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Ann Thorac Surg 2001;71:S311-S314
© 2001 The Society of Thoracic Surgeons

Bioprosthetic valves and conduits: new developments

Pericardial and porcine stentless aortic valves: are they hemodynamically different?

Xu Y. Jin, MD, PhDa, Stephen Westaby, FRCSa

a Department of Cardiac Surgery, Oxford Heart Centre, Oxford Radcliffe Hospitals, Oxford, United Kingdom

Address reprint requests to Dr Jin, Department of Cardiac Surgery, Oxford Heart Centre, John Radcliffe Hospital, Oxford, OX3 9DU, UK
e-mail: x.y.jin{at}btinternet.com

Presented at the VIII International Symposium on Cardiac Bioprostheses, Cancun, Mexico, Nov 3–5, 2000.


   Abstract
Top
 Abstract
Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
 References
 
Background. We sought to compare the early hemodynamic performance of pericardial stentless aortic valves with that of well-established porcine stentless aortic prostheses.

Methods. A total of 169 patients (97 men and 72 women, aged 73 ± 6 years) undergoing aortic valve replacement received either a pericardial (Pericarbon, Sorin Biomedica, Saluggia, Italy; n = 89) or a porcine (Freestyle, Medtronic, n = 80) stentless aortic valve. Aortic valve hemodynamics and root dynamism were assessed by Doppler echocardiography at discharge and 12 months after implantation.

Results. Clinical demographic data, valve size (24.0 ± 1.9 vs 24.6 ± 2.3 mm), and body surface area (1.85 ± 0.19 vs 1.80 ± 0.19 m2) did not differ between porcine and pericardial valves (both p > 0.05). The 1-year postoperative mean valve pressure gradient (4.2 ± 2.6 vs 3.7 ± 2.6 mm Hg), effective orifice area (2.2 ± 0.8 vs 2.2 ± 0.8 cm2), and left ventricular ejection fraction (62 ± 13 vs 63 ± 13, %) also did not differ (all p > 0.05). However, at discharge, systolic increase in aortic sinus diameter was significantly greater in pericardial valves than in porcine ones (7.7 ± 5.7 vs 4.9% ± 4.2%, p < 0.01). Furthermore, pericardial valves had a greater slope of effective orifice area–systolic aortic flow relationship (0.89 ± 0.07 vs 0.70 ± 0.06, cm2/100 mL/s, p < 0.01).

Conclusions. Nonprosthetic thin-walled pericardial valves appear to offer better aortic root dynamism and more efficient hemodynamics than those of porcine valves immediately after implant. At 1-year follow-up, however, both types of stentless valves provide equally excellent hemodynamics. The clinical choice between the two will depend on their long-term durability.


   Introduction
Top
 Abstract
 Introduction
Material and methods
 Results
 Comment
 Acknowledgments
 References
 
Over the last decade, superior hemodynamics, better clinical outcome, and more promising long-term durability of stentless porcine aortic valves have made them a better alternative to their stented counterparts [1–4]. Bovine pericardial stentless valves, however, have not been widely used in clinical practice, despite that they have been available for clinical implant in Europe [5]. Early clinical experiences in pericardial stentless aortic valve have been encouraging [5–7]; this study aimed to define detailed hemodynamic characteristics of bovine pericardial stentless aortic valve, to compare it with better established porcine stentless valves, and to elucidate the underlying physiologic mechanism for possible differences between the two.


   Material and methods
Top
 Abstract
 Introduction
 Material and methods
Results
 Comment
 Acknowledgments
 References
 
Patients
A total of 169 patients who had undergone aortic valve replacement with either a stentless porcine valve (Freestyle valve, Medtronic Inc, Minneapolis, MN; n = 80) or a stentless bovine pericardial valve (Pericarbon stentless valve, Sorin Biomedica; n = 89) were prospectively studied by echocardiography at discharge from the hospital (within 2 weeks, n = 169), and at 1 year (6 to 20 months, n = 138) follow-up. The study was conducted as part of routine clinical follow-up of valve surgery patients and ongoing heart valve research program, which was approved by the local ethics committee. There were 97 men and 72 women, with a mean age (± SD) of 73 ± 6 years at operation. The original aortic valve disease was predominantly stenotic in 151 patients and regurgitant in 18 patients. Patients with mild or greater stentless valve regurgitation were not included in this study. Indications for stentless valve replacement were similar to those for a stented bioprosthesis. All of the implantations were performed by the same surgeon (S.W.) using a modified subcoronary implantation technique. The surgical methods have been previously described in detail [7, 8]. In brief, after excision of the diseased native aortic valve, the stentless valve was sewn in a subcoronary position using interrupted 4-0 Ethibond sutures (Ethicon, Somerville, NJ) for the proximal suture line. The upper suture line was fashioned using continuous 4-0 Prolene sutures (Ethicon, Somerville, NJ) after sculpting of the left and right coronary sinuses. The sizes of the stentless valves ranged from 21 to 29 mm, with a mean size of 24.4 ± 2.0 mm. A total of 35% of patients had concomitant coronary artery disease that required a mean of 2 ± 1 bypass grafts.

Echocardiographic study
Prospective transthoracic echocardiography was performed and interpreted by the same echocardiologist. The methods have been previously described in detail [9]. In brief, from the parasternal left ventricular long axis view, the diameter of the outflow tract was measured from a two-dimensional image in early systole. Standard M-mode echocardiography of left ventricle was performed and recorded, and left ventricular ejection fraction was calculated. The diameter of aortic sinus and ascending root was also measured by M-mode echo at the onset of QRS and the midejection, respectively. Thus maximal systolic dimensional increase at both levels was determined. From an apical five-chamber view, flow velocities in the central left ventricular outflow tract and those across the stentless valve were recorded on videotape at a speed of 100 mm/s for off-line analysis [9, 10]. The peak, mean, time-integral, and total duration of systolic flow velocities were measured. Left ventricular stroke volume was calculated as the product of cross-sectional area and flow velocity time-integral of the left ventricular outflow tract. Cardiac index was thus the product of stroke volume and heart rate, and was indexed to body surface area. Mean systolic aortic flow rate (mL/s) was calculated from left ventricular stroke volume divided by total duration of systolic flow velocity across the stentless valve. Valve effective orifice area was calculated by the continuity equation. Peak and mean pressure drop across the stentless aortic valve were calculated using the modified Bernoulli equation by taking both the subvalvular (V1) and valvular (V2) flow velocities as follows: transvalvular pressure drop = 4(V22 - V12), in mm Hg. Mean values for each measurement were derived from three beats in patients in sinus rhythm, and from five beats in those in nonsinus rhythm. Systolic and diastolic blood pressure of systemic circulation was also recorded during each echocardiography.

Statistical analysis
Echocardiographic and hemodynamic data are presented as mean ± 1 SD. Data were analyzed using Minitab statistic software (Release 11 for Windows, 1996; Minitab Inc, Philadelphia, PA) [11]. The Student t test or {chi}2 test was used to examine possible difference in valve hemodynamics or clinical data between the two valve groups. Linear regression analysis was used to examine possible correlations between valve effective orifice area and mean systolic aortic flow. The residual of orifice area as determined by regression analysis was further examined with the Student t test between the two valve groups at early and late follow-up, respectively. A p value less than 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Material and methods
 Results
Comment
 Acknowledgments
 References
 
Comparison of clinical demographic data between two valve groups
There was no significant difference between the porcine and pericardial valves, with respect to patients’ age (73 ± 6 vs 74 ± 6 years), gender, aortic valve pathology (AS/AR: 70/10 vs 81/8), body surface area (1.85 ± 0.19 vs 1.80 ± 0.19, m2), implanted valve size (24.0 ± 1.9 vs 24.6 ± 2.3 mm), and follow-up time (0.65 ± 0.72 vs 0.65 ± 0.96, months) at discharge or (14.3 ± 7.0 vs 13.1 ± 6.0 month) 1 year after the operation (all p > 0.05).

Comparison of systemic hemodynamics
There was no significant difference between the two valve groups in term of heart rate (78 ± 13 vs 80 ± 19, bpm), systemic blood pressure (138 ± 20 vs 138 ± 18 mm Hg), cardiac index (2.85 ± 0.90 vs 2.75 ± 0.90 L/m2), and left ventricular ejection fraction (55 ± 16 vs 57% ± 15%) at discharge, or 1 year after the operation (all p > 0.05).

Comparison of hemodynamic performance between porcine and pericardial stentless valves
At discharge, the stentless valve effective orifice area and mean transvalvular pressure gradient did not differ between the two groups. However, porcine valves had a higher flow velocity at both outflow tract and valve levels, and a higher peak pressure gradient and mean systolic flow rate than those of pericardial valves (all p < 0.05), despite the fact that outflow tract diameter did not differ (Table 1). At 1 year follow-up, effective orifice area, pressure gradients, and outflow tract diameter remained undistinguishable between the two groups. Systolic flow velocity and flow rate were no longer different between the two groups (Table 2).


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  		Table 1. Comparison of Hemodynamic Performance of Stentless Aortic Valves at Discharge (Mean ± SD)

 

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  		Table 2. Comparison of Hemodynamic Performance of Stentless Aortic Valves at 1-Year Follow-up (Mean ± SD)

 
Effects of valve type on the relationship of effective orifice area and systolic flow
Effective orifice area of stentless valves correlated significantly with mean systolic aortic flow rate at both discharge and 1 year follow-up, with a slope of 0.760, 0.759, cm2 per 100 mL/s and a correlation coefficiency of 0.80, 0.82, respectively (both p < 0.001). However, from regression analysis at discharge, we found that the residual effective orifice area of pericardial valves was significantly greater than that of porcine valves (0.109 ± 0.54 vs –0.122 ± 0.478, cm2, p = 0.004). By 1 year follow-up, the difference in residual effective orifice area was no longer statistically significant (0.078 ± 0.464 vs -0.063 ± 0.441, cm2, p = 0.070). Nevertheless, these findings suggest that the linear relationship of effective orifice area and systolic flow were different at discharge between pericardial and porcine valves. Taking further linear regression analysis within each valve group, we found that the slope between effective orifice area and systolic flow rate of pericardial valves was significantly greater than that of porcine valves (0.887 ± 0.068 vs 0.699 ± 0.055, cm2 per 100 mL/s, p = 0.003, Fig 1). However, there was no statistically significant intercept in either group.



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  		Fig 1. Slope of linear regression of effective orifice area (EOA) with mean systolic aortic flow for pericardial and porcine valves at discharge, respectively. Note that the slope of pericardial valves is steeper than that of porcine valves (p = 0.003), implying that the former have a greater increase in effective orifice area at a given increase in systolic aortic flow.

 
Effects of valve type on aortic root dynamics
Systolic and diastolic blood pressure, as well as dimensional changes at the ascending root did not differ between pericardial and porcine valves at discharge (2.3 ± 3.3 vs 1.8% ± 2.7%) or 1 year follow-up (2.8 ± 3.8 vs 3.6% ± 4.1%); in both cases, p was greater than 0.05. Systolic dimensional increase at the aortic sinus level, however, was significantly greater in pericardial valves than that in porcine valves at discharge (7.7 ± 5.7 vs 4.9 ± 4.2, %, p = 0.001). However, the same measure did not differ at 1 year follow-up (8.0 ± 6.0 vs 6.9 ± 4.9, %, p > 0.05).


   Comment
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
Acknowledgments
 References
 
The Sorin Pericarbon stentless aortic valve (Sorin Biomedica, Saluggia, Italy) is constructed from two separate sheets of glutaraldehyde-treated bovine pericardium [12]. The thin-walled pericardial stentless valve is made without any prosthetic material, and is much softer and more pliable than the Freestyle porcine stentless valves. Limited clinical experience of Pericarbon stentless valve reported previously showed favorable results [5–7]. However, detailed flow characteristics and prospective comparison with better established porcine stentless valves were not available in the literature. Overall, both types of stentless valve provide a comparable hemodynamic performance at 1 year follow-up. The subtle but significantly better performance of pericardial valves at discharge does illustrate some important principles that will have implications for improving valve design.

Physiologic considerations
With entirely comparable clinical demographic profiles and systemic circulation, pericardial stentless valve is associated with a more distensible aortic sinus and a greater effective orifice area at a given systolic flow (Fig 1). A better hemodynamic performance of pericardial valves versus porcine ones had previously been demonstrated in stented valve setting [13] and was attributed to a better opening characteristic of pericardial leaflets than of porcine ones. Although the same advantage may also be applicable to the stentless pericardial valves, a higher ratio of internal to external diameter and more dynamic aortic root function, because of a thin wall and absence of prosthetic material, will further contribute to their hemodynamic efficiency. This may explain our findings that pericardial valves are associated with a greater effective orifice area at given systolic flow than porcine ones at discharge. At 1 year follow-up, when the aortic root dynamism in porcine valves has improved to the level of pericardial valves, the hemodynamic difference between the two groups became insignificant.

In bioprosthetic aortic valves, it is well recognized that effective orifice area is a function of systolic flow [13]. Comparison of effective orifice area between different biological valves will have to take systolic flow into consideration [14]. In this study, we confirmed that effective orifice area of both pericardial and porcine stentless valves significantly correlated with systolic flow. At discharge, however, systolic flow differs between the two groups. A direct comparison of effective orifice area between the two groups has obvious limitations. Our approach was therefore to compare the residual value of effective orifice area from the pooled linear regression analysis. We were thus able to differentiate the two valve groups in complex clinical hemodynamic circumstances, and also justified the rationale for a direct comparison of the slopes of linear regression between the two valve groups.

Limitations of the study
This study was focused on the stentless valve hemodynamics early after implantation. Thus, only normal functioning and competent valves were studied. Patients in the two groups were matched by clinical demographic data, but were not prospectively randomized. However, given the consistent performance of same investigators, the study is unlikely to have any significant bias toward either of the valves.

In summary, the present study examined in vivo flow characteristics of pericardial stentless valve in detail. Nonprosthetic and thin-walled pericardial valve appears to better preserve the dynamism of native aortic root and thus offers more efficient hemodynamics than that of porcine valves immediately after implant. At 1 year follow-up, the hemodynamic performance and aortic root dynamics of both valves become equally excellent. The clinical choice between pericardial and porcine stentless valve will largely depend on their durability.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
References
 
The authors thank Medtronic Inc, USA and Sorin Biomedica, Italy, for their support to the Oxford Heart Valve Research Program.


   References
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
 References
 

  1. Jin X.Y., Zhang Z.M., Gibson D.G., Yacoub M.H., Pepper J.R. Effects of valve substitute on changes in left ventricular function and hypertrophy after aortic valve replacement. Ann Thorac Surg 1996;62:683-690.[Abstract/Free Full Text]
  2. Westaby S., Horton M., Jin X.Y., Katsumata T., Li H., Grunkemeir G. Survival advantage of stentless aortic bioprosthesis. Ann Thorac Surg 2000;70:785-790.[Abstract/Free Full Text]
  3. David T.E., Feindel C.M., Scully H.E., Bos J., Rakowski H. Aortic valve replacement with stentless porcine aortic valves: a ten-year experience. J Heart Valve Dis 1998;7:250-254.[Medline]
  4. Westaby S., Huysmans H.A., David T.E. Stentless aortic bioprosthesis: compelling data from the Second International Symposium. Ann Thorac Surg 1998;65:235-240.[Abstract/Free Full Text]
  5. Casabona R., De Paulis R., Zattera G.F., et al. Stentless porcine and pericardial valve in aortic position. Ann Thorac Surg 1992;54:681-685.[Abstract]
  6. Rubay J.E., Khoury G.E.I., Buche M. Aortic valve replacement with the stentless bovine pericardial valve: early experience with the Sorin valve. In: Huysmans H., David T., Westaby S., eds. Stentless bioprostheses, 2nd ed. Oxford: ISIS Medical Media, 1999:47-52.
  7. Westaby S., Jin X.Y., Vaccari G., Katsumata T. The Sorin Stentless pericardial valve: implant technique and hemodynamic profile. Semin Thorac Cardiovasc Surg 1999;11(Suppl I):62-68.[Medline]
  8. Westaby S., Jin X.Y., Katsumata T., Arifi A., Braidley P. Stentless aortic valve replacement: versatility of the porcine root. J Thorac Cardiovasc Surg 1998;116:477-484.[Abstract/Free Full Text]
  9. Jin X.Y., Westaby S., Gibson D.G., Pillai R., Taggart D. Left ventricular remodelling and improvement in Freestyle stentless valve haemodynamics. Euro J Cardiothorac Surg 1997;12:63-69.[Abstract]
  10. Jin X.Y., Westaby S. Echocardiographic assessment of Freestyle aortic root with particular reference to aortic regurgitation. In: Huysmans H., David T., Westaby S., eds. Stentless bioprostheses, 2nd ed. Oxford: ISIS Medical Media, 1999:175-181.
  11. Minitab Inc. Minitab reference manual, release 12, Windows version. Philadelphia: Minitab Inc, 1997:2.26–2.28.
  12. Stacchino C., Bona G., Rinaldi S., Vallana F. Design and performance characteristics of the pericarbon stentless valve. J Heart Valve Dis 1995;4(Suppl I):S102-S115.
  13. Cosgrove D.M., Lytle B.W., Williams G.W. Hemodynamic performance of the Carpentier-Edwards pericardial valve in the aortic position in vivo. Circulation 1985;72(Suppl II):II146-II152.
  14. Cosgrove D.M., Lytle B.W., Gill C.C., et al. In vivo hemodynamic comparison of porcine and pericardial valves. J Thorac Cardiovasc Surg 1985;89:358-368.[Abstract]



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