Pathophysiology of fetal intrauterine central shunts in high-risk pregnancies : a prospective observational Doppler study.

Abstract

The primary objective of antenatal assessment and monitoring is to ensure wellbeing of the fetus and the mother. There are different methods of assessment during pregnancy and in labour. Doppler ultrasound is one of the tests widely used in clinical practice in the evaluation of pregnancies that are at a greater risk of developing maternal or fetal complications due to uteroplacental insufficiency. Doppler ultrasound enables evaluation of sequential changes in circulatory haemodynamics in the fetus by evaluation of the fetus for signs of brain sparing and severity of redistribution of circulation. Recognition of abnormal Doppler flow patterns helps the clinician to optimise the appropriate timing of delivery. Identification of the ‘high risk’ fetus, before any changes of fetal compromise become evident, still remains one of the major dilemmas in contemporary clinical practice. This thesis seeks to explore the role of Doppler monitoring fetal intrauterine central shunts as a method of identifying the ‘high-risk’ fetus before any other established parameters, such as, fetal biometry, fetal weight or flow waveforms in umbilical artery become abnormal. This thesis also evaluates the role of serial Doppler monitoring of fetal central shunts in those fetuses where IUGR has been established. This is based on the premise that the intrauterine shunts are present in fetal circulation to work closely with the placenta to ensure appropriate nutrition and oxygenation of the fetus, bypassing the lungs. Four prospective longitudinal studies were designed to evaluate the role of fetal intrauterine shunts in adaptive response mechanisms in cardiovascular stress. Two models were taken into consideration: an ‘acute cardiovascular stress’ model and a ‘chronic cardiovascular stress’ model. To study the ‘response to acute cardiovascular stress’ in high-risk fetuses, a cohort of mothers undergoing fetal intrauterine transfusion for fetal anaemia were selected. These fetuses were scanned immediately before and after transfusion, and Doppler flows through all the intrauterine shunts were documented and compared with fetoplacental and cerebral circulation. To study the ‘response to chronic cardiovascular stress’, a prospective longitudinal observational study was designed and the sequence of changes in Doppler ultrasound of the fetal central shunts studied and compared with the Doppler flow waveforms of normal pregnancies with a group of pregnancies complicated by uteroplacental insufficiency. Normograms were designed for all the Doppler parameters and flows from adverse pregnancy outcomes were compared to the normogram. The pregnancy outcomes in the longitudinal study were correlated with placental pathology. Our study showed that although changes were demonstrated in the flow patterns within central shunts, these changes were not statistically significant in the ‘acute cardiovascular stress model’, suggesting that there may be other haemodynamic alterations in acute cardiovascular stress. However, in the ‘chronic cardiovascular stress model’, the results suggest that the intrauterine cardiac shunts may play an important role in redistribution of fetal flows in early stages of growth restriction, suggesting that Doppler ultrasound monitoring of foramen ovale can be potentially used as a screening tool to identify high-risk fetuses as early as 16 weeks.