Reply

Abstract

The statement by Archer et al. not to forget aortocaval compression of the intra-abdominal veins of pregnant women in the supine position during maternal sonography is very relevant. This issue indeed deals with one of the fundamental characteristics of venous hemodynamics during pregnancy, which differs from non-pregnant conditions. It is well known that under normal physiological conditions, venous return equals cardiac output. Therefore, a change of venous return will automatically influence cardiac output and, as such, modulation of venous return is an important physiological mechanism involved in control of cardiac output. Contrary to arteries and arterial flow, external pressure to veins is an important determinant of venous return1. This characteristic of venous return becomes particularly relevant in the abdomen of a pregnant woman. During pregnancy, a gradual increase of intraperitoneal pressure has been reported2, which is associated with a rise of intravenous pressure in the femoral veins3. In a review on vena cava syndrome4, large interindividual variability was reported in cardiovascular reflex responses associated with the supine hypotensive syndrome. Others, including Archer et al., found a high degree of variation in cardiac output, especially in the third trimester5. Regulation and control of venous return and cardiac output therefore seems to be one of the major physiological challenges with which the female body must deal during pregnancy, and this is particularly reflected in the cardiovascular reflex response to maternal position. Evidence is growing that some pregestational or gestation-induced conditions of the venous compartment, and/or the cardiovascular system as a whole, interfere with this function and may predispose to specific gestational disorders, such as pre-eclampsia or fetal growth restriction6-8. The hemodynamic response to cardiovascular challenge by position change or exercise is one way of assessing the adaptive capacities of the maternal cardiovascular system9. This assessment should also be considered in maternal sonography, while respecting fetal safety conditions. Other methods to explore the functionality of the venous compartment have been reported, but, bearing in mind the complexity of reflex responses to biological variables such as position, breathing, diurne and pregnancy, it is easy to understand that these studies have always been technically difficult. Therefore, methodologic standardization is of major importance in studies of venous hemodynamics, especially when it comes to the application of ultrasound—and Doppler—technology10. In order to conduct these so-called cardiovascular challenge tests, new technologies such as electrical cardiometry may indeed be very useful, as Archer et al. have illustrated. However, validation studies on their application in experimental or clinical settings are required11. W. Gyselaers*, * Department of Obstetrics & Gynaecology, Ziekenhuis Oost Limburg, Schiepse Bos 6, B-3600 Genk, Belgium; Department of Physiology, Hasselt University, Diepenbeek, Belgium