Peripheral Blood Flow Intensity in Maternal Kidneys and Correlation with Blood Pressure

Abstract

Objective: This article aimed to compare the peripheral blood flow intensity (BFI) in the maternal right and left kidney in uncomplicated third trimester pregnancies and to study possible associations between parenchymal BFI in the kidneys and maternal, labor and new-born characteristics. Methods:We conducted a prospective cohort study in third trimester pregnancies at Trondheim University Hospital, Norway, from January to April 2018. The BFI in the maternal kidneys was examined with color Doppler ultrasound and the peripheral BFI was calculated using the Pixel Flux-method which is a method based on pixel wise calculation of color in an image. Velocity was coded by the color bar in the ultrasound device. The peripheral BFI was calculated as the product of velocity and area encoded by all colored pixels divided by all pixels within the cortical layer by a software examining the color in each pixel of a standardized region of interest of the renal parenchyma in video-clips through a complete heart cycle. BFI (cm/s) was thus calculated as follows: BFI = mean area colored pixels _ mean velocity colored pixels mean area all pixels of the ROI The primary outcome measure was peripheral BFI in the renal cortex. We also examined associations between BFI in the entire cortex with maternal age, body mass index, blood pressure, pH in the umbilical artery, Apgar score after 5 minutes and birthweight. Results: In all, 51 pregnant women were included in the study, but 17 were excluded, mainly due to movement artifacts from the pulsating aorta, leaving 34women in the final study population.We found significantly lower BFI in the cortex of the left kidney compared with the right kidney, 0.37cm/s versus 0.69cm/s, respectively (P = 0.04). Dividing into regions, the BFI was significantly lower in the proximal 25% of the left renal cortex compared to the corresponding right side (P=0.01), and in the proximal 50% cortex (P=0.02), but the differences were not significantly different in the distal 25% (P=0.06) or in the distal 50% (P=0.20) of the renal cortex. We observed a significant negative correlation between peripheral BFI in the left kidney and both systolic blood pressure (r=_0.38; P=0.03) and diastolic blood pressure (r=_0.36; P=0.04), and no significant correlations between BFI with maternal age, body mass index, blood pressure, pH in the umbilical artery, Apgar score after 5 minutes and birthweight in the left kidney (P>0.05). We did not observe any significant correlations between BFI in the right kidney and maternal or new-born characteristics (P>0.05). Conclusion: The BFI in the left renal cortex was lower compared with the right renal cortex, and BFI in the left renal cortex was negatively correlated with blood pressure, but not significantly correlated with maternal age, body mass index or newborn characteristics. We did not observe any significant correlations between BFI in the right kidney and maternal or new-born characteristics.