Antiemetics

Chapter 182 Antiemetics

Michael D. Willard, DVM, MS, DACVIM

• Centrally acting antiemetics are generally much more effective than peripheral-acting antiemetics. Centrally acting antiemetics that work on the medullary vomiting center are often more effective than those that act just at the chemoreceptor trigger zone.

• Promazine derivatives (e.g., chlorpromazine, prochlorperazine) are effective centrally acting antiemetics, but they can cause hypotension due to α-adrenergic blocking activity.

• Metoclopramide works at the chemoreceptor trigger zone and is also a gastric prokinetic. Metoclopramide can sometimes cause abnormal behavior and even vomiting (possibly due to excessive gastric prokinetic activity).

• The 5-HT₃ receptor antagonists ondansetron and dolasetron are among the most effective antiemetics used in dogs and cats, and they have very few side effects.

• Maropitant is a newly approved centrally acting antiemetic for use in dogs, but clinical experience with this drug is limited.

INTRODUCTION

Antiemetics are indicated primarily in patients that are vomiting so much that it is difficult to maintain fluid or electrolyte homeostasis, or patients with severe nausea that is obviously diminishing their quality of life (Table 182-1). Depending on individual particulars, it is sometimes appropriate to allow a patient to vomit once or twice a day in order to see if other therapy is having a beneficial effect on the underlying cause. Typical indications include pancreatitis, gastritis or enteritis, peritonitis, hepatic disease, renal insufficiency, and patients that are recumbent or at high risk for aspiration pneumonia. These drugs are usually ineffective in patients with gastrointestinal (GI) obstruction. Parenteral administration is preferred because oral administration may be ineffective if the drug is vomited before it is absorbed.

Table 182-1 Centrally Acting Antiemetics Commonly Used in Dogs and Cats

Drug	Dosage	Special Considerations
Chlorpromazine	0.5 mg/kg IM, SC q8-12h	Can cause hypotension and sedation
Prochlorperazine	0.1 to 0.5 mg/kg IM, SC q8-12h	—
Metoclopramide	0.1 to 0.5 mg/kg IV, IM, PO q8-12h or CRI of 1 to 2 mg/kg IV q24h	Potent gastric prokinetic Can cause extrapyramidal effects if overdosed
Ondansetron	0.5 to 1 mg/kg IV, PO q12-24h	—
Granisetron	1 mg/kg IM q8-24h	—
Dolasetron	0.6 to 1 mg/kg IV, SC, PO q24h	—
Maropitant	1 mg/kg SC, q24h or 2 mg/kg PO q24h	Newly approved drug for dogs

CRI, Constant rate infusion; IM, intramuscular; IV, intravenous; PO, per os; SC, subcutaneous.

PROMAZINE DERIVATIVES

Promazine derivatives are considered broad-spectrum, centrally acting antiemetics,^1-3 being effective against most causes of nausea except inner ear problems; plus, they are inexpensive. They have antidopaminergic and antihistaminic effects that block the chemoreceptor trigger zone, and at higher dosages they block the medullary vomiting center. These drugs also have anticholinergic, antispasmodic, and α-adrenergic blocking effects. The promazine derivatives used most commonly as antiemetics in small animal veterinary medicine are chlorpromazine, prochlorperazine,⁴ and acepromazine. Chlorpromazine typically is used at 0.1 to 0.5 mg/kg IV, SC, or IM q6-8h, and prochlorperazine is used at 0.1 to 0.5 mg/kg IV or SC q8-12h. The antiemetic effect of these drugs is typically evident at dosages far below those causing sedation; however, varying degrees of vasodilation may occur, producing hypotension. Therefore caution is necessary with the use of these drugs in dehydrated or hypotensive patients, and intravenous fluid therapy may be necessary.

Promazine drugs have been reported to cause increases in central venous pressure and changes in heart rate (bradycardia or tachycardia), and they possess antiarrhythmic qualities in the dog. These drugs were previously reported to lower the seizure threshold, but this has recently been called into doubt.⁵ The promazines are metabolized by the liver and can cause central nervous system (CNS) signs in patients with substantial hepatic insufficiency, especially in patients with congenital portosystemic shunts. It has been suggested that prochlorperazine, and perhaps other promazine derivatives, not be used concurrently with metoclopramide because these drugs may potentiate extrapyramidal effects.⁴

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Tags: Small Animal Critical Care Medicine

Sep 10, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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