ABSTRACT
BACKGROUND: COVID-19-related ARDS is characterized by severe hypoxemia with initially preserved lung compliance and impaired ventilation/perfusion (VÌ/QÌ) matching. PEEP can increase end-expiratory lung volume, but its effect on VÌ/QÌ 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 VÌ/QÌ. Respiratory mechanics measurements, shunt, and VÌ/QÌ mismatch (low VÌ/QÌ and high VÌ/QÌ) 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 VÌ/QÌ. We also investigated if PEEP might have a different effect on VÌ/QÌ mismatch in 2 different respiratory mechanics phenotypes, that is, high elastance/low compliance (phenotype 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 VÌ/QÌ, high VÌ/QÌ, and alveolar dead space were not significantly influenced, 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 mechanical ventilation for > 48 h, PEEP had a heterogeneous effect on VÌ/QÌ mismatch and, on average, higher levels were not able to reduce shunt. The subject's compliance could influence the effect of PEEP on VÌ/QÌ mismatch since an increased shunt was observed in subjects with lower compliance, whereas the opposite occurred in those with higher compliance.