I used Phenylephrine for years in a critical care unit for blood pressure control. Pharmacy makes a solution of Phenylephrine and a liter of saline . Central lines are best because PIV's can extravasate and you lose the effect. On a pump you slowly dial up the drug until you get the number you want on the arterial line. Someone who had lost BP or dangerously low could be bumped back up quickly while fluids were going in and causation determined. Phenylephrine first, Norepinephrine if that fails, Epinephrine next, and as a last resort a Vasopressin drip. Ephedrine could be rarely used but as we knew from bitter experience the saying was "Leave them dead on Levophed"