Effect of Lateral Tilt Angle on the Volume of the Abdominal Aorta and Inferior Vena Cava in Pregnant and Nonpregnant Women Determined by Magnetic Resonance Imaging

We read with interest the article by Higuchi et al.¹  in which the authors, using magnetic resonance imaging, study the effect of the gravid uterus compression on the aorta and vena cava volumes at 15°, 30°, and 45° left-lateral tilt angles, showing no reduction in the aortic volume and no benefit on the observed cava compression below 30° degrees.

We believe that the study design does not actually allow for any clinically meaningful conclusions from a practical point of view. In fact, we do not know the important contribution of the pregnancy mechanical factors.

Although the study was aimed to answer practical questions regarding the management of the gravid women at the operating table, from the obstetrical perspective, the study is too simplistic to achieve its objectives. First, the authors do not take into account the fetal weight percentile,²  a crucial parameter when weight-related compression is considered in pregnancy. In fact, normality range for fetal weight at 39 weeks spans from 2,500 g (third percentile limit for fetal growth restriction) to nearly 3,800 g (90th percentile limit for fetal macrosomia).^2,3  Therefore, even normal fetuses may present with a weight difference of nearly 1,500 g, and this may be increased in severely growth-restricted or macrosomic fetuses up to 2,000 to 2,500 g.²  Second, in their analysis, they ignore the ponderal importance of accessory structures such as the placenta⁴  and amniotic fluid,⁵  which may also account for notorious differences among different pregnancies. Third, the study lumps different gestational ages together without noticing the existence of differences between 37 and 39 weeks that could reach in case of the fetal 50th percentile up to 350 g.² 

Most fundamentally, the research question asked could gloss over the importance of the effect of lateral tilt angle in pregnant women with supine hypotensive syndrome receiving intravenous fluid.

As a result of all these considerations, any proposal based only on the tilt angle is obviously incomplete. Because the missing data can be easily evaluated with ultrasound, especially the estimated fetal weight,⁶  we would like to propose the authors a multivariate analysis, adding all fetal and maternal ponderal factors, to accurately determine the precise circumstances that should be considered in the hemodynamic management of the term gravid women, especially in the subset of obstetrical patients in whom most need to know the real effect of tilt angle on the relieve of possibly aorta and inferior vena caval compression. We are talking about the pregnant women with supine hypotensive syndrome.

The authors declare no competing interests.