114 THE EFFECTS OF OUABAIN IN LAMBS WITH DEPRESSED MYO CARDIAL FUNCTION

Abstract

The inotropic effects of digoxin have been difficult to demonstrate in healthy fetal and newborn lambs. We examined the physiologic effects of digoxin in six newborn lambs in whom myocardial function had been depressed by halothane anesthesia (0.5-1.0%). Resting measurements of arterial and venous pressures, cardiac output, left ventricular dp/dt, and the left ventricular pre-ejection period (PEP) to left ventricular ejection time (LVET) ratio were made prior to ouabain administration (50 μgm·kg−1 bolus followed by 0.05 μgm·kg−1 ·min.−1 for 45 minutes). At rest, mean cardiac output (369 ml·kg−1 ·min.−1), dp/dt (3833 mm Hg·sec.−1), PEP/LVET (0.306) and stroke work (1320 mm Hg·ml) were not affected by ouabain. Forty-eight hours later, halothane anesthesia via controlled ventilation reduced cardiac output to 262 ml·kg−1·min.−1, increased PEP/LVET to 0.411, reduced stroke work to 766 mm Hg·ml and reduced left ventricular dp/dt to 2758 mm Hg·sec.−1; heart rate and systemic vascular resistance did not change. Ouabain administration during the halothane anesthesia improved myocardial function indices to levels not different from control values. These studies suggest the apparent resistance of healthy, immature sheep to the inotropic effects of cardiac glycosides may reflect their already high level of resting function rather than end organ resistance to drug action.