Pharmacologic control of acute vomiting
Initial nonspecific management of vomiting includes NPO (in minor cases a 6-12 hour period of nothing per os may be all that
is required), fluid support, and antiemetics. Initial feeding includes small portions of a low fat, single source protein
diet starting 6-12 hours after vomiting has ceased. Drugs used to control vomiting will be discussed here.
The most effective antiemetics are those that act at both the vomiting center and the chemoreceptor trigger zone. Vomiting
is a protective reflex and when it occurs only occasionally treatment is not generally required. However, patients that continue
to vomit should be given antiemetics to help reduce fluid loss, pain and discomfort.
For many years I strongly favored chlorpromazine (Thorazine), a phenothiazine drug, as the first choice for pharmacologic
control of vomiting in most cases. The HT-3 receptor antagonists ondansetron (Zofran) and dolasetron (Anzemet) have also
been highly effective antiemetic drugs for a variety of causes of vomiting. Metoclopramide (Reglan) is a reasonably good
central antiemetic drug for dogs but not for cats. Maropitant (Cerenia) is a superior broad spectrum antiemetic drug and
is now recognized as an excellent first choice for control of vomiting in dogs. Studies and clinical experience have now
also shown maropitant to be an effective and safe antiemetic drug for cats. While it is labeled only for dogs, clinical experience
has shown it is safe to use the drug in cats as well. Maropitant is also the first choice for prevention of motion sickness
vomiting in both dogs and cats.
Phenothiazine antiemetics (chlorpromazine, prochlorperazine) have a broad spectrum effect and are effective in controlling
vomiting due to a variety of causes. Chlorpromazine acts on the emetic center, chemoreceptor trigger zone, and on peripheral
receptors. It is also thought to function as a calcium channel antagonist. This effect decreases cyclic AMP concentrations
in intestinal epithelial cells which leads to decreased intestinal epithelial cell secretion. Further, chlorpromazine has
minimal anticholinergic effects. The recommended dose is 0.1 to 0.25 mg/lb IM or SC SID - TID as needed to control vomiting.
At this dose there is a minimal sedative effect. Any sedation resulting from use of chlorpromazine, unless pronounced, is
not considered a deleterious side effect, and in fact this is often considered a beneficial effect through decreasing the
discomfort and distress that can be associated with nausea. Chlorpromazine is an excellent choice for control of nausea.
Patient comfort should always be a priority.
A potential side effect of phenothiazine drugs is hypotension, which can result from an alpha-adrenergic blocking action,
causing arteriolar vasodilation. This is of minimal concern in well-hydrated patients, and in dehydrated patients it is readily
controlled with intravenous fluid support. For patients with vomiting due to renal or liver disease that are already depressed,
the dosage of chlorpromazine is often reduced to 0.1-0.15 mb/lb SID-BID. This lower dose is often effective for controlling
vomiting and is not likely to cause significantly more sedation.
Metoclopramide (Reglan) is a gastric prokinetic drug that also has central antiemetic effect. Metoclopramide increases gastric
and proximal small intestinal motility and emptying without causing acid secretion, decreases enterogastric reflux, and provides
inhibition of the chemoreceptor trigger zone. The central antiemetic effect is mediated through antagonism of dopaminergic
D2 receptors in the chemoreceptor trigger zone of the medulla to inhibit vomiting induced by drugs, toxins, metabolic disease,
and acid-base imbalances. Metoclopramide is a less effective central antiemetic drug in cats than in dogs because serotonin
receptors, rather than dopaminergic receptors, predominate in the CTZ of cats. For vomiting in cats, I generally usually
use metoclopramide only if a promotility effect is desired. Chlorpromazine, dolasetron, ondansetron, or maropitant should
be used as a first or second choice to control acute frequent vomiting in cats. Parvovirus can cause gastric hypomotility
and therefore the promotility effects of metoclopramide may prove beneficial. However, maropitant, dolasetron, or ondansetron
are likely to be more effective than metoclopramide.
The recommended injectable dose of metoclopramide is 0.1 to 0.25 mg/lb IM or SC given TID to QID as needed. Metoclopramide
can also be given IV as a constant rate infusion (0.5 - 1.0 mg/lb over 24 hours). Metoclopramide should not be used if gastric
outlet obstruction or GI perforation is suspected, or in patients with a seizure disorder. Whereas in the past chlorpromazine
and metoclopramide were occasionally used together in dogs in which neither drug was effective in significantly reducing the
frequency of vomiting when used alone; currently, ondansetron, dolasetron, or maropitant are preferred as single agent therapy
for control of acute, severe emesis. It is possible that the combination of chlorpromazine and metoclopramide may potentiate
side effects that may result from use of either drug individually. Animals that are treated with a combination of chlorpromazine
and metoclopramide are observed carefully for nervous-type behavior or significant depression. My preference at this time,
if both chlorpromazine and metoclopramide are ineffective when given individually, or if there is severe vomiting that does
not respond to whichever of these drugs is used first, is to institute dolasetron (Anzemet) or ondansetron (Zofran) therapy
(see later discussion).