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J Thorac Cardiovasc Surg 1998;116:477-481
© 1998 Mosby, Inc.

Surgery for Adult Cardiovascular Disease

Valve replacement with a stentless bioprosthesis: Versatility of theporcine aortic root

From the Department of Cardiac Surgery, Oxford Heart Centre, The JohnRadcliffe Hospital, Oxford, United Kingdom.

Received for publication Jan 22, 1998. Revisions requested April 8, 1998; revisions received April 24, 1998. Accepted for publication June 2, 1998. Address for reprints: Stephen Westaby, BSc, FRCS, MS, Oxford HeartCentre, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UnitedKingdom.

	Abstract

Top Abstract Introduction Patients and methods Results Discussion References

 
Objective: Stentless valves conveyimportant hemodynamic benefits but are used selectively depending on aortic rootstructure. The Freestyle valve (Medtronic, Inc, Minneapolis, Minn) is aversatile device that can be implanted by different methods depending onoperating conditions. We aimed to demonstrate that a stentless valve could beused in every patient without increased risk of morbidity or mortality. Wedocumented the effects of this valve on clinical outcome and left ventricularmechanics.
Methods: The Freestyle valvewas implanted by the modified subcoronary method into 200 consecutive unselectedpatients who received a tissue valve in the aortic position and by rootreplacement in 2. Forty-three percent were older than 75 years. Forty percentunderwent coronary bypass. Detailed clinical and echocardiographic follow-up(Food and Drug Administration protocol) was used out to 3 years.
Results: Mean ischemic time was 43 ± 6 minutes forisolated aortic valve replacement and 63 ± 14 minutes with concomitantprocedures. Thirty-day mortality was 6%, none of the deaths being valverelated. Hemodynamic function improved progressively with falling valvegradients and increased effective orifice areas. Left ventricular mass fellwithin normal limits over 2 years, but at 3 years there was a non-valve-relatedupswing. No instances of valve thrombosis, hemolysis, or paravalvular leak werenoted. Less than 5% had mild to moderate aortic regurgitation.
Conclusions: The Freestyle valve can be used in virtuallyevery patient with aortic valve disease and provides superlative hemodynamicoutcome. Hospital mortality and morbidity are similar to those reported forstented valves in an elderly population. (J Thorac Cardiovasc Surg1998;116:477-84)

	Introduction

Top Abstract Introduction Patients and methods Results Discussion References

 
Aortic valve disease in elderly patients is dominated by calcific aorticstenosis. For worthwhile event-free survival at great age it is important toachieve rapid improvement in ventricular mechanics, functional class, andquality of life without surgical or valve-related morbidity. ^1,2The type of prosthesis has an important bearing on postoperative leftventricular function, particularly in smaller sizes. ^3,4Stentless aortic bioprostheses convey important hemodynamic benefits, incontrast to the nonphysiologic flow profile and residual pressure gradientsacross mechanical valves or stented bioprostheses. ^5,6Just as homograft or pulmonary autograft aortic valve replacement results inrapid resolution of left ventricular hypertrophy in the young, similar benefitsare achieved for the elderly with stentless xenografts. ^1,6Nevertheless, their widespread use has been restricted by perceived difficultiesof implantation with prolonged myocardial ischemic and cardiopulmonary bypasstimes. Consequently, we sought to address the criticism that stentless valvesare suitable only for selected patients without aortic root calcification ordiscrepancy between the diameters of the anulus and sinotubular junction. Thisarticle describes our experience, including the learning curve, in 202consecutive unselected patients, operated on by 1 surgeon (S.W.), who received abioprosthesis in the aortic position. During this series no patient older than70 years received a mechanical valve, and no patient received a stentedbioprosthesis.

	Patients and methods

Top Abstract Introduction Patients and methods Results Discussion References

 
The series comprised 115 male (57%) and 87 female (43%)patients aged 30 to 86 years. Mean age at implantation was 73 years. Only 18patients (9%) were younger than 65 years. These were patients whodeclined a mechanical valve or would not accept anticoagulation with warfarin.Eighty-seven patients (43%) were older than 75 years of age. One hundredforty-two (70%) were in New York Heart Association (NYHA) class III orIV. One hundred sixty-one patients (80%) were in sinus rhythm and 2 had apre-existing pacemaker for heart block. The remainder had atrial fibrillation orflutter. One hundred ten patients (55%) had coronary artery disease and81 (40%) underwent concomitant coronary artery surgery. One hundredeighty-four patients (91%) had critical aortic stenosis, of whom 47 (23%)had an element of aortic regurgitation. The remainder were operated on forsevere aortic regurgitation. Two patients had active native valve endocarditisand 1 had an acute type A dissection. ⁷Eleven patients underwent reoperation for a failed bioprosthesis (n = 9), mechanical valve (n =1), or homograft (n = 1). With informedconsent and ethical committee approval, all patients were prospectively enteredinto a clinical trial approved by the US Food and Drug Administration (FDA) withdetailed serial echocardiographic follow-up.

Surgical methods
In 200 patients the porcine aortic cylinder (Freestyle valve, Medtronic,Inc, Minneapolis, Minn) was implanted by the modified subcoronary techniquedescribed by the author ⁸ (Fig.1, A to D). This method was used irrespective of thedegree and extent of calcification or discrepancy between annular size andsinotubular junction for these patients. Two patients underwent aortic rootreplacement for "porcelain" aorta, which followed dense mediastinalirradiation for breast cancer.

View larger version (54K): [in this window] [in a new window]   Fig. 1. Technical aspects ofFreestyle valve implantation. A, Transverseaortotomy with stay sutures in place and the first valve sutures at the base ofthe inner leaflet triangle. Valve size is chosen to fit the anulus and fill theaortic root. B, Simple interrupted valve suturesin the anulus and valve inflow. The porcine coronaries are closer together thanin the human coronary ostia. C, The porcineaortic sinuses (left and right) are removed after the valve inflow sutures aretied. This ensures accurate apposition without commissural distortion.D, The outflow suture line joins the crest ofthe trimmed Freestyle valve to the transverse aortotomy. In the event of anoccluded right coronary ostium and coronary bypass, the right coronary sinusneed not be excised. Closure of the aortotomy effectively "suspends"the valve within the aortic root and prevents cusp prolapse.

View larger version (37K): [in this window] [in a new window]   Fig. 2. Potential for atechnical error, which produces an elevated transvalvular gradient afterFreestyle valve implantation. If the distance between the valve anulus and rightcoronary ostium is less than the height of the inflow cloth beneath the porcineright coronary, the cloth may be buckled. This can be avoided by rotating theporcine right coronary sinus to the human noncoronary sinus.RCA, Right coronary artery;LCA, left coronary artery.

We initially elected to prescribe warfarin for all patients for 3 months,aiming for an international normalized ratio (INR) of 2.0 to 2.5 for patients insinus rhythm. Those in atrial fibrillation were administered anticoagulantscontinuously to maintain an INR of 2.5 to 3.5 unless atrial fibrillationreverted to sinus rhythm. However, when 2 patients older than 80 years hadanticoagulant-related cerebral hemorrhage (Table I), we reverted to a policy ofusing only aspirin, 75 mg daily, for patients in sinus rhythm.In total, 84 patients (42%) received warfarin and 29 (15%)received neither warfarin nor aspirin.

Echocardiography
Transthoracic echocardiography was performed at the time of dischargefrom the hospital and then at 6 months, 12 months, and annually. Echocardiogramswere recorded with the Toshiba 380A echocardiographic system, with a 2.5-MHZphased-array transducer (Toshiba Corp, Tokyo, Japan). The protocols weredescribed previously in detail. ⁹Standard left ventricular M-mode echocardiograms were recorded and stored onvideotape at a speed of 50 mm/s, with simultaneous electrocardiogram andphonocardiogram. From an apical 5-chamber view, flow velocities in the outflowtract (2.5 MHZ pulsed Doppler ultrasonography) and the maximum velocity acrossthe stentless valve (2.5 MHZ continuous wave Doppler ultrasonography) wererecorded at a speed of 100 mm/s for off-line analysis. ^10,11Systemic blood pressure was also recorded noninvasively by the Hewlett-Packard66s hemodynamic monitoring system (Hewlett-Packard Company, Andover, Mass).

Mean values for each measurement were derived from 3 heartbeats inpatients in sinus rhythm and from 5 beats in those with atrial fibrillation or aVVI pacemaker. End-diastolic dimension, septum thickness, posterior wallthickness, and end-systolic dimension were measured from M-mode echocardiograms.Dimensional shortening fraction, ejection fraction, and the ratio of the wallthickness to cavity radius at end-diastole were determined according to thecriteria of the American Society of Echocardiography. ¹² Left ventricular muscle mass wascalculated ^12,13 and indexed to the body surfacearea. Mean flow velocities and the time integral of systolic flow velocities inthe left ventricular outflow tract and those of the aortic valve were derivedfrom the Doppler recordings. ^10,11,14Left ventricular stroke volume was calculated as the product of thecross-sectional area and flow-velocity time-integral in the outflow tract. Theeffective orifice area of the aortic valve was calculated by the continuityequation ¹¹ and indexed to thecross-sectional area of the outflow tract. Mean pressure drop across the aorticvalve was calculated from the simplified Bernoulli equation by taking thesubvalvular (V₁) and valvular (V₂) mean velocities (meanpressure drop = 4 (V₂² -V₁²),in millimeters of mercury. ¹¹Left ventricular stroke volume index and cardiac index were calculated fromstroke volume (LVSV), heart rate, and body surface area. Left ventricular strokework, measured in millijoules, was determined by LVSV x (meanarterial pressure + mean net aortic valve pressure drop) x 0.0136 x9.8 and indexed to body surface area (millijoules per square meter). ⁹ Myocardial stroke work was defined asglobal stroke work divided by muscle mass volume (millijoules per cubicmillimeter). ¹⁵

Statistical analysis
Echocardiographic and hemodynamic data are presented as mean ± 1standard deviation. Data were analyzed by means of MINITAB statistical software(Release 11 for Windows, 1996; Minitab Inc, State College, Pa). ¹⁶ One-way analysis of variance wasperformed to test the significance of changes in each measurement over thefollow-up time. When this was significant, a further comparison of 95%confidence intervals with respect to the discharge echocardiographic study wascarried out by means of Dunnett's method, with an overall error rate of 0.05 andan individual error rate of 0.02. The changes in valve performance, leftventricular mass index, and ejection fraction over follow-up time and acrossdifferent valve sizes were examined by 2-way analysis of variance.

	Results

Top Abstract Introduction Patients and methods Results Discussion References

 
For the 113 patients undergoing isolated aortic valve replacement, themean crossclamp time was 43 ± 6.0 minutes (range, 34-60 minutes). In 85patients undergoing concomitant procedures the crossclamp time was 63 ±14 minutes (range, 34-123 minutes). In no patient was an implant abandoned orthe valve size changed.

Clinical outcomes
Twelve deaths occurred (6% hospital mortality) within 30 days ofthe operation (Table I). Three perioperative deaths occurred as a result ofacute myocardial infarction in elderly patients (>75 years) with aorticstenosis, left ventricular hypertrophy, and severe diffuse coronary arterydisease. Two patients died on the second postoperative day, 1 of an extensiveperioperative cerebrovascular accident (CVA) and the second of left ventricularfailure; the second death occurred in a moribund octogenarian (NYHA class IV)with aortic regurgitation and a "bovine" heart. The remainingpatients died of stroke, mesenteric ischemia, or acute myocardial infarctionbetween 8 and 21 days after the operation. A further 3 patients (allpreoperatively in NYHA class IV or V) required intra-aortic balloon pump supportfor between 2 and 7 days after the operation for impaired left ventricularfunction without perioperative myocardial infarction. These patients survivedwith fully competent aortic valves.

There were 16 (8%) late deaths, 4 of which were cardiac in origin.Two occurred as a result of myocardial infarction 2 and 8 months after theoperation, 1 was caused by acute type A dissection 30 months after valvereplacement, and the last was caused by a stroke 4 months after the operation.Because this last patient, 77 years old, did not have an autopsy, the death wasconsidered to be related to the study valve. Other deaths occurred as a resultof malignant disease (n = 4), primaryrespiratory problems (n = 3), rupturedabdominal aortic aneurysm (n = 1), or wereunexplained sudden deaths, presumed arrhythmia in patients all of whom underwentautopsy to rule out other valve-related disease. Of the 194 hospital survivors,143 (74%) remained in sinus rhythm. There were 39 patients (20%)in atrial fibrillation, and 10 new patients required a pacemaker (5%). Of111 patients assessed at 1 year, 81% were in NYHA class I and 19%were in NYHA class II. Valve-related morbidity is summarized in Table I. Allneurologic events and anticoagulant-related hemorrhage are included in thistable.

Hemodynamic performance of the Freestyle valve
From the 194 patients discharged from the hospital, 497 echocardiogramswere obtained with adequate image quality for comprehensive hemodynamicanalysis. This included 146 echoes at discharge, 108 at 6 months, and 109, 84,and 50 echoes at 1, 2, and 3 to 4 years, respectively, after implantation. Thesefigures corresponded to 77%, 81%, 94%, 98%, and 90%of scheduled echocardiographic follow-up, respectively. The overall changes inleft ventricular hemodynamics and Freestyle aortic valve performance aresummarized in Table II. A significant fallin heart rate 6 months after aortic valve replacement was accompanied by areciprocal increase in global stroke volume index. By 24 months systemic bloodpressure had increased significantly. An increase in the flow velocity timeintegral in the left ventricular outflow tract was noted at 12 months, but thecross-sectional area of the outflow tract remained unchanged. In contrast, thepeak flow velocity across the stentless valve fell significantly at 6 monthsafter implantation though its velocity time integral was unchanged. Thus theeffective valve orifice area progressively increased from 1.9 to 2.2 cm².This represented 56% of outflow tract area at hospital discharge, risingto 67% by 3 years. This corresponded with a fall in calculatedtransvalvular mean pressure gradient from 7.5 to 5.2 mm Hg (Table II). Less than5% of patients had more than trivial aortic regurgitation, and in nopatient was aortic regurgitation progressive.

	Discussion

Top Abstract Introduction Patients and methods Results Discussion References

The new stentless aortic bioprostheses provide improved hemodynamics,rapid resolution of left ventricular hypertrophy, and the promise of improveddurability with mitigation from early calcification through biochemicaltreatments. ^19-21 Superlative valve function andavoidance of anticoagulation with warfarin are compelling arguments for the useof stentless bioprostheses in elderly patients with aortic stenosis. ²² The structure of the valve andaortic root is highly variable in this group of patients. Many have poststenoticdilatation with a discrepancy between anulus and sinotubular junctiondimensions. Others have widespread calcification in the aortic sinuses, often incircumferential distribution around the coronary ostia. There is wide variationin the angle (90 to 180 degrees) between the coronary ostia, and in bicuspidvalves the coronary arteries may be diametrically opposite in the aortic root.Despite this, all patients accrue significant benefit from valve replacementwith a stentless prosthesis, and we have shown in a consecutive series that theFreestyle porcine aortic root can be used by one technique or another invirtually every patient. The implantation method we describe is reproducible,safe, and effective by accommodating variability in the aortic root. In somepatients the ascending aorta must be tailored to the size of the Freestyleoutflow, and in the event of very severe calcification aortic root replacementmay be performed. In our experience this is rarely necessary and collective datashow that root replacement is associated with a higher operative mortality(Medtronic submission of collective data to the FDA, 1997).

This consecutive series without patient selection included many elderlypatients (NYHA classes IV or V) with left ventricular failure, unstable angina,endocarditis, or aortic dissection who were treated on an emergency basis. Ahospital mortality of 6% is not excessive in this context and no deathswere attributed to the use of a stentless rather than a stented aorticprosthesis. ^23,24 With a well-organized operation(median ischemic time of 43 minutes for isolated valve replacement) there is noreason for stentless valve usage to contribute to perioperative mortality. ²⁵ On the contrary, our medianextubation time for all patients was less than 3 hours with hospital dischargeto home between 4 and 6 days after the operation. ²⁶ The fact that 10 patients (5%)required a pacemaker after the operation was partly due to pre-existingsecond-degree heart block (in 2 in whom a pacemaker was used electively) andpartly through new complete heart block. This may be caused by injury to theconducting bundle during decalcification of the anulus.

Detailed clinical and echocardiographic follow-up confirms theeffectiveness of cardiac and systemic physiologic rehabilitation with astentless valve. Very low transvalvular gradients translate into rapidresolution of left ventricular hypertrophy, return to NYHA class I, and a lowincidence of valve-related complications. ^9,20 In 1 patient who had acute type Adissection 2 years after the operation, the appearance of the Freestyle valveclosely resembled that of a normal human valve (Fig. 3), and there was nocalcification in the xenograft aortic wall.

View larger version (130K): [in this window] [in a new window]   Fig. 3. Freestyle valve 2 yearsafter implantation (photographed during an operation for acute type Adissection. The valve was conserved).

Surgery for aortic stenosis is one of medicine's great success stories,but there is increasing realization that the type of valve prosthesis has animportant bearing on outcome. Given the unequivocal differences inrehabilitation of the left ventricle after use of a stentless versus stentedxenograft, it becomes progressively more difficult to justify the use offirst-generation technology.

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