Structure of the ventricular myocardium and conduction system

Abstract

Three centuries ago Robert Lower stated that the ventricular myocardium is a longitudinal band of muscle that is wrapped around the heart chambers in the manner of a complex bandage. Since that time many investigators have attempted to unwind the myocardial bundles, but no two patterns are alike. These discrepancies may result from the fact that the muscle bands can not be dissected until the extremely tough collagenous network of the heart is dissolved. Unfortunately, the remaining muscle fibers then resemble a soft ill-defined mass that may be unwound in many different patterns. A new technique for the analysis of the architecture of tissues has provided data on ventricular structure that differ markedly from earlier analyses. These studies have been based on the well established anatomic relationship that each cluster of approximately 100 muscle fibers is enclosed, together with blood capillaries and extracapillary fluids, in an epimysial fibrous connective tissue capsule. 1 Capsular enclosure implies that an extracapsular cleft separates adjacent capsules (Fig. 1). The geometrical relationships of these clefts can be examined by intramuscular (not intravascular) injection of various materials. The capsules exclude the entry of the injected materials into the intracapsular spaces, and they enter only the extracapsular clefts. As a result, the injected material outlines the orientation of each enclosed bundle of muscle fibers. Casts of the extracapsular clefts of the heart and of other tissues are produced by intraparenchymal injection of monomeric liquid methyl methacrylate. This monomer, which is less viscous than water, enters the wall of the myocardium with the ease of an intramuscular injection, and spreads freely along the natural extracapsular clefts. Hardening of the plastic provides an in situ preparation that outlines the directions of the muscle bundles. The bundles of the heart do not follow bandage-like courses. Instead, the ventricular myocardium appears to be composed of sets of concentric muscle bands arranged in incomplete disks. The disks form great circles around the center of the ventricular spheroid. 2 These arrangements can provide mechanically for the adjustment of tension development to load, and thereby adjust oxygen uptake to tensile load. 3 Address for reprints: Simon Rodbard, MD, Department of Cardiology, City of Hope Medical Center, Duarte, Calif, 91010. Compartments: The plastic injection technique demonstrates that the ventricle is separated into nine discrete compartments (Fig. 2). For example, plastic injected into the free wall of the right ventricle remains in this free wall. After the plastic hardens, digestion of the injected heart in alkali releases a cast of the extracapsular clefts of only the free wall of the right ventricle and its lymphatic drainage system. The remainder of the heart receives no plastic and is digested without residhe. Inspection of isolated casts of the free wall of the right ventricle shows it to be a discrete compartment consisting of many disks of muscle bundles. Injections of color-coded plastic are used to identify and demonstrate other ventricular compartments. Thus, injection of yellow plastic into the left ventricular free wall forms an isolated cast of this free wall. Injection of green plastic into the free wall of the right ventricle, and of yellow plastic into the free wall of the left ventricle, produces separate but interdigitated casts of the two free walls. When the