The forces generated within the musculature of the left ventricular wall

Abstract

Objectives: To test the hypothesis that two populations of myocardial fibres-fibres aligned parallel to the surfaces of the wall and an additional population of fibres that extend obliquely through the wall-when working in concert produce a dualistic, self stabilising arrangement. Methods: Assessment of tensile forces in the walls of seven porcine hearts by using needle probes. Ventricular diameter was measured with microsonometry and the intracavitary pressure through a fluid filled catheter. Positive inotropism was induced by dopamine, and negative inotropism by thiopental. The preload was raised by volume load and lowered by withdrawal of blood. Afterload was increased by inflation of a balloon in the aortic root. The anatomical orientation of the fibres was established subsequently in histological sections. Results: The forces in the fibres parallel to the surface decreased 20235% during systolic shrinkage of the ventricle, during negative inotropism, and during ventricular unloading. They increased 10230% on positive inotropic stimulation and with augmentation in preload and afterload. The forces in the oblique transmural fibres increased 8265% during systole, on positive inotropic medication, with an increase in afterload and during ventricular shrinkage, and decreased 36% on negative inotropic medication. There was a delay of up to 147 ms in the drop in activity during relaxation in the oblique transmural fibres. Conclusion: Although the two populations of myocardial fibres are densely interwoven, it is possible to distinguish their functions with force probes. The delayed drop in force during relaxation in obliquely oriented fibres indicates that they are hindered in their shortening to an extent that parallels any increase in mural thickness. The transmural fibres, therefore, contribute to stiffening of the ventricular wall and hence to confining ventricular compliance.

The length-force behavior and operating length range of squid muscle varies as a function of position in the mantle wall

“…Thompson et al (Thompson et al, 2010a) showed in D. pealeii that the inner surface of the mantle experiences 1.3 to 1.6 times greater circumferential strain than the outer surface at any time point during the exhalant phase of a jet. Because circular fibers are oriented in transverse planes of the mantle and also appear to follow a perfectly circumferential trajectory, there are no architectural specializations, such as those found in mammalian cardiac ventricle muscle (Lunkenheimer et al, 2004;Anderson et al, 2006;Anderson et al, 2009) or vertebrate skeletal muscles (Alexander, 1969;Rome and Sosnicki, 1991;Wakeling and Johnston, 1999;Azizi et al, 2008) that could permit all of the fibers to experience approximately the same range of strains during jetting. Thus, the transmural gradient of circumferential strain in the mantle may require muscle fibers from the inner surface to operate farther left on the ascending limb of the length-tension curve at any instant during mantle contraction.…”

Pneumatic Distension of Ventricular Mural Architecture Validated Histologically

“…These are the cardiomyocytes believed by Krehl [18] to provide the driving force for ventricular emptying. Perhaps more importantly, the quantitative, as opposed to qualitative, observations we were able to make using an automated system that avoids the need for subjective interpretation [16] confirm that a significant proportion of cardiomyocytes are aligned in a transmural fashion across the ventricular walls [11]. These measurements provided no support for the notion that all cardiomyocytes are aligned in a virtually tangential fashion.…”

“…The marked heterogeneity in the extent of aggregation of the individual cardiomyocytes, furthermore, indicates a local "fingerprint" for each ventricular region, these regions representing the "segments" as identified echocardiographically [17], thus pointing to the well-recognized regional variability in antagonistic function of the wall [11, 12, 20 -22]. The simultaneous coexistence of dilating and constrictive activity in the ventricular myocardium was measured previously, a feature which prevails throughout the cardiac cycle [11]. We suggest that this mechanism, which was described as antagonism, is designed to control the mean ventricular shape and size.…”

“…Global function is the joint effect of the highly variable patterns of local wall motion, which also means that an almost deliberate pattern of even partially disturbed local functions may fail to disrupt global function [27]. An essential component of this global function is the antagonism provided between the tangential and intruding populations of cardiomyocytes, thus underscoring the co-existence of unloading and auxotonic systolic forces [11,28]. It is axiomatic that knowledge of ventricular mural architecture is essential when assessing the clinical findings now revealed using computed tomography or magnetic resonance imaging [2].…”

Effects of incremental beta-blocker dosing on myocardial mechanics of the human left ventricle: MRI 3D-tagging insight into pharmacodynamics supports theory of inner antagonism

“…2A, left). According to direct force measurements, tangential netting components generate the unloading type of force signal, which slowly decays after having reached an early systolic peak (11). This signal proves to be relatively insensitive to negative inotropic therapy (10).…”