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Ann Thorac Surg 2002;74:1107-1113
© 2002 The Society of Thoracic Surgeons
a Department of Medicine, Division of Cardiology, University of Michigan, Ann Arbor, Michigan, USA
Accepted for publication May 29, 2002.
* Address reprint requests to Dr Bach, L3119 Women’s -0273, 1500 E. Medical Center Dr, Ann Arbor, MI 48109 USA
e-mail: dbach{at}umich.edu
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Abstract |
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 TopFootnotes Abstract IntroductionPatients and methodsResultsCommentReferences |
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Methods. The long-term multicenter study of the Freestyle stentless aortic bioprosthesis includes 500 consecutive patients implanted using the subcoronary and 162 using the full root technique. Clinical outcomes and echocardiographic hemodynamics were compared through 5 years.
Results. There were no differences between groups in time to death, valve-related death, or reoperation. The incidence of operative death was higher in the full root than in the subcoronary group (odds ratio 3.97, p = 0.001). Patients in the subcoronary group were more likely to have New York Heart Association functional class III or IV symptoms at 1 year (1.7% versus 0%, p = 0.04) and 5 years postoperatively (4.4% versus 0%, p = 0.02). Mean gradient was lower (p = 0.0004) and effective orifice area larger (p = 0.04) in the full root group. Left ventricular mass index decreased in both groups. The preponderance of patients in both groups had no or trivial aortic regurgitation through 5 years.
Conclusions. Full root implantation of the Freestyle stentless aortic bioprosthesis was associated with higher operative mortality, but somewhat better hemodynamics, functional class, and freedom from aortic regurgitation. Higher operative mortality argues against the empiric replacement of the ascending aorta in the absence of aortic root pathology. In appropriately selected patients, both implant techniques are viable alternatives for valve implantation.
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Introduction |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentReferences |
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Hemodynamic performance and freedom from aortic regurgitation (AR) are influenced by the method of implantation of allograft valves [8]. Whether the implant technique of stentless xenografts similarly affects hemodynamic performance, and whether the implant technique of either allograft or stentless xenograft aortic valves impact clinical outcomes, require further investigation. The purpose of the present study was to determine whether implant technique of the Freestyle aortic root bioprosthesis impacts clinical outcomes, hemodynamic performance, and freedom from AR.
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Patients and methods |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentReferences |
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Echocardiography
Echocardiograms were obtained at the respective investigational centers, at time points coinciding with those used for clinical data. Analyses were performed using standard clinical criteria. Mean transvalvular gradient was calculated using the modified Bernoulli equation, correcting for proximal velocity [10]. Effective orifice area (EOA) was calculated using the continuity equation [10]. The left ventricular (LV) mass index was calculated as the ratio of LV mass-to-body surface area; LV mass was calculated using the modified American Society of Echocardiography cube method [11]: 
, where (IVS), (LVIDD), and (PW) are standard measures (in centimeters) of interventricular septum (IS), LV internal diameter in diastole (LVIDD), and posterior wall (PW), respectively. The AR was graded as absent, trivial, mild, moderate, or severe based on standard clinical criteria, and included assessment of jet width, circumference, and eccentricity [12].
Statistical methods
The 
2 test was used to determine differences in demographic and preoperative factors between implant groups. Factors considered in the analysis were patient sex (male, female); age at implant (
70, >70 years); history of diabetes, hypertension, hyperlipidemia, tobacco use, angina pectoris, myocardial infarction, coronary disease, transient ischemic event or stroke, and congestive heart failure (yes, no); New York Heart Association (NYHA) class (I/II, III/IV); left atrial enlargement (yes, no); LV enlargement (yes, no); LV hypertrophy (yes, no); LV ejection fraction (50%, >50%); valve size (19/21 mm, 23 mm, 25 mm, 27 mm); history of previous valve replacement (yes, no) or previous cardiac operation (yes, no); presence of aortic aneurysm (yes, no), ascending aorta calcified (yes, no), or any abnormality of the ascending aorta (yes, no); any concomitant procedure (yes, no); concomitant coronary artery bypass grafting (yes, no); cross-clamp time; site of operation (hospitals A through H); and one of first 20 implants for implanting surgeon (yes, no).
Clinical outcomes investigated were death, cardiac death, operative death, valve-related or unexplained death, reoperation, length of hospital stay after implant, and NYHA class at 1 and 5 years postoperatively. Cox proportional hazard models were used to test for differences between groups in death, cardiac death, valve-related or unexplained death, and reoperation. Demographic and preoperative factors found significantly different between implant groups were evaluated to determine association with each outcome using a log-rank test, and significant factors were included as covariates in the final Cox proportional hazards model for each outcome. A logistic regression model was developed to test for differences in early death. Demographic and preoperative factors found significantly different between groups were evaluated to determine association with early death using a 2 test, and significant factors were included as covariates in the logistic regression model. The 2 tests were used to determine whether demographic and preoperative factors found significantly different between groups were associated with NYHA class at 1 year and at 5 years. An analysis of covariance was used to test for differences between groups in hospital length of stay. Student’s t tests were used to determine whether there were differences in length of stay associated with demographic and preoperative factors found significantly different between implant groups. Factors associated with differences in length of stay were included as covariates in the analysis of covariance.
Hemodynamic data investigated included mean transvalvular gradient, EOA, AR, and regression of indexed LV mass. Repeated measures of analysis were performed to test for differences in mean gradient, EOA, and LV mass index across time, among valve sizes, and between implant techniques. Echocardiograms at discharge, 3 to 6 months, 1-, 2-, 3-, and 4-year evaluations were included in the analysis of mean gradient and EOA; echocardiograms at the discharge, 3 to 6 months, 1-, 2-, and 3-year evaluations were included in the analysis of LV mass index. To evaluate the change in severity of AR, a comparison was made between the severity at discharge and the severity at 1 year. The numbers of patients were tabulated with a decrease in severity, with no change, or with an increase in severity, and the distribution of patients among these three groups was compared between implant techniques using a 2 test. The same procedure was used to test for differences in severity at the discharge and 5-year evaluations.
Statistical analysis was performed using the SAS (SAS Institute, Inc, Cary, NC) statistical software. Differences were considered significant at a value of p less than 0.05.
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Results |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentReferences |
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All patients implanted at hospital H underwent subcoronary implantation, and were older (75.9% versus 58.6% >70 years) and more likely to undergo concomitant procedures (66.2% versus 48.4%) than patients implanted at other sites. Removing site as a possible confounding variable, concomitant procedures was associated with a higher rate of operative death (risk ratio 3.41, p = 0.002). Operative death remained significantly higher among patients implanted using the full root than among those implanted using the subcoronary technique (odds ratio 2.31, p = 0.02).
Cardiac death
There was a significant difference in time to cardiac death for preoperative factors including age more than 70 years (risk ratio 3.41, p = 0.004), myocardial infarction (risk ratio 3.75, p < 0.0001), valve size 19 or 21 mm (risk ratio 4.45, p < 0.0001), concomitant surgical procedure (risk ratio 3.33, p = 0.002), and hospital D (risk ratio 0.10, p = 0.002). Adjusting for these factors, there was a significant difference (p < 0.0001) in time to cardiac death by implant technique. Freedom from cardiac death at 1 year and 5 years postoperatively was 96.1% and 92.3%, respectively, for patients implanted using the subcoronary technique, and 93.8% and 88.8% for patients implanted using the full root technique.
Valve-related death
There was a significant difference in time to valve-related or unexplained death for implantation at hospital F (risk ratio 3.44, p = 0.028). There was no difference (p = 0.65) in time to valve-related death by implant technique. Freedom from valve-related death or unexplained death at 1 year and 5 years was 98.3% and 96.5%, respectively, for patients implanted using the subcoronary technique, and 98.6% and 94.6% for patients implanted using the full root technique.
Reoperation
There was a significant difference in time to reoperation for abnormal ascending aorta (risk ratio 9.38, p = 0.035) and hospital A (risk ratio 3.69, p = 0.046). There was no difference (p = 0.78) in time to reoperation by implant technique. Freedom from reoperation at 1 year and 5 years was 98.9% and 96.9%, respectively, for patients implanted using the subcoronary technique, and 100% and 97.7% for patients implanted using the full root technique.
NYHA class
Data on NYHA class were available for 559 patients at 1 year, and for 122 patients at 5 years postoperatively. There were no clinical factors associated with NYHA class at 1 year or 5 years postoperatively (p > 0.05). However, the prevalence of NYHA class III or IV status 1 year postoperatively was higher among patients implanted using the subcoronary technique (7 of 422 [1.7%]) than among those implanted using the full root technique (0 of 137 [0%], p = 0.04). At 5 years, NYHA class III or IV status was more prevalent among patients in the subcoronary (4 of 92 [4.4%]) than in the full root group (0 of 30 [0%], p = 0.02).
Length of stay
There was a significant difference in length of stay for preoperative NYHA class III or IV (11.6 ± 11.1 days) versus class I or II (9.5 ± 11.8 days, p = 0.03) status. There was no difference in length of stay for patients implanted using the subcoronary (11.1 ± 10.6 days, median 8.0 days) compared with the full root (11.0 ± 13.4 days, median 7.0 days, p = 0.89) implant technique.
Hemodynamics
Mean gradient
As anticipated, gradients were lower for larger than for smaller valve sizes (p = 0.0001). In addition, gradients were lower among patients implanted using the full root technique than among those implanted using the subcoronary technique (p = 0.0004). Gradients decreased significantly over time for both groups (p = 0.0001). However, the decrease in mean gradient was larger among patients implanted using the subcoronary than the full root technique. Mean gradients for patients with a 21-mm valve decreased from 13.4 ± 5.0 mm Hg at discharge to 9.9 ± 5.9 mm Hg at 1 year postoperatively in the subcoronary group, and from 8.0 ± 3.8 to 6.6 ± 4.6 mm Hg in the full root implant group; mean gradients among patients with a 27-mm valve decreased from 8.2 ± 4.5 to 4.9 ± 3.1 mm Hg over the same period in the subcoronary group, and from 4.7 ± 2.9 to 3.6 ± 1.6 mm Hg in the full root group. Gradients by valve size and implant technique at 1 year postoperatively are shown in Table 4.
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Left ventricular mass index
The LV mass index at baseline was not statistically different between implant groups (p = 0.97), although larger valve sizes were associated with greater baseline LV mass index (p = 0.07). The LV mass index across time is shown in Figure 2.
The LV mass index decreased significantly over time for both implants groups (p = 0.0001). However, the decrease was of greater magnitude in the full root than in the subcoronary group (p = 0.08).
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Comment |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentReferences |
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Clinical outcomes
Clinical outcomes were reasonably good in both implant groups, with high rates of survival free of adverse events through 5 years. (Operative mortality is discussed later.) There were no significant differences between implant groups with respect to death, valve-related or unexplained death, reoperation, or hospital length of stay. Freedom from cardiac death was somewhat higher among patients undergoing subcoronary than full root implantation. However, elimination of surgical site as a factor in the regression model resulted in inclusion of functional class as statistically associated with cardiac death, with no significant difference in time to cardiac death between implant groups. It is likely that surgical site and NYHA class are confounded variables (specific sites had more patients with advanced heart failure), and that clinical factors likely contributed to the differences in cardiac death observed between implant techniques.
Preoperative hyperlipidemia was paradoxically associated with a lower risk of death. This likely reflects a difference in the diagnosis rather than the presence of hyperlipidemia. Because assessment of lipids was not mandated, it is likely that patients carrying the diagnosis of hyperlipidemia were receiving appropriate therapy, whereas some patients not carrying the diagnosis had unrecognized, untreated disease.
Operative mortality
Operative mortality was high in both implant groups. Although the study population included a fairly high proportion of patients with clinical or operative factors associated with increased risk, the observed rate of operative mortality is concerning.
In the interest of reporting the longest possible clinical follow-up, the study population represents the earliest experience among the implanting institutions. An attempt was made to address whether operator inexperience contributed to high operative mortality. The logistic regression model defined operator experience as within or not within the first 20 implants per surgeon. Using this model, as well as a model using the first 20 implants per site, operative mortality did not decrease with increased operator experience. However, a report on the experience of a single surgeon at another institution demonstrated that operative mortality decreased substantially with increasing experience, despite increasing complexity of operation and higher patient risks [14]. Specifically, from 1997 to 1999, 30-day mortality among a cohort of 266 consecutive patients undergoing Freestyle aortic valve replacement decreased from 7.5% to 3.4%, whereas surgical complexity scores and the prevalence of comorbidities increased. Operative mortality in this cohort was higher in association with full root than subcoronary implant technique (4.5% versus 3.0% overall); however, operator experience was associated with a statistically significant decrease in mortality in both groups. Together with the present report, these data suggest that the interaction between operator experience and early mortality changes over more than the first 20 implants per surgeon.
The present report gives an estimate of operative mortality reflective of early experience both for individual surgeons and for the valve prosthesis. With experience, operative mortality decreases, and is likely closer to 4.0% to 4.5% for full root and 2.5% to 3% for subcoronary implantation [14]. Although assessment of future operative mortality is beyond the scope of this work, it should be noted that training for new surgeons who implant stentless bioprostheses incorporates lessons learned in the past 10 years. As such, operative mortality for new implanters may be lower and the learning curve likely "steeper" than that reported in the present work.
Study limitations
The present study was observational. As such, multiple factors influenced implant technique, including surgeon preference as well as patient-related variables. There were clear differences between study sites, with some performing only subcoronary and others performing only full root procedures. Site was used as a clinical variable in the regression model in an attempt to recognize regional differences in patients, clinical practice, and surgical practice. Differences between sites underscore the heterogeneity of the study population. However, inasmuch as it is impossible to perform a study that randomly assigns patients to the two implant techniques, the present study reports outcomes reflective of clinical practice as the valve was used at a variety of institutions.
There were insufficient data in the present study to test the root inclusion technique. The preponderance of subcoronary valves were implanted using the modified subcoronary technique; as such, data in the present study should not be extrapolated to other stentless bioprostheses implanted using a complete subcoronary technique.
Small numbers of patients may have precluded detection of small differences between implant groups. In addition, statistically significant differences between groups for some variables may be of limited clinical importance. Finally, longer term data and functional data are required to better measure late differences between implant techniques. In particular, the impact of implant technique on complications and mortality at reoperation was not addressed in the present study.
In conclusion, both the subcoronary and the full root implantation techniques of the Freestyle stentless aortic bioprosthesis are associated with good hemodynamic and clinical outcomes through 5 years. Differences observed between implant techniques included higher operative mortality and subsequent cardiac mortality among patients undergoing full root implantation, but with somewhat more favorable hemodynamics, lower prevalence of AR, and better NYHA functional class on follow-up. Higher operative mortality associated with the full root technique argues against empiric replacement of the ascending aorta in the absence of aortic root pathology. In appropriately selected patients, both the full root and the subcoronary implant techniques appear to be viable alternatives for valve implantation.
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Footnotes |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentReferences |
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* Dr Cartier passed away on Jan 2, 2001. 
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Appendix |
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References |
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TopFootnotesAbstractIntroductionPatients and methodsResultsCommentReferences |
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degran K., Brodin L. Effects of exercise on Doppler-derived pressure difference, valve resistance, and effective orifice area in different aortic valve prostheses of similar size. Am J Cardiol 1999;83:619-622.[Medline]
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mild AR.