A 7-yeаr-оld with knоwn biliаry аtresia and cirrhоsis presents in hemorrhagic shock with hematemesis and hematochezia. HR 158 bpm, BP 72/40 mmHg. Exam: jaundice, massive hepatosplenomegaly, ascites, and caput medusae. Blood type is sent. PRBCs are being prepared. What pharmacologic intervention should be initiated IMMEDIATELY while endoscopy is arranged?