A. Choudhary, S. Maheshwari, N. Paliwal, S. Ujwal, and G. Singariya, “Comparative study of fentanyl versus tramadol as adjuvant with low dose local anaesthetic ropivacaine (0.1%) for epidural labour analgesia”, Int J Res Med Sci, vol. 8, no. 9, pp. 3309–3314, Aug. 2020.