Effects of acute preload and afterload stress on myocardial function in newborn and adult sheep

Unanesthetized, chronically instrumented newborn and adult sheep were subjected to acute circulatory preload stress (rapid administration of 30 ml X kg-1 isotonic saline) and afterload stress (intravenous administration of 0.2 mg X kg-1 methoxamine). Variables of circulatory status and myocardial function were monitored continuously during experimental manipulations. Preload stress elevated arterial and venous pressures, reduced the preejection period (PEP) to left ventricular ejection time (LVET) ratio (PEP/LVET), and increased the rate of left ventricular pressure rise (dp/dt) to a comparable extent in newborn and adult subjects. Afterload stress increased arterial and venous pressures to a comparable extent in newborns and adults; the percent changes in PEP (25), LVET (16), PEP/LVET (12) and dp/dt (-21) in newborns were more pronounced than those in adults.