Variation in Arterial CO2 is a Stronger Determinant of Brain Tissue Oxygenation Than its Synchronous Value in Critically Ill Patients With Acute Brain Injury.

OBJECTIVES

In critically ill patients with acute brain injury (ABI), maintaining intracranial pressure (ICP) and brain tissue oxygenation (PbtO2) within targets could prevent secondary neurologic injury. Tight control of CO2 (Paco2), a potent regulator of cerebrovascular tone, is generally advocated, but its vasomotor effect may be short-lived. Our aim was to compare the effect of the synchronous Paco2 vs. its variation from a previous baseline on PbtO2 and ICP.

DESIGN

We performed a post hoc analysis of a prospective cohort study collecting arterial blood gas (ABG) values alongside synchronous neuromonitoring variables. Linear mixed-effects models were fitted to evaluate the association between Paco2 and/or Paco2 variation from its average of the last 8-24 hr vs. PbtO2 and ICP, while controlling for cerebral perfusion pressure and Pao2.

SETTING

Mixed medical-surgical ICU of Sacré-Coeur Hospital, an academic trauma center in Montreal, Canada.

PATIENTS

All consecutive adult patients admitted for ABI with ICP and PbtO2 monitoring between May 2017 and November 2021.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

We included 78 patients with 3047 ABG-neuromonitoring couplets. The model using the variation of Paco2 from its average of the last 24 hr displayed the best performance for the prediction of PbtO2 (coefficient 0.37; 95% CI 0.21-0.53). The strongest predictor of ICP was the variation of Paco2 from its average of the last 8 hr (coefficient 0.17; 95% CI 0.10-0.23).

CONCLUSIONS

Variation in Paco2 from baseline is a more significant determinant of PbtO2 and ICP than the absolute Paco2 value at a given time. There may be a baseline vasomotor reset when patients are exposed to a given level of CO2 for 8 to 24 hr. Therefore, sustained intentional manipulation of Paco2 is unlikely to have lasting effects and slower correction rates of high or low Paco2 could help prevent brain tissue hypoxia or intracranial hypertension, respectively.