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Ann Thorac Surg 2001;71:86-91
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

The CryoLife O’Brien stentless porcine aortic bioprosthesis: 5-year follow-up

^a Department of Cardiothoracic Surgery, General Hospital "Santa Maria della Misericordia," Udine, Italy
^b Department of Cardiology, General Hospital "Santa Maria della Misericordia," Udine, Italy

Accepted for publication May 6, 2000.

Address reprint requests to Dr Gelsomino, U. O. Cardiotoracica, Azienda Ospedaliera Santa Maria della Misericordia, Piazzale Santa Maria della Misericordia, 33100 Udine, Italy
e-mail: sandrogelsomino{at}virgilio.it

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. Mortality, morbidity, complication rates, and echo hemodynamic results using the Cryolife O’Brien stentless aortic bioprosthesis over a 5-year period are reported.

Methods. The stentless valve was implanted in 97 conscecutive patients, 54 male and 43 female, mean age 70.9 ± 6.5 years. All patients underwent preoperative, discharge (early study), 6-month (intermediate study), and late (18.3 ± 10.4 months) echocardiography.

Results. The actuarial 5-year survival rate was 93.9% ± 3%. Aortic regurgitation was absent in 95.5%, mild in 3.4%, and moderate in 1.1%. Peak and mean systolic gradients were significantly lower at discharge (p < 0.001) and at the 6-month follow-up (p < 0.001) but did not significantly fall further at the late study (p = NS). The effective orifice area index at discharge (p < 0.001) and at 6 months (p < 0.001) differed significantly from preoperative values, but variations at late study were not significant (p = NS). Left ventricular mass index decreased early postoperatively (p < 0.001) and at 6-month assessment (p < 0.001) with a further significant reduction at late echocardiography (p = 0.04).

Conclusions. The 5-year results of this stentless valve showed a low rate of valve-related complications with excellent hemodynamic performance in all valve sizes.

	Introduction

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Stented prostheses in aortic position have the disadvantages of high transvalvular gradients and incomplete regression of left ventricular hypertrophy [1], which has been widely demonstrated to affect long-term ventricular function adversely after aortic valve replacement [2].

Stentless valves are credited with hemodynamics superior to those of stented bioprostheses [3]. Stentless aortic valve replacement also led to regression of left ventricular hypertrophy [4], which in turn could translate into better patient survival.

The aim of the present study was to examine the results of using the CryoLife O’Brien stentless aortic porcine valve over a 5-year period.

	Material and methods

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Among 625 patients undergoing aortic valve replacement in our institution between November 1993 and November 1998, 97 (15.5%) consecutive patients had a Cryolife O’Brien stentless valve inserted in the aortic position. Mean age was 70.9 ± 6.5 years (range, 50 to 83). A total of 80 patients (80.4%) were more than 65 years of age; 54 (55.7%) were male and 43 (44.3%) were female. Mean body surface area (BSA) was 1.74 ± 0.17 (range, 1.10 to 2.14). Seventeen patients (17.5%) were in New York Heart Association functional class IV, 65 (67%) were in functional class III, and 15 (15.5%) were in functional class II. Perioperative data for the patients are listed in Table 1. All operations were performed by the same experienced surgeon. Dilation of the sinotubular junction and extensive root calcification were exclusion criteria.

Patients routinely received warfarin sodium (Coumadin; Du Pont Pharmaceuticals, Wilmington, DE) for 3 months after the operation. At recent follow-up 34 (38.2%) patients continued to receive anticoagulation therapy because of atrial fibrillation.

Study valve
The CryoLife O’Brien stentless porcine aortic prosthesis (CryoLife International, Marietta, GA) is of a composite design, constructed with noncoronary leaflets obtained from three porcine valves. Leaflets are carefully excised from valves already fixed in glutaraldehyde under very low or near zero pressure. Individual noncoronary leaflets are matched for size and symmetry to ensure a synchronous opening and to promote maximal leaflet coaptation. The matched set of leaflets is sutured together along the free edges of the porcine aortic wall at the leaflet commissures. The base of the valve is finished with a blanket stitch to ensure its integrity. No reinforcement with Dacron (C.R. Bard, Haverhill, PA) is necessary, a significant difference when compared to other stentless valves.

The surgical technique that we used has already been extensively described [5]. Basically the valve is secured in a supraannular position with one single suture line. The selected valve is one size larger than the measured aortic annular diameter.

Echocardiography
All patients underwent preoperative transthoracic echocardiography. The referring cardiologist assessed valve functions in all survivors with serial transthoracic echocardiograms, at discharge (early study), and at 6 months (intermediate study). Furthermore, 82 of 92 survivors had echocardiography at an mean of 18.3 ± 10.4 months (range, 6 to 44 months) postoperatively (late study).

Transvalvular gradients and left ventricular mass were available for all patients. At each follow-up, M-mode, two-dimensional, and Doppler echocardiography were performed. Standard apical, parasternal and subcostal views were obtained. The following variables were measured: left ventricular end-systolic and end-diastolic diameters and fractional shortening, interventricular septum and posterior wall thickness, and maximum and mean flow velocity across the stentless valve. Additional variables were calculated as discussed below.

Left ventricular ejection fraction was calculated from Teichholz formulas.

Left ventricular mass (LVM) was calculated from ventricular septum thickness (ST [cm]) in diastole, the posterior wall thickness (WT [cm]) in diastole and the left ventricular end-diastolic diameter (LVEDD [cm]), with the American Society of Echocardiography cube method [6, 7], as follows:

Maximum and mean aortic valve gradients were calculated by a modified Bernoulli equation; the effective orifice area (EOA) was calculated by the continuity equation; values of EOA and of left ventricular mass were indexed for body surface area, and the results were averaged.

Complication rates for primary and secondary events
Operative and long-term mortality and morbidity were collected during the 5-year follow-up period using the guidelines of Edmunds and colleagues [8] for reporting morbidity and mortality after cardiac valvular operations. Follow-up information was obtained during outpatient clinic appointments or by telephone interviews made between 1 and 60 months (mean 31.3 ± 15.5 months) postoperatively. Follow-up of 94 hospital and 92 late survivors was 100% complete.

Statistical analysis
The Statistical Package for the Social Sciences, version PC 8.0 (SPSS, Chicago, IL) was used to perform data analyses. Continuous variables were expressed as means and standard deviations. Categorical data were given as percentages. Paired and unpaired Student’s t tests were used as appropriate to analyze continuous data, and the ² and Fisher’s exact test were used to analyze discrete data.

Repeated analysis of variance (ANOVA) measures were used to make comparison of the three assessment points. Multiple group comparison was performed using the Bonferroni and Tukey post hoc tests.

Comparison of continuous variables between valves size groups was made using either pooled or separate variance assumption, after testing of homogeneity of variance with the Levine test.

Univariate and multivariate analyses (general linear models) were performed to identify predictors of early (within 30 days of operation) and late deaths.

The small number of postoperative events limited the statistical power of multivariate analyses.

Death and event-free survival estimates were calculated by the product-limit method of Kaplan and Meier and reported with 95% confidence limits; the Mantel-Cox (log-rank) test was used to test the hypothesis that there was no difference in survival among groups. Cox proportional hazards models were used to examine the predictive value of preoperative and operative variables considered to be associated with outcome; the importance of covariates was evaluated singly and in combination using stepwise procedures. In all cases p values less than 0.05 were considered to be significant.

	Results

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The 30-day mortality rate was 3.1% (3 of 97 patients). Univariate analysis of risk factors identified a decreased preoperative mean gradient (p = 0.005) as the sole variable associated with perioperative mortality. The average preoperative mean gradient in survivors was 61.2 ± 26.5 mm Hg (range, 10 to 150) compared with 25.5 ± 9.7 mm Hg (range, 22 to 40) in patients who died perioperatively (p = 0.009). In the low preoperative gradient subgroup one patient had a left ventricular ejection fraction of less than 30% and two patients had associated coronary artery disease (CAD).

Multivariate analysis identified two significant variables associated with perioperative mortality. These included an associated CAD (p = 0.003) and a low mean preoperative gradient (0.009).

There were 2 late deaths (2.2%), 1 of which was cardiac related. Actuarial survival at 5 years was 93.9% ± 3% (Fig 1). By univariate analysis, the presence of CAD (p = 0.016) and a previous myocardial infarction (p = 0.017) were associated with reduced overall survival. Only the presence of CAD (p = 0.002) remained significantly related to reduced overall survival by multivariate analysis. The 5-year survival in patients without CAD (Fig 2) was much better than that for patients with significant CAD (100% and 87.6% ± 4%, respectively, p = 0.03). Among the 92 hospital survivors there were two (2.2%) thromboembolic incidents; 5-year actuarial freedom from thromboembolism was 97.8% ± 5%. Two patients (2.1%) had episodes of endocarditis, and the actuarial freedom from endocarditis was 96.8% ± 5% at 5 years. There were no episodes of significant anticoagulation-related hemorrhage. Two valves (2.1%) were explanted because of structural deterioration but none for endocarditis. In the first case the early deterioration (6 months postoperatively) of the valve was due to technical failure; in the other case a retraction of a cusp was responsible for valve incompetence after 47 months from implantation. Actuarial freedom from structural deterioration at 5 years was 90.2% ± 4%.

View larger version (17K): [in this window] [in a new window]   Fig 1. Actuarial 5-year survival after aortic valve replacement with Cryolife O’Brien stentless bioprosthesis (with 95% confidence interval).

View larger version (21K): [in this window] [in a new window]   Fig 2. The 5-year survival of patients without coronary artery disease (broken line) was significantly better than that of patients with this disease (solid line).

View larger version (13K): [in this window] [in a new window]   Fig 3. (A) Actuarial freedom from all events at 5 years. (B) Hazard function shows a slowly rising incidence over time.

Echocardiographic data are shown in Table 2
Aortic insufficiency was absent in 66 (74.2%) patients and was graded trivial in 19 (21.3%), mild in 3 (3.4%), and moderate in 1 (1.1%). Left ventricular ejection fraction was significantly higher at early than preoperative study (p < 0.001) and showed a further but not significant increment at intermediate and late controls (p = NS).

Left ventricular end-diastolic diameter decreased at the early follow-up (p = 0.01). Variations at the intermediate and late studies were not significant (p = NS). Left ventricular end-systolic diameter was lower at discharge than preoperatively (p = 0.04). No further significant improvements were noticed at successive follow-ups (p = NS). Peak and mean systolic gradients across the Cryolife O’Brien stentless valve (Fig 4) were significantly lower at discharge (p < 0.001) and at the 6-month follow-up (p < 0.001) but not significantly lower at the late than intermediate follow-up (p = NS).

View larger version (24K): [in this window] [in a new window]   Fig 4. Mean gradients by valve size at the various points of the study.

Left ventricular mass index decreased by 18.7% at discharge, 8% after 6 months, and again 3.3% at late study.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
The implantation of a stentless aortic valve was first reported in 1965 by Binet and associates [9] and in 1966 by O’Brien and Clareborough [10]. With the development of tissue fixation with glutaraldehyde and availability of easy handling stented valves, the interest in stentless porcine valves diminished, although stented bioprostheses in the aortic position had the disadvantage of stent-related mechanical stress on the leaflets during the cardiac cycle and restrictive hemodynamics with relatively high residual transvalvular gradients in the small valve sizes [11].

To avoid some of the drawbacks of stented valves, an increasing number of surgeons began using aortic homografts. Ross [12] in 1962 and Barrat-Boyes [13] in 1964 independently began implanting homograft valves with the technique described by Duran and Gunning [14]. The availability of homografts, however, is limited. In 1988 David and colleagues [15] reintroduced the use of stentless glutaraldehyde-fixed aortic porcine xenografts. The composite porcine stentless valve first reported by O’Brien and Clarebrough [10] in 1966 was reintroduced in 1991 using glutaraldehyde preservation [16].

Today, all reports on the use of stentless valves agree that, despite a more demanding technique, stentless valves show excellent hemodynamics even in patients with a small aortic root. In addition, regression of left ventricular hypertrophy is more complete when compared to that achieved with stented valves. The issue could be significant because, for patients undergoing aortic valve replacement, incomplete left ventricular mass regression has been shown to reduce 10-year survival significantly [17]. The presence of elevated residual gradients due to a possible patient prosthesis mismatch continues to be the most important determinant of persistent left ventricular hypertrophy. The absence of a stent and a larger available flow area of the valve ensure low residual obstruction, as demonstrated by low residual gradients and a rapid regression of left ventricular hypertrophy even in patients with small aortic roots.

In the present study of patients undergoing aortic valve replacement with the Cryolife O’Brien stentless valve, among the 92 survivors the EOA index early after implantation was more than 1.05 cm²/m² in 41 patients (45.5%), 0.86 to 1.05 cm²/m² in 37 patients (41.1%), and 0.66 to 0.85 cm²/m² in 12 patients (13.4%). Based on the definition (EOA index < 0.85cm²/m²) of prosthesis–patient mismatch [18–20] its early prevalence in this series was therefore 12.2% with an incidence for 21-mm valves of 3.3% (3 patients) and for 23-mm valves of 8.9% (8 patients). A significantly larger EOA index was revealed late postoperatively as being more than 1.05 cm²/m² in 63 patients (70%), 0.86 to 1.05 cm²/m² in 24 patients (26.7%), and 0.66 to 0.85 cm²/m² in 3 patients (3.3%) with a 21-mm valve. Residual pressure gradients, decreasing significantly early postoperatively, were extremely low at late echocardiography, and a significant reduction in left ventricular mass and left ventricular mass index all along the observation period demonstrated an excellent left ventricular mass regression long term after aortic valve replacement with the Cryolife O’Brien stentless valve. This advantageous left ventricular remodeling may translate into better patient survival [21, 22].

Although the issue of durability remains unresolved and further investigations are needed to assess the long-term results, our 5-year data for the Cryolife O’Brien stentless bioprosthesis have been excellent in terms of clinical and hemodynamic performance. This valve seems to be a good choice for the patient undergoing aortic valve replacement with small aortic annulus and already selected for a biological prosthesis.

	Acknowledgments

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We gratefully acknowledge the help of Dr Ulrik Hvass (Hôpital Bichat, Paris, France) for his valuable professional suggestions and critical review of the manuscript. We thank Dr Orlando Parise for the statistical analysis.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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