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Commentary: How to achieve perfection

  • Christoph Haller
    Correspondence
    Address for reprints: Christoph Haller, MD, Division of Cardiovascular Surgery, The Hospital for Sick Children, 555 University Ave, Toronto, Ontario, Canada, M5G 1X8.
    Affiliations
    Department of Cardiovascular Surgery, The Labatt Family Heart Centre, The Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada
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  • Shi-Joon Yoo
    Affiliations
    Department of Diagnostic Imaging and The Labatt Family Heart Centre, The Hospital for Sick Children, University of Toronto, Toronto, Canada
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      Aortic arch reconstruction is a key element of stage I palliation. We still struggle to define the ideal aortic geometry, its effect on outcome, and how to reliably achieve these characteristics.
      Figure thumbnail fx1
      3D MRI reconstruction of a near-ideal neoaorta in a patient after Fontan completion.
      See Article page 1791.
      An abundance of literature has been published on the Norwood type aortic arch reconstruction in hypoplastic left heart syndrome (HLHS). Many surgical techniques have been described, all claiming improvements of either neoaortic flow pattern, risk of aortic arch obstruction, recoarctation and pulmonary artery compromise, avoidance of nonautologous patch material, surgical convenience, or other aspects of the procedure.
      • Hasegawa T.
      • Oshima Y.
      • Maruo A.
      • Matsuhisa H.
      • Tanaka A.
      • Noda R.
      • et al.
      Aortic arch geometry after the Norwood procedure: the value of arch angle augmentation.
      • Haller C.
      • Chetan D.
      • Saedi A.
      • Parker R.
      • Van Arsdell G.S.
      • Caldarone C.A.
      • et al.
      Geometry of the aortic arch after initial hybrid or Norwood palliation.
      • Burkhart H.M.
      • Ashburn D.A.
      • Konstantinov I.E.
      • De Oliveira N.C.
      • Benson L.
      • Williams W.G.
      • et al.
      Interdigitating arch reconstruction eliminates recurrent coarctation after the Norwood procedure.
      • Haller C.
      • Chetan D.
      • Saedi A.
      • Parker R.
      • Van Arsdell G.S.
      • Honjo O.
      Geometry and growth of the reconstructed aorta in patients with hypoplastic left heart syndrome and variants.
      It is increasingly clear that despite the wealth of data, we are still struggling to objectify what seems obvious: The ideal neoaortic arch after the Norwood procedure should have smooth tapering and near-homogenous wall properties while avoiding unnecessary angulation and leaving sufficient space for the branch pulmonary arteries and bronchi.
      In this issue of the Journal, Schäfer and colleagues
      • Schäfer M.
      • Di Maria M.V.
      • Jaggers J.
      • Stone M.L.
      • Ivy D.D.
      • Barker A.J.
      • et al.
      High-degree Norwood neoaortic tapering is associated with abnormal flow conduction and elevated flow-mediated energy loss..
      report 4-dimensional flow magnetic resonance imaging (MRI) results in patients with HLHS after Norwood aortic arch reconstruction. They analyzed flow patterns with regard to the neoaortic arch geometry and found that a group with greater degree of tapering—defined as a larger variance in neoaortic diameters across the thoracic aorta—was associated with differences in energy loss, even when indexed to stroke volume, cardiac index, or right ventricular power. This was confirmed in segments of the aorta and across the whole thoracic aorta, respectively. Furthermore, the authors found a correlation of larger aortic arch dimensions as well as correlation of the degree of tapering with energy loss.
      The authors use an elegant approach at defining the hemodynamic impact of the reconstructed aortic arch after the Norwood procedure. As opposed to purely geometric assessments, 4-dimensinal flow MRI measurements offer a comprehensive analysis of the flow dynamics, energy loss, and wall stress resulting from the abnormal shape. Unfortunately, describing the complex 3-dimensional geometry of the aortic arch remains challenging. In particular, abrupt changes of the aortic diameter, folds, kinks, and general wall irregularities are not accounted for with the 4 measurements of the ascending aorta, transverse arch, isthmus, and descending aorta. These uncaptured morphologic characteristics, however, may be much more important determinants of disturbed blood flow and associated energy loss. A previous analysis of ours showed that proximal kinking or folding accounted for as many reinterventions as distal recoarctation.
      • Haller C.
      • Chetan D.
      • Saedi A.
      • Parker R.
      • Van Arsdell G.S.
      • Honjo O.
      Geometry and growth of the reconstructed aorta in patients with hypoplastic left heart syndrome and variants.
      Furthermore, the aortic tapering and its association with energy loss described by the authors is affected by the inhomogeneous patient population. Patients in the group with higher arch tapering (H) had larger body surface area compared with patients with lesser tapering (L). This is likely primarily due to the standard deviation being used for grouping, as it is based on absolute aortic diameter measurements, which tend to show larger discrepancy across measurements at later stages of palliation. Indexing to stroke volume or cardiac index is not ideal either, as hearts of patients in interstage I were generating output for an in-parallel circulation as opposed to patients after stage II. Assuming that the stroke volume in group H was greater compared with patients in group L and considering that the in-parallel circulation of some patients in group L suggests that the blood volume delivered to the systemic circulation was lower than the actual stroke volume, the differences in energy loss indexed by stroke volume could be misleading. As opposed to the described increased absolute energy loss and increased stroke volume adjusted energy loss in group H, the energy loss adjusted to cardiac index was actually lower in that group. Whether the relatively small differences in energy loss are ultimately of clinical importance can be questioned as well.
      Despite these limitations, the study by Schäfer and colleagues highlights the importance of the aortic arch reconstruction in HLHS. The complexity of the reconstructed aortic arch geometry and the influence of a multitude of factors, such as ventricular function, size of ventricular outflow, compliance of native and reconstructed neoaortic tissue, etc, are further complicating a clear definition and analysis of the ideal neoaortic arch. In addition, even if we succeeded in defining the perfect aortic arch dimensions in each individual patient with HLHS, it will remain a challenge to achieve these exact dimensions in a reproducible manner. We have previously proposed to generate custom templates of the reconstructed aortic arch based on echocardiographic parameters to guide surgeons intraoperatively.
      • Haller C.
      • Yoo S.J.
      • Van Arsdell G.
      • Honjo O.
      Three-dimensional construction of tissue casting molds for aortic arch reconstruction in hypoplastic left heart syndrome.
      However, many anatomical characteristics are predetermined by the size of the Damus-Kaye-Stansel anastomosis, distal aortic dimensions, and the aortopulmonary window. Within these limitations, details of tapering and an approach to allow reproducible results have yet to be achieved. The general ideal shape of the aorta is subjectively well understood, although not objectively defined, and likely every surgeon is always aiming to achieve that. The reality, however, is that this is not an easy feat. Maybe it is time to leverage MRI flow studies, computational fluid dynamics, and 3-dimensional printing to generate individualized patch material to optimize Norwood aortic arch reconstructions.

      References

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        • Noda R.
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        Aortic arch geometry after the Norwood procedure: the value of arch angle augmentation.
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        • Chetan D.
        • Saedi A.
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        Geometry of the aortic arch after initial hybrid or Norwood palliation.
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        Interdigitating arch reconstruction eliminates recurrent coarctation after the Norwood procedure.
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        • Saedi A.
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        Three-dimensional construction of tissue casting molds for aortic arch reconstruction in hypoplastic left heart syndrome.
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      Linked Article

      • High-degree Norwood neoaortic tapering is associated with abnormal flow conduction and elevated flow-mediated energy loss
        The Journal of Thoracic and Cardiovascular SurgeryVol. 162Issue 6
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          The Norwood neoaortic arch biomechanical properties are abnormal due to reduced vessel wall compliance and abnormal geometry. Others have previously described neoaortic geometric distortion by the degree of diameter reduction (tapering) and associated this with mismatched ventricular-neoaortic coupling, abnormal flow hemodynamic parameters, and worse patient outcome. Our purposes were to investigate the influence of neoaortic tapering (ie, diameter reduction) on flow-mediated viscous energy loss (EL′) in post-Norwood palliated hypoplastic left heart syndrome patients, and correlate flow-geometry with single ventricle power generation.
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