VPT 311: General and Systemic Veterinary Pharmacology (2+1)

OSMOTIC PURGATIVES

• Osmotic cathartics are nonabsorbable or poorly absorbable salts or polymers that osmotically retain water in the intestinal lumen. Solutes that are not absorbed in the intestine retain water osmotically and distend the bowel – increase peristalsis indirectly.
• All inorganic salts used as osmotic purgatives have similar action differ only in dose, palatability and risk of systemic toxicity.
• They have a rapid onset of action that begins in the small intestine.
• Osmotic cathartics are the cathartics of choice for elimination of poisons.
• Magnesium sulfate and magnesium oxide are administered orally 5-15g; bitter in taste
• Sodium sulfate 10-15g is administered orally or via a stomach tube as a 6% solution
• Sodium phosphate 6-12 g
• Sodium potassium tartrate (Rochelles salt) 8-15g
• Polythelene glycol-electrolyte solutions are administered orally prior to colonoscopy
• Sodium phosphate and sodium tartrate mixtures are administered rectally to dogs. In cats, they produce hyperphosphatemia and should not be used.
• The salts mentioned above are dissolved in 100-200ml of wter, produce 1-2 fluid evacuation within 1-23 hours with mild cramping, casue nearly complete emptying of bowels.
• Smaller doses have milder laxative action.
• Magnesium ions release cholecystokinin which may aid purgative action of magnesium salts.
• Magnesium salts are contraindicated in renal insufficiency, while sodium salts should not be used in patients suffering form Congestive Heart Failure and other sodium retaining states.
• Repeated use of saline purgatives can cause fluid and electrolyte imbalance.

Lactulose

• It is a semisynthetic disaccharide of fructose and lactose which is not digested or absorbed form small intestine but retains water.
• It is broken down in the colon by the colonic bacteria to osmotically more active products.
• Flatulence is common, cramps occur in few cases. Nausea may be produced becsue of unpleasant taste.
• It is not preferred for constipation.
• Lactulose causes reduction of blood ammonia by 25-50% in patients with hepatic encephalopathy.
• The breakdown products of lactulose are acidic and reduce the pH of stools.
• Ammonia produced by bacteria in colon in converted to ionized ammonium and is not absorbed.