Evidence has shown a high incidence of spontaneous hyperventilation in patients with aneurysmal subarachnoid hemorrhage (aSAH), which is associated with poor outcomes. It's well established that the hypocapnia caused by hyperventilation leads to cerebral vasoconstriction, reduces the cerebral blood flow, and decreases the cerebral blood volume and intracranial pressure consequently. However, persistent cerebral vascular constriction increases the risk of cerebral ischemia; therefore, maintaining a partial pressure of arterial carbon dioxide (PaCO2) in the range of 35-40mmHg is recommended to minimize the hazard hypocapnia.~There's still no standard method to deal with spontaneous hyperventilation. Based on the pharmacology and clinical experience, remifentanil seems to be an ideal choice since it could inhibit the respiratory rate in a dose-dependent fashion. As one of the most used short-acting opioids, remifentanil could prolong the expiratory time, meanwhile not influencing the inspiratory time and respiratory drive, consequently decreasing respiratory rate and maintaining the tidal volume.~In this exploratory physiology study, the investigators will test the hypothesis that spontaneous hyperventilation could be suppressed by titrating the dose of remifentanil and could be realized as the target of PaCO2. The investigators will determine the optimal amount of remifentanil, which fulfills the criteria of efficacy and safety, and evaluate its effect on the cerebral blood flow velocity of both the middle cerebral artery and internal carotid artery.
Although spontaneous hyperventilation patients with aneurysmal subarachnoid hemorrhage closely associated with poor outcomes, the standard therapy remains unavailable. Remifentanil has the pharmacological characterization of respiratory inhibition, mainly prolonging the expiratory time and decreasing the respiratory rate while preserving the respiratory drive. The investigators hypothesis that spontaneous hyperventilation could be corrected by titrating the dose of remifentanil and cerebral blood flow will augment during this process.