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

Abstract

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 (CRS ) and decreased dead space to tidal volume fraction (Vd/Vt). Increasing CRS reflects improved lung volume, whereas decreasing Vd/Vt reflects improved ventilation/perfusion matching. An increasing expiratory time constant (RCEXP ) reflects both, changes in CRS and alterations in tissue resistance. Whether RCEXP might reflect corresponding changes in Vd/Vt better than CRS 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 cmH2O. PEEP was increased stepwise between 5 and 20 cmH2O with 2 minutes allowed for Vd/Vt equilibration. RCEXP , Vd/Vt and CRS 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. CRS 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 cmH2O of PEEP whereas Vd/Vt was the lowest at 10 cmH2O of PEEP and CRS was the highest at PEEP of 15 cmH2O.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.