Monitoring thoracic fluid content using bioelectrical impedance spectroscopy and Cole modeling
Heart failure is a chronic disease marked by frequent hospitalizations due to pulmonary fluid congestion. Monitoring the thoracic fluid status may favor the detection of fluid congestion in an early stage and enable targeted preventive measures. Bioelectrical impedance spectroscopy (BIS) has been used in combination with the Cole model for monitoring body composition including fluid status. The model parameters reflect intracellular and extracellular fluid volume as well as cell sizes, types and interactions. Transthoracic BIS may be a suitable approach to monitoring variations in thoracic fluid content.
The aim of this study was to identify BIS measures, which can be derived based on the Cole model, that are sensitive to early stages of thoracic fluid accumulation. We simulated this medical condition in healthy subjects by shifting a part of the whole blood from the periphery towards the thorax. The redistribution of blood was achieved non-invasively through leg compression using inflatable leg sleeves. We acquired BIS data before, during and after compression of the legs and examined the effect of thoracic fluid variations on parameters derived based on the Cole model and on geometrical properties of the impedance arc. Indicator dilution measurements obtained through cardiac magnetic resonance imaging were used as a reference for the changes in pulmonary fluid volume.
Eight healthy subjects were included in the study. The Cole model parameters of the study group at baseline were: R0 = 51.4 ± 6.7 Ω, R∞ = 25.0 ± 7.0 Ω, fc = 49.0 ± 10.5 kHz, α = 0.687 ± 0.027, the resistances of individual fluid compartments were RE = 51.4 ± 6.7 Ω, RI = 50.5 ± 22.9 Ω, the fluid distribution ratio was K = 1.1 ± 0.3, and the radius, area and depression of the arc’s center were: R = 15.7 ± 1.3 Ω, XC = −8.5 ± 1.5 Ω, A = 134.0 ± 15.6 Ω2. The effect of leg compression was a relatively small, reversible increase in pulmonary blood volume of 90 ± 57 mL. We observed significant changes in parameters associated with intracellular, extracellular and total fluid volume (R0: -1.5 ± 0.9 %, p < 0.01; R∞: −2.1 ± 1.1, p < 0.01; RI: −2.6 ± 1.6 %, p < 0.01), and in the arc’s geometrical properties (R: -1.6 ± 1.3 %, p < 0.05; XC: −1.7 ± 1.5 %, p < 0.05, A: −2.9 ± 1.2 %, p < 0.01). K and the parameters associated with tissue structure fc and α remained stable.
Transthoracic BIS is sensitive to small variations in intra-thoracic blood volume, in particular the resistances of fluid compartments and the geometric properties of the impedance arc. Taken together with previous studies, our findings suggest that R0 may be a suitable parameter to monitor congestion. Use of additional parameters such as RI, K, XC, fc and α may enable the discrimination between different types and stages of thoracic fluid accumulation and should be the focus of future research.