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Ann Thorac Surg 1998;66:805-809
© 1998 The Society of Thoracic Surgeons

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Original articles: Cardiovascular

Fifth-year hemodynamic performance of the prima stentless aortic valve

^a Department of Cardiac Surgery, Oxford Heart Centre, John Radcliffe Hospital, Oxford, England, United Kingdom

Accepted for publication April 17, 1998.

Address reprint requests to Mr Pillai, Oxford Heart Centre, John Radcliffe Hospital, Oxford, OX3 9DU, England

Presented in part at the VII International Symposium on Cardiac Bioprostheses, Barcelona, Spain, June 13–16, 1997.

	Abstract

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Background. The medium-term hemodynamic performance of stentless valves has not been widely reported, particularly in comparison with in vitro studies. Therefore, we have assessed prospectively the hemodynamics of the Edwards Prima valve in its fifth year after implantation in the aortic position, and compared the results with those at 1 month after implantation and also with in vitro data.

Methods. Thirty-five patients (age, 77 ± 6 years; 19 men) were prospectively studied by Doppler echocardiography at 1 month and 52 ± 8 months after implantation of a Prima stentless valve. Valve hemodynamics were assessed by measuring the mean pressure gradient, mean valve resistance, and effective orifice area. Left ventricular systolic function was quantified by ejection fraction, the degree of hypertrophy by ventricular mass index, and the ratio of ventricular wall thickness to cavity radius as a measure of ventricular geometry.

Results. With a mean valve size of 24.6 ± 2.2 mm in the fifth year after implantation, the mean pressure gradient was 6.2 ± 3.5 mm Hg, the mean valve resistance, 29 ± 16 dyne · s^-1 · cm^-5), and the effective orifice area was 2.05 ± 0.50 cm². Compared with 1 month after operation, there was a 47% decrease in mean valve resistance (p = 0.002) and a 39% increase in effective orifice area (p = 0.001). Furthermore, both effective orifice area and mean valve resistance in the fifth year did not differ from their in vitro counterparts, whereas the left ventricular ejection fraction (0.64 ± 0.14), the left ventricular mass index (119 ± 49 g/m²), and the ratio of ventricular wall thickness to cavity radius (0.44 ± 0.13) were within the normal range.

Conclusions. This study suggests that the Prima valve is a reliable stentless aortic bioprosthesis. This is supported by a favorable medium-term clinical outcome, durable hemodynamic performance, and normal mean values of left ventricular ejection fraction and mass index in the fifth year after implantation.

	Introduction

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
In recent years, stentless bioprosthetic valve replacement for aortic valve disease has attracted increasing clinical interest, and several large series have reported encouraging results at a mean follow up of 2 to 3 years [1–3]. Since 1991, a cohort of 50 patients who had received an Edwards Prima stentless porcine aortic valve have been prospectively followed up in our center. Its early performance has previously been reported [4]. In the present study, we assess its fifth-year hemodynamic performance by echocardiography, and compare these results with those at discharge, as well as with data from an in vitro study.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Fifty patients with aortic valve disease underwent aortic valve replacement with the Prima stentless porcine valve (Baxter Inc, Irvine, CA). All patients gave informed consent to participate in this prospective study, which was approved by the Central Oxford Research Ethics Committee. According to the initial study protocols, participants were scheduled to have clinical and echocardiographic follow-up at the time of hospital discharge and then annually. By the end of April 1997, of the initial 50 patients, there were eight late deaths. Four of these resulted from preexisting myocardial disease, and one from complications of endocarditis. The cause of death in the remaining three is unknown. Of 42 survivors, 7 patients did not attend the latest echocardiographic study because of advanced age and long traveling distance.

Therefore, 35 patients underwent detailed echocardiographic assessment between June 1996 to April 1997 and constitute the population of the hemodynamic study. There were 19 men and 16 women, with a mean age of 77 ± 6 years (mean ± standard deviation) at late follow-up. The cause of aortic valve disease was stenosis in 28 patients, regurgitation in 2, and mixed in 5. Indications for stentless valve replacement were similar to those for a stented bioprosthesis. Implantation methods have been previously described in detail [4]. In brief, after excision of the diseased native valve, the Prima valve was sewn in a subcoronary position using interrupted 4-0 Ethibond (Ethicon, Somerville, NJ) sutures for the proximal suture line. The upper suture line was fashioned using continuous 4-0 Prolene (Ethicon) after sculpturing of the left and right coronary sinuses. The mean cross-clamp time was about 70 minutes and the bypass time, 90 minutes. The size of the Prima valve implanted ranged from 19 to 29 mm, with a mean size of 24.6 ± 2.2 mm. Eight patients had coronary artery disease and were concomitantly grafted. The follow-up period is 36 to 72 months (mean, 52 ± 9 months). At the latest follow-up, 31 patients were in New York Heart Association functional class I, 2 patients in II, and 2 in III. Apart from 5 patients in atrial fibrillation and 2 with a VVI pacemaker inserted late, all were in sinus rhythm.

Echocardiographic study
Transthoracic echocardiography was performed using a Toshiba 380A Ultrasound System (Toshiba Corp, Tochigi-Ken, Japan), with a 2.5-MHz phased-array transducer. From the parasternal left ventricular long axis view, the diameter of the outflow tract was measured from a two-dimensional image in early systole [5]. Standard left ventricular M-mode echocardiograms [6] were recorded and stored on video tape at a speed of 50 to 100 mm/s, with simultaneous recordings of electrocardiogram and phonocardiogram. From an apical five-chamber view (from which the outflow tract and aortic valve can be imaged parallel to the Doppler ultrasound beam), flow velocities in the outflow tract (2.5-MHZ pulsed Doppler) and that across the stentless valve (2.5-MHZ continuous wave Doppler) were recorded at a speed of 100 mm/s for off-line analysis. Aortic regurgitation was semiquantified as none (0/4), trivial to mild (1/4), moderate (2/4), and moderate severe (3/4), or severe (4/4), according to the width and the length of regurgitant jet with respect to those of left ventricular outflow tract, from the parasternal and apical five-chamber views [7]. Systemic blood pressure was recorded noninvasively by the Hewlett Packard (Andover, MA) 66S hemodynamic monitoring system. Body surface area was calculated from the height and weight.

Measurements and calculation
Mean values for each measurement were derived from three heart beats in patients in sinus rhythm, and from five beats in those in atrial fibrillation or with an internal pacemaker.

Left ventricular cavity size and wall thickness
End-diastolic dimension, septal thickness, posterior wall thickness, and end-systolic dimension were measured from M-mode echocardiograms. The ratio of wall thickness to cavity radius at end-diastole, and left ventricular muscle mass were calculated according to the formula of the American Society of Echocardiography [6, 8]. Muscle mass was indexed to body surface area.

Hemodynamics of stentless aortic valve
Systolic peak flow velocities and its time integral in the left ventricular outflow tract, and those of the aortic valve were measured by Doppler recordings [2, 9, 10]. Left ventricular stroke volume (LVSV) was calculated as the product of the cross-sectional area and flow velocity time integral in the outflow tract. The effective orifice area of the aortic valve was calculated by the continuity equation (stroke volume divided by valve flow velocity time integral) [9, 11]. Peak and mean pressure decreases across the aortic valve were calculated using the modified Bernoulli equation by taking the subvavular (V₁) and valvular (V₂) flow velocities (transvalvular pressure drop = 4(V₂² - V₁²), in mm Hg. Valve resistance was defined as mean transvavular pressure decrease divided by the ejection flow rate, and expressed in dyne·s^-1 · cm^-5 [5, 9].

Left ventricular hemodynamics and systolic function
Global stroke volume index and cardiac index were calculated from stroke volume (LVSV), heart rate, and body surface area. Global stroke work, measured in millijoules, was determined by LVSV x (mean arterial pressure + mean aortic valve pressure drop) x 0.0136 x 9.8, and indexed to body surface area (millijoules per square meter) [11, 12]. Left ventricular ejection fraction was calculated from end-diastolic and end-systolic dimensions using the formula by Teichholz and colleagues [13].

Statistical analysis
Echocardiographic and hemodynamic data are presented as mean ± one standard deviation. Data were analyzed using Minitab statistic software (release 11 for Windows, 1996; Minitab Inc, Philadelphia, PA) [14]. One-way analysis of variance was performed to test the significance of changes in each measurement with respect to valve size. Paired t test was used to determine the significance of changes in valve hemodynamics from discharge to latest follow-up, and the distribution of the changes across valve sizes were further tested by one-way analysis of variance. The possible differences between median value of each valve size of in vitro test and that of individual patient at late in vivo follow-up were tested by paired t test. A p value less than 0.05 was considered statistically significant.

	Results

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Medium-term valve hemodynamics
Hemodynamic data from the 35 patients at late follow-up are presented in Table 1, along with values for individual valve sizes. With a mean valve size of 24.6 mm, the effective orifice area was 2.05 cm²; the mean pressure drop across the valve was 6.2 mm Hg, and the mean valve resistance was 29 dyne · s^-1 · cm^-5. Body surface area, heart rate, and valve orifice area showed significant variation with valve sizes, but not cardiac index, transvalvular flow velocity, valve pressure drop, or valve resistance (Table 1).

Time-related changes in valve hemodynamics
In comparison with the early discharge study, there was a significant increase in left ventricular stroke volume (p < 0.001), and a decrease in heart rate (p = 0.001) at late follow-up. Over the same time period, peak transvalvular pressure drop and mean valvular resistance both decreased, and effective orifice area index increased (p = 0.026, 0.002, 0.001, respectively). The decrease in mean transvalvular pressure drop (from 8.1 to 6.5 mm Hg) was insignificant (p = 0.110). The incidence of transvalvular regurgitation was low at discharge, and had not increased significantly at late follow up (Table 2).

	Comment

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

Comparison of in vivo valve hemodynamics
In 1991, four European centers participated in the initial clinical trial of assessing the Prima stentless valve. The early hemodynamic assessment between different centers showed varying results, particularly regarding the mean transvalvular gradient and effective orifice area. Dossche and colleagues [17] reported a mean transvalvular gradient of 12 to 15 mm Hg across the Prima valve at discharge, and found that the Prima valve had similar gradients to those of a stented porcine valve, both being higher than the gradient across an aortic homografts, a finding inconsistent with an invasive study [16]. There were limitations to their study; it was nonrandomized and clinical data with respect to age, cause of valve disease, and number of patient receiving each valve type were not comparable. Taking these limitations into account, it appears that the mean valve pressure gradient and effective orifice area apparently changed little over the 24 months of follow-up [18]. More recently, Bortolotti and colleagues [19] reported a peak systolic gradient of 30 mm Hg 6 months after Prima valve implantation. They attributed the gradients, in part, to inward folding of the Dacron cloth beneath the right coronary ostium. Our early report [4] showed a mean transvalvular pressure gradient at discharge of 8.6 mm Hg with mean valve size of 24 mm, a value comparable to the findings of other stentless valves [7, 11, 15]. The possible explanation for the discrepancies in Prima valve performance between different centers is not entirely clear. Our implantation technique was a subcoronary freehand approach, with the level of the inlet suture line in the outflow tract, after thorough decalcification of the aortic ring and fully scalloping the porcine valve sinuses. We believe these key points may have played a favorable role in determining subsequent hemodynamics of the Prima valve in our series.

The decrease in mean systolic pressure gradient (from 8.1 to 6.2 mm Hg) during the follow-up period did not reach statistical significance, largely because stroke volume increased. Not surprisingly, therefore, mean valve resistance decreased by 47% and effective orifice area increased by 39%. The extent of this improvement in Prima valve hemodynamics is consistent with that reported for other stentless valves [7, 11]. Mean values of left ventricular cavity size, mass index, and geometry index of Prima valve patients at late follow-up are all comparable to those reported after insertion of stentless valves [11]. The wide range of standard deviation of ejection fraction and mass index, however, demonstrates considerable interpatient variations, whose precise determinants need to be investigated in the future. Mild aortic incompetence detected by color Doppler is not an uncommon finding after aortic homograft or stentless porcine valve implantation [17]. Its physiologic significance and impact on long-term ventricular function and valve durability remain to be fully defined. In our study, the incidence of mild aortic regurgitation was low and did not change significantly from discharge to late follow-up. Four patients have moderate (2/4) aortic regurgitation, which has not progressed; therefore, there is no indication for reoperation.

Comparison of in vivo and in vitro performance of the prima valve
As a new bioprosthesis, the Prima valve was subject to comprehensive in vitro testing with pulsatile flow [20]. These data offer an objective reference for clinical (in vivo) assessment. Previous experience with stented bioprostheses have shown that the in vivo valve effective orifice area is 90% to 94% of its in vitro counterpart by 2 years after implantation [21,22]. However, from 2 to 5 years, it decreases to less than 80% of its in vitro counterpart, together with an increase in mean transvalvular pressure gradient. This change probably represents early degeneration of valve function, which precedes clinical symptoms or signs of valve malfunction [22]. The corresponding changes for a stentless valve differs from these in two ways. Although it has a greater in vivo effective orifice area than a stented one of the same size [15], it is correpondingly less than its in vivo counterpart early after implantation [11]. This incomplete utilization of valve orifice area has been attributed in part to impaired left ventricular function causing inefficient flow dynamics early after operation [11]. Two years later, as ventricular function significantly improves, the ratio of in vivo orifice area to its in vitro counterpart increases from 60% to 85%, and transvalvular pressure gradient decreases to 5 mm Hg [11]. Longer term data comparing in vivo and in vitro hemodynamics of the stentless valve is still not available in the literature. We believe that the maintenance of effective orifice area and mean valve resistance at fifth year (in vivo) follow-up suggests that these early degenerative changes seen with stented valves are not occurring with the Prima valve. When considered along with the improvement in in vivo hemodynamics during follow-up, our findings support the idea that a stentless design may offer a more sustainable and fuller utilization of the effective orifice area than those of a stented valve [1].

In conclusion, fifth year assessment of the Prima valve demonstrates that it is a reliable stentless bioprosthesis. It offers favorable medium-term clinical outcome and durable hemodynamic performance. Implantation techniques, however, may affect its hemodynamics, and a more versatile valve design should be considered for future development.

	Acknowledgments

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
This study was supported in part by CardioVascular Group, Baxter Healthcare Corporation, Irvine, CA. We thank Dr Derek Gibson of Royal Brompton Harefield Hospital for reviewing the manuscript.

	References

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 

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