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Ann Thorac Surg 1997;63:362-366
© 1997 The Society of Thoracic Surgeons

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Original Article: Cardiovascular

Hemodynamic Performance of Small Aortic Valve Bioprostheses: Is There a Difference?

Section of Cardiovascular Surgery and Division of Cardiovascular Diseases, Mayo Clinic and Foundation, Rochester, Minnesota

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Background. There is the potential for left ventricular outflow obstruction when small aortic valve bioprostheses are employed in normal-sized or large adults. It has been hoped that bovine pericardial valves would improve hemodynamic performance in the smaller tissue valve sizes.

Methods. To determine in vivo hemodynamic performance of heterograft aortic valve prostheses, we analyzed echocardiographic data from patients receiving 21- or 23-mm Carpentier-Edwards pericardial, Medtronic Intact, and Carpentier-Edwards porcine bioprostheses. In addition, data from 19-mm Carpentier-Edwards pericardial valves were included for comparison of hemodynamic performance between valve sizes. Doppler echocardiography was performed in 151 patients within 2 weeks of operation. Left ventricular outflow gradient was derived from continuous Doppler measurements of flow velocity, and effective orifice area was calculated by the continuity equation.

Results. There were statistically significant differences in hemodynamic performance of different sized prostheses for each valve type (effective orifice area, p < 0.01; valvular gradient, p < 0.03). There were, however, no significant differences in effective orifice area or mean gradient for different valve types within each size category.

Conclusions. The in vivo hemodynamic performance of these three different aortic valve heterograft bioprostheses is similar. Patient–prosthesis mismatch with heterograft prostheses, as demonstrated by the indexed effective orifice area can be avoided by appropriate sizing and use of annular enlarging techniques when necessary.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Despite the steady evolutionary improvement in prosthetic heart valve design, hemodynamic performance of prostheses does not yet match that of native valves. Hemodynamic performance of small stent-mounted heterograft prosthetic valves may be suboptimal. The bulk of the heterograft itself as well as the stent, sewing ring, and other material to which the graft is attached contribute to this [1]. Bioprostheses constructed with bovine pericardium were hoped to have larger effective orifice areas compared with stented porcine heterograft valves with similar sewing ring diameters; however, there are few studies that directly compare the in vivo hemodynamic performance of these basic valve designs. Because of the paucity of comparative data on small bioprosthetic valves [2], we undertook the following study to evaluate hemodynamic performance of three different valve designs.

The Carpentier-Edwards (CE) pericardial bioprosthesis is a low-profile trileaflet valve composed of bovine pericardium that is preserved in a buffered glutaraldehyde solution and mounted on a flexible frame [3]. It was introduced in 1981 and became available for use in the United States in 1991. The hemodynamic characteristics of the pericardial bioprostheses have been reported to be better than those of the porcine valves [2, 4, 5].

The Medtronic Intact aortic valve, developed in 1983 and recently discontinued, is a porcine bioprosthesis that is fixed in low-pressure glutaraldehyde to retain collagen crimp and leaflet pliability. Due to the increased elasticity of its leaflets and presence of impregnated anticalcium agents, it was hoped that the prosthesis would have a prolonged durability over other glutaraldehyde-fixed bioprostheses. Also, the in vitro hemodynamic characteristics of porcine bioprostheses prepared with low-pressure fixation were better than those of other tissue valves in one study of the 27-mm size [2].

These CE pericardial and Medtronic aortic prostheses were compared with our standard aortic bioprosthesis, the CE porcine valve. This is a glutaraldehyde-fixed porcine tissue valve, mounted on a flexible stent with a slightly asymmetric annulus to minimize the effect of the muscular shelf on the orifice of the porcine right coronary leaflet.

The purpose of the present study was to evaluate and compare the in vivo hemodynamic performance of the small sizes of three aortic valve prostheses using postoperative echocardiographic data.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Patient Selection
From 1990 through 1994, 1,574 patients underwent aortic valve replacement at the Mayo Clinic, and 897 patients received bioprostheses. Among these, 151 patients received bioprostheses that were 23 mm or less in size, which represented the study group. Patients undergoing combined mitral valve replacement were not included in the study group. The valves used included the CE porcine (model 2625), the CE pericardial (model 2800), and the Medtronic Intact prostheses. Aortic bioprostheses are routinely placed in the supraannular position at our institution, although the technique was not always stated in the operative note (pledgeted, interrupted, horizontal mattress sutures passed through the annulus from the ventricular side to the aortic side, and then through the valve sewing ring). Clinical data and follow-up information were obtained from medical records and telephone conversations with all patients. For purpose of comparison, we analyzed the Doppler echocardiographic study performed most recently after operation; in most patients (90%) this was obtained before hospital dismissal after aortic valve replacement. None of the patients were symptomatic or was suspected of having prosthetic valve dysfunction at the time of echocardiographic examination. Choice of type of bioprosthesis was at the discretion of the operating surgeon.

Doppler Echocardiography
Complete two-dimensional, transthoracic spectral Doppler, and color-flow studies were performed using commercially available ultrasound instruments (Acuson 128 XP and Hewlett Packard Sonos %). The aortic prostheses were imaged with particular attention focused on cusp thickness and motion, sewing ring stability, and the presence of vegetations or valve bed abnormalities (such as pseudoaneurysm). The time velocity integral (TVI) for the left ventricular outflow tract (LVOT) was obtained from pulsed-wave Doppler interrogation in apical long-axis format.

During acquisition of the LVOT velocity spectrum, special care was taken to position the sample volume to avoid the region of flow convergence immediately below the prosthesis. Each prosthesis was evaluated with continuous-wave Doppler echocardiography using a nonimaging probe in multiple positions including apical, right parasternal (with patient in the right lateral decubitus position), left parasternal, subcostal, suprasternal, and right supraclavicular. In each position the transducer was manipulated to obtain the maximum velocity spectrum (using both video and audio feedback).

The prostheses were evaluated for regurgitation by color-flow imaging of the LVOT in parasternal long- and short-axis as well as apical long-axis formats. Regurgitation was also evaluated during the continuous-wave Doppler examination from the apical position.

All hemodynamic measurements were made on-line; at least three cycles were averaged for patients in sinus rhythm, and at least five cycle for patients with irregular rhythms. The mean aortic valve gradient was measured using continuous-wave Doppler velocity spectra from the transducer position that yielded the maximum velocity across the prosthesis. These spectra were electronically traced on the video screen and the mean gradient was calculated, using the instrument's software, from the simplified Bernoulli equation. This same trace also yielded the TVI for the prosthesis. The TVI of the LVOT was obtained by electronically tracing the velocity spectra from the LVOT pulsed-wave Doppler examination. The effective orifice area (EOA) of the prosthesis was calculated according to the continuity equation:

(1)

where D = diameter of the left ventricular outflow tract, V_lvot = the velocity time integral of flow from the left ventricular outflow tract, and V_av = the velocity time integral of flow through the aortic valve. Prosthetic TVI = TVI of the aortic valve, and SRd = sewing ring diameter of the prosthetic valve. To provide uniformity, the sewing ring diameter was used for all valve area measurements.

The degree of valvular regurgitation was graded as none (grade 0), trivial or mild (grade 1), moderate (grade 2), moderately severe (grade 3), and severe (grade 4). Regurgitation was identified by visual assessment of the proportion of the LVOT, immediately below the prosthesis, occupied by regurgitant color-flow signals [6, 7]. Semiquantitation was assisted by noting the density of high-velocity regurgitant signals in the continuous-wave Doppler spectrum and also by noting the degree of diastolic flow reversal in the descending thoracic aorta during pulsed-wave examination [7].

Data Analysis
All variables are expressed as mean ± standard deviation except where otherwise noted. Differences in hemodynamic values between the valve groups were tested for significance with the Student-Newman-Keuls test and the Bonferroni (Dunn) t test.

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Average age of the 151 patients was 77 years (range, 21 to 98 years) and 58% were female. Preoperatively, 97% of patients had New York Heart Association class III or IV disability. The cause of aortic valve disease was stenosis in 82% of patients, mixed stenosis and regurgitation in 16%, and regurgitation in 2% of patients. Twenty-two patients (14.6%) had undergone prior cardiac operations. At the time of aortic valve replacement, 73 patients (48.3%) underwent concomitant coronary artery bypass and 16 (10.6%) had concomitant septal myectomy; pericardial patches were used in 23 (15.2%) to augment the aortic valve annulus [8]. Clinical features are stratified by valve type and size in Table 1.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
The in vivo hemodynamic performance of small bioprosthetic aortic valves is becoming increasingly relevant. Surgical pathology of aortic valve disease is changing; previously, bicuspid aortic valves were the primary cause of aortic valve stenosis and regurgitation in patients having valve replacement [9]. In current practice, the most common pathologic finding is senescent calcification of otherwise normal tricuspid valves [10, 11]. Changes in the prevalence of the surgical pathology of the aortic valve almost certainly reflect the changes in the age of patients referred for valve replacement. More elderly patients with aortic valve disease are being accepted for operation, and senescent aortic valve stenosis is very prevalent in this population [12]. The outlook for operative survivors is excellent, with good relief of symptoms and an expected survival that is equal to the norm for an elderly group of patients [13]. Tissue valves are the preferred prosthesis in the elderly due to the patient's relatively shorter life expectancy, good durability of tissue prostheses in the elderly, and the increased potential for complications from long-term anticoagulation with age.

Senescent calcification of the aortic valve is not associated with dilatation of the aortic valve annulus or proximal ascending aorta. Indeed, the aortic valve annulus is frequently small, especially in women who have a small body surface area. Thus, there is a need to understand the hemodynamic performance of small aortic bioprostheses.

Using postoperative echocardiographic analysis, we found that, for the 21- and 23-mm sizes, there is no significant difference in transvalvular gradients or EOAs between the CE pericardial, Medtronic Intact, or CE porcine bioprostheses.

The in vivo performance of the CE pericardial valve has been studied by others, and our results are similar to these reports. Frater and associates [14] and Salomon and colleagues [15] reported results of echocardiographic evaluation of smaller CE pericardial valves in the aortic position at 4.8 years and 7 years postoperatively, respectively. They both reported slightly smaller mean gradients, but also slightly smaller EOAs, for each valve size, as compared with our results. Cosgrove and associates [16] and Pelletier and colleagues [17] reported intraoperative hemodynamic evaluation of the in vivo performance of the CE pericardial valve. The measured mean gradient and calculated EOA for each valve size in these studies were similar to our findings.

Cosgrove and co-workers [4] compared the in vivo hemodynamic performance of CE pericardial and standard CE porcine valves for the larger (25-mm) aortic prostheses. Their group used intraoperative measurements and found that the larger CE pericardial valves were less obstructive, with a larger EOA, compared with similarly sized CE porcine valves. However, they did not evaluate the smaller aortic bioprostheses. An in vitro study of 27-mm valves found pericardial valves to also have better hemodynamic characteristics than porcine valves [2]. Our study provides data directly comparing the different valve types at the smaller sizes, and also compares the in vivo performance of tissue bioprostheses.

Some limitations of the present study should be acknowledged. First, it might be argued that the Doppler echocardiographic method is not sufficiently sensitive to detect small differences in mean gradients or orifice areas among the different models of bioprostheses. Over the past decade, the Doppler technique has become the established method for quantifying aortic valve stenosis and for observing its progression.

The continuity equation can be applied to the assessment of prosthetic valves in the aortic position with a high level of accuracy and can be useful in the diagnosis of bioprosthetic aortic valve stenosis as evidenced by previous studies [18, 19]. Further, bioprosthetic aortic valves have a central flow, which simplifies assumptions and measurement of peak velocity. There is close correlation between mean aortic valve gradient measured during cardiac catheterization and that measured by continuous-wave Doppler echocardiography as evidenced by the study by Burstow and associates [20], who found a correlation coefficient of 0.94. In our study, valves areas were calculated by the simplified continuity equation, using the sewing ring diameter for approximation of the left ventricular outflow diameter. This version of the continuity equation has been validated against the more complete form [21]. Use of the sewing ring approximation for calculation of prosthetic valve EOA has also been validated [18, 19], but this approximation may lead to a slight increase in EOA compared with calculations made using direct measurement of the LVOT diameter (as we found when compared with some other echocardiographic studies) [14, 15]. This difference should not affect our results, because we used the sewing ring diameter for measurement in all three valve types, and our purpose in this report is to allow in vivo comparison of the valve types. Furthermore, the bioprosthetic valves at our institution are routinely placed in the supraannular position; thus, any differences based on different suturing techniques are unlikely to be a factor.

Indeed, our data confirm the usefulness of Doppler echocardiography for comparative studies of aortic valve bioprostheses. For each valve model, we found significant differences in orifice areas and transvalvular gradients comparing 21-mm and 23-mm sizes.

A second potential criticism of our study is that hemodynamic assessments were made in the resting state early after operation. It is possible that minor differences between valve models in mean gradients or orifice areas might become manifest when transvalvular flow is increased by exercise or catecholamine infusion. It seems, however, unlikely that any clinically important differences in valve function would be missed by these studies; indeed, there were no real trends in hemodynamic performance favoring one heterograft design over the other. Additionally, resting hemodynamics are considered adequate for evaluation of native aortic valve disease and for follow-up of prosthetic valves, and in clinical practice, additional maneuvers to increase transvalvular flow add little to hemodynamic assessment of valves except in conditions where resting cardiac output is markedly reduced.

The third potential limitation of our study is that the CE porcine valves were placed over a slightly earlier time period than the Intact and CE pericardial prostheses. It is unlikely that this affects the results, because the methods used to evaluate the performance of each of the valve types are similar. In the patients studied we did not find any statistical difference between the different valve types. In addition, to establish uniformity, we compared multiple demographic variables for the different valve groups. They were very similar, and there was no statistically significant difference in age, New York Heart Association class, body surface area, ejection fraction, aortic valve area, or cardiac output for the different groups. Thus, comparisons between groups would seem justified.

The small aortic annulus presents the potential for iatrogenic "valve prosthesis–patient mismatch" [22]. The sewing ring of a prosthetic valve reduces its EOA to one that is smaller than the native orifice, resulting in a valve that may not provide the necessary clinical or hemodynamic benefit to a patient. An alternative to the placement of a small valve in the narrow aortic annulus is the use of an annulus-enlarging procedure to accommodate a larger valve. We have had good results with this technique [8].

Pelletier and associates [23] compared the porcine and pericardial valves in the aortic position and found that freedom from reoperation and rates of complications were similar for the groups 6 years postoperatively. Cosgrove and colleagues [24] have recently reported excellent durability for the CE pericardial valve at 10 years. Our follow-up for the CE pericardial prosthesis is relatively short at this time, and we are not able to make any statements about the durability of the different valve types. Also, the significance of transprosthetic gradients and their influence on long-term survival of patients is not yet clear. Further long-term studies need to be carried out to answer these questions.

In conclusion, in vivo hemodynamic performance of the smaller sizes of these three aortic valve heterograft prostheses is similar. Prosthesis–patient mismatch with heterograft prostheses can be avoided by appropriate sizing and use of annular enlarging techniques when necessary.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Presented at the Poster Session of the Thirty-first Annual Meeting of The Society of Thoracic Surgeons, Palm Springs, CA, Jan 30–Feb 1, 1995.

Address reprint requests to Dr Daly, Section of Cardiovascular Surgery, Mayo Clinic and Mayo Foundation, 200 First St SW, Rochester, MN 55905.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 

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This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Monica L. McDonald Richard C. Daly Hartzell V. Schaff Charles J. Mullany Fletcher A. Miller James J. Morris Thomas A. Orszulak Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by McDonald, M. L. Articles by Orszulak, T. A. Search for Related Content PubMed PubMed Citation Articles by McDonald, M. L. Articles by Orszulak, T. A.