Ann Thorac Surg 2005;80:198-203
© 2005 The Society of Thoracic Surgeons
Original article: Cardiovascular
Midterm Results After Aortic Valve Replacement With a Stentless Bioprosthesis Aortic Valve
Ivo Martinovic, MDa,*,
Manfred Everlien, MDa,
Ibrahim Farah, MDa,
Thomas Wittlinger, MDa,
Igor Knez, MD, PhDb,
Hans Greve, MD, PhDa,
Paul Vogt, MD, PhDb
a Department of Cardiothoracic Surgery, Klinikum Krefeld, Krefeld, Germany
b Department of Cardiovascular Surgery, Justus Liebig University, Giessen, Germany
Accepted for publication January 7, 2005.
* Address reprint requests to Dr Martinovic, Am Oberend 11, 47809 Krefeld, Germany (Email: ivomartino{at}aol.com).
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Abstract
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BACKGROUND: It is suggested that a simplified implant model Cryolife-O’Brien (CryoLife International, Kennesaw, GA) offers less satisfactory outcome compared with standard stentless models. This study was conducted to prospectively evaluate the midterm results after aortic valve replacement with the Cryolife-O’Brien stentless bioprosthesis.
METHODS: In 1996, a prospective clinical trial using different stentless valves was initiated. From September 1996 through September 2002, 156 consecutive patients with a mean age of 74.5 years underwent aortic valve replacement with the Cryolife-O’Brien porcine stentless bioprosthesis. The predominant aortic valve lesion was stenosis in 128 and insufficiency in 28 cases. Patients have been followed from 2 to 72 months (mean, 42 months). Echocardiography was performed by one echocardiographer preoperatively, intraoperatively, postoperatively at discharge, 2 to 6 months later, and annually thereafter.
RESULTS: Sixty percent of patients received a valve 25 mm in diameter or larger; 39% had concomittant coronary bypass grafting. The 30-day operative mortality rate was 6.4%. Ten late nonvalve-related deaths have occurred. Severe aortic insufficiency caused by oversizing leads to early reoperation in 3 patients. The peak and mean systolic gradients decreased significantly during the first 12 months after implantation, p < 0.001, and the effective valve areas increased significantly during this time interval p < 0.001. At 5 years, ten patients have moderate aortic insufficiency. The actuarial survival at 5 years was 88 ± 3%. The rate for freedom from endocarditis was 100% and for freedom from thromboembolic events was 94%.
CONCLUSIONS: Midterm results after aortic valve replacement with the Cryolife-O‘Brien stentless bioprosthesis are encouraging. Good hemodynamics have been coupled with low rate of valve-related complications, thus representing a very good alternative to conventional stented bioprostheses.
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Introduction
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The advantages of conventional stented bioprostheses over mechanical decreased the rate of valve-related complications regarding embolism and anticoagulation. On the other side, the obstructive nature of the stent is supposed to be responsible for higher residual transvalvular gradients, which may have an important impact on the clinical outcome, remodeling of the left ventricle, valve durability, structural deterioration, and reoperation rate [1]. David and associates [1, 2] revived the concept of direct insertion of stentless porcine heterografts into the aortic root.
The objectives of a stentless bioprosthesis design were to improve hemodynamic performance and increase the durability of bioprosthetic valves. Aortic valve replacement with stentless valves provides superior hemodynamic results to stented valves because the obstruction caused by the stent and the sewing cuff is eliminated. Additionally, the aortic sinuses of the host may allow for better dissipation of the mechanical stress to which the leaflets are subjected during diastole, thus enhancing durability of the bioprosthesis [2]. New fixation and antimineralization procedures lead to further improvement of these bioprostheses. However, in some studies it has been suggested that a simplified implant model, Cryolife-O’Brien Model 300 (CryoLife International Inc, Kennesaw, GA) offers less satisfactory outcome compared with standard stentless models [3]. This work represents a detailed standardized study that prospectively evaluated the hemodynamic performance of the O’Brien bioprosthesis by serial echocardiographies by the same echocardiographer and the clinical outcome of 156 patients who underwent aortic valve replacement (AVR) in six years.
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Material and Methods
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From September 1996 through September 2002, 156 consecutive patients with a mean age of 74.5 ± 3 years underwent aortic valve replacement with the Cryolife-O’Brien Model 300 porcine stentless bioprosthesis. All these replacements were performed by the same surgeon. During the last 6 years, 1,057 aortic prostheses were implanted, except double-valve replacements. In 510 patients a mechanical valve was used, and in 547, a tissue valve. A total of 317 stentless aortic tissue valves were implanted by two surgeons. The amount of stentless valves was approximately 58% of all tissue valves. Primarily, these devices were used rarely because of the more difficult implantation technique than in stented bioprostheses. Having more experience, we started with implantation of these devices in older patients and in combined procedures. Almost 50% of implanted devices were Cryolife-O’Brien stentless bioprosthesis and another half were Medtronic Freestyle (Medtronic, Minneapolis, MN) and Toronto SPV St. Jude Medical (St. Jude Medical, St. Paul, MN) prostheses. There were no special indications for selection of each bioprosthesis except in 18 patients suffering from a disease of the ascending aorta in addition, or in redo procedure because of endocarditis or valve degeneration in which the Medtronic Freestyle prosthesis in full-root technique (n = 85) was implanted. The contraindications for the implantation of the Cryolife-O’Brien bioprosthesis were excessive calcification of the aortic root and aortic root aneurysm. Patients have been followed up from 2 to 72 months (mean, 42 months). Echocardiography was performed by the same echocardiographer preoperatively, intraoperatively, postoperatively at discharge, 3 to 6 months later, and annually thereafter. Mean values for each echocardiographic measurement were derived from 3 consecutive heart beats in patients with sinus rhythmus and from 5 beats in those in atrial fibrillation or a VVI pacemaker (Ventritex Inc, Sunnyvale, CA). The effective orifice area (EOA) of the aortic valve was calculated by the continuity equation and the mean transvalvular gradient at rest was derived from the simplified Bernoulli equation accounting for the flow velocity across the left ventricular outflow tract [4]. Echocardiographic classification of aortic insufficiency was in accordance with the criteria described by Perry and colleagues [5]. Selected preoperative patient characteristics are summarized in Table 1. The surgical procedure consists of a subcoronary supraannular technique, which has been described previously [6], and remained constant throughout the study period. Surgical intraoperative variables, including aortic cross-clamp time, cardiopulmonary bypass time, and valve sizes are listed in Table 2. Morbid and fatal valve-related events were categorized as structural valve deterioration, nonstructural valve dysfunction, thromboembolism, anticoagulant-related bleeding events, prosthetic valve endocarditis, reoperation, valve-related mortality, and all valve-related morbidity and mortality, according to the framework devised by the Society for Thoracic Surgeons and the American Association for Thoracic Surgery ad hoc committee [7].
Statistical Analysis
Data from individual patients were expressed as mean ± standard deviation, and that of important ratios as ± 70% confidence limits. Comparisons between continous variables were performed by repeated-measures analysis of variance (ANOVA) to detect any significant changes (in mean transvalvular gradient across the aortic valve prosthesis, EOA, over time) followed, if statistically significant, by a two-tailed paired Student’s t test with Bonferroni correction. The acturial estimates were used to describe the time-related event-free rates from death and other valve-related complications, and the variability of these estimates is indicated by ± standard error of the mean.
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Results
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Early Outcome, Mortality, and Late Survival Rates
Sixty percent of patients received a valve 25 mm in diameter or larger. Operative mortality rate at the time of surgery was 0.6%. Rethoracotomy for bleeding had to be performed in 3 patients. One of these patients required prolonged mechanical ventilation. Reintubation for respiratory failure had to be performed in 4 patients, all on the first operative day. New-onset atrioventricular (AV) block occurred in 6 patients; in 5 of them regular conduction was completely restored after a maximum of 7 days. The remaining 1 patient required permanent pacemaker implantation before discharge. Transient confusion was observed in 4 patients and it had resolved until discharge in all of them. None of the patients developed mediastinitis. The 30-day mortality rate was 6.4 ± 2%. In the last consecutive 128 patients the 30-day mortality rate was 3.1%. In the first 28 patients there were 6 in-hospital deaths (21.4%) with no death being directly valve related. Postmortem examination was performed in all 6 patients and no technical or valve failure was demonstrated. Three of these patients underwent concomittant coronary artery bypass grafting procedure and another 3 patients had also associated morbidity. Causes of death were respiratory failure requiring prolonged mechanical ventilation and subsequent pneumonia in 2, perioperative stroke with severe neurological deficit in 1 patient being readied for hospital discharge, and prolonged intensive care unit stay because of low output syndrome after myocardial infarction complicated by multiple organ failure in 3 patients with concomittant coronary artery bypass grafting procedure. The overall actuarial survival rates were 94 ± 2% and 87 ± 3% after 1 and 6 years, respectively (Fig 1).
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Fig 1. Overall actuarial survives estimates for all patients. Number of patients at risk for each time interval is in parentheses.
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Ten late deaths, none related to the valve have occurred (Table 3). Three of them were cardiac-related with two patients suffering a fatal myocardial infarction 1 and 4 years after AVR. The third patient had sudden death three years after AVR, which was most likely related to an arrhythmic event. Freedom from cardiac mortality, when noncardiac deaths were excluded, was 94 ± 2%, and 92 ± 2% at 1 and 5 years, respectively (Fig 2). The rate for freedom from reoperation at six years was 98% ± 1% (Fig 3). At last follow-up, 96% of patients were in New York Heart Association class I or II, with a mean of 1.09 ± 0.32 (versus 3.11 ± 0.5 preoperatively, p < 0.001).
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Fig 2. Long-term actuarial estimates of freedom from cardiac deaths. Number of patients at risk for each time interval is in parentheses.
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Fig 3. Long-term actuarial estimates of freedom from reoperation. Number of patients at risk for each time interval is in parentheses.
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Valve-Related Complications
There has been no single incidence of the structural failure in this time frame as indicated by clinical examination or echocardiographic evaluation. Severe aortic insufficiency leads to early reoperation, because of technical reasons, in 3 patients. All of the valves were well-positioned but two of them had central valvular incompetence caused by large disparity between sinotubular and annular diameters. The third patient had paravalvular incompetence and a torn suture was found intraoperatively at the site where the aortic annulus had significant calcification. This one was refixated and two others were replaced with the same model having a diameter one number smaller. The actuarial rate for freedom from endocarditis was 100% at 5 years, respectively. After implantation, permanent anticoagulation with warfarin was prescribed only if additional atrial arrhythmia was present. The actuarial rate for freedom from thromboembolic events was 96 ± 3% and 94 ± 3% at 1 and 5 years, respectively. There were 8 documented thromboembolic events. Of these, 2 occurred in the perioperative period, of which 1 was fatal after mobilization of the patient. There were 2 late strokes and 4 late transient ischemic attacks, two at 2 and 4 years and two at 4 years and 11 months, of which none was fatal. A mean age in six patients with late thromboembolic events was 80.5 years. Three of these patients developed atrial arrhythmia close to the time of the events and hemodynamic significant internal carotid stenosis was diagnosed in 4 patients. Another 7 patients developed late atrial arrhythmias, in 5 of them warfarin therapy was initiated. No patient developed a clinically important anticoagulant-related bleeding event.
Echocardiography and Hemodynamic Data
Figure 4 and Table 4 summarize the changes in mean aortic transvalvular gradient as a function of time for all patients preoperatively, at discharge, at 6 months, and annually thereafter, respectively. Table 5 summarizes the changes in EOA as a function of time for all patients preoperatively, at discharge, at 6 months, and annually thereafter. As shown there was consistent decreases in mean aortic gradient as a function of time up to 1-year follow-up. Thereafter the values appeared to plateau. There was a significant increase in EOA in the postoperative series and again at 3 to 6 months. Analysis of variance showed significantly larger EOA in larger valves, which also was consistent throughout the follow-up period.
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Fig 4. Decrease of mean systolic gradients during follow-up from preoperative value to postoperative. n = number of patients. (preop [156] = 48.8 ± 12; 6 mo [147] = 7.8 ± 2.5; 1 yr [145] = 7.2 ± 2.4; 2 yr [133] = 5.8 ± 2.2; 3 yr [116] = 4.9 ± 2.1; 4 yr [96] = 4.8 ± 2; 5 yr [76] = 5.6 ± 2.5.)
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Aortic Insufficiency and Other Signs of Valve Failure
At 1-year follow-up, 92% (133 of 145 patients) had no aortic insufficiency, 5% (7 of 145 patients) had trivial, and 3% (5 of 145 patients) had mild aortic insufficiency. At 3 years, 75% (87 of 116 patients) had no aortic regurgitation, 13% (15 of 116 patients) had trivial, and 11% (13 of 116 patients) had mild to moderate aortic regurgitation. At 5 years, 72% (55 of 76 patients) , 14% (11 of 76 patients), and 13% (10 of 76 patients) had no, trivial, and moderate aortic regurgitation, respectively. The last group contains the same patients that progressed from mild to moderate over time and remained stable. Three of these patients are from the twelve patients with the oversizing by 2 sizes and that could be a possible cause of aortic insufficiency. The aortic valve remained pliable in all patients, and the leaflets did not change their thickness or show evidence of calcification as assessed by echocardiography.
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Comment
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The aim of aortic valve replacement is the relief of symptoms associated with abnormal hemodynamics and the eventual normalization of left ventricular function. A prosthesis that closely resembles the native valve will be most effective in achieving this aim. Although homograft aortic valves may thus represent the best alternative to the native aortic valve, their availibility is limited [8]. The stentless xenograft may be a suitable alternative. Comparative cohort studies [1, 9] have shown that stentless valves have significantly lower gradients and larger effective orifice areas than conventional stented bioprostheses and are also superior to mechanical bileaflet prostheses [10].
Gross and colleagues [11] have reported, despite slightly higher transvalvular gradients, excellent midterm results achieved with stentless valves when compared with homografts. By elimination of a rigid sewing ring, the dynamic nature of the aortic root may be maintained after AVR with this device [12]. Maintaining normal aortic root function may, at least in part, be responsible for the excellent hemodynamic performances of both stentless valves as well as with aortic homografts [13]. Stentless aortic bioprostheses were reintroduced nearly 14 years ago [14, 15] in order to overcome the limited durability observed after AVR with stented bioprostheses.
Long-term follow-up data now suggest that endurance of stentless valves are comparable with that of stented bioprostheses. Because several models of stentless xenografts have been introduced gradually in clinical practice, the existence of any beneficial effect of these valves needs to be verified for every xenograft model. Recently, Desai and colleagues [16] reported about 200 patients with excellent early clinical results after AVR with Toronto SPV St. Jude Medical and a significant increase in hazard for structural valve deterioration that occurred in late follow-up. Actuarial freedom from structural valve deterioration was 98.8% at 5 years and declined to 77.9% at 10 years. Luciani and colleagues [3] have suggested unsatisfactory early outcome with the Cryolife-O’Brien stentless bioprosthesis compared with two other stentless xenografts, although midterm survival after stentless aortic valve replacement was good with all three xenografts. Regression analysis showed the O’Brien type of xenograft to be predictive of valve-related events (p = 0.02), valve deterioration (p = 0.001), and reoperation (p = 0.001) during follow-up. Early deaths were 4% (2 of 49 patients). Survival at 4 years was 85% ± 8% compared with 90% ± 3 at 4 years in the present study; freedom from valve-related events was 70% ± 8% (p = 0.004), while freedom from valve deterioration was 73% ± 8% (p = 0.001) compared with 97 ± 2 and 100% in the present study, respectively. At follow-up, reintervention on the xenograft was necessary in 6 of 49 patients (technical cause, 3; valve tear, 2; pannus, 1) compared with 3 reinterventions in our group.The Cryolife-O’Brien stentless bioprosthesis has been implanted in a large number of patients, mostly over the age of 70 years (85%) with follow-up extending more than 6 years in some of them. There are sufficient data in patients at 5-year follow-up to allow some inferences with regard to the performance of this bioprostheses for replacement of the aortic valve. In a recent article, Hvass and colleagues [17] reported about 650 patients (19% concomittant coronary and 10% mitral procedures) with actuarial survival of 76 ± 6% at 10 years. In the present study, there was a minimal operative risk of 0.6% compared with other published rates of 0.8 to 8% [18, 19]. Although most of the patients were elderly and almost half of them with concomittant coronary artery disease, acceptable 30-day mortality of 6.4% was seen to be comparable with values reported elsewhere [17, 20, 21]. There was a remarkable improvement in functional capacity after operation with 95% of patients in functional New York Heart Association (NYHA) class I or II. Eighty-five percent of the patients were in class III or IV before operation. This remarkable improvement is likely related to excellent hemodynamic performance of the bioprosthesis, probably because of its unique design. The selection of the three noncoronary leaflets removes the unfavorable muscle-based right coronary leaflet characteristics of the pig valve, such that the effective orifice area is maximized. Because there is no need for external Dacron support or any additional material of biological or synthetic type at the annulus level of the xenograft, any unnecessery increase in the thickness of the xenograft and any artificially induced stiffness or stress on the tissue can be avoided [22]. Some of the early postoperative gradients are not significantly different from those that can be achieved from comparable size stented valves.
This study shows that the low transvalvular gradients have reached their significantly decreased maximum at 3-year follow-up and were sustained at 4-year follow-up. Effective valve area has been consistently good even in small size valves. Low transvalvular gradient and large effective orifice area, even in 21-mm valves, reduce the possibility of patient to prosthesis size mismatch. Gelsomino and colleagues [23] showed that left ventricular hypertrophy resolves rapidly after the aortic valve is replaced with this bioprosthesis. The Cryolife-O’Brien stentless bioprosthesis not only has good forward flow hemodynamics but also can be inserted with a high degree of diastolic competence. The bioprosthesis was competent, or only mildly incompetent, in 98% of patients.
When considering the age (32% octogenarians at follow-up) and associated morbidity of the patients with thromboembolic events, the thromboembolic rate for the Cryolife-O’Brien stentless bioprosthesis was low. Patients were not given warfarin unless there was persistent atrial fibrillation or flutter. Early thromboembolism was noted in only 1.5% of patients. The rate for freedom from endocarditis was 100% at 6-year follow-up. The endocarditis freedom rates in some major studies utilizing current stented bioprostheses in the aortic position show the values between 94% and 97% at five years [24, 25]. This indicates that the Cryolife-O’Brien stentless bioprosthesis is quite resistant to infection, possibly due to the fact that this device has no synthetic materials other than suture, thereby reducing the possibility of foreign body reaction and infection. Technical problems in the initial phase with a central, transvalvular gradient causing unacceptable hemodynamic performance was the major cause of early reoperation requiring bioprosthesis explantation in 2 patients. Paravalvular leak not requiring bioprosthesis explantation well refixation was observed in one patient. In the present study, structural deterioration has not been observed during this time interval and it is comparable with a freedom from structural deterioration of 99.75% in patients aged greater than or equal to 65 years (mean age, 76 ± 8 years [17].
The design of the Cryolife-O’Brien stentless bioprosthesis allows the valve to be implanted in an effective supraannular position, above the annulus and sutured to the aortic wall. This allows the valve to be oversized, such that a prosthesis larger than the measured host annulus diameter can be installed, independently of the indicated size. For example, in the present study oversizing by one size (2 mm) was used in most patients and by two sizes in 12 patients.
In conclusion, our evaluation of 156 Cryolife-O’Brien bioprostheses permits us to make a statement that this device is a very good alternative to conventional stented bioprostheses. It has a good hemodynamics that may improve left ventricular function. This improvement in ventricular function may positively reflect on long-term survival. The midterm clinical and hemodynamic results with the Cryolife-O’Brien stentless bioprosthesis are encouraging but further follow-up is needed to determine the valve durability.
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References
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Abstract
Introduction
