 This research is being done to compare two methods of giving fentanyl, a narcotic often given to patients following brain surgery and determine if one method has more side effects than the other. Both of these methods are available in the postoperative treatment of pain. This research also is being done to determine if patients receiving narcotic pain medicine will benefit from additional monitoring of carbon dioxide levels. Since narcotic pain medicines can slow down breathing, The investigators want to see if measuring exhaled carbon dioxide levels will help identify a slower breathing rate and improve safety. 