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In adult humans and pig models, strain rate (SR) correlates strongly with invasive measures of contractility but does not demonstrate a force-frequency relationship, which is a fundamental behavior of myocardial contraction. Given the considerable maturational changes of the myocardium from fetal stages to adulthood, extrapolation of adult findings to the young heart may not be appropriate. We sought to evaluate the SR response of the immature heart to increased heart rate (HR) and inotropic stimulation.The study consisted of two parts. In part 1, children without obvious structural or functional cardiac abnormalities were evaluated following successful radiofrequency ablation. Echocardiography was performed at baseline HR and then with atrial pacing and isoprenaline infusion titrated to achieve 130% of baseline HR. Left ventricular (LV) speckle tracking–derived SR and tissue Doppler isovolumic acceleration (IVA, a load-independent marker of contractility) were measured. In part 2, young piglets were submitted to atrial pacing at 200, 230, and 260 bpm. Invasive LV dP/dt was assessed, and speckle tracking–derived SR was measured at all stages. Repeated measures analysis of variance was used for comparison with baseline values.In part 1, the LV SR and IVA (septal and lateral) in 23 children (ages 7.8-17.5 years) increased significantly with pacing and isoprenaline infusion (P < .001). In part 2, SR and invasive dP/dt increased significantly with increasing HR in young piglets (1-17 days; P < .01 and P < .001, respectively). Both LV SR and dP/dt plateaued at highest HRs concurrent with the decrease in LV end-diastolic dimension from baseline (73.0% ± 9.9% of the baseline value at 260 bpm; P < .001).SR in children is augmented with chronotropic and inotropic stimulation and in young infant piglets with chronotropic stimulation; in both children and piglets it has a force-frequency relationship, a behavior that is consistent with a measure of contractility.Validation of left ventricular speckle tracking–derived strain rate in young hearts.Evidence of appropriate strain rate response to chronotropy and inotropy in this age group.Demonstration of force-frequency relationship, in contrast to adult studies.