Impact of mild hypercapnia in critically ill patients with metabolic acidosis

Purpose: Clinical trials focusing on critically ill patients with metabolic acidosis, a common exclusion criterion is the presence of a PaCO₂ > 45 mmHg. The aim of this study was to assess the impact of mild hypercapnia on patient characteristics, severity, and clinical outcomes in critically ill patients with metabolic acidosis. Material and methods: Multicentre, retrospective, observational study conducted in 12 intensive care units (ICUs) in Queensland, Australia. Patients with metabolic acidosis and concurrent vasopressor requirement were included and the exposure of interest was the PaCO₂ level at the time of meeting the eligibility criteria divided in two groups: PaCO₂ ≤ 45 mmHg and PaCO₂ 46–50 mmHg. Primary clinical outcome was major adverse kidney events within 30 days (MAKE30). Results: We studied 5601 patients, with 3605 (64.4 %) in the PaCO₂ ≤ 45 mmHg group and 1996 (35.6 %) in the PaCO₂ 46–50 mmHg group. The incidence of MAKE30 was lower in the PaCO₂ 46–50 mmHg group (29 % vs. 34 %; OR, 0.79 [95 %CI, 0.69 to 0.90]; p < 0.001) as was the use of renal replacement therapy, and the incidence of acute kidney injury. After adjustment for confounders, no outcome was different between the groups. The maximum fall of pH associated with an increase of 1 mmHg of PaCO₂ in the PaCO₂ 46–50 mmHg group was 0.006. Conclusion: In patients with metabolic acidosis, after adjustment for potential confounders, mild hypercapnia does not increase the MAKE-30 rate and does not have a major impact on pH.