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Ann Thorac Surg 1999;67:1609-1615
© 1999 The Society of Thoracic Surgeons

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Original Articles

Aortic root replacement with the freestyle stentless porcine aortic root bioprosthesis

^a Department of Cardiothoracic Surgery, Wake Forest University School of Medicine/Baptist Medical Center, Winston-Salem, North Carolina, USA
^b Section of Cardiology, Wake Forest University School of Medicine/Baptist Medical Center, Winston-Salem, North Carolina, USA

Address reprint requests to Dr Kon, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1096;
e-mail: nkon{at}wfubmc.edu

Presented at the Forty-fifth Annual Meeting of the Southern Thoracic Surgical Association, Orlando, FL, November 12–14, 1998.

	Abstract

Top Abstract Introduction Patients and methods Results Comment References

 
Background. Stentless porcine prosthetic valves offer several advantages over traditional valves. Among these are superior hemodynamics, laminar flow patterns, lack of need for anticoagulation and perhaps improved durability.

Methods. One hundred and twelve patients were operated on from September 17, 1992 to April 13, 1998 as part of a multi-center worldwide investigation. All patients received a total aortic root replacement. Patients were evaluated postoperatively at discharge, 3 to 6 months, and yearly by clinical exam and color flow Doppler echocardiography.

Results. There were 4 deaths either in the hospital or within 30 days after surgery for an operative mortality of 3.6%. No patients experienced structural valve deterioration, non-structural valve deterioration, paravalvular leak, unacceptable hemodynamic performance, or postoperative endocarditis. The linearized rates for survival and thromboembolic complications at 5 years were 82.8% and 90.5% respectively. Excellent hemodynamic function is demonstrated by very low gradients, large EOA, and an exceedingly low incidence of any aortic regurgitation.

Conclusions. The Medtronic Freestyle aortic root bioprosthesis can be used safely to replace the aortic root for aortic valve and aortic root pathology. Root replacement allows optimal hemodynamic performance with no significant aortic regurgitation. Early and intermediate results are encouraging, but further follow-up is needed to determine valve durability.

	Introduction

Top Abstract Introduction Patients and methods Results Comment References

 
The ideal valve substitute for the aortic valve should have superior hemodynamics, no pressure gradients, no leaks, and laminar flow. It should be easy to implant, silent, biocompatible, and resistant to thromboembolic (TE) problems. Lastly, it should be durable enough to last the patient’s lifetime.

Cryopreserved aortic allografts exhibit many of the ideal valve substitute features, and provide a gold standard for the hemodynamic function of biologic aortic valve substitutes. Use of allograft valves has been limited, not only by availability, but by the technical demands required for competent valve implantation. Several aortic allograft and pulmonary autograft studies have shown the root replacement to be the most reliable implant technique to achieve optimal hemodynamic function.

In an effort to achieve similar results to aortic allografts, David and associates demonstrated the feasibility of implanting glutaraldehyde fixed stentless porcine valves [1]. Over the last 10 years, excellent hemodynamic performance has been demonstrated repeatedly with several types of stentless valves [2–5]. Improved durability over stented bioprostheses is anticipated based on the adverse effects of stent mounting reported in experimental animal studies [6] and with allograft valves [7].

The Medtronic Freestyle aortic root bioprosthesis was approved for investigational use by the Food and Drug Administration in July 1992, and for clinical use in November 1997. It has a stentless design analogous to an aortic allograft. The leaflets are treated with an anti–mineralization agent (AOA) to mitigate calcification, and are fixed stress free to retain their structural integrity. The porcine aortic wall is fixed at 40 mm Hg pressure to prevent sinus shrinkage. These modifications may improve durability.

The Freestyle aortic root bioprosthesis behaves like an aortic allograft or pulmonary autograft and can be inserted by the freehand complete subcoronary, modified subcoronary, inclusion root, and free–standing total aortic root replacement techniques. This study reports our 5 year experience implanting the Freestyle valve as a free–standing total aortic root replacement.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment References

 
Wake Forest University Baptist Medical Center was selected in 1992 to serve as one of the United States study centers to begin implanting the Freestyle investigational valve. Before initiation of the study at our hospital, the institution’s investigational review board approved the study design. Written informed consent was obtained for each patient before operation prior to November 1997 FDA approval.

From September 17, 1992 to April 13, 1998, 112 patients received the Freestyle aortic root bioprosthesis. Preoperative clinical data including age at implant, gender, cardiac rhythm, and NYHA class are listed in Table 1. All 5 available sizes were implanted (19, 21, 23, 25, 27). The distribution is shown in Table 2. Table 3 lists the various preoperative risk factors in this elderly patient population. Etiology of the explanted aortic valve dysfunction, incidence of a concomitant procedure, and length of patient follow–up are listed in Table 4 , Table 5 , and Table 6 , respectively.

Surgical technique
The free–standing total aortic root replacement technique was chosen in all 112 patients. The aorta is transected just above the sino–tubular junction (Fig 1 ). Both coronary ostia are mobilized on generous buttons of aortic wall (Fig 2 ). The remaining tissue of each sinus of Valsalva and the diseased aortic valve are excised. The proximal, or inflow anastomosis is accomplished using 28 to 35 simple interrupted sutures of 3–0 braided Dacron tied around a 1 mm strip of Teflon felt (Fig 3 ). The sutures are placed in a single plane in the left ventricular outflow tract to conform to the round inflow of the prosthesis. The coronary arteries on their buttons of aortic wall are sewn end to side to the corresponding sinus of Valsalva of the bioprosthesis with a continuous 5–0 polypropylene suture (Fig 4 ). The distal end of the bioprosthesis is sewn end–to–end to the aorta with a continuous 5–0 polypropylene suture to complete the root replacement (Fig 5 ).

View larger version (23K): [in this window] [in a new window]   Fig 1. The aorta is transected just above the sinotubular junction.

View larger version (21K): [in this window] [in a new window]   Fig 2. Both coronary ostia are mobilized on generous buttons of aortic wall.

View larger version (39K): [in this window] [in a new window]   Fig 3. 28–35 simple interrupted sutures of braided dacron are placed in the proximal inflow of the heart and aortic root bioprosthesis.

View larger version (35K): [in this window] [in a new window]   Fig 4. The coronary ostia are reimplanted into the bioprosthesis with a continuous 5-0 polypropylene suture.

View larger version (26K): [in this window] [in a new window]   Fig 5. The distal end of the bioprosthesis is sewn to the aorta with a 5-0 polypropylene suture end-to-end to complete the root replacement.

Data analysis
Data preoperatively and at each postoperative clinical and echocardiographic evaluation were placed in the Freestyle database at Medtronic, Inc. Descriptive statistics are used to summarize the preoperative, operative, follow–up clinical, and hemodynamic data. The number of patients, mean, and standard deviation are provided for categorical data. The life table method is used to estimate survival and freedom from adverse events.

	Results

Top Abstract Introduction Patients and methods Results Comment References

 
Patient improvement from preoperative to 1 year postoperative in NYHA class for congestive heart failure is depicted in Table 7. On clinical examination at the 1–year postoperative interval, 92.8% of patients were in class 1 and 4.8% were in class 2. No patients reaching 1 year were in class 3 or 4.

The operative mortality (30 day or in hospital) was 3.6%. Survival over the 5 year follow–up is shown in (Fig 6 ). No patients had hemolysis, unacceptable hemodynamic performance, paravalvular leak, structural, or nonstructural valve deterioration. Freedom from reoperation and postoperative endocarditis were both 100% at 5 years (Fig 7 , Fig 8 ). Four thromboembolic events occurred in the first 30 days, 2 resulting in permanent neurologic deficits. Two were transient. There was also 1 late transient deficit. The linearized rate for freedom from TE events is shown in (Fig 9 ).

View larger version (12K): [in this window] [in a new window]   Fig 6. The linearized rate of freedom from death following aortic root replacement.

View larger version (11K): [in this window] [in a new window]   Fig 7. The linearized rate of freedom from re-operation following aortic root replacement.

View larger version (13K): [in this window] [in a new window]   Fig 8. The linearized rate of freedom from endocarditis following aortic root replacement.

View larger version (15K): [in this window] [in a new window]   Fig 9. The linearized rate of freedom from thromboembolic events following aortic root replacement.

View larger version (13K): [in this window] [in a new window]   Fig 10. LV mass index reduction (g/m2) following TRR with Freestyle.

View larger version (23K): [in this window] [in a new window]   Fig 11. Incidence of aortic regurgitation following aortic root replacement.

	Comment

Top Abstract Introduction Patients and methods Results Comment References

The superior hemodynamics and improved late clinical outcomes for stentless porcine bioprostheses have been demonstrated [4, 11–15]. The disadvantages of a stent include an increase in stress forces associated with leaflet opening and closing, the possibility of leaflet abrasion adjacent to the cloth covering the stent. The potential for paravalvular leak, and most importantly, the obstructive mass that the stent imposes within the aortic root. These factors lead to obstructive hemodynamics and turbulent flow. Adverse effects of stent mounting on valve durability have been demonstrated [7].

The main advantage of a stent is that it allows for perfect valve mounting and thus eliminates the risk of implanting an incompetent valve. Previous studies with pulmonary autografts [16, 17], aortic allografts [18, 19], and stentless porcine valves [4] have shown that the technical errors associated with freehand (non–stented) valve insertion can be eliminated using root replacement techniques. This is especially true when dealing with complex aortic root pathology. In this study, we chose to utilize the total root replacement technique in all circumstances regardless of the aortic valve or aortic root pathology.

For example, sinus of Valsalva and ascending aortic disease whether dilated, aneurysmal, or calcific could be handled simply by removal of the diseased tissue and expanded the number of instances in which we could use this valve. The Freestyle aortic root bioprosthesis was also extended with a segment of Dacron graft to replace the remainder of the ascending aorta and/or aortic arch when necessary.

Replacement of the entire aortic root did not prove to be a more complicated or more dangerous procedure in this series. An operative mortality rate of 3.6% shows no increased risk with this technique compared to other studies using stented valves or the subcoronary technique for stentless valves [12–15, 20–23]. Although cross–clamp times were longer than one sees for implantation of a stented bioprosthesis, they are still well within acceptable limits considering the modern day myocardial preservation techniques employed.

A root replacement operation requires more operative time than implanting a stented valve, but in some respects is easier and more reliable. Exposure is enhanced when the root is removed and the coronary ostia are mobilized out of the way. Each suture is simple to place, making malalignment less likely. Only 2 patients in this series were re–explored for bleeding which was easily controlled and did not involve the aortic root. Reliability is documented by the least incidence of aortic insufficiency and lower early gradients (discharge and at 3 to 6 months) when directly compared to a subcoronary implant technique [4]. Further evidence of implant reliability with a root replacement technique is demonstrated in this study by no findings of hemolysis, paravalvular leak, structural or non–structural valve deterioration, unacceptable hemodynamic performance, or postoperative explantation. Utilization of the root replacement technique with aortic allografts has been associated with greater durability when compared to other implant techniques [24].

The subcoronary implant technique has been employed in most previous reports on stentless porcine valves. These studies have consistently shown a decrease in gradient and increase in EOA over time. Early and late gradients are both low in our study and do not change over time. This discrepancy is most likely related to absorption of hematoma in the potential space between the porcine and native aortic walls which occurs when performing a subcoronary implant. Aortic root and left ventricular outflow tract remodeling with time may also occur over the first year after implantation [3, 5, 8, 12, 13, 16, 17].

Five of 112 patients in the present study experienced TE complications. In no instance was thrombus seen on any echocardiogram of an implanted Freestyle aortic valve. Two patients suffered permanent neurologic deficits, while 3 patients had only transient deficits. These TE rates (Fig 4) are not unexpected given the preoperative risk factors listed in Table 3.

The clinical and echocardiographic follow–up data in this study shows continuous low gradients and no aortic insufficiency. This translates into beneficial effects on clinical outcome and resolution of left ventricular hypertrophy. By 1–year post–op, 92.8% of patients were in NYHA class 1 and 4.8% of patients were in NYHA class 2, and is indicative of the excellent quality of life these elderly patients enjoy postoperatively. At the left ventricular level, echo studies document rapid regression of hypertrophy, a relative decrease in wall thickness, and a return to normal left ventricular dimensions.

In addition to improved quality of life, improved survival after aortic valve surgery with a stentless valve compared to a stented one has been shown in a case matched study [15]. In this case matched study, the Toronto SPV demonstrated a 5–year survival of 92%. Five–year survival in this study was 83%. The discrepancy is related to the patient’s age at implant. Mean age in the Toronto SPV study was 62 years, while mean age in this study was 72 years. In most reports on late outcomes after AVR with stented biologic valves, the actuarial survival at 5 years ranged from 70% to 80% [21–23].

Compelling evidence now exists for the use of stentless valves in aortic valve surgery. We prefer the total aortic root replacement technique because it is a safe and effective technique to implant a stentless valve regardless of the aortic valve or aortic root pathology. Optimal valve function is insured. Short and intermediate term results are excellent. Long–term follow–up is still required to determine durability.

	References

Top Abstract Introduction Patients and methods Results Comment References

 

1. David T.E., Ropchan G.C., Butany J.W. Aortic valve replacement with stentless porcine bioprosthesis. J Cardiac Surg 1988;3:501-505.[Medline]
2. David T.E., Pollick C., Bos J. Aortic valve replacement with stentless porcine aortic bioprosthesis. J Thorac Cardiovasc Surg 1990;99:113-118.[Abstract]
3. Del Rizzo D.F., Goldman B.S., Joyner C.P., et al. Initial clinical experience with the Toronto stentless porcine valve. J Cardiac Surg 1994;9:379-385.[Medline]
4. Kon N.D., Westaby S., Amarasena N., et al. Comparison of implant techniques using the Freestyle stentless porcine aortic valve. Ann Thorac Surg 1995;59:857-862.[Abstract/Free Full Text]
5. Jin X.Y., Westaby S., Gibson D.G., et al. Left ventricular remodeling and improvement in Freestyle stentless valve hemodynamics. Eur J Cardiothorac Surg 1997;12:63-69.[Abstract]
6. Hazekamp M.G., Goffin Y.A., Huysmans H.A. The value of the stentless biovalve prosthesis. Eur J Cardiothorac Surg 1993;7:514-519.[Abstract]
7. Angell W.W., Oury J.H., Lamberti J.J., et al. Durability of the viable aortic allograft. J Thorac Cardiovasc Surg 1989;98:48-56.[Abstract]
8. Barratt–Boyes B.G., Jaffe W.M., Ko P.H., et al. The zero pressure fixed Medtronic Intact porcine valve: an 8.5–year review. J Heart Valve Dis 1993;2:604-611.[Medline]
9. Christie G.W. Anatomy of aortic heart valve leaflets: the influence of glutaraldehyde fixation on function. Eur J Cardiothorac Surg 1992(Suppl 1):S25-S33.
10. Gott J.P., Pan–Chin, Dorsey L.M.A., et al. Calcification of porcine valves: a successful new method of antimineralization. Ann Thorac Surg 1991;53:207-216.
11. Westaby S., Huysmans H.A., David T.E. Stentless aortic bioprostheses: compelling data from the Second International Symposium. Ann Thorac Surg 1998;65:235-240.[Abstract/Free Full Text]
12. Cartier P.C., Metras J., Dumesnil J.G., et al. Clinical and hemodynamic performance of the Freestyle aortic root bioprosthesis. Ann Thorac Surg 1999;67:345-351.[Abstract/Free Full Text]
13. Westaby S., Amarasena N., Long V., et al. Time–related hemodynamic changes after aortic replacement with the Freestyle stentless xenograft. Ann Thorac Surg 1995;60:1633-1639.[Abstract/Free Full Text]
14. Sintek C.F., Fletcher A.D., Khonsari S. Stentless porcine aortic root: valve of choice for the elderly patient with small aortic root. J Thorac Cardiovasc Surg 1995;109:871-876.[Abstract]
15. David T.E., Puschmann R., Ivanov J., et al. Aortic valve replacement with stentless and stented porcine valves: A case–match study. J Thorac Cardiovasc Surg 1998;116:236-241.[Abstract/Free Full Text]
16. Kouchoukos N.T., Dávila–Román V.G., Spray T.L., et al. Replacement of the aortic root with a pulmonary autograft in children and young adults with aortic–valve disease. N Engl J Med 1994;330:1-6.[Abstract/Free Full Text]
17. Elkins R.C., Santangelo K., Stelzer P., et al. Pulmonary autograft replacement of the aortic valve: an evolution of technique. J Cardiac Surg 1992;7:108-116.[Medline]
18. O’Brien M.P., McGiffin D.C., Stafford E.G. Allograft aortic valve implantation: techniques for all types of aortic valve and root pathology. Ann Thorac Surg 1989;48:600-609.[Abstract]
19. Daicoff G.R., Botero L.M., Quintessenza J.A. Allograft replacement of the aortic valve versus the miniroot and valve. Ann Thorac Surg 1993;55:855-859.[Abstract]
20. Westaby S., Jin X.Y., Katsumata T., et al. Valve replacement with a stentless bioprosthesis: versatility of the porcine aortic root. J Thorac Caradiovasc Surg 1998;116:477-484.[Abstract/Free Full Text]
21. Cosgrove D.M., Lytle B.W., Taylor P.C., et al. The Carpentier–Edwards pericardial aortic valve: ten–year results. J Thorac Cardiovasc Surg 1995;110:651-662.[Abstract/Free Full Text]
22. Akins C.W., Carroll D.L., Buckley M.J., et al. Late results with Carpentier–Edwards porcine bioprosthesis. Circulation 1990;82(Suppl):IV65-IV74.
23. David T.E., Armstrong S., Sun Z. The Hancock II bioprosthesis at ten years. Ann Thorac Surg 1995;60:S229-S234.
24. Lund O., Yacoub M., Chandrasekaran V., et al. Primary aortic valve replacement with homografts over 25 years: valve and procedure–related determinants of outcome. J Thorac Cardiovasc Surg 1999;117:77-91.[Abstract/Free Full Text]

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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): Neal D. Kon A. Robert Cordell Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kon, N. D. Articles by Kitzman, D. W. Search for Related Content PubMed PubMed Citation Articles by Kon, N. D. Articles by Kitzman, D. W.