 Balanced anesthesia combining volatile anesthetics and opioids is commonly used in modern anesthesia. The purpose of this study is to compare two methods of administering desflurane, an volatile anesthetic during balanced anesthesia. The investigators hypothesized that administering desflurane at 1 MAC minimum alveolar concentration during balanced anesthesia will be more beneficial in terms of stability of the patients vital signs and hypnotic depth than administering desflurane according to the patients BIS bispectral index value. 