Zahid, Fahad, et al. “INTRATHECAL TRAMADOL AS AN ADJUVANT IN SUBARACHNOID BLOCK TO PROLONG THE DURATION OF ANALGESIA: Tramadol As Adjuvant in Subarachnoid Block”. Pakistan Armed Forces Medical Journal, vol. 67, no. 4, Aug. 2017, pp. 534-39, https://pafmj.org/PAFMJ/article/view/640.