The primary question is what effect routine N2O exposure has on the latter compared with non-N2O anesthesia in elderly people who either have or do not have subclinical cobalamin deficiency. It will help answer whether or not the combination can help explain the increased risk of cognitive problems after surgery in elderly patients, and by extension whether preoperative cobalamin testing and treatment may be indicated in the elderly. It will also test whether genetic predisposition affects the described problems.