Physiological and quantitative CT-scan characterization of COVID-19 and typical ARDS: a matched cohort study

Abstract

Purpose: To investigate whether COVID-19-ARDS differs from all-cause ARDS.

Methods: Thirty-two consecutive, mechanically ventilated COVID-19-ARDS patients were compared to two historical ARDS sub-populations 1:1 matched for PaO₂/FiO₂ or for compliance of the respiratory system. Gas exchange, hemodynamics and respiratory mechanics were recorded at 5 and 15 cmH₂O PEEP. CT scan variables were measured at 5 cmH₂O PEEP.

Results: Anthropometric characteristics were similar in COVID-19-ARDS, PaO₂/FiO₂-matched-ARDS and Compliance-matched-ARDS. The PaO₂/FiO₂-matched-ARDS and COVID-19-ARDS populations (both with PaO₂/FiO₂ 106 ± 59 mmHg) had different respiratory system compliances (Crs) (39 ± 11 vs 49.9 ± 15.4 ml/cmH₂O, p = 0.03). The Compliance-matched-ARDS and COVID-19-ARDS had similar Crs (50.1 ± 15.7 and 49.9 ± 15.4 ml/cmH₂O, respectively) but significantly lower PaO₂/FiO₂ for the same Crs (160 ± 62 vs 106.5 ± 59.6 mmHg, p < 0.001). The three populations had similar lung weights but COVID-19-ARDS had significantly higher lung gas volume (PaO₂/FiO₂-matched-ARDS 930 ± 644 ml, COVID-19-ARDS 1670 ± 791 ml and Compliance-matched-ARDS 1301 ± 627 ml, p < 0.05). The venous admixture was significantly related to the non-aerated tissue in PaO₂/FiO₂-matched-ARDS and Compliance-matched-ARDS (p < 0.001) but unrelated in COVID-19-ARDS (p = 0.75), suggesting that hypoxemia was not only due to the extent of non-aerated tissue. Increasing PEEP from 5 to 15 cmH₂O improved oxygenation in all groups. However, while lung mechanics and dead space improved in PaO₂/FiO₂-matched-ARDS, suggesting recruitment as primary mechanism, they remained unmodified or worsened in COVID-19-ARDS and Compliance-matched-ARDS, suggesting lower recruitment potential and/or blood flow redistribution.

Conclusions: COVID-19-ARDS is a subset of ARDS characterized overall by higher compliance and lung gas volume for a given PaO₂/FiO₂, at least when considered within the timeframe of our study.

Keywords: ARDS; COVID-19; CT scan; Mechanical ventilation; Respiratory system mechanics.

Fig. 1

a Respiratory system compliance in COVID-19-ARDS (orange) and in PaO₂/FiO₂-matched-ARDS populations (blue). The matched PaO₂/FiO₂ ratios were similar (COVID-19-ARDS = 106 ± 59 mmHg, PaO₂/FiO₂-matched-ARDS = 106 ± 60 mmHg). Note that, for the same PaO₂/FiO₂ ratio, the Crs in COVID-19-ARDS is significantly higher (~ 11 ml/cmH₂O) than in PaO₂/FiO₂-matched-ARDS (median values 49.5 and 38.4 ml/cmH₂O, respectively). b PaO₂/FiO₂ ratio in COVID-19-ARDS (orange) and in Compliance-matched-ARDS populations (blue). The matched respiratory system compliance values were similar (COVID-19-ARDS = 49.9 ± 15.4 ml/cmH₂O, Compliance-matched-ARDS  = 50.1 ± 15.7 ml/cmH₂O). Note that, for the same Crs, the PaO₂/FiO₂ ratio in Compliance-matched-ARDS is significantly higher (~ 70 mmHg) than in COVID-19-ARDS (median values 155.0 and 85.4 mmHg, respectively)

Fig. 2

Lung gas volume measured in the 10 equally spaced lung segments along the sterno-vertebral axis (level 1 = closest to the sternum, level 10 = closest to the vertebra). The gas volume of both the PaO₂/FiO₂-matched-ARDS (dark blue) and Compliance-matched-ARDS (light blue) was significantly different from COVID-19-ARDS (p < 0.001 and p = 0.043, respectively). Note that the gas volume was higher in COVID-19-ARDS, even compared to the Compliance-matched-ARDS. The extent of the differences is particularly evident in the most dependent lung regions, where the gas volume at each level was even more than double in COVID-19-ARDS than in PaO₂/FiO₂-matched-ARDS

Fig. 3

Venous admixture as a function of the fraction of non-aerated tissue, in PaO₂/FiO₂-matched-ARDS (PF-ARDS, left panel), COVID-19-ARDS (CARDS, middle panel) and Compliance-matched-ARDS (Crs-ARDS, right panel). As shown, in PaO₂/FiO₂-matched-ARDS and Compliance-matched-ARDS, the venous admixture increases proportionally with similar slopes (0.83 and 0.89, respectively) with the increase fraction of non-aerated tissue, implying a coupling between the shunt fraction and the fraction of non-aerated tissue. In contrast, in COVID-19-ARDS, the two variables were uncoupled. The relationships followed the regression equations: PaO₂/FiO₂-matched-ARDS, venous admixture = 0.83 × fraction of non-aerated tissue + 0.14, p = 0.003, R² = 0.32 (22 observations). CARDS, venous admixture = − 0.07 × fraction of non-aerated tissue + 0.5, p = 0.75, R² = − 0.03 (29 observations). Crs-ARDS, venous admixture = 0.89 × fraction of non-aerated tissue + 0.13, p = 0.004, R² = 0.35 (19 observations). Missing data were due to the lack of central venous blood samples