A 29-year-old woman has a 1.4 L haemorrhage after an uncomplicated vaginal birth. The placenta is complete and there is no vaginal or cervical tear. Despite uterine massage, an oxytocin infusion and tranexamic acid, the uterus remains poorly contracted; her blood pressure is 132/78 mmHg and she has severe asthma. Which one of the following is the most appropriate next treatment?