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Ann Thorac Surg 2000;69:817-822
© 2000 The Society of Thoracic Surgeons

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Original Articles

Comparative rest and exercise hemodynamics of 23-mm stentless versus 23-mm stented aortic bioprostheses

^a Department of Cardiology and Angiology, University Hospitals Homburg, Homburg/Saar, Germany
^b Department of Thoracic and Cardiovascular Surgery, University Hospitals Homburg, Homburg/Saar, Germany

Address reprint requests to Dr Fries, Medical Clinic III, Department of Cardiology and Angiology, University Hospitals Homburg, 66421 Homburg/Saar, Germany
e-mail: fries{at}med-in.uni-sb.de

	Abstract

Top Abstract Introduction Patients and methods Results Comment References

 
Background. The hemodynamic superiority of stentless valves at rest has been generally accepted, but there is a lack of studies on exercise hemodynamics.

Methods. We assessed aortic valve hemodynamics at rest and during exercise in 10 patients with a 23-mm stentless aortic bioprosthesis (Medtronic Freestyle; Medtronic Europe SA/NV, St. Stevens Woluwe, Belgium), in 10 patients with a 23-mm stented aortic bioprosthesis (Carpentier-Edwards, SAV, model 2650; Baxter Edwards AG, Horw, Switzerland), and in 10 healthy volunteers (control group) by means of Doppler echocardiography.

Results. Gradients at rest and gradients on comparable maximum exercise levels were significantly lower in patients with stentless valves compared to those with stented valves (rest: 6 ± 2/11 ± 4 mm Hg [mean/peak] versus 12 ± 3/21 ± 10 mm Hg; exercise: 9 ± 3/18 ± 6 mm Hg [mean/peak] versus 22 ± 8/40 ± 11 mm Hg). Patients with stentless valves revealed, in comparison to healthy young men, significantly higher gradients, but the small gradient difference of 3/7 mm Hg (mean/peak) at rest remained nearly unchanged throughout the exercise protocol (4/8 mm Hg [mean/peak] at 25 W, 4/9 mm Hg at 50 W and 4/9 mm Hg at 75 W). In contrast, the gradient difference between patients with stented and stentless valves increased significantly from one exercise level to the next (6/12 mm Hg [mean/peak] at rest, 8/14 mm Hg at 25 W, 12/17 mm Hg at 50 W, and 15/25 mm Hg at 75 W).

Conclusions. A stentless aortic bioprosthesis seems to be an appropriate aortic valve substitute, especially in patients who perform regular physical exercise.

	Introduction

Top Abstract Introduction Patients and methods Results Comment References

 
Limited durability has remained a major drawback of stent-mounted porcine bioprostheses, mainly due to primary tissue failure. It has been shown that the stent of a bioprosthesis is a major factor governing stress on the tissue component [1, 2]. Thus, stentless bioprostheses, first used by Binet and associates in the early 1960s [3], represent a potentially more durable alternative to stented xenografts. Lack of availability and a more complex operation technique has restricted the use of stentless valves until now.

Recent hemodynamic studies at rest have shown that stentless porcine xenografts in the aortic position are superior to conventional stented bioprostheses [4–7]. However, hemodynamic performance at rest is not truly representative of a patient’s daily activities. The exercise hemodynamics of stentless bioprostheses may essentially contribute to the excellent left ventricular remodeling, which has been demonstrated after stentless aortic valve replacement [8–11]. In order to elucidate these issues, we compared the hemodynamic performance of a 23-mm stentless aortic bioprosthesis to a stented 23-mm counterpart, at rest and during exercise, by means of Doppler echocardiography.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment References

 
We assessed aortic valve hemodynamics at rest and during exercise in 3 groups of individuals. Group 1 was the control group and consisted of 10 healthy volunteers. We chose young people in order to ensure optimum transaortic flow conditions in the control group. Group 2 consisted of 10 patients who had aortic valve replacement with a stentless aortic bioprosthesis (Medtronic Freestyle; Medtronic Europe SA/NV, St. Stevens Woluwe, Belgium), while group 3 consisted of 10 patients with a stented aortic bioprosthesis (Carpentier-Edwards, SAV, model 2650; Baxter Edwards AG, Horw, Switzerland). Consecutive patients who had received a 23-mm valve at least 3 months before the investigation were included in the study. Written informed consent was obtained from all patients and none refused to participate.

The decision to compare the 23-mm Carpentier-Edwards valve to the 23-mm Medtronic Freestyle valve was not taken because of the identical manufacturer‘s labeled size, which may differ significantly from measured internal and external diameters [12]. The selection of our study groups was taken as a consequence of the clinical setting in which an aortic bioprosthesis should be implanted, and where one may chose between a stentless and a stented valve. Intraoperatively we used an independent valve sizer and found a 23-mm supraannular porcine Carpentier-Edwards valve suitable in cases in which a 23-mm Medtronic Freestyle valve could be implanted by the full root replacement technique.

Operative procedures
In all patients, St. Thomas’ solution was used for cardioplegia. Carpentier-Edwards valves were inserted using standard operating techniques. A transverse aortotomy was performed, the diseased aortic valve completely excised, and the stented bioprosthesis inserted in a supraannular position using interrupted Teflon supported mattress sutures.

The Medtronic Freestyle valve was inserted using a total root replacement technique [13]. The aorta was transected just above the sinotubular ridge, both coronary ostia were mobilized with buttons of aortic wall, and the diseased aortic valve was removed. The stentless bioprosthesis was connected to the left ventricular outflow tract with a continuous suture (Prolene; Ethicon, Hamburg, Germany) incorporating a strip of pericardium as a basal external reinforcement of the annulus. Using continuous Prolene sutures, the coronary arteries were then implanted into the corresponding sinuses of Valsalva of the bioprosthesis. Finally the distal porcine root was anastomosed to the ascending aorta.

Echocardiography
The Ultramark 9 (Advanced Technology Laboratories, Bothell, WA) was used for all measurements. Left ventricular end-systolic and end-diastolic dimensions and thickness of left ventricular posterior wall and interventricular septum were assessed in the short axis of a parasternal view by multiple M-mode measurements with calculation of fractional shortening. Left ventricular mass was calculated as "ASE-cube" using the formula of Troy [14].

Aortic valve maximum Doppler velocity was measured with a continuous-wave transducer (ATL 2.00 MHz CW, PN 4000-0307-03; Advanced Technology Laboratories). Particular care was taken to obtain the highest possible velocity by variation of the acoustic window and transducer orientation. The modified Bernoulli equation was used to calculate peak gradients (p = 4 V², where V = peak transvalvular flow velocity), and mean pressure gradients were obtained by averaging the cardiac cycle measurement which resulted in the highest peak gradient.

Exercise protocol
During bicycle exercise, patients sat on a seat reclined in a 50° position (Ergo-Metrics 900 L; Ergoline, Bitz, Germany). The starting workload was 25 W, and then was increased by 25 W every 3 minutes until symptomatic termination of the test occurred (dyspnea, muscular fatigue). To facilitate Doppler measurements during exercise, the chest site where optimum Doppler waveforms were recorded was marked before starting exercise. In case of unsatisfactory Doppler signal, the whole bicycle unit was tilted slightly to the left side until optimal measurements were obtained. Measurements were performed during the last minute of each 3-minute workload level. Blood pressure and heart rate were measured noninvasively every minute using a sphygmomanometer cuff fixed on the right arm.

Statistical methods
Results were expressed as mean ± standard deviation. A Mann-Whitney U-test was used for comparative analysis of continuous variables in 2 groups of patients. Analysis of variance was used for comparison of all 3 patient groups. Statistical significance was established at p less than or equal to 0.05.

	Results

Top Abstract Introduction Patients and methods Results Comment References

 
Clinical patient characteristics
Demographics of the patients and controls are shown in Table 1. There were no significant differences between the 2 groups of patients with stentless and stented valve replacement, except for left ventricular mass at time of the investigation.

Rest and exercise hemodynamics
Rest and maximum exercise hemodynamics of patients with stentless valves compared to those with stented valves are shown in Table 2. All patients reached a 50 W workload. The 75 W exercise level was reached by 5 of 10 patients with Carpentier-Edwards valves, and by 4 of 10 patients with the Freestyle valve. Focusing on the differences in maximum peak and mean transvalvular gradients at rest, as well as during exercise, superiority of the stentless valve is clearly evident.

View larger version (24K): [in this window] [in a new window]   Fig 1. Average peak gradients (± standard deviation) of the study patients at rest (before and 5 minutes after exercise) and during exercise.

View larger version (25K): [in this window] [in a new window]   Fig 2. Average mean gradients (± standard deviation) of the study patients at rest (before and 5 minutes after exercise) and during exercise.

View larger version (36K): [in this window] [in a new window]   Fig 3. Difference of average peak and mean gradients between patients with Freestyle valves and the control group, and between patients with Carpentier-Edwards valves and those with Freestyle valves.

	Comment

Top Abstract Introduction Patients and methods Results Comment References

Methodology
Simultaneous Doppler and catheterization studies have shown that continuous-wave Doppler ultrasound can accurately predict pressure gradients across prosthetic valves in the aortic position [15]. By use of the short form of the Bernouli equation gradients are estimated 3 to 5 mm Hg higher than after correction for left ventricular outflow tract (LVOT) velocity. This overestimation occurs to the same extent in resting and exercise gradients [16, 17]. As we focused on Doppler gradient differences between patient groups such systematic overestimation is not of great importance and does not impair the clinical significance of our results. Correcting the Bernoulli equation for LVOT velocity requires repeated PW-Doppler measurements after exact positioning of the measure volume of the LVOT by means of B-mode echocardiography (2-dimensional picture). Taking into account the increasing patient movements during exercise, and the short time window for measurements at each exercise level (1 minute), in our experience, this cannot be performed reliably. For the same reason, we did not try to calculate further LVOT flow dependent variables, such as effective orifice area, stroke volume, and cardiac output.

Rest hemodynamics
We found a resting peak gradient of 21 ± 10 mm Hg (mean gradient 12 ± 3 mm Hg) in 10 patients with 23-mm Carpentier-Edwards valves. Previously reported peak gradients for normally functioning Carpentier-Edwards valves are comparable to our findings (8 to 23 mm Hg) [16, 18–21]. The relatively wide range of measured values in these studies may be explained by inclusion of patients with different valve sizes and partly by differences in methodology.

In 10 patients with 23-mm Medtronic Freestyle valves we determined a peak gradient of 11 ± 4 mm Hg and a mean gradient of 6 ± 2 mm Hg. These are almost exactly the same values as reported by Westaby and colleagues [11] in a series of 17 patients with the same valve and valve diameter (peak gradient: 12 ± 7 mm Hg, mean gradient: 5 ± 2 mm Hg). Data are confirmed by Sintek and colleagues [6] and Jin and associates [9] for the Medtronic Freestyle valve, and by other investigaters for a comparable type of stentless porcine valve, such as the Toronto SPV [4, 8, 22, 23] and the Biocor valve [17, 24].

Exercise hemodynamics
The mean exercise gradient in our patients with Carpentier-Edwards valves (22 ± 8 mm Hg) is confirmed in the few studies including exercise measurements in patients with stented bioprostheses. Craver and colleagues [25] reported in 10 patients with a 23-mm modified orifice Hancock valve, a mean pressure gradient during exercise of 28 ± 16 mm Hg, and in 6 patients with 23-mm Carpentier-Edwards valves, Chaitman and colleagues [18] reported a mean gradient of 29 ± 5 mm Hg. Jaffe and coworkers [26] found a mean transvalvular gradient of 28 ± 8 mm Hg in 7 patients with 23-mm Medtronic Intact valves.

By contrast, there are no data published concerning transvalvular gradients during bicycle exercise in the Medtronic Freestyle valve. We measured mean gradients of 9 ± 3 mm Hg and peak gradients of 18 ± 6 mm Hg at a workload of 70 ± 23 W. In comparison to healthy young men, gradients were only slightly higher (3/7 mm Hg (mean/peak) at rest and 4/9 mm Hg during exercise, Figs 1 and 2). These differences were statistically significant, but may be physiologically negligible. Taking into account natural aortic valve degeneration with age (elasticity loss and sclerosis), it can be speculated that there would be insignificant or no gradient differences when comparing our patients with Freestyle valves to healthy subjects of comparable age.

Our results with the Freestyle valve are comparable to findings in a different type of stentless aortic bioprosthesis (Biocor valve). Eriksson and colleagues [17] reported 26 patients with 19 to 25-mm extended Biocor valves with gradients of 8 ± 3/12 ± 4 mm Hg (mean/peak) at rest and 15 ± 6/24 ± 8 mm Hg (mean/peak) during bicycle exercise (median 60 W) 15 months postoperatively. Donatelli and coworkers [24] studied 7 patients with 23 to 27-mm Biocor valves and measured gradients of 6 ± 2/11 ± 3 mm Hg (mean/peak) at rest and 9 ± 2/15 ± 5 mm Hg (mean/peak) after exercise (modified Bruce protocol), 6 months postoperatively.

Difference of average gradients between study groups
As in previous studies, gradients were usually measured immediately after and not during exercise. Our data provide new information concerning transaortic gradients in different types of aortic valves, on different exercise levels. Interestingly, the difference of average gradients between patients with Freestyle valves and healthy subjects remained almost unchanged when comparing values at rest (3/7 mm Hg [mean/peak]) and during exercise (4/8 mm Hg [mean/peak] at 25 W, 4/9 mm Hg at 50 W and 4/9 mm Hg at 75 W). By contrast the difference of average gradients between patients with Carpentier-Edwards valves and those with Freestyle valves increased significantly from one exercise level to the next (6/12 mm Hg [mean/peak] at rest, 8/14 mm Hg at 25 W, 12/17 mm Hg at 50 W and 15/25 mm Hg at 75 W).

This finding means that resting hemodynamics may be representative for exercise hemodynamics in stentless but not in stented bioprostheses. In the latter, hemodynamic performance significantly decreases with increasing activity. In our series the average gradient difference between patients with Carpentier-Edwards and those with Medtronic Freestyle valves was at a workload of 75 W, more than twice as high as at rest (Fig 3). This may explain why left ventricular remodeling is so much better in patients with stentless valves, as compared to those with stented valves [8–11]. Better regression of left ventricular hypertrophy is confirmed in our patients. When comparing left ventricular mass measurements preoperatively and at time of the exercise test, it is evident that left ventricular mass decreased significantly better in patients with stentless valves (Table 1).

The reason for the significantly sharper increase of transprosthetic gradients in patients with Carpentier-Edwards valves remains unclear. We cannot suppose that there were any differences in the increase of cardiac output or stroke volume in the study groups. Each individual patient had a normally sized and normally functioning left ventricle, had either no coronary artery disease or were completely revascularized, and showed adequately increasing heart rate and blood pressure during exercise. Furthermore, the symptom limiting exercise was muscular fatigue (and not angina or dyspnea) in all patients. Thus we can expect comparable increase in cardiac output during exercise in all 3 study groups.

Differences in geometrical orifice area cannot explain better hemodynamic performance of the Freestyle valve as the internal diameter (at the valve base) of a 23-mm Carpentier-Edwards porcine valve (21-mm) is even larger than the internal diameter of a 23-mm Freestyle valve (20-mm). By contrast, according to lower gradients and reflecting a less impaired transprosthetic blood flow, in vivo and in vitro measurements of effective orifice area reveal significantly smaller values in stented valves [4–6, 22]. Variables influencing transprosthetic flow in favor of the stentless valve design may be valve shape and surface, as well as distensibility of the aortic root. During exercise a possible increase of effective orifice area may also be of importance, but data concerning this issue are conflicting and methodologically problematic, as formulas for calculation of effective orifice area are flow dependent [27].

As different implantation techniques may influence transaortic flow characteristics, our results might be slightly different if we had used the root inclusion or subcoronary technique. However, taking into account that many surgeons use stentless valves up to two sizes larger as stented valves in the same patient, we could have compared the 23-mm Carpentier-Edwards valve with a 25 or 27-mm Medtronic Freestyle, and our results in favor of the stentless valve would have been even more impressive.

The hemodynamic performance of stentless aortic bioprostheses implanted in elderly patients, by the full root replacement technique, is slightly inferior to that of natural valves in healthy young men. The small, and probably physiologically insignificant, gradient difference between stentless valves and natural valves at rest remains unchanged during exercise. By contrast, the already significant gradient difference between patients with stented bioprostheses and those with stentless grafts increases even further during exercise. Our data strongly support the suggestion that, from the hemodynamic point of view, a stentless aortic bioprosthesis is an appropriate aortic valve substitute, especially in patients with a smaller aortic root and in those performing regular physical activity.

	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): Olaf Wendler Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Fries, R. Articles by Schäfers, H.-J. Search for Related Content PubMed PubMed Citation Articles by Fries, R. Articles by Schäfers, H.-J.