Objective:The present study aimed to compare the effect of periradial injection of Papaverine versus Nitroglycerine on radial artery diameter prior to cannulation in cardiac surgery patients. We hoped that periradial injection will facilitate the radial artery cannulation and decrease its spasm. Design:This is a prospective randomized controlled trial that included ninety patients scheduled for elective cardiac surgery. Interventions:Patients were randomly assigned into one of three groups: One group received a peri-radial subcutaneous injection of Papaverine and Lignocaine 2% (30 participants); the second group received a peri-radial subcutaneous injection of Nitroglycerine and Lignocaine 2% (30 participants) and a control group that received a peri-radial subcutaneous injection of Lignocaine 2% (30 participants). Measurements and Main Results:Theradial artery diameter was measured by ultrasound before the injection and 20 minutes following injection, and the radial pulse palpability score was measured on a score of 1 (being weak) to 3 (being strongly palpable) before the injection and 20 minutes after.We observed a significant increase in radial artery diameter after injection of Papaverine (p < 0.001) and Nitroglycerine (p < 0.001), compared to baseline values, while there was no significant change in the control group. The changes in the Papaverine group were significantly higher (p= 0.003) than that observed in the Nitroglycerine group.The palpatory score of the radial artery was significantly higher in the Papaverine group than the Nitroglycerine group and both are significantly higher than the control group. Conclusions:Papaverine achieved significant increases in radial artery diameter and palpability score.