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Ann Thorac Surg 1998;66:425-430
© 1998 The Society of Thoracic Surgeons

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Original articles: cardiovascular

Aortic valve replacement with the biocor PSB stentless xenograft

^a Division of Cardiac Surgery, University of Verona, Verona, Italy

Accepted for publication March 5, 1998.

Address reprint requests to Dr Luciani, Division of Cardiac Surgery, University of Verona, O.C.M. Piazzale Stefani 1, Verona, 37126 Italy

Presented in part at the VII International Symposium on Cardiac Bioprostheses, Sitges, Spain, June 13–15, 1997.

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. The midterm clinical results after aortic valve replacement with the Biocor PSB stentless xenograft on all patients operated between October 1992 and October 1996 were reviewed.

Methods. One hundred six patients, aged 70 ± 6 years, had aortic valve replacement for aortic stenosis (67%), regurgitation (11%), or both (22%). Associated procedures were done in 49 patients (46%), including coronary artery bypass in 30 patients, mitral valve repair/replacement in 16, and ascending aorta replacement in 5 patients. Aortic cross-clamp and cardiopulmonary bypass times were 96 ± 24 and 129 ± 31 minutes, respectively.

Results. There were 3 (3%) early deaths due to low output (2 patients) and cerebrovascular accident (1 patient). Follow-up of survivors ranged from 6 to 66 months (mean, 39 ± 14 months). Survival was 94% ± 2% and 90% ± 3% at 1 and 5 years. There were 5 late deaths due to cardiac cause (2), cancer (2), and pulmonary embolism (1 patient). No patient had structural valve deterioration, whereas 100% and 95% ± 3% were free from valve-related events at 1 and 5 years. There were two reoperations due to narrowing of the left coronary ostium and endocarditis, with an actuarial freedom from reoperation of 99% ± 1% and 98 ± 1% at 1 and 5 years, respectively. Functional results demonstrated a mean peak transprosthetic gradient of 16 ± 12 mm Hg, with only 1 patient (1%) with a 55 mm Hg gradient. No cases of valve regurgitation greater than mild were recorded at follow-up. Assessment of New York Heart Association functional class demonstrated a significant improvement (2.9 ± 0.6 versus 1.4 ± 0.7; p = 0.01). All patients were free from anticoagulation.

Conclusions. Aortic valve replacement using the Biocor PSB stentless xenograft offers excellent midterm survival, negligible valve deterioration, and a very low rate of valve-related events, which are comparable to estimates reported with other models of stentless xenografts and currently available stented xenografts. Hemodynamic performance is favorable and quality of life satisfactory.

	Introduction

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During the past years a revived interest has developed in the use of stentless prostheses as substitutes for the aortic valve [1–5]. Because of the ability to adapt to the native aortic root and reproduce the normal anatomy, more durable function with stentless xenografts is expected, as previously shown for aortic homografts [6]. Similarly to aortic homografts, early hemodynamic performance has been shown to be superior to stented bioprostheses and comparable with human cadaveric valves [7, 8]. Despite encouraging early and midterm results observed with a variety of stentless valve models recently introduced in the market [1–5], a paucity of data exists with the use of the Biocor Stentless Bioprosthesis [3, 9].

The present study reviews the clinical experience with more than 100 implants using the Biocor Stentless Bioprosthesis during a 5-year time interval.

	Material and methods

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Patient population
Starting in May 1992, three different kinds of stentless bioprostheses were routinely used at our institution: (1) the Biocor PSB valve (Biocor Industria e Pesquisa Ltda, Belo Horizonte, MG, Brazil); (2) the Toronto SPV valve (St. Jude Medical, Inc, St Paul, MN); and (3) the O’Brien-Angell valve (Bravo Cardiovascular model 300; Cryolife, Atlanta, GA). The Biocor PSB aortic valve was the first valve used for aortic valve replacement at the University of Verona. Patients were selected according to the same criteria used for traditional porcine stented valves: (1) age older than 65 years, (2) contraindication to warfarin, and (3) in younger patients, preference of a xenograft rather than a homograft or autograft. Associated anomalies were never considered as a contraindication to the use of stentless valve. The entire group of patients (106) undergoing aortic valve replacement was not randomized; therefore, the choice to implant a stentless valve rather than a stented one was left to the surgeon at the time of operation. The demographic variables of the population are as follows: 50 men and 56 women; age, 70 ± 6 years; stenosis, 72; regurgitation, 12; mixed, 19; endocarditis, 7; redo aortic valve replacement, 3; and New York Heart Association functional class, 2.9 ± 0.6.

The Biocor PSB valve is a composite valve made of selected individual porcine cusps, avoiding leaflets with muscular bands. The leaflets are tanned with glutaraldehyde with zero pressure and sutured to a strip of bovine pericardium that is shaped in the form of a conduit and scalloped, above and below the insertion of the valve leaflets, to fit the aortic root.

Technique of implantation
The technique of implantation described by the manufacturer suggests to size the valve at the annular level. Instead, we adopted the concept of David and colleagues [1] and preferred to consider the sizing at the sinotubular junction. We performed the inflow suture line either with the use of interrupted 4-0 multifilament polyester suture (aortic annulus, 23 mm) or with three running 4-0 polypropylene sutures (aortic annulus, 25 mm). The outflow suture line was completed using three running 4-0 polypropylene sutures. Operations were performed under moderately hypothermic cardiopulmonary bypass. Before January 1995, pharmacologic cardiac arrest was obtained with the St. Thomas II cold cardioplegia. Thereafter, cold blood cardioplegia was routinely used. Incidence of associated procedures ranked high in this group of patients, with coronary revascularizations as the most common (Table 1). In case of coexistent mitral valve disease not amenable to repair, mitral valve replacement was done using stented bioprostheses. Other associated procedures consisted of ascending aorta replacement or carotid thromboendarterectomy. No oral antiplatelet or anticoagulant drugs were given to the patients because of the stentless xenograft implant.

Statistical analysis
Continuous variables were expressed as mean value ± standard deviation, and discrete variables as percentage of total. Actuarial life-table estimates were constructed using the Kaplan-Meier method for the following events: (1) survival, (2) freedom from cardiac death, (3) freedom from any event, (4) freedom from valve-related events, (5) freedom from reoperation, and (6) freedom from structural and nonstructural deterioration. These events were defined in agreement with the reporting on cardiac valvular operations guidelines [10]. To identify risk factors for time-related occurrence of events after aortic valve replacement, we analyzed several variables with the Cox regression multivariable analysis [11]. Variables included age, sex, indication for operation, endocarditis as a cause of aortic insufficiency, presence of a bicuspid aortic valve, associated coronary artery disease, name of surgeon, need for associated procedures, size of xenograft valve, duration of aortic cross-clamp time, use of blood cardioplegia, and immediate postoperative transprosthetic peak pressure gradient. A p value less than 0.05 was considered significant.

	Results

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Survival
There were three (3%) early deaths; 2 patients died as a result of low output syndrome and 1 to a cerebrovascular accident. Follow-up of survivors ranged from 6 to 66 months (mean, 39 ± 14 months). There was a total of five deaths (5%) during the follow-up period; malignancy (2 patients), cardiac cause (2), and pulmonary embolism (1 patient) accounted for these casualties. Actuarial 1- and 5-year survival estimates were 94% ± 2% and 90% ± 3%, respectively (Fig 1). Analysis of actuarial freedom from cardiac death was 99% ± 1% and 95% ± 2% at the same time intervals.

View larger version (15K): [in this window] [in a new window]   Fig 1. Actuarial survival after aortic valve replacement with the Biocor PSB stentless valve. Patients at risk are reported in parentheses.

Valve-related events
There were only two (3%) valve-related adverse events during follow-up. One patient suffered from a cerebral ischemia probably due to embolism, as mentioned above, and a second patient underwent reoperation because of endocarditis. Because of its early onset, the digestive bleeding event reported above could not be directly related to the prosthetic valve device. At follow-up there were no other embolic events and no patient was on oral anticoagulation. The actuarial freedom from valve-related events was 100% and 95% ± 3% at 1 and 5 years, respectively (Fig 2).

View larger version (14K): [in this window] [in a new window]   Fig 2. Actuarial valve-related event-free survival after aortic valve replacement with the Biocor PSB stentless valve. Patients at risk are reported in parentheses.

Valve deterioration
One case of critical transprosthetic obstruction (1%) was classified as nonstructural valve deterioration. The patient presented with a peak transprosthetic pressure gradient of 55 mm Hg 6 months after operation, in the absence of morphologic or dynamic anomalies of the valve at bidimensional echocardiographic imaging. No evidence of progression of the obstruction was demonstrated at each subsequent echocardiographic examination. Considering the satisfactory clinical conditions of the patient who remained in New York Heart Association functional class I, no further intervention was thus far decided. The actuarial 1- and 5-year freedom from valve deterioration was 99% ± 1% and 99% ± 1%, as depicted in Figure 3.

View larger version (15K): [in this window] [in a new window]   Fig 3. Actuarial freedom from valve deterioration after aortic valve replacement with the Biocor PSB stentless valve. Patients at risk are reported in parentheses.

View larger version (15K): [in this window] [in a new window]   Fig 4. Actuarial freedom from reoperation after aortic valve replacement with the Biocor PSB stentless valve. Patients at risk are reported in parentheses.

Because of the low mortality and morbidity, analysis of risk factors failed to demonstrate any relationship between the variables considered and the adverse events after aortic valve replacement.

	Comment

Top Abstract Introduction Material and methods Results Comment References

 
Stentless bioprostheses are similar to freehand homografts in their morphology and technique of implantation [1–6]. These valves have been designed to adapt to the aortic root and restore the normal anatomy of this complex unit. The ideal matching with the native aortic root and the absence of a stent should allow more physiologic excursions of valvular cusps with a consequent reduction of mechanical stress [12–14]. This in turn is expected to permit longer durability because of a slower rate of calcification, as observed with aortic homografts [6]. The larger valve orifice gives a further advantage. Along with the wide excursions of the cusps, this larger orifice can attain more favorable hemodynamic performance with lower gradients, due to larger dynamic orifices and greater potential for regression of left ventricular hypertrophy [14]. Therefore, these valves should be more suitable than conventional bioprostheses for implantation in small aortic roots. At present, manufacturers can offer a wide choice of stentless bioprostheses that differ even substantially among them because of manufacturing, sizing, and technique of implantation [1–5]. At the beginning of our experience, Biocor Stentless Bioprostheses were the only commercially available stentless valves in Italy. Other products soon followed. From the start, we observed an evident ease of implantation, the possibility of adaptation to virtually every aortic root size, and satisfactory hemodynamic results. In particular, two structural features of the bioprosthesis were found to be uniquely advantageous: the presence of a scalloped design both at the upper and the lower margins of the pericardial support, allowing for a inferior suture line that closely follows the profile of the native aortic valve leaflets, and the unique character of the pericardium, offering both greater ease of needle penetration and greater potential for resistance to infectious processes. The latter property of the Biocor PSB valve is what led us to its use in selected cases of aortic valve endocarditis, when homograft valves were unavailable, as reported below.

The learning curve showed to be less steep than we expected. There were no complications due to improper technique of implantation and resulting in early graft dysfunction. Although well aware of the manufacturers’ suggestion on how to size the valve at the level of the aortic annulus [3], we preferred to follow the technique of David and colleagues [15] by first sizing at the sinotubular junction followed by its matching with the annular diameter. In this way, distortion of the graft can be minimized by avoiding to implant the prosthesis in the presence of a greatly asymmetrical aortic root, that is if the diameters of the annulus and the sinotubular junction differ by more than 10%. During the time interval of the present study, only 5 patients were excluded due to unfavorable anatomic conditions of the aortic root. Implant of a stented aortic xenograft proved simpler in these patients.

Implantation of a Biocor PSB, as well as of other stentless valves, is technically more demanding and needs longer ischemic times compared with traditional bioprostheses. Nevertheless, no impact on the early survival was apparent in our series, as reported by other researchers using different types of stentless valves [1, 2, 4, 5]. This finding is of relevance considering the older age of our patient population and the frequent need for associated surgical procedures. The present results are, however, at variance with the two previously reported studies on the Biocor PSB valve, which showed an operative mortality of 5% and 11%, respectively [3, 9]. Although difference in early mortality may be explained by differences in patient population in the study by Vrandecic and associates [3], technical mishaps resulting in atrioventricular conduction disturbance may account for the mortality in the study by Casabona and associates [9]. On the basis of all of the above observations and on our experience, it is fair to conclude that the Biocor PSB stentless valve per se will not result in greater operative mortality. Behavior of these valves during the time course of our observation can be considered extremely encouraging with an overall negligible incidence of early and late complications. No thromboembolic events and only one minor bleeding event were recorded over a 5-year period. Only 1 patient required reoperation for prosthetic valve endocarditis. On the other hand, because of its unique construction of exclusively biological materials, a Biocor stentless valve was deliberately used in 7 of our 106 patients as a substitute for native aortic valve disrupted by endocarditis. Durable sterilization of the infectious process was achieved in these patients, resulting in no need for reoperation at follow-up. These results favorably compare with the two existing reports on the Biocor PSB valve. In the study by Vrandecic and colleagues [3], both perioperative morbidity (12% versus 1% in the present series) and prevalence of reoperation on the valve (4% versus 1% in the present series) were significantly greater during a 43-month observation period. The relative impact of the younger age at aortic valve replacement (36 versus 70 years) and of the higher prevalence of redo aortic valve replacement (20% versus 3%), when compared with our study, needs to be taken into account. Morbidity was also higher in the experience of Casabona and associates [9] with 27 Biocor PSB implants, resulting in a 20% prevalence of complete atrioventricular block and need for permanent pacing. However, as specified above these data must be credited to complications related to the implant technique rather than to the device itself.

Early in our experience, 1 patient needed reoperation 5 months after valve implantation due of obstruction of the left coronary artery ostium resulting in resting angina. This phenomenon was initially interpreted with concern, because of the suspicion of a granulomatous reaction of the aortic endothelium against the pericardial conduit of the prosthesis. At the time of reoperation, planned to bypass the left coronary artery, diffuse thickening of the aortic wall was disclosed. The prosthesis, however, was intact and the upper suture line was covered with a regular endothelial lining, which was not found to lay close to the coronary ostium. The latter showed a pinpoint narrowing and was uniformly stenotic without evidence of discrete tissue thickening. Histology of the aortic wall failed to show specific anomalies. Since that episode no other case of such a complication was observed.

Transprosthetic pressure gradients, which contrary to most published series on stentless bioprostheses were expressed as peak values, confirm the observations with other stentless valves models showing low values, both at the time of discharge and in postoperative controls [4, 5, 16]. The apparent trend toward higher peak pressure gradients with the size 21-mm prostheses needs to be interpreted with caution. Indeed, the inclusion in the estimate of the failed xenograft, which presented a high peak gradient (>55 mm Hg), could easily lead to overestimation of the reported result on a limited number of patients. A strict control of the rate of ventricular mass regression [17] was not available for a significant number of our patients in the present experience. The clinical relevance of any left ventricular mass change that were to be detected in a series of mostly senile patients with a high percentage of associated coronary artery disease, like the present, is, however, uncertain. Unfortunately, a lack of information exists regarding the early and late functional performance of Biocor PSB. The only available data come from a single institution, with a cohort of patients completely different from ours because of much younger age and a high percentage of rheumatic disease [3]. Highly impressive results, both in terms of progressive increase of the dynamic valve orifice and reduction of the ventricular mass, have been observed in patients undergoing aortic valve replacement using the Toronto SPV [16, 17]. Furthermore, more recent observations done on patients with Toronto SPV valves seem to confirm superior performance of stentless bioprostheses even during stress conditions, as their dynamic behavior permits to maintain low gradients with high flows during inotropic stimulation [18]. Follow-up resting gradients after aortic valve replacement using the Biocor PSB in our series favorably compare with previously published data using other stentless valves, including the Toronto SPV [2, 4, 5, 16]. It is thus conceivable that the hemodynamic performance of the Biocor PSB xenograft under stress conditions may also resemble the result obtained using the Toronto SPV valve, although currently the evidence is not available.

In the present experience, only 1 patient with "nonstructural valve dysfunction," defined as a transvalvular gradient more than 50 mm Hg in the absence of tissue deterioration, was observed. This complication occurred early in this series in a patient who received a 21-mm diameter prosthesis, having a rather narrow outflow tract. It is our impression that the significant residual degree of obstruction might be secondary to an oversizing of the prosthesis rather than to malfunction. Aside from this isolated case, follow-up function of the Biocor PSB appeared comparable, both in terms of clinical outcome and of peak transvalvular gradients, with the standards set by the Toronto SPV valve [15–17, 19, 20].

In conclusion, several relevant considerations can be drawn from the present study, which comprises a large clinical experience with the Biocor PSB valve. The Biocor PSB is a stentless bioprosthesis that can be used as a substitute for the aortic valve, with virtually no limitation as it relates to aortic root anatomy and dimensions. Technique of implantation is easily reproducible and allows to avoid gross mistakes even in the very early learning phase. Impact of endocarditis, thromboembolic complications, and nonstructural dysfunction up to 5 years of follow-up remains negligible, whereas structural dysfunction has not been observed. Resting hemodynamic results suggest favorable hemodynamic performance of this bioprosthesis, even in the presence of small aortic annuli. Only prolonged (ie, longer than 10 years) and careful monitoring of the Biocor PSB stentless valve will prove if more physiologic excursions of cusps will translate into longer durability of clinical relevance.

	References

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