Association between mild hypercapnia and hospital mortality in patients admitted to the intensive care unit after cardiac arrest: A retrospective study.

BACKGROUND

Mild hypercapnia may increase cerebral oxygenation and attenuate cerebral injury in post-cardiac arrest patients. However, its association with hospital mortality has not been evaluated.

METHODS

We conducted a retrospective multi-center study of prospectively collected data of all cardiac arrest patients admitted to the ICU between 2014 and 2015. Different kinds of arterial carbon dioxide tension (PaCO), including time-weighted mean PaCO, mean PaCO, admission PaCO and proportion of time spent in four PaCO categories (hypocapnia, normocapnia, mild hypercapnia, and severe hypercapnia) were used to explore the association with outcomes. Restricted cubic splines models were built to evaluate the association between PaCO and odds ratio for hospital mortality in overall population and subgroups of different pH levels (acidosis, normal pH and alkalosis).

RESULTS

2783 post-cardiac arrest patients in 150 ICUs were included. 933 (33.5%) were classified into the hypocapnia (PaCO < 35 mmHg), 1088 (39.1%) into the normocapnia (35-45 mmHg), 472 (17%) into the mild hypercapnia (45-55 mmHg) and 390 (10.4%) into the severe hypercapnia (>55 mmHg) group. Compared with normocapnia, mild hypercapnia was not associated with higher hospital survival probability (OR 1.08 [95% CI 0.84-1.38, p = 0.558]). Time spent in the normocapnia was associated with good outcome (OR 0.98 [95% CI 0.97-0.99, p < 0.001], for every 5 percentage point increase in time), but mild hypercapnia was not (OR 1 [95% CI 0.98-1.01, p = 0.542]). Cox-proportional hazards models supported these findings. Associations between PaCO and hospital mortality were not statistically significant in normal pH and alkalosis subgroups.

CONCLUSIONS

PaCO has a U-shaped association with odds ratio for hospital mortality, with mild hypercapnia not having a higher hospital survival probability than normocapnia in post-cardiac arrest patients.