Uma Mandal, Indranil Halder, and Keya Chakraborty. “Comparative Study of Bupivacaine-Fentanyl Versus Bupivacaine-Clonidine in Patients Undergoing Spinal Anaesthesia for Lower Abdominal Surgery”. International Journal of Health and Clinical Research, vol. 5, no. 2, Jan. 2022, pp. 843-9, https://ijhcr.com/index.php/ijhcr/article/view/4926.