015 - Validation of kidney oxygenation monitoring with NIRS in preterm neonates: kidney location, depth, and laterality differences

Publication Number: 15.7055

Matthew W. Harer, University of Wisconsin School of Medicine and Public Health, Waunakee, WI, United States; Lauren Gadek, Northwestern University The Feinberg School of Medicine, Chicago, IL, United States; Kate S. Walker, University of Wisconsin School of Medicine and Public Health, Madison, WI, United States; Shayla Schwingle, University of Wisconsin School of Medicine and Public Health, Madison, WI, United States; Cassandra E.. Nelson, UW Madison Dept of Pediatrics PCRC, Madison, WI, United States; Paige E. Condit, University of Wisconsin School of Medicine and Public Health, Madison, WI, United States

Background: Near infrared spectroscopy (NIRS) is used in preterm neonates to monitor kidney oxygenation. Sensors on the flank emit an arc of light 2-3 cm deep, but the depth of kidney tissue in preterm neonates is unknown. Anatomic and blood flow differences could create variances in oxygenation to the right and left kidneys.

Objective: Assess differences between simultaneous right left kidney oxygenation and kidney location/depth with ultrasound. We hypothesized there would not be clinically significant differences in right left kidney oxygenation and that some neonates would have kidneys < 2 cm deep.

Design/Methods: Preterm neonates < 32 weeks gestational age (GA) were enrolled in a prospective observational NIRS monitoring study. NIRS monitoring was performed with neonatal 5100 INVOS monitors and neonatal sensors. Two epochs of three hours of simultaneous bilateral kidney monitoring were performed (5-8 days and 9-14 days). Point of care kidney ultrasound was performed once between days 5-12. A linear probe from a Sonosite SII machine was placed below the last rib and above the iliac crest at T12-L2 and three measurements were performed – skin to superior kidney, skin to inferior kidney, and spine to kidney.

Results: 99 neonates had an ultrasound performed (median GA 29 wks (IQR 27-30), median birth weight (BW) 1260g (IQR 1030-1470)). There were no significant right left differences in median skin to superior kidney depth (0.39 and 0.38 cm; p=0.62) or spine to kidney distance (1.06 and 1.02 cm; p=0.31). There was a difference in right left skin to inferior kidney depth (2.15 and 2.04 cm; p=0.02; Figure 1). Correlation to GA, BW, and length are shown in Table 1. 42/99 (43%) of left and 26/99 (26%) of right measurements had an inferior kidney depth < 2 cm (p=0.02).

87 neonates had bilateral monitoring during the Epoch 1 and 78 during Epoch 2 (median GA 29 wks (IQR 27-30), median BW 1260g (IQR 1020-1450)). There was higher oxygenation in the left kidney in a combined analysis of both epochs (65.6 vs. 62.7, p< 0.01). Both GA and sex analyses showed higher oxygenation on the left compared to right (p < 0.05). Figure 2 shows the number of neonates with 10, 10-20, and >20-point differences in simultaneous right left kidney oxygenation. Individual differences of >10 percentage points were seen in 37% of neonates.

Conclusion(s): Important anatomic differences are seen in kidney depth, creating the possibility that more than just kidney oxygenation is being measured in the most premature neonates. Laterality differences in kidney oxygenation exist and should be considered when assessing kidney oxygenation in preterm neonates.

Figure 1 - Right and Left Kidney Comparison
Right left comparison.pdfPoint of care kidney ultrasound was performed once between days 5-12. A linear probe from a Sonosite SII machine was placed below the last rib and above the iliac crest at T12-L2 and three measurements were performed – skin to superior kidney, skin to inferior kidney, and spine to kidney. There were no right left differences in skin to superior kidney or spine to kidney distance but there was a difference in right left skin to inferior kidney distance (p=0.02; Mann Whitney test).

Table 1 - Correlation of Point of Care Ultrasound kidney measurements to neonatal birth length, birth weight, and gestational age
Table 1.pdfEach of the 3 point of care kidney ultrasound measurements on the right and left were compared to birth length, birth weight, and gestational age at birth. The strongest correlation of ultrasound measurements are birth weight and birth length (Spearman r).

Figure 2 - Point Differences in Simultaneous Right Left Kidney Oxygenation Monitoring
Point Differences.pdfThis Figure shows the number of neonates with 10, 10-20, and >20-point differences in simultaneous right left kidney oxygenation. Individual differences of >10 percentage points were seen in 37% of neonates.