Pulmonary vascular permeability index should not be called extravascular lung water pulmonary blood volume ratio

In their recent paper, Michelle Chew and colleagues [1] evaluated the performance accuracy of three extravascular lung water (EVLW) ratios in patients with shock at risk for or with acute lung injury (ALI). They indexed EVLW to actual body weight (ABW), predicted body weight (PBW) and pulmonary blood volume (PBV), and confirmed that all EVLW ratios were higher in patients with ALI and in non-survivors and correlated with respiratory parameters.

Although these findings are interesting, they deserve some comments. First, whereas indexing EVLW to PBW may reduce the number of patients with acute respiratory distress syndrome (ARDS) with normal EVLW, in this population, the EVLW/ABW index performed best [2]. Furthermore, the absence of a link between EVLW and PaO₂/FiO₂ (calculated r2 of 0.14 to 0.23) could explain that these indices identified only 33% of ALI and 38% of ARDS cases. Second, the EVLW/PBV terminology appears very unusual. The transpulmonary thermodilution technique determines cardiac output, intrathoracic thermal volume (ITTV), pulmonary thermal volume (PTV) and global end-diastolic volume (Global end-diastolic volume (GEDV) = ITTV - PTV). The estimated blood volume of the chest (Intrathoracic blood volume (ITBV) = GEDV + PBV) permits the calculation of PBV (ITBV = 1.25 × GEDV - 28.4 ml) [3]. Importantly, the PiCCO monitor automatically provides, at each calibration, an estimation of the pulmonary vascular permeability index (PVPI), based on the ratio of EVLW to PBV, which reflects alveolar-capillary barrier permeability and is of importance for the mechanisms of pulmonary edema [4, 5].

Thus, I believe that it would be more pertinent to use the PVPI instead of the EVLW/PBV ratio and that it is time to promote an international consensus on this.