Serum Beta-Endorphin Response to Stress Before and After Operation Under Fentanyl Anesthesia in Neonates, Infants and Preschool Children - Mirilas, P.; Mentessidou, A.; Kontis, E.; Antypa, E.; Makedou, A.; Petropoulos, A. S. (European Journal of Pediatric Surgery, 2010)