Does the thoracic fluid content reflect lung water and cardiac preload?

Abstract

Whether the thoracic fluid content (TFC) estimated by bioimpedance actually reflects the thoracic water content and tracks its changes has not been tested yet. We compared TFC changes induced by volume expansion with those of well-established markers of cardiac preload including the global end-diastolic volume indexed (GEDVI) estimated by transpulmonary thermodilution (TPTD) and central venous pressure (CVP). We assessed the relationship between TFC and extravascular lung water indexed (EVLWI) estimated by TPTD in patients with acute respiratory distress syndrome (ARDS). In 42 patients with a TPTD device (PiCCO2), we measured GEDVI, EVLWI and CVP before and after a 500-mL fluid bolus (Fluid group). In 23 different patients with ARDS, we measured the day-to-day changes in TFC and EVLWI (ARDS group). In the Fluid group (42 measurements), TFC increased significantly during fluid infusion. There was no correlation between the fluid-induced changes in GEDVI or CVP and the fluid-induced changes in TFC (p = 0.71 and p = 0.09, respectively). In the ARDS group, 124 measurements were performed, representing 101 changes between timepoints. There was no correlation between the percent changes in GEDVI, EVLWI, the sum of GEDVI + EVLWI and the percent changes in TFC (p = 0.52, 0.40 and 0.55, respectively). Considering all 208 measurements performed, absolute values of EVLWI were correlated with absolute values of TFC (r = 0.21, p = 0.04). There was no correlation between the absolute values of GEDVI and the sum of GEDVI + EVLWI and those of TFC (p = 0.33 and 0.39, respectively). Still considering all measurements performed, the percentage of error of cardiac index measured by bioreactance compared to TPTD was 131%. In critically ill patients, TFC measured by bioimpedance does not follow the changes induced by a fluid bolus of well-known markers of cardiac preload. It also does not follow the changes in EVLWI observed in patients with ARDS. While absolute values of TFC are weakly correlated with those of EVLWI, it is determined neither by GEDVI, nor by the sum of both.

Supplementary Information: The online version contains supplementary material available at 10.1007/s10877-025-01335-6.

Keywords: Acute respiratory distress syndrome; Biompedance; Cardiac preload; Extravascular lung water; Hemodynamic monitoring; Thoracic fluid content; Transpulmonary thermodilution; Volume expansion.

Fig. 1

Correlation between the changes in thoracic fluid content (TFC) and in global end-diastolic volume indexed (GEDVI) induced by fluid infusion in the Fluid group

Fig. 2

Correlation between the absolute values of thoracic fluid content (TFC) and the sum of global end-diastolic volume indexed (GEDVI) and extravascular lung water indexed (EVLWI) considering all measurements performed in the study. Patients of the Fluid group: dark blue (n = 84), patients of the EVLW group: light blue (n = 124). The dotted line indicates the regression line. N = 208, p = 0.39