Pharmacology · Cardiovascular

Class IV Agents and Other Nodal Modulators

Verapamil, diltiazem, adenosine, digoxin, and magnesium sulfate

Verapamil and diltiazem both block L-type calcium channels in nodal tissue, slowing atrioventricular conduction and prolonging the PR interval — diltiazem is preferred for sustained rate control in atrial fibrillation because it has substantially less negative inotropic effect and is better tolerated with borderline left ventricular function; both are contraindicated in heart failure with reduced ejection fraction, wide-complex tachycardia of unknown origin, and Wolff-Parkinson-White.
Adenosine activates A1 receptors in the atrioventricular node, opening a potassium channel that hyperpolarizes nodal cells and transiently blocks conduction for 10–20 seconds; its half-life under 10 seconds makes it first-line for acute termination of atrioventricular nodal reentrant and reentrant tachycardias and a diagnostic tool in wide-complex tachycardia; methylxanthines block its effect and dipyridamole potentiates it.
Magnesium sulfate is first-line for torsades de pointes regardless of serum magnesium level — it suppresses early afterdepolarizations by blocking calcium current; toxicity progresses from loss of deep tendon reflexes to respiratory paralysis and is reversed with calcium gluconate.
Digoxin slows the atrioventricular node through vagal tone enhancement, making it ineffective during exercise or high sympathetic states — it is an adjunct for atrial fibrillation rate control, not monotherapy; amiodarone, verapamil, and quinidine all raise digoxin levels; life-threatening toxicity produces bidirectional ventricular tachycardia and is treated with digoxin-specific antibody fragments.
In Wolff-Parkinson-White syndrome with atrial fibrillation, all nodal-blocking agents (adenosine, verapamil, diltiazem, digoxin, beta-blockers) are absolutely contraindicated — blocking the atrioventricular node forces conduction entirely through the accessory pathway, which can sustain 200–300 beats per minute and degenerate into ventricular fibrillation; treatment is IV procainamide or electrical cardioversion.

Class IV Agents — Non-Dihydropyridine Calcium Channel Blockers

Class IV

Verapamil

Blocks L-type calcium channels in nodal tissue
Slows atrioventricular node; prolongs PR interval
Indications: atrioventricular nodal reentrant tachycardia / atrioventricular reentrant tachycardia termination (second-line to adenosine); rate control in atrial fibrillation; fascicular ventricular tachycardia
More negative inotropy than diltiazem
Avoid in heart failure with reduced ejection fraction, wide-complex tachycardia of unknown origin, Wolff-Parkinson-White

Class IV

Diltiazem

Same atrioventricular nodal blocking mechanism as verapamil
Substantially less negative inotropic effect
Preferred for sustained atrial fibrillation rate control (continuous infusion possible)
Better tolerated with borderline left ventricular function
Same contraindications as verapamil: heart failure with reduced ejection fraction, Wolff-Parkinson-White, wide-complex tachycardia

Adenosine and Magnesium Sulfate

Unclassified

Adenosine

A1 receptor agonist → transient atrioventricular nodal block (10 to 20 seconds)
Half-life under 10 seconds; rapid bolus required
First-line for atrioventricular nodal reentrant tachycardia / atrioventricular reentrant tachycardia; diagnostic tool in wide-complex tachycardia
Adverse effects: flushing, chest tightness, dyspnea (transient and universal)
Contraindicated in Wolff-Parkinson-White with atrial fibrillation and severe asthma
Methylxanthines (caffeine, theophylline) block effect; dipyridamole potentiates

Electrolyte Agent

Magnesium Sulfate

Suppresses early afterdepolarizations by blocking calcium current
First-line for torsades de pointes regardless of serum magnesium level
Adjunct in digoxin toxicity (give slowly to avoid worsening atrioventricular block)
Empiric use in refractory ventricular fibrillation
Toxicity: loss of deep tendon reflexes → respiratory paralysis
Antidote: calcium gluconate

Digoxin

Cardiac Glycoside

Mechanism and Indications

Inhibits sodium-potassium pump → raises intracellular calcium → positive inotropy
Atrioventricular nodal slowing via vagotonic mechanism (ineffective during exercise)
Atrial fibrillation rate control adjunct (not monotherapy); heart failure with reduced ejection fraction (reduces hospitalizations, not mortality)
Renally eliminated; dose reduction in renal impairment
Key interactions: amiodarone, verapamil, quinidine all raise digoxin levels

Cardiac Glycoside

Toxicity Recognition

Precipitated by hypokalemia, hypomagnesemia, renal impairment, advanced age
Non-cardiac: nausea, vomiting, yellow-green visual disturbances (xanthopsia)
Cardiac: bradyarrhythmias, atrioventricular block, bidirectional ventricular tachycardia (severe)
Treatment: stop digoxin; correct hypokalemia; avoid calcium
Digoxin-specific antibody fragments (Digibind/DigiFab) for life-threatening toxicity

Critical Rule — Wolff-Parkinson-White Syndrome with Atrial Fibrillation

Adenosine, verapamil, diltiazem, digoxin, and beta-blockers are ALL contraindicated in pre-excited atrial fibrillation or flutter in Wolff-Parkinson-White syndrome. Blocking the atrioventricular node redirects all conduction through the accessory pathway, which can conduct atrial fibrillation at 200 to 300 beats per minute and degenerate into ventricular fibrillation. Treatment: intravenous procainamide or electrical cardioversion only.

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