Ann Thorac Surg 1997;63:1128-1132
© 1997 The Society of Thoracic Surgeons
Original Article: Cardiovascular
Dynamic Evaluation of the 21-mm Medtronic Intact Aortic Bioprosthesis by Dobutamine Echocardiography
Isaac Kadir, FRCS,
Mohammad Bashar Izzat, FRCS,
Peter Wilde, FRCR,
Barnaby Reeves, DPhil,
Alan J. Bryan, FRCS,
Gianni D. Angelini, FRCS
Bristol Heart Institute, Department of Clinical Radiology, and Research & Development Support Unit, University of Bristol, Bristol, United Kingdom
Accepted for publication November 7, 1996.
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Abstract
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Background. High residual transvalvular gradients have been reported with the use of small Medtronic Intact aortic valve prostheses. The aim of this study was to evaluate the hemodynamic performance of 21-mm prostheses using dobutamine Doppler echocardiography.
Methods. Ten patients (7 women; mean age, 79 years) who had undergone aortic valve replacement with 21-mm Medtronic Intact prostheses 19.1 ± 9.9 (standard deviation) months previously were studied. Dobutamine infusion was started at a rate of 5 µg•kg-1•min-1 and increased to 10 and 20 µg•kg-1•min-1 at 15-minute intervals. Pulsed and continuous-wave Doppler studies were performed at rest and at the end of each stage. Effective orifice area, performance index, and discharge coefficient of each valve were calculated, and peak and mean velocity and pressure drop across the prostheses were measured. Cardiac output was determined by Doppler measurement of flow in the left ventricular outflow tract.
Results. Dobutamine stress increased heart rate and cardiac output by 68% and 65%, respectively (both p < 0.005), and mean transvalvular gradient increased from 19.1 ± 5.1 mm Hg at rest to 33.2 ± 7.7 mm Hg at maximum stress (p < 0.0001). Regression analyses demonstrated that maximum-stress gradient was independent of all variables except resting gradients (p < 0.004). Body surface area had no effect on the changes in cardiac output, effective orifice area, or transprosthetic gradient at maximum stress.
Conclusions. These data show that the 21-mm Medtronic Intact aortic prosthesis exhibits acceptable hemodynamic performance. Transvalvular gradients remained within a clinically acceptable range, both at rest and at maximum stress. Moreover, overall hemodynamic performance suggests that patient-prosthesis mismatch is unlikely to be a problem of clinical importance when this prosthesis is used.
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Introduction
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The Medtronic Intact valve (Medtronic, Inc, Minneapolis, MN) was the first zero-pressure glutaraldehyde-fixed bioprosthesis to be introduced. This is a high-profile porcine valve mounted on a flexible creep-resistant Celcon stent, and treated with toluidine blue as an anticalcification agent [1]. Satisfactory medium-term durability of the Intact bioprosthesis, comparable with that of other porcine bioprostheses, has been documented [2, 3]. Recent reports on this valve, however, have demonstrated high transvalvular gradients when a small prosthesis is inserted in the aortic position [4, 5], and some authors have even advised against the use of prostheses smaller than 23 mm [6], considering that significant gradients across the left ventricular outflow tract may place persistently high demands on the left ventricle and delay the regression of left ventricular hypertrophy [7]. In this study we sought to evaluate the in vivo hemodynamic performance of small (21-mm) Medtronic Intact aortic prostheses by means of dobutamine Doppler echocardiography.
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Material and Methods
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Ten consecutive patients (7 women; mean age, 79 ± 5 [standard deviation] years; range 71 to 86) who underwent isolated aortic valve replacement for aortic stenosis with a 21-mm Medtronic Intact bioprosthesis at our institution were studied. Body surface area was 1.75 ± 0.16 (standard deviation) m2. The patients were studied 19.1 ± 9.9 (standard deviation) months postoperatively; 9 were in New York Heart Association functional class I, and 1 was in class II. The left ventricular outflow tract diameter was 18.5 ± 1.9 (standard deviation) mm. All patients were in sinus rhythm and receiving no regular medications. Preoperative coronary angiography did not demonstrate any significant coronary artery disease in any subject.
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Dobutamine-Stress Protocol
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The study protocol was approved by the United Bristol Healthcare Trust Ethics Committee, and written informed consent was obtained from all subjects. Patients underwent stress echocardiography after a 3-hour fast. After a detailed history and physical examination to exclude the presence of any contraindication to stress testing [8], complete prestress two-dimensional echocardiography was performed to exclude bioprosthesis malfunction, other valvular disease, or severe left ventricular dysfunction. Apical four-chamber views were then acquired from which baseline (rest) Doppler measurements of transvalvar flow were obtained (see below).
Using a peripheral venous cannula, a graded infusion of dobutamine was administered intravenously at increments of 5, 10, and 20 µg•kg-1•min-1 at 15-minute intervals. During the study, patients underwent continuous electrocardiographic monitoring, and blood pressure was recorded at 5-minute intervals with an automated cuff. Criteria for stopping the dobutamine infusion included (1) hypotension (systolic blood pressure < 100 mm Hg), (2) dyspnea, or (3) significant ventricular or supraventricular arrhythmias. Repeat (stress) Doppler measurements were obtained before each incremental increase in the infusion rate. After the completion of the final assessment at a dose of 20 µg•kg-1•min-1 (maximum stress), dobutamine infusion was discontinued, and the patient was monitored for a minimum of 20 minutes or until heart rate (HR) had returned to prestress values.
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Doppler Measurements and Calculations
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All tests were performed by an experienced echocardiographer. Echocardiography was carried out using an Aloka SSD-830 ultrasound system with a 2.5-MHz transducer (Aloka, Japan) with facilities for continuous-wave and pulsed-wave Doppler. Parasternal long-axis views were obtained, and the early-systolic diameter (D) of the left ventricular outflow tract (LVOT) was measured just below the prosthetic valve using an inner-edge-to-inner-edge method. For each patient, an average of three diameter measurements was used. The LVOT cross-sectional area (CSA) was calculated as CSA = 
•D2/4.
The pulsed-wave Doppler cursor was then placed in the left ventricular outflow tract immediately proximal to the aortic valve, and pulsed-Doppler flow velocity was recorded. Peak and mean velocities in the LVOT were then measured. Cardiac output (CO) was calculated as follows: CO = VTI•CSA•HR, where VTI is the velocity time integral in the LVOT, and HR is in beats per minute.
Systolic valve flow (Q) was also calculated as follows: Q = CO/(Systolic ejection period•HR). Flow velocity across the bioprosthesis was obtained by means of continuous-wave Doppler echography from the apical view. Great care was taken to orient the transducer so that the angle between the Doppler cursor and LVOT was as close to 0 degrees as possible, and to obtain the highest possible velocity signal. Peak velocity was measured, averaging from three velocity envelopes, and mean velocity was calculated by on-line averaging of the instantaneous velocities measured throughout the velocity complexes. Measurements were made in triplicate in each stage to ensure reproducibility. The modified Bernoulli equation was used to calculate peak and mean pressure drop (gradient) across the bioprosthesis as follows: 
P = 4(VCW2 - VPW2), where P is pressure drop and VCW and VPW are the velocities (peak or mean) across the valve (using continuous-wave Doppler) and in the LVOT (using pulsed-wave Doppler), respectively. Velocity ratio (VR), the ratio of mean subaortic to mean transvalvular velocity, gives an approximate guide to orifice behavior, independent of measurements of LVOT diameter [9]. The prosthetic valve effective orifice area (EOA) was calculated using the modified continuity equation as follows: EOA = CSA•VR.
This simplified equation has shown an extremely good correlation with that of the original continuity equation [10]. The EOA index (EOAI), a measure of how well the flow area of the valve matches the body size, is calculated as follows: EOAI = EOA/BSA, where BSA is patient's body surface area. This index is used to detect mismatch between valve size and body surface area. The discharge coefficient (Cd), a measure of how effectively the valve uses its nominal flow area, is calculated as follows: Cd = EOA/AOA, where AOA is the actual (nominal) orifice area, as provided by the manufacturer (Fig 1
). Performance index (PI), a measure of how effectively the external dimension of the valve is used in providing forward flow, is calculated as follows: PI = EOA/SRA, where SRA is the sewing ring area of the prosthesis, as provided by the manufacturer.
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Fig 1. . The Medtronic Intact valve. (A) Annulus diameter = 21 mm. (B) Sewing ring diameter = 27 mm. (C) Aortic profile = 15.7 mm. The minimum calibrated orifice area is 1.25 cm2.
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Statistical Analysis
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Parameters were calculated for each patient at each level of dobutamine infusion, and data are presented as mean ± standard deviation. Only changes in CO, P, and EOA were analyzed, because all other parameters are derived from these and, therefore, are not independent. Paired t tests of rest and maximum stress measures were used to determine the effect of dobutamine stress on these variables. A regression equation for predicting P at maximum stress was also determined by regression modeling.
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Results
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All patients had good left ventricular function, and no impairment in regional myocardial contractility with dobutamine stress could be detected in any patient. Dobutamine infusion was well tolerated; the only side effect was the development of infrequent atrial or ventricular ectopic beats (40% of patients). Significant increases in HR, CO, and Q (Fig 2), and a significant decrease in mean arterial blood pressure (Table 1), were observed when comparing rest with maximum dobutamine-induced stress.
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Fig 2. . Increase in systolic valve flow and mean transprosthetic gradient with increasing doses of dobutamine. Data are presented as mean ± standard error.
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As described above, differences between rest and maximum stress were analyzed using a paired t test for the variables that were directly measured, ie, mean arterial blood pressure, CO, P, and EOA. Mean arterial blood pressure decreased significantly at maximum stress (mean difference = -20 mm Hg; 95% confidence interval = -31.8 to -8 mm Hg; p = 0.004), whereas CO (mean difference = 2.8 L/min, 95% confidence interval = 1.2 to 4.4 L/min, p = 0.003) and P (mean difference = 14.1 mm Hg, 95% confidence interval = 10.8 to 17.4 mm Hg, p < 0.0001) increased significantly (see Fig 2
). Effective orifice area did not change significantly between rest and maximum stress (mean difference = 0.12 mm2, 95% confidence interval = -0.06 to -0.30 mm2, p = 0.18).
Regression analyses were carried out, with P at maximum stress as the dependent variable and other variables (age, BSA, mean arterial blood pressure, CO, EOA, and P at rest) as predictors. In univariate regressions, only P at rest was a significant predictor. This regression gave a reasonable fit (r2(adj) = 62%, F = 15.8, degrees of freedom = 1 and 8, p < 0.004): stress P = 9.75 + 1.23 (rest P). No combination of stress P with any other predictor variables gave a better fit. This regression equation suggests that maximum-stress P is independent of all the variables except rest P. The lack of association between BSA and maximum-stress P (univariate regression: r2 (adj) = <1%, t = 0.78, p = 0.46; bivariate regression, r2 (adj) = 61%; t for BSA = 0.88, p = 0.41) is of particular interest. It might be argued, however, that BSA is nevertheless important if it predicts rest P and through this relationship, maximum stress P. A univariate regression of BSA on rest Ps was performed to investigate this possibility and demonstrated that there is no significant relationship (r2(adj) < 1%; t = 0.29; p = 0.78).
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Comment
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The implantation of valve prostheses in small aortic annuli is tempered by concerns over their hemodynamic performance [4]. Both mechanical and bioprosthetic valves demonstrate the so-called patient-prosthesis mismatch, where valve components (mainly the sewing ring in bioprostheses) reduce its EOA to one that is smaller than the native aortic orifice, potentially producing a hemodynamically significant functional stenosis. This is thought to occur more frequently when a small prosthesis is inserted in a patient with a large BSA [11, 12]. It is important, therefore, when selecting a replacement prosthetic valve to choose a prosthesis that exhibits the best utilization of its orifice area to minimize the consequences of patient-prosthesis mismatch that might offset the benefits of the procedure [12].
In addition to valve type and size, Ps are dependent on flow across the prosthesis [13, 14]. Indeed, even in a valve prosthesis that demonstrates acceptable hemodynamic performance at rest, a rise in stroke volume (eg, during exercise) may considerably increase the pressure-drop across the valve, unmasking suboptimal valvular function [15, 16]. To obtain meaningful data, therefore, the hemodynamic performance of an aortic replacement valve should be evaluated under a range of flow conditions. We have previously reported the utility of dobutamine-Doppler echocardiography in the evaluation of aortic prostheses [17, 18]. With this approach, changes in Ps can be continuously documented while CO is manipulated by increasing doses of dobutamine, thereby providing paired flow-gradient data in a well-controlled fashion.
The hemodynamic performance of the Medtronic Intact prosthesis has received little attention; moreover, some studies have reported data obtained through assembling patients with more than one valve size [2, 6]. Recently, compromise of the EOA and elevation of P were demonstrated with smaller Intact aortic bioprostheses, both in vitro [4] and in vivo [5, 19]. Still, Mullany and colleagues [20] reported a mean resting P of 18 mm Hg in 17 patients with 21-mm Medtronic Intact aortic prostheses, identical to a mean gradient of 17.6 mm Hg in a similar group of patients who had a 21-mm Standard Carpentier-Edwards bioprosthesis implanted.
This current study presents a detailed analysis of the hemodynamic performance of small Medtronic Intact aortic prostheses at variable flow conditions. In this group of patients, the average calculated EOA was 1.04 ± 0.21 cm2 at rest and 1.16 ± 0.29 cm2 at maximum stress, which is identical to previously reported in vitro measurements [4].
From the hemodynamic data of valve behavior at rest and the expected increase in CO with exercise, an EOAI greater than 0.9 cm2/m2 has been predicted as a requirement to minimize postoperative Ps [11, 21]. Similar to previous reports [22], EOAI at rest in this group of patients was 0.62 ± 0.16 cm2/m2, less than the minimum requirement implied above. Yet, this was not associated with unacceptably high Ps, even at high-flow conditions (33.2 ± 7.7 mm Hg). This indicates that the orifice of the Medtronic Intact bioprosthesis is effectively utilized to provide forward flow, and therefore generate only small gradients across a given orifice area.
The detailed data analyses showed no evidence of an effect of BSA on P, change in P with stress, or EOA. The sample size in this study is small, but these findings suggests that patient-prosthesis mismatch may not be a problem of clinical significance when this prosthesis is used.
In conclusion, the 21-mm Medtronic Intact aortic bioprosthesis appeared to have adequate hemodynamic performance, with moderate P generation under both rest and stress conditions. Maximum-stress Ps were highly predictable and directly dependent on rest Ps. The acceptance of this P should be considered individually for each patient, taking into account parameters like age and preexisting aortic gradient, balanced against the potential increased risk if aortic root enlargement is considered.
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Acknowledgments
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We gratefully acknowledge the assistance of Mrs Catherine Walsh in performing the echocardiographic examinations.
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Footnotes
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Address reprint requests to Mr Kadir, Bristol Heart Institute, Bristol Royal Infirmary, Bristol BS2 8HW, United Kingdom.
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References
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Abstract
Introduction
