Physiological characteristics of diameter pulses in the fetal descending aorta.

A study was performed to establish the dynamic behavior in situ of the descending aorta in the human fetus. The pulsatile movements of the vessel walls were recorded in 36 clinically normal fetuses using a real-time phase-locked ultrasonic system measuring echo movements with high spatial and temporal resolution. The mean amplitude of the diameter pulsations was similar in the 28th - 35th week to that in the 36th - 40th week but, as the apparent diastolic diameter increased, the pulse-elicited increment of the cross-sectional area was 29% larger in the older group. The diastolic diameter decreased (p less than 0.001) and the amplitude of the diameter pulsation increased (p less than 0.001) with prolongation of the previous beat interval. The diameter pulse velocity was positively correlated to gestational age (p less than 0.001), ranging between 1.35 and 2.89 m/sec. The incremental phase of the pulse curve diminished in duration with increasing distance from the heart (p less than 0.001) and with decreasing duration of the previous beat interval (p less than 0.001). The maximum slope of the incremental phase was higher in the more distal part of the aorta (p less than 0.001). The study demonstrates that the diameter pulses contain useful information on the cardiovascular dynamics and provides evidence that the Frank-Starling mechanism is effective also in the human fetus.