Effect of nitrous oxide on intracranial pressure after cranial-dural closure in patients undergoing craniotomy.

After cranial-dural closure, nitrous oxide (N2O) may diffuse into and expand an entrapped volume of intracranial air, thereby increasing intracranial pressure (ICP). We performed a prospective clinical study to determine the effect of continuation of N2O after dural closure on ICP in patients undergoing craniotomies. Patients were randomly assigned in a 1.5:1 ratio into a group in which N2O was continued after dural closure (N2O, n = 15) or a group in which N2O was discontinued and replaced with nitrogen (N2, n = 9) at the time of dural closure. PaCO2 was normal prior to closure, and end-tidal PCO2 was kept constant after dural closure. Ipsilateral ICP was recorded at 5-min intervals after dural closure until completion of skin closure and immediately postoperatively. Presence of intracranial air was determined by head computed tomography scan within the first postoperative hour. ICP at the time of dural closure did not differ between the groups (N2O: 3 +/- 2 mmHg vs. N2:5 +/- 1 mmHg). Intraoperatively, ICP did not change after dural closure, regardless of whether N2O was continued or discontinued. Postoperatively, ICP was reduced, with a significant decrease in ICP (P less than 0.01) observed only in the N2O group. Postoperative computed tomography scans demonstrated the presence of intracranial air in all patients, with most exhibiting a mild to moderate degree of pneumocephalus. We conclude that continuation of N2O after dural closure did not affect ICP during the craniotomy closure. These results suggest that it is not necessary to discontinue N2O prior to dural closure for reasons of avoiding expansion of intracranial air and increasing ICP.