The postural impact of the gravid uterus on the renal veins and its clinical significance.

OBJECTIVE

The mechanical impact of the gravid uterus on the vasculature is well known, but its role in hypertensive disorders of pregnancy (HDP) is incompletely understood. Prior studies have demonstrated an increase in blood pressure greater than 20 mmHg in pregnant participants upon rolling from the left lateral posture (LLD) to supine is highly predictive of preeclampsia, but the use and mechanism of this test remain underexplored. Here, we assess the impact of posture on renal venous caliber, its relation to HDP, and the potential for renal venous compression to occur in the real-world environment.

METHODS

We performed a retrospective study of 161 pregnant participants (22-40 weeks gestation) with previously acquired magnetic resonance imaging to measure renal vein caliber. We assessed sleeping and waking posture via a two-part cross-sectional survey of 147 pregnant and non-pregnant participants. Renal vein diameters were analyzed using repeated measures mixed-effects analysis with Tukey's post hoc test for multiple comparisons. Renal vein ratios and sleep posture vulnerability (a metric of the potential for renal venous compression) were analyzed using one-way ANOVA with Tukey's post hoc test for multiple comparisons. Renal vein ratios were additionally analyzed using multivariate linear regression with a main-effects weight least squares model weighted by 1/y.

RESULTS

Left renal vein diameter ratios were larger in supine (45.8 ± 26.9%) than LLD (21.7 ± 35.8%, p < 0.01), but not right lateral decubitus (RLD, 32.2 ± 32.6%, p = 0.19). The study was underpowered to assess differences in participants with HDP: few participants underwent MRI in RLD (n = 17 normotensive, n = 1 preeclamptic, n = 0 gestationally hypertensive) or LLD (n = 23 normotensive, n = 0 preeclamptic, n = 2 gestationally hypertensive), but a multivariate linear regression demonstrated a significant effect of gestational age and maternal posture on left renal vein diameter. Self-reported sleeping posture demonstrated most pregnant participants (94.5%) fell asleep on their side, but up to 23.8% woke on their back.

CONCLUSION

These results highlight the potential for renal venous compression and the flow-protective potential of the left lateral posture. Imaging data demonstrates renal venous compression in the supine posture and self-reported sleep data demonstrate that patients may inadvertently sleep supine even when falling asleep on their side. While most patients likely have a collateral network to accommodate renal venous compression, those who do not may be vulnerable to HDP. A longitudinal patient-matched prospective study is needed to fully dissect this underlying mechanism.