The cardiovascular effects of isoproterenol in the preterm newborn lamb

We studied the effects of an isoproterenol infusion on cardiac output and the distribution of blood flow in 5 preterm newborn lambs. Cardiac output and regional organ blood flow were measured by injecting radionuclide-labeled microspheres into the superior vena cava and left ventricle. We found that an average infusion of 0.5 microgram/kg X min of isoproterenol increased cardiac output 73% which correlated closely with an increase in stroke volume, but not with HR. The increase in systemic blood flow was distributed exclusively to the myocardium and skeletal muscle, bone, and skin. We conclude that isoproterenol increases cardiac output in preterm newborn lambs similarly to adults and that the stroke volume of the immature heart is more changeable than previously thought.