Umapathy, P., and M. Ayathullah. “Onset and Duration of Epidural Analgesia With Bupivacaine and Bupivacaine Plus Fentanyl: A Comparative Study”. International Journal of Research in Medical Sciences, vol. 5, no. 7, June 2017, pp. 3103-6, doi:10.18203/2320-6012.ijrms20172995.