AntiCholinergic Drugs | Parasympatholytic Drugs | Dr Najeeb
Dr. Najeeb Lectures
Overview
This video provides a comprehensive overview of anticholinergic drugs, also known as parasympatholytic or cholinergic antagonist drugs. It explains their mechanism of action, which involves binding to cholinergic receptors without causing stimulation, thereby blocking the effects of acetylcholine. The video categorizes these drugs into anti-muscarinic and anti-nicotinic types, further differentiating them into selective and non-selective blockers. It details the actions of atropine, a prototype anti-muscarinic drug, on various organ systems including the eyes, gastrointestinal tract, respiratory system, urinary bladder, and cardiovascular system. Therapeutic uses, side effects, and antidotal applications, particularly in cases of organophosphate poisoning, are thoroughly discussed, along with the concept of atropine toxicity symptoms.
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Chapters
- Anticholinergic drugs block cholinergic receptors.
- Also known as parasympatholytic or cholinergic antagonist drugs.
- They bind to receptors but do not stimulate them, preventing acetylcholine's action.
- Distinction between agonists (bind and stimulate) and antagonists (bind but do not stimulate).
- Classified based on the receptors they block: anti-muscarinic and anti-nicotinic.
- Anti-muscarinic drugs block muscarinic receptors (M1-M5).
- Anti-nicotinic drugs block nicotinic receptors.
- Sub-classification into selective and non-selective blockers for muscarinic receptors.
- Examples: Atropine (non-selective), Pyrenzapine (M1 selective), Darifenacin (M3 selective).
- Cause mydriasis (pupil dilation) by blocking sphincter pupillae.
- Cause cycloplegia (paralysis of ciliary muscle), leading to blurred vision for near objects.
- Contraindicated in narrow-angle glaucoma due to risk of acute angle closure.
- Therapeutic uses: refractive error determination, fundoscopy, iridocyclitis management.
- Shorter-acting alternatives like tropicamide and cyclopentolate exist.
- Reduce lacrimal secretions, causing dry eyes (xerophthalmia).
- Reduce salivary secretions, causing dry mouth (xerostomia).
- Decrease respiratory and nasal secretions.
- Inhibit sweating (anhidrosis), leading to increased body temperature (hot as a hair).
- Can cause cutaneous vasodilation (atropine flush).
- Cause bronchodilation by blocking parasympathetic-mediated bronchoconstriction.
- Used historically for asthma; inhaled ipratropium used for COPD.
- Reduce gastrointestinal motility, acting as antispasmodics.
- Can reduce gastric acid secretion (e.g., M1 selective pyrenzapine for ulcers).
- Reduce hypermotility of the urinary bladder (e.g., darifenacin for stress incontinence).
- Low doses may initially cause bradycardia due to M1 receptor blockade.
- Moderate doses cause tachycardia by blocking M2 receptors on the SA node.
- Increase conduction velocity through the AV node (positive dromotropic effect).
- Therapeutic uses: symptomatic sinus bradycardia, partial heart block.
- Used in posterior/inferior myocardial infarction associated with bradycardia or heart block.
- Pre-operative medication to reduce secretions and prevent vagal reflexes.
- Used to reverse neuromuscular blockade after surgery (in conjunction with acetylcholinesterase inhibitors).
- Antidote for poisoning by muscarinic stimulants (e.g., organophosphates, certain mushrooms, physostigmine overdose).
- Atropine toxicity symptoms: blind as a bat, dry as a bone, hot as a hair, red as a beet.
Key takeaways
- Anticholinergic drugs block acetylcholine's action by binding to cholinergic receptors without activating them.
- They are broadly classified into anti-muscarinic and anti-nicotinic agents.
- Atropine, a prototype anti-muscarinic, has widespread effects on the eyes, secretions, GI tract, respiratory system, and heart.
- Ocular use requires caution in patients with narrow-angle glaucoma.
- Side effects of atropine toxicity include mydriasis, cycloplegia, dry mouth and eyes, anhidrosis, tachycardia, and potential hyperthermia.
- These drugs are valuable as antispasmodics, in managing bradycardia, and as antidotes for cholinergic excess.
- Pre-operative administration can prevent complications related to secretions and vagal reflexes.
- The effects of anticholinergics are often characterized by the 'blind as a bat, dry as a bone, hot as a hair, red as a beet' mnemonic.