Biophysics of the developing heart. III. A comparison of the left ventricular dynamics of the fetal and neonatal lamb heart.

Changes in heart rate, left ventricular dimensions, and inotropic state of chronically instrumented lambs from in utero to neonatal life are described and analyzed. Six lambs were instrumented from 5 to 23 days prior to parturition and studied prior to and after birth. Heart rate, aortic systolic and diastolic pressure, left ventricular end-diastolic and peak systolic pressure, and left ventricular minor axis end-diastolic (EDD) and end-systolic dimension (ESD) were monitored. The maximum rate of rise of left ventricular pressure (Pmax), percentage fractional shortening [(EDD - ESD divided by EDD) x 100%; %FS], and the Pmax -interval ratio (this ratio of Pmax of the postextrasystolic potentiated systole to Pmax of the previous regular systole is independent of volume) described contractility. Subsequent to birth: heart rate, aortic systolic and diastolic pressure and left ventricular dimensions increased; during spontaneous rhythm and at equal fetal and neonatal heart rates and ventricular dimensions, %FS and P max increased significantly; and P max -interval ratio changed significantly. An increase in myocardial inotropic state occurs with birth. This enhancement is in addition to the effects of the increase in heart rate and end-diastolic volume that occur with the adaptation to birth.