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Ann Thorac Surg 2008;86:64-70. doi:10.1016/j.athoracsur.2008.01.085
© 2008 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Mechanical Aortic Valve Prostheses in the Small Aortic Root: Top Hat Versus Standard CarboMedics Aortic Valve

Suzanne Roedler, MD*, Martin Czerny, MD, Jan Neuhauser, MD, Daniel Zimpfer, MD, Roman Gottardi, MD, Daniela Dunkler, PhD, Ernst Wolner, MD, Michael Grimm, MD

Department of Cardiothoracic Surgery, University of Vienna Medical School, Vienna, Austria

Accepted for publication January 23, 2008.

* Address correspondence to Dr Roedler, Department of Cardiothoracic Surgery, University of Vienna Medical School, Waehringer Guertel 18-20, Vienna, A-1090, Austria (Email: suzanne.roedler{at}netway.at).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 References
 
Background: The purpose of this study was to evaluate outcome in patients with a small aortic root receiving either a standard CarboMedics mechanical aortic valve or a Top Hat CarboMedics valve (CarboMedics, Austin, TX), specifically designed for the small aortic root.

Methods: Between 1986 and 2006, 316 consecutive patients underwent 19- or 21-mm mechanical aortic valve replacement, receiving either a CarboMedics Top Hat bileaflet valve (n = 56; mean age, 66 ± 14 years) or a standard CarboMedics aortic valve replacement (n = 260; mean age, 60 ± 13 years) at our institution based on institutional indications for the choice of type of valve prostheses. Median follow-up time was 83.5 months. We studied survival, valve-related and non–valve-related events, and hemodynamic performance by serial echocardiographic follow-up studies.

Results: In-hospital mortality was 8.9% in the Top Hat group and 10.0% in the standard group (p = 0.354). Five- and ten-year survival in patients in the Top Hat group was 83% and 67%, respectively. Five- and ten-year survival in the standard group was 73% and 59%, respectively (log-rank = 0.331). There were no differences in regard to valve-related and non–valve-related events. Cox regression analysis revealed age (hazard ratio, 1.045; 95% confidence interval, 1.026 to 1.066), previous cardiac surgery (hazard ratio, 1.812; 95% confidence interval, 1.101 to 2.982), additional procedures at the time of valve replacement (hazard ratio, 2.604; 95% confidence interval, 1.651 to 4.108), New York Heart Association class IV (hazard ratio, 3.645; 95% confidence interval, 1.214 to 10.945), and severely impaired left ventricular ejection fraction (hazard ratio, 2.253; 95% confidence interval, 1.289 to 3.941) to be independent predictors of survival.

Conclusions: Mechanical aortic valve replacement in the small aortic root is associated with substantial perioperative mortality, in particular in the subset of patients requiring additional cardiac surgical procedures. Nevertheless, long-term outcome is satisfying. Because the type of prosthesis does not predict outcome in the multivariate Cox model, we conclude that use of the smaller Top Hat prosthesis can be recommended for the challenging cohort of patients with a small aortic root.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 References
 
Mechanical aortic valve replacement remains the therapy of first choice predominantly in younger patients with hemodynamically significant aortic valve disease. With increasing age, biologic aortic valve replacement outperforms mechanical aortic valve replacement with regard to the risk-benefit ratio of continuing anticoagulation [1, 2].

Patients with the intraoperative finding of a small aortic root represent a selected group of high-risk patients in the cohort of patients requiring aortic valve replacement [3, 4]. The use of small prostheses is known to be an independent risk factor for short-term and long-term outcomes [5]. In elderly patients with small aortic roots, the fashion of the biologic valve may substantially mismatch with the diameters of the native aortic valve ring as well as with the sinuses of Valsalva. Therefore, usage of a hemodynamically advantageous mechanical aortic valve may represent an attractive alternative. The CarboMedics Top Hat bileaflet valve, owing to its particular supraannular design, permits implantation of a larger valve, thereby balancing potential patient-prosthesis mismatch [6, 7].

The aim of the study was to evaluate long-term outcome in patients with a small aortic root receiving either a standard CarboMedics mechanical aortic valve or a Top Hat CarboMedics valve, specifically designed for the small aortic root.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 References
 
Study Population
Between March 1993 and January 2006, 56 patients (mean age, 66 ± 14 years) underwent mechanical aortic valve replacement with the CarboMedics Top Hat bileaflet valve (CarboMedics, Austin, TX) at our institution. These patients were compared with 260 patients (mean age, 60 ± 13 years) in whom a standard CarboMedics prosthesis was implanted. Demographic data and clinical risk factors of both groups are shown in Table 1. Median numeric EuroSCORE was 6.1 ± 3.1 in the Top Hat group and 6.3 ± 2.8 in the standard group. Median logistic EuroSCORE was 7.4 ± 3.3 in the Top Hat group and 7.5 ± 3.0 in the standard group [8, 9]. Because the Top Hat valve has been available for clinical use only since 1993, no 20-year data with this valve can be provided. The institutional review board approved the study and waived the need for patient consent.


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  		Table 1 Demographics and Clinical Risk Factors
 
Echocardiography
Standard M-mode dimensions were collected according to the European Society of Echocardiography. All Doppler measurements were obtained as the average of at least three cycles in patients with sinus rhythm or more than five cycles in those with atrial fibrillation. The peak and mean gradients across the prosthesis were calculated according to the modified Bernoulli equation. The effective orifice area was calculated by using the mean of the continuity equitation (LVOT2 x 0.785 x TVI1)/TVI2, where LVOT is the diameter of the left ventricular outflow tract and TVI1 and TVI2 are the time-velocity integrals at the left ventricular outflow tract and across the aortic valve, respectively. Body surface area was calculated according to the Dubois formula. Preoperative echocardiography was performed in all patients. Preoperative ejection fraction averaged 0.53 ± 0.11, range 0.24 to 0.68 versus 0.55 ± 0.09, range 0.26 to 0.70 (p = 0.586), with 9% of patients having a preoperative ejection fraction less than 0.30 in the Top Hat group and 0.08 in the standard group (p = 0.234). Preoperative hemodynamic data, collected at transthoracic echocardiography, as well as valve sizes implanted are reported in Table 2.


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  		Table 2 Preoperative Hemodynamic Data and Valve Sizes Implanted
 
Surgical Technique
Median sternotomy was performed in all cases. Cardiopulmonary bypass was established through cannulation of the cranial ascending aorta as well as of the right atrium by means of a two-stage venous cannula. Systemic cooling to 32°C core temperature was performed. Blood cardioplegia at 4°C with a ratio of 1:4 (Buckberg's cardioplegic solution to blood) was initiated in an antegrade fashion and repeated through the coronary sinus every 20 minutes. Prosthetic size was selected according to the size of the aortic annulus. The decision as to which prosthesis to use was left to the individual surgeon. Both prostheses were implanted using 2-0 pledgeted mattress sutures with the pledgets placed on the ventricular site of the annulus. Furthermore, if the diameter of the annulus was critical, sutures at the noncoronary sinus were placed above the native annulus and pledgets were placed outside the sinus of Valsalva to increase space.

Postoperative anticoagulation was performed according to institutional standards. After postoperative day 5, all patients received warfarin at daily updated dosages according to international normalized ratios (range, 2.5 to 3.5).

Follow-Up
Postoperative clinical and echocardiographic evaluations were performed to a strict algorithm requiring serial follow-up examinations after 6 and 12 months and annually thereafter. Clinical follow-up was complete in patients receiving the Top Hat valve and was 97% complete with the standard valve. Echocardiographic follow-up was complete in 85% of patients in the Top Hat group and in 90% of patients in the standard group (p = 0.513).

The following events were collected: in-hospital mortality, early hemodynamic characteristics, survival, and event-free survival, as well as late hemodynamic characteristics. These data were compared with the results of standard CarboMedics (19 or 21 mm) aortic valve (CarboMedics) replacement. In- hospital and late deaths as well as valve-related events were strictly defined according to the published guidelines of The Society of Thoracic Surgeons. Furthermore, a Cox regression analysis was performed to identify independent predictors of survival.

Statistical Analysis
The SPSS statistical software (version 11, SPSS Inc, Chicago, IL) was used for statistical analysis. Continuous variables are expressed as mean ± standard deviation; categorical variables are expressed as counts and percentages. Paired Student's t test or the paired Wilcoxon rank-sum test in case of asymmetric distributed data was used to compare preoperative echocardiographic data with data collected at follow-up. The Kaplan–Meier statistics were used to calculate survival rates and freedom from valve-related complications, and the log-rank test was used to determine intergroup differences. Finally, we used univariate and multivariate Cox regression analysis to identify independent predictors of survival and functional status.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 References
 
Clinical Results
Fifty percent of patients of the Top Hat group underwent combined procedures, including coronary artery bypass grafting in 18 patients, mitral valve replacement in 13 patients, mitral valve reconstruction in 4 patients, tricuspid valve replacement in 1 patient, and tricuspid valve reconstruction in 2 patients. Forty-one percent of patients of the standard group underwent combined procedures, including coronary artery bypass grafting in 55 patients, mitral valve replacement in 48 patients, and mitral valve reconstructions in 3 patients, as well as 5 tricuspid valve reconstructions.

In-hospital mortality was 8.9% in the Top Hat group and 10.0% in the standard group (p = 0.354). In the Top Hat group 2 patients died as a result of multiorgan failure, 1 patient as a result of functional obstruction of the left main ostium, and 2 patients as a result of low cardiac output failure. In the standard group, 7 patients died as a result of multiorgan failure, 7 patients as a result of low output cardiac failure, 4 patients as a result of malignant arrhythmia, 3 patients as a result of sudden cardiac death, and 5 patients as a result of non–cardiac-related reasons.

Survival
Five-year survival in the Top Hat group was 83% and 67% after 10 years. Five-year survival was 73% in the standard group and 59% after 10 years (log-rank = 0.331; Fig 1).


Figure 1
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  		Fig 1. Freedom from mortality and freedom from mortality or morbidity according to groups. (CM = standard CarboMedics; SAV = supraannular valve [Top Hat].)

 
Valve-Related Events
Freedom from thromboembolic events was 97% ± 2% in the Top Hat group at 5 and 10 years. Freedom from thromboembolic events was 96% ± 3% in the standard group at 5 years and 92% ± 7% at 10 years (log-rank = 0.148). Freedom from anticoagulation-related hemorrhage was 89% ± 11% in the Top Hat group at 5 and 10 years. Freedom from anticoagulation-related hemorrhage was 92% ± 8% in the standard group at 5 years and 84% ± 15% at 10 years (log-rank = 0.576). Freedom from prosthetic valve endocarditis was 98% ± 2% in the Top Hat group at 5 and 10 years. Freedom from prosthetic valve endocarditis was 100% in the standard group at 5 and 10 years (log-rank = 0.157). Freedom from reoperation was 98% ± 1.8% in the Top Hat group at 5 and 10 years (log-rank = 0.354). Freedom from reoperation was 99% ± 3.6% in the standard group at 5 and 10 years. Freedom from paravalvular leakage in the Top Hat group was 100% after 5 years and 93% ± 5% after 10 years, and 98% ± 1% and 95% ± 5%, respectively, in the standard group (log-rank = 0.611; Figs 2–5). Go Go Go Indications for reoperation were paravalvular leakage in 1 patient in the Top Hat group and in 3 patients in the standard group. One patient in the standard group had a sinus of Valsalva aneurysm, which was the indication for reoperation.


Figure 2
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  		Fig 2. Freedom from anticoagulation-related hemorrhage according to groups. (CM = standard CarboMedics; Pts = patients; SAV = supraannular valve [Top Hat].)

 

Figure 3
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  		Fig 3. Freedom from paravalvular leaks according to groups. (CM = standard CarboMedics; Pts = patients; SAV = supraannular valve [Top Hat].)

 

Figure 4
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  		Fig 4. Freedom from endocarditis according to groups. (CM = standard CarboMedics; Pts = patients; SAV = supraannular valve [Top Hat].)

 

Figure 5
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  		Fig 5. Freedom thromboembolic events according to groups. (CM = standard CarboMedics; Pts = patients; SAV = supraannular valve [Top Hat].)

 
Early and Late Hemodynamic Characteristics
Early and late hemodynamic characteristics are depicted in Table 3 at discharge, at 5 years, and at 10 years after surgery.


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  		Table 3 Early and Late Hemodynamic Characteristics
 
Cox Regression Analysis
Cox regression analysis revealed age (hazard ratio, 1.045; 95% confidence interval, 1.026 to 1.066), previous cardiac surgery (hazard ratio, 1.812; 95% confidence interval, 1.101 to 2.982), additional procedures at the time of valve replacement (hazard ratio, 2.604; 95% confidence interval, 1.651 to 4.108), New York Heart Association stage IV (hazard ratio, 3.645; 95% confidence interval, 1.214 to 10.945), and severely impaired left ventricular ejection fraction (hazard ratio, 2.253; 95% confidence interval, 1.289 to 3.941) to be independent predictors of survival (Table 4).


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  		Table 4 Cox Proportional Hazards Model—Survival
 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
References
 
Mechanical aortic valve replacement in the small aortic root is associated with substantial perioperative mortality, in particular in the subset of patients requiring additional cardiac surgical procedures. Nevertheless, long-term outcome is satisfying. Because the type of prosthesis does not predict outcome in the multivariate Cox model, we conclude that the use of the smaller Top Hat prosthesis can be recommended for the challenging cohort of patients with a small aortic root.

Concomitant operations such as coronary artery bypass grafting or double-valve replacement in this series were high, thereby underlining the substantial rate of comorbidities in this patient cohort. Furthermore, 20% of patients had already undergone previous cardiac surgery. Our rate of mortality in both groups is higher than in other series [10–12]. However, the factors previously mentioned explain these results. Furthermore, median numeric as well as logistic EuroSCORE numbers indicated elevated risk [8, 9].

Five- and ten-year survival rates were comparable with those of other series in both groups, indicating a regular long-term course in these patients after successful repair of their limiting initial disease [10–12]. Five- and ten-year survival rates in patients after Top Hat aortic valve replacement were marginally higher as compared with standard CarboMedics aortic valve replacement. We do think that the rather low number of patients may be responsible for this finding.

Freedom from thromboembolic events was low in both groups throughout the entire study period, thereby indicating the favorable hemodynamics of the concept of both valves themselves. As the modification of the Top Hat valve involves only the sewing cuff, no substantial differences should be expected. Furthermore, freedom from anticoagulation-related hemorrhage was acceptably low in this series. This fact can be attributed to the meticulously conducted anticoagulation regimen during follow-up. Prosthetic valve endocarditis was rarely observed, which may be attributed to aggressive decalcification of the aortic annulus during surgery, thereby avoiding remaining calcified tissue within the aortic root. Furthermore, all patients are instructed to take a liberal antibiotic prophylaxis when undergoing any kind of invasive treatment. In our opinion, the aggressive decalcification of the annulus during surgery has also contributed to our very low rate of paravalvular leakage during the study period in both groups. Finally, the rate of reoperations was low in both groups, and indications for reoperations were not related to any kind of valve failure but attributable to the progression of other valve or coronary artery diseases.

Echocardiographic follow-up was performed at discharge, at 6 and 12 months, and annually thereafter. Our echocardiographic results do not differ substantially from previously published series [13, 14]. As the basic principle of the design of the valve has not been modified and the difference between both valve types merely consists in the alteration of the sewing cuff, these findings are reasonable. However, after 10 years the mean gradients tended to rise again. Evidence exists that resting gradients are directly correlated to body surface area [15]. Therefore, as these patients get older and are thereby more limited in their physical abilities, their body weight and, consequently, their body surface area may increase and explain our findings.

Cox regression analysis revealed age, previous cardiac surgery, additional procedures at the time of valve replacement, and New York Heart Association stage IV as well as severely impaired left ventricular ejection fraction as independent predictors of survival. Besides left ventricular ejection fraction, age is known to be a very strong independent predictor of survival irrespective of the type of operation performed [16, 17]. Most likely this can be attributed to the fact that the elderly are generally more frail and have a diminished physiologic reserve when compared with their younger counterparts. Contradictory results exist with regard to the impact of previous cardiac surgery on survival [18–20]. In our series, previous cardiac surgery turned out to be an independent predictor of survival. This finding can be explained by the fact that the majority of these patients had primary cardiac surgery related to ischemic heart disease and already had an impaired left ventricular function at the time of redo surgery.

Furthermore, additional procedures at the time of aortic valve replacement independently predicted survival. At first glance this seems somewhat confusing, as an elective combined procedure does not implicate substantial additional risk. When analyzing this particular patient cohort, we noticed that the majority of these patients had been scheduled for isolated aortic valve replacement. However, because of intraoperative complications such as right heart failure as a result of functional obstruction of the right ostium or new onset of severe mitral valve insufficiency as a result of alteration of mitral valve geometry during aortic valve replacement, these procedures became necessary during the operation itself. In this context our results seem reasonable.

In the patient who had functional obstruction of the left coronary ostium, the following mechanism was hypothesized. This patient had a severely calcified sinus of Valsalva as well as a low offspring of the left ostium. Through the high profile of the Top Hat valve together with the noncompliant sinus of Valsalva, flow disturbance depending on different loading conditions may have occurred, leading to this fatal event. With regard to the patients requiring additional coronary artery bypass grafting to the right coronary artery, we hypothesize a similar mechanism. Furthermore, if repeated defibrillation after release of the aortic cross-clamp is required, normalization of the electrocardiogram is prolonged, or clinical or echocardiographic right ventricular dysfunction is anticipated, we are very liberal in revascularizing the right coronary artery to avoid any upcoming perfusion problem.

Presumably, the Top Hat group would have performed substantially worse if these patients had received the smaller standard valve, as patient-prosthesis mismatch would have become a concerning issue [21]. Furthermore, at the beginning of the introduction of the valve, concerns were raised with regard to the high sewing cuff and the potential threat to the coronary ostia. Despite the fact that we have observed such a dramatic event in 1 patient with obstruction of the left ostium, this dreadful complication is rare and the incidence presumably not higher than in any other mechanical valve type. Furthermore, hemodynamics are favorable as the direct comparison of our echocardiographic data between the two valve types has shown. Finally, patients with substantial narrowing of the left ventricular outflow tract mimicking or representing a hypertrophic obstructive cardiomyopathy are not included in this series as it represents a completely different pathologic entity. No patient in this series underwent myocardiectomy of a hypertrophic septum in this series. Thereby, this potential bias was avoided.

Finally, New York Heart Association stage IV as well as severely impaired left ventricular ejection fraction turned out to be independent predictors of survival. Without doubt, the majority of patients in New York Heart Association stage IV have a severely impaired left ventricular ejection fraction. Furthermore, additional left ventricular remodeling in this particular subset of patients is limited, and many of these patients die because of sudden cardiac-related death during follow-up [22].

This study implicates all relevant main limitations of a retrospective, nonrandomized analysis. The primary limitation is that the compared types of valves are not randomly assigned. However, this is based on the clinical situation that the Top Hat is the valve of last choice when no other prosthesis fits in. Furthermore, it remains speculative how patients receiving a Top Hat valve would perform with a standard prosthesis, presumably very poorly. It should be noted that in 80% of patients with a regular sizing of 19 mm, implantation of a 21-mm Top Hat was feasible. Nevertheless, all relevant factors, including preoperative, postoperative, and follow-up data, were collected prospectively according to highly standardized methods. Additionally, this study represents a single-center experience. Therefore, further analyses are warranted to generalize these results.

Furthermore, the implantation rate of the Top Hat valve is low. This can be attributed to several factors. First, several mechanical valve types are used in our institution, and the Top Hat is in fact a valve specially provided for the very patients with a small aortic root. Furthermore, root enlargement techniques, having not been widely available in previous years, are now being applied, thereby reducing the number of patients for this specific indication. Finally, the Sorin Mitroflow valve has not been used in our setting until recently, turning out to be the ideal valve in elderly patients with a small aortic root, thereby neutralizing the necessity of a mechanical valve in the aortic position in these patients.

In summary, mechanical aortic valve replacement in the small aortic root is associated with substantial perioperative mortality, in particular in the subset of patients requiring additional cardiac surgical procedures. Nevertheless, long-term outcome is satisfying. Because the type of prosthesis does not predict outcome in the multivariate Cox model, we conclude that the use of the smaller Top Hat prosthesis can be recommended for the challenging cohort of patients with a small aortic root.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 References
 

  1. Milano A, Guglieimi C, De Carlo M, et al. Valve related complications in elderly patients with biological and mechanical aortic valves Ann Thorac Surg 1998;66(Suppl):S82-S87.[Medline]
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  8. Nashef SA, Roques F, Michel P, Gauducheau E, Lemeshow S, Salomon R. European system for cardiac operative risk evaluation (EuroSCORE) Eur J Cardiothorac Surg 1999;16:9-13.[Abstract/Free Full Text]
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M. Czerny and S. Roedler
Reply.
Ann. Thorac. Surg., February 1, 2009; 87(2): 677 - 677.
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 Ann. Thorac. Surg.Home page
H. Elsayed
Predicting complications of top hat prosthesis in aortic valve replacements: suspicion can save lives.
Ann. Thorac. Surg., February 1, 2009; 87(2): 676 - 677.
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