Assessment of Resuscitation Efforts in Animals

Once the fluid therapy plan is underway, ongoing assessment is critical. If adequate fluids have been administered and reasonable resuscitation end points 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

• hypocalcemia

• 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 can be checked to see whether CVP is near the end points assigned (see the table Resuscitation End Points). 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 hydroxyethyl starch given over 5–15 minutes. If 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 end points.

Ultrasonography, with an experienced ultrasonographer, may be useful to assess cardiac function and/or volume status in select patients; most notably, the caudal vena cava collapsibility index (CVCCI) has been reported to identify patients who will be responsive to fluids (1). The CVCCI is collected by ultrasonographically examining the vena cava, just caudal to where it crosses the diaphragm. The maximum and minimum diameters of the CVC are measured during the expiratory and inspiratory phase, respectively The CVCCI is then calculated with the following formula:

(Max diameter − Min diameter) / Max diameter × 100

A cutoff value of 27% in dogs and 31% in cats was reasonably sensitive and specific to identify patients who will be responsive to fluids. However, even a subjective assessment of intravascular volume may be useful.

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 an IV CRI; dopamine (2–15 mcg/kg/min), norepinephrine (0.05–2 mcg/kg/min), vasopressin (1–5 mU/kg/min), and phenylephrine (1–3 mcg/kg/min) are common options. If cardiac contractility is known to be low, dobutamine (2–10 mcg/kg/min) may be administered. These medications are gradually decreased once blood pressure has stabilized.