Introduction toAntiarrhythmics, Antihyperlipidemics & Cardiac Drugs
This unit covers drugs that directly target the heart and blood. Antiarrhythmics restore the normal rhythmic beating of the heart by modifying ion channel function. Antihyperlipidemics lower dangerous cholesterol and lipid levels to prevent atherosclerosis. Coagulants and Anticoagulants maintain the delicate balance of blood clotting. Drugs for CHF strengthen the failing heart’s contraction. Understanding the chemistry behind each mechanism is paramount.
Syllabus & Topics
- 1Antiarrhythmic Drugs – Introduction: Cardiac arrhythmia = abnormal heart rhythm. Vaughan-Williams Classification: Class I (Na⁺ channel blockers), Class II (β-blockers), Class III (K⁺ channel blockers), Class IV (Ca²⁺ channel blockers).
- 2Class IA – Na⁺ Channel Blockers (Moderate): Quinidine sulphate (stereoisomer of quinine, prolongs AP duration + ERP), Procainamide HCl (amide analogue of procaine, SLE-like syndrome on chronic use), Disopyramide phosphate (anticholinergic side effects, negative inotropic).
- 3Class IB – Na⁺ Channel Blockers (Weak): Phenytoin sodium (also anticonvulsant – dual use), Lidocaine HCl (IV only, drug of choice for ventricular arrhythmias post-MI), Tocainide HCl (oral analogue of lidocaine), Mexiletine HCl (oral, structurally related to lidocaine).
- 4Class IC – Na⁺ Channel Blockers (Strong): Lorcainide HCl – potent Na⁺ channel blockade, slows conduction markedly. Reserved for life-threatening arrhythmias due to proarrhythmic risk.
- 5Class III – K⁺ Channel Blockers: Amiodarone (blocks K⁺, Na⁺, Ca²⁺ channels + β-receptors – broad spectrum. Contains iodine → thyroid dysfunction, pulmonary fibrosis. Very long half-life ~40 days), Sotalol (non-selective β-blocker + Class III activity).
- 6Antihyperlipidemic – Fibric Acid Derivatives: Clofibrate – activates PPARα → increases lipoprotein lipase activity → ↓triglycerides, ↑HDL. Used primarily for hypertriglyceridemia.
- 7Antihyperlipidemic – HMG-CoA Reductase Inhibitors (Statins): Lovastatin (first statin, derived from Aspergillus terreus fungus) – competitive inhibition of HMG-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis → ↓LDL dramatically. Side effects: myopathy, rhabdomyolysis.
- 8Antihyperlipidemic – Bile Acid Sequestrants: Cholestyramine and Colestipol – large cationic exchange resins that bind bile acids in the intestinal lumen preventing their reabsorption → liver converts more cholesterol to bile acids → ↓plasma LDL. Not absorbed, GI side effects.
- 9Coagulants – Vitamin K Analogues: Menadione (Vitamin K3, synthetic, water-soluble) and Acetomenadione – essential cofactor for hepatic synthesis of clotting factors II, VII, IX, X. Used to reverse bleeding from over-anticoagulation.
- 10Anticoagulants – Oral: Warfarin (coumarin derivative – inhibits Vitamin K epoxide reductase → ↓functional clotting factors II, VII, IX, X. Narrow therapeutic index, monitored by INR/PT). Anisindione (indanedione derivative, similar mechanism).
- 11Antiplatelet Agent: Clopidogrel – irreversibly blocks P2Y12 ADP receptor on platelets → inhibits platelet aggregation. Prodrug requiring CYP450 activation. Used post-MI, post-stent placement.
- 12Drugs for CHF – Cardiac Glycosides: Digoxin (from Digitalis lanata) and Digitoxin (from Digitalis purpurea). Mechanism: inhibit Na⁺/K⁺-ATPase → ↑intracellular Na⁺ → Na⁺/Ca²⁺ exchanger reverses → ↑intracellular Ca²⁺ → ↑contractility (positive inotropic effect). Narrow therapeutic index → digitalis toxicity.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. Why is Lidocaine Given IV Only for Arrhythmias?
Lidocaine undergoes extensive first-pass hepatic metabolism (over 70%) when administered orally, resulting in very low bioavailability. Therefore, it must be given intravenously for treating cardiac arrhythmias. Oral analogues such as Tocainide and Mexiletine were developed to overcome this limitation.
Q2. Why Does Amiodarone Cause Thyroid Problems?
Amiodarone contains approximately 37% iodine by weight. During metabolism, it releases large amounts of iodine, which disrupts normal thyroid hormone synthesis. This may lead to hypothyroidism via the Wolff–Chaikoff effect (iodine-induced inhibition of thyroid hormone synthesis) or hyperthyroidism via the Jod–Basedow phenomenon (iodine-induced excess hormone production in susceptible individuals).
Q3. How Do Statins Lower Cholesterol?
Statins such as Lovastatin are structural analogues of HMG-CoA and competitively inhibit HMG-CoA reductase, the rate-limiting enzyme in hepatic cholesterol biosynthesis. Reduced liver cholesterol levels stimulate upregulation of LDL receptors, increasing clearance of LDL from the bloodstream and significantly lowering plasma LDL cholesterol.
Q4. What is the Mechanism of Digoxin Toxicity?
Digoxin inhibits the Na⁺/K⁺-ATPase pump. At toxic levels, excessive intracellular Ca²⁺ accumulates, leading to delayed afterdepolarizations (DADs) and triggered arrhythmias. Hypokalemia increases toxicity because potassium competes with digoxin for binding to the ATPase. Clinical manifestations include nausea, visual disturbances (xanthopsia—yellow halos), and potentially fatal arrhythmias.
Q5. Why is Warfarin’s Therapeutic Index So Narrow?
Warfarin is approximately 99% protein-bound, and minor displacement by other drugs can significantly raise free drug levels. It is metabolized primarily by CYP2C9, and genetic variations cause substantial inter-individual dose variability. Its anticoagulant effect is delayed because it depends on depletion of existing clotting factors, making dose adjustment challenging. Overdose may result in severe hemorrhage.