Can we quantify pulmonary edema?
The last 2 decades have witnessed the introduction and evolution of the transpulmonary thermodilution (TPTD) technique.  This enables the quantification of pulmonary edema by extravascular lung water index (ELWI) and pulmonary vascular permeability index (PVPI). PVPI enables differential diagnosis of the origin of pulmonary edema: cardiogenic or permeability induced. ELWI and PVPI may be used as criteria indicating the risk of fluid administration. 
Extravascular lung water is an accumulation of fluid in the interstitium of the lung tissue and / or the alveoli. Pulmonary vascular permeability index (PVPI) is an indirect reflection of the integrity of the alveolo-capillary barrier.
Clinical issue: Chest x-rays do not reliably reflect the level of pulmonary edema
The usual clinical approach to assess pulmonary edema is the evaluation of a chest x-ray. This is very challenging as the chest x-ray is a density measurement, influenced by all compartments in the chest like bones, muscles, tissue layers, blood, blood vessels, air, tissue edema, pleural effusion and possibly also pulmonary edema. Thus, in clinical studies it was demonstrated that chest x-ray evaluation for pulmonary edema is very inaccurate when comparing it to the direct quantification by transpulmonary thermodilution. , , .
Cardiogenic or permeability pulmonary edema?
PVPI is an indirect reflection of the integrity of the alveolo-capillary
barrier. It is calculated from the relation between EWLI and Pulmonary Blood Volume (PBV), the ratio of the fluid volume that is out of the vessels over the fluid volume that remains in the vessels. 
Cardiogenic pulmonary edema
Hydrostatic pressure increases as a result of intravascular fluid overload.
This causes fluids to leak into the extravascular space.
Permeability pulmonary edema
Vascular permeability is increased by an inflammatory reaction caused for example, by sepsis. This leads to the increased transfer of fluids, electrolytes
and proteins from the intravascular to the extravascular space, even
with a normal to low intravascular fluid status and hydrostatic pressure.
Interaction of ELWI and PVPI is of clinical benefit
The need to identify and quantify pulmonary edema in complex syndromes such as ARDS can influence outcomes. Pulmonary edema accumulation impairs respiratory gas exchange, resulting in respiratory distress. The parameters ELWI and PVPI can be used to sensitively quantify pulmonary edema at the bedside. It also allows the evaluation of the severity of ARDS.
ARDS management based on PiCCO improves outcome
In the context of ARDS, studies suggest that management based on protocols including ELWI measurements is safe, leads to a lower cumulative fluid balance, improves ICU mortality, and reduces the duration of mechanical ventilation, of ICU stay and treatment costs.
Improvement of the oxygenation (PaO2/FiO2) in ARDS patients over 7 days is significantly better in patients with hemodynamic management based on PiCCO parameters.
Sakka S, Ruhl C, Pfeiffer U, Beale R, McLuckie A, Reinhart K, Meier-Hellmann A. Assessment of cardiac preload and extravascular lung water by single transpulmonary thermodilution. Intensive Care Medicine 2000;26(2):180-7
Tagami T, Eng Hock Ong M. Extravascular lung water measurements in acute respiratory distress syndrome: why, how, and when?
Curr Opin Crit Care 2018;24(3):209-215
Kuzkov V et al. Extravascular lung water after pneumonectomy and one-lung ventilation in sheep. Crit Care Med 2007; 35(6): 1550-1559
Katzenelson R et al. Accuracy of transpulmonary thermodilution versus gravimetric measurement of extravascular lung water. Crit Care Med 2004; 32(7): 1550-1554
Tagami T, Kushimoto S, Yamamoto Y, et al. Validation of extravascular lung water measurement by single transpulmonary thermodilution: human autopsy study.
Crit Care. 2010;14(5):R162
Brown L, Calfee C, Howard J, Craig T, Matthay M, McAuley D. Comparison of thermodilution measured extravascularlung water with chest radiographic assessment of pulmonary oedema in patients with acute lung injury.
Ann Intensive Care. 2013;3(1):25
Saugel B, Ringmaier S, Holzapfel K, et al. Physical examination, central venous pressure, and chest radiography for the prediction of transpulmonary thermodilution-derived hemodynamic parameters in critically ill patients: a prospective trial.
J Crit Care. 2011;26(4):402-410
Lemson J, van Die LE, Hemelaar AE, van der Hoeven JG. Extravascular lung water index measurement in critically ill children does not correlate with a chest x-ray score of pulmonary edema. Crit Care. 2010;14(3):R105
Jozwiak M, Teboul JL, Monnet X. Extravascular lung water in critical care: recent advances and clinical applications. Ann Intensive Care. 2015;5(1):38
Yuanbo Z, Jin W, Fei S, et al. ICU management based on PiCCO parameters reduces duration of mechanical ventilation and ICU length of stay in patients with severe thoracic trauma and acute respiratory distress syndrome. Ann Intensive Care. 2016;6:113