High-normal P(a)CO(2) values might be associated with worse outcome in patients with subarachnoid hemorrhage – a retrospective cohort study

BACKGROUND: While both hypercapnia and hypocapnia are harmful in patients with subarachnoid hemorrhage (SAH), it is unknown whether high-normal P(a)CO(2) values are better than low-normal values. We hypothesized that high-normal P(a)CO(2) 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 P(a)CO(2) on favorable outcome, defined as modified Rankin scale score < 3 at discharge. Primary endpoint was the difference in the proportion of P(a)CO(2) values above 40 mmHg in relation to all measured P(a)CO(2) 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). P(a)CO(2) 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 P(a)CO(2) 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 P(a)CO(2) remained an independent predictor of outcome (OR 0.05, 95% CI 0.00–0.81, p = 0.035). CONCLUSIONS: A higher proportion of P(a)CO(2) 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.