Re‐Setting the Gold Standard

In this issue of the Journal, Mori and his colleagues provide a truly spectacular account of the anatomy of the inferior pyramidal space.1 The precise morphology of this area of the heart, in the past, has been difficult to understand, yet it occupies a pivotal position from the stance of the clinical electrophysiologist. It is the more remarkable, therefore, that the elucidation of the anatomical arrangement is based on clinical imaging. The images provided using multi-detector computed tomography reveal the overlapping of the wall of the right atrium relative to the musculature of the underlying ventricular mass. It is this feature that is the key to understanding of this area. Equally importantly, the anatomical details, shown in attitudinally appropriate fashion, also serve to clarify the relationships of the inferior pyramidal space to the adjacent superior and inferior paraseptal parts of the right atrioventricular junction. Taken together, the images show that we now need to re-assess the techniques considered to provide the “gold standard” for understanding cardiac anatomy. The quality of the images provided by Mori and his associates, of course, does not detract from the important information still to be provided for the clinical electrophysiologist by investigations using autopsied specimens, such as the one undertaken by Cabrera and Sanchez-Quintana, and published as “Education in Heart.”2 In that synthesis, the Spanish workers also reviewed the multiple studies in which they had collaborated with myself, and subsequently with Siew Yen Ho as she continued to demonstrate the importance of the understanding of cardiac morphology in her ongoing studies at the Royal Brompton Hospital. Similar studies demonstrating the importance of autopsy investigations had been provided by Asirvatham and his colleagues at the Mayo Clinic, including an impressively extensive investigation of the myocardial sleeves of the great veins and coronary sinus.3 Even the excellent account of Cabrera and Sanchez-Quintana,2 however, based on their dissections of multiple autopsied hearts, does not provide the detail regarding the architecture and relationships of the inferior pyramidal space when compared to the images produced by Mori and his colleagues.1 There is little doubt that the interpretations provided by the Japanese group are demonstrating the “real thing.” Comparisons can be made between their images, such as their Figure 6, and my own dissections as reproduced in our textbook “Wilcox's Surgical Anatomy of the Heart” (Figs 5.11 and 5.13).4 In this regard, I should declare my bias, since I had been exchanging e-mails with Dr. Mori regarding additional investigations conducted by he and his colleagues regarding the location of the cardiac valves within the cardiac silhouette. I had also been reviewing earlier versions of this publication. I had asked him, during the review process, to correlate the images with the dissections as shown in our textbook. As can be seen, the results are quite spectacular. They show in superb detail the superior continuation of the fibrofatty tissues of the inferior atrioventricular groove, demonstrating its position as the “meat” sandwiched between the atrial and ventricular layers of the inferior pyramidal space. This nuance had escaped us in our earlier studies, since we had interpreted the floor of the triangle of Koch, by virtue of the offsetting of the hinge of the tricuspid valve relative to that of the mitral valve, as representing an atrioventricular muscular septum. Only subsequently, having performed dissections such as that shown in Figure 5.13 of “Wilcox's Surgical Anatomy,” did we come to appreciate that, although it was possible to pass a needle directly from the vestibule of the right atrium to the outflow tract of the left ventricle, the course of the needle extended through the insulating tissues contained within the inferior pyramidal space. We should, of course, have appreciated this fact much earlier, since the pioneering studies of Sealy and Gallagher had focused attention on the inferior pyramidal space, albeit that they did describe the space as being “septal.”5 The very fact that Sealy was able to ablate surgically accessory muscular atrioventricular connections through the space, nonetheless, should have alerted us to the fact that insulating tissues forming its floor did not separate directly the cardiac cavities, and hence did not form a true septal structure.6 It is the very fact that these features can now be demonstrated with even better accuracy during life than in the autopsy room that tells us that we now need to reset the gold standard for the establishment of cardiac anatomy. Appreciation of this fact is also not new. The writing has been on the wall for quite some time. As long ago as 2007, Saremi and Krishnan showed that cardiac computed tomography was capable of demonstrating the anatomic landmarks to the conduction tissues.7 At that stage, furthermore, they were employing a 64 slice machine to produce their images. Technology has improved since then, as have the images demonstrating cardiac anatomy. In a subsequent investigation, in which he collaborated with Ho and Sanchez-Quintana, Saremi showed how he was able to demonstrate the anatomy of the right ventricular outflow tract with just as much accuracy as the morphologists.8 In that review, furthermore, the investigators correctly pointed out that existence of the so-called outlet septum was questionable in the otherwise normally formed heart. This is because very little, if any, of the muscular tissue derived from the outflow cushions during cardiac development interposes between the subpulmonary and subaortic outflow tracts. The muscular ventricular septum, including its crest, is present from the outset of development.9 Saremi and his colleagues8 did point out that histology might still be necessary to distinguish the true extent of a muscular outlet septum. Even histology, nonetheless, may not give the correct answer, since Acosta and colleagues, including Sanchez-Quintana and Cabrera, have identified the crest of the muscular ventricular septum as the “outlet septum” in their latest publication.10 There is then one additional reason for considering the images as produced by analysis of multi-detector computer tomographic imaging as the new “gold standard.” This is because, as I emphasized at the outset, the images are displayed in attitudinally appropriate fashion. The need to describe the heart as seen during life was itself emphasized many years ago now by McAlpine in his spectacular atlas.11 Although claiming to follow his lead, we neglected for some time to use the attitudinal approach in our own initial ventures into production of atlases.12 It was the promptings of Cosio that brought home to me the clinical importance of the attitudinal approach.13 It is the more disappointing, therefore, to observe the reluctance of many electrophysiologists to adopt the appropriate way of describing the cardiac components. With the increasing use of computed tomography to display cardiac anatomy during life, the use of attitudinally appropriate terms must surely become more widespread. When viewing the images provided by Mori and his colleagues,1 it is obvious that the apex of the triangle of Koch points upwards, and that the so-called anterior and posterior septal components are located superiorly and inferiorly, being paraseptal to boot. Increasing use of computed tomography will also show that the so-called posterior descending artery is really inferior and interventricular. In similar fashion, it will become clear that the papillary muscles supporting the leaflets of the mitral valve are located infero-septally, and supero-laterally, rather than “postero-medially” and “antero-laterally” as they are currently described. Whether cardiologists are prepared to grasp the nettle, and describe structures as they are seen during life, remains to be seen. As is shown by the superb studies of Mori and colleagues,1 along with those of Saremi and his associates,7, 8 they no longer have any excuse to ignore the evidence provided by their own eyes.