Calculation of the normal range of extravascular lung water

Excerpt

A recent article by Takashi Tagami and colleagues [1] investigated the correlation between extravascular lung water (EVLW) and postmortem lung weight, as well as the normal range of the EVLW. Since it is ethically inappropriate to conduct invasive procedures using the PiCCO monitoring system in healthy individuals, the real normal range of EVLW remains unreported. To solve this problem, the authors built a linear correlation equation from the EVLW measurements and related postmortem lung weight, then substituted the lung weight factor in the equation with normal values of lung weight reported in another large study. This is an ingenious study design. However, there is one issue worthy of consideration when interpreting the results. The population in this study was critically ill patients, including a significant number of patients with lung injury. Thus, it is not surprising that the EVLW is greater in the study population compared to the normal one, as well as the lung weight (1,320 g versus 878 g in male or 636 g in female). Nevertheless, the equation was obtained from the critically ill patients with lung injury, and cannot be extrapolated to healthy individuals. That is because the proportion of EVLW in lung weight is greater in patients with lung injury than in normal lung. The correlation coefficient should be modified if the normal lung weight is substituted into the equation. Although the difference in correlation coefficients between respiratory failure and non-respiratory failure groups was not statistically significant, it can be explained by the limited sample size, which is subject to type II error. …