Assessment of the Determinants of the Thoracic Fluid Content Measured by the Bioimpedance-based Starling System in Critically Ill Patients

The Starling system is a completely non-invasive hemodynamic monitoring technique. It measures cardiac output through bioreactance. It also measures the mean transthoracic electric impedance (Z0) from which the thoracic fluid content (TFC) can be derived. The TFC is the only other variable that is provided by the system besides cardiac output and that is not inferred from it. The TFC is supposed to reflect the total content of fluid in the thorax and may include two of the variables that are commonly used for hemodynamic monitoring: the extravascular lung water (EVLW), which is the volume of lung edema, and the global end-diastolic volume (GEDV), which is a static index of cardiac preload. Cardiac preload is an important physiological variable; a prerequisite for TFC as a reflect cardiac preload would be that its changes are tightly linked to the changes in well-established markers of cardiac preload, as the GEDV measured by transpulmonary thermodilution, or the central venous pressure (CVP). Nevertheless, this has never been demonstrated. In addition, TFC includes the volume of EVLW, which is the volume of fluid contend in the lungs outside the pulmonary vessels. Nevertheless, the degree to which TFC is influenced by absolute values and changes in EVLW has never been established. The objective of the study is to establish the determinants of TFC among well-known hemodynamic variables including EVLW and GEDV, as well the value of TFC as a marker of cardiac preload and lung water.