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Compendium June 2002 Drug Therapy/GI Disease 453 Table 1. Emetic Drugs Drug Dose Comment 3% Hydrogen peroxide Dogs and cats: 5–10 ml PO, one to two doses as needed Fatal aspiration of foam is possible in cats Ipecac syrup Dogs: 1–2.5 ml/kg PO; cats: 1–3.3 Not to exceed 15 ml/dog; dogs and ml/kg PO (may cause toxicity and death) cats: dose may be repeated once but product must be removed via gastric lavage if vomiting does not occur Apomorphine hydrochloride Dogs: 0.04 mg/kg IV, 0.08–0.1 mg/kg IM or SC, or 0.25 mg (conjunctival sac) Not recommended for use in cats Xylazine Cats: 0.44 mg/kg IM Not effective in dogs; emesis in cats should occur within 15 min; sedation lasts 1–2 hr EMETICS Emetics are used to induce vomiting in dogs and cats for early treatment of ingestion of noncorrosive toxins. Within the first 30 minutes to 1 hour after toxic ingestion, emetics can help eliminate the toxin from the stomach before they are absorbed in the small intestine. Emesis should never be induced in patients that are unconscious, experiencing a seizure, or having respiratory difficulty. Reflux emesis can be induced by instilling warm water (5 to 10 ml/kg) into the stomach via a stomach tube. This will dilute the toxin and initiate the vomiting reflex by stimulating peripheral receptors in the stomach that respond to distention. Gastric mucosa irritants can also induce the vomiting reflex peripherally. Orally administered hypertonic saline solution or table salt, 3% hydrogen peroxide solution, or ipecac syrup or powder (i.e., emetine) can be effective and administered at home prior to an emergency visit (Table 1). Ipecac in cats can be toxic, particularly with the administration of multiple doses that can be absorbed systemically. Hydrogen peroxide can foam and cause fatal aspiration in cats; therefore, it should be used with caution in this species. 1 Centrally acting drugs are more potent stimulants of the vomiting reflex. Apomorphine hydrochloride, an opioid and synthetic derivative of morphine, stimulates the CTZ by acting on dopamine receptors. 1 It is available as a powder inside a capsule and can be mixed in sterile water and given subcutaneously. The tablet form, which is more expensive, can be used in the conjunctival sac for quick absorption. Oral administration usually requires a much higher dose because of decreased oral bioavailability. 1 Likewise, giving subsequent doses does not improve the vomiting effect but may actually suppress the emetic center. Although considered only a mild CNS depressant, apomorphine hydrochloride should not be used in animals with depressed CNS conditions. Apomorphine hydrochloride is not recommended for use in cats because they need a higher dose to induce vomiting, and this can cause excitement and hyperactivity of the CNS. 1 Xylazine, an α 2-agonist, is the drug of choice to stimulate emesis in cats. 1 Although it is used more commonly for its sedation/analgesic properties, it can be used at a lower dose to induce vomiting in cats. In dogs, xylazine is not as consistent at inducing emesis. There are no reports on the use of medetomidine, a drug with similar actions, to induce emesis in either species. ANTIEMETICS Vomiting occurs in dogs and cats as a result of many GI and non-GI disorders. Suppression of vomiting is indicated when the underlying cause is known and serves to prevent further metabolic alterations caused by the loss of fluid, gastric acids, and electrolytes. Although treatment of the underlying cause is always the primary objective, the use of antiemetics for the interim to prevent further debilitation is beneficial. Suppressing vomition is contraindicated in patients with pyloric or intestinal obstruction. Dopamine Antagonists Metoclopramide, a dopamine antagonist, blocks dopamine receptors at the CTZ 2 (Table 2). It is suspected that at higher doses, antagonism is at the 5-HT 3 receptors. This agent is reported to be 20 times more potent than phenothiazines, although this difference in efficacy has not been documented in dogs and cats. 3 Metoclopramide also has prokinetic effects, which may help minimize vomiting by increasing upper GI motility, increasing lower esophageal sphincter tone, decreasing pyloric sphincter tone, and promoting antegrade contraction of the gastric antrum. A side effect is CNS stimulation that presents as irritability or nervousness, particularly in cats. The antidote for this side effect is diphenhydramine hydrochloride
454 Small Animal/Exotics Compendium June 2002 Table 2. Antiemetic Drugs Drug Dose Comment Metoclopramide Dogs and cats: 0.1–0.5 mg/kg PO or Use lower doses with renal failure; SC q8h, 0.02 mg/kg IV q1h, or 1–2 common side effects are anxiety and mg/kg/day IV by CRI restlessness in dogs and cats Domperidone Dogs and cats: 0.1–0.5 mg/kg IM or IV q12h or 2–5 mg/kg PO q12h Not available in the United States Ondansetron Dogs and cats: 0.1–0.2 mg/kg SC q8h, No side effects are known; further 0.1–1.0 mg/kg q12–24h PO, 0.5 mg/kg IV load, or 0.5 mg/kg IV infusion q1h investigation is needed in dogs and cats Dolasetron Dogs and cats: to prevent nausea or Can be combined with metoclopramide vomiting—0.6 mg/kg/day IV or PO; for optimal prevention of chemotherapyvomiting—1 mg/kg/day IV or PO induced nausea and vomiting in dogs and cats Chlorpromazine Dogs: 0.2–0.4 mg/kg IM or SC q6–8h, 3.3 mg/kg PO q6–8h; cats: 0.5 mg/kg IM or SC q6–8h, 2.0–4.0 mg/kg/day PO May cause sedation Prochlorperazine Dogs: 0.25–0.5 mg/kg IM q8–12h or 1 mg/kg PO q12h; cats: 0.13 mg/kg IM q12h or 0.5 mg/kg PO q12h with food May cause sedation in dogs and cats Trifluoperazine Dogs and cats: 0.03 mg/kg IM q12h May cause sedation in dogs and cats Cyclizine hydrochloride Dogs and cats: 4 mg/kg IM q8h For vestibular disease–induced emesis Meclizine hydrochloride Dogs 10 kg: 2–6 mg/kg/day PO; cats: 4 mg/kg/day PO For vestibular disease–induced emesis Diphenhydramine hydrochloride Dogs and cats: 2–5 mg/kg PO q6–8h, For vestibular disease–induced emesis 2 mg/kg IM or IV (slowly) q12h CRI = constant-rate infusion. (1 mg/kg IV) in dogs and cats. 4 Contraindications are GI hemorrhage and obstruction. Other dopamine antagonists that have been available but less commonly used are domperidone and the butyrophenone derivatives haloperidol and droperidol. Domperidone, which is similar to metoclopramide in activity, is currently not available in the United States for use in dogs and cats. The butyrophenones cause potent tranquilizer side effects and seizures in preepileptic patients. Serotonin Antagonists Serotonin antagonists, also known as 5-HT 3 antagonists, are a relatively new class of antiemetic drug that blocks stimulation at the CTZ and in the intestine to prevent vomiting. 5 The neurotransmitter serotonin acts by increasing the secretion of acetylcholine at distal ganglia, thereby stimulating the CTZ to induce vomiting. Serotonin antagonists block this mechanism and are considered the most potent inhibitors of vomiting. They are particularly useful for preventing vomiting induced by chemotherapy, parvovirus, and causes that are resistant to other therapies. 5-HT 3 antagonists that are available include ondansetron, tropisetron, granisetron, azesetron, and dolasetron. Ondansetron and dolasetron have been reported to be very effective in both dogs and cats 6 ; however, the high cost of these drugs limits their use to chemotherapy patients or patients unresponsive to other medications. Side effects have not been reported in dogs and cats; however, indepth study of these drugs in companion animals have not been published. Phenothiazines Phenothiazines control vomiting by inhibiting the CTZ as a result of their antidopaminergic and antihistaminergic effects. 2 Phenothiazine derivatives used as antiemetics in small animals include chlorpromazine, prochlorperazine, triflupromazine, perphenazine, trifluoperazine, and mepazine. Side effects associated with phenothiazines are sedation, hypotension caused by peripheral α blockade, and seizures in preepileptic patients. Glucocorticoids Glucocorticoids are used in human medicine to con-
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