Fig. 2

A The pneumonia severity index (PSI), lung infiltration volume (LIV), C-reactive protein (CRP) concentration in the blood, cytokine concentration in the ELF and plasma, provided therapies and comorbidities were recorded for individual patients in order of LIV, including deceased patients (marked by †). B Age, CRP in blood, total cytokine concentration in the ELF and plasma, LIV, and PSI were analyzed. (1) Comparisons among the chronological groups based on when they developed AHRF due to COVID-19. (2) Comparisons among the groups stratified by the severity of pneumonia using the PSI. (3) Comparison among the groups after patients were stratified by the severity of pneumonia using the LIV. The boxplots show the median, individual data points (colored dots), and whisker lines extending to Q1–1.5 × IQR and Q3 + 1.5 × IQR or the last data point within these values. Points outside these limits are considered outliers. Significance (†p < 0.05) was determined using the Kruskal‒Wallis test with Bonferroni correction for multiple comparisons. AHRF Acute hypoxemic respiratory failure, CRP C-reactive protein, ELF Epithelial lining fluid, IQR Interquartile range, LIV Lung infiltration volume (%) [17, 18], PSI Pneumonia severity index [15]