculated with the modified continuity equation. Systolic valve flow, effective orifice area index to BSA, discharge coefficient, and performance index were also calculated as detailed previously [2, 3].

Statistical analyses
Parameters were calculated for each patient at each level of dobutamine infusion, and data are presented as mean ± standard deviation. Only changes in cardiac output, transprosthetic gradients, and effective orifice area were analyzed because all other parameters were derived from these and, hence, are not independent.

One-way analyses of variance were carried out to test for differences in demographics and baseline clinical characteristics between groups of patients with different prostheses. Analysis of maximum stress performance was carried out by multiple linear regression modeling, fitting dummy variables for different prosthesis types using the St. Jude Medical 19-mm mechanical prosthesis as the baseline category for regression analyses because it had the largest sample size. Analyses were carried out both using raw data and after applying a natural logarithmic transformation. Because analyses using raw data yielded residuals that were more normally distributed (ie, the regression models fitted the raw data better), only these findings are reported here.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
The characteristics of the patient sample are described in Table 1. There were no differences in mean BSA, resting heart rate, or blood pressure between the groups of patients with different valve types (F = 1.87, p = 0.11; F = 0.39, p = 0.85; and F = 1.53, p = 0.20, respectively). All patients had good left ventricular function, dobutamine infusion was well tolerated, and no impairment in regional myocardial contractility with dobutamine stress could be detected in any patient. Dobutamine infusion resulted in a significant increase in heart rate, cardiac output, and systolic valve flow, which were comparable between the groups (Table 2). With the increase in transprosthetic flow, there were significant increases in mean transprosthetic gradients in all groups (Table 3). Values of discharge coefficients and performance indices for the various prostheses were reported previously [2–6].

The relationship between BSA and gradient at rest is summarized as: . Despite its significance (95% confidence interval = 0.54 to 2.19, p = 0.002), this was only a small increase and accounted for a mere 4% of the variance in gradient. For example, using the St. Jude Medical instead of the CarboMedics prosthesis in a 19-mm annulus contributes more to the increase in gradient than a 0.7-m² increase in BSA.

	Comment

Top Abstract Introduction Material and methods Results Comment References

 
By definition, patient–prosthesis mismatch is considered to be present when the in vivo prosthetic valve effective orifice area is less than that of a native human valve [1]. Accordingly, at least in theory, patient–prosthesis mismatch exists with all aortic valve prostheses, even those of contemporary design, because these continue to represent a compromise to some degree compared to normal human aortic valves. With many large size valve prostheses, however, this mismatch is mild in severity and of no clinical significance. It was mainly the use of small-sized aortic valve prostheses, chiefly in the era of caged-ball prostheses, which had raised concerns about the presence of significant residual gradients, with the potential detrimental sequelae to the left ventricle [1]. Even at present, however, the approach to aortic valve replacement in the presence of a small aortic annulus remains controversial.

Our studies on the in vivo performance of modern small-sized aortic valve prostheses demonstrated that gradients across several, but not all, types of prostheses were clinically acceptable, even under stress conditions, indicating that many modern small-sized prostheses use their nominal orifice areas more effectively compared to those of older designs [2–6]. Our findings were in agreement with recent clinical studies that have documented the safety of several modern small-sized aortic valve prostheses, with similar early and late survival rates to that of larger sized prostheses [7–11]. The documented comparable regression of myocardial hypertrophy and improvement in left ventricular function after aortic valve replacement using small and larger prostheses further emphasized the favorable performance of small-sized prostheses [7, 8, 11–13].

The problem of patient–prostheses mismatch was presumed to occur more often in patients with large BSAs, in whom a high cardiac output across a small orifice area may produce high transprosthetic gradients [1, 14, 15]. Hence, the calculated effective orifice area of a specific prosthesis has frequently been corrected for BSA to relate its hemodynamic performance to an individual patient. Moreover, from the hemodynamic data of valve behavior at rest and the expected increase in cardiac output with exercise, an effective orifice area index of more than 0.9 cm²/m² has been predicted as a requirement to minimize postoperative transprosthetic gradients [1, 16]. This precaution is still often observed today, although it was based on reports evaluating first generation biological and mechanical prostheses, and notwithstanding the scarcity of evidence in its support. Indeed, to date there are no controlled or randomized data in the surgical literature to suggest that implantation of a small valve or patient–prosthesis mismatch reduces long-term survival.

The present analysis demonstrates that the change (ie, increase) in transprosthetic gradients with stress is comparable among all six prostheses under assessment; hence, the only variable that influences transprosthetic gradient at stress was gradient at rest. Further investigation of each prosthesis using patient’s characteristics, hemodynamic variables, and valve idiosyncrasies (type, material, or size) as predictors showed that the main predictor of transprosthetic gradient is the characteristics of each particular valve. This result, together with the findings of other studies [9, 17, 18], clearly reflects the dependence of the valve hemodynamics on certain design characteristics.

Special attention should be paid to discrepancies between tissue annulus diameters of different prostheses when using their comparisons for selecting the better prosthesis type [19]. For example, we found that higher gradients were associated with the 19-mm St. Jude Medical compared with the 19-mm CarboMedics bileaflet prostheses. The diameter of a labeled 19-mm CarboMedics prosthesis, however, is actually 19.8 mm, making it unusable in a patient with a 19-mm annulus. Hence, in the clinical circumstances, the two valve prostheses are hardly comparable.

The most notable finding from the present analysis is that, although BSA was also a significant predictor of transprosthetic gradient at rest, it accounted only for about 4% of the variance in gradient, which did not appear to be clinically significant. A relatively narrow range of BSA in the patient group studied is one explanation for the poor association. However, it is more conceivable that patient–prosthesis mismatch is negligible with modern small-sized aortic valve prostheses. This finding helps to explain recent studies that have demonstrated similar long-term survival rates after aortic valve replacement in patients with large BSAs and their smaller counterparts [11], and comparable regression of left ventricular hypertrophy in patients with different mismatches between valve prostheses and BSA [13].

In conclusion, in the presence of a narrowed aortic annulus, decision making is based on careful evaluation of several factors: patient’s age and lifestyle, the familiarity of the surgeon with root-enlarging and homograft valve techniques, and the type of prostheses available. At present, several modern small-sized aortic prostheses allow for clinically acceptable hemodynamics in annuli that may have traditionally called for aortic root enlargement. Furthermore, the perception that these prostheses can be used without concern for leaving significant residual gradients, regardless of the patient’s BSA, is of notable clinical relevance, and consideration should be given to using this information in daily surgical practice.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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