Arterial carbon dioxide tension and outcome in patients admitted to the intensive care unit after cardiac arrest

Background: Arterial carbon dioxide tension (PaCO₂) affects neuronal function and cerebral blood flow. However, its association with outcome in patients admitted to intensive care unit (ICU) after cardiac arrest (CA) has not been evaluated. Methods and results: Observational cohort study using data from the Australian New Zealand (ANZ) Intensive Care Society Adult-Patient-Database (ANZICS-APD). Outcomes analyses were adjusted for illness severity, co-morbidities, hypothermia, treatment limitations, age, year of admission, glucose, source of admission, PaO₂ and propensity score.We studied 16,542 consecutive patients admitted to 125 ANZ ICUs after CA between 2000 and 2011. Using the APD-PaCO₂ (obtained within 24h of ICU admission), 3010 (18.2%) were classified into the hypo- (PaCO₂<35mmHg), 6705 (40.5%) into the normo- (35-45mmHg) and 6827 (41.3%) into the hypercapnia (>45mmHg) group. The hypocapnia group, compared with the normocapnia group, had a trend toward higher in-hospital mortality (OR 1.12 [95% CI 1.00-1.24, p=0.04]), lower rate of discharge home (OR 0.81 [0.70-0.94, p<0.01]) and higher likelihood of fulfilling composite adverse outcome of death and no discharge home (OR 1.23 [1.10-1.37, p<0.001]). In contrast, the hypercapnia group had similar in-hospital mortality (OR 1.06 [0.97-1.15, p=0.19]) but higher rate of discharge home among survivors (OR 1.16 [1.03-1.32, p=0.01]) and similar likelihood of fulfilling the composite outcome (OR 0.97 [0.89-1.06, p=0.52]). Cox-proportional hazards modelling supported these findings. Conclusions: Hypo- and hypercapnia are common after ICU admission post-CA. Compared with normocapnia, hypocapnia was independently associated with worse clinical outcomes and hypercapnia a greater likelihood of discharge home among survivors.