Studies of the fetal left ventricle have shown that it is capable of more than doubling its output (LVO) during oxygen ventilation. This increase is associated with decreases in afterload (arterial elastance, or Ea), increases in preload (end-diastolic volume, or EDV) and no apparent increase in contractility (LV end-systolic elastance, or Ees). The relative importance of improved mechanical coupling (the relationship between contractility and afterload, or Ees/Ea) and load coupling (the relationship between afterload and preload, or Ea/EDV) to this increase is not known. We studied 10 late-term fetal sheep acutely, with indwelling intracardiac volume and pressure catheters and vascular occluder balloons to compare these relationships and their effects on LV performance in the non-ventilated and ventilated fetal sheep. In both states we increased and decreased Ea with occlusion of the aortic isthmus and ductus arteriosus respectively, and increased Ees with dobutamine infusion. We derived Ees by transient occlusion of the inferior vena cava in all conditions. We applied multiple regression techniques to determine the effect of any intervention (p<0.05 being significant). As expected, oxygen ventilation was associated with a near doubling of LVO, which increased up to 218% from baseline with the addition of dobutamine. The increase in LVO was associated with a decrease in Ea of 44%, but with a similar decrease in Ees (45%), so that mechanical coupling did not change. Moreover, Ees/Ea was in the optimal range in the non-ventilated fetus(0.89±0.04). Conversely, EDV increased by 32%, so that Ea/EDV decreased to 59% baseline. Isolated decreases and increases in Ea by vascular occlusions to alter mechanical coupling did not result in changes in LVO. We conclude that disadvantageous mechanical coupling does not exist in the non-ventilated fetus, and that changes do not occur to explain the large increase in LVO with ventilation. Rather, load coupling changes dramatically, suggesting that improved afterload-preload matching is responsible for that increase.
