Objective To study the relation between diameter pulse (pressure pulse) and flow velocity waveforms in the fetal descending aorta in fetuses with intrauterine growth retardation and acceleration.
Design Blood flow through a tubular system can be expressed by the ratio of blood pressure to vascular resistance. Doppler ultrasound and a phase locked loop echo tracking system coupled to a B-mode ultrasonic imager (central frequency 3.5 MHz) were used to assess downstream resistance and change in blood pressure, respectively.
Setting Tertiary referral unit in a teaching hospital
Participants Serial study between 21 and 40 weeks of 22 women with normally grown fetuses at intervals of four weeks; 25 women with small for gestational age fetuses with uteroplacental insufficiency (high umbilical artery pulsatility index); and six women with large for gestational age fetuses.
Main outcome measures We measured the maximum systolic and minimum diastolic diameter of the fetal descending aorta (the pulse amplitude) and then calculated the pulsatile waveform time integral above the least diastolic diameter (pulsatile area) and total waveform time integral (perfusion area).
Results Normal fetal growth was associated with an increase in systolic and diastolic diameters in the fetal descending aorta with advancing gestational age. Pulse amplitude, pulsatile and perfusion areas increased with gestational age. The increased pulse amplitude and increased pulsatile area in association with a decrease of the pulsatility index in the descending aorta during the second trimester suggested that pulse pressure and stroke volume were increased. In the group with intrauterine growth retardation, systolic and diastolic diameters of the descending aorta and perfusion area were within the normal range. Moreover, there was an increase in the diastolic diameter per unit fetal weight and a decrease in the pulsatile area. In the large for gestational age fetuses, there was an increase in the pulse amplitude and pulsatile area.
Conclusion These findings suggest that in growth restricted fetuses there is an increase in diastolic pressure and a reduction in stroke volume, while in large for gestational age fetuses there is an increase in the pulse pressure and stroke volume. It is possible that measurement of pressure pulse waveform in relation to Doppler velocity waveform may be used to infer changes of blood flow.