High-normal PaCO2 values might be associated with worse outcome in patients with subarachnoid hemorrhage – a retrospective cohort study

Abstract Background While both hypercapnia and hypocapnia are harmful in patients with subarachnoid hemorrhage (SAH), it is unknown whether high-normal PaCO2 values are better than low-normal values. We hypothesized that high-normal PaCO2 values have more detrimental than beneficial effects on outcome. Methods Consecutive patients with aneurysmal subarachnoid hemorrhage (aSAH) requiring mechanical ventilation treated in a tertiary care university hospital were retrospectively analyzed regarding the influence of PaCO2 on favorable outcome, defined as modified Rankin scale score < 3 at discharge. Primary endpoint was the difference in the proportion of PaCO2 values above 40 mmHg in relation to all measured PaCO2 values between patients with favorable and unfavorable outcome. Results 150 patients were included. Median age was 57 years (p25:50, p75:64), median Hunt-Hess score was 4 (p25:3, p75:5). PaCO2 values were mainly within normal range (median 39.0, p25:37.5, p75:41.4). Patients with favorable outcome had a lower proportion of high-normal PaCO2 values above 40 mmHg compared to patients with unfavorable outcome (0.21 (p25:0.13, p75:0.50) vs. 0.4 (p25:0.29, p75:0.59)) resulting in a lower chance for favorable outcome (OR 0.04, 95% CI 0.00–0.55, p = 0.017). In multivariable analysis adjusted for Hunt-Hess score, pneumonia and length of stay, elevated PaCO2 remained an independent predictor of outcome (OR 0.05, 95% CI 0.00–0.81, p = 0.035). Conclusions A higher proportion of PaCO2 values above 40 mmHg was an independent predictor of outcome in patients with aSAH in our study. The results need to be confirmed in a prospective trial.