PERINATAL LEFT-VENTRICULAR PERFORMANCE IN FETAL SHEEP - INTERACTION BETWEEN OXYGEN VENTILATION AND CONTRACTILITY

Left ventricular (LV) output nearly triples at birth, in association w ith increases in serum catecholamines. Similar increases in catecholam ines in utero, however, do not increase output. We hypothesized that c atecholamines increase contractility in utero, but that output cannot increase until LV loading conditions are changed by oxygen ventilation , To address this hypothesis, we studied nine fetal sheep acutely plac ed in a warm water bath (40 degrees C). Conductance and manometric cat heters were placed in the LV to generate pressure-volume loops during caval occlusion in the nonventilated and oxygen-ventilated states, eac h under control, dobutamine, and propranolol conditions. Contractility was estimated by the end-systolic pressure-volume relationship, prelo ad by end-diastolic volume, and afterload by arterial elastance, Oxyge n ventilation increased LV output 1.4-fold, despite a decrease in cont ractility to about three-fourths of the nonventilated value. Heart rat e remained constant, whereas preload increased and afterload decreased significantly, During oxygen ventilation, dobutamine increased output to 2.3 times the control, nonventilated value, associated with increa ses in contractility and heart rate and no change in preload and after load. Although dobutamine increased contractility and heart rate simil arly in the nonventilated and oxygen ventilated states, output increas ed significantly more during ventilation, Similarly, propranolol decre ased contractility and heart rate equally in both states, but output d ecreased far more during ventilation. Thus, oxygen ventilation is asso ciated with advantageous changes in LV load such that the positive ino tropic and chronotropic effects of dobutamine are translated into grea ter increases in LV output.