DOPPLER INVESTIGATION OF THE FETAL CIRCULATION

Doppler ultrasound provides a non-invasive method for the study of fetal haemodynamics. Animal studies on effects of hypoxia have provided evidence of a redistribution of cardiac output to favour perfusion of the fetal heart, adrenals and brain, at the expense of the carcass, gut and kidneys. This paper summarises the changes in fetal Doppler parameters with advancing gestation. Furthermore, it examines the alterations in fetal haemodynamics in relation to fetal blood oxygen tension in samples obtained by cordocentesis from small for gestational age (SGA) fetuses. In SGA fetuses increased downstream impedence to flow in the umbilical artery, as demonstrated by the absence of end-diastolic frequencies in the flow velocity waveforms (FVW), is associated with fetal hypoxia (figure 2). This presumably reflects the underlying derangement of placental structure and function. The impedence to flow and mean blood velocity were also measured in FVWs from the descending thoracic aorta and common carotid artery, obtained by pulsed Doppler ultrasound, and from the middle cerebral and renal arteries obtained by colour flow imaging. There were significant correlations between the degree of fetal hypoxia and alterations in Doppler parameters, which were compatible with the brain sparing effect (figures 3, 4). Thus, in fetal hypoxia impedence to flow in the common carotid and middle cerebral arteries was decreased, whereas impedence in the aorta and renal artery was increased. There were simultaneous alterations in the mean blood velocity in the opposite direction to those in impedence.