Abstract 18023: In vivo Validation of Area Change Ratio to Estimate Myofiber Strain in the Heart

Abstract

Background: Area change ratio (AC) using the 3-D speckle tracking echocardiography is emerging as a mechanical index that approximates myofiber shortening by incorporating both circumferential and longitudinal shortening. However, AC has not been validated against myofiber strain in the heart in vivo , and its physiological significance remains unclear. Methods and Results: We studied the time course of AC in the anterior mid-left ventricular wall of normal canine heart in vivo (n=14) during atrial pacing over the entire cardiac cycle using transmurally implanted markers and biplane cineradiography (8 msec/frame). AC was calculated as the myocardial area change relative to the elemental material area on the circumferential- longitudinal plane at the reference configuration (= end diastole). AC was compared with myofiber strain that was determined from transmural fiber orientations directly measured in the heart tissue. The time course of both AC and myofiber strain was determined in the subepicardial, midwall, and subendocardial layers. The time course of AC and myofiber strain was significantly different (P<0.05), and the difference was more pronounced towards the endocardium (Figure). AC consistently overestimated myofiber shortening. The timing of the peak AC was significantly delayed compared to that of the peak myofiber shortening (P<0.05). Conclusions: AC is significantly different from myofiber strain both in magnitude and timing in vivo. AC overestimates myofiber shortening, and the overestimation is worse towards the endocardial layers, likely resulting from crossfiber deformation that is inherently included in AC. The significant delay in estimating the peak myofiber shortening may hinder accurate optimization and/or responder identification for cardiac resynchronization therapy.