Application of multi-frequency cardiometabolic risk bioimpedance - PDF document

Beyond BMI and Waist Circumference in obesity management and Application of multi-frequency cardiometabolic risk bioimpedance analysis to the • Reduction in fat mass management of patients with • Maintenance of lean body mass (fat-free mass) • Reduction of central fat deposition, esp. visceral fat obesity and metabolic disorders • BMI is uninformative for these aspects of body composition Lindsay Plank • BMI cut-offs for overweight/obesity are problematic in non-European populations Department of Surgery • WC does not identify visceral fat University of Auckland • Bioimpedance analysis: a tool for evaluating fat Auckland, New Zealand mass, skeletal muscle mass, visceral fat, their changes with weight loss, and their relationships to cardiometabolic disorders? Percent fat vs BMI Percent fat vs BMI Bioelectrical impedance devices measure impedance at one or more frequencies 60 AI M Hand-to-foot Leg-to-leg Women E Standing Standing PI 50 or (or supine arm-to-arm) 40 Body fat, % 30 Impedance Constant Impedance (derived from 20 ~ alternating measured voltage) current 10 Constant Obese current 0 They don’t ‘measure’ water volumes or fat mass. 15 20 25 30 35 40 45 50 These must be estimated in some way from impedance BMI, kg/m 2 values Rush, Freitas, Plank Br J Nutr 2009; 102: 632 Human body is modelled as a single cylinder Basic principle For a cylindrical conductor full of electrolyte: Conductor length ~ 95% Height Impedance Z = ρ L/A ( ρ = electrolyte resistivity) i.e, Z = ρ L 2 /(AL) and volume of electrolyte = AL Length x Length Electrolyte volume = ρ Impedance Height x Height (impedance index) Total fluid volume ∝ Impedance, Z Stahn et al Handbook of Anthropometry 2012

To increase the strength of the relationship between the impedance index and fluid volume: weight, age, sex added as predictors ECW Typically, a gold-standard determination of total body water C M allows an equation to be developed for predicting this volume Water R ECW from H 2 /Z, weight, age, sex R ICW ICW Total body fat Water 73% of FFM Impedance (Z) consists of both resistive (R) and H 2 /Z, weight, age, sex will Fat-free reactive (X, cellular tissue) components mass predict FFM as measured by UWW, DXA or 4-C model, Cell and tissue interfaces act like capacitors and allowing TBF estimation Protein their impedance to current flow (termed reactance) is frequency dependent Mineral Carbohydrate A large number of equations have been published - they tend to be specific for the populations on which they were developed HF At intermediate frequencies, measured impedance is Cells current a combination of resistance (R) and reactance (X) ICW Reactance causes the current to lag behind the voltage creating a phase shift: LF ECW current Voltage Magnitude (mV, mA) Current Capacitors present very high impedance to low frequency current – at zero frequency no current through X cells and Z 0 = R ECW (determination of ECW volume) Z Time At high frequencies, current penetrates cells – at infinite frequency, Z ∞ = parallel combination of R ECW and R ICW phase shift R Phase (determination of TBW volume) - can be calculated as an angle angle Stahn et al Handbook of Anthropometry 2012 φ = arctan (X/R) Bioelectrical Impedance Analysis Single Frequency BIA Most widely used approach since development of the technology in Single frequency Typically 50kHz 1979 by RJL Systems. (SFBIA) Empirical eqns In addition to TBW and FFM estimation, the close relationship between ECW and TBW in health allows ECW estimation. Increasing complexity, Originally developed for supine measurement, standing leg-to-leg Typically 2-7 Multi-frequency devices have proliferated. device size, selected freqs (MFBIA) cost Empirical eqns Contribution to Z H 2 /Z highly correlated for ~50% whole body, legs and Model-fitting Bioimpedance arms. Less population spectroscopy (~50% total ~5% Leg-to-leg and arm-to-arm specific mass) (BIS) measurements have similar predictive ability as ~45% whole body.

Comparative evaluation of leg-to-leg and traditional Recommendations for reliable supine hand-to-foot devices whole-body measurements leg-to-leg traditional • Conditions should be same as those under which the equations or model used were developed • Check device calibration regularly DXA • Fast (except water) and avoid alcohol, caffeine and exercise for FFM at least 8 h (kg) • Empty bladder before measurement r = 0.89 • Swab skin with alcohol r = 0.95 SEE = 6.1kg SEE = 4.3kg • Proximal electrode positioning is critical H 2 /Z H 2 /Z • Avoid excessively warm or cold ambient temperature (Addition of age and sex to regression models improved R 2 to similar • Arms ~30 deg from trunk; legs ~45 deg separation (limbs values) straight); towels/blankets for obese • Consistency vital for serial measurements (time of day) Nunez C et al Med Sci Sports Exer 1997;29:524 2 Raw impedance data (resistance, reactance) at 50 • Supine for at least 10 min before measurement kHz provide useful information: (follow manufacturer’s instructions – Quadscan manual suggests 3-4 min) � Phase angle (PA) Z X Phase angle 13 Ω increase over 1 h – translates to 500mL φ reduction in TBW R Change (Z measured supine after in Z ( Ω ) � Bivariate vector analysis (BIVA) standing for 1 h) Shirreffs & Maughan Eur J Appl Physiol 1994;69:461 (independent of regression equations) Time (min) Time ( min) Standardization crucial Phase angle Multi-frequency BIA � Depends on X which is a measure of cellular mass and cellular integrity Bodystat Quadscan 4000 � Depends on R which is a measure of fluid status � Low PA occurs in malnutrition and poor cellular Impedance measured at: function 5, 50, 100, 200 kHz Low PA (at 50 kHz) Liver cirrhosis predicts poor survival in Z 200 ECW liver disease, HIV/AIDS, Prediction Marker, PM = ∝ Z 5 TBW lung, breast, colorectal, pancreatic cancer Increasing PM generally indicates poorer health (fluid retention) FFM estimated from 50 kHz impedance data Selberg & Selberg Eur J Appl Physiol 2002;86:509

Type 2 diabetics undergoing bariatric surgery 160 50 140 40 120 100 Weight BMI 30 Pre-surgery characteristics 80 Wgt 20 (kg) 60 40 10 20 0 0 Baseline 12 months Baseline 12 months Male : Female 32 : 37 Age (y) 47.0 ± 6.9 (20 − 56) DXA 70 DXA 60 BIA 60 BIA 50 Weight (kg) 116.2 ± 19.4 (82.0 − 165.7) 50 Fat 40 40 %fat 30 BMI (kg/m 2 ) 40.0 ± 6.0 (28.5 − 60.4) (kg) 30 20 20 10 10 0 0 mean ± SD (range) Baseline 12 months Baseline 12 months DXA 100 BIA 80 Body FFM 60 composition (kg) 40 20 Bodystat Quadscan 4000 GE-Lunar iDXA 0 Baseline 12 months 8 Women 30 70 Men 60 25 6 TBW Changes: ECW 50 20 (L) W: P<0.0001 (L) 40 4 15 PA M: P=0.002 30 Changes: 10 Changes: 20 (deg) 2 W: P<0.0001 W: P<0.0001 5 10 M: P<0.0001 M: P<0.0001 0 0 0 Baseline 12 months Baseline 12 months Baseline 12 months Women 40 Women Men 1.0 Men 30 0.8 Changes: Changes: ICW PM 20 W: P<0.0001 0.6 W: P<0.0001 (L) M: P=0.0001 M: P=0.005 0.4 10 0.2 0 Baseline 12 months 0.0 Baseline 12 months Baseline Body Fat Women 12 month Body Fat Women 60 12 100 20 10 50 15 8 80 6 5.3 10 2SD BIA 8.0 40 BIA 2SD 17.5% 14.0% 4 minus 5 minus BIA BIA 30 2 60 0.2 mean 0.2 mean DXA 0 DXA 0 0.4% 0.7% 20 -2 -5 40 -7.6 -2SD -4 -4.9 -2SD 10 r=0.958 -13.9% r=0.966 -10 -16.2% -6 20 -15 0 -8 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 0 10 20 30 40 50 60 30 40 50 60 70 80 90 100 Average DXA and BIA Average DXA and BIA DXA DXA Baseline Body Fat Men 12 month Body Fat Men 50 4 100 2 1.8 2 2SD 0 -0.8 2SD 7.2% -1.6% 40 80 -2 0 BIA BIA -4 -2 30 BIA minus BIA minus -4.1 -6 -6.4 mean -4 mean 60 -16.8% -13.5% DXA DXA 20 -8 -6 40 -10 -8 -12.0 10 r=0.975 -2SD -10.0 r=0.931 -12 -10 -2SD -25% -41% 20 -14 0 -12 20 30 40 50 60 70 80 90 100 20 40 60 80 100 0 10 20 30 40 50 5 10 15 20 25 30 35 40 45 DXA Average DXA and BIA DXA Average DXA and BIA