CO 2 in ARDS: Pragmatic Pa CO 2 –pH Exposure Patterns and Bedside Monitoring Indices

ARDS is frequently accompanied by elevated arterial carbon dioxide tension ( P a CO 2 ), often accentuated by lung-protective ventilation. Although hypercapnia has long been viewed as a tolerable trade-off to limit ventilator-induced lung injury, contemporary clinical data suggest that its clinical interpretation depends on the exposure pattern, including magnitude and duration, the accompanying acid/base status, and the mechanisms underlying CO₂ retention. In ARDS, elevated Pa CO 2 may reflect reduced effective alveolar ventilation under lung-protective constraints, increased physiologic dead space, or both, and these mechanisms have different bedside implications. In this narrative review, we summarize pragmatic Pa CO 2 –pH ranges observed or suggested in cohorts across ARDS severity strata and propose a bedside traffic-light framework that integrates Pa CO 2 × pH × exposure time to support individualized decision-making without implying guideline-level thresholds. We also review capnography-derived indices, including the arterial–end-tidal carbon dioxide gradient ( Pa CO 2 −P_ETCO2), the normalized arterial–end-tidal carbon dioxide gradient [( Pa CO 2 −P_ETCO2)/ Pa CO 2 ], the end-tidal-to-arterial CO₂ ratio (P_ETCO2/ Pa CO 2 ), the ventilatory ratio, and V_E/V̇_CO2, which may help estimate and track dead space and V/Q mismatch over time. Prospective studies are needed to determine whether strategies informed by CO₂–pH patterns and ventilatory efficiency improve patient-centered outcomes.