Heart failure is a chronic disease marked by frequenthospitalizations due to pulmonary fluid congestion. Monitoringthe thoracic fluid status may favor the detection of fluidcongestion in an early stage and enable targeted preventivemeasures. Bioelectrical impedance spectroscopy (BIS) has beenused in combination with the Cole model for monitoring bodycomposition including fluid status. The model parameters reflectintracellular and extracellular fluid volume as well as cell sizes,types and interactions. Transthoracic BIS may be a suitableapproach to monitoring variations in thoracic fluid content.The aim of this study was to identify BIS measures, which canbe derived based on the Cole model, that are sensitive to earlystages of thoracic fluid accumulation. We simulated this medicalcondition in healthy subjects by shifting a part of the whole bloodfrom the periphery towards the thorax. The redistribution ofblood was achieved non-invasively through leg compression usinginflatable leg sleeves. We acquired BIS data before, during andafter compression of the legs and examined the effect of thoracicfluid variations on parameters derived based on the Cole modeland on geometrical properties of the impedance arc. Indicatordilution measurements obtained through cardiac magneticresonance imaging were used as a reference for the changes inpulmonary fluid volume.Eight healthy subjects were included in the study. The Colemodel 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, theresistances of individual fluid compartments were RE = 51.4 ± 6.7Ω, 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 ofleg compression was a relatively small, reversible increase inpulmonary blood volume of 90 ± 57 mL. We observed significantchanges in parameters associated with intracellular, extracellularand 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 geometricalproperties (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 withtissue structure fc and α remained stable.Transthoracic BIS is sensitive to small variations in intrathoracicblood volume, in particular the resistances of fluidcompartments and the geometric properties of the impedancearc. Taken together with previous studies, our findings suggestthat R0 may be a suitable parameter to monitor congestion. Use ofadditional parameters such as RI, K, XC, fc and α may enable thediscrimination between different types and stages of thoracic fluidaccumulation and should be the focus of future research.