CO₂ in ARDS: Pragmatic PaCO2-pH Exposure Patterns and Bedside Monitoring Indices

ARDS is frequently accompanied by elevated arterial carbon dioxide tension (${P_a}_{{\mathrm{CO}}_2}$), 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 ${\mathrm{Pa}}_{{\mathrm{CO}}_2}$ 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 ${\mathrm{Pa}}_{{\mathrm{CO}}_2}$-pH ranges observed or suggested in cohorts across ARDS severity strata and propose a bedside traffic-light framework that integrates ${\mathrm{Pa}}_{{\mathrm{CO}}_2}$ × 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 (${\mathrm{Pa}}_{{\mathrm{CO}}_2}$-P_ETCO2), the normalized arterial-end-tidal carbon dioxide gradient [(${\mathrm{Pa}}_{{\mathrm{CO}}_2}$-P_ETCO2)/${\mathrm{Pa}}_{{\mathrm{CO}}_2}$], the end-tidal-to-arterial CO₂ ratio (P_ETCO2/${\mathrm{Pa}}_{{\mathrm{CO}}_2}$), 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.