The fetal aortic pressure pulse waveform in normal and compromised pregnancy.

OBJECTIVE

To study the arterial pressure waveform in the descending thoracic aorta during pregnancy in both normal and compromised fetuses.

DESIGN

The pressure pulsation waveform propagated along the vascular tree, and acting laterally on the arterial wall, produces a corresponding change in the vessel diameter. The distance between diametrically opposite points of the aortic lumen was followed using a phase locked loop echo tracking system coupled to a B-mode ultrasonic imager (central frequency 3.5 MHz).

SETTING

Tertiary referral unit, teaching hospital.

PARTICIPANTS

A cross-sectional study of 80 normal fetuses between 20 and 40 weeks yielded normal data. We studied 58 women with evidence of potential fetal compromise (high umbilical artery systolic: diastolic ratio).

MAIN OUTCOME MEASURES

From the aortic diameter waveform we measured the maximum systolic and minimum diastolic dimension and calculated pulse amplitude. The first derivative of the aortic diameter waveform identified the incisura of aortic and pulmonary valve closure and was used to time the end of ventricular ejection and systole.

RESULTS

In normal pregnancy there was an increase in systolic and diastolic diameter and pulse amplitude with advancing gestation. Ventricular ejection time was constant. In the fetal compromised group the absolute systolic and diastolic diameters were within the normal range, but diastolic diameter per unit fetal weight was increased. There was a decrease in pulse amplitude as a percentage of diastolic diameter and an increase in the diastolic systolic diameter ratio. Fetal outcome was examined in relation to the diastolic systolic diameter ratio. Those with a high ratio (above 90th centile of normal group) exhibited significantly more adverse indices of fetal outcome.

CONCLUSIONS

The fetal aortic pressure pulse waveform was represented by the vessel diameter waveform. In fetal compromise reduced pulse amplitude and increased diastolic to systolic diameter ratio suggest corresponding changes in arterial pressure pulse. We suggest these are the response of the cardiac pump to increased afterload imposed by the high umbilical placental vascular resistance.