Once the fluid therapy plan is underway, ongoing assessment is critical. If adequate fluids have been administered and reasonable resuscitation endpoints have not been reached, several causes should be considered; these variables should be rapidly assessed and corrected:
inadequate volume administration
ongoing fluid losses from hemorrhage
third space fluid losses
heart disease or pericardial fluid accumulation
severe vasodilation or vasoconstriction
organ ischemia
hypoglycemia
hypokalemia
arrhythmias
severe acidemia or alkalemia
anemia or hypoxemia
decreased venous return
hypothermia
intracranial disease
hypoadrenocorticism or critical illness-related corticosteroid insufficiency
If a central venous pressure (CVP) line is available, it may be checked to see whether CVP is near the endpoints assigned ( Resuscitation Endpoints). If not, or if no CVP is available, a fluid challenge can be given. This typically consists of a rapid IV infusion (10–15 mL/kg) of crystalloids and/or a rapid IV infusion (5 mL/kg) of a hydroxyethyl starch. If the perfusion parameters improve with this challenge, then the likely cause of the nonresponsive shock is inadequate volume, and colloids are titrated to reach the desired endpoints. Ultrasonography, with an experienced ultrasonographer, may be useful to assess cardiac function and/or volume status in select patients.
If fluid volume appears adequate, underlying etiologies have been addressed and treated, and the animal is still hypotensive, vasopressors can be used. These medications are often started at the lower end of the dose and titrated up quickly to obtain a blood pressure that supports organ function, then delivered as a continuous rate infusion: dopamine (2–15 mg/kg/minute), norepinephrine (0.05–2 mcg/kg/minute), vasopressin (1–5mU/kg/minute), and phenylephrine (1–3 mcg/kg/minute) are common options. If cardiac contractility is known to be low, dobutamine (2–10 mcg/kg/minute) may be administered. These medications are gradually reduced once blood pressure has stabilized.
