Asian Cardiovasc Thorac Ann 2003;11:37-41
© 2003 Asia Publishing EXchange Ltd
Risk in Elderly Patients After Stentless Versus Stented Aortic Valve Surgery
Jürgen Ennker, MD,
Ulrich Rosendahl, MD,
Ina Carolin Ennker, MD,
Stefan Bauer, MD,
Ines Florath, PhD
Heart Institute Lahr/Baden, Lahr, Germany
For reprint information contact: Jürgen Ennker, MD Tel: 49 7821 92 5101 Fax: 49 7821 925110 email: ennker{at}heart-lahr.com Heart Institute Lahr/Baden, Lahr D-77933, Germany.
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ABSTRACT
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Recent studies suggest that the hemodynamic advantage of stentless bioprostheses over the stented type improves long-term survival after aortic valve replacement, but the more complex and time-consuming implantation technique may increase the risks of operative death and postoperative complications. Between April 1996 and June 2001, 519 patients with a mean age of 76 ± 5 years underwent aortic valve replacement using a stentless (Medtronic Freestyle, n = 277) or stented bioprosthesis (Medtronic Mosaic, n = 242). Multiple logistic regression analysis considering different patient populations revealed no increased risk of operative death, postoperative complications, or neurological impairment after implantation of a stentless bioprosthesis. Survival curves in respect of 367 patients who underwent aortic valve replacement up to September 2000 and were followed up for 3 years were not different (p = 0.98). As the patients were elderly, improved survival due to implantation of a stentless valve could not be demonstrated within this time span.
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INTRODUCTION
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The increase in age of the population in western developed countries and progress in the diagnosis of aortic valve disease by modern noninvasive techniques have resulted in a higher rate of aortic valve replacement (AVR) in the elderly. Tissue valves are especially appropriate in elderly patients because of the advantages of avoiding anticoagulation and the risk of thromboembolic complications, and the reduced incidence of structural failure in this age group.1 The durability of stented bioprostheses is limited by the valve design; continuous stress on valve cusps suspended from a rigid frame results in structural failure and the need for reoperation. In contrast, stentless valves lack the structural problem of the stented type and should have a longer lifespan. In addition, stentless valves show a more physiological flow pattern and a larger inner diameter because of the absence of stents, resulting in a greater effective orifice area. Consequent benefits such as lower residual mean transvalvular pressure gradients after AVR and faster resolution of hypertrophy have been reported.2–5 Since left ventricular hypertrophy causes cardiac arrhythmias and increases the risk of death, a positive impact on clinical outcome and quality of life would be expected after AVR with a stentless bioprosthesis.6 A significant survival benefit for patients with stentless rather than stented valves has been demonstrated.7–9 A disadvantage of stentless bioprostheses is the more demanding implantation technique that may influence the duration of operation and, therefore, the operative risk and likelihood of postoperative complications. To evaluate these risks, we used logistic regression modeling.
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PATIENTS AND METHODS
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Between April 1996 and June 2001, 277 patients underwent AVR with the stentless Freestyle Aortic Root Bioprosthesis (Medtronic, Inc., Minneapolis, MN, USA) using a subcoronary implantation technique, while 242 patients had AVR with the stented Mosaic bioprosthesis (Medtronic, Inc.). Patient characteristics are shown in Table 1
, and age profiles of both groups are illustrated in Figure 1.Patients receiving a Freestyle bioprosthesis more often had a 23-mm valve implanted, whereas those in the Mosaic group more frequently received a 21-mm valve (Figure 2). To determine survival, 367 patients undergoing surgery between April 1996 and September 2000 were followed up for 3 years by mailed questionnaires. Follow-up was 98% complete and comprised 484 patient-years.
All patients underwent AVR via a complete sternotomy and standard extracorporal circulation. Myocardial protection was accomplished with moderate hypothermia no lower than 32°C, and retrograde cold blood cardioplegia. The stented Mosaic valve was inserted according to standard techniques in an epiannular position. The stentless Freestyle prosthesis was implanted by a subcoronary technique (with or without scalloping the noncoronary sinus) described previously.10 No prosthesis was undersized. The type of prosthesis was chosen according to the surgeon’s preference. Patients with extensive calcification of the aortic wall or other anatomic findings making the implantation of a stentless valve difficult or dangerous, received a stented bioprosthesis.
Operative mortality was defined as death within 30 days postoperatively. Operative risks and postoperative complications were compared using logistic regression because the two groups were different with regard to preoperative status. The variables considered as potential confounders were: age, sex, concomitant coronary artery bypass grafting (CABG) or mitral valve replacement, redo AVR, New York Heart Association functional class, preoperative myocardial infarction, hyperlipidemia, hypertension, diabetes mellitus, body mass index, body surface area, height, previous CABG, a permanent pacemaker, need for dialysis, peripheral arterial occlusive disease, and concomitant pulmonary, neurological and renal diseases. Statistical analyses were performed using SPSS software (SPSS, Inc., Chicago, IL, USA). All variables were subjected to univariate testing by the chi-squared or likelihood-ratio test. Variables showing a difference with a p value of 0.25 or less were included in the model until the deviance of the model did not decrease significantly further. Using the final model, improvement due to inclusion of the valve type (0 for Mosaic, 1 for Freestyle) was tested. To determine survival, the Kaplan-Meier method and Cox regression were used. Model building and testing were carried out as described for logistic regression modeling.
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RESULTS
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Operative mortality (within 30 days) was 2.6% for patients after isolated AVR with the stentless Freestyle bioprosthesis (4.1% in cases of concomitant CABG). For those undergoing isolated AVR with a stented Mosaic bioprosthesis, the mortality was 4.2% (9% with concomitant CABG). The crossclamp and cardio– pulmonary bypass times were significantly shorter in the Mosaic group than the Freestyle group (70 ± 22 min versus 113 ± 32 min, p < 0.001; 83 ± 21 min versus 127 ± 35 min, p < 0.001, respectively). Risk factors for operative death, which were contained in the final model, are shown in Table 2. Although crossclamp and cardiopulmonary bypass times were significantly shorter in the Mosaic group, the model did not improve significantly when these variables were included (p = 0.48 and p = 0.1, respectively). Implantation of the stentless Freestyle valve did not increase the operative risk (p = 0.43).
Table 3 shows the postoperative complications after AVR. On comparing individual complications, no major differences were found. However, overall, more patients in the Mosaic group had neurological disorders, perioperative myocardial infarction, low output syndrome, pericardial effusion, pleural effusion, respiratory insufficiency, reintubation, resuscitation, and redo thoracotomy. Logistic regression did not reveal any difference in the risk of postoperative complications in patients receiving a Freestyle or Mosaic valve (p = 0.46). The risk factors identified were diabetes mellitus (p = 0.03; odds ratio: 1.7), age > 74 years (p = 0.005; odds ratio: 2.1), comorbidity (p = 0.03; odds ratio: 1.7), preoperative infection (p = 0.04; odds ratio: 2.8), and myocardial infarction (p = 0.11; odds ratio: 1.7). Although the statistic of Wald for the risk of myocardial infarction was not significant, by including this variable in the model, the deviance decreased significantly (p < 0.005). Risk factors for postoperative neurological disorders after AVR with a bioprosthesis were diabetes mellitus (p = 0.002; odds ratio: 1.9), age > 74 years (p = 0.002; odds ratio: 1.8), body mass index < 24 (p = 0.001; odds ratio: 2.0), and emergency procedure (p = 0.016; odds ratio: 1.6). The implantation of a stentless Freestyle prosthesis did not increase this risk (p = 0.31).
The survival curves (including operative mortality) were not different (log rank: p = 0.95) for patients after AVR with Freestyle or Mosaic valves (Figure 3). The curves for survival after AVR with either bioprosthesis were in accordance with the expected survival for 76-year-old men and women in the healthy German population in the year 1999 (Figure 3). Crossing of the two survival curves indicates some confounding. In this case, the log-rank test has no power. Cox regression considering potential confounders also failed to reveal any difference between the two survival curves (p = 0.94). An analysis of older and younger age groups did not disclose any difference (p = 0.93). Risk factors for midterm survival were identified as diabetes mellitus (p = 0.002; odds ratio: 2.6), previous CABG (p = 0.017, odds ratio: 12.5) and preoperative New York Heart Association class III and IV (p = 0.009; odds ratio: 3).
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Figure 3. Kaplan-Meier survival curves for 367 patients with stented (thick broken line) or stentless bioprostheses (thick line), and the expected survival for women (upper thin line) and men (lower thin line) in the healthy population (Zensor indicates patients still living at the endpoint of the study).
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DISCUSSION
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The human aortic valve does not have stents, nor does any other valve in nature. Therefore, commercially available stented aortic valves are somewhat unphysiological. The most physiological way of replacing the human aortic valve would be to use a stentless homograft valve. The limited availability of homografts and the issues of consistent anticalcification treatment as well as reliable quality of such grafts have reduced the supply to negligible quantities compared to the overall number of valves implanted. Commercially available quality-controlled stentless porcine valves offering theoretically similar physiological advantages as homografts, have been increasingly used for AVR during the last decade. The intermediate and long-term results with such valves have not yet been established. Furthermore, they require a more complex implantation technique, potentially leading to a higher perioperative complication rate, which is still controversial. Consequently, stentless bioprostheses are reserved for older patients in whom the potential benefits of this type of valve might not truly materialize. The intention of this investigation was to determine whether the implantation of stentless rather than stented valves affected the outcome in a high-risk population of older patients; if not, the implantation of stentless valves might be safely performed in younger populations, with anticipation of good long-term results. Santini and colleagues11 described mortality rates of 8% for the implantation of stentless valves and 5% for stented valves in patients aged 73 to 77 years. Westaby and colleagues9 reported 8% mortality after AVR with stentless valves and 12% for stented bioprostheses in a patient population with a mean age of 73 years.
Previous studies have not answered the question of whether there is a difference in operative risk for implantation of various biological valve types. Most have been retrospective observational studies in which the two analyzed study arms differed in preoperative status, as they were not randomized in respect of valve type. Usually, only mortality rates after AVR with stentless and stented bioprostheses were compared. Logistic regression is the only method capable of testing the differences between potential confounders according to risk, which may vary among study populations. Using multiple logistic regression, we determined previous CABG and an emergency procedure as risk factors for operative death, which is in accordance with other studies.12,13 Further risk factors have been identified as poor left ventricular function, hypertension, and concomitant surgical procedures.14,15 Some investigators found concomitant CABG to be a risk factor, whereas others did not.16–19
Although patients receiving a stented bioprosthesis were more ill than those given the stentless type, and the implantation of a stentless bioprosthesis is regarded as more complex and time consuming, no differences have been observed in operative mortality or the risks of postoperative complications or neurological disorders.20 Because of the larger effective orifice areas of stentless valves, more rapid regression of left ventricular hypertrophy and longer survival times could be expected. Recently, significantly different survival curves with a higher survival rate at 5 to 8 years for stentless valves were reported.7,9 Del Rizzo and colleagues8 found a significant difference in 5-year survival only in patients younger 75 years, while Vrandecic and colleagues20 did not detect any significant difference during a follow-up of 10 years in patients with a mean age of 47 years.
In this study where the mean age was 76 years, we found no difference in 3-year-survival, even for patients younger than 75 years. However, the survival curves for patients after AVR with stentless and stented bioprostheses were within the expected survival of the healthy population. In spite of the more complex and time-consuming operative technique, resulting in longer crossclamp and cardiopulmonary bypass times, the risks of operative death and postoperative complications were not increased after AVR with a stentless bioprosthesis. In the hands of an experienced surgeon, implantation of a stentless aortic valve does not increase perioperative risk or negatively affect long-term outcome even in very old patients. As the long-term results of implantation of stentless valves appear to be satisfactory, these valves may be considered more often for AVR in younger patients.
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Footnotes
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Presented at the 15th Biennial Congress of the Association of Thoracic & Cardiovascular Surgeons of Asia, December 6–9, 2001, Mumbai, India.
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REFERENCES
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ABSTRACT
INTRODUCTION
