Nephrotic syndrome (NS) is a kidney disease characterized by the massive leakage of proteins into the urine, with consequent hypoalbuminemia and oedema formation [
Availability of a simple, inexpensive, and harmless method for routine assessment of volume status could provide new and clinically useful information to clinicians in the treatment of children with NS. Bioelectrical impedance spectroscopy (BIS) may prove to be such a method. BIS is noninvasive, harmless, quick, simple, and an inexpensive method that can be performed with a portable instrument. These features make it suitable for routine use.
Bioelectrical impedance is the opposition (impedance) of the body tissue against the flow of electrical current. The bioimpedance depends on the body composition, especially the distribution of water and nonwater in the body. Scales claiming to measure not just your weight but also your body composition make use of bioimpedance; however, most of these devices measure only at a single frequency. BIS measures bioimpedance at a wide range of frequencies, giving more complete and more reliable information.
BIS has recently been used to demonstrate increased volumes of extracellular water (ECW) in NS patients compared to controls [
While bioimpedance-based approaches for quantitative estimation of water volumes exist, these methods are based on prediction equations, which are prone to bias and imprecision. When only a single frequency is measured, the prediction equations are typically simple and purely empirical [
An alternative approach to using prediction equations may be the use of BIS resistance parameters, either as the resistances (R) or as the so-called resistance indices (RI) [
Briefly, to the BIS apparatus, the body can be represented by the electrical circuit shown in Figure
Model representing the electrical properties of body tissue [
Generally, there is a growing interest from the scientific community in the use of raw impedance data as indices of body water volume, especially in patients with altered body water distribution [
The aims of the present study were to investigate how BIS measurements can reflect disease status in NS, including both resistances (R
Abbreviations and concepts used in this paper.
Abbreviation | Description |
---|---|
| |
NS | Nephrotic syndrome |
SFBIA | Single-frequency bioimpedance analysis |
BIS | Bioelectric impedance spectroscopy (many frequencies) |
| |
ANS | Children with active NS (n = 8) |
ANS | Subgroup of ANS, same children as NSR group (n = 5) |
NSR | Children from ANS group re-studied at the time of NS remission (n = 5) |
HC | Healthy control children (n = 38) |
| |
ECW | Extracellular water (L) |
ICW | Intracellular water (L) |
TBW | Total body water (TBW = ECW + ICW) |
| |
Z | Impedance for alternating current (Ohm, Ω). |
R | Resistance (Ω) |
| |
R | R of the extracellular water (ECW) |
R | R of the intracellular water (ICW) |
R | R of the total body water (TBW, measured at infinite frequency) |
C | Cell membrane capacitance (nanofarad, nF) |
| |
RI | RI = H2/R (cm2/Ω) of the ECW |
RI | RI of the ICW |
RI | RI of the TBW |
Standardized testing and reporting procedures were followed as far as possible [
Eight children (7 boys, 1 girl, age range: 2-10 years) with active NS (ANS group) were enrolled at the Department of Paediatrics and Adolescent Medicine, Aarhus University Hospital, Denmark. Inclusion criteria were the presence of NS defined by proteinuria >40 mg/m2/day, plasma albumin <25 g/L, oedema, and hyperlipidemia. Exclusion criteria were low plasma levels of C3-complement, postinfectious glomerulonephritis, and vasculitis such as Henoch-Schönlein nephritis of specific glomerulonephritis as, for example, dense deposit disease. Five of the ANS patients (ANS
For comparison, impedance measurements were also made in 38 healthy control children (HC group) (23 boys, 15 girls, age range: 2-10 years). These controls were taken from a previously published dataset [
Informed consent was obtained from the subjects’ parents or legal guardians before study enrolment. The study was performed in accordance with the Helsinki Declaration and was approved by the Central Denmark Region Committees on Health Research Ethics (case number: 1-10-72-17-12).
Participants were not fasting but had refrained from intense physical exercise four hours prior to measurements and had been resting in the supine position for 5 min before measurement and remained at rest (no movement) during measurement. There were no restrictions on voiding. Participants remained clothed with only hands and feet uncovered with the body positioned with the arms and legs abducted at a 30-45° angle from the trunk.
BIS measurements were performed with a Xitron 4200, HYDRA BIS device (Xitron Technologies, San Diego, CA, USA), which measures the impedance at 50 frequencies in the range from 5 to 1000 kHz. All measurements were performed in accordance with and earlier published paper [
Briefly, skin surface Ag-AgCl ECG-style gel electrodes were placed at wrists and ankle for whole-body measurements, and the skin was cleaned with alcohol (ethanol 75%) before the placement of electrodes. Measurements were performed with participants lying supine on a nonconductive surface (hospital bed/examination table) and abducted arms and legs. Intertwining of cables and similar sources of electrical interference was avoided.
Calibration of the device was tested every week with an electronic verification module (TS4201), supplied by the manufacturer (Xitron Technologies, San Diego, CA, USA).
All measurements were made at room temperature (21° to 25°C), between 08:30 and 15:30, by the same trained operator, and performed in triplicate with electrodes remaining in place between measurements. The total mean measurement time was 7 min, covering patient preparation and BIS measurements.
The quality of the impedance data was checked by visual inspection and statistical analysis of the Cole plots, using the ImpediMed SFB7 Multi-Frequency Analysis software (Version 5.4.0.3, Brisbane QLD, Australia) as described previously [
The precision of triplicate measurements made in each NS patient was based on the electrical parameters R
The average CV% showed to be low for R
Resistance varies with the length and cross-sectional area of the conductor. For current passing through a body, neither path length nor cross-sectional area is well-defined. However, a resistance index (RI) can be calculated, which takes body size into account and is roughly proportional to the water volume [
Weight and height were measured by trained personnel before impedance measurements were performed. Weight was measured on digital scales, with light clothes to within 0.1 kg. Height was measured without shoes, to the nearest 0.5 cm using a stadiometer. All measurements were made in duplicate with mean values used.
Resting venous blood samples and blood pressures (Carescape V100 Monitor, GE Healthcare, USA) were collected in the ANS patients before commencing medical treatment. Blood chemistry was performed by trained biomedical laboratory scientists in an accredited hospital laboratory.
Results were presented as mean ± standard deviation (SD), after test for normality, using Q-Q plots and statistical tests (Shapiro-Wilk and Kolmogorov-Smirnow) [
A paired two-tailed Student’s t-test was applied to determine differences in impedance data between patients with active NS and at remission (ANS
Statistical significance was set at a
All statistical tests and graphical illustrations were prepared using the statistical software MedCalc ® (Version 17.9.7, Medcalc Software, Ostend, Belgium).
Table
Characteristics of the subjects enrolled in the study.
Parameter | ANS | ANS | NSR | HC |
---|---|---|---|---|
Sex (M/F) | 7/1 | 4/1 | 4/1 | 23/15 |
Age (years) | 6.9 ± 3.1 | 6.8 ± 3.1 | 7.7 ± 3.8 | 7.5 ± 2.2 |
Study weight (kg) | 31.3 ± 17.1 | 28.5 ± 9.6 | 28.3 ± 10.4 | 25.3 ± 6.2 |
Height (cm) | 120.7 ± 21.1 | 120.3 ± 21.8 | 126.1 ± 26.2 | 126.4 ± 14.2 |
BMI (kg/m2) | 20.1 ± 4.6 | 19.1 ± 1.5 | 17.3 ± 2.0 | 15.6 ± 1.2 |
Data are means ± SD; for group abbreviations, see Table
Table
Clinical data for the ANS patients.
Parameters / Patients | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
---|---|---|---|---|---|---|---|---|
Sex | M | F | M | M | M | M | M | M |
Age (years, months) | 5,2 | 7, 8 | 4,11 | 2,2 | 8,8 | 9,11 | 11,2 | 4,3 |
Blood pressure (mmHg) | 109/71 | 130/93 | 109/68 | 110/70 | 116/77 | 133/73 | 109/56 | 88/62 |
Hypertension† (yes/no) | yes | yes | no | yes | no | yes | no | no |
Pulse (bpm) | 155 | 77 | 92 | 109 | 104 | 99 | 102 | 97 |
P-Albumin (g/L) | 7 | 9 | 9 | 10 | 9 | 9 | 4 | 10 |
P-sodium (mmol/L) | 131 | 132 | 137 | 139 | 129 | 137 | 127 | 141 |
P-potassium (mmol/L) | 3.5 | 4.9 | 5.5 | 3.5 | 5.0 | 3.8 | 3.9 | 3.6 |
P-creatinine ( | 21 | 71 | 24 | 18 | 50 | 56 | 50 | 30 |
eGFR‡ (ml/min/1.73m2) | 195 | 65 | 167 | 177 | 94 | 94 | 108 | 123 |
B-hemoglobin (mmol/L) | 8.5 | 9.3 | 8.1 | 8.5 | 9.4 | 8.7 | 10.6 | 7.9 |
†Hypertension defined as blood pressure above the 95% percentile for high and gender; ‡eGFR estimated by the Schwartz formula [
Impedance measurements were only possible in five patients during remission because of repeated relapse in two of the patients and transition to another hospital of one patient.
One ANS patient was notable for being an apparent outlier for weight (Figure
Relationship between resistance indices and age (left panel) and weight (right panel); ∆: ANS, ▲: NSR, and ○: HC, see Table
Relationship between cell membrane capacitance (C
Data are presented in Table
Measured resistances (R), corresponding resistance indices (RI), and cell membrane capacitances (C
Parameters | ANS | ANS | NSR | HC |
---|---|---|---|---|
R | 447.1 ± 48.9 | 420.2 ± 43.5 | 752.6 ± 71.7 | 816.3 ± 73.8 |
R | 1871.6 ± 182.3 | 1919.7 ± 176.2 | 1799.5 ± 239.5 | 1922.3 ± 224.0 |
R | 359.7 ± 33.3 | 344.1 ± 31.6 | 528.6 ± 46.0 | 572.3 ± 53.2 |
RI | 34.9 ± 11.7 | 37.2 ± 11.4 | 22.1 ± 8.4 | 20.0 ± 4.7 |
RI | 8.4 ± 3.3 | 8.3 ± 3.0 | 9.5 ± 3.7 | 8.6 ± 2.2 |
RI | 43.3 ± 14.8 | 45.5 ± 14.3 | 31.6 ± 11.6 | 28.6 ± 6.9 |
C | 0.53 ± 0.21 | 0.47 ± 0.13 | 0.82 ± 0.34 | 0.68 ± 0.20 |
Data are means ± SD; for group and parameter descriptions, see Table
Statistical comparison of results from Table
Parameters | ANS | ANS vs HC | NSR vs HC |
---|---|---|---|
R | 0.0006 | <0.0001 | ns |
R | ns | ns | ns |
R | 0.0006 | <0.0001 | ns |
RI | 0.0270 | 0.0183 | ns |
RI | ns | ns | ns |
RI | 0.0028 | 0.0402 | ns |
C | 0.0307 | 0.0479 | ns |
For group and impedance descriptions, see Table
Overall, the results showed no significant differences between the children in submission (NSR group) compared to the healthy control children (HC group). Regarding ICW, the corresponding BIS parameter (R
The mean values of R
Correspondingly, the resistance indices RI
Cell membrane capacitance (C
To the authors’ knowledge, this paper demonstrated for the first time the relation between changes in disease status and cell membrane capacitance in paediatric patients with NS. These results were obtained using impedance spectroscopy, which allows the calculation of resistance at the optimal frequencies for ECW (
In the present study, BIS clearly indicated a higher ECW in the ANS patients compared to both the controls (HC group) and patients at remission (NSR group). Importantly, these findings accord with observations made in previous studies in NS patients [
As described in the Introduction, the use of BIS-modelled resistances avoids the assumptions inherent in derived equations for predicting fluid volumes. R
In contrast to the resistance values, resistance indices (RIs) standardize the values according to the subject’s height. RIs have been shown to be an accurate reflection of absolute water volumes in children [
Consideration of basic BIS parameters can include electrical parameters other than resistance. Specifically, charge differences over cell membranes results in a capacitive effect [
Even though there is no reason to suspect cell death or cell destruction in connection with NS, our results indicates that the cell membrane can be affected by the disease, despite the fact that ICW seems unaffected. This may indicate that there are components of the disease that affect the membrane negatively, e.g., inflammation and immune system activation [
Some limitations to the study should be acknowledged. Only a small number of patients were available for study. This is a consequence of the low incidence of the disease with only around 2 new cases per 100,000 [
Impedance measurements were performed as a wrist-ankle (whole-body) analysis. This arrangement does not make it possible to determine how the individual segments or discrete regions of the body contribute to the overall impedance with respect to NS, i.e., to identify in which of the segments/part where the greatest impedance changes occur. To obtain such information, it is necessary to perform focal or segmental measurements [
This study shows how simple resistance indices could be used to assess changes in disease status in children with NS. This approach avoids invoking assumptions underpinning prediction of absolute volumes. More widely, such indices may prove to be useful screening tools for detection of fluid compartment imbalance in a clinical setting where the volume status of at-risk patients needs to be evaluated. In addition, it was observed that changes in C
The data used to support the findings of this study are available from the corresponding author upon request.
The study was performed in accordance with the Helsinki Declaration and was approved by the Central Denmark Region Committees on Health Research Ethics (case number: 1-10-72-17-12).
Informed consent was obtained from the subjects’ parents or legal guardians before study enrolment.
Author Leigh C. Ward provides consultancy services to ImpediMed Ltd. ImpediMed Ltd. had no involvement in the preparation of this manuscript. Other authors have no conflicts of interest to declare concerning this work.
We would like to thank the children and their parents for participation in the study and the clinical staff at the Department of Paediatrics and Adolescent Medicine at Aarhus University Hospital for their important support during this study.