To the Editor We read with interest the article by Tiruvoipati et al.1 We recognize and appreciate their data analysis of a large cohort that assessed the association of hypercapnic acidosis with cerebral injury. However, given the inherent limitations to retrospective data sets, we question whether hypercarbic acidosis yields deleterious outcomes and whether serum pH normalization improves neurological outcomes. In the article, the authors defined compensated hypercapnia as an elevated partial pressure of carbon dioxide (Pco2 > 45 mm Hg) with a normal pH level (7.35-7.45) and hypercapnic acidosis (Pco2 > 45 mm Hg) with a pH level of less than 7.35. They excluded patients with a metabolic acidosis or alkalosis from their analysis, but it is unclear how they defined these 2 acid-base states. Patients with compensated hypercapnia may represent a group of patients with competing acid-base disorders or a cohort of patients who experienced a less grievous initial physiological insult, whereas hypercapnic acidosis may actually reflect an increased severity of injury. As such, patients’ pH levels and compensatory status likely represent dependent variables rather than causal agents.