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Saturday, August 9, 2025

Group V antiarrhythmics


Generic and Brand Names

  • Adenosine — Adenocard, Adenoscan

  • Digoxin — Lanoxin, Digitek

  • Magnesium sulfate — Various generic formulations

  • Ivabradine — Corlanor (in some regions)

  • Other agents — Vary by region and indication

Class

  • Group V antiarrhythmics (Vaughan Williams classification)

  • “Miscellaneous” agents that do not fit into Groups I–IV

  • Act via unique mechanisms outside of sodium channel, beta-blocker, potassium channel, or calcium channel blockade

Mechanism of Action

  • Adenosine: Activates adenosine A1 receptors in the atrioventricular (AV) node, increasing potassium efflux and hyperpolarizing the cell membrane; transiently blocks AV node conduction

  • Digoxin: Inhibits Na⁺/K⁺-ATPase pump, increasing intracellular calcium via sodium-calcium exchange; increases vagal tone, slowing AV conduction

  • Magnesium sulfate: Stabilizes cardiac cell membranes; modulates ion flux, particularly calcium; useful in torsades de pointes and digitalis-induced arrhythmias

  • Ivabradine: Selective inhibitor of the funny current (If) in the sinoatrial (SA) node, reducing heart rate without affecting contractility

Indications

  • Adenosine: Acute termination of paroxysmal supraventricular tachycardia (PSVT), including AV nodal re-entrant tachycardia (AVNRT)

  • Digoxin: Rate control in atrial fibrillation and atrial flutter (especially with heart failure); certain supraventricular arrhythmias in paediatric patients

  • Magnesium sulfate: Torsades de pointes; hypomagnesemia-associated arrhythmias; digitalis toxicity

  • Ivabradine: Inappropriate sinus tachycardia; selected cases of heart failure with high resting heart rate (not first-line antiarrhythmic use)

Dosage and Administration

  • Adenosine:

    • Rapid IV bolus 6 mg; if no response within 1–2 min, give 12 mg; may repeat once more at 12 mg

    • Follow immediately with a rapid saline flush

  • Digoxin:

    • Oral or IV; loading dose (digitalisation) followed by maintenance based on age, weight, renal function

    • Narrow therapeutic index — dosing must be individualized

  • Magnesium sulfate:

    • IV 1–2 g over 15 min for torsades; adjust for renal impairment

  • Ivabradine:

    • Oral; start at 5 mg twice daily, adjust after 2 weeks based on heart rate response

Monitoring

  • Adenosine: Continuous ECG during administration; watch for transient asystole

  • Digoxin: Serum digoxin levels (therapeutic 0.5–2 ng/mL), renal function, electrolytes (K⁺, Mg²⁺, Ca²⁺), ECG

  • Magnesium sulfate: Serum magnesium, ECG, reflexes (to monitor for toxicity)

  • Ivabradine: Resting heart rate, ECG (for bradycardia, QT prolongation), visual disturbances

Contraindications

  • Adenosine: 2nd/3rd-degree AV block without pacemaker, sick sinus syndrome, severe hypotension, bronchospastic lung disease (relative)

  • Digoxin: Ventricular fibrillation, known digoxin hypersensitivity

  • Magnesium sulfate: Heart block, myocardial damage, hypermagnesemia

  • Ivabradine: Acute decompensated heart failure, severe bradycardia, SA block, 3rd-degree AV block, strong CYP3A4 inhibitors

Precautions

  • Adenosine: May precipitate bronchospasm in asthma/COPD; transient flushing and chest discomfort are common

  • Digoxin: Risk of toxicity increased with hypokalaemia, hypomagnesemia, hypercalcaemia, and renal impairment

  • Magnesium sulfate: Use caution in myasthenia gravis, renal insufficiency

  • Ivabradine: Avoid in patients with atrial fibrillation without rate control; monitor for luminous visual phenomena (phosphenes)

Adverse Effects

  • Adenosine: Transient asystole, flushing, chest pain, dyspnoea, hypotension

  • Digoxin: Nausea, vomiting, confusion, visual changes (yellow/green halos), arrhythmias (esp. ventricular)

  • Magnesium sulfate: Hypotension, flushing, loss of deep tendon reflexes, respiratory depression at high levels

  • Ivabradine: Bradycardia, atrial fibrillation, luminous visual phenomena

Drug Interactions

  • Adenosine: Dipyridamole potentiates effects; methylxanthines (theophylline, caffeine) antagonize effects

  • Digoxin: Many — amiodarone, verapamil, macrolides, and others increase levels; electrolyte disturbances increase toxicity risk

  • Magnesium sulfate: CNS depressants, neuromuscular blockers may enhance adverse effects

  • Ivabradine: Strong CYP3A4 inhibitors/inducers affect plasma levels; additive bradycardia with beta-blockers, CCBs

Overdose

  • Adenosine: Extremely short half-life; effects resolve rapidly

  • Digoxin: Life-threatening arrhythmias, hyperkalaemia; treat with digoxin-specific antibody fragments

  • Magnesium sulfate: Loss of reflexes, respiratory depression, cardiac arrest; treat with IV calcium gluconate

  • Ivabradine: Severe bradycardia; treat with atropine, temporary pacing if needed

Patient Counselling

  • Adenosine: Warn about transient flushing, chest discomfort, brief heart pause sensation

  • Digoxin: Take at the same time each day; report nausea, vision changes, palpitations; avoid abrupt changes in potassium intake

  • Magnesium sulfate: Used acutely in hospital setting; explain purpose and monitoring

  • Ivabradine: Take with food; report any visual disturbances or marked slowing of pulse




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