Catecholamines. Catecholamines increase heart rate and cardiac contractility, and some are potent vasoconstrictors (e.g. dopamine, norepinephrine). Notably, catecholamines alter the intravascular volume-expanding effects of fluid therapy.44,45 Drugs that activate alpha receptors (e.g. dopamine, norepinephrine, and phenylephrine) and induce vasoconstriction reduce blood volume expansion, while those that
activate beta receptors augment blood volume expansion.46 Since optimization of blood pressure and blood flow is the goal in a hypotensive anesthetized animal, the administration
of a mixed alpha-beta agonist (e.g. dopamine) is the more rational therapeutic choice for maintaining arterial blood pressure.46
Final considerations. Finally, it should be emphasized that anesthetic-induced or -associated hypotension is best treated by decreasing or stopping
the administration of anesthetic drugs. A potent analgesic (e.g. fentanyl) can be administered to reduce the requirement for inhalant or injectable anesthesia.47 A vasoconstrictor (e.g. dopamine, norepinephrine) or positive inotrope (e.g. dopamine, dobutamine) may be required to restore and maintain blood vessel tone or cardiac contractile function, blood flow,
and arterial blood pressure, respectively, in animals that do not respond to fluid therapy due to vasodilatation or poor cardiac
contractile function.48
CONCLUSION
Perioperative fluid therapy in animals is not innocuous and should be considered identical to drug therapy—individualized
and goal-directed. Differences in the types of fluids (crystalloid or colloids) available determine their effects upon electrolyte
and acid-base balance and their ability to restore or maintain arterial blood pressure and tissue perfusion.
Isotonic balanced crystalloids maintain water requirements, blood osmolality, tissue perfusion, and acid-base balance in otherwise
normal healthy surgical candidates. Colloid solutions are more effective for treating low arterial blood pressure, poor tissue
perfusion, and blood loss. Both crystalloids and colloids are dilutional to substances; they do not contain (e.g. hemoglobin) and should be dosed in order to prevent excessive hemodilution (hemoglobin < 7 g/dl) and avoid fluid overload.
Thus, accurate assessment of fluid status (i.e. fluid responsiveness) and careful definition of targets at all stages of the perioperative period are needed to improve clinical
outcomes.
Fluid administration is essential for optimizing tissue perfusion. The determination of PVI helps to predict fluid therapy
responsiveness and to guide fluid replacement during anesthesia in dogs and cats.
Acknowledgments
The author thanks Dava Cazzolli, DVM, DACVECC; Jaime Chandler, DVM, DACVECC; Michelle Albino, LVT, CVT (Anesth); and Yukie
Ueyama, DVM, for their assistance in abstracting data.
William Muir, DVM, MS, PhD, DACVA, DACVECC
VCPCS
338 W. 7th Ave.
Columbus, OH 43201
For references, see http://dvm360.com/FluidTherapy3Refs.
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