The role of canine superficial ventricular muscle fibers in endocardial impulse distribution.

Abstract

Thin sections of canine right and left ventricular endocardium and myocardium were studied in a tissue bath to compare conduction properties of intraventricular specialized conducting tissue [Purkinje fibers (PF)], the superficial layers of subendocardial ventricular muscle (SVM), and the deeper ventricular muscle (DVM) below this level. The study was carried out because of observations that some areas of the endocardium, which are devoid of either specialized conducting tissue or of PF-VM junctions between specialized conducting tissue and ventricular muscle, conduct relatively rapidly, favoring specific orientations of propagation. Preparations containing PF, SVM, and DVM were studied electrophysiologically and histologically. A technique of stripping limited areas of endocardium was used to expose DVM in order to determine its intrinsic calculated conduction velocity. In 12 preparations, the average calculated conduction velocity in PF was 1.62 m/sec, and the average in DVM was 0.26 m/sec. The SVM conduction velocity was intermediate between the two, averaging 0.98 m/sec when propagation was parallel to SVM fiber orientation. Conduction velocity transverse to SVM fiber orientation was not significantly different from DVM conduction velocity. Histologically, the most superficial layers of VM were oriented uniformly in the direction of rapid subendocardial conduction, in contrast to DVM fibers in which orientation varied. It is concluded that the geometric arrangement of SVM fibers may provide a means for rapid subendocardial conduction and impulse distribution at a conduction velocity intermediate between PF and DVM in areas devoid of specialized conducting tissue.

Beneficial Effect on Cardiac Resynchronization From Left Ventricular Endocardial Pacing Is Mediated by Early Access to High Conduction Velocity Tissue

“…For the latter, fiber angles varied transmurally and were fitted to canine histological data (Data Supplement; Figure To model the experimentally observed fast endocardial conduction (FEC) in the LV, 7,12 a 1-mm region of LV endocardial tissue, referred to herein as the FEC layer, was introduced into the model. 11 The conductivity of this FEC layer was isotropic with g l and g t increased to 0.3764 S/mm and resulted in a CV of 1.2 m/s.…”

“…9 The exact mechanisms underlying the decreases in ATs for ENDO-CRT BV versus EPI-CRT BV have not been fully elucidated. It has been hypothesized that the differences are caused by (1) shorter activation path lengths in ENDO-CRT BV allowing earlier activation in remote regions 6,7,9 ; (2) earlier activation of fast-conducting endocardial tissue, which has been shown to have a higher conduction velocity (CV) than epicardial tissue in canines 7,11,12 and in humans 13 ; and (3) bulk tissue anisotropy, which may be important in dyssynchronous HF patients where major fiber remodeling and redistribution of gap junction proteins, such as Connexin 43, are known to occur. 9,14,15 To test the role and relative importance of these 3 hypotheses, we used computational models of tissue excitation to quantify the mechanisms listed above in terms of their impact on ATs and to investigate if the combined or individual effects of these mechanisms can plausibly explain the observed reduction in ATs with LV endocardial pacing.…”

Left Ventricular Septal and Left Ventricular Apical Pacing Chronically Maintain Cardiac Contractile Coordination, Pump Function and Efficiency

“…This fast endocardial impulse conduction is in agreement with earlier findings. 21 Thirty-year-old measurements by Myerberg et al 22 indicate that this high conduction velocity may be attributed at least partly to subendocardial nonPurkinje fibers. Our promising findings on chronic LV apical pacing are in line with superior hemodynamic performance in an acute pediatric pacing study.…”

Left Ventricular Endocardial Pacing Improves Resynchronization Therapy in Canine Left Bundle-Branch Hearts

“…Subendocardial non-Purkinje fibers conduct impulses faster than midmyocardial or epicardial fibers, especially in the longitudinal direction. 12 Indeed, the large difference in base-to-apex conduction times between epicardial and endocardial CRT (Figure 3) suggests that more rapid spread of the activation wave front longitudinally may be an important component of endocardial CRT. A third factor contributing to faster activation during endocardial LV pacing is the more rapid transmural conduction from endocardium to epicardium than in the opposite direction.…”