GI pharmacology Flashcards
Function of purgatives
Accelerate transit through the intestine Used for alleviation of constipation
Define constipation
Infrequent or difficult evacuation of faeces
Includes: infrequent bowel movement, difficuly during defaecation, sensation on incomplete bowel evacuation
In sever cases - faecal impaction causes bowel obstruction. Presents as vomiting and tender abdomen with paradoxical diarrhoea where soft tool from the small intestine bypasses the matter in the colon
Name 4 causes of constipation
Diet
Hormones
SIde effects of medication (opiods)
Illness
Classes of purgatives
Bulk laxatives (increase volume of non-absorbable residue)
Osmotic laxatives (increase stool water content)
Faecal sottneers (alter faecal consistency)
Stimulant purgatives (increase GI motility)
Bulk laxatives
Polysaccharide polymers e.g. methylcellulose, bran, agar that are not digested in the upper Gi tract. They form a bulky mass in the gut lumen and retain water.
This softens the faecal material (improves consisteny) and promotes increased motility
Take 1-3 days to work, no side effects. First line treatment in constipation and IBS
Osmotic laxatives
Consist of poorly absorbed solutes: MgSO4, MgOH and lactulose.
Produce an osmotic load, the agents trap increased volumes of fluid in the lumen
The transfer of gut contents through the small intestine increases.
An abnormally large volume of fluid enters the colon, which causes distension and purgation.
Use of saline purgatives
MgSO4 and MgOH result in potent, rapid purgation within 1hr
Used in bowel prep for surgery
Can cause dehydration and electrolyte depletion. Not used in children or renal patients.
How does Lactulose cause purgation?
Lactulose is a semi-synthetic dissacharide.
Colonic bacteria convert it into fructose and galactose which are poorly absorbed. Fermentation produces lactic acid and acetic acid, which draws water into the lumen.
Takes 1-3 days to work. Used in negating constipating effects of opiods
Side effects: abdominal cramps, flatulence, nausea, electrolyte disturbance
Faecal softners
Surface active compounds that act in the GI tract as a detergent to produce softer faeces.
Act slowly (3-5 days)
Stimulant purgatives
Act by increaseing the electrolyte (and water secretion) by the mucosa, and icnrasing peristalsis
Bisacodyl: Stimulates rectal mucosa, resulting in mass movements and defaecation in 15-30mins. Given as a suppository, used for bowel prep
Senna: Anthracene derivative combine with sugars. Passes unchanged into the colon where bacteria release anthracine. These are absorbed and directly act on the myenteric plexus. Inhibits motlility in the proximal colon and increases contraction at the distal colon. Force of mass movements and peristalsis is increased. Act oevr 8hrs, taken at night. Overuse can lead to an atonic colon.
Cathartic colon
Anatomical and physiological changes in the colon that occur with chronic use of stimulant laxatives
Results in laxative dependency and tachyphylaxis (high dose of laxatives required)
Can lead to fluid and electrolye imbalance, steatorrhoea, vitamin and mineral deficencies.
Symptoms: bloating, feeling of fullness, abdominal pain, incomlete defaecation
Types of diarrhoea
Secretory: Increased active secretion or inhibition of absorption. No structural damage to the GIT
Osmotic: Loss of water due to a heavy osmotic load e.g. maldigestion (Coeliacs) where nutrients remain in the lumen, pulling water into the lumen
Motility: Abnormally high Gi motility decreases time available for absorption of nutrients and water. Can occur in diabetic neuropathy
Inflammatory diarrhoea: Damage to the mucosal lining or brush border leads to a passive loss of protein-rich fluids, decreased ability to absorb lost fluids. Caused by infections and AI disease
Treatment of diarrhoea
Maintenance of fluid/electrolyte balance
Anti-diarrhoeal agents
Anti-infective agents (Severe Campylobacter infections - erythromycin/ciprofloxacin)
Antimotility agents used in diarrhoea
Opiods: codeine and loperamide
Loperamide has selective action on the GIT. Acts on µ-opiod receptors in the myenteric plexus. Increases tone and rhythmic contrations of the colon, but reduces propulsive activity. Contracts sphincters.
This reduces abdominal cramps and shortens duration of illness.
Used to treat symptoms of uncomplicated diarrhoea in adults.
Chronic use can lead to paralytic ileus, constipation, cramps and dizziness
Antispasmodic drugs used in diarrhoea
Muscarinic receptor antagonists e.g. atropine, hyoscine
Inhibit parasympatheic activity.
Used in bowel colic, abdominal cramps assocaited with diarrhoea.
Side effects: dry mouth, blurred vision, urinary retention.
