A 30-year-old G3P2 woman with a history of hypertension presents to the birthing floor in labor. Following a prolonged labor and delivery with no fetal complications, she continues to bleed vaginally but remains afebrile. On bimanual examination, her uterus is soft, boggy, and enlarged. There are no visible lacerations. Uterine massage only slightly decreases the hemorrhage, and oxytocin is only mildly effective. Which of the following is the next best step in mgt: