Small Animal Anesthesia Parts I and Ii

Anticholinergics Anticholinergics are competitive antagonists of acetylcholine at post ganglionic parasympathetic muscurinic receptors. Stimulation of muscurinic receptors induces salivation, pupillary constriction, bronchoconstriction, gastric acid secretion, gastrointestinal motility, and slowing of the heart rate. The two most commonly used anticholinergics in veterinary medicine are atropine and glycopyrrolate (Table 1). These two drugs differ in their duration of action, with the effects of glycopyrrolate lasting at least twice as long as atropine. Furthermore, glycopyrrolate does not cross the intact blood brain barrier or the placenta. Anticholinergics increase myocardial oxygen consumption by increasing heart rate. and can precipitate cardiac arrhythmias and decrease the threshold for ventricular fibrillation. For these reasons anticholinergics should not be used as a routine part of the anesthetic regimen, since many critically ill patients can not tolerate increases in myocardial oxygen consumption or alterations of cardiac rhythm. Anticholinergics are indicated for specific situations. For example, animals with high resting vagal tone, or high vagal tone resulting from narcotic or alpha-2 administration, those with excessive upper airway secretions, or in those animals in which cardiac output is dependent on maintenance of a normal heart rate. This includes neonates or individuals with cardiac tamponade.