In situ physiological study of the developing heart

Studies on cardiac muscle and whole heart isolated from the fetal and newborn lamb and adult sheep reveal striking age-dependent differences in cardiac ultrastructure, passive and active length-tension properties, force-velocity relationships, the compliance characteristics of both ventricles, myocardial energetics, and in the development of myocardial autonomic control. Isometric force development and both the extent and velocity of shortening at any load are reduced in the fetus when compared with the adult because of age-related changes in the proportion of myocardial tissue consisting of myofilaments. The distensibility characteristics of both the left (LV) and right ventricles (RV) are comparable in the fetal lamb close to term. In the early newborn period the RV has compliance characteristics similar to the RV of the fetus, whereas the LV alters its stress-strain characteristics to resemble the adult. Most recently, methods have been developed for the chronic evaluation of fetal left ventricular function. Internal LV dimensions and pressures are monitored continuously from midgestation beyond delivery.