Estimation of total-body and limb muscle mass in hemodialysis patients by using multifrequency bioimpedance spectroscopy

¹ From the Renal Research Institute, New York, NY (GAK, FZ, SS, CK, MKK, and NWL); the Division of Nephrology, Department of Medicine, University of California, Davis, Davis, CA (GAK); the Research Service, VA Northern California Health Care System, Mather, CA (GAK); and St Lukes Hospital, New York, NY (SBH and JW)

² Supported by a grant from the Renal Research Institute, by grant no. PO1-DK42618 from the National Institutes of Health (to SBH), and by the Research Service of the US Department of Veterans Affairs.

³ Reprints not available. Address correspondence to GA Kaysen, Division of Nephrology, University of California, Davis, Genome and Biomedical Sciences Facility, 451 East Health Sciences Drive, Davis, CA 95616. E-mail: gakaysen{at}ucdavis.edu.

ABSTRACT  
Background: Skeletal muscle mass can be measured noninvasively with magnetic resonance imaging (MRI), but this is time-consuming and expensive.

Objective: We evaluated the use of multifrequency bioimpedance spectroscopy (BIS) measurements of intracellular volume (ICV) to model total-body skeletal muscle mass (TBMM) and limb skeletal muscle mass in hemodialysis patients.

Design: TBMM was measured by MRI in 20 male and 18 female hemodialysis patients with a median (range) age of 54 y (33–73 y), weight of 78.9 kg (43.2–120 kg), and body mass index (BMI; in kg/m²) of 27.3 (19.4–46.6). We measured total body water (TBW) by using D₂O dilution, extracellular volume (ECV) as bromide space, and ICV as TBW minus bromide space. Total body potassium (TBK) measured as ⁴⁰K was used as an independent model of TBMM. BIS was used to measure whole-body TBW (ankle to wrist) and TBW in the arms and legs. BIS-estimated ICV was used to construct models to calculate limb muscle mass and TBMM. The latter was compared with models derived from isotopic methods.

Results: BIS yielded a model for TBMM [TBMM = 9.52 + 0.331 x ICV + 2.77 (male) + 0.180 x weight (kg) – 0.133 x age] (R² = 0.937, P < 0.0001) as precise as TBK-measured TBMM [TBMM = 1.29 + 0.00453 x TBK (mEq) + 1.46 (male) + 0.144 x weight (kg) – 0.0565 x age] (R² = 0.930, P < 0.0001) or isotopic methods. BIS models were also developed for measuring leg and arm muscle mass.

Conclusion: BIS provides an estimate of TBMM that correlates well with isotopic methods in approximating values obtained by MRI and can be used to estimate limb muscle mass.

Key Words: Body composition • body cell mass • nutrition • extracellular fluid • total body water • bioimpedance spectroscopy • hemodialysis patients

INTRODUCTION  
The evaluation of nutritional status in dialysis patients is both important and quantitatively difficult. Changes in body composition consistent with malnutrition are frequent in long-term hemodialysis patients and are powerful predictors of mortality (1, 2). Anthropometric measurements of nutritional status frequently are operator dependent. Formulas using those measurements to calculate total body water (TBW) are imprecise and in poor agreement with direct measurements provided by isotope dilution, especially in the obese and in dialysis patients. Body mass index (BMI) is associated with a low relative risk of mortality in hemodialysis patients (3). Body composition may differ in patients having the same BMI. Creatinine can be used to model the relative effects of fat and muscle mass (3), but the availability of an independent method of assessing muscle mass could have value in distinguishing the contributions of muscle and fat to morbidity associated with different BMIs.

Magnetic resonance imaging (MRI) provides a precise measure of the volume of fat, muscle, and bone compartments, depending on the number of slices (4). Volumes are then transformed to masses. Muscle mass is 1.04 times muscle volume (5, 6).

We designed this study to establish whether regression models could be developed to estimate whole-body muscle mass as well as limb skeletal muscle mass in hemodialysis patients by using bioimpedance spectroscopy (BIS) and whether the results obtained were not significantly different from those obtained by using other standard and accepted techniques. Measurement of the quantity of potassium in the TBK also defines the intracellular volume (ICV) (7). The clinical use of measurement of TBK is limited, however, by the availability of instrumentation. Isotopic methods also provide precise evaluations of intracellular and extracellular compartments. Estimation of TBW by using the dilution of isotopically labeled water (D₂O) combined with evaluation of extracellular space by using bromide allows the calculation of ICV by subtraction.

Because of its availability and simplicity, bioelectrical impedance analysis (BIA) has significant potential as a complement to standard anthropometric techniques for assessing the nutritional status of dialysis patients (8). Whereas single-frequency BIA provides data that are useful for predicting mortality in populations (9, 10), the output data (phase angle and reactance) are not readily comparable to other methods that provide estimates of physiologically recognizable compartments, such as ICV, extracellular volume (ECV), and fat-free mass (8). Furthermore, BIA relies on a single frequency, 50 kHz (11).

Isotopic methods are restricted to measurement of whole-body fluid volumes. In contrast, both MRI and BIS (depending on electrode placement) can measure regional tissue compartments or fluid distribution. Thus, we also sought to ascertain whether limb muscle mass as measured by MRI could be modeled by measurements of ICV in the arms and legs.

SUBJECTS AND METHODS  
Subjects
Forty-two hemodialysis patients (19 F, 23 M) aged >18 y were chosen so as to encompass a wide range of BMIs and ages. All subjects had been on maintenance hemodialysis for 3 mo before the study.

All subjects provided written informed consent to their participation. The institutional review boards of St Luke's, Beth Israel, and the University of California, Davis, hospitals approved the study protocol.

Methods
On the day of a regularly scheduled hemodialysis session, the subjects were studied 3 h before initiation of the dialysis treatment. The study began with the administration of D₂O and sodium bromide, as described below. After the administration of these isotopes, the subject was then placed in the 4-pi counter. On exiting the counter, the patient was placed in a recumbent position as described below for measurement of bioimpedance. After this measurement, the patient underwent an MRI scan to measure muscle mass. The patient was then transported to the dialysis unit, where blood was obtained before the initiation of dialysis to measure D₂O and bromide.

After an overnight fast, body weight was measured to the nearest 0.1 kg (Weight Tronix, Pinbrook, NJ), and height was measured to the nearest 0.5 cm by using a stadiometer (Holtain, Crosswell, United Kingdom). A sample of blood (5 mL) was obtained for measurement of baseline deuterium and bromide. D₂O at a dose of 500 mg/kg body wt and 0.06 g of 4 mol NaBr/L were then given by mouth. Whole-body MRI scans were prepared by using a 1.5 Tesla scanner (6X Horizon; General Electric, Milwaukee, WI) to evaluate muscle mass (12). The 4-pi whole-body counter was used to measure ⁴⁰K (13).

Both whole-body (wrist to ankle) and segmental BIS (SBIS, Xitron 4200; Xitron, San Diego, CA) measurements that used multiple frequencies from 5 kHz to 1000 kHz were recorded before dialysis to include defined segmental BIS in the leg and arm on the nonaccess side of the body in those patients having an arm access. In those patients with a catheter for dialysis access, the dominant arm was chosen. Two electrodes were placed on the hand, wrist, and foot for injecting current at a frequency between 5 kHz and 1 MHz. Two electrodes were placed on the wrist and ankle for whole-body BIS measurement, and another 2 electrodes were placed on the shoulder and greater trochanter to measure the fluid compartments in the arm (between electrodes at wrist and shoulder) and leg (between electrodes at great trochanter and ankle). A switch was used to transfer data accumulation from the whole body and segments to the Xitron device (4, 12). Raw data derived from these measurements included resistance, reactance, phase angle, and impedance. A program provided by Xitron based on the Cole-Cole model was used to calculate segmental and wrist-to-ankle extracellular and intracellular resistance (R_E and R_I, respectively (14). Calculations of wrist-to-ankle ECV and ICV were based on the variables R_E, R_I, and body weight and included constants for resistivity, body geometry, and body density (14). Previously published equations (14, 15) were used to calculate ECV and ICV (see below). TBW was calculated as the sum of ECV and ICV by using the whole-body measurements alone.

Patients were then transported to the hemodialysis unit and again weighed at 3 h after the initial dose of D₂O and bromide; blood for measurement of both D₂O and bromide was obtained before dialysis. At that time, immediately before the initiation of dialysis, an additional 5 mL blood was drawn for measurement of both D₂O and bromide so that their respective volumes of distribution could be determined by isotope dilution (16). The dose concentration in the collected specimen is measured on a single-frequency infrared spectrophotometer after the specimen is lyophilized. The TBW volume is calculated by dividing the dose by the net D₂O concentration in specimen. The precision for the TBW measurement by this method is ± 1.5%. Measurements of bromide concentration are carried out on an HPLC system (16). The CV of measurement of bromide dilution is 1.4%.

Wrist-to-ankle (whole-body) ECV, ICV, and TBW (ECV+ICV) were calculated as follows (14):

RESULTS  
Complete measurements were available in 38 of the subjects (20 men, 18 women) (Table 1). The patients were from the multiethnic area of Upper Harlem in New York City: 33 patients were African American, 3 were Hispanic, and 2 were Asian. Twelve of the patients were diabetic.

View this table:
TABLE 1. Body composition of 38 hemodialysis patients measured immediately before hemodialysis¹

 
Several models were fitted with the MRI measurement of muscle volume by using either isotopically derived data (D₂O and bromide), TBK data, or data derived from anthropometric measures and BIS (Table 2). Total mean (±SD) skeletal muscle as measured by MRI was 24.0 ± 1.1 kg (range: 12.6–38.4 kg) (Table 1). Muscle represented 30.6% of body weight (range: 20.9 –41.8%). TBW measurements obtained by using D₂O dilution (Table 1) and by using BIS were significantly correlated. (R² = 0.785, P < 0.0001). No bias was evident by Bland-Altman analysis.

View this table:
TABLE 2. Models for estimation of total-body muscle mass (TBMM)¹

 
ECV measurements calculated from the bromide volume of distribution and from whole-body BIS also were not significantly different, and they correlated with one another (Table 1 TBW in dialysis patients may vary independently of intracellular water because of failed renal function and the resulting expansion of ECV. Furthermore, ECV, even in a given dialysis patient, will vary according to the time since the previous dialysis treatment. Accordingly, we based our BIS model on measurements of ICV alone. With the use of this measure, the best-fit multiple regression model included sex, weight, and age (Table 2 and Figure 1). The coefficients of the regression models are presented in tabular form. Median TBMM estimated by the BIS model was 27.2 kg (
View larger version (17K):
FIGURE 1.. Correlation between total-body muscle mass (TBMM) measured by magnetic resonance imaging (MRI) and a regression model developed by using ankle-to-wrist multifrequency bioimpedance spectroscopy (BIS), age, sex, and weight (whole-body BIS model). The equation used was TBMM (kg) = 9.52 + 0.331 x BIS-derived ICV (L) + 2.77 (male) + 0.180 x weight (kg) –0.113 x age (y). The results of this equation give a value of SE = 1.85 kg (R² = 0.937, P < 0.0001). The dependent variable was TBMM measured by using MRI. ICV, intracellular volume.

 
Bland-Altman analysis found bias (R² = 0.259, P < 0.001) (Figure 2). Median TBMM estimated by the TBK model was 25.5 kg (
View larger version (16K):
FIGURE 2.. Bland-Altman plot of the difference between magnetic resonance imaging (MRI) and bioimpedance spectroscopy (BIS) estimates of total-body skeletal muscle mass (TBMM) model versus the average value for total body muscle computed by the model and TBMM measured with MRI [  
Bromide space (ECV) was eliminated from the regression model when the D₂O-measured volume of distribution was included. Much as was seen with BIS analysis, when TBW (volume of distribution of D₂O) was intentionally omitted from the regression model, ICV calculated as the difference between D₂O and bromide distribution volume (ICV bromide) provided a model that did not differ significantly from that achieved by the other methods (Table 2 The correlation between TBMM measured with MRI and measured by using the BIS model described above did not differ significantly from that obtained by using TBK plus the same demographic variables (ie, weight, sex, and age) or from that obtained with the model that used isotope dilution plus age, sex, and weight (Table 2). Thus, ICV measured either with BIS or with isotope dilution provided a model that closely correlated with TBMM measured with MRI. In patients studied to establish the precision of BIS measurements and the variability introduced by time since the previous dialysis treatment, weight was 77.7 ± 15.9 kg (range: 56.2–102.4 kg), BMI was 27.0 ± 0.9 (range: 19.4–46.6), and ICV was 18.5 ± 5.0 L (range: 10.8–26.7 L). ICV was measured 3 times in each patient. The CV of the measurement of ICV was 3.78%. ICV was independent of the length of time after the previous dialysis. ICV measured 1 d after a dialysis treatment was 18.6 ± 1.54 L, and that measured 2 d after treatment was 18.5 L ± 1.61 L (P = 0.599, paired t test).

Understanding that sex, weight, and age and could predict TBMM without additional information on TBW or ICV, we modeled TBMM versus TBK (mEq), isotope dilution methods, and the measurement of ICV by using BIS alone to establish the specific precision of each volume measurement and by including BIS in the absence of other information, as shown in the following equation:

DISCUSSION  
Skeletal muscle mass represents a large fraction of ICV. Thus, it is possible to construct models using isotopic or bioelectrical measures of ICV that model MRI measures of muscle mass. Because of the nature of isotopic measurements, limb ICV cannot be measured easily or at all. In contrast, BIS lends itself to measurements of both whole-body fluid volumes and volumes of defined anatomic compartments—in this case, limb ICV. This study shows that, in a cohort of mostly African American hemodialysis patients, multifrequency BIS can be used to estimate TBMM in addition to muscle mass of both the arms and legs.

Because potassium is largely an intracellular cation, skeletal muscle mass can be estimated by measuring TBK in healthy adults (17). Dialysis patients may accumulate potassium, and thus one may question the validity of TBK in predicting muscle mass in these patients. Because >95% TBK is intracellular at a concentration 40 times that in the extracellular compartment, the modest increase in extracellular potassium that might be encountered in dialysis patients should have little effect on the use of this measure (18). A second potential problem that might confound results in patients with chronic kidney disease is that of alteration of intracellular electrolyte composition. Cotton et al (19) reported that patients with creatinine clearances < 6.3 mL/min had low resting transmembrane potential differences and low intracellular potassium. However, hemodialysis 3 times/wk was sufficient to restore intracellular potassium to a concentration that did not differ significantly from that in normal subjects (155 ± 4.9 and 155 ± 4.3 mEq/L, respectively).

Empirical models that rely on measures of TBW have been developed to describe total protein mass (20). Hemodialysis patients, however, accrue extracellular fluid between dialysis treatments. Although we were able to construct a model of precision whose results did not differ significantly between the use of isotope dilution and BIS measures of TBW, it is possible that the variability of ECV in dialysis patients may make such models vary as functions of the time since the previous dialysis treatment. Models of TBMM developed by using BIS measures of ICV had regression coefficients that did not differ significantly from those obtained by using TBK or isotope dilution estimates of ECV. Similarly, measurement of ICV in either the arms or legs provided models that correlated well with measured limb muscle mass.

We have developed regression models by using multifrequency BIS with regression coefficients similar to those obtained by using isotopic techniques when they were compared with independently measured values for muscle mass. In addition, we have established that segmental measurements of limb muscle mass provided by multifrequency BIS estimates of ICV correlate closely with MRI measures of limb muscle mass. Bias in the relations is introduced by including various demographic or anthropometric measures in the regression model, not by using the BIS measures of volume. Estimates of limb muscle mass obtained by using BIS volume measurements are without notable bias. Whereas isotopic methods are not capable of analyzing compartmental muscle masses, BIS is of sufficient quality to provide this information. This information could allow for quantitative structural evaluation of rehabilitation efforts after injury to associated joints. Furthermore, we have established that ICV does not vary significantly with intradialytic periods, which would be of clinical interest.

Whereas BMI and adiposity have been found to be associated with low mortality among dialysis patients, indexes associated with lean body mass, such as volume of distribution of urea, independently predict survival (21, 22). Similarly, TBK, a measure of ICV, positively predicts survival in chronic disease (23–25).

Because visceral volume increases at a lower rate than does muscle mass (26), muscle mass contributes a greater fraction of tissue mass as body cell mass increases (27). This is of some importance, because the resting energy expenditure of skeletal muscle is significantly less than that of organ tissues (28), and, thus, energy needs as a fraction of the body mass would be expected to decline as body cell mass increases (27, 29). Indexes associated with greater muscle mass likewise predict survival among patients with end-stage renal disease (30).

All of the models we developed that use measures of ICW alone implicitly assume that a fixed fraction of body cell mass is composed of muscle within a group of persons with a similar body cell mass. The addition of weight, sex, and age provides more information; however, all measurements other than MRI must be used with the understanding that muscle mass may differ from that obtained by using the model in subjects whose body morphometry differs widely from average values.

Measurement of lean body mass in dialysis patients is confounded by the expansion of extracellular fluid, a component of lean body mass. More precise measures of ICV should provide a better measure of somatic protein stores (body cell mass rather than lean body mass, which includes ECV and bone) than does measurement of lean body mass alone. Anthropometric measurements are limited by operator variability and precision (31). BIA has been found to be useful in monitoring the nutritional status of dialysis patients (8). However, the use of BIA in dialysis patients has mostly relied on measurement of phase angle, and those studies have not been extended to segmental analysis (8, 32).

Nutritional status can be evaluated longitudinally by the monitoring of serum albumin. In contrast to the measurement of serum protein, the estimation of intracellular mass presents more of a challenge. Measurement of TBK by using ⁴⁰K or of total body nitrogen by using neutron activation provides excellent measures of body cell mass (33); however, both procedures are time-consuming, expensive, and not widely available. Evaluation of TBW by isotope dilution of extracellular space using bromide (34, 35), chloride (34), inulin (36), or sulfate (34) also is not easily applicable clinically. MRI also provides a measure of specific extracellular and intracellular compartments (28, 37). Measurement of the quantity of potassium either in the total body or regionally also defines the volume of intracellular water (7).

We have calibrated whole-body BIS with other measures of body composition, specifically ⁴⁰K, MRI muscle, and isotope dilution techniques of intracellular water and extracellular water that correlates sufficiently well to isotopic measures to be useful in describing body composition. In contrast to isotopic and MRI measurements, BIS measurements are noninvasive and inexpensive, and the procedure is portable. The precision of BIS in estimating ECV and ICV is sufficiently good (published CV: 3–6%; 38, 39) to be useful in longitudinal analysis of individual patients by providing the capability of measuring muscle mass regionally or segmentally in longitudinal studies, in rehabilitation after surgery or injury, and after training in the gymnasium.

ACKNOWLEDGMENTS  
GAK conceived of and designed the project, designed the protocol, did the regression modeling, and participated in manuscript preparation. FZ supervised the acquisition of the bioimpedance spectroscopy (BIS) data; analyzed the BIS data; developed the concept of regional BIS analysis, including the mathematical model to measure intracellular volume, extracellular volume, and total water in individual body segments; and participated in manuscript preparation. SS recruited patients, and SS and CK performed the BIS measurements. SBH analyzed the magnetic resonance imaging data. JW supervised and performed the total body potassium measurements as well as measurements of both D₂O and bromide volumes of distribution. MKK supervised the acquisition of data for the last 20 patients studied and participated in manuscript preparation. NWL participated in the study concept and design and participated in manuscript preparation. None of the authors had any personal or financial conflict of interest.

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