Evaluation of time constant, dead space and compliance to determine PEEP in COVID-19 ARDS: a prospective observational study

Multiple variables exist to identify optimal positive end-expiratory pressure (PEEP) to keep alveolar recruitment in acute respiratory distress syndrome (ARDS). These include increased respiratory system compliance (C-RS) and decreased dead space to tidal volume fraction (Vd/Vt). Increasing C-RS reflects improved lung volume, whereas decreasing Vd/Vt reflects improved ventilation/perfusion matching. An increasing expiratory time constant (RCEXP) reflects both, changes in C-RS and alterations in tissue resistance. Whether RCEXP might reflect corresponding changes in Vd/Vt better than C-RS during alveolar recruitment is unknown. This prospective observational study examined the correlation between these variables during ascending PEEP titration in patients with novel Coronavirus disease (COVID-19) related ARDS. PEEP titrations were performed in ten patients with COVID-19 ARDS under passive, pressure-controlled ventilation with a fixed driving pressure of 14 cmH(2)O. PEEP was increased stepwise between 5 and 20 cmH(2)O with 2 minutes allowed for Vd/Vt equilibration. RCEXP, Vd/Vt and C-RS were recorded at each PEEP level and statistically assessed.The overall correlation between Vd/Vt and RCEXP was -0.72 (95% CI: -0.57 to -0.82); p < 0.0001. C-RS had a weaker correlation with Vd/Vt (-0.47 (95% CI: -0.25 to -0.64); p < 0.0001). RCEXP was the highest at 12 cmH(2)O of PEEP whereas Vd/Vt was the lowest at 10 cmH(2)O of PEEP and C-RS was the highest at PEEP of 15 cmH(2)O. Both parameters of exhalation, Vd/Vt and RCEXP, are strongly correlated which likely reflects corresponding mechanical and global ventilation/perfusion responses during ascending PEEP titration.