Heterogeneity of Ventilation/Perfusion Mismatch at Different Levels of PEEP and in Mechanical Phenotypes of COVID-19 ARDS

BACKGROUND: COVID-19-related ARDS is characterized by severe hypoxemia with initially preserved lung compliance and impaired ventilation/perfusion (V /Q) matching. PEEP can increase end-expiratory lung volume, but its effect on V/Q mismatch in COVID-19-related ARDS is not clear. METHODS: We enrolled intubated and mechanically ventilated subjects with COVID-19 ARDS and used the automatic lung parameter estimator (ALPE) to measure V /Q. Respiratory mechanics measurements, shunt, and V/Q mismatch (low V/Q and high V /Q) were collected at 3 PEEP levels (clinical PEEP = intermediate PEEP, low PEEP [clinical -50%], and high PEEP [clinical + 50%]). A mixed-effect model was used to evaluate the impact of PEEP on V /Q. We also investigated if PEEP might have a different effect on V/Q mismatch in 2 different respiratory mechanics phenotypes, that is, high elastance/low compliance (pheno-type H) and low elastance/high compliance (phenotype L). RESULTS: Seventeen subjects with COVID-related ARDS age 66 [60-71] y with a PaO2/FIO2 of 141 +/- 74 mm Hg were studied at low PEEP = 5.6 +/- 2.2 cm H2O, intermediate PEEP = 10.6 +/- 3.8 cm H2O, and high PEEP = 15 +/- 5 cm H2O. Shunt, low V /Q, high V /Q, and alveolar dead space were not significantly influ-enced, on average, by PEEP. Respiratory system compliance decreased significantly when increasing PEEP without significant variation of PaO2/FIO2 (P = .26). In the 2 phenotypes, PEEP had opposite effects on shunt, with a decrease in the phenotype L and an increase in phenotype H (P = .048). CONCLUSIONS: In subjects with COVID-related ARDS placed on invasive me-chanical ventilation for > 48 h, PEEP had a heterogeneous effect on V/Q mismatch and, on aver-age, higher levels were not able to reduce shunt. The subject's compliance could influence the effect of PEEP on V/Q mismatch since an increased shunt was observed in subjects with lower compliance, whereas the opposite occurred in those with higher compliance.