Left Ventricular Mechanics
Each myocardial layer contributes to longitudinal, circumferential, and radial deformation. Longitudinal deformation is largely dependent on a well functioning subendocardial fiber layer, while circumferential deformation of the left ventricle (LV) is predominately dependent on the subepicardial fiber layer5. LV shearing forces occur within the three-dimensional myofiber orientation causing the largest shear force in the circumferential-longitudinal plane referred to as LV twist or torsional deformation. The rotation of LV base and apex in opposite directions, combined with twist shearing forces, are responsible for translating a mere 15-20% maximal myocyte length shortening into an ultimate LV cavity volume reduction of over half. These same shear forces are responsible for untwisting during diastolic recoil. Left ventricular diastolic untwisting releases stored mechanical energy and creates a vacuum effect facilitate movement of blood from the left atrium (LA) to the LV.
The predominant myocardial mechanics of HFpEF or restrictive cardiomyopathies involve subendocardial fiber dysfunction leading to decrements in longitudinal strain. Epicardial layer derived circumferential strain and torsional shear forces remain relatively preserved or even increased to supranormal values early in the disease course 6. These compensatory circumferential and twist mechanics allow for preservation of ejection fraction. On the other hand, constrictive pericarditis, a common HFpEF mimic, leads to a reciprocal situation of decreased circumferential and torsion forces while longitudinal forces are preserved 7. These cardiac mechanics can serve as an adjunct to traditional echocardiographic signs differentiating features of constrictive versus restrictive physiology, as clinical presentation may be similar between disease states.