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Ann Thorac Surg 2001;72:49-53
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Hemodynamic performance of the 21-mm Sorin Bicarbon mechanical aortic prostheses using dobutamine Doppler echocardiography

Accepted for publication March 23, 2001.

Address reprint requests to Dr Angelini, Bristol Heart Institute, Department of Cardiac Surgery, Bristol Royal Infirmary, Bristol BS2 8HW, United Kingdom
e-mail: g.d.angelini{at}bristol.ac.uk

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. Small-sized mechanical aortic prostheses are commonly associated with generation of high transvalvular gradients, particularly in patients with large body surface area, and can result in patient–prosthesis mismatch. This study evaluates the hemodynamic performance of 21-mm Sorin Bicarbon bileaflet mechanical prostheses using dobutamine stress echocardiography.

Methods. Fourteen patients (7 women; mean age, 63 ± 8 years) who had undergone aortic valve replacement with a 21-mm Sorin Bicarbon bileaflet mechanical prosthesis 32.4 ± 5.1 months previously were studied. After a resting Doppler echocardiogram, a dobutamine infusion was started at a rate of 5 µg · kg^-1 · min^-1 and increased to 30 µg · kg^-1 · min^-1 at 15-minute intervals. Pulsed- and continuous-wave Doppler echocardiographic studies were performed at rest and at the end of each increment of dobutamine. Both peak and mean velocity and pressure gradient across the prostheses were measured, and effective orifice area, discharge coefficient, and performance index were calculated.

Results. Dobutamine stress increased heart rate and cardiac output by 83% and 81%, respectively (both p < 0.0001), and mean transvalvular gradient increased from 15.6 ± 5.5 mm Hg at rest to 35.4 ± 11.9 mm Hg at maximum stress (p < 0.0001). Although the indexed effective orifice area was significantly lower in patients with a larger body surface area, this was not associated with any significant pressure gradient. The performance index of this valve was unchanged throughout the study. Regression analyses demonstrated that the mean transvalvular gradient at maximum stress was independent of all variables except resting gradient (p = 0.05). Body surface area had no association with the changes in cardiac output, transvalvular gradient at maximum stress, and effective orifice area.

Conclusions. These data show that the 21-mm Sorin Bicarbon bileaflet mechanical prosthesis offers an excellent hemodynamic performance with full utilization of its available orifice when implanted in the aortic position. The lack of significant transvalvular gradient in patients with a larger body surface area suggests that patient–prosthesis mismatch is highly unlikely when this prosthesis is used.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
The Sorin Bicarbon mechanical valve is a third-generation bileaflet prosthesis that has been in widespread use in the United Kingdom since 1991 [1, 2]. In vitro studies of this valve in the aortic position when compared with the St. Jude, Carbomedics, and modified Edwards-Duromedics revealed that it had the lowest pressure difference with regard to both pulsatile and nonpulsatile flow [3]. Valve analysis under laboratory conditions, however, does not accurately reflect in vivo performance. Furthermore, Doppler echocardiographic assessment in the resting supine patient is unable to demonstrate the hemodynamic state during exercise when a high gradient across the valve could be generated under conditions of increased cardiac output [4]. Although satisfactory intermediate clinical results with this valve have been published, no studies have yet addressed the in vivo hemodynamics under conditions of increased cardiac output [5, 6]. This study aims, therefore, to characterize the hemodynamic performance of the size 21-mm Sorin Bicarbon bileaflet mechanical prosthesis when implanted in the aortic position using dobutamine stress echocardiography [7–9].

	Material and methods

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Patient population
Fourteen patients (7 women) with aortic valve stenosis who underwent aortic replacement with the 21-mm Sorin Bicarbon (Saluggia, Italy) bileaflet mechanical prosthesis (32.4 ± 5.1 months previously) were recruited into the study. The mean age was 63 ± 8 years (range, 44 to 73 years). Mean body surface area was 1.88 ± 0.20 m². Nine patients underwent isolated aortic valve replacement, and 3 patients received coronary artery bypass grafting at the same time. Two patients underwent combined mitral (valve repair and valve replacement with 27-mm Sorin Bicarbon bileaflet) and aortic procedures. The mean left ventricular outflow tract diameter was 19.8 ± 2.7mm. At the time of the study, 12 patients were in New York Heart Association functional class I and 2 patients were in class II. All patients were in sinus rhythm, and taking no medication except warfarin.

Dobutamine stress protocol
The study protocol was approved by the United Bristol Healthcare Trust Ethics Committee, and written informed consent was obtained from all patients undergoing the study. The dobutamine stress protocol has been previously reported [6–8]. Apical four-chamber views were obtained, and baseline (rest) Doppler measurements of transvalvular flow and gradient were performed. A peripheral venous cannula was used for graded infusion of dobutamine. Increments of 5, 10, 20, and 30 µg · kg^-1 · min^-1 of dobutamine at 15-minute intervals were used. During the course of the study, patients received continuous electrocardiographic monitoring with blood pressure being measured at 5-minute intervals. Criteria for discontinuing the dobutamine infusion included hypotension with systolic pressure less than 100 mm Hg, dyspnea, or development of significant ventricular or supraventricular arrhythmias. Echocardiographic Doppler measurements were obtained just before each incremental increase in the infusion rate.

Doppler measurements and calculations
An experienced echocardiographer performed all tests so as to minimize interobserver variation. Parasternal long-axis views were obtained, and the early systolic diameter (D) of the left ventricular outflow tract 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 adopted. The left ventricular outflow tract cross-sectional area (CSA, in square meters) was calculated as follows: 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-wave Doppler flow velocity was recorded. Peak and mean velocities in the left ventricular outflow tract were then measured. Cardiac output (CO, in liters per minute) was calculated as follows: CO = VTI x CSA x HR, where VTI is the velocity time integral in the left ventricular outflow tract, and HR is heart rate in beats per minute.

Flow velocity across the valve was obtained by means of continuous-wave Doppler after interrogation from multiple windows. 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. The modified Bernoulli equation was used to calculate pressure drop (gradient, in millimeters of mercury) across the prosthesis as follows: P = 4 (V_CW² - V_PW²) where P is pressure drop, and V_CW and V_PW are the velocities (peak or mean) across the valve (using continuous-wave Doppler) and in the left ventricular outflow tract (using pulsed-wave Doppler), respectively. The mean pressure drop was calculated by applying the modified Bernoulli equation at multiple instantaneous velocities throughout the velocity profile. Velocity ratio (VR), the ratio of mean subaortic to mean transaortic velocity, gives an approximate guide to orifice behavior, independent of measurements of left ventricular outflow tract diameter [10].

The prosthetic valve effective orifice area (EOA, in square centimeters) was calculated using the modified continuity equation as follows: EOA = CSA x VR. This simplified equation has shown an extremely good correlation with that of the original continuity equation [11]. 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 the patient’s body surface area. The discharge coefficient (C_d), a measure of how effectively the valve uses its nominal flow area, is calculated as follows: C_d = EOA / AOA, where AOA is the actual (nominal) orifice area, as provided by the manufacturer. Performance index (PI), a measure of how effective 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.

Statistical analysis
The above-mentioned variables were calculated for each patient at each level of dobutamine infusion, and the data are presented as mean ± standard deviation. Paired Student’s t tests of the rest and maximum stress measurements were used to determine the effect of dobutamine stress on these variables. Regression analyses were carried out with P at maximum stress as the dependent variable and other variables (age, BSA, CO, mean arterial blood pressure, EOA, and P at rest) as predictors.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
Prestress echocardiography showed good left ventricular function in all patients. Four patients experienced occasional atrial or ventricular ectopics beats. The hemodynamic and Doppler data at rest and with dobutamine stress are presented in Table 1. Heart rate increased by 83% with dobutamine from a resting rate of 69 ± 15 beats/min to 126 ± 29 beats/min (p < 0.0001). Cardiac output increased by 81% from 5.7 ± 1.8 L/min at rest to 10.3 ± 3.4 L/min with maximum dobutamine (p < 0.0001). There was a correspondingly significant increase in systolic valve flow with maximum dobutamine compared with rest (p < 0.0001). Consistent with the actions of dobutamine, there was a significant fall in mean blood pressure at maximum infusion (resting, 104 ± 14 mm Hg; maximum dobutamine, 85 ± 16 mm Hg; mean difference, -18.2 mm Hg; p = 0.0009).

Because patient–prosthesis mismatch is more likely when a small valve is inserted in a patient with a larger BSA, the study population was subdivided into those with a BSA greater (n = 6) or less than 2 m² (n = 8); (Table 2). In those with a BSA less than 2 m² (4 men, 4 women), the EOAI at rest was unchanged when compared with maximum stress (1.01 ± 0.52 cm²/m² versus 1.08 ± 0.44 cm²/m², respectively; p = 0.38). In these patients mean gradient was 13 ± 4.1 mm Hg at rest, which increased to 31.1 ± 8.9 mm Hg at maximum stress (p < 0.0001). Patients with a BSA greater than 2 m² (3 men, 3 women) had a smaller EOAI (0.65 ± 0.15 cm²/m² at rest; 0.68 ± 0.28 cm²/m² at maximum stress; p = 0.48), but the mean gradient was not unduly increased (20.4 ± 4.5 mm Hg at rest and 45.0 ± 13.1 mm Hg at maximum stress; p = 0.02). Performance index was similar at rest or maximum stress in patients with BSA greater or less than 2 m².

	Comment

Top Abstract Introduction Material and methods Results Comment References

 
The implantation of valve prostheses in small aortic annuli is tempered by concerns over their hemodynamic performance [12]. Several Doppler echocardiography studies have shown that most mechanical and bioprosthetic valves are at least mildly stenotic and that relatively high transprosthetic gradients can be observed after the operation, despite normal prosthetic function [4, 13–15]. This is thought to occur more frequently when a small prosthesis is inserted in a patient with a large BSA, resulting in patient–prosthesis mismatch [16]. Physiologically, it must be remembered that transprosthetic gradients are essentially determined by both the EOA and transvalvular flow. Transvalvular flow, in turn, is related to cardiac output, which at rest is largely determined by BSA. Mismatch, therefore, is deemed to occur when the indexed EOA is reduced (ie, when the size of the prosthetic orifice is too small in relation to the patient’s body size) and is manifested as the problem of high transprosthetic gradients [12].

It has been demonstrated that to avoid any significant gradient at rest or on exercise the indexed EOA should ideally be not less than 0.85 to 0.90 cm²/m² [17]. Therefore one of the objectives of aortic valve replacement should be to ensure that the indexed EOA after the operation is above this level to avoid residual stenosis. New prosthetic valves are currently tested before marketing with in vitro rigs and by cardiac catheterization. When a valve prosthesis is therefore first introduced into clinical use, there are no normal standards to refer to in the interpretation of echocardiographic studies of that prosthetic valve or to adhere to recommendations for the prevention of patient–prosthesis mismatch [18].

In this in vivo study of a 21-mm Sorin Bicarbon bileaflet mechanical aortic valve using dobutamine echocardiography, hemodynamic data including EOAI and transprosthetic gradients are obtained under a range of flow conditions (cardiac output). Calculation of the EOA by the continuity equation represents a measure of valve performance that is independent of flow. The 21-mm Sorin bileaflet valve showed a mean EOA at rest (1.57 ± 0.71 cm²) larger than that reported for the same size St. Jude (1.51 ± 0.59 cm²) or Carbomedics (1.20 ± 0.62 cm²) bileaflet valve studied with the same methodology [8]. The stress value for the Sorin bileaflet valve was (1.74 ± 0.66 cm²) compared with the St. Jude (1.40 ± 0.61 cm²) or the Carbomedics (1.23 ± 0.47 cm²). It is interesting to note that in the resting echocardiographic study by Badano and colleagues [19], similar findings were reported for the 23-mm Sorin bileaflet valve when compared with the same size St. Jude or Carbomedics valves [19]. These findings confirm the in vitro data that the Sorin Bicarbon valve has an internal diameter that exceeds that of the Carbomedics or the St. Jude Medical valves of the same size [3]. One of the innovative design features of the Sorin Bicarbon valve is its curvilinear leaflets, which allow the subdivision of the valve area available for flow into three hydrodynamically equivalent orifices. A consequence of this is a more laminar flow with a lower energy loss and maximum utilization of the valve orifice.

In our unselected group of patients, EOAI at rest was 0.88 ± 0.45 cm²/m², which increased to 0.96 ± 0.43 cm²/m² when the cardiac output increased by 81% (maximum stress). These values are associated with relatively mild transprosthetic gradient generation (15.6 ± 5.5 mm Hg at rest and 35.4 ± 11.9 mm Hg at maximum stress). In an in vivo resting Doppler study of fifty-nine 21-mm Sorin bileaflet mechanical valves in the aortic position, EOAI was reported as 0.93 cm²/m² (range, 0.69 to 1.24 cm²/m²), with a mean gradient of 13 mm Hg (range, 7 to 21 mm Hg) [19]. In this study mean transvalvular flow was 197 mL/s (range, 129 to 295 mL/s), yet in our study, whereas mean transvalvular flow was much higher (276 ± 95 mL/s), there was no appreciable difference in mean gradient. This suggests that in the entire group of patient studied, patient–prosthesis mismatch was not an issue of clinical relevance.

The excellent performance of the 21-mm Sorin bileaflet valve was further demonstrated when the study group was subdivided into those with a BSA less than 2 m² and those with a BSA greater than 2 m² (Table 2). In those with a BSA less than 2 m², the EOAI was 1.01 ± 0.52 cm²/m² and was associated with a very low mean transprosthetic gradient of 13.0 ± 4.1 mm Hg. In those patients with a BSA greater than 2 m², although the EOAI was significantly reduced (0.65 ± 0.15 cm²/m²), there was still only a relatively minor increase in mean transprosthetic gradient (20.4 ± 4.5 mm Hg).

The performance index of this valve changed relatively little with maximum stress, suggesting full opening of the valve at a low-pressure differential and maximal utilization of its orifice area.

In conclusion, the 21-mm Sorin Bicarbon bileaflet mechanical aortic valve appeared to have an excellent hemodynamic performance with relatively insignificant pressure gradient generation under both rest and stress conditions. The low transvalvular gradients even in patients with large BSA suggest that patient–prosthesis mismatch is not a clinical problem when this prosthesis is used in the aortic position.

	References

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This Article Abstract Full Text (PDF) 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): Isaac Kadir Alan J. Bryan Gianni D. Angelini Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kadir, I. Articles by Angelini, G. D. Search for Related Content PubMed PubMed Citation Articles by Kadir, I. Articles by Angelini, G. D. Related Collections Valve disease