Mechanisms of oxygenation responses to proning and recruitment in COVID-19 pneumonia

Abstract

Purpose: This study aimed at investigating the mechanisms underlying the oxygenation response to proning and recruitment maneuvers in coronavirus disease 2019 (COVID-19) pneumonia.

Methods: Twenty-five patients with COVID-19 pneumonia, at variable times since admission (from 1 to 3 weeks), underwent computed tomography (CT) lung scans, gas-exchange and lung-mechanics measurement in supine and prone positions at 5 cmH₂O and during recruiting maneuver (supine, 35 cmH₂O). Within the non-aerated tissue, we differentiated the atelectatic and consolidated tissue (recruitable and non-recruitable at 35 cmH₂O of airway pressure). Positive/negative response to proning/recruitment was defined as increase/decrease of PaO₂/FiO₂. Apparent perfusion ratio was computed as venous admixture/non aerated tissue fraction.

Results: The average values of venous admixture and PaO₂/FiO₂ ratio were similar in supine-5 and prone-5. However, the PaO₂/FiO₂ changes (increasing in 65% of the patients and decreasing in 35%, from supine to prone) correlated with the balance between resolution of dorsal atelectasis and formation of ventral atelectasis (p = 0.002). Dorsal consolidated tissue determined this balance, being inversely related with dorsal recruitment (p = 0.012). From supine-5 to supine-35, the apparent perfusion ratio increased from 1.38 ± 0.71 to 2.15 ± 1.15 (p = 0.004) while PaO₂/FiO₂ ratio increased in 52% and decreased in 48% of patients. Non-responders had consolidated tissue fraction of 0.27 ± 0.1 vs. 0.18 ± 0.1 in the responding cohort (p = 0.04). Consolidated tissue, PaCO₂ and respiratory system elastance were higher in patients assessed late (all p < 0.05), suggesting, all together, "fibrotic-like" changes of the lung over time.

Conclusion: The amount of consolidated tissue was higher in patients assessed during the third week and determined the oxygenation responses following pronation and recruitment maneuvers.

Keywords: ARDS; COVID-19; Lung recruitment; Mechanical ventilation; Prone position.

Fig. 1

Upper panels: Tissue mass distributions of normally aerated (A), poorly aerated (B) and non-aerated tissues (C), as a function of lung segments (mean ± se) along the sterno (segment 1)-vertebral (segment 10) axis, in supine-5 (blue) and prone-5 positions (red). A The normally aerated tissue, in supine position, was more distributed in the ventral regions (segments 1 to 5, 179 gr ± 56, SD) and decreased in dorsal regions (segments 5 to 10, 122 gr ± 74, SD) (p < 0.001). In prone position, in contrast, it was less distributed in the ventral regions (segments 1 to 5, 122 gr ± 60, SD) and more in dorsal regions (segments 5 to 10, 200 gr ± 84, SD) (p < 0.001). B The poorly aerated tissue, in supine position, was less distributed in the ventral regions (segments 1 to 5, 161 gr ± 94, SD) and increased in dorsal regions (segments 5 to 10, 320 gr ± 112, SD) (p < 0.001). Similarly, in prone position it was less distributed in the ventral regions (segments 1 to 5, 211 gr ± 87, SD) and more in dorsal regions (segments 5 to 10, 298 gr ± 101, SD) (p < 0.001). C: the non-aerated tissue, in supine position, was markedly less distributed in the ventral regions (segments 1 to 5, 73 gr ± 92, SD) than in dorsal regions (segments 5 to 10, 427 gr ± 254, SD) (p < 0.001). Similarly, in prone position it was less distributed in the ventral regions (segments 1 to 5, 169 gr ± 154, SD) and more in dorsal regions (segments 5 to 10, 294 gr ± 209, SD) (p < 0.001). Note that differences in column heights between prone and supine from 1 to 5 indicate the formation of ventral atelectasis, while from segments 6 to 10 it indicates the disappearance of the dorsal atelectasis. Lower panels: Tissue mass distributions of normally aerated (A), poorly aerated (B) and non-aerated tissues (C), as a function of lung segments (mean ± se) along the sterno (segment 1)-vertebral (segment 10) axis, in supine-5 (blue) and supine-35 (okra yellow). D The normally aerated tissue was greater in supine-35 than in supine-5, in each of the ten segments (total normally aerated tissue 535 gr ± 171 SD vs 302 gr ± 116 SD, respectively, p < 0.001). E The poorly aerated tissue, was similar in supine-35 and in supine-5 and similarly distributed in each of the ten segments (total poorly aerated tissue 439 gr ± 189 SD vs 481 gr ± 154 SD, respectively). F: the non-aerated tissue was greater in supine-5 than in supine-35, in each of the ten segments (total non-aerated tissue 499 gr ± 328 SD vs 323 gr ± 249 SD, respectively, p < 0.05). Note that the height of the red columns represents the consolidated tissue and the difference between supine-5 and supine-35 columns represents the atelectatic tissue prevalent in the dorsal lung segments (from 5 to 10)

Fig. 2

Distributions of atelectatic (blue columns) and consolidated tissue (okra yellow columns) in the ten lung segments (mean ± se) along the sterno-vertebral axis, as a function of time elapsed from admission to the study day. A First week (n = 10); B, second week (n = 10) and C, third week (n = 5). As shown, there was a significant increase of consolidated tissue overtime (p < 0.05 at the repeated measures ANOVA), while the decrease of atelectatic tissue did not reach the statistical significance