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Interact CardioVasc Thorac Surg 2008;7:1-5. doi:10.1510/icvts.2007.162412
© 2008 European Association of Cardio-Thoracic Surgery
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Work in progress report - Congenital

Comparative computational fluid dynamic study of two distal Contegra conduit anastomoses{star}

Antonio F. Cornoa,* and Elizabeth S. Mickaily-Huberb

a Alder Hey Children Hospital, Eaton Road, Liverpool, L12 2AP, UK
b CFS Engineering, Lausanne, Switzerland

Received 2 July 2007; received in revised form 4 September 2007; accepted 5 September 2007

{star} The authors acknowledge Medtronic Ltd., UK for funding of the CFD study.

Freedom of investigation: The authors had full control of the design of the study, methods used, outcome measurements, analysis of data, and production of the written report.

*Corresponding author. Tel.: +44-151-2525713; fax: +44-151-2525643.

E-mail address: Antonio.Corno{at}rlc.nhs.uk (A.F. Corno).


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 Acknowledgements
 References
 
A computational fluid dynamic (CFD) study compared two configurations of distal anastomosis of 12 mm Contegra® conduit: conventional ‘circular’ vs. oblique ‘elliptical’ anastomosis extended on the left PA, evaluating pressure and velocity profiles, and shear stress, from PA origin to the bifurcation. ‘Elliptical’ anastomosis provides larger (=54% difference) cross-sectional area than ‘circular’ anastomosis. Velocity contours showed important stagnation at PA bifurcation in ‘circular’ anastomosis and minimal in ‘elliptical’ configuration, where fluid flow occurred preferentially in left PA. Pressure contours showed peak pressure zone at bifurcation in ‘circular’ anastomosis, while ‘elliptical’ exhibited more uniform pressure distribution. Shear stress distribution was more homogeneous in ‘elliptical’ than in ‘circular’ anastomosis. At bifurcation and in right PA artery velocity and pressure were higher for ‘circular’ than ‘elliptical’ anastomosis, while in left PA velocity was much higher for ‘elliptical’ anastomosis. CFD study demonstrates more homogeneous pressure, velocity and shear stress distributions for ‘elliptical’ compared to ‘circular’ anastomosis at PA bifurcation, and preferential flow in left PA. CFD results suggest that clinical application of ‘elliptical’ anastomosis, with cross-sectional area larger than conventional ‘circular’ anastomosis, may reduce incidence and degree of distal stenosis, particularly for small size conduits.

Key Words: Biological valved conduit; Computational fluid dynamics; Congenital heart disease; Distal stenosis; Right ventricular outflow tract


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 Acknowledgements
 References
 
Contegra® (Medtronic Inc., Minneapolis, MN) conduit provided encouraging results as right ventricular outflow tract reconstruction in complex congenital heart defects [1–8] and as pulmonary valve replacement during Ross operation [1, 2, 4, 6–8].

A disturbing sequence of publications recently reported a variable incidence of stenosis at level of conduit distal anastomosis [6–12].

To investigate the potential role of the surgical technique in the occurrence of distal stenosis, a CFD study has been designed to compare two different configurations of distal anastomosis of the conduit.

The working hypothesis was that ‘elliptical’ configuration provides a larger connection than conventional ‘circular’ anastomosis, with a lower resistance to flow, and thereby provides beneficial effects towards reduction of incidence and degree of distal stenosis.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Results
 4. Discussion
 5. Conclusions
 Acknowledgements
 References
 
A 12 mm Contegra® conduit has been used as the model for a CFD study, because the literature indicates late stenosis particularly for smaller Contegra® diameters [5–11].

Two types of distal anastomosis of conduit to PA have been evaluated (Fig. 1):

  1. conventional end-to-end ‘circular’ anastomosis
  2. oblique ‘elliptical’ anastomosis, with incision extended on anterior aspect of left PA, and an oblique tailoring of the distal end of conduit


Figure 1
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  		Fig. 1. Schematic representation of the ‘circular’ and of the ‘elliptical’ configuration of the distal anastomosis.

 
A cross-sectional area obtained with two surgical techniques was measured at location of anastomosis of conduit to PA using a grid overlaid on cross sections taken at precisely the same planar cut for both configurations.

2.1. Computational fluid dynamics (CFD) model

A geometrical model for PA was designed with proximal section at level of pulmonary valve, and distal section taken upstream with respect to PA bifurcation.

Our overall geometrical representation of normal PA bifurcation is based on a previously reported study [13].

Blood was modeled as Newtonian fluid having viscosity= 4x10–3 Pa·s and density=1060 kg/m3. Assumed flow rate was 1.5 l/min, corresponding to laminar Reynolds number of 700. A steady-state, with fully open pulmonary valve, was considered as first-step to solving a complex problem. Calculations were performed using a respected code NSMB. Both geometrical domains were discretized into 548288 finite-volume cells, with 24 blocks for ‘circular’ type and 38 blocks for ‘elliptic’ type. Implicit time scheme was implemented with 4th-order central scheme in space. No-slip boundary condition was used on artery walls, inlet velocity at PA trunk was set at 0.22 m/s corresponding to required flow rate, and atmospheric pressure condition was set at artery outlet regions allowing pressure to develop naturally in flow field. All calculations were made on PC cluster composed of four Pentium 4, 3.6 GHz, and processors having 3 Gigabytes of memory each. Convergence was ascertained when the L2-residues of pressure and velocity solutions were stabilized, corresponding to about 20,000 n-step iterations.

CFD model evaluated pressure and velocity profiles, and shear stress. Pressure and velocity values obtained have been compared along three arbitrary lines, respectively, from origin of main PA to its bifurcation (line A), and downstream at outlet of right (line B) and left (line C) PA (Fig. 2). Values of pressures (with respect to outlet value) and velocity taken at fixed points of 5 mm intervals for line A (PA bifurcation), and 2 mm intervals for lines B (right PA) and C (left PA), have been compared to evaluate percentage difference between the two configurations.


Figure 2
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  		Fig. 2. Three arbitrary lines, respectively from the origin of the main pulmonary artery to the pulmonary arteries bifurcation (line A), and downstream at the outlet of the right (line B) and left (line C) pulmonary artery, to compare pressure and velocity values obtained for the two techniques of anastomosis.

 

   3. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
4. Discussion
 5. Conclusions
 Acknowledgements
 References
 
A cross-sectional area at location of anastomosis measured 110 mm2 for ‘circular’ type of anastomosis and 169 mm2 for ‘elliptical’, with a difference=54% in favour of ‘elliptical’ configuration.

3.1. Computational fluid dynamics (CFD) model

Comparison of velocity contours showed stagnation at PA bifurcation in both configurations: navy blue recirculation zones, well evident in ‘circular’ type of anastomosis, were minimized in ‘elliptical’ type, where fluid flow occurred preferentially in left PA. A comparison of pressure contours showed a peak pressure zone at bifurcation in ‘circular’ type of anastomosis, while ‘elliptical’ geometry exhibited more uniform pressure distribution. Shear stress evaluation demonstrated zones of high stress at anastomosis and around PA bifurcation in both techniques. Nevertheless, shear stress distribution, as well as all other parameters, was more homogeneous in ‘elliptical’ type of anastomosis than in ‘circular’ geometry (Fig. 3).


Figure 3
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  		Fig. 3. Comparison of velocity contours, pressure contours, and shear stress contours.

 
On line A (PA bifurcation) values of velocity were higher for ‘circular’ than for ‘elliptical’ configuration, as well as values of pressure, with exception of last corresponding to site of conduit anastomosis. On line B (right PA) values of velocity and pressure drop were higher for ‘circular’ than for ‘elliptical’ anastomosis. On line C (left PA) values of pressure drop were higher for ‘circular’ than for ‘elliptical’ anastomosis almost everywhere, while values of velocity were much higher for ‘elliptical’ anastomosis because of preferential flow condition created by this geometry (Fig. 4).


Figure 4
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  		Fig. 4. Values of velocity and pressure for the line A (pulmonary artery bifurcation), for the line B (right pulmonary artery), and for the line C (left pulmonary artery).

 
Calculated distribution of total pulmonary flow (=1.488 l/min) between right and left PA was the following:
–in ‘circular’ configuration 0.636 l/min (=42.7%) for right PA and 0.852 l/min (=57.3%) for left PA
–in ‘elliptical’ configuration 0.606 l/min (=40.7%) for right PA and 0.882 l/min (=59.3%) for left PA

Calculated difference failed to reach statistical significance.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
5. Conclusions
 Acknowledgements
 References
 
The introduction of Contegra® as an alternative to homografts has been supported by several proven advantages [1–8].

The most frequently reported complications have been thrombus, particularly in correspondence of the valve [4, 12] and premature valve incompetence, generally correlated with elevated PA pressures and resistance [4–6, 11, 12].

The most disturbing complication, with a reported incidence variable between 6% and 50%, has been stenosis at distal anastomosis, with proximal conduit dilatation, and occasionally formation of aneurysm or pseudo-aneurysm, particularly in infants requiring small size conduits and after repair of truncus arteriosus [6–12].

The increasing rate of stenosis at distal anastomosis has also been observed as a progressive problem in time [5, 6, 9, 11, 12]. This observation contrasts with other studies reporting mild pressure gradients remaining stable during follow-up [2, 3], or increasing only in younger patients with small size conduits [5, 6, 8].

4.1. Potential mechanisms of distal stenosis

  1. Presence of hypoplasia and/or distal stenosis of PA branches
    Establishment of normal pulmonary blood flow can unmask the presence of peripheral stenosis [5, discussion of 6, 7, 8, 11]
  2. Discrepancy in size between conduit and PA
    Patient–conduit mismatch definitely has a role in occurrence of distal stenosis, particularly in infants with relatively small PAs [5–7, 11].
  3. Type of surgical technique
    Potential surgical reasons responsible for inducing distal stenosis are tailoring of distal end of conduit, incision on PA, suture technique with purse-stringing running sutures, and excessive length of conduit creating a kinking at distal anastomosis [3, discussion of 6, 7, discussion of 8, 1].
  4. Local immunologic/inflammatory reaction
    This mechanism has been denied by observers who at re-operations have found absent active inflammatory reaction, valve leaflets remaining pliable, and integrity of Contegra® conduit [6, 8]. Other investigators have instead described local immunologic reaction [discussion of 6, 14], with peri-adventitial inflammation [14].
  5. Local peel formation
    Peel formation has been reported in correspondence of distal anastomosis, very similar to neo-intima found in vascular grafts, with excessive intimal peel formation and severe peri-graft scarring reaction [6, 9, 11, 12].
  6. Thrombosis
    Thrombus formation has been considered as one of the factors potentially responsible for distal stenosis, particularly in the absence of anti-platelet and/or anticoagulant treatments [6–8] or after inadequate rinsing of glutaraldehyde used for conduit preservation [discussion of 8].
  7. Combination of two or more of the above
    Multi-factorial cause of stenosis in correspondence of distal anastomosis is of course the most appealing hypothesis.

4.2. Surgical technique

The surgical technique has been taken into consideration since our first report of Contegra® as pulmonary valve replacement during Ross operation [1]. Our later study with multi-gated CT-scan showed that values of diameters of implanted Contegra® conduit at level of proximal and distal anastomosis, as well as valve, remained unchanged during follow-up [8]. Disturbingly, pressure gradients recorded in these patients were all at the location of distal anastomosis, but without increase during follow-up [8].

After these observations a different surgical technique has been introduced, following the same principle applied in the repair of tetralogy of Fallot where a trans-annular patch is required, tailoring the distal end of Contegra® conduit in oblique fashion, and opening the PA with an incision extended into left PA. The resulting end-to-end connection, instead of being ‘circular’ like in the conventional technique, becomes ‘elliptical’ (Fig. 1).

Not only the advantages of ‘elliptical’ vs. ‘circular’ configuration should be intuitive, but should repeat the clinical experiences obtained with vascular anastomosis in general. In fact the measurement of the cross-sectional areas confirmed that ‘elliptical’ configuration provides a substantially larger (=54% difference) cross-sectional area than conventional ‘circular’ anastomosis.

In our CFD model, designed following previous studies on distribution of pulmonary blood flow [13, 15], the evaluation of pressure, velocity, and shear stress contours show more homogeneous flow distributions in ‘elliptical’ than in ‘circular’ type of anastomosis (Fig. 3). The comparison of curves of pressure and velocity also shows more favourable flow distribution for ‘elliptical’ than for ‘circular’ type of anastomosis, with preferential flow in left PA (Fig. 4). Despite preferential flow towards left PA observed with ‘elliptical’ anastomosis, flow distribution remained quite homogeneous, without any statistically significant difference compared to ‘circular’ anastomosis.

4.3. Limits of the study

  1. Single size conduit (12 mm) with similar size (12 mm) of main PA has been used. Occurrence of distal stenosis has been reported more frequently in small conduits, and discrepancy in size between conduit and PA has already been proved to be one of the factors predisposing distal stenosis.
  2. Fixed pulmonary blood flow (1.5 l/min) has been considered. We have calculated the highest flow possible in an infant suitable for 12 mm conduit implantation.
  3. Only end-to-end types of surgical techniques have been studied. The end-to-side type of anastomosis has been excluded not to complicate the calculations because of simultaneous presence of antegrade pulmonary blood flow.
  4. Steady-state calculations were performed assuming fully open pulmonary valve. Our study did not account for pulsatile nature of flow or for difference in compliance between graft and native tissue, because it constitutes first-step CFD approach to a complex problem. The potential effect of these variables would equally affect both configurations.
  5. The current study makes no attempt to account for tissue variation due to grafting, as this would require Fluid Structure Interaction study, substantially more complex and computation intensive.
  6. The problem of Contegra® distal stenosis generally occurs as late complication, while this is an acute study. We might speculate that creating a larger and hemodynamically more favourable anastomosis could reduce the incidence and severity of this late complication, independently from the problem of late occurrence of fibro-intimal hyperplasia or peel formation.
  7. To date clinical validation has not been yet reported. After having seen the CFD and clinical advantages of the ‘elliptical’ technique, we are not considering a return to ‘circular’ anastomosis for a prospective clinical comparison.


   5. Conclusions
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
Acknowledgements
 References
 
Our CFD study demonstrates more homogeneous velocity, pressure and shear stress distributions for ‘elliptical’ than for ‘circular’ type of anastomosis at PA bifurcation, with flow preferentially occurring in left PA. This induced homogeneity of flow precludes substantial flow recirculation which is conducive to stagnation and thus, by extrapolation, to distal stenotic formation.

These CFD results provide preliminary proof that clinical application of the ‘elliptical’ anastomosis with larger cross-sectional area may reduce the incidence and degree of distal stenosis, particularly for small size conduits.


   Acknowledgements
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 Acknowledgements
References
 
The authors acknowledge Benoît Pigneur (Institut Supérieur Industriel, Brussels, Belgium) for the initial conceptualization of the geometries using CATIA, Roland von Kaenel and Hélène Zipper (CFS Engineering, Lausanne, Switzerland) for meshing, and Thomas Ludwig (SMR, Bienne, Switzerland) for technical assistance.


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 Acknowledgements
 References
 

  1. Corno AF, Hurni M, Griffin H, Jeanrenaud X, von Segesser LK. Glutaraldehyde-fixed bovine jugular vein as a substitute for the pulmonary valve in the Ross operation. J Thorac Cardiovasc Surg 2001; 122:493–494.[Free Full Text]
  2. Corno AF, Hurni M, Griffin H, Galal OM, Payot M, Sekarski N, Tozzi P, von Segesser LK. Bovine jugular vein as right ventricle-to-pulmonary artery valved conduit. J Heart Valve Dis 2002; 11:242–247.[Medline]
  3. Breymann T, Thies WR, Boething D, Goerg R, Blanz U, Koerfer R. Bovine valved venous xenografts for RVOT reconstruction: results after 71 implantations. Eur J Cardiothorac Surg 2002; 21:703–710.[Abstract/Free Full Text]
  4. Tiete AR, Sachweh JS, Roemer U, Kozlik-Feldmann R, Reichart B, Daebritz SH. Right ventricular outflow tract reconstruction with the Contegra bovine jugular vein conduit: a word of caution. Ann Thorac Surg 2004; 77:2151–2156.[Abstract/Free Full Text]
  5. Karamlou T, Blackstone EH, Hawkins JA, Jacobs ML, Kanter KR, Brown JW, Mavroudis C, Caldarone CA, Williams WG, McCrindle BW. Can pulmonary conduit dysfunction and failure be reduced in infants and children less than age 2 years at initial implantation. J Thorac Cardiovasc Surg 2006; 132:829–838.[Abstract/Free Full Text]
  6. Meyns B, van Garsse L, Boshoff D, Eyskens B, Mertens L, Gewillig M, Fieuws S, Verbeken E, Daenen W. The Contegra conduit in the right ventricular outflow tract induces supravalvular stenosis. J Thorac Cardiovasc Surg 2004; 128:834–840.[Abstract/Free Full Text]
  7. Brown JW, Ruzmetov M, Rodefeld MD, Vijay P, Darragh RK. Valved bovine jugular vein conduits for right ventricular outflow tract reconstruction in children: an attractive alternative to pulmonary homograft. Ann Thorac Surg 2006; 82:909–916.[Abstract/Free Full Text]
  8. Corno AF, Qanadli SD, Sekarski N, Artemisia S, Hurni M, Tozzi P, von Segesser LK. Bovine valved xenograft in pulmonary position: medium-term follow-up with excellent hemodynamics and freedom from calcifications. Ann Thorac Surg 2004; 78:1382–1388.[Abstract/Free Full Text]
  9. Göber V, Berdat P, Pavlovic M, Pfammatter JP, Carrel TP. Adverse mid-term outcome following RVOT reconstruction using the Contegra valved bovine jugular vein. Ann Thorac Surg 2005; 79:625–631.[Abstract/Free Full Text]
  10. Kadner A, Dave H, Stallmach T, Turina M, Prêtre R. Formation of a stenotic fibrotic membrane at the distal anastomosis of bovine jugular vein grafts (Contegra) after right ventricular outflow tract reconstruction. J Thorac Cardiovasc Surg 2004; 127:285–286.[Free Full Text]
  11. Rastan AJ, Walther T, Daehnert I, Hambsch J, Mohr FW, Janousek J, Kostelka M. Bovine jugular vein conduit for right ventricular outflow tract reconstruction: evaluation of risk factors for mid-term outcome. Ann Thorac Surg 2006; 82:1308–1315.[Abstract/Free Full Text]
  12. Shebani SO, McGuirk S, Beghai M, Stickley J, De Giovanni JV, Bu'lock FA, Barron DJ, Brawn WJ. Right ventricular outflow tract reconstruction using Contegra valved conduit: natural history and conduit performance under pressure. Eur J Cardiothorac Surg 2006; 29:397–405.[Abstract/Free Full Text]
  13. Tang T, Chiu IS, Chen HC, Cheng KY, Chen SJ. Comparison of pulmonary arterial flow phenomena in spiral and Lecompte models by computational fluid dynamics. J Thorac Cardiovasc Surg 2001; 122:529–534.[Abstract/Free Full Text]
  14. Bottio T, Vida VL, Stellin G, Thiene G, Casarotto D, Gerosa G. Expected freedom from structural degeneration and patient outgrowth for the bovine jugular vein conduit: is it possible to calculate a safe rate for children. Ann Thorac Surg 2003; 76:2167–2168.[Free Full Text]
  15. Corno AF, Prosi M, Fridez P, Zunino P, Quarteroni A, von Segesser LK. The non-circular shape of FloWatch®-PAB prevents the need for pulmonary artery reconstruction after banding. Computational fluid dynamics and clinical correlations. Eur J Cardiothorac Surg 2006; 29:93–99.[Abstract/Free Full Text]




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