#536 Impact of adipose tissue on estimating extracellular resistance in hemodialysis patients using a two-compartment model

Field: Medicine (Health Sciences)

Abstract Background and Aims Determination of fluid status in dialysis patients is a key in clinical practice. Although multi-frequency bioimpedance spectroscopy (BIS) is a valuable technique in clinical research for assessing body composition and fluid status, it presents challenges when estimating body hydration in routine clinical settings for individual patients. Standard BIS method is based on a two-compartment (2C) model, which processes raw resistance and reactance data to account for extracellular and intracellular volumes yet excluding adipose tissue (fat). We proposed an electrical equivalent circuit for a three-compartment (3C) model. The aim of this study was to evaluate whether adipose tissue influence the accuracy of extracellular resistance by the 2C model. Method Magnetic resonance imaging (MRI) and bioimpedance measurements (Hydra 4200, Xitron Technologies) were performed within 10 cm of the calf in a group of hemodialysis patients prior to dialysis. Calf circumference at maximal (Cmax) and minimal (Cmin) cross-sectional areas within a 10 cm distance were measured, respectively. The standard 2C model (Cole model, Fig. 1A) was used to calculate extracellular resistance (RE-2C), intracellular resistance (RI-2C), and capacitance (CM) using the Xitron program. We propose a 3C model that incorporates fat resistance (RF) and capacitance (CF) through curve-fitting methods (Zhu et al., Physiol Meas, 26:S133–S143, 2005) to calculate RF, RE-3C and RI-3C in the calf (Fig. 1B). The calf geometric volume (VCalf = L*C2/4π) can be calculated by the average circumference (C = (Cmax + Cmin)/2) and the length (L = 10 cm) of the calf. Calf fat-free mass (cFFM) was calculated as the difference between VCalf and adipose tissue (ATCalf). The thickness of ATCalf was calculated as the difference in cross-sectional area between VCalf and cFFM. Linear regression analysis was used to determine the relationship between the variables. Results Eighteen HD patients (5 females; age 56.1 ± 10.7 years; weight 79.6 ± 17.9 kg) were studied. Absolute RF values were strongly negatively correlated with ATCalf as determined by MRI (R2 = 0.69, P &lt; 0.0001; Fig. 2). RE-2C was significantly higher than RE-3C (45.5 ± 8.3 vs. 42.8 ± 7.1 Ω; P &lt; 0.01). On the other hand, there was no statistical difference in intracellular resistance between RI-2C and RI-3C (60.8 ± 18.4 vs. 57.9 ± 20.9 Ω; P = 0.23). RE-2C correlated strongly with RE-3C (R2 = 0.77, P &lt; 0.0001); however, RE-2C systematically overestimated values compared to RE-3C (Fig. 3). Additionally, RE-2C was positively correlated with the thickness of ATCalf (Fig. 4). Moreover, the difference between RE-2C and RE-3C was found to be inversely associated with cFFM (Fig. 5). Conclusion This study demonstrated that higher extracellular resistance as estimated by the 2C model is associated with greater adipose tissue thickness. Since overestimating resistance can lead to an underestimation of body fluid, the effect of adipose tissue on calculations of body fluid in individuals cannot be ignored. Our study found that the accuracy of extracellular resistance estimated by the two-compartment (2C) model (Cole model) may be affected in obese subjects, as the fat component is ignored in the 2C model. Adding a third component, the 3C model may improve the accuracy of fluid estimation using bioimpedance. It is important to note that the 3C model presented in this study assumes that skin impedance does not affect body bioimpedance measurements. However, this assumption may not hold true for patients with very dry or very wet skin. Incorporating skin impedance into the model could further enhance the accuracy of bioimpedance measurements.