Introduction: Current AHA guideline recommends fixed target PaCO2 range of 40~45 mmHg after ROSC. However, normal physiologic response to metabolic acidosis is respiratory compensation and its loss can lead to severe acidosis and hemodynamic instability. We developed and tested a new hyper/hypoventilation index with time-weighted manner. Material & Methods: This is a retrospective study of a single academic hospital from Jan, 2008 to July, 2014. Patients who survived more than 24 hours after ROSC were included. Basic Utstein elements and ABGA results during the first 24 hours after ROSC were collected. Absolute hypoventilation exposure (AHE) and relative hypoventilation exposure (RHE) were calculated using fixed PaCO2 and calculated PaCO2 level. Results: 232 patients were included. Univariable logistic regression demonstrated that both RHE and AHE were significantly associated with neurologic outcomes. However, RHE was more predictable for neurologic outcomes than AHE (0.73 vs 0.59 in AUROC, respectively, p<0.001) and was an independent risk factor. RHE was associated with mean arterial pressure(MAP), and there was partial mediation effect of exposure to low MAP in increased RHE. Conclusion: Relative hypoventilation exposure was an independent predictor for poor neurologic outcome in OHCA patients. Increased RHE was associated with mean arterial pressure level.