Transmural myocardial mechanics during isovolumic contraction.
OBJECTIVE
We sought to resolve the 3-dimensional transmural heterogeneity in myocardial mechanics observed during the isovolumic contraction (IC) phase.


BACKGROUND
Although myocardial deformation during IC is expected to be little, recent tissue Doppler imaging studies suggest dynamic myocardial motions during this phase with biphasic longitudinal tissue velocities in left ventricular (LV) long-axis views. A unifying understanding of myocardial mechanics that would account for these dynamic aspects of IC is lacking.


METHODS
We determined the time course of 3-dimensional finite strains in the anterior LV of 14 adult mongrel dogs in vivo during IC and ejection with biplane cineradiography of implanted transmural markers. Transmural fiber orientations were histologically measured in the heart tissue postmortem. The strain time course was determined in the subepicardial, midwall, and subendocardial layers referenced to the end-diastolic configuration.


RESULTS
During IC, there was circumferential stretch in the subepicardial layer, whereas circumferential shortening was observed in the midwall and the subendocardial layer. There was significant longitudinal shortening and wall thickening across the wall. Although longitudinal tissue velocity showed a biphasic profile; tissue deformation in the longitudinal as well as other directions was almost linear during IC. Subendocardial fibers shortened, whereas subepicardial fibers lengthened. During ejection, all strain components showed a significant change over time that was greater in magnitude than that of IC. Significant transmural gradient was observed in all normal strains.


CONCLUSIONS
IC is a dynamic phase characterized by deformation in circumferential, longitudinal, and radial directions. Tissue mechanics during IC, including fiber shortening, appear uninterrupted by rapid longitudinal motion created by mitral valve closure. This study is the first to report layer-dependent deformation of circumferential strain, which results from layer-dependent deformation of myofibers during IC. Complex myofiber mechanics provide the mechanism of brief clockwise LV rotation (untwisting) and significant wall thickening during IC within the isovolumic constraint.
Department of Medicine and Bioengineering, University of California, San Diego, CA, USA. ha8000@gmail.com
Hiroshi
H
Ashikaga
Tycho I G
TI
van der Spoel
Benjamin A
BA
Coppola
Jeffrey H
JH
Omens
D000818:Animals; D001696:Biomechanical Phenomena; D002304:Cardiac Pacing, Artificial; D002933:Cineradiography; D004285:Dogs; D006352:Heart Ventricles / Q000000981:diagnostic imaging*; D021621:Imaging, Three-Dimensional; D008955:Models, Cardiovascular; D009200:Myocardial Contraction; D011857:Radiographic Image Interpretation, Computer-Assisted; D012399:Rotation; D013314:Stress, Mechanical; D013997:Time Factors; D016277:Ventricular Function, Left