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Ann Thorac Surg 2003;76:1564-1570
© 2003 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Are mechanical valves with enhanced inner diameter advantageous in the small sized aortic annulus?

^a Departments of Cardiothoracic and Vascular Surgery, Jena, Germany
^b Anesthesiology and Intensive Care Medicine,, Jena, Germany
^c and Medical Statistics and Documentation, Friedrich-Schiller-University Hospital Jena, Jena, Germany

Accepted for publication April 18, 2003.

^* Address reprint requests to Dr Albes, Department of Cardiovascular Surgery, Heart-Center-Brandenburg, Ladeburger Strasse 17, 16321, Bernau, Germany
e-mail: j.albes{at}immanuel.de

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
BACKGROUND: Mechanical bileaflet valves with enhanced inner diameter may offer superior hemodynamic properties in patients with a small aortic annulus. The aim of this clinical study was to compare these valves with standard bileaflet prostheses in vivo.

METHODS: Mechanical aortic valve replacement for combined stenosis and regurgitation was performed in 47 patients with standard CarboMedics prostheses (CM: 21 mm, 23 mm, 25 mm) and two types of diameter enhanced St. Jude Medical prostheses (SJM-AHPJ: 21 mm, 23 mm, 25 mm; SJM-Regent: 21 mm, 23 mm). Transvalvular mean gradients (TVG) were assessed intraoperatively by means of transesophageal echocardiography (TVG_TEE) and simultaneous direct pressure monitoring of the left ventricle and the ascending aorta (TVG_CATH), as well as early (3 months) and late (9 months) postoperatively by means of transthoracic echocardiography (TVG_TTE). Left ventricular muscle mass was assessed preoperatively, early, and late postoperatively to evaluate remodeling capacity.

RESULTS: In all valve types and sizes, both TVG assessments exhibited consistent findings. Small-sized conventional valves of 21 mm showed a marked initial TVG. In contrast, both valve types with enhanced inner diameter exhibited significantly lower TVG comparable with those achieved with larger valves (TVG_CATH CM 21 mm, 15.6 ± 3.9 mm Hg; SJM-AHPJ 21 mm, 11.9 ± 1.6 mm Hg; SJM-Regent 21 mm, 9.9 ± 1.1 mm Hg; CM 23 mm, 7.8 ± 0.8 mm Hg; SJM-AHPJ 23 mm, 7.7 ± 1.4 mm Hg; SJM-Regent 23 mm, 9.5 ± 1.8 mm Hg). During the postoperative course TVG remained constant in all valve types and sizes. Left ventricular muscle mass, however, diminished markedly in all valves without exhibiting significant differences between size matched valve types.

CONCLUSIONS: In patients with a small aortic annulus, who require a 21-mm valve, diameter-enhanced prostheses provide lower transvalvular gradients than conventional valves. However, in the intermediate clinical course, appropriate left ventricular remodeling occurred in all patients independent of the size and the type of the valve.

	Introduction

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Bileaflet valves represent the most widely implanted mechanical prostheses worldwide [1]. In the aortic position, a small annulus often requires an implantation of a valve with a smaller diameter than appropriate for adequate perfusion of the body at rest and on exertion [2]. The resulting mismatch can yield a remaining nonphysiological transvalvular gradient, leading to the development of left ventricular hypertrophy, or impedance of left ventricular remodeling with the consequence of subsequent irreversible myocardial damage [3]. Annulus augmentation plasty can provide adequate space for a valve with a body size–matched diameter [4]. This procedure is demanding and thus not liberally performed. A valve with a larger effective opening area, however, can serve as an avenue out of this dilemma. The industry has therefore developed valves with an enlarged inner diameter [5]. Those valves come close to the hemodynamic performance of a next larger-sized valve. An already established design has currently been modified in order to further enlarge the inner diameter [6]. The purpose of this clinical study was therefore to analyze the initial and medium- term performance of conventional and diameter-enhanced valves in the small-sized annulus.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Forty-seven patients (mean age, 64.4 ± 10.7 years; 32 men, 15 women) were identified who required implantation of a mechanical aortic valve with and without concomitant coronary bypass surgery. Patients who required simultaneous procedures of the mitral or tricuspid valve as well as the ascending aorta, additional measures at the aortic root such as annulus augmentation plasty, or septal myectomy because of left ventricular outflow tract obstruction (LVOTO) were not enrolled. Patients with chronic atrial fibrillation, patients with markedly reduced left ventricular function (ejection fraction < 45%), and patients with previous open-heart surgery were also not enrolled. All patients gave written informed consent in concordance with the local ethical committee. The patients were stratified according to the prevailing type of aortic disease (isolated stenosis: 33 patients, 70.2%; prevailing stenosis: 5 patients, 10.6%; prevailing insufficiency: 7 patients, 14.9%; isolated insufficiency: 2 patients, 4.3%). They were randomly assigned to one of the studied valve types while valve sizes were chosen according to the individual size of the native annulus. Three rotatable valve types of different sizes were investigated in eight groups: SJM-AHPJ 21 mm (n = 6); SJM-AHPJ 23 mm (n = 6); SJM-AHPJ 25 mm (n = 6); SJM-Regent 21 mm (n = 6); SJM-Regent 23 mm (n = 6) (St. Jude Medical Inc, St. Paul, MN) and CarboMedics Standard A500 Series (CarboMedics) 21 mm (n = 5); CarboMedics 23 mm (n = 6); and CarboMedics 25 mm (n = 6) (Sulzer-CarboMedics Inc, Austin, TX) (Table 1).

Intraoperative hemodynamic assessment
In all patients' body surface areas (BSA, m²) were computed. Invasive pressure monitoring was performed using a monitoring set (Combitrans, B. Braun AG, Melsungen, Germany) and standard monitor (Ohmeda AS/3, Datex, Helsinki, Finland). Before weaning from cardiopulmonary bypass, a catheter routinely used for left atrial pressure measurement (LAP 1251 3.6 F, 1.2-mm diameter, 51-cm length, Jostra AG, Hirrlingen, Germany) was introduced through the right upper pulmonary vein and advanced into the left ventricle through the mitral valve. Aortic pressure was measured 5 to 7 cm above the implanted valve in the ascending aorta by means of a standard 3.5 F cannula for peripheral veins (Sterican, B. Braun AG) and a standard pressure line (Combidyn 1.5 x 2.7 mm, 150-cm length, B. Braun AG). Bipolar atrial and right ventricular temporary pacemaker leads (Osypka GmbH, Rheinfelden-Herten, Germany) were implanted and connected with an external pacemaker (5388 Temporary Pacemaker, Medtronic GmbH, Düsseldorf, Germany).

After weaning from cardiopulmonary bypass, pressures and cardiac output were measured at 100 beats per minute heart rate with either sinus rhythm or atrial pacing. Patients with intractable atrial flutter or fibrillation or necessity of sequential atrial and ventricular pacing were excluded from the study. After calibration, simultaneous pressure measurements of the left ventricle and aorta were performed. Analog data were digitalized and further processed in a personal computer. Mean systolic transvalvular pressure gradient (TVG_CATH; mm Hg) was computed [7]. Transesophageal echocardiography was performed with a Sonos 5500 and a 6.2-MHz multiplane probe (Philips Medical Systems, Andover, MA). Flow velocity (m/s) was measured by means of continuous-wave Doppler echocardiography utilizing a transgastric alignment, which allowed for a central axial transprosthetic view. Simultaneous assessment of the velocity in the left ventricular outflow tract and ascending aorta was performed using the double envelope technique. Mean systolic transvalvular echocardiographic pressure gradient (TVG_TEE) was derived by the simplified Bernoulli equation. Effective orifice area (EOA; cm²) was computed by means of the Gorlin formula. Effective orifice area index (EOAI; cm²/m²) was derived in order to correct EOA for body surface area (EOAI=EOA/BSA). The performance index (PI) of valves, which describes the ratio between effective orifice area and the outer diameter of the valve including the sewing ring (mounting area), was assessed (PI=EOA/mounting area) [8]. Cardiac output (CO; L/min) was measured with an indwelling catheter (Swan-Ganz-Catheter, Baxter Deutschland GmbH, Unterschleissheim, Germany) using a thermodilution technique. Mean values were computed from three subsequent measurements at the respective time. Cardiac index (CI; L/min/m² BSA) was computed.

Postoperative hemodynamic course
Left ventricular ejection fractions (EF; %) were assessed preoperatively, after 3 months, and after 9 months postoperatively, whereas mean systolic transvalvular gradients (TVG_TTE; mm Hg) were assessed after 3 months and after 9 months postoperatively utilizing transthoracic echocardiography (TTE) with an S4 probe (Sonos 5500, Philips Medical Systems).

Remodeling
Total left ventricular muscle mass (LVMM; g) was assessed preoperatively, after 3 months, and after 9 months utilizing transthoracic echocardiography with an S4 probe (Sonos 5500, Philips Medical Systems). Left ventricular muscle mass index (LVMMI; g/m²) was computed to correct for individual body surface area (BSA; m²).

The stratified types of prevaling aortic disease were statistically compared with the extent of preoperative muscle mass. Effective orifice area index as well as gender were correlated with postoperative LVMMI regression independent of the implanted valve types and sizes.

Statistical analysis
Data were analyzed by means of SPSS 10.0 (SPSS Inc, Chicago, IL) for Windows (Microsoft Corp, Redmond, WA) software package on a standard personal computer. All data were analyzed for normal distribution. Normally distributed values were analyzed by means of analysis of variance (ANOVA) and post hoc comparison with Tukey-HSD (honestly significant difference) adjustment. Nonnormally distributed values were analyzed by means of Kruskal-Wallis test and subsequent Mann-Whitney test. Comparison of left ventricular geometry values assessed during the clinical course of an individual group was performed using the differences between the respective time points by means of a paired Student's t test. For correlation of echocardiography with direct pressure measurement and EOAI as well as gender with LVMMI regression, a Bland-Altman plot was created, linear regression analysis was performed, and the Pearson product-moment correlation coefficient was computed. Significance was assumed if p was less than 0.05. All data are presented as means ± standard deviation (SD).

	Results

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Patient characteristics
Mean age did not differ significantly between the eight groups, whereas significant differences of BSA occurred corresponding with the implanted size of the valve. Cardiac index showed a mean of 3.2 ± 0.6 l/min/m² at 100 beats per minute without exhibiting significant differences between the groups (Table 2). The majority of our patients (70.2%) suffered from a combined aortic disease. The prevailing type of aortic disease did not influence the extent of preoperative muscle mass (ANOVA, p = 0.865).

View larger version (18K): [in this window] [in a new window]   Fig 1. Regression analysis. Correlation of mean transvalvular gradients obtained with direct pressure measurement (TVGCATH; mm Hg) and echocardiographic Doppler assessment (TVGTEE; mm Hg) is shown. Correlation is indicated by a regression line ± 95% confidence interval (dashed lines) (correlation coefficient, 0.84; R2, 0.71).

Effective orifice area (EOA) and effective orifice area index (eoai)
Measured EOAs were consistently smaller than the geometrical orifice area provided by the manufacturer (Table 1). EOA as well as EOAI increased with the diameter of the valves. Interestingly, no significant differences were found between EOA and EOAI of all three different valve types of the same size (Table 3).

Performance index (PI)
Peformance index increased with the size of the valve without showing significant differences between the size-matched valve types. However, the 23-mm Regent valve exhibited the highest observed PI, which differed significantly from the PI of the CM 21-mm and the CM 25-mm valves (Table 3).

Postoperative hemodynamic course
In all patients, the initially obtained TVG did not change significantly during the entire postoperative course. Left ventricular ejection fraction increased in all groups early after surgery. Whereas the total analysis revealed a significant change, the analysis of the individual groups failed to exhibit significance at the 5% level (Table 4).

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

Whereas in vitro hemodynamic studies unequivocally provide clear data endorsing the hydrodynamic properties of mechanical valves, the transfer of these findings into the in vivo situation and thus the clinical setting remains difficult. Transesophageal echocardiography is widely accepted to provide an adequate initial hemodynamic assessment of valvular prostheses [13]. However, some authors describe limitations of Doppler flow measurements [14]. As an alternative, direct intraoperative pressure monitoring can provide additional insights and may serve to validate echocardiographic findings [15, 16].

Currently, it is not entirely elucidated to what extent initial hemodynamic differences influence the clinical course, in which left ventricular remodeling appears to be an important aspect. After restoration of proper valve function, left ventricular hypertrophy should diminish, indicating a reduction of volume and afterload of the left ventricle [6]. The aim of our study was therefore to evaluate the proposed advantages of diameter-enhanced valves by means of an intraoperative in vivo hemodynamic evaluation with two independent methods and the impact on left ventricular performance and remodeling in the early and intermediate clinical course. The study was restricted to valves that allow for an annular implantation to guarantee comparability. Therefore, valves that can be positioned in a supraannular fashion, such as the "top hat" CarboMedics valve, were not included. A wide variety of valves with a conventional inner/outer-diameter ratio exist. However, in order to remain concise and to avoid impedance of statistical accuracy, we limited the total number of investigated groups. Thus, both diameter-enhanced valve types were compared with one conventional valve.

All valves were oriented in a perpendicular angle of the hinge axis to a virtual axis between both coronary ostia. This was achieved either directly at implantation or with a subsequent valve rotation maneuver. We could recently show that even a slightly 20° tilted position of bileaflet valves resulted in an impairment of hemodynamic properties, which could be compensated by rotation into the previously mentioned position [17]. We analyzed the influence of the type of aortic disease on preoperative left ventricular muscle mass. Interestingly, neither a prevailing stenosis nor regurgitation showed a statistical influence on the extent of preoperative LVMM.

Our study revealed that in the in vivo setting, hydrodynamic advantages are not as obvious as ex vivo. However, it was evident that diameter-enhanced valves do provide lower initial and chronic transvalvular gradients than valves with a conventional design. Although the Regent valve exhibited the lowest gradients in our study, significant differences between both diameter-enhanced valve types could not be demonstrated. Perhaps, the rather small percentage of enhancement achieved by the latest design modification did not find an appropriate hemodynamic reflection in the clinical situation. The PI has been evaluated in order to look at the impact of a small rim design on the resulting hemodynamic performance [18]. Theoretically, the more slender the rim, the higher the chance to implant a valve of the next larger size number in a given native annulus. The observed results in our study, however, were inconsistent because a significant improvement of PI was only statistically evident for the 23-mm-sized Regent valve. However, the surgeon may prefer to choose the valve with the largest available inner diameter that fits into a given native annulus as long as it is technically feasible.

We used two different methods to assess the initial hemodynamic performance and could show that both methods correlated closely. One, therefore, validates the other. However, we detected systematically higher gradients using the Doppler method. This can be explained by the flow velocity profile of a mechanical bileaflet valve, which exhibits a tripartite flow pattern with a small central part and two larger lateral parts [18, 19]. Whereas about 80% of the total flow passes through the lateral apertures, the flow through the central opening accounts for only 20%. Therefore, the flow velocity of the central part is higher than the velocity of the lateral openings when measured directly downstream. A Doppler signal obtained from this particular area may, therefore, overestimate the real resulting average velocity across the entire area, thereby providing false too-high "localized" gradients [15, 18, 19]. However, as we observed consistently higher values by Doppler than by catheter-derived measurements as well as an excellent correlation, we believe that Doppler assessment is reliable and may be used as the method of choice for routine intraoperative hemodynamic assessment. The physician, however, should keep in mind the tendency to an overestimation of the velocity. Not much is known regarding gender differences in LVMM regression. Although other studies failed to show a correlation of gender with LVMM regression [20], in our study, male patients exhibited a significantly faster muscle mass reduction than the female cohort. Hypothetically, differences in the hormonal state as well as in the quantity and quality of myocyte receptor expression may serve as an explanation [21].

In theory, one can assume that lower transvalvular gradients may eventually lead to a faster, more pronounced remodeling of the left ventricle. It has already been shown that significant remodeling does occur after implantation of diameter-enhanced SJM-Regent valves [6]. A clinical study that elucidates the remodeling capacity of both diameter-enhanced SJM valve designs in comparison with a conventional bileaflet valve design, however, has not been conducted yet. We found clear evidence that replacement of a severely destructed native valve by an artificial valve is indeed followed by a marked remodeling, as observed in the regression of LVMM. As expected, the initially measured transvalvular gradients remained rather fixed throughout the entire observation period. To our surprise, however, our study failed to demonstrate a statistical advantage of diameter-enhanced valves in terms of a more pronounced remodeling than conventional valves. The differences in the hemodynamic performance of conventional valves and diameter-enhanced valves were obviously not sufficient to influence LVMM regression significantly. One must therefore assume that a conventional design is adequate for proper recovery of an altered hypertrophic left ventricle in the intermediate course. However, these findings may be different when looking at the long-term course. Small differences in the hemodynamic properties may then gain momentum for significant changes, although this remains speculative as long as respective studies are not at hand. In contrast, we saw the most significant LVMM regression occurring already in the first 3 months postoperatively and an only moderate further regression during the following 6 months.

At our institution, 19-mm valves are not implanted in adult patients. Instead, we perform an annulus augmentation plasty in order to achieve conditions, which are at least adequate for an implantation of a 21-mm-sized valve. However, in a study by Medalion and associates, even 19-mm mechanical valves have been shown to be a reasonable option for the elderly female patient with a body surface area in the proximity of 1.5 m² [22].

We conclude from our experience that current bileaflet valve designs can be safely implanted and provide adequate hemodynamic properties that allow for a reasonable remodeling of the left ventricle. Diameter-enhanced valves exhibit hemodynamic advantages in vivo, which, however, do not result in a particularly beneficial early clinical course. It therefore remains the surgeon's preference to choose the valve design he or she feels most comfortable with in the patient's individual anatomical situation. Doppler assessment appeared to be as reliable as direct pressure measurement. Being less invasive, it can be recommended as the method of choice for intraoperative hemodynamic assessment.

	Acknowledgments

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The authors are most grateful for the financial support of the study provided by the St. Jude Medical Corporation, St. Paul, MN.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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