Venous blood flow in maternal kidneys in third trimester of pregnancy

Objectives: Maternal intra-abdominal pressure and hemodynamics change during pregnancy. The left renal vein may be compressed between the uterus and the spine and aorta, causing congestion and impaired venous return from the left kidney during late pregnancy. The aim of this study was to compare venous and arterial blood flow between the right and left kidney in the third trimester in women without known pregnancy complications.Methods: We conducted a prospective cohort study in 50 women with uncomplicated third-trimester pregnancies at Trondheim University Hospital, Norway, from January to April 2018. The arterial and venous blood flow were examined with pulsed wave Doppler in the hilum of the kidneys and the cross section of the area (CSA) of the vessels was measured from 3D acquisitions. Two diameters of the main vein and artery were measured after rotating the image of the vessels in the C-plane to be as circular as possible. CSA was calculated as π×(mean diameter/2)². Blood flow volume (ml/minute) in the vessels were calculated as 0.5 × TAmax (cm/s)×CSA (cm²)×60. The main outcome was venous and arterial blood flow volumes, and secondary outcomes were maximum velocity (Vmax), minimum velocity (Vmin), pulsatile index (PI), time-averaged maximum flow (TAmax) and renal interolobar vein impedance index (RIVI). We also examined possible associations between blood flow and maternal age, BMI and blood pressure.Results: We observed differences in venous flow parameters between the two kidneys. The mean total flow volume in the renal veins was 274 ml/min in the left vein versus 358 ml/min in the right vein (p=.10). Vmax, TAmax, PI, and RIVI were all significantly lower in the left renal vein. No differences in arterial blood flow between the two kidneys were found. BMI was negatively correlated to flow in the left renal vein (r= -0.28; p<.05), but not associated to flow in the right renal vein.Conclusion: We found that venous flow pattern differs between left and right renal veins in uncomplicated late pregnancies, but the total flow was not significantly different. New studies should be done in women with preeclampsia.