The role of Doppler technology in the evaluation of fetal hypoxia.

Failing intrauterine support to the fetus can lead to intrauterine growth retardation (IUGR) and hypoxia and it is associated with a high risk of perinatal morbidity and mortality. The main effects of moderate to severe hypoxia on the fetus are different degrees of blood flow redistribution and reduction of oxygen consumption to maintain oxygen delivery to the central organs at the expenses of peripheral organs. The redistribution results in a 'brain sparing' effect. Recently, a Doppler ultrasonic technology (continuous wave, pulsed wave and colour flow imaging) has been developed for the non invasive measurement of flow. Doppler velocimetry detects the flow velocity waveform (FVW) which reflects the cardiac output, the vascular compliance and the resistance to the flow in a defined point of the vessel. Velocity waveform indices or even simpler criteria, such as the presence or absence of diastolic flow or flow reverse during diastole, have been applied to a number of fetal vessels. A significant relationship exists between blood oxygen, systemic lactic acidosis (determined by cordocentesis) and increase PI values in umbilical artery (UA), thoracic aorta (TA) and renal artery (RA). Moreover, in experimental animals during steady state hypoxia, several cardiovascular parameters are affected (heart rate/cardiac output decreases and blood pressure increases) while placental flow don't show a significant variation thus suggesting a raise in placental vascular resistance. Redistribution of the flow may be reliably evaluated by the cerebroplacental ratio (i.e. ratio between PI of MCA and PI of UA, c/p). The phenomenon of absent or reverse end diastolic flow (ARED) has been related to severe fetal hypoxia and this abnormal finding is suggested to occur before FHR monitoring shows evidence of fetal distress. Doppler ultrasound, in combination with ultrasonic imaging, offers the possibility of assessing fetal hypoxia in some detail, without significant discomfort for the patient and without any known risk. This functional assessment of the fetus offers that possibility of furthering our knowledge of mechanisms involved and the consequences of the low oxygen level in the fetus and of the effects of various therapeutic measures.