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Other Antiarrhythmic Drugs

For medical students2 min readUpdated 2026-10-10

The group of other antiarrhythmic agents includes drugs with unique mechanisms of action that do not fit into the classical Vaughan-Williams classification. These agents include the endogenous nucleoside adenosine, cardiac glycosides, and electrolyte supplements containing potassium and magnesium. They are used for strictly specific rhythm disorders, ranging from supraventricular tachycardias to atrial fibrillation.

AdenosineHas a half-life of only about 10 seconds, with effects lasting up to 1 minute.
DigoxinDecreases ventricular rate in tachy-form atrial fibrillation.
Magnesium sulfateThe drug of choice for intravenous termination of *Torsades de Pointes*.
Adverse effectAdministration of adenosine can cause frightening, yet transient, asystole.

Adenosine: Ultra-Short Action

Adenosine (Adenosine) is a natural (endogenous) nucleoside that acts as an agonist at adenosine $A_1$ receptors. Its key pharmacokinetic feature is an extremely short half-life of about 10 seconds. The total duration of its effect does not exceed one minute. This is because the substance is instantly taken up by erythrocytes via a specific nucleoside transporter and rapidly degraded by vascular endothelial enzymes. For this reason, the drug is administered exclusively via rapid intravenous bolus.

At the molecular level, stimulation of $A_1$ receptors leads to the opening of coupled potassium channels. $K^+$ ions flow out of the cell, causing membrane hyperpolarization. Electrophysiologically, this manifests as suppression of sinus node automaticity, decreased myocardial contractility, and profound depression of conduction in the atrioventricular (AV) node.

The drug induces transient AV nodal blockade, which interrupts the re-entry circuit between the atria and ventricles. Therefore, adenosine is used exclusively to terminate supraventricular tachyarrhythmias, including those associated with Wolff-Parkinson-White (WPW) syndrome. Due to its mechanism of action, potential side effects include transient asystole and respiratory disturbances (dyspnea).

Cardiac Glycosides in Arrhythmias

A prominent representative of this group is digoxin (a digitalis preparation). Its primary indication in antiarrhythmic therapy is the tachy-form of atrial fibrillation (atrial flutter and fibrillation).

In atrial fibrillation, atrial automaticity is pathologically elevated, chaotic re-entry circuits occur, and atrial contraction rates reach up to 300 beats per minute. The natural filtering properties of the AV node allow only every second or fourth impulse to pass. However, without treatment, the ventricular rate remains high—around 150 beats per minute—requiring medical management.

Digoxin exerts a negative dromotropic effect, meaning it decreases AV nodal conduction. By creating incomplete conduction block, it converts tachy-form atrial fibrillation into a normosystolic form. The clinical result is the normalization of the ventricular rate.

With overdose or high doses, complete heart block may occur. At the atrial level, fibrillation may transition to flutter. The ventricles then switch to an autonomous (idioventricular) rhythm, with pacemaker cells in the bundle of His driving a rate of 40–60 beats per minute. Hemodynamically, the rhythm becomes slower but more coordinated, improving cardiac pumping function.

Potassium and Magnesium Preparations

Electrolytes play a critical role in maintaining the stability of cardiomyocyte cell membranes. Clinical practice utilizes potassium chloride (most effective in intravenous polarizing solutions) and combination products, such as potassium-magnesium asparaginate (Panangin, Asparkam).

They are indicated for tachyarrhythmias and extrasystoles under one key condition: the rhythm disturbances must be caused by hypokalemia or hypomagnesemia (reduced blood concentrations of these ions). These agents are administered both orally (per os) and intravenously. Adverse effects include neurological symptoms (paresthesias), gastrointestinal disturbances (dyspepsia), and cardiac complications (conduction depression up to AV block).

Magnesium sulfate holds a special place in emergency practice. When administered intravenously, it is the specific agent for terminating life-threatening polymorphic ventricular tachycardia, specifically Torsades de Pointes.

Frequently asked questions

What are the contraindications to adenosine?

Contraindications to adenosine include severe bronchial asthma, angina pectoris, and Wolff-Parkinson-White (WPW) syndrome combined with atrial fibrillation (AF).

  • Severe bronchial asthma — a direct contraindication.
  • Angina pectoris — a direct contraindication.
  • WPW syndrome combined with AF — contraindicated because impulses will travel down accessory pathways, potentially triggering ventricular fibrillation.
What non-cardiac symptoms characterize cardiac glycoside toxicity?

Cardiac glycoside toxicity is characterized by the following non-cardiac manifestations:

  • Nausea and vomiting — can be signs of digitalis toxicity in digoxin overdose.
  • Visual disturbances — listed among the side effects of digoxin.
  • Hyperkalemia — listed among the side effects of digoxin.
Which drugs have clinically significant interactions with digoxin?

Digoxin has clinically significant interactions with the following drugs and drug classes:

  • Quinidine — inhibits P-glycoprotein, increasing blood concentrations of digoxin and raising the risk of digitalis toxicity.
  • Verapamil — a P-glycoprotein inhibitor; combining digoxin with verapamil increases the risk of digitalis toxicity.
  • Amiodarone — combining digoxin with amiodarone increases the risk of digitalis toxicity.
  • Phenobarbital, rifampin, phenytoin — inducers of hepatic microsomal enzymes; accelerate the metabolism and inactivation of drugs, which can weaken the therapeutic effect of digoxin.
  • Ivabradine — requires caution when co-administered with digoxin.
Why is adenosine administered only as a rapid IV bolus?

Due to its pharmacokinetics: the drug is instantly taken up by erythrocytes and broken down by endothelial enzymes. Its half-life is about 10 seconds, so slow administration would cause it to be degraded before reaching the heart.

How does digoxin help in atrial fibrillation?

It exerts a negative dromotropic effect, suppressing conduction in the AV node. This filters out excess impulses coming from the atria and reduces the ventricular rate to normal values.

When are potassium and magnesium preparations effective against arrhythmias?

Only when the rhythm disorder is directly linked to a reduced blood concentration of these ions (hypokalemia or hypomagnesemia).

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