THE FETAL AORTIC PRESSURE PULSE WAVE-FORM IN NORMAL AND COMPROMISED PREGNANCY

Objective To study the arterial pressure waveform in the descending th oracic aorta during pregnancy in both normal and compromised fetuses. Design The pressure pulsation waveform propagated along the vascular t ree, and acting laterally on the arterial wall, produces a correspondi ng change in the vessel diameter. The distance between diametrically o pposite points of the aortic lumen was followed using a phase locked l oop echo tracking system coupled to a B-mode ultrasonic imager (centra l frequency 3.5 MHz). Setting Tertiary referral unit, teaching hospita l. Participants A cross-sectional study of 80 normal fetuses between 2 0 and 40 weeks yielded normal data. We studied 58 women with evidence of potential fetal compromise (high umbilical artery systolic:diastoli c ratio). Main outcome measures From the aortic diameter waveform we m easured the maximum systolic and minimum diastolic dimension and calcu lated pulse amplitude. The first derivative of the aortic diameter wav eform identified the incisura of aortic and pulmonary valve closure an d was used to time the end of ventricular ejection and systole. Result s In normal pregnancy there was an increase in systolic and diastolic diameter and pulse amplitude with advancing gestation. Ventricular eje ction time was constant. In the fetal compromised group the absolute s ystolic and diastolic diameters were within the normal range, but dias tolic diameter per unit fetal weight was increased. There was a decrea se in pulse amplitude as a percentage of diastolic diameter and an inc rease in the diastolic systolic diameter ratio. Fetal outcome was exam ined in relation to the diastolic systolic diameter ratio. Those with a high ratio (above 90th centile of normal group) exhibited significan tly more adverse indices of fetal outcome. Conclusions The fetal aorti c pressure pulse waveform was represented by the vessel diameter wavef orm. In fetal compromise reduced pulse amplitude and increased diastol ic to systolic diameter ratio suggest corresponding changes in arteria l pressure pulse. We suggest these are the response of the cardiac pum p to increased afterload imposed by the high umbilical placental vascu lar resistance.