Heterogeneity of Ventilation/Perfusion Mismatch at Different Levels of PEEP and in Respiratory Mechanics 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 5 intermediate PEEP, low PEEP [clinical 2 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 P _aO2/F _IO2 of 141 6 74 mm Hg were studied at low PEEP 5 5.6 6 2.2 cm H ₂O, intermediate PEEP 5 10.6 6 3.8 cm H ₂O, and high PEEP 5 15 6 5 cm H ₂ O. 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 P _aO2/F _IO2 (P 5.26). In the 2 phenotypes, PEEP had opposite effects on shunt, with a decrease in the phenotype L and an increase in phenotype H (P 5.048). CONCLUSIONS: In subjects with COVID-related ARDS placed on invasive mechanical 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.