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.