It is essential that initial treatment of systemic hypotension always be aimed at correction of the underlying physiologic problem: decreased preload, cardiac dysfunction, or peripheral vasodilation. Differentiation of cardiac and noncardiac causes of systemic hypotension is a critical first step. If the animal is hypovolemic, intravenous fluids and/or blood products should be administered until euvolemia has been attained. If hypovolemia is severe enough to cause hypotension, a shock bolus should be given, up to 60 to 90 mL/kg for dogs and 45 to 60 mL/kg for cats (given incrementally as you know!).
If the animal remains hypotensive once euvolaemia has been achieved, the use of pressors should be considered. Commonly used pressors for treating vasodilation include dopamine (5-15 mcg/kg/min), epinephrine (0.05-1 mcg/kg/min), norepinephrine (0.1-1 mcg/kg/min), or phenylephrine (0.5-5 mcg/kg/min), administered for their alpha-agonist effects, as constant-rate IV infusions. Only phenylephrine is a pure alpha-agonist; the others have varying degrees of beta effects in addition to their alpha effects. Vasopressin (0.5-5 mU/kg/min) also can be used in cases with vasodilatory shock and may be especially useful in cases of sepsis/SIRS as the vessels can become refractory to catecholamines. These drugs need to be titrated to effect, requiring frequent blood pressure monitoring. They should never be used in place of adequate volume expansion, because most patients with hypovolemic shock already have compensatory vasoconstriction.
My first choice drug wise would probably be norepinephrine.
Hope that helps.
Scott x
