A 1-week-old term neonate is undergoing an exploratory laparotomy for duodenal atresia. The patient’s vital signs include a heart rate of 170 bpm, a systolic blood pressure of 40 mmHg, and capillary refill time of 7 seconds. The patient has already received a 60mL/kg bolus of isotonic saline. Administration of which of the following is the next best step in the management of this patient?