Extracellular volume and glomerular filtration rate in children with chronic kidney disease

Abstract

Background and objectives: Extracellular volume (ECV) is the fluid contained in all noncellular compartments of the body and is a quantity tightly controlled by the kidney. Thus, there is a strong link between ECV and kidney function.

Design, setting, participants & measurements: The Chronic Kidney Disease in Children (CKiD) study uses injected iohexol to obtain direct measures of GFR. Direct calculation of ECV was viable from GFR studies using descriptors of the disappearance curves. Using linear regression methods on the log-transformed variables, markers of size (height and weight) and biomarkers of kidney disease (serum creatinine, blood urea nitrogen, and cystatin C) were assessed for their relationships with ECV normalized to body weight (ECV/wt). The relationship to hypertension (systolic BP >95th percentile for age, sex, and height) was also assessed.

Results: Data from 790 iohexol studies with medians for GFR=43.4 ml/min per 1.73 m(2), weight=35 kg, and height=1.4 m were used. The median ECV was 8.6 L, and the median ECV/wt was 0.23 L/kg. ECV was found to be a function of height (m) and weight (kg) according to the relationship ECV=√weight×height. Biomarkers of kidney disease yielded significant relationships with ECV/wt, but the strength of association was small. No significant association between ECV/wt and hypertension was found.

Conclusions: ECV has relevance to studies of chronic kidney disease, is related to biomarkers, and can be easily estimated from the square root of weight and height.

© 2011 by the American Society of Nephrology

Figures

Figure 1.

Distribution of ECV/wt by age (in the 788 person-visits) and Tanner stage(in the 756 person-visits) from the CKiD cohort visits 1 and 2. The median values of ECV/wt across categories of age (from left to right) were 0.26, 0.24, 0.23, and 0.21 L/kg and across categories of Tanner stage (from left to right) were 0.24, 0.23, 0.23, 0.22, and 0.20 L/kg.

Figure 2.

Plot of ECV versus BSA illustrating the correlation between the two measures of body size. ECV can be estimated from height and weight in a similar manner to BSA.

Figure 3.

Distribution of ECV/wt by categories of greater than the 95th percentile and less than or equal to the 95th percentile of systolic BP (normalized for height and age) in 756 person-visits from CKiD cohort visits 1 and 2.