A patient in the ED reports 30 min of severe, crushing, substernal chest pain. BP is 110/70, HR is 58/min, and monitor shows regular sinus bradycardia. Pt. has received 325mg Aspirin, O2 4LPM via NC, & 3 sublingual nitro 5 min apart, but continues to have severe pain. What agent is given next (if there are no contraindications)?