How can fluid status be measured?
The last 2 decades have witnessed the introduction and evolution of the transpulmonary thermodilution (TPTD) technique for measuring EVLW and PVPI. (tagami) This gives two new tools extravascular lung water index (EVLI) and pulmonary vascular permeability index (PVPI). EVLWI 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.
The accuracy of transpulmonary thermodilution for the quantification of pulmonary edema has been confirmed in several experimental and clinical studies (kuzkov, tagami, katzenelson)
Can we quantify pulmonary edema?
Examples of chest X-rays that do not 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, plural 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 (Brown, Saugel, Lemson).
How can this be of a clinical benefit?
The need to identify and quantify pulmonary edema in complex syndromes such as ARDS can influence outcomes. Regardless of the cause, this ELWI accumulation impairs respiratory gas exchange, resulting in respiratory distress. Newer tools such as EVLWI and PVPI can be used to establish the existence, evaluate the severity and identify the nature of ARDS.
What does this all mean?
In the context of ARDS, some small studies suggest that management based on protocols including EVLWI measurements is safe, leads to a lower cumulative fluid balance, improves ICU mortality, and reduces the duration of mechanical ventilation and of ICU stay.
ARDS is progressive with an early treatment window that can be exploited. EVLW and PVPI measurement will open the door to future ARDS clinical practice and research, and have potential to be included in the future ARDS definition.
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