Abstract:
Abstract Background: Recent literature has found a significant association between supine sleep position and stillbirth in late pregnancy. In this position the gravid uterus completely obstructs the inferior vena cava, however, this is compensated for by the collateral venous system providing an alternative route for blood flow back to the systemic circulation. A small number of women experience supine hypotensive syndrome, thought to be due to inadequate collateral circulation Objective: To review the literature describing the anatomy of the collateral venous system and its variations. Search Strategy: A systematic review was conducted using the electronic databases: Medline, Embase, Scopus and Google Scholar. Selection Criteria: Relevant anatomical and radiological literature studying the azygos and ascending lumbar veins in particular were reviewed. Search limited to adult and human studies only. Results: The collateral venous system is categorised into superficial, intermediate and deep systems, the latter being the dominant pathway. The azygos pathway provides an immediate collateral circulation in the event of acute inferior vena caval obstruction. The abdominal portion of this pathway has not been well studied and there are certain variations that may render this pathway ineffective. Conclusion: The collateral venous system provides an alternative route for blood to flow back into the systemic circulation when occlusion of the inferior vena cava occurs in the supine position in late pregnancy. However, there are various anatomical variations that may render this pathway ineffective and this may have implications for late pregnancy stillbirth.