While the heart size maintains a constant proportion to body size, heart function parameters, such as heart rate and cardiac output, show a more complex scaling pattern. How these phenomena affect the long-axis left ventricular (LV) function is unknown. We studied 10 mice, 15 rats, 6 rabbits, 8 mongrel dogs and 38 human volunteers. Doppler tissue echocardiography data were postprocessed to reconstruct mitral annulus (MA) peak systolic velocity and displacement. The relationship between MA peak velocity, MA displacement and LV ejection time, and LV end-diastolic volume (and mass) were fit to an allometric (power-law) equation Y = kMβ. LV mass varied from 0.062 to 255 g, while end-diastolic volume varied from 0.014 to 205 ml. β values of the relation between LV ejection time and LV end-diastolic volume and mass were 0.247 ± 0.017 and 0.267 ± 0.018, respectively. β values of the relationship between MA displacement and LV end-diastolic volume and mass were 0.358 ± 0.047 and 0.390 ± 0.051 (P < 0.023 versus β of LV ejection time). β values of the relationship between MA peak systolic velocity and LV end-diastolic volume and mass were 0.096 ± 0.012 and 0.100 ± 0.013, respectively (P < 0.0001 versus 0). Finally, β values of the relationship between the long-to-short axis displacement ratio and LV end-diastolic volume and mass were 0.077 ± 0.017 and 0.086 ± 0.019 (P < 0.0001 versus 0). We conclude that MA velocity, displacement, and long-to-short axis displacement ratio scale allometrically to heart size. This reduces the relative long-axis contribution to heart function in small mammals.
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