Understanding the pathophysiology of intra-uterine growth retardation: the role of the ‘lower limb reflex’ in redistribution of blood flow

Abstract

Doppler ultrasound was used to investigate the circulatory redistribution and underlying reflex responses of fetal cardiovascular compensation in 30 small-for-gestational age (SGA) fetuses. The utero-placental bed, umbilical artery and vein, thoracic and abdominal aorta, internal and external cerebral arteries were evaluated. The values were compared to reference ranges constructed from 135 normal pregnancies, correlated to fetal blood gases obtained by cordocentesis and compared to the outcomes. In Group I (mortality and morbidity), all fetuses had loss of end-diastolic frequencies (L-EDF) in the abdominal aorta (100%), but only 20 (87%) and 13 (56%) had L-EDF in the thoracic aorta and umbilical artery respectively. High vascular resistance in the placental bed and low impedance in the middle cerebral and common carotid arteries was found in 14 (61%), 12 (52%) and 20 (87%) fetuses, respectively. In Group II (Healthy infants) two fetuses had high utero-placental vascular resistance and one had brain-sparing. Doppler indices did not always reflect fetal hypoxaemia demonstrating that redistribution in SGA fetuses may not be triggered by a fall in pO 2, and that hypoxaemia is an associated pathology but may not be the underlying cause. It is postulated that redistribution in SGA fetuses is regulated by reflex mechanisms (the ‘lower limb reflex’) which result in severe vasoconstriction in the abdominal aorta, mesentery and carcass, favouring the brain and cardiac muscles. This mechanism explains the good predictive value of L-EDF in the abdominal aorta for poor neonatal outcome (sensitivity, specificity and positive predictive value, all 100%). In SGA, Doppler measurements are better predictors of outcome than fetal blood gases, as they reflect underlying fetal compensatory reflex mechanisms.