Ratanasuwan, P., S. Nonphiaraj, S. Pongjanyakul, W. Somdee, and W. Taesiri. “Efficacy of Combination of Ketamine and Morphine for Intravenous Patient Controlled- Analgesia in Upper Abdominal Surgery at Srinagarind Hospital”. Srinagarind Medical Journal, vol. 30, no. 5, Oct. 2015, p. 67, https://li01.tci-thaijo.org/index.php/SRIMEDJ/article/view/40017.