What is the Clinical Significance of Ventricular Mural Antagonism?

Recent morphological studies provide evidence that the ventricular walls are arranged as a 3D meshwork of aggregated cardiomyocyte chains, exhibiting marked local structural variations. In contrary to previous findings, up to two-fifths of the chains are found to have a partially transmural alignment, thus deviating from the prevailing tangential orientation. Upon contraction, they produce, in addition to a tangential force, a radial force component that counteracts ventricular constriction and aids widening of the ventricular cavity. In experimental studies, we have provided evidence for the existence of such forces, which are auxotonic in nature. This is in contrast to the tangentially aligned myocytes that produce constrictive forces, which are unloading in nature. The ventricular myocardium is, therefore, able to function in an antagonistic fashion, with the prevailing constrictive forces acting simultaneously with a dilatory force component. The ratio of constrictive to dilating force varies locally according to the specific mural architecture. Such antagonism acts according to local demands to preserve the ventricular shape, store the elastic energy that drives the fast late systolic dilation and apportion mural motion to facilitate the spiralling nature of intracavitary flow. Intracavitary pressure and flow dynamics are thus governed concurrently by ventricular constrictive and dilative force components. Antagonistic activity, however, increases deleteriously in states of cardiac disease, such as hypertrophy and fibrosis. ß-blockade at low dosage acts selectively to temper the auxotonic forces

Assessment of the Helical Ventricular Myocardial Band Using Standard Echocardiography

n the discussion of their recent article, Hayabuchi and his colleagues[1] acknowledge that the “helical myocardial band” remains controversial. In the accompanying editorial, Buckberg harbored no such doubts.[2] Are the limited echocardiographic findings illustrated truly sufficient for Hayabuchi and his colleagues to conclude that there is a “helical ventricular myocardial band”?[1] They refer to a model that Torrent-Guasp had carved out of the ventricular muscular mass by disrupting myriads of myocardial branches, suggesting moreover that this band is freely moveable on itself. The histological studies produced by Hort[3] and Feneis,[4] however, provided evidence that the ventricular cone does not have discrete origins and insertions of the cardiomyocytes as found in skeletal muscle. Pettigrew had demonstrated more than a century ago[5] the multiple interleaving sheets of cardiomyocytes to be found within the cone. Lev and Simkins,[6] cited by Buckberg, also had emphasized that the cone can be dissected at the whim of the prosector, as achieved by Torrent-Guasp when subjectively producing the preparations now modeled by Buckberg.[7] Our investigations, cited by Hayabuchi and colleagues,[1] endorse the works of Feneis[3] and Hort.[4] The histological findings show no obvious anatomical substrate, other than the obvious change in alignment of the aggregated chains of cardiomyocytes, to explain the echocardiographic feature emphasized by the Japanese workers. They certainly provide none that represent a substantial proportion of the width of the septum, as the echocardiograms seem to suggest. The echogenic band is seen in the equatorial and basal regions of each of the walls of the left ventricle when viewed from the apex. No such band is seen when the ventricular mass is viewed using the parasternal window. We suggest that the echogenic band represents an area of distinct myocyte orientation within the continuous mesh of the septum, where the reflected ultrasound is perpendicular to the dominant orientation of the cardiomyocytes, thus giving maximum intensity compared with the surrounding tissue. The echogenic band, when viewed from the apex, therefore, is likely to represent no more than the chains of cardiomyocytes located within the mid-wall of the ventricular cone which are aligned circumferentially. The concept of the helical ventricular myocardial band does not model the circumferential orientation in this region. There are further problems, however, with the concepts advanced by Buckberg,[2] His inferences are based on imaging systems that measure only strain, as opposed to assessing the local development of force. The onset of shortening is not identical with the onset of contraction, so it is his mistake to interpret late shortening as delayed contraction. We have shown that within the ventricular cone, there are extended zones in which the myocardium contracts auxotonically, that is, the force increases during systole.[8] The features of such auxotonic contraction are delayed onset, restricted shortening, and delayed termination