The American journal of physiology
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Changes in cardiac output (Qco), heart rate, right atrial pressure, (Pra), and mean systemic pressure (Pms) in response to blood volume changes were measured in chronically prepared fetal sheep. With a 10% decrease in blood volume, fetal cardiac output, measured with the microsphere technique, decreased significantly from 592 +/- 28 to 471 +/- 32 ml . min-1 . kg-1. Heart rate changed little from control animals (163 +/- 5) to those with decreased volume (161 +/- 10 beats/min). ⋯ Heart rate again changed little (153 +/- 9 beats/min). The fact that cardiac output rose only a small amount, whereas right atrial pressure rose sharply with an increased blood volume, suggests that the fetal heart is operating near the upper limit of its Starling function curve. As a result, there is very limited cardiac reserve for increases in fetal cardiac output.