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1 Department of Cadiovascular and Thoracic Surgery, Stanford University, School of Medicine, Stanford, CA, USA
2 Department of Biomedical Engineering, Texas A&M University, College Station, TX, USA
3 Department of Cadiovascular and Thoracic Surgery, Stanford University, School of Medicine, Stanford, CA, USA; Laboratory of Cardiovascular Physiology and Biophysics, Research Institute of Palo Alto Medical Foundation, Palo Alto, CA, USA
* To whom correspondence should be addressed. E-mail: allencheng{at}stanford.edu.
The constant volume property of contracting cardiac muscle has been invoked in models of heart wall mechanics which predict that systolic subendocardial LV wall thickening must significantly exceed subepicardial thickening. To examine this prediction, we implanted arrays of radiopaque markers to measure lateral equatorial wall transmural strains and global and regional LV geometry in 7 sheep and studied the 4-D dynamics of these arrays using biplane videofluoroscopy (60 Hz) in anesthetized, intact animals, one- and eight-weeks after surgery. A transmural gradient of systolic lateral wall thickening was observed at 1-week (linear regression, p=0.009), but was no longer present at 8-weeks (p=0.243). Referenced to end-diastole, group mean (±SD) end-systolic radial subepicardial, midwall, and subendocardial wall thickening strains were, respectively, 0.08±0.08, 0.14±0.08, and 0.22±0.12 at 1-week and 0.19±0.07 (p=0.02; 1- vs. 8-weeks), 0.20±0.04, and 0.23±0.07 at 8-weeks. With the exception of an 8 ml (7%) increase in EDV (p=0.04) from 1- to 8-weeks, LV shape and hemodynamics were otherwise unchanged. We conclude that equivalent hemodynamics can be generated by the left ventricle with or without a transmural gradient of systolic wall thickening in this region and thus such a gradient is unlikely to be a fundamental property of the contracting LV myocardium. We discuss some implications of these findings to mechanisms involved in systolic wall thickening.
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