Vet360 Vet360 Vol 05 Issue 03 - Page 30

CPD ARTICLE Metoclopramide Metoclopramide is often used as an alternative to maropitant due to its additional weak prokinetic effects on the proximal gastrointestinal tract. This dopamine antagonist is a very effective anti-emetic which acts both centrally and peripherally. By increasing the tone and amplitude of gastric contractions and increasing pressure within the gastroesophageal sphincter, metoclopramide counteracts vomiting. Metoclopramide is very useful for the treatment of conditions such as oesophagitis, gastric motility disorders but is contraindicated in patients with suspected gastrointestinal obstructions. For this reason, maropitant may be a safer empirical antiemetic to use in suspected self-limiting cases where an intestinal foreign body can’t be excluded as a possible cause. The antiemetic effect of metoclopramide appears to be more pronounced in dogs than cats. This is probably because there are only a few dopamine receptors present in the central nervous system of cats. Metoclopramide is thus a poor antiemetic choice (and apomorphine a poor emetic choice) in this species as both require dopamine receptors. Metoclopramide is contraindicated in patients with gastrointestinal haemorrhage, obstruction, or perforation and must be used with caution in patients with cranial trauma and phaeochromocytoma. Central nervous system signs are the most common side effect seen with metoclopramide use which may manifest subtly as mild sedation or behaviour change or overt extrapyramidal signs. Metoclopramide is used in dogs as an intermittent bolus at 0.2 – 0.5 mg/kg, PO, SC or IM, q6-8h or as a constant rate infusion at 0.01 – 0.09 mg/kg/hour. Most package inserts claim that metoclopramide is light sensitive but a study looking at the admixture of metoclopramide with different solutions, including 0.9% NaCl, showed that the drug remains physically and chemically stable for at least 48 hours with a negligible effect of lighting on drug stability 7 . Unfortunately, here is relatively little information available regarding the pharmacokinetics of ondansetron in dogs and cats to define clear clinical applications and indications for the use of this drug, especially when considering that it is expensive. However, ondansetron appears to be well-tolerated in dogs and cats. Due to the efficacy and relative safety of the aforementioned drugs, the use of phenothiazine derivatives such as chlorpromazine and prochlorperazine have fallen out of favour. Phenothiazine derivatives causes hypotension, which may be dangerous in cases suffering from haemodynamic instability or dehydration, and they also cause tranquilisation even at low doses, which interferes with the assessment of a patient. ANTACIDS The integrity of the gastric mucousa is dependent on a delicate balance between ulcerogenic factors and the counteracting defence mechanisms of the gastrointestinal tract. Ulceration of the gastric mucousa may be associated with nonsteroidal anti- inflammatory drug use, excessive gastrin release, excessive histamine release associated with mast cell tumours, hepatic dysfunction, chronic kidney disease, pancreatitis and many more. The primary endogenous ulcerogenic factors are hydrochloric acid and pepsin. Gastric mucousal defence mechanisms compose of a pre-epithelial mucous-bicarbonate-phospholipid barrier, the epithelial barrier and continuous cell renewal of epithelial cells, continuous mucosal blood flow through gastric microvasculature, an endothelial barrier, sensory innervation and the generation of vasodilatory prostaglandins and vasodilatory nitric oxide. Both the preventative and repair mechanisms are dependent on mucousal prostaglandins. Ulceration therefore occurs in the presence of excessive ulcerogenic factors or impaired defence mechanisms. Ondansetron is the third, commonly used antiemetic at the practitioner’s disposal. This serotonergic antagonist has historically been used in cases receiving chemotherapy due to its potent effect of blocking neuronal transmission in the chemoreceptor trigger zone and vagal afferent nerves. As a premedication for chemotherapy, ondansetron is given at 0.5 - 1.0 mg/kg, PO, 30 minutes prior to administration of the chemotherapy. Antacids act to antagonise acid production, promote healing or potentiate the natural protective mechanisms of the gastric mucousa. Acid production is stimulated via neuronal and hormonal pathways mediated via the binding of gastrin, acetylcholine, histamine, and prostaglandins to specific receptors on acid-producing parietal cells. Hydrochloric acid is eventually produced from hydrogen molecules secreted by proton pumps on the luminal surface of the parietal cells. The mediators of this physiological process are targeted for reducing acid production as demonstrated in Figure 2. Its use, at lower doses, has also been described in cases with gastrointestinal causes of vomiting In dogs with canine parvoviral enteritis doses of 0.1 – 0.15mg/ kg, slowly IV, q12h have been recommended. The invention of catheterless radiotelemetric pH monitoring devices have facilitated the accurate and noninvasive measurement intragastric pH in dogs and cats since 2002. This has resulted in great advances Ondansetron vet360 Issue 03 | JULY 2018 | 30