Drugs Affecting Gut Motility Flashcards by Minhee Kim

What are the types of physiological control of gut motility

Myogenic
Neurological
Hormonal

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What is myogenic control of gut motility

Rhythmic contractions of intestinal smooth muscle
Interstitial cells of Cajal act as pacemaker cells
Depolarisations spread through gap junctions

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What is neuronal control of gut motility

Enteric Plexus - network of autonomic fibres in intestinal wall
Cholinergic nerves increase force of contraction
Non-adrenergic inhibitory nerves decrease contraction

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What is hormonal control of motility

Peptide hormones secreted by endocrine cells in intestinal mucosa
CCK
Secretin
Gastrin
Paracrine - prostaglandin, histamine, somatostatin

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What are the steps of vomiting reflex

Contraction of pyloric sphincter
Relaxation of cardia and oesophagus
Contraction of abdominal wall and diaphragm
Expulsion of gastric contents

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What are triggers of vomiting reflex

RICP
Vestibular apparatus : rotational movement
Chemoreceptor Trigger Zone: toxins, Drugs, electrolytes
Higher cortical centres: pain, sight, smell
GI tract: stretch, inflammation

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What is vomiting centre

Area in medulla responsible for control+coordination of muscles involved in vomiting reflex

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What is Area Postrema

Part of vomiting centre containing CTZ
Located in base of 4th ventricle
Thus Outside BBB and exposed to blood vomiting triggers in peripheral circulation

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What are neurotransmitters involved in vomiting reflex

Vestibular apparatus: H1 receptor
CTZ: D2, 5HT3
GI: D2
Vomiting centre: ACh, H1, 5HT3

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What are types of antiemetic drugs

D2 antagonist 
5HT3 antagonist
ACh antagonist 
H1 antagonist 
Cannabinoids 
Benzodiazepines

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Give examples of D2 antagonists

Domperidone
Metoclopromide
Phenothiazine

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What is mechanism of action of D2 antagonists

Antagonise D2 receptor at Area postrema

Antagonise D2 at stomach - slow gastric emptying

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What are indications of D2 antagonists

Acute N+V (esp if L-dopa induced)

PONV

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What are side effects of D2 antagonists

Galactorrhoea (loss of dopaminergic inhibition of prolactin release)
Dystonia (rare)

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What are PK properties of D2 antagonists

Oral, PR
Extensive first pass metabolism
Does not cross BBB

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Give examples of 5HT3 antagonists

Ondansetron

Granisetron

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What is mechanism of action of 5HT3 antagonists

Antagonise 5HT3 at area postrema

Antagonise 5HT3 receptors of vagus afferent fibres in GIT

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What are indications of 5HT3 antagonists

Chemotherapy (1st line)
Radiation sickness
PONV (esp opioid induced)

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What are side effects of 5HT3 antagonists

Headache
Constipation
Flushing

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How are 5HT3 antagonists administered

Enhanced effect if given with dose of corticosteroids

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What is mechanism of action of metoclopramide

Antagonise D2 at area postrema
Antagonise D2 at stomach - slow gastric emptying
Anticholinergic (GI)
Antagonise 5HT3 at vagal afferent fibres of GIT

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What are indications of metoclopramide

Acute N+V (first line)

PONV (gastric cause)

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How is metoclopramide administered

Oral

TDS - T1/2 4hrs

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What are side effects of metoclopramide

Extrapyramidal side effects - CI in PD

Galactorrhoea

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Give examples of ACh antagonists

Hyoscine

What is mechanism of action of ACh antagonists

Antagonise mAChR at vestibular apparatus Anticholinergic at GIT - inhibit gastric secretions

What are indications of ACh antagonists

Motion sickness | N+V in Obstruction

What are side effects of ACh antagonists

Systemic anticholinergic effects

Give examples of H1 antagonists

Cyclizine | Promethiazine

What is mechanism of action if H1 antagonists

Antagonise H1 at vomiting centre

What are indications of H1 antagonists

Acute N+V

What are side effects of H1 antagonists

Sedation | QT prolongation

What are principles of management of constipation

Lifestyle: increase fluids, fibre, exercise Treat underlying cause: dehydration, DM, PD, opioids, diuretics, anticholinergics Pharmacological: laxatives

What are the types of laxatives

``` Stimulant (soft stool) Osmotically active (hard stool) Bulking agent (hard stool) Faecal softeners (hard stool) ```

Give examples of bulking laxatives

Ispaghula | Cellulose

What is mechanism of action of bulking agents

``` Insoluble agents draw water in Bowel distension Stimulate stretch receptors Stimulate peristalsis Takes days to take effect ```

What are indications of bulking agents

Hard stools

What are side effects of bulking agents

Risk of obstruction - CI in adhesions and ulcers Dehydration Flatulence

Give examples to faecal softeners

Arachis oil | Glycerol

What is mechanism of action of faecal softeners

Lubricate stool | Assist evacuation of stool

What are indications of faecal softeners

Hard stools Adhesions Anal fissure Haemorrhoids

What are side effects of faecal softeners

Safe | But not effective

Give examples of osmotically active laxatives

Mg, Na salts Lactulose Macrogol (Movicol)

What is mechanism of action of osmotically active laxatives

Draw water into lumen Cause bowel distension Stimulate stretch receptors in bowel wall Stimulate peristalsis

What are indications of Mg and Na salts

Urgent relief | Resistant constipation

How is Mg and Na salts administered

What is mechanism of action of lactulose

Broken down by colonic flora into acetic and lactic acid Acetic and lactic acid are osmotically active Acetic and lactic acid bind+trap ammonia in gut

What are indications of lactulose

1st line hospital constipation | Liver failure - prevent encephalopathy

Give examples of stimulant laxatives

Senna Codanthramer Bisacodyl

What is mechanism of action of stimulant laxatives

Stimulate enteric plexus Stimulate peristalsis directly Cause electrolyte and fluid retention, stimulate stretch receptors thus peristalsis

What are indications of stimulant laxatives

Urgent treatment: Surgical preparation Impaction

What are side effects of stimulant laxatives

``` Abdominal cramps Hypokalaemia Colonic atony (chronic use) Melenosis Coli (pigmentation; overuse) ```

What are principles of management of diarrhoea

Treat cause: infection, IBD, IBS, bile, pancreatic enzymes Supportive: fluids, electrolyte replacement Anti-diarrhoea Drugs: for symptom relief

Give examples of anti-motility drugs

Loperamide

What is mechanism of action of loperamide

``` Opioid analogue Bind to opioid receptors on bowel wall Decreased motility Increased transit time Increased water absorption Increased anal tone ```

What are indications of loperamide

Chronic diarrhoea

What are side effects of loperamide

Risk of toxic megacolon - CI in IBD

What are types of anti-diarrhoea drugs

Anti-motility Bulk-forming laxatives Cholestyramine Pancreatic enzymes

What is mechanism of action of bulk forming agents for diarrhoea

Absorb water

What are indications of bulk forming agents for diarrhoea

IBS with chronic diarrhoea+constipation

What is mechanism of action of cholestyramine

Bile acid sequestration

What are indications of cholestyramine

Diarrhoea secondary to bile acid malabsorption: Crohn’s Ileal resection

Give examples of pancreatic enzymes

Pancreatase

What are indications of pancreatase

Diarrhoea secondary to malabsorption from lack of pancreatic enzymes: Chronic pancreatitis Cystic fibrosis

What is IBS

Chronic and relapsing disorder of GI function without physical/biochemical abnormality

What is the diagnostic criteria of IBS

6 months of: Abdominal pain relieved by defecation Abdominal bloating Change in bowel habit: Constipation, diarrhoea

What are principles of management of IBS

Lifestyle: increase fluid, limit fibre, limit caffeine Diarrhoea: loperamide Constipation: ispaghul Pain: peppermint oil, mebeverine

What is mechanism of action of mebeverine

Antispasmodic agent Anticholinergic effects on gut wall Relax intestinal smooth muscle Relieves pain from muscle spasms