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Strain softening in rat left ventricular myocardium
J L Emery1, J H Omens, A D McCulloch
1Department of Bioengineering, University of California, San Diego, La Jolla 92093-0412, USA.
Journal of Biomechanical Engineering
|February 1, 1997
Summary
Strain softening, not viscoelasticity, explains reduced left ventricular stiffness during preconditioning. This phenomenon, known as the Mullins effect, involves damage to elastic components in heart tissue.
Area of Science:
- Cardiovascular Physiology
- Biomaterials Science
- Cardiac Mechanics
Background:
- The preconditioning phenomenon in the myocardium is characterized by reduced left ventricular stiffness.
- The underlying mechanisms, particularly the role of strain-history dependence, remain incompletely understood.
- Previous explanations have focused on viscoelastic properties of cardiac tissue.
Purpose of the Study:
- To investigate if strain softening (Mullins effect) can account for the reduced left ventricular stiffness observed during myocardial preconditioning.
- To differentiate the contributions of strain softening versus viscoelasticity in cardiac tissue behavior under varying pressure loads.
Main Methods:
- Passive pressure-volume relationships were measured in isolated, arrested rat hearts during controlled inflation-deflation cycles.
- Experiments involved varying maximum inflation pressures to assess changes in ventricular stiffness and volume.
- A computational model was developed, first incorporating quasi-linear viscoelasticity, then modified with a strain softening theory (Johnson and Beatty, 1992).
Main Results:
- Increasing maximum inflation pressure led to decreased left ventricular stiffness, especially in the diastolic range.
- Left ventricular volume at 10 mmHg increased significantly with prior pressure history and magnitude, reaching up to 350% of unloaded volume.
- The viscoelastic model failed to replicate the observed reduction in stiffness; the strain softening model accurately reproduced the experimental data.
Conclusions:
- Strain softening, attributed to potential damage in elastic components, provides a better explanation for myocardial preconditioning and reduced ventricular stiffness than viscoelastic effects.
- The Mullins effect is a key factor in understanding the mechanical behavior of the myocardium following cyclic loading.
- This finding has implications for understanding cardiac tissue mechanics and potential therapeutic interventions.