***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
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    ***EXAMPLES*** 
    Input: BACKGROUND The hemodynamic superiority of  stentless valves  at rest has been generally accepted , but there is a lack of studies on exercise hemodynamics . METHODS We assessed aortic valve hemodynamics at rest and during exercise in 10 patients with a 23-mm stentless aortic bioprosthesis ( Medtronic Freestyle ; Medtronic Europe SA/NV , St. Stevens Woluwe , Belgium ) , in 10 patients with a 23-mm stented aortic bioprosthesis ( Carpentier-Edwards , SAV , model 2650 ; Baxter Edwards AG , Horw , Switzerl and ) , and in 10 healthy volunteers ( control group ) by means of Doppler echocardiography .  RESULTS Gradients at rest and gradients on comparable maximum exercise levels were significantly lower in patients with stentless valves compared to those with stented valves ( rest : 6 + /- 2/11 + /- 4 mm Hg [ mean/peak ] versus 12 + /- 3/21 + /- 10 mm Hg ; exercise : 9 + /- 3/18 + /- 6 mm Hg [ mean/peak ] versus 22 + /- 8/40 + /- 11 mm Hg ) . Patients with stentless valves  revealed , in comparison to healthy young men , significantly higher gradients , but the small gradient difference of 3/7 mm Hg ( mean/peak ) at rest remained nearly unchanged throughout the exercise protocol ( 4/8 mm Hg [ mean/peak ] at 25 W , 4/9 mm Hg at 50 W and 4/9 mm Hg at 75 W ) . In contrast , the gradient difference between patients with stented and stentless valves  increased significantly from one exercise level to the next ( 6/12 mm Hg [ mean/peak ] at rest , 8/14 mm Hg at 25 W , 12/17 mm Hg at 50 W , and 15/25 mm Hg at 75 W ) . CONCLUSIONS A stentless aortic bioprosthesis seems to be an appropriate aortic valve substitute , especially in patients who perform regular physical exercise Mechanical and conventional stented bioprostheses , because of need for anticoagulants , hemodynamic characteristics and long-term durability , do not represent the optimal heart valve replacement device . In this study we compared the hemodynamic function of a  stented aortic bioprosthesis ( St Jude Bioimplant ) , implanted in 10 patients , with a  stentless porcine aortic bioprosthesis ( Biocor )  implanted in 7 patients , by means of Doppler echocardiography early postoperative at rest ( T1 ) and 6 months later at rest ( T2a ) and during exercise ( T2b ) . Mean and peak systolic gradients across stentless porcine prostheses  were significantly lower than across stented bioprostheses ( T1 p = 0.008 and p = 0.004 ; T2a p < 0.0001 and p < 0.0001 ; T2b p < 0.0001 and p < 0.0001 , respectively ) . Our results show that systolic and diastolic mechanical stress on biological components of a glutaraldehyde-fixed stentless porcine aortic bioprosthesis is much lesser than on stented bioprostheses . This feature has appeared evident at rest and much more after exercise testing . The reduction of systolic and diastolic stress is expected to determine lower calcification degree and longer durability of stentless porcine aortic bioprostheses . Moreover , aortic valve replacement by means of stentless bioprostheses allows the implantation of a 1 size ( 2 mm ) larger device , appearing favourable especially in small aortic annulus . On the basis of these promising results we suggest that stentless bioprostheses are a valid alternative to stented bioprostheses for aortic valve replacement . However , patient population is too small and the follow-up is too short to draw a definite statement about long-term hemodynamic performance of this device OBJECTIVES Although stentless aortic bioprostheses are believed to offer improved outcomes , benefits remain unsubstantiated . The aim of our study was to compare stentless with stented bioprostheses , with regard to postoperative changes in left ventricular mass and hemodynamic performance , in the elderly patient . METHODS Forty patients with aortic stenoses , over the age of 75 years , were r and omized to receive either the stented Perimount ( n=20 ) or the stentless Prima Plus ( n=20 ) bioprosthesis .  Left ventricular mass regression , effective orifice area , ejection fraction and mean gradients were evaluated at discharge , 6 months and 1 year after surgery . RESULTS Overall a significant decrease in left ventricular mass was found 1 year postoperatively . However , there was no significant difference in the rate of left ventricular mass regression between the groups . Furthermore , 1 year postoperatively , the hemodynamic performance of the valves and the change in the ejection fraction did not differ between the groups . CONCLUSIONS Our study shows that in a r and omized cohort of elderly patients with aortic stenosis , we were not able to detect significant differences , with regard to hemodynamic performance and regression of left ventricular mass , between the stentless and stented valve groups . To our surprise , previously reported findings of non-r and omized trials that showed faster and more complete regression of left ventricular mass and hemodynamic benefits of stentless valves were not reproducible Background — Aortic valve replacement ( AVR ) is the established treatment for severe aortic stenosis . In response to the long-term results of aortic homografts , stentless porcine valves were introduced as an alternative low-resistance valve . We conducted a r and omized trial comparing a stentless with a stented porcine valve in adults with severe aortic stenosis . Methods and Results — The primary outcome was change in left ventricular mass index ( LVMI ) measured by transthoracic echocardiography and , in a subset , by cardiovascular MR . Measurements were taken before valve replacement and at 6 and 12 months . Patients undergoing AVR with an aortic annulus ≤25 mm in diameter were r and omly allocated to a stentless ( n=93 ) or a stented supra-annular ( n=97 ) valve . There were no significant differences in mean LVMI between the stentless versus stented groups at baseline ( 176±62 and 182±63 g/m2 , respectively ) or at 6 months ( 142±49 and 131±45 g/m2 , respectively ) , although within-group changes from baseline to 6 months were highly significant . Changes in LVMI measured by cardiovascular MR ( n=38 ) were consistent with the echo findings . There was a greater reduction in peak aortic velocity ( P<0.001 ) and a greater increase in indexed effective orifice area ( P<0.001 ) in the stentless group than in the stented group . There were no differences in clinical outcomes between the 2 valve groups . Conclusions — Despite significant differences in indexed effective orifice area and peak flow velocity in favor of the stentless valve , there were similar reductions in left ventricular mass at 6 months with both stented and stentless valves , which persisted at 12 months BACKGROUND Residual left ventricular hypertrophy adversely affects long-term outcome after aortic valve replacement . A stentless biological valve in the aortic position has been shown to offer a better hemodynamic profile than a stented one . However , it remains to be defined whether this difference is translated into inter-mediate-term effects on left ventricular structure and function . METHODS One hundred thirty-seven patients receiving single aortic valve replacement ( 52 with concomitant coronary artery bypass graft ) were enrolled in this study . Ninety-eight were men , and the mean age was 68 years ( range , 55 to 90 years ) . Of the 137 patients , 39 had an aortic homograft  , 72 a Toronto stentless porcine valve , and 26 had a stented porcine or bileaflet mechanical valve , with mean valve size of 25 + /- 2.5 mm ( mean + /- st and ard deviation ) . Left ventricular muscle mass and function were assessed by M-mode echocardiography performed before and 0.5 , 6 , 12 , 24 , and 36 months after operation , and recorded on paper for off-line digitizing . Peak valve prosthesis pressure gradients were quantified by continuous wave Doppler . RESULTS A total of 330 echocardiograms obtained during this study were adequate for computer digitizing . Clinical data , preoperative left ventricular function , and hypertrophy were similar between the three groups . Significant improvement in left ventricular function and major regression of left ventricular hypertrophy had occurred in the entire population by 6 months after operation . Multivariate analysis of variance showed that patients with previous aortic regurgitation had a larger left ventricular cavity size ( p < 0.001 ) and greater mass index ( p = 0.001 ) postoperatively than those with previous aortic stenosis . In addition , peak valvular gradient was lower ( p < 0.001 ) , mass index less ( p < 0.001 ) , and left ventricular function more normal both systolic , by a greater peak velocity of dimension shortening ( p = 0.05 ) and wall thickening ( p = 0.002 ) , and diastolic , by a greater peak velocity of dimension lengthening ( p = 0.046 ) , with an aortic homograft or stentless porcine valve compared with a mechanical or stented biological valve . There was no significant difference in peak valve gradient , left ventricular mass index , or function between the aortic homograft and the stentless porcine valve . Age , sex , and concomitant coronary artery bypass graft , as well as aortic cross-clamp time , cardioplegia method , and valve size all proved to be insignificant determinants of postoperative left ventricular hypertrophy or function . CONCLUSIONS In the first 2 years after implantation , the superior hemodynamic performance of aortic homograft and stentless porcine valve appears to result in more extensive regression of ventricular hypertrophy and greater improvement of left ventricular function than occurs with a mechanical or stented biological valve . These findings encourage the use of a stentless biological valve in older patients requiring aortic valve replacement , and a larger scale long-term r and omized study of stentless versus stented biological valve or mechanical valve seems warranted A number of recent reports have suggested that valve design has an influence on long-term survival  after aortic valve replacement ( AVR ) . The present study examines the clinical results of two large multicenter trials of the Hancock II stented and Medtronic Freestyle stentless bioprosthetic valves . In all cases a subcoronary implant technique was used . All patients were operated on between 1991 and 1994 , and all patients were monitored for 5 years . The 5-year actuarial survival rate was 86 % for Freestyle and 77 % for Hancock patients ( P<.05 ) . Late mortality occurred in 40 Freestyle patients ( 8.8 % ) compared with 36 Hancock II patients ( 16.1 % , P = .0074 ) . Cox 's proportional hazard models were used to identify independent predictors of 5-year survival for the two groups . Hazard ratio ( HR ) and P value are presented in parentheses . The final analysis included the interactive variable valve*age . The result ant model indicates that the interaction of age and valve type has a significant effect on outcome . In patients < or = 60 years of age at the time of operation , AVR with a Hancock II valves was associated with nearly a fivefold risk of death compared with patients of the same age who received a Freestyle valve ( HR = 4.97 , P = .0004 ) . However , this survival advantage decreased by 50 % with each decade thereafter ( valve*age interaction HR = 0.50 , P = .0027 ) . Our data indicate that AVR with a stentless valve confers a survival advantage to the patient . The probability of dying within 5 years of surgery is 50 % greater in patients who received a Hancock II versus a Freestyle valve ( HR = 1.50 , P = .0442 ) . However , this survival advantage is most prominent in younger patients ( < 60 years of age ) where the probability of death is fivefold greater with Hancock than Freestyle valves . With advancing age the benefits of stentless valves are diminished The goal of this study was to analyze regression of left ventricular hypertrophy after r and omization to conventional biological versus stentless aortic valve replacement . Stentless ( Freestyle , Toronto , n = 106 ) or conventional biological aortic valves ( Carpentier-Edwards , n = 74 ) were evaluated prospect ively . Preoperatively there were no differences with regard to aortic valve pathology , left ventricular function , and pressure gradients between the two patient groups . The patient annulus index ( 13.55 vs. 13.46 mm ; NS ) measured intraoperatively was used as baseline for further comparison . Postoperatively , left ventricular mass index was 213+/-77 g/m2 ( stentless ) compared with 202+/-72 ( conventional group ) g/m2 ( NS ) , whereas after 6 months it was 141+/-41 g/m2 in the stentless and 170+/-43 g/m2 in the conventional group ( P<.05 ) 
    Output:
    
Our primary analyses revealed no differences between groups for assessment s of LV mass or mean transvalvular gradients .
Secondary analyses showed stentless valves to have lower peak gradients .
Stentless valves did not display hemodynamic benefit in terms of LV mass regression or postoperative mean gradients , but do appear to display superior hemodynamics in terms of peak gradients .