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Ann Thorac Surg 1995;60:1633-1638
© 1995 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Time-Related Hemodynamic Changes After Aortic Replacement With the Freestyle Stentless Xenograft

Departments of Cardiac Surgery and Cardiology, Oxford Heart Centre, John Radcliffe Hospital, Oxford, England

	Abstract

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 
Background. The Freestyle valve is a porcine aortic root fixed with net zero pressure across the cusps and treated with the anticalcification agent alpha-aminooleic acid. We evaluated the hemodynamic function of this stentless valve.

Methods. We implanted the valve into 80 consecutive patients scheduled to receive a bioprosthesis. Sixty-nine patients were more than 70 years old and 14 had calcified aortic sinuses. The ``cylinder within a cylinder'' implantation technique was used. Twenty-seven patients received coronary grafts, and 3 had mitral repair. All patients were studied echocardiographically during week 1 and 41 were studied at 6 months.

Results. Median ischemic time for isolated aortic valve replacement was 44 minutes. There were four hospital deaths, none valve related. No patient had more than trivial aortic regurgitation. Mean systolic gradients for valve sizes 21 to 25 mm fell significantly with time (p 0.05) due to remodeling of the porcine root and early regression of left ventricular hypertrophy. Late hemodynamic function was equivalent to an aortic homograft.

Conclusions. The Freestyle valve is easily implanted and has excellent hemodynamics. Transvalvular gradients decrease progressively.

	Introduction

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 
See also page 1639.

With mean transvalvular gradients of 10 mm Hg or less for most sizes, the freehand-sewn aortic homograft provides the gold standard for hemodynamic function in biological valves [1, 2]. Limited availability of human valves, together with poor preservation and storage methods, caused Binet and associates [3] to adopt the porcine aortic root as an alternative to the homograft in 1965. In turn surgical difficulties led to stent mounting of both xenograft and homograft valves, and the hybrid of biological and mechanical structures became known as a ``bioprosthesis'' [4–6]. The stent and sewing ring of a bioprosthesis reduce the potential size of the valve implant, limit the orifice area, and increase the transvalvular gradient [7]. Stented porcine xenografts have limited durability because of calcification and primary tissue failure [8, 9]. Stent-mounted aortic homografts suffer the same fate, with accelerated failure within 8 years [10]. Fatigue testing shows that the stent of a bioprosthesis is the major factor governing stress on the tissue component [11–13]. Clearly the best stent for a glutaraldehyde-fixed porcine xenograft is its native aortic root.

Interest in stentless valves was renewed by David and associates [14] in 1988, when hemodynamic tests again showed very small resting gradients. Much larger valves could be inserted without a stent and sewing ring. The Freestyle valve (Medtronic, Minneapolis, MN) is a porcine aortic root fixed with diastolic pressure on the sinuses (40 mm Hg) and net zero pressure across the cusps [15]. The aortic wall retains about 10% of its distensibility after glutaraldehyde treatment. It was our hypothesis that better coaption between the valve cylinder and native aorta with time would improve hemodynamics. This, together with antimineralization treatment, may lead to a lower rate of stress-mediated calcification and increase durability [16]. We examined this hypothesis by recording the changes in transvalvular gradients during the first 6 months after implantation.

	Patients and Methods

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 
The Freestyle Valve
The Medtronic Freestyle aortic root bioprosthesis incorporates the natural valve and approximately 2 cm of aorta with the right and left coronary arteries ligated. The inflow aspect of the valve is covered with a thin layer of Dacron cloth, which increases to a depth of 4 to 5 mm over the muscle bar adjacent to the right coronary sinus. The tissue is cross-linked in buffered glutaraldehyde solution with a zero pressure differential across the valve cusps. Preservation of the structure of the natural valve sinuses decreases the mechanical stress on the cusps by dissipation of energy to the aortic root during closure. Zero-pressure fixation retains the natural collagen crimp, which aids in resisting cyclic fatigue [17]. Alpha-aminooleic acid, a derivative of the naturally occurring oleic acid, reduces the potential for leaflet calcification [15, 16]. This does not influence the mechanical properties, histologic structure, or biocompatibility of the porcine root, which is supplied sterile in a buffered 0.2% glutaraldehyde solution. In vitro effective orifice area for different sized valves is shown in Table 1. Size for size the effective orifice area (in vitro) of the Freestyle 23-mm valve is equivalent in size to that of the 31-mm Hancock valve (manufacturer's data).

Sixty-nine patients (74%) had impaired left ventricular function (ejection fraction < 0.40) and 39 (48%) were in New York Heart Association functional class III or IV. Eleven (26%) were in atrial fibrillation. Nine (11%) had suffered previous myocardial infarction and 10 (12%) had congestive cardiac failure. In addition, 14 (16%) had chronic obstructive airways disease with a forced expiratory volume less than 1.5 L.

No stented bioprosthesis was used in any patient after the beginning of this investigation, and no patient greater than 70 years received a mechanical valve. Twenty-seven patients with coronary artery disease required coronary bypass grafts and 3 patients underwent repair of the mitral or tricuspid valves in association with aortic replacement. The Food and Drug Administration investigative guidelines did not permit replacement of the mitral or tricuspid valves. Informed consent was obtained from all patients, and the study was approved by the hospital's ethics committee.

Technical Aspects of Freestyle Valve Implantation
The implantation technique, described as the ``cylinder within a cylinder'' method, fits the porcine aortic root snugly within the patient's root and preserves the spatial orientation of the valve commissures. Glutaraldehyde preservation stiffens the porcine cylinder, and with cloth covering of the muscle bar and inflow, the surgical procedure is less complex than aortic homograft insertion. The steps in the surgical procedure are illustrated in Fig 1.

View larger version (2K): [in this window] [in a new window]   Fig 1. . The ``cylinder within a cylinder'' method for Freestyle valve implantation. (A) The aorta is opened with a transverse aortotomy 2 to 3 mm above the sinotubular junction. Stay sutures are applied to display the coronary ostia, the native valve is excised, and the annulus size is determined. (B) Simple interrupted valve sutures are inserted. At the commissures these are placed at the subcommissural level to maintain the inflow in a single plane. The valve is oriented so that the porcine coronary ostia approximate to the human coronary ostia. (C) The valve sutures are tied before the porcine coronary sinuses are excised. (D) The porcine coronary sinuses are excised to align directly with the human coronary ostia. The height of the valve cylinder is trimmed to equate with the depth of the human sinuses. (E) The outflow suture line is performed with a single double-armed 4/0 Prolene (Ethicon, Somerville, NJ) suture, which joins the crest of the Freestyle valve to the transected aorta. The suture line is brought beneath the coronary ostia. If the right coronary ostium is occluded within 2 cm the porcine right coronary sinus is not excised. (F) The Freestyle valve in situ.

Thorough decalcification was performed to maximize the aortic valve orifice, and calcium was removed from the aortic sinuses where necessary.

Sizing of the xenograft cylinder was critical to achieve an optimum result. It was particularly important not to distort the inflow aspect of the valve by oversizing while obtaining a snug fit between the porcine aortic cylinder and the human valve sinuses to maximize valve area. For the Freestyle valve the annulus itself is of primary importance because the patient's sinotubular junction can be tailored to the circumference of the outflow of the valve chosen. The circumference of the porcine outflow must be maintained to prevent distortion of the valve commissures and aortic regurgitation. Careful judgment is required for orientation of the Freestyle valve because the porcine coronary ostia are more closely approximated than those of the human. (It is unwise to excise the porcine coronary ostia from their sinuses before the inflow sutures of the valve are secured).

The porcine root is inserted using interrupted inflow sutures of 2/0 Ticron (Davis & Geck, Danbury, CT) and a single running suture of 4/0 polypropylene to the outflow aspect. The crest of the porcine outflow is sutured to the transverse aortotomy and the suture line is carried below the coronary ostia, allowing adequate clearance between the inflow cloth and coronary orifice. Closure of the transverse aortotomy reinforces the outflow suture line.

There were three modifications to this method according to individual circumstances: (1) If the right coronary artery was completely occluded by atheroma within 2 cm of its origin, then the porcine right coronary sinus was not scalloped, and the coronary artery was bypassed. When a right coronary occlusion occurred more distally, the right coronary ostium was left patent so that the sinoatrial branch and small branches to the anterior aspect of the right ventricle were preserved. (2) When the root was extensively calcified, a cutting needle was used for the outflow suture line and the sutures were passed perpendicularly through the aortic wall. This provided a more secure implantation than tangential bites in a calcified sinus. (3) In 2 female patients with a small aortic root, the distance between the annulus and the right coronary ostium was limited. In this circumstance the Freestyle valve was rotated so that the elevated cloth covering the muscle bar did not lie beneath the right coronary ostium. Early in the series attempts to accommodate the cloth beneath the right coronary ostium caused an element of folding of the Dacron, which impinged on the valve orifice (Fig 2).

View larger version (41K): [in this window] [in a new window]   Fig 2. . Mechanism for elevated early transvalvular gradient in patients with a small aortic root and short distance between the annulus and right coronary ostium. The gradient regresses with time. (LCA = left coronary artery; RCA = right coronary artery.)

Postoperative Investigations
These were carried out according to strict guidelines for follow-up by the United States Food and Drug Administration. Patient demographics and clinical events were recorded for all patients. Preoperative and postoperative blood samples were taken to evaluate hemolysis. Detailed two-dimensional echocardiography was performed before hospital discharge (day 5 or 6) and then at 6 months postoperatively. Mean and peak systolic gradients were recorded together with the degree of aortic regurgitation.

	Results

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 
Ischemic and cardiopulmonary bypass times for isolated aortic valve replacement and aortic valve replacement plus coronary bypass grafting (n = 27) or another valve procedure (n = 3) are shown in Table 2.

There were six late non-valve-related deaths due to myocardial infarction (2), cerebral hemorrhage (2), ruptured abdominal aortic aneurysm, and carcinoma of the pancreas. These patients were aged 72 to 84 years. Postmortem examination up to 8 months postoperatively showed the Freestyle valve to completely fill the human sinuses with no intervening hematoma. There remained a clear plane of separation between the porcine and human aortic wall. Of the 70 surviving patients, 41 have been studied echocardiographically at 6 months and 8 at 1 year. All surviving patients have shown progressive clinical improvement to New York Heart Association class I or II at 6 months. There has been no hemolysis or endocarditis in this series.

The mean and peak systolic gradients for different valve sizes during week 1 and at 6 months are shown in Table 3. By 6 months there was a consistent reduction in mean systolic gradient for all patients with 21- to 25-mm valves. The highest 6-month gradient (recorded in a 21-mm valve) was 15 mm Hg. This occurred in a female patient with a heavily calcified aortic root in whom the cloth-covered muscle bar was not rotated away from the right coronary ostium (see Fig 2). The highest gradients both during week 1 (up to 23 mm Hg) and at 6 months (up to 15 mm Hg) were in small, heavily calcified aortic roots, but these were acceptable in comparison with gradients for equivalent-sized stented bioprostheses.

	Comment

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 
This consecutive series of 80 predominantly elderly patients who received a bioprosthesis illustrates that the Freestyle valve can be inserted safely in most patients irrespective of the condition of the aortic root. So far we have not had the opportunity to use the valve in a patient with endocarditis. For isolated aortic valve replacement the duration of cardiopulmonary bypass was less than 1 hour, and the last 30 isolated aortic valve replacements were performed with ischemic times of less than 45 minutes. Consequently, use of the Freestyle valve is unlikely to increase the operative risk for patients who require an associated procedure.

The ``cylinder within a cylinder'' method is simple and reproducible, and avoids aortic regurgitation. It is easier to maintain the spatial orientation of the valve commissures and circumference of the valve outflow than with an elliptic aortotomy, which disrupts and distorts the sinotubular junction and noncoronary sinus [18]. The valve chosen should provide a snug fit within the human aortic root so that the outflow of the Freestyle valve can be sutured to the crest of the transverse aortotomy without splaying the commissures. When there is a discrepancy in size, the native aorta is tailored to fit the xenograft outflow. There is a learning curve in this operation both for valve sizing and the technique of implantation. The stiffened glutaraldehyde-preserved aortic sinuses ``sit up'' within the human sinuses and facilitate implantation. Fewer valve sutures are required than for an aortic homograft because of the inflow cloth cover. Orientation of the valve must take into account clearance of the inflow Dacron cloth beneath the coronary ostia. The potential for distortion of the inflow cloth covering the muscle bar (situated below the human right coronary ostium) was recognized when a 20-mm Hg gradient was identified for a 21-mm valve in a female patient with a small aortic root. The 4- to 5-mm height of the Dacron cloth may be difficult to accommodate below the right coronary ostium, particularly in a heavily calcified root, where care must be taken not to damage the bundle of His by over-excavating calcium from the valve annulus. The problem is avoided by rotating the Freestyle valve to site the elevated inflow cloth in the noncoronary sinus. We have not experienced any problem accommodating the inflow cloth beneath the left coronary ostium.

When the aortic cross-clamp is released the partially distensible porcine root should completely fill the globular human sinuses. There is otherwise the potential for hematoma to collect between the two cylinders, and this may partly account for early gradients across the valve, which subsequently regress. Distention of the native aortic root should compensate for intramural hematoma except when distention is limited by calcification. The inflammatory response to operation might also contribute to higher early gradients and be expected to resolve.

By 6 months postoperatively echocardiography showed the hemodynamic function of the Freestyle valve to be directly equivalent to that of an aortic homograft as long as the technical aspects of implantation were satisfactory. Aortic regurgitation occurs less frequently than with the aortic homograft, and there was no significant regurgitation in our patients. By maintaining the circumference of the Freestyle outflow, the commissures cannot be distorted. In contrast, with the scalloped Toronto valve, size mismatch can result in aortic regurgitation [19, 20].

We suspect that the overall reduction in mean systolic gradient with time occurs through expansion and remodeling of the porcine root to snugly fit the human sinuses, together with resolution of perivalvular hematoma and the acute inflammatory response. Early regression of left ventricular hypertrophy also occurs. This last mechanism is seen with most valve replacement devices, but there is evidence that those with excellent hemodynamics (such as the aortic homograft or the Toronto valve) lead to more rapid regression in left ventricular hypertrophy than stented valves with higher valve gradients [21, 22]. There have been conflicting reports of long-term function in other stentless porcine xenografts. The hemodynamic performance of the Toronto valve (St. Jude, Minneapolis, MN) is also equivalent to that of the aortic homograft [19, 20], whereas early reports for the Prima valve (Baxter, Irvine, CA) show late gradients equivalent to those of the stented Carpentier valve, thereby offering no hemodynamic advantage [23]. These differences are as likely to be related to surgical technique as to valve design.

It remains to be seen whether methods of preparation and preservation can influence durability and differentiate between existing stentless valves. Broom and Thomson [24] have defined mechanisms for achieving increased durability by retaining the structural characteristics of fresh porcine valve tissue. Zero-pressure leaflet fixation may contribute to durability as appears to be the case for the Intact bioprosthesis (Medtronic) [25]. This process retains the natural collagen crimp of the valve cusp and maintains the tissue's shock-absorbing capacity, which aids in resisting cyclic fatigue [15]. This feature, together with the improved hemodynamics of the Freestyle valve over stented valves of equivalent diameter, may translate into improved durability. For patients older than 70 years we do not consider it an advantage to scallop the noncoronary sinus of the porcine xenograft. As yet there is no evidence to suggest that the porcine aortic cusp will function longer in a pathologic human sinus than in its own sinus. Many surgeons perform freehand implantation of an aortic homograft leaving the noncoronary sinus intact, and this has not adversely influenced the results of homograft implantation. Animal studies have demonstrated that the alpha-aminooleic acid processing delays leaflet calcification [15, 16]. Eventual calcification of the aortic wall is inevitable and may complicate reoperation. However, the combination of improved hemodynamics, zero-pressure fixation and antimineralization treatment may improve durability so that patients more than 70 years old have only a very small risk of reoperation.

The Freestyle valve is versatile in that it can be used for aortic root replacement. This method is preferred by some surgeons, although this may eventually lead to a higher operative mortality for elderly patients [26]. In addition, reoperations will be difficult unless a generous button of recipient coronary tissue is left to facilitate remobilization of the coronary ostia. Transvalvular gradients in the postoperative period were smaller with root replacement (which allows a larger valve to be inserted), but at 6 months the transvalvular gradients were identical [26]. This supports the concept that progressive remodeling of the Freestyle valve occurs to fit the human aortic sinuses.

In summary, our preliminary experience has shown that with the surgical method described, the Freestyle stentless valve is suitable for most patients in whom an aortic bioprosthesis is preferred. Calcification of the aortic root does not exclude use of the Freestyle valve. There is a learning curve in both valve sizing and orientation according to the circumstances of the human anatomy. Valve gradients decrease progressively with time, probably for reasons other than regression of left ventricular hypertrophy.

	Footnotes

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 
Presented at the Thirty-first Annual Meeting of The Society of Thoracic Surgeons, Palm Springs, CA, Jan 30-Feb 1, 1995.

Address reprint requests to Mr Westaby, Oxford Heart Centre, John Radcliffe Hospital, Oxford, OX3 9DU, England.

	References

Top Footnotes Abstract Introduction Patients and Methods Results Comment References

 

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