The purpose of this study is to evaluate the effect of ketamine infusion for reduction of neuropathic pain and improvement of quality of life after lumbar surgery.

Neuropathic pain defined as 'pain caused by a lesion or disease of the somatosensory system'. Neuropathic pain is 20 - 35% of low back pain.

Ketamine is categorized as N-methyl D-aspartate receptor antagonist. Ketamine has an analgesic effect in low dose and anesthetic effect in relative high dose. Also, ketamine is used in chronic neuropathic pain as postherpetic neuralgia., phantom pain, central neuropathic pain, complex regional pain syndrome, ischemic pain.

In this study, ketamine will be infused during lumbar surgery in randomized, double-blind and double-blind parallel group control trial.

Inclusion Criteria:

• Undergoing lumbar spine surgery due to back pain patients
• American Physical Status Classification I or II

Exclusion Criteria:

• less than 3 month of back pain
• past history of operation at lumbar area
• past history of glucoma, drug abuse, alcohol abuse
• pregnancy