PaCO₂ as a Possible Treatable Trait in Acute Respiratory Failure: A Scoping Review

Acute respiratory failure (ARF) often leads to ICU admission, ventilatory support, illness, and death. The usual classification into hypoxemic and hypercapnic types does not capture its full complexity. Precision medicine uses the concept of "treatable traits" to guide care based on traits that are clinically relevant, identifiable, measurable, and possibly changeable. Arterial carbon dioxide pressure (PaCO₂) reflects factors like alveolar ventilation, dead space, respiratory mechanics, and how patients respond to ventilatory support. This makes it clinically relevant in selected situations. We carried out a scoping review using PRISMA-ScR and JBI guidelines to summarize evidence on hypocapnia and hypercapnia as prognostic, stratification, or clinically relevant variables during respiratory support. We searched PubMed/MEDLINE, ScienceDirect, and Web of Science (1994-2025), and checked references by hand. Thirty-four studies met our criteria and were grouped into four areas: pre-intubation or early acute presentation, non-invasive support (NIV/HFNC), invasive mechanical ventilation (IMV), and weaning or post-extubation. In summary, hypocapnia was linked to worse outcomes or failure of support in hypoxemic or cardiogenic cases. Hypercapnia helped identify patients who benefited from NIV, such as those with chronic obstructive pulmonary disease or obesity hypoventilation. For IMV, the effects depended on the presence and severity of acidosis and on its duration. Overall, PaCO₂ showed context-dependent clinical relevance, acting mainly as a prognostic or stratification marker and, in narrower settings, as a variable that may inform monitoring or support decisions. This review provides a pragmatic framework for interpreting PaCO₂ across respiratory support contexts and highlights the need for safe and clinically meaningful targets.