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Supplement: Waltham International Symposium

Extracellular Water and Total Body Water Estimated by Multifrequency Bioelectrical Impedance Analysis in Healthy Cats: A Cross-Validation Study

Department of Molecular Biosciences, School of Veterinary Medicine, University of California–Davis, Davis, CA and ^* U.S. Department of Agriculture Western Human Nutrition Research Center, Davis, CA

²To whom correspondence should be addressed. E-mail: denise.elliott{at}effem.com.

KEY WORDS: • multifrequency bioelectrical impedance analysis • body composition • total body water • extracellular water • bioimpedance

EXPANDED ABSTRACT

Multifrequency bioelectrical impedance analysis (MF-BIA) is emerging as a simple, noninvasive routine clinical procedure that allows the rapid and frequent evaluation of total body water (TBW) and extracellular water (ECW) (1) The application of MF-BIA to determine TBW and ECW in adult healthy cats has been evaluated (2). In that study, the MF-BIA prediction of TBW in 20 healthy adult cats was r = 0.84, and the standard error of the estimate (SEE) was 0.26 L, or 9.96%. Similarly, the MF-BIA prediction of ECW in 20 healthy adult cats was r = 0.91, and SEE was 0.07 L, or 6.87%. The purpose of this study was to cross-validate the MF-BIA method by comparing the relationship between MF-BIA with TBW determined by deuterium oxide (D₂O) dilution and ECW determined by bromide (Br) space in a group of cats with a diverse range of body weights and body condition scores.

	MATERIALS AND METHODS

TOP MATERIALS AND METHODS RESULTS LITERATURE CITED

 
The experimental protocols adhered to the NIH guidelines and were approved by the Animal Use and Care Administration Advisory Committee of the University of California at Davis. Fifty-six adult domestic shorthair cats (11 M, 35 F, 5 MC, 5 FS) were evaluated. All cats were considered to be healthy on the basis of physical examination. The cats were weighed to the nearest 0.01 kg and body condition was scored using a five-category subjective body condition scoring system, where 1 = thin, 2 = lean, 3 = ideal body condition, 4 = heavy, 5 = obese. Morphometric measurements were obtained on the right side of the cat with the cat in a normal standing position using a flexible tape graduated in millimeters (2). The height was recorded from the ground to the dorsal border of the scapula (scapula height) and the ground to the dorsal border of the pelvis (pelvic height). The length was determined from the external occipital protuberance to the base of the tail (head to tail), from the lateral canthus of the eye to the base of the tail (eye to tail) and from the tip of the nose to the base of the tail (nose to tail).

Blood samples were collected for background concentrations of deuterium and bromide. Tracer doses of D₂O (0.4 g/kg BW, deuterium oxide 99.8 APE; Cambridge Isotope Laboratories, Andover, MA) and NaBr (30 mg/kg BW, NaBr, ACS reagent assay > 99%; Sigma Chemical, St. Louis, MO) were administered intravenously after food and water were withheld for 24 h. An additional venous blood sample was obtained after a 90-min equilibration period to determine the concentration of the dilutional indicators. Concentrations of D₂O in condensed water were determined in duplicate using Fourier transform infrared spectroscopy (3). Bromide concentrations were assessed by high-pressure liquid chromatography (4).

Each cat was anesthetized with ketamine (1 mg/kg IV, to effect, Ketalar; Parke-Davis, Morris Plains, NJ), and diazepam (0.5 mg/kg IV, to effect, Valium; Roche Laboratories, Nutley, NJ) and positioned on a nonconductive table. Intradermal tetrapolar platinum electrodes (Grass platinum subdermal 27-gauge needle electrodes; Astro-Med, West Warwick, RI) were positioned at the external occipital protuberance on and perpendicular to the dorsal midline and the tail base on and perpendicular to the dorsal midline (body); the lateral condyle of the right humerus and the lateral aspect of the proximal tibia at the level of the femorotibial joint (ipsilateral); and the lateral condyle of the left humerus and the lateral aspect of the proximal tibia at the level of the femorotibial joint (contralateral) (2). MF-BIA measurements were performed with a Hydra ECF/ICF Bioimpedance Analyzer connected to the corresponding intradermal electrodes while the cat was positioned in sternal recumbence. Ten measurements of resistance (R) and reactance (X_c) were obtained, and the corresponding impedance and phase angle were computed from R and X_c at 50 frequencies, ranging from 5 to 1000 kHz. MF-BIA measurements were repeated with the cat positioned in left lateral recumbence, using all electrode arrays. The path length between each of the tetrapolar electrode configurations was recorded using a flexible tape graduated in millimeters. Six MF-BIA configurations were evaluated: sternal ipsilateral (SI), sternal contralateral (SC), sternal body (SB), left lateral ipsilateral (LLRI), left lateral contralateral (LLRC) and left lateral body (LLRB).

The generated Z- and two-spectral data for each electrode array and body position were fitted to an enhanced version of the Cole–Cole model of current conduction through heterogeneous biological tissues using iterative nonlinear curve-fitting algorithms derived for use with the Hydra Bioimpedance Spectrometer (1). The enhanced modeling program extends the original Cole–Cole model to allow for the frequency-invariant time delays caused by the speed at which electrical information is transferred through a conductor to yield the resistance of extracellular fluid (R_E) and intracellular fluid (R_I), respectively (1). The apparent resistivity constants of the ECW and the ICW were determined by the iterative prediction of V_ICW/V_ECW in a companion study of 20 healthy adult cats (2). The values for ECW and ICW were predicted from the modeled R_E and R_I using equations formulated from Hanai’s mixture theory, which describe the effects of nonconductive material on the apparent resistivity of the surrounding conductive fluid (1).

In addition to descriptive statistics, relationships among variables were determined by analysis of variance, general linear models procedure and least-squares linear regression techniques using standard statistical software (Statistical Analysis System, SAS Institute Inc, Cary, NC). Bland–Altman plots were constructed to display differences between the MF-BIA predicted and the D₂O determined TBW, and the MF-BIA predicted and the Br determined ECW. All data were expressed as means ± SE unless otherwise noted. Any value of P < 0.05 was considered significant.

	RESULTS

TOP MATERIALS AND METHODS RESULTS LITERATURE CITED

 
The mean age of the 56 healthy cats was 4.40 ± 0.38 y, mean body weight was 4.69 ± 0.19 kg and mean body condition score was 3.5 ± 0.1. The mean D₂O determined TBW of the 56 cats was 2.45 ± 0.08 L (median 2.42 L, range 1.44–4.32 L), and the volume of the Br determined ECW was 0.93 ± 0.03 L (median 0.92 L, range 0.52–1.51 L). There was a significant difference (P < 0.049) using the sternal ipsilateral path length MF-BIA predicted TBW and the TBW determined by deuterium oxide dilution. There were no significant differences between the remaining MF-BIA predicted TBW at any configuration and the TBW determined by deuterium oxide dilution. There were no significant differences between any configurations of the MF-BIA predicted ECW and bromide space.

The MF-BIA configuration that had the smallest SEE compared with that of the D₂O determined TBW was the left lateral recumbence body path length (LLRBPL) configuration (r = 0.86, SEE = 0.24 L). The difference between the D₂O determined and the MF-BIA (LLRBPL) determined TBW was 0.16 ± 0.04 L, and the limits of agreement were -0.41 to 0.74 L (Fig. 1). Conversely, the MF-BIA configuration that had the largest SEE compared with that of the D₂O determined TBW was the left lateral recumbence contralateral eye to tail (LLRCET) configuration (r = 0.65, SEE = 0.45 L). The difference between the D₂O determined and the MF-BIA (LLRCET) determined TBW was -0.01 ± 0.06 L, and the limits of agreement were -0.94 to 0.92 L (Fig. 1).

View larger version (16K): [in this window] [in a new window]   FIGURE 1 Representative Bland–Altman plots illustrating the narrowest and widest difference between total body water (TBW) estimated by multifrequency bioelectrical impedance analysis (BIA) and deuterium oxide (D2O) dilution plotted against the mean of the two methods. (A) Left lateral body path length. (B) Left lateral contralateral path from eye-to-tail configuration [see companion report (2) for diagram]. Solid line is the mean difference (bias); dashed lines are the limits of agreement (mean difference ± 1.96 SD).

View larger version (15K): [in this window] [in a new window]   FIGURE 2 Representative Bland–Altman plots illustrating the narrowest and widest difference between extracellular water (ECW) estimated by multifrequency bioelectrical impedance analysis (BIA) and sodium bromide (Br) dilution plotted against the mean of the two methods. (A) Left lateral body path length. (B) Sternal body from eye-to- tail. Solid line is the mean difference (bias); dashed lines are the limits of agreement (mean difference ± 1.96 SD).

	FOOTNOTES

 
¹ Presented as part of the Waltham International Symposium: Pet Nutrition Coming of Age held in Vancouver, Canada, August 6–7, 2001. This symposium and the publication of symposium proceedings were sponsored by the Waltham Centre for Pet Nutrition. Guest editors for this supplement were James G. Morris, University of California, Davis, Ivan H. Burger, consultant to Mars UK Limited, Carl L. Keen, University of California, Davis, and D’Ann Finley, University of California, Davis.

³ Current address: Waltham USA, 3250 East 44th St., Vernon, CA 90058.

⁴ Abbreviations used: BCM, body cell mass; Br, bromide; D₂O, deuterium oxide; ECW, extracellular water; FFM, fat-free mass; FM, fat mass; F, female; FS, female spayed; ICW, intracellular water; LLRB, left lateral body; LLRC, left lateral contralateral; LLRI, left lateral ipsilateral; M, male; MC, male castrated; MF-BIA, multifrequency bioelectrical impedance analysis; R, resistance; R_E, resistance of the extracellular water; R_I, resistance of the intracellular water; SB, sternal body; SC, sternal contralateral; SI, sternal ipsilateral TBW, total body water; X_c, reactance.

	LITERATURE CITED

TOP MATERIALS AND METHODS RESULTS LITERATURE CITED

 

1. De Lorenzo, A., Andreoli, A., Matthie, J. R. & Withers, P. O. (1997) Predicting body cell mass with bioimpedance by using theoretical methods: a technological review. J. Appl. Physiol. 82:1542-1558.[Abstract/Free Full Text]

2. Elliott, D. A., Backus, R. C., VanLoan, M. D. & Rogers, Q. R. (2002) Evaluation of multifrequency bioelectrical impedance analysis for the assessment of extracellular and total body water in healthy cats. J. Nutr. 132:1757S-1759S.[Abstract/Free Full Text]

3. Backus, R. C., Havel, P. J., Gingerich, R. L. & Rogers, Q. R. (2000) Relationship between serum leptin immunoreactivity and body fat mass as estimated by use of a novel gas-phase Fourier transform infrared spectroscopy deuterium dilution method in cats. Am. J. Vet. Res. 61:796-801.[Medline]

4. Miller, M. E., Cosgriff, J. M. & Forbes, G. B. (1989) Bromide space determination using anion-exchange chromatography for measurement of bromide. Am. J. Clin. Nutr. 50:168-171.[Abstract/Free Full Text]

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