In vivo validation of longitudinal-circumferential area change ratio to estimate myofiber shortening in the heart.

The aim of this paper was to validate area change ratio (%AC) against myofiber shortening (%λ(f)) in the heart in vivo. %AC is emerging as a mechanical index that may approximate %λ(f) by incorporating both circumferential and longitudinal shortening. However, the physiological significance of % AC remains unclear. We studied the time course of %AC in the anterior midleft 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 ms/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 %λ(f) that was determined from the transmural fiber orientation directly measured in the heart tissue. The time course of both %AC and %λ(f) was determined in the subepicardial, midwall, and subendocardial layers. The time course of %AC and %λ(f) was significantly different, and the difference was more pronounced towards the endocardium. %AC consistently overestimated %λ(f). The timing of the peak %AC was significantly delayed compared to that of the peak %λ(f). We conclude that %AC is significantly different from %λ(f) both in magnitude and timing in vivo. %AC overestimates %λ(f), and the overestimation is worse toward the endocardial layers. This may be a potentially important limitation when applying %AC to optimization and responder identification for cardiac resynchronization therapy.