magnetic resonance investigation of blood flow

7/27/2019 Magnetic Resonance Investigation of Blood Flow

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Magnetic Resonance Investigation of Blood FlowAfter Aortic Valve Bypass (Apicoaortic Conduit)Craig E. Stauffer, BA, Jean Jeudy, MD, Mehrdad Ghoreishi, MD, Crystal Vliek, MD,Cindi Young, Bartley Griffith, MD, and James S. Gammie, MD

Divisions of Cardiac Surgery, Radiology, and Medicine, University of Maryland Medical Center, Baltimore, Maryland

Background. Aortic valve bypass (AVB, apicoaorticconduit) is an alternative to aortic valve replacement(AVR) for high-risk patients with aortic stenosis (AS).The redistribution of blood flow after AVB has beenpoorly characterized. In order to understand cardiovas-cular physiology after AVB, we performed cardiac mag-netic resonance (CMR) imaging of AVB recipients.Methods. Fifteen patients with symptomatic AS under-

went beating-heart AVB. Electrocardiography-gated two-dimensional phase-contrast velocity mapping CMR im-aging was conducted on each patient. Instantaneous flowwas acquired at discrete intervals within the cardiac cycleand ventricular function and volumes were evaluated.Five age-matched patients without aortic valve diseaseserved as controls.Results. Conduit flow (as a percent of total cardiac

output) was 65% 5%. Ejection fraction was unchanged

compared with before AVB (50% 17% versus 57% 13%; p 0.91). Ventricular volumes and cardiac indiceswere within normal limits and similar to those values incontrols (cardiac index 2.9 1.0 versus 2.3 L/min/m2; p0.26; end-diastolic volume index 59 17 mL versus 55 20 mL; p 0.66; end-systolic volume index, 25 12versus 25 18 mL; p 0.91; stroke volume index, 33 11versus 30 6 mL; p 0.57 for AVB and control patients,

respectively). There was a small degree of retrogradeblood flow in the descending aorta above the level of theconduit insertion (10% 8% of cardiac output).Conclusions. Aortic valve bypass results in a predict-

able blood flow distribution between the native aortaand conduit and is associated with normal ventricularvolumes and function.

(Ann Thorac Surg 2011;92:13328) 2011 by The Society of Thoracic Surgeons

Aortic valve replacement (AVR) improves the qualityof life and extends survival in patients with symp-tomatic aortic stenosis (AS) [1]. AVR requires mediansternotomy, aortic cross-clamping, debridement of thediseased native valve, and cardioplegic cardiac arrest, allof which contribute to the risk of morbidity and mortality[2], particularly in the elderly population and those withsignificant comorbidities [1, 3]. At least 30% of patientswith symptomatic severe AS are not referred for surgicalprocedures because they are deemed too high risk. Leftuntreated, symptomatic AS is associated with dismalsurvival rates [47].

Aortic valve bypass (AVB, apicoaortic conduit) is analternative to conventional AVR for high-risk patientswith AS. AVB is performed on the beating heart through

a small left thoracotomy (Fig 1). AVB effectively relievesleft ventricular outflow obstruction of AS through thecreation of a second left ventricular outflow tract (LVOT)[8, 9]. Although AVB has been performed clinically foralmost 50 years, the relative distribution of blood flowafter the introduction of a second LVOT has been incom-pletely characterized [10].

Cardiovascular magnetic resonance (CMR) imaging isa powerful tool that can precisely image cardiovascular

anatomic structures as well as provide qualitative andquantitative assessment of blood and myocardial motionin vivo [11-14]. We used CMR imaging to characterize thedistribution of thoracic blood flow and ventricular func-

tion in patients after AVB.

Patients and Methods

Patient Selection

A retrospective chart review of all patients who under-went AVB at the University of Maryland Medical Center

was performed. All patients who had postoperative CMRimaging studies were included. Patients received preop-erative and predismissal transthoracic Doppler echocar-

diography in a core laboratory [15]. Elderly control pa-tients were recruited to undergo CMR imaging. Thisstudy was approved by the Institutional Review Board of

the University of Maryland Medical Center, protocolHP-00045095.

Accepted for publication April 18, 2011.

Presented at the Fifty-seventh Annual Meeting of the Southern ThoracicSurgical Association, Orlando, FL, Nov 36, 2010.

Address correspondence to Dr Gammie, University of Maryland Medical

Center, N4W94, 22 S Greene St, Baltimore, MD 21201; e-mail: [email protected].

Dr Gammie discloses that he has a financial relation-ship with Correx, Inc.

2011 by The Society of Thoracic Surgeons 0003-4975/$36.00Published by Elsevier Inc doi:10.1016/j.athoracsur.2011.04.069
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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Operative Technique

Aortic valve bypass conduits consisting of the apical

connector (Hancock Apical Left Ventricle ConnectorModel 174A, Medtronic Inc, Minneapolis, MN), a stent-less porcine valve (Freestyle Aortic Root Bioprosthesis,Medtronic, Inc) and a woven Dacron (polyethylene te-rephthalate) graft were prepared on the back table at thetime of operation. A small left anterior sixth interspacethoracotomy was performed to provide access to boththe apex of the left ventricle and the descending thoracicaorta. The distal anastomosis between the conduit andthe descending aorta was performed with the use of apartial occluding clamp. A stab wound was created 2 cmlateral to the true apex of the left ventricle, followed by acoring procedure in preparation for the apical anastomo-

sis. Once the myocardial plug was removed, the apicalconnector was inserted and tied securely in place [2].

CMR Imaging

Cardiovascular magnetic resonance imaging was per-formed using a 1.5 T Siemens CMR system (SiemensMedical Solutions USA, Inc, Malvern, PA). Through-plane phase-contrast cines were generated by usingbreath-holding, retrospectively electrocardiography-gated two-dimensional phase-contrast velocity mappingof specific positions of the vasculature (Fig 2). Each cinewas composed of 30 images taken over the course of 1cardiac cycle, which were looped to recreate physiologicmotion. Two-dimensional cut planes were manually po-sitioned at the following locations: ascending aorta, 1 cmsuperior to the sinotubular junction; mid-descendingthoracic aorta, 3 cm superior to the conduit-aortic anas-tomosis; distal descending thoracic aorta, 3 cm inferior tothe conduit-aortic anastomosis; conduit, 3 cm distal to theinsertion of the apical connector in the apex.

All CMR cine acquisitions were processed offline usinganalysis software (Argus, Siemens Medical SolutionsUSA, Inc) at a devoted workstation and all analyses werecompleted by the same operator to eliminate variability.Quantitative blood flow and velocity measurements weregenerated for each two-dimensional cut plane by tracingthe endothelial border of the corresponding vessel tocreate a region of interest that encompassed the lumen ofthe vessel.

Left ventricular function was assessed by CMR-derived short-axis cine imaging using steady-state freeprecession technique. Regions of interest were created bytracing the endocardial and epicardial borders of the leftventricle for each patient at end-systole and end-diastole.Analysis software provided quantitative data on leftventricular function and volumes, including end-diastolic volume, end-systolic volume, stroke volume,ejection fraction, and cardiac output. Ventricular functionand volume data were normalized to body surface area.

Statistical Analyses

Statistical analysis was performed with JMP 8.0 (SASInstitute, Cary NC). Distribution of patient characteristicswas summarized with means and standard deviation forcontinuous variables and for categorical variables. Thearea under the curve of mean flows of the distinct pointswere measured using Matlab (version 2009b, Math-Works, Natick, MA). A simple ttest was used to calculatestatistical significance (Table 1).

Fig 1. An aortic valve bypass (apicoaortic conduit).

Fig 2. (A and B) Through-plane phase-contrast images of the ascending aorta.

1333Ann Thorac Surg STAUFFER ET AL2011;92:1332 8 BLOOD FLOW AFTER AORTIC VALVE BYPASS


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Results

Between August 2008 and March 2010, 15 high-risk patientswith symptomatic severe AS who were treated with beat-ing-heart AVB without cardiopulmonary bypass under-went imaging with CMR (Table 2). The mean patient agewas 85 years. Five elderly control patients without aorticvalve disease also underwent imaging; the mean age inthese subjects was 69 years. Consistent with our evolutiontoward the use of progressively smaller apical conduits, 1patient received a 20-mm apical connector,3 patients received18-mm connectors, 1 patient received a 16-mm connector, 4patients received 14-mm connectors, and 6 patients received12-mm connectors. Valve size and Dacron graft size variedwith apical connector size: 19 mm valves and 18 mm Dacron

grafts were used with 12 mm, 14 mm and 16 mm apicalconnectors; 21 mm valves and 20 mm Dacron grafts were usedwith 18 mm apical connectors, and 23 mm valves and 22 mmDacron grafts were used with 20 mm apical connectors.

Twelve patients underwent CMR imaging within 2weeks of operation (mean, 7.6 2.7 days; range, 4 to 14days), 2 patients underwent imaging 2 years after AVB,and 1 patient underwent imaging 5 years after AVB.

In all cases the native aortic valve opened and ante-grade blood flow was observed in the ascending aorta.There was no evidence of important native or conduitvalve insufficiency as evidenced by continuous forwardflow in the ascending aorta and conduits of each patient.

Conduit flow (as a percent of total cardiac output) was65% 5% (Fig 3). There was retrograde blood flow in thedescending aorta above the level of the conduit insertion,although the magnitude of retrograde flow was small(8% 8 of cardiac output) (Fig 3).

Flow in the ascending aorta in control patients and netforward flow (the sum of ascending aortic and conduit flow)in the patients who underwent AVB was nearly identical(AVB flow 107% of control flow; p 0.57) (Fig 4).

Conduit flow did not appear to vary as a function of theapical connector size. There was significant variability inascending aortic blood flow, although there did not appearto be a correlation with conduit size (Fig 5). No left ventric-ular pseudoaneurysms or intraaortic thrombi were noted.

Table 1. Summary and Statistics of AVB VentricularFunction

AVB Control p Value

Mean preoperativeejection fraction

50% ( 17) 59% ( 15) . . .

Mean postoperativeejection fraction 57% (

13) . . . 0.091

End-diastolic volumeindex

59 mL( 17)

55 mL( 20)

0.066

End systolic volumeindex

25 mL( 12)

25 mL( 18)

0.091

Stroke volume index 33 mL( 11)

30 mL ( 6) 0.057

Cardiac index 2.9 L/min/m2 ( 1)

2.3 L/min/m2 ( 1)

0.026

AVB aortic valve bypass.

Table2.PatientDemographicandCMRImagingDerivedVentricularFunction

Treatment

Body

Surface

Area(m2)

Age

Preoperative

AorticValve

Area(cm

2)

CMRImaging

(days

postoperative)

Preoperative

Echo-

d

erived

E

jection

Fraction(%)

Postoperative

CMR

Imaging

derived

Ejection

Fraction(%)

Ejection

Fraction

(%)

End-

d

iastolic

V

olume

Index

End

Systolic

Volume

Index

Systolic

Volume

Index

Total

Cardiac

Output

Cardiac

Index

AVBM

ean(standard

deviation)

2(0.2

)

86(7.4

)

0.6

(0.1

7)

8(510)

51(16)

58(13)

11(11)59(17)

26(12)

33(12)

5.3

3(1.7

9)

2.9

(1.0

2)

Control

Mean

(standard

deviation)

2.0

1(0.2

)

69(5.7

)

...

...

...

59(15)

...

55(20)

25(18)

30(6)

4.6

4(0.8

2)

2.3

4(0.5

4)

AVB

aorticvalvebypass;

CMR

card

iovascularmagneticresonance.

1334 STAUFFER ET AL Ann Thorac SurgBLOOD FLOW AFTER AORTIC VALVE BYPASS 2011;92:1332 8


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Echocardiographically determined preoperative ejec-tion fraction was 51 16 (30% to 75%). Ejection fraction,stroke volume, end-diastolic volume, and end-systolicvolume were within normal limits after AVB and weresimilar to these values in controls (Table 1).

Comment

The principal findings of this investigation include aconsistent split of cardiac output between the conduit

and the native LVOT, preserved ventricular function

after AVB, and ventricular volumes and function thatwere identical to an age-matched control group.

Cardiovascular magnetic resonance imaging is ideallysuited for assessment of the physiologic effects of AVBconduit insertion for a number of reasons. CMR imagingprovides minimally invasive high-resolution, real-timeimages without the use of ionizing radiation or iodinatedcontrast media [16, 17]. In addition CMR imaging pro-vides precise quantitation of blood flow that is not

available with CT or echocardiography.

Fig 3. Thoracic blood flow in patients whounderwent AVB operation (n 15). (AVB aortic valve bypass; BSA body surfacearea.)

Fig 4. Distribution of thoracic blood flow; col-ored lines represent control patients (n 5)and black line variants are patients who un-derwent AVB operation (n 15). (AVB aortic valve bypass; BSA body surfacearea.)



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Aortic valve bypass conduit insertion resulted in con-sistent blood flow split between the conduit and theascending aorta. We found that 65% of ventricular out-flow was directed through the valved conduit and 35%through the native aortic valve. This is consistent withprevious echocardiographic data obtained by our group[15]. The range of relative conduit flow was small andpredictable and did not seem to vary as a function of theconduit size. Total stroke volume in patients who haveundergone AVB, as computed by both area under thecurve (Fig 3) and CMR imaging derived stroke volumes(Table 1) was equivalent to that in control patients. Al-though some degree of retrograde blood flow was found inthe region of the descending aorta superior to the distalanastomosis, this represented a relatively small percentage

of the total cardiac output and was significantly smallerthan the amount of forward flow in the ascending aorta.These data suggest that all cerebral blood flow to the brainafter insertion of an AVB conduit is derived from antegradeblood flow across the native valve rather than retrogradeflow through the conduit. These findings are consistentwith previous results of computational modeling of AVBfluid dynamics, which demonstrated the presence of asmall degree of retrograde flow in the descending aortaabove the insertion of the valved conduit for larger diame-ter conduits [8]. In that study all cerebral blood flow arosefrom antegrade flow in the ascending aorta. Importantly,the magnitude of cerebral blood flow was unchanged

before and after insertion of the AVB conduit.We also confirmed preserved left ventricular functionwith no significant change compared with controls after theoperation despite the removal of the apical myocardial plugand insertion of a rigid connector into the left ventricularapex (Table 1). Although others have reported thrombusformation with AVB [18], we did not see evidence of this inthe conduit, native aorta, or ventricle in any patient.

In summary, AVB resulted in a consistent split of flowbetween the native LVOT and the AVB conduit. AVBeffectively treats AS without cardiopulmonary bypass,cardioplegic cardiac arrest, or manipulation of the nativevalve or the ascending aorta, and is associated withnormal postoperative ventricular function and sizes.

References

1. Varadarajan P, Kapoor N, Bansal RC, Pai RG. Survival inelderly patients with severe aortic stenosis is dramaticallyimproved by aortic valve replacement: Results from a cohortof 277 patients aged or 80 years. Eur J Cardiothorac Surg2006;30:7227.

2. Gammie JS, Krowsoski LS, Brown JM, et al. Aortic valvebypass surgery: midterm clinical outcomes in a high-riskaortic stenosis population. Circulation 2008;118:14606.

3. Bloomstein LZ, Gielchinsky I, Bernstein AD, et al. Aorticvalve replacement in geriatric patients: determinants ofin-hospital mortality. Ann Thorac Surg 2001;71:597600.

4. Turina J, Hess O, Sepulcri F, Krayenbuehl HP. Spontaneouscourse of aortic valve disease. Eur Heart J 1987;8:47183.

5. Iivanainen AM, Lindroos M, Tilvis R, Heikkila J, Kupari M.Natural history of aortic valve stenosis of varying severity inthe elderly. Am J Cardiol 1996;78:97101.

6. Pai RG, Kapoor R, Bansal RC, Varadarajan P. Malignantnatural history of asymptomatic severe aortic stenosis: ben-efit of aortic valve replacement. Ann Thorac Surg 2006;82:211622.

7. Christensen KL, Ivarsen HR, Thuesen L, Kristensen B.Aortic valve stenosis: fatal natural history despite normal leftventricular function and low invasive peak-to-peak pressuregradients. Cardiology 2004;102:14751.

8. Balaras E, Cha KS, Griffith BP, Gammie JS. Treatment ofaortic stenosis with aortic valve bypass (apicoaortic conduit)surgery: an assessment using computational modeling.

J Thorac Cardiovasc Surg 2009;137:680 7.9. Brown JW, Gammie JS. Off pump aortic valve bypass using

a valved apicalaortic conduit. Operat Tech Thorac Cardio-vasc Surg 2007;12:8594.

10. Gammie JS, Brown JW, Brown JM, et al Aortic valve bypassfor the high-risk patient with aortic stenosis. Ann Thorac

Surg 2006; 81:160510.11. Bogren HG, Mohiaddin RH, Yang GZ, Kilner PJ, Firmin DN.

Magnetic resonance velocity vector mapping of blood flowin thoracic aortic aneurysms and grafts. J Thorac CardiovascSurg 1995;110:70414.

12. Kilner PJ, Yang GZ, Mohiaddin RH, Firmin DH, LongmoreDB, et al. Helical andretrograde secondary flowpatterns in theaortic arch studied by three-directional magnetic resonancevelocity mapping. Circulation 1993;88(5 Pt 1):223547.

13. Markl M, Harloff A, Bley TA, et al. Time-resolved 3D MRvelocity mapping at 3T: improved navigator-gated assess-ment of vascular anatomy and blood flow. J Magn ResonImaging 2007;25:82431.

14. Pennell DJ, Sechtem UP, Higgins CP, et al. Clinical indica-tions for cardiovascular magnetic resonance (CMR): Consen-sus Panel Report. J Cardiovasc Magn Reson 2004;6:72765.

Fig 5. Flow assessed as a function of apicalconnector diameter. (A) Ascending aorta; (B)conduit. (BSA body surface area.)



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15. Vliek CJ, Balaras E, Li S, et al. Early and midterm hemody-namics after aortic valve bypass (apicoaortic conduit) sur-gery. Ann Thorac Surg 90:136 43.

16. Kumar A, Patton DJ, Friedrich MG. The emerging clinicalrole of cardiovascular magnetic resonance imaging. Can

J Cardiol 26:31322.

17. Kirchin MA, Runge VM. Contrast agents for magnetic reso-nance imaging: safety update. Top Magn Reson Imaging2003;14:42635.

18. Parsa CJ, Milano CA, Proia AD, Mackensen GB, Hughes GC. Apreviously unreported complication of apicoaortic conduit forsevere aortic stenosis Ann Thorac Surg 2009; 87:9278.

DISCUSSION

DR VINOD THOURANI (Atlanta, GA): That was an excellentpresentation. I congratulate you, especially at your level as athird-year med student to present so eloquently. I havefollowed your work also at Maryland closely since we also atEmory are one of the sites that are performing aortic valvebypass in those patients which we can not enroll in thetranscatheter valve program. In these high-risk patients, Ithink that this surgery is a viable option such that youperform a thoracotomy so as to avoid a redo-median sternot-omy and avoid previous coronary bypass grafts or the possi-bility of patient-prosthesis mismatch.

The question I have for you is, I am currently using an 18Medtronic Hancock conduit with a 21 Medtronic freestyle rootinterposed in the conduit towards the elbow portion closer to theleft ventricle. Thus far, we have not switched to a smallerconduit. What valve do you use for the 14 conduit?

MR STAUFFER: We use a 19 mm stentless (Freestyle) valve withthe 14 mm as well as with the 12 mm connectors.

DR JOHN BROWN (Indianapolis, IN): Craig, that was anexcellent presentation. It has been gratifying for me to see DrGammie and your institution add some real science to thisprocedure. This is an operation I have been doing for 35 yearsand have done more than 100 patients right now, and I find itvery useful in that patient population where going back into the

sternum is unattractive for the patient and unattractive for thesurgeon.Where does this operation fit, in your opinion, and either you

or Jim can address this, now that transcatheter aortic valveimplantation (TAVI) is available? Which patients should beoffered the aortic valve replacement (AVR) operation? It sort ofgets into the debate that we heard earlier this morning. Whenthis operation is available in certain centers, when should it beoffered to patients as opposed to other modalities to treat aorticstenosis in this high-risk patient population?

MR STAUFFER: We believe that aortic valve bypass (AVB)offers a number of advantages that make it a more attractivealternative for patients who are either unable or unsuitable forAVR.

As I presented, AVB can be performed on the beating heart,making this a great alternative for patients who are unable toundergo cardiopulmonary bypass or cardioplegic arrest. AVBalso has the potential to decrease the risk of perioperative andpostoperative strokes as there is no manipulation of the ascend-ing aorta or native valve during AVB.

DR THOURANI: John, I just want to comment on it a little bit,because at Emory we have been lucky somewhat to be able tohave TAVI and AVB at the same time. So during that same timeperiod we have had about 100 transcatheter valve patientsimplanted, roughly, and we have had about 2025 apical con-duits performed. In our high-risk aortic stenosis algorithm, ourfirst choice has been to go to TAVI; ither transfemoral ortransapical. The patients overall seem to tolerate the small

femoral incision and mini-thoracotomy without the need forcardiopulmonary bypass extremely well.

For the future, if the patient can not have TAVI, most likely anAVB is the most reasonable procedure of choice in that patientcohort. It will also be dependent on the indications approved bythe FDA for TAVI and also what CMS will pay for in terms ofoff-label use of TAVI. Again, I want to stress that this is just mypersonal experience over the past two-and-a-half-year timeperiod having both procedures available to me.

DR GAMMIE: John, I would like to thank you for that excellentquestion as well as your mentorship. I learned this operationfrom you, and it has been an absolute pleasure collaboratingwith you over the last six or seven years.

I would like to point out that in the PARTNER trial there wasa 5% risk of stroke, and our aggregate experience at Indiana andMaryland has shown a very low risk of stroke with aortic valvebypass, and I think that is a key differentiator. When you do anAVB, you do not mess with the ascending aorta, you are notblowing up a balloon in a stenotic valve, and, I think we saw apresentation here yesterday demonstrating that there is a sig-nificant embolic load with TAVI. We dont know the durabilityof a catheter-based stent-mounted valve, and, to the contrary,with aortic valve bypass, John, you have three patients walkingaround now more than a quarter of a century after an aorticvalve bypass operation.

And we can frankly now do this operation through a verysmall incision. Several weeks ago we did our first robotic aorticvalve bypass where we accomplished the distal anastomosiswith a robot, which enabled us to do aortic valve bypass througha very small incision.

And the final point I would make, particularly with one of thestent-mounted valves, there is a very high incidence of pace-maker implantation after surgery, and for obvious reasons, wehave never had to implant a pacemaker after this operation. SoI think there is an awful lot of enthusiasm about TAVI, but weremain optimistic that AVB may offers some significant benefitscompared to that, and with additional instrumentation, we thinkthat we will be able to accomplish this operation in a trulyminimally invasive fashion.

And I also want to congratulate Craig on a superb effort and

presentation. Thanks.

DR THORALF SUNDT (Rochester, MN): If you have got timefor one more question, and, Jim, you may want to answer this, Iam trying to see where this approach fits vis--vis TAVI. As welook at those patients proposed for entry into the trials, there arereally two reasons, I think, to go to TAVI. One is that it is atechnically difficult operation to do it the conventional way. Forexample porcelain aorta is a good reason to go to TAVI. And yetporcelain aorta really is the reason that we at Mayo have mostoften used the apicoaortic conduit. I agree is a neat operationand I like it too. So for that indication, clearly these two arecompeting approaches. But the other reason that we go toTAVI is the perioperative risk of conventional operation; TAVI ismeant to be a lower risk option than a conventional operation. Do



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you think that is also true for apico-aortic conduit? Is it a lower-riskapproach? For example, I have got a patient with cirrhosis to dowhen I get back that was turned down by the committee for thePARTNER trial because on the one hand his predicted risk is toolow, and yet his one-year mortality is too high. So hedoesnt fit intothe PARTNER trial. So I have to choose the operation that is goingto be the lowest perioperative risk for him. Do you think I shoulddo him through a median sternotomy the regular way or should Ido an apico-aortic conduit? Which will have a lower perioperativerisk? Does that question make sense?

DR GAMMIE: I think the advantage of AVB in that case is thatyou avoid a couple of hour bypass run, and I think in a patientwith cirrhosis, that is a significant advantage. But this is still anoperation. And the one thing that we have learned, certainly inthe frail patient that cant give you a firm handshake and thathas lost 20 pounds, they wont do well with any intervention thatyou do. But in the case of your patient, I would absolutely choosethe aortic valve bypass.

DR SUNDT: Thanks.

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2011 by The Society of Thoracic Surgeons Ann Thorac Surg 2011;92:1338 0003-4975/$36.00Published by Elsevier Inc
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