Fan Y, Ji M, Zang L, Wang W, Yin Q, Xu J, Yang J. Comparison of epidural tramadol-ropivacaine and fentanyl-ropivacaine for labor analgesia: A prospective randomized study. ujms [Internet]. 2011Nov.9 [cited 2024Dec.3];116(4):252–257. Available from: https://ujms.net/index.php/ujms/article/view/6059