Experimentally induced changes in heart rate alter umbilicoplacental hemodynamics in fetal sheep

In the anesthetized sheep fetus, we have accelerated heart rate by atrial pacing (n = 6) or caused decelerations by stimulating the vagus nerve (n = 7). Alterations in heart rate were linearly related to changes in the ratio of the diastolic minimum (D) to systolic maximum (S) of flow and velocity, and to changes in the flow pulsatility index in the umbilical artery. Decelerations slightly increased vascular resistance, and decreased blood flow and mean arterial pressure, but accelerations had no significant effect. The index of impedance was unaltered by either intervention. The major factor mediating the influence of heart rate on blood flow pulsatility in the umbilical artery was the aortic blood pressure pulsatility. An equation was derived to adjust flow D/S to a standardized heart rate. Results showed that heart rate correction can affect the interpretation of experimental data.