Extracellular Volume and Glomerular Filtration Rate in Children with Chronic Kidney Disease

Abstract

SummaryBackground 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 Ͼ95th percentile for age, sex, and height) was also assessed.Results Data from 790 iohexol studies with medians for GFR ϭ 43.4 ml/min per 1.73 m 2 , weight ϭ 35 kg, and height ϭ 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 ϭ ͌ weight ϫ 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.

Insights from the Chronic Kidney Disease in Children (CKiD) Study

“…To date, 22 studies have been published from data collected in CKiD, 15 of which address these four clinical domains. The remainder address methodological issues of measurement of kidney function (3)(4)(5)(6) or methodologic issues related to the analysis of longitudinal data (7)(8)(9). The purpose of this interim review is to summarize these initial reports.…”

Estimation of extracellular fluid volume in children

“…When this relation was applied to the children of groups 1 and 2, eECV was 5.88 × BSA 1.20 (r00.93) and 5.89 × BSA 1.24 (r 00.94), respectively (Fig. 6), compared to eECV06.83 × BSA 1.24 (r00.95) in the study of Abraham et al [5]. These three equations give values of eECVat a BSA of 1.73 m 2 of 11.4, 11.6 and 13.5 l, respectively.…”

“…Many equations have been developed to estimate various body fluid volumes from height and weight, including total body water [1], blood volume [2] and extracellular fluid volume (ECV) [3][4][5][6][7], but few have been developed for children.…”

“…This author is aware of four equations for estimating ECV that are applicable to children, two of which were developed specifically for children [4,5] and two developed to encompass both children and adults [6,7].…”

Is Body Surface Area the Appropriate Index for Glomerular Filtration Rate?

“…(Bird, 2003;Peters, 1992Peters, , 1994bPeters, , 2000 Furthermore, several scientists showed a high correlation between BSA and ECV. (Abraham et al, 2011;Newman, 1944;Peters, 2004;White & Strydom, 1991) One could therefore argue that since this equivalence exists, either ECV or BSA can be used to index the GFR. Various studies suggested that ECV is at least a more appropriate index for GFR in children.…”