INS AND OUTS OF FOAL FLUID THERAPY
PAMELA WILKINS, DVM, MS, PHD, DACVIM-LA, DACVECC University of Illinois, Champaign-Urbana IL USA
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Dobutamine is useful after patients are volume replete for support of cardiac output. The dose range is between 2 to 20 ug/kg/min, provided as a constant rate infusion, with best results generally obtained between 2 and 5 ug/kg/min. If larger doses are needed, a better approach might be to add a pressor to the treatment plan. DOPAMINE Dopamine has dopaminergic activity at low doses, β1 and β2 activity at moderate doses, and β1 activity at high doses. Dopamine causes norepinephrine release, which has led to the suggestion that this is its major mode of action at higher doses. At doses greater than 20 ug/kg/min, intrapulmonary shunting, pulmonary venous vasoconstriction, and reduced splanchic perfusion may occur. Dopamine also produces natriuresis at lower doses through a direct effect on renal tubules. For these reasons, dopamine has fallen out of favor in human critical care and at many veterinary referral institutions. NOREPINEPHRINE Norepinephrine has α1 and β1 activity but variable β2 activity, resulting in potent vasopressor effects; it has inotropic and chronotropic effects, but its chronotropic effect usually is blunted by vagal reflexes slowing the
heart rate induced by the increase in blood pressure. In many critical care units, norepinephrine has become a pressor of choice and frequently is used along with dobutamine. Evidence suggests that splanchic perfusion is maintained better with norepinephrine than with some other pressors and norepinephrine is frequently paired with dobutamine. The dose range is 0.2 to 2.0 ug/kg/min, although larger doses have been used when necessary in certain patients. EPINEPHRINE Epinephrine has α1, α2, β1, and β2 activity; activity predominates and results in increased cardiac output and decreased peripheral resistance at low doses. Epinephrine has been associated with hyperglycemia, hypokalemia, lipolysis, increased lactate concentration, and increased platelet aggregation. The effect on renal function is controversial. Use of epinephrine usually is limited to those patients not responding to other pressors. VASOPRESSIN Vasopressin (antidiuretic hormone) is a pressor gaining a great deal of attention in the critical care literature. Vasopressin appears to be depleted from the neurohypophysis in septic shock, and short-term administration
of vasopressin spares conventional vasopressor use, in addition to improving some measures of renal function. Low-dose vasopressin infusion (0.5-2.0 mU/kg/min) increases mean arterial pressure, systemic vascular resistance, and urine output in patients with vasodilatory septic shock that are hyporesponsive to catecholamines. These data indicate that low-dose vasopressin infusions may be useful in treating hypotension in patients with septic shock.
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