Background
Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, was initiated to be used clinically for anesthetic induction and maintenance in the 1970s [Theoretically, the co-administration of subanesthetic ketamine during general anesthesia could deepen the anesthetic depth. A series of studies have confirmed that low-dose ketamine reduces the consumption of propofol [S-ketamine (induction dose 0.5–1 mg/kg i.v.), the pure dextrorotatory enantiomer of ketamine, has an approximately two-fold higher sedative potency compared with ketamine, associated with a stronger effect at the NMDA receptor [