Discussion and Suggestions for Research

The acute “supine hypotensive syndrome of pregnancy” is well known, but the possibility that chronic and unrecognized obstruction of the inferior vena cava by the gravid uterus could lead to serious fetal, placental and myometrial complications has been neglected. Hemodynamically significant obstruction of the inferior vena cava decreases maternal cardiac output, and these decreases can be used as a signal which alerts us to the underlying obstruction of the inferior vena cava and uterine venous return. Techniques have been developed for easily identifying trends or sudden changes in maternal cardiac output and reliable detection of the direction of change of cardiac output is sufficient for detecting and correcting obstruction of the inferior vena cava. Correct absolute values for cardiac output are not needed for this simple yet potentially important application. Obstruction of uterine venous return at the vena cava not only decreases intervillous space perfusion, but also increases intervillous space pressure, and this increased intervillous space pressure may be as—or more—detrimental to fetal oxygenation than decreased intervillous perfusion per se. Obviously, unobstructed uterine venous return is not the only requirement for a healthy pregnant woman and a healthy fetus and newborn. A healthy pregnancy depends on a system of four separate hemodynamic “components”: 1) unobstructed uterine venous return, 2) a healthy maternal heart, 3) well remodeled spiral arteries gently and abundantly perfusing unobstructed intervillous spaces and 4) low myometrial tissue pressure (since all perfusion of the placenta and myometrium passes through the myometrium). These four hemodynamic components are arranged in series, which means that obstruction of uterine venous return will impair the overall success of the entire four-component system. Hence, this book argues that obstruction of uterine venous return does not act in isolation, but rather may act to exacerbate problems in the other three components of the system, such as a) myocardial dysfunction, b) inadequate spiral artery remodeling, or c) a larger uterus which has increased myometrial tissue pressure due to Laplace’s Law. Even though obstruction of uterine venous return may be only one of several interrelated factors which contribute to a problem pregnancy, it nevertheless deserves prompt investigation because it may be the only factor in a precarious pregnancy which can be fixed easily and immediately. Several different technologies can be used to trend maternal cardiac output in order to detect obstruction of the inferior vena cava, but for research and clinical use in large numbers of patients, technologies should be developed and chosen which are 1) safe, non-invasive, continuous and hands-free, 2) easy to learn, apply and interpret, and 3) well-tolerated by pregnant women for prolonged use during normal daily activities in the absence of medical or research personnel. If “cardiac output-guided maternal positioning” is shown to improve pregnancy outcomes, the eventual goal of device development should be wearable devices which give immediate and easy-to-understand feedback to the pregnant woman that her current body position may be unfavorable for optimal uterine perfusion and that she should change her position—for example, by turning onto her left side rather than lying supine. The ideas in this book may be relevant to a wide range of pregnancy-related pathologies, namely fetal growth restriction, preeclampsia, preterm birth, dysfunctional labor and uterine atony. Or they may simply be theories which are not true, and which will be disproven by further research. But these ideas deserve prompt further investigation because—if they are true—they could be the source of breakthroughs in our efforts to improve maternal and fetal outcomes.