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Ann Thorac Surg 2006;82:1379-1384
© 2006 The Society of Thoracic Surgeons

Original Articles: Cardiovascular

Root Replacement Using Stentless Valves in the Small Aortic Root: A Propensity Score Analysis

Takashi Kunihara, MD, PhD, Kathrin Schmidt, MD, Petra Glombitza, MD, Vachtang Dzindzibadze, MD, Henning Lausberg, MD, Hans-Joachim Schäfers, MD, PhD*

Department of Thoracic and Cardiovascular Surgery, University Hospital Homburg, Homburg, Germany

Accepted for publication May 5, 2006.

* Address correspondence to Dr Schäfers, Department of Thoracic and Cardiovascular Surgery, University Hospital Homburg, Kirrberger Strasse 1, Homburg/Saar, D-66421, Germany (Email: chhjsc{at}uniklinik-saarland.de).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 References
 
BACKGROUND: Root replacement using a stentless bioprosthesis may be the optimal approach to avoid patient-prosthesis mismatch in patients with a small aortic root. Primary root replacement, however, is considered to be associated with increased surgical risk. We compared early outcome of full root replacement with a stentless bioprosthesis with that of aortic valve replacement with a stented bioprosthesis using propensity score-matching analysis.

METHODS: Of 231 patients undergoing elective, first-time aortic valve replacement with a small root (≤22 mm), 120 patients were selected using propensity score-matching analysis. They underwent either root replacement using a 23-mm stentless bioprosthesis (stentless group, n = 60) or supra-annular aortic valve replacement using a 21-mm stented bioprosthesis (stented group, n = 60). Preoperative characteristics and frequency of concomitant operations were identical.

RESULTS: Duration of operation (196 ± 54 versus 174 ± 49 minutes), cardiopulmonary bypass (112 ± 36 versus 91 ± 33 minutes), and aortic cross-clamping (76 ± 21 versus 61 ± 21 minutes) were significantly longer in the stentless group. However, the need for perioperative transfusion and the incidence of postoperative reexploration for bleeding (3% versus 8%) was lower, and ventilation time was shorter. Mean duration of intensive care and hospital stay were also significantly shorter (2.3 ± 1.7 versus 4.0 ± 3.9 days, 8.9 ± 3.1 versus 12.4 ± 5.7 days). In-hospital mortality was identical (5% each). No independent predictor for in-hospital mortality was identified.

CONCLUSIONS: Full root replacement using a stentless bioprosthesis does not increase postoperative morbidity or mortality of aortic valve replacement and may be advantageous in patients with a small aortic root.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 References
 
In the recent decade, the stentless bioprosthesis has become an increasingly accepted alternative in aortic valve replacement (AVR) [1, 2]. It is definitely a good alternative to other options for acute infective endocarditis with aortic annular abscess and acute type A dissection involving the aortic root.

There is some evidence that a stentless bioprosthesis provides more physiologic hemodynamics compared with stented valve substitutes [3–6]. That may be of particular importance for patients with a small aortic root, in whom implantation of a stented valve substitute may result in patient-prosthesis mismatch (PPM) with its associated adverse outcome [7–9]. Subcoronary implantation of stentless valves is still preferred to avoid or minimize the risk of bleeding [10, 11]. This mode of implantation, however, has been associated with residual transvalvular gradients, effective orifice areas not superior to those of stented bioprostheses [12], and late aortic regurgitation [10, 11, 13]. Implantation by full root replacement, on the other hand, allows for insertion of almost any valve size and provides almost normal hemodynamic function both at rest and during exercise, and without early or midterm aortic regurgitation [11, 14]. This type of implantation has not been generally accepted, most likely owing to concerns over increased perioperative morbidity and mortality. We have routinely implanted stentless valves using full root replacement to prevent PPM and distortion of the prosthesis. We have reported good outcome for different types of aortic valve pathology and excellent hemodynamic function [15, 16]. It is unclear, however, whether this approach is comparable to conventional AVR in patients with a small aortic root.

In this report, we analyze the clinical outcome of root replacement using a stentless bioprosthesis with that of AVR using a stented bioprosthesis in patients with small aortic root. To minimize selection bias, we used a propensity score-matching analysis [17].


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 References
 
From February 1996 to June 2004, 231 patients with aortic valve stenosis and an aortic ring size of 22 mm or less underwent elective AVR in our institution. This investigation, including follow-up studies, was accepted by the local Ethics Committee. In all patients, informed consent was obtained for the anonymous use of their data, including follow-up information. Replacement was performed using either full root technique with a stentless porcine bioprosthesis sized 23 mm (n = 84) or a stented bioprosthesis sized 21 mm in the supra-annular position (n = 147). There were 35 male and 196 female patients, and their age ranged from 46 to 87 years (mean, 75 ± 6). Stentless bioprostheses used were Freestyle (Medtronic, Minneapolis, Minnesota) in 59 cases and Prima plus (Edwards Lifesciences, Irvine, California) in 25 cases. Stented bioprostheses used were Perimount (Edwards Lifesciences; n = 130) and Mosaic (Medtronic; n = 17). Patients with acute infective endocarditis or who had had previous AVR were excluded. Concomitant cardiac/aortic surgery was performed in 140 patients (61%).

Variables analyzed were sex, age, height, weight, body surface area, aortic valve stenosis, pressure gradient, aortic valve insufficiency and its grade, smoking history, hypertension, hyperlipidemia, diabetes mellitus, peripheral arterial disease, carotid artery stenosis, cerebrovascular disease, chronic obstructive pulmonary disease, chronic renal failure, sinus rhythm, pacemaker, left ventricular (LV) end-diastolic pressure, and LV ejection fraction as preoperative variables. Concomitant procedure, intraoperative transfusion, intraoperative use of intra-aortic balloon pumping, duration of operation, cardiopulmonary bypass, aortic cross-clamping, and hypothermic circulatory arrest were analyzed as intraoperative variables. Postoperative infection, arrhythmia, need for electronical cardioversion, sinus rhythm at discharge, new pacemaker implantation, low output syndrome, pleural or pericardial effusion (required drainage), duration of mechanical ventilation, stroke, confusion, reexploration, postoperative drain discharge, postoperative or perioperative transfusion, intensive care unit stay, postoperative hospital stay, and in-hospital death were evaluated as postoperative endpoints.

Propensity Score-Matching Analysis
To minimize patient selection bias in this retrospective nonrandomized study, we used a propensity score-matching analysis to evaluate the pure effect of operative procedure on postoperative morbidity and mortality. Using multivariate logistic regression analysis, the probability of being assigned to a stentless bioprosthesis over a stented one was calculated from baseline characteristics. A p value less than 0.20 was defined for selecting variables for entry into the final model. Selected variables were peak-to-peak pressure gradient over the aortic valve, mitral valve insufficiency, history of smoking, syncope, hyperlipidemia, peripheral arterial disease, chronic obstructive pulmonary disease, renal insufficiency, sinus rhythm, and implantation of pacemaker. Using these covariables, a propensity score was calculated for each patient. Finally, each patient who underwent full root replacement using a stentless bioprosthesis was matched to 1 patient who underwent AVR using stented prosthesis with the closest propensity score. The maximum difference of propensity score for a matching was less than 0.05. Using this technique, comparable patient cohorts (60 patients from each groups) were identified for the final analysis (stentless group and stented group). Baseline characteristics of propensity-matched pairs were almost identical, including body size, hemodynamics, cardiac function, and cardiac risk factors (Table 1). In this subgroup, stentless bioprostheses used were Freestyle (Medtronic) in 41 and Prima plus (Edwards Lifesciences) in 19 cases. Stented bioprostheses used were Perimount (Edwards Lifesciences) in 56 cases and Mosaic (Medtronic) in 4 cases.


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  		Table 1. Patient Characteristics
 
Anesthesia and Cardiopulmonary Bypass
All patients were treated by a standardized intravenous anesthesia regimen using propofol and sufentanyl. We used a nonheparin-coated circuit (Raumedic; Rehau AG & Co, Rehau, Germany), a roller pump (Stöckert Instrumente GmbH, München, Germany), a membrane oxygenator (Quadrox; Jostra Medizintechnik GmbH & Co KG, Hirrlingen, Germany), and an arterial filter (Quart, Jostra Medizintechnik GmbH & Co KG) for cardiopulmonary bypass. The cardiopulmonary bypass system was primed with lactated Ringer's solution mainly. Alpha-stat strategy was applied during cardiopulmonary bypass.

Operative Technique
All operations were performed through a full median sternotomy. Cardiopulmonary bypass was established using ascending aortic and right atrial cannulation. During core cooling, the ascending aorta was cross-clamped, and cold blood cardioplegia was given directly into the coronary ostia. The Hegar sizers were used to determine the diameter of the aortic ring in all patients. For implantation of stented valves, the commercial sizers were then utilized for final choice of the prosthesis. For stentless valves, a 23-mm size was chosen irrespective of the exact diameter of the root. In patients without aortic arch involvement, the operations were performed with a core temperature of 32°C to 33°C. In patients who underwent concomitant coronary artery bypass graft surgery or mitral valve repair/replacement, distal anastomoses of coronary artery bypass grafting or the mitral procedure were performed before AVR. For arch involvement, AVR was performed during core cooling. When core temperature reached to 18°C to 20°C, aortic arch replacement was performed under hypothermic circulatory arrest. The remaining procedures were performed during rewarming.

We attempted to use a 21-mm stented bioprosthesis or larger in aortic position. When a 21-mm stented bioprosthesis could not be implanted or the body surface area of the patient exceeded 1.8 m2, we implanted a 23-mm stentless bioprosthesis using full root replacement unless there severe calcifications of the aortic root close to the coronary ostia. For root replacement, the stentless bioprosthesis was connected to the left ventricular outflow tract using a running 4-0 polypropylene suture with a pericardial strip for reinforcement. The coronary ostia were reattached directly to the prosthesis using a running 5-0 polypropylene suture in all patients. For AVR with a stented bioprosthesis, the prosthetic valve was implanted in supra-annular position using interrupted mattress sutures with pledgeted 2-0 Ethibond. No aortic root enlargement procedures were performed.

Hemodynamic Performance
Twenty patients from each of the two groups were randomly selected, and the hemodynamic performance of aortic prosthesis was measured using transthoracic echocardiography (HDI 3000; ATL Technologies, Hagen, Germany) late after implantation. For these measurements, the mean interval between operation and echocardiography was 27 ± 25 months. There was no difference in patient characteristics between the two groups.

Statistical Analysis
All values are expressed as mean ± SD. Statistical analysis was performed using StatView 5.0 (SAS Institute, Cary, North Carolina). Student's t test was used for comparison of the continuous variables and the {chi}2 test was used for comparison of frequencies between the groups. A p value of less than 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 References
 
Concomitant procedures were performed equally in both groups. The need for intraoperative transfusion was significantly more frequent in the stentless group (30%) than in the stented group (8%; p = 0.0054). Fresh frozen plasma was given to 8 patients in the stentless group (1.9 ± 0.6 units), and to no patient in the stented group. The duration of operation, cardiopulmonary bypass, and aortic cross-clamping was significantly longer in the stentless group than in the stented group (196 ± 54 minutes versus 174 ± 49 minutes, p = 0.0231, 112 ± 36 minutes versus 91 ± 33 minutes, p = 0.0013, and 76 ± 21 minutes versus 61 ± 21 minutes, p = 0.0001, respectively; Table 2).


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  		Table 2. Operative Characteristics
 
Postoperative mediastinal drain discharge was less in stentless group (442 ± 303 mL versus 635 ± 709 mL, p = 0.0575), although this difference was not significant. Postoperatively, significantly more red blood cell transfusions were given in the stented group (1.7 ± 1.0 units versus 3.7 ± 2.9 units; p = 0.0090). The combined number of intraoperative and postoperative transfusions was significantly lower in the stentless group (2.1 ± 1.4 units versus 3.6 ± 2.6 units, p = 0.0159). The incidence of reexploration for bleeding was comparable between the groups (3% versus 8%; p = 0.4360; Table 3).


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  		Table 3. Postoperative Variables
 
There was a trend toward shorter duration of postoperative mechanical ventilation in the stentless group (0.8 ± 1.2 days versus 1.5 ± 2.4 days; p = 0.0598). The duration of intensive care unit stay (2.3 ± 1.7 days versus 4.0 ± 3.9 days, p = 0.0029) and postoperative in-hospital stay (8.9 ± 3.1 days versus 12.4 ± 5.7 days, p < 0.0001) were significantly longer in the stented group. The incidence of postoperative stroke was identical between the groups (7% versus 5%, p > 0.9999). Two patients in the stented group (3%) required pacemaker implantation versus none in the stentless group. There was no significant difference in incidence of low output syndrome (10% versus 5%, p = 0.4882), and in-hospital mortality was comparable in both groups (5% each; Table 3). Univariate logistic regression analysis identified aortic valve insufficiency grade, new pacemaker implantation, duration of mechanical ventilation, and postoperative low output syndrome as independent predictors for in-hospital mortality. However, by multivariate logistic regression analysis, none of them was identified as an independent predictor for in-hospital mortality (Table 4).


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  		Table 4. Univariate and Multivariate Logistic Regression Analyses of Independent Predictors for In-Hospital Mortality
 
Echocardiography late after the operation revealed that peak and mean pressure gradients over the aortic prosthesis were significantly lower in the stentless group (11.2 ± 1.6 mm Hg, 6.9 ± 1.6 mm Hg) than in the stented group (17.8 ± 5.0 mm Hg, 10.8 ± 3.4 mm Hg; p = 0.0095, p = 0.0245, respectively; Fig 1). This difference did not have significant impact on the physiologic status of patients. Clinically and echocardiographically, no patient had evidence of moderate or severe PPM.


Figure 1
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  		Fig 1. Transvalvular pressure gradient of two prostheses measured by transthoracic echocardiography in late follow-up. (Black bars = stentless valve; white bars = stented valve.)

 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
References
 
Aortic valve replacement is the only effective therapy for patients with critical aortic valve stenosis. The goal of surgery for these patients is to restore aortic valve function to a degree that results in complete regression of LV hypertrophy and LV mass to normal levels. That leads to improvement of LV function and markedly improved long-term survival [3–6]. Valve substitutes, however, leave transvalvular gradients that are almost always higher than the physiologic systolic gradients of the aortic valve [3, 4, 16]. These gradients have been considered to depend on valve size, but the relationship between valve size and size or body surface area of the patient is the really important factor. This relationship has been called patient-prosthesis match. A non-ideal match—functionally nothing else but an aortic stenosis—has been defined as patient-prosthesis mismatch (PPM) [7–9] and has been quantified by indexed effective orifice area of the prosthesis. Severe mismatch and, to a lesser extent, moderate mismatch have been found to be associated with increased early and possibly late mortality [9, 18, 19]. Blackstone and coauthors [20] questioned the effect of PPM in a thorough retrospective analysis of a large patient population. Despite severe PPM being present in a very small proportion of the patients, they showed an increase in operative mortality. An effect on long-term survival could not be shown.

It is thus agreed that severe PPM increases early mortality, whereas effect on late results is less clear. Clinically, severe PPM appears to be associated with a higher incidence of late symptoms of heart failure and less regression of LV hypertrophy as determined by echocardiography [6, 7]. Patients with a small aortic root are known to be particularly prone to the occurrence of PPM. Thus, the prosthesis implanted must be carefully selected to avoid PPM. In view of this mixed evidence, however, only valves or implant procedures that do not increase the perioperative risk of AVR seem acceptable.

Current mechanical valves have provided low systolic gradients and acceptable orifice areas in patients with a small aortic root, but require anticoagulation with the known risk of hemorrhagic complications in the older patients [21]. In patients such as the cohort we studied with a mean age of 75 years, mechanical valves do not seem to be a reasonable option.

Stented bioprostheses are generally accepted for the elderly patient [8] because of their ease of implantation and reproducible performance without need of anticoagulation. Nevertheless, they seem to be prone to the development of PPM in some patients with a small aortic root and larger body surface area. It has been reported that 19-mm valves are associated with an increased risk of operative mortality [22], and—while we realize that this does not take all evidence on PPM into consideration—we have tried to abandon the use of 19-mm valves for the average-sized adult.

One possibility to avoid PPM in patients with a small aortic root is an aortic ring enlargement procedure [23, 24], which allows implantation of a larger valve. This procedure has been found to be associated with prolonged myocardial ischemia and perioperative bleeding problems [25]. It may also be technically difficult when the aortic root is fragile, which is frequently seen in the elderly patients. It is thus unclear whether ring enlargement results in adequate orifice areas without increasing operative risk.

Stentless valves theoretically seem to avoid the problem of PPM completely owing to their superior hemodynamic performance [26]. More complete resolution of LV hypertrophy and LV mass has been found [3–6], thus possibly improved long-term prognosis [27, 28]. Others have observed that transvalvular pressure gradients after subcoronary implantation of stentless bioprostheses were not significantly lower than those seen in stented valve substitutes [12]. Implantation of stentless valves by root replacement, however, resulted in almost physiologic systolic gradients at rest and under conditions of exercise [11, 14, 16]. Therefore, stentless valves, if implanted as full root replacement, can indeed lead to physiologic function of the prosthesis and thus avoid the problem of PPM altogether.

As a clear advantage of full root technique is that a valve of almost any size can be implanted, we have routinely used root replacement for implantation of stentless bioprostheses and found acceptable surgical outcome and superior hemodynamics [15, 16]. It has been also reported that full root implantation of stentless bioprosthesis provides better hemodynamics, functional class, and freedom from aortic insufficiency [10, 11, 13]. In addition, if long-term durability of the valve is affected by coaptation of the valve leaflets, a full root implantation may be advantageous to enhance durability of the valve [2, 13]. Of course, many more years of detailed follow-up will be necessary to confirm this issue. Despite these apparent advantages of root replacement, concerns over increased early morbidity and mortality have kept many surgeons from using this approach.

As expected, we found myocardial ischemia prolonged with implantation of stentless valves. Nevertheless, this difference averaged only 15 minutes using a single continuous suture to the left ventricular outflow tract, and ischemia was more prolonged through the need of concomitant procedures. This additional duration of myocardial ischemia has been shown to have little effect on postoperative outcome [1, 29]. Neither the incidence of reexploration for bleeding nor the amount of postoperative drain discharge was increased in the stentless group. The increased number of intraoperative transfusions in the stentless group was more than outweighed by significantly fewer postoperative transfusions and most likely reflects a treatment effect. Interestingly, the total number of perioperative transfusions was significantly lower in the patients treated by stentless valves. Not only morbidity, but also mortality was not negatively influenced by valve type, both in the entire study sample (9.5% in the stentless group, 11.6% in the stented group, p = 0.7940) and in the propensity score-matched groups. Interestingly, postoperative recovery was significantly faster after implantation of stentless valves. Since this valve has been part of clinical routine for more than 10 years and this investigation is a purely retrospective, treatment bias regarding intensive care and hospital stay appear unlikely.

Based on our results, we cannot draw any conclusions regarding long-term data, such as durability of a stentless bioprosthesis. At present, only results up to 10 years have been reported, which have suggested good clinical performance and durability of a stentless valve even compared with the homograft [11, 13, 30]. As yet, few long-term data are available on stentless valves implanted by full root replacement. While we could demonstrate superior hemodynamic performance of stentless valves over stented valves in the studied patients, this population in insufficient to provide any information on long-term benefit of stentless valves. That, however, requires a much larger patient cohort that needs to be stratified or corrected for the many variables that have been shown to influence long-term survival and clinical status [28]. Our main objective was to clarify whether full root implantation of a stentless bioprosthesis was associated with increased risk of periperative morbidity or mortality. We found no increase in morbidity or mortality, which is in contrast to previous reports [10]. Since patient-related risk factors seem to be predominant in determining the risk of death [10], selection bias may have had an influence that could have been minimized or eliminated by the use of propensity score-matching. However, our results warrant confirmation by larger prospective randomized clinical trials with stented or stentless bioprostheses.

In conclusion, for patients with a small aortic root, full root replacement using a stentless bioprosthesis requires a longer duration of the procedure without increasing the risk of postoperative complications. A full root replacement using a stentless bioprosthesis appears a safe, useful, and promising alternative to AVR using stented valve substitutes for patients with a small aortic root.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 

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