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Digitalis Toxicity

Intoxicatio glycosidica

For medical students2 min readUpdated 2026-10-10

Digitalis toxicity develops due to the tendency of cardiac glycosides toward cumulative accumulation and their narrow therapeutic index. The condition manifests with severe cardiac and extracardiac disturbances requiring prompt electrolyte correction and specific antidotal therapy.

Main DangerVentricular fibrillation rapidly progressing to asystole

Antidote of ChoiceDigoxin-specific antibody fragments

Enzyme RestorationUnithiol as a sulfhydryl group donor

Risk FactorHypokalemia and hypomagnesemia induced by diuretics

Causes and Mechanisms

Cardiac glycoside overdose occurs due to their ability to undergo tissue accumulation in the body, combined with an extremely narrow therapeutic index. Manifestations are divided into cardiac and extracardiac signs. Concomitant hypokalemia and hypomagnesemia—frequently developing in patients taking loop or thiazide diuretics—play a major role in provoking arrhythmias.

Cardiac Manifestations and Arrhythmias

Cardiovascular involvement during toxicity is life-threatening:

Ventricular premature beats (extrasystoles): arise from increased automaticity driven by intracellular ion imbalance (hypokalemia and intracellular hypercalcemia). The rhythm frequently takes the form of bigeminy (an extrasystole following every normal contraction) or trigeminy (after every two normal beats).
Atrioventricular (AV) block: develops due to a negative dromotropic effect caused by enhanced vagal tone on the heart.
Ventricular fibrillation: chaotic contractions of myocardial fibers at a rate of 450–600 per minute, rapidly progressing to asystole and cardiac arrest. This is the most common cause of mortality.

Treatment Principles and Antidotal Therapy

Managing an overdose requires a comprehensive approach:

Specific therapy: Digoxin-specific antibody fragments are the most effective agents.
Reversal of AV block: Atropine is administered to block cardiac $M_2$ muscarinic receptors, eliminate the inhibitory vagal (n. vagus) effect on the atrioventricular node, and improve conduction.
Electrolyte balance correction: Potassium chloride and combination products (such as potassium-magnesium aspartate) are used. Magnesium ions act as $Na^+,K^+$-ATPase activators, ensuring potassium transport into cardiomyocytes.
Calcium chelation: Disodium edetate (EDTA) is administered intravenously to chelate excess $Ca^{2+}$ ions in the blood.
Antidotal therapy: Unithiol, acting as a donor of thiol (-SH) groups, forms disulfide bridges with the glycoside, freeing the intrinsic thiol groups of $Na^+,K^+$-ATPase and restoring its transport function.

Management of Extrasystoles

When ventricular extrasystoles develop, class IB sodium channel blockers, specifically phenytoin and lidocaine, are prescribed. The choice of these drugs is justified by their preferential action on ventricular tissue, effectively stabilizing the cardiac rhythm.

Mnemonic

Antidote chain: "Antibodies clear digoxin, Atropine relieves vagal block, Unithiol rescues enzymes via thiol groups, and EDTA binds calcium."

Frequently asked questions

What extracardiac manifestations occur in digitalis toxicity?

Digitalis toxicity produces systemic extracardiac adverse effects affecting the gastrointestinal, visual, and nervous systems. These manifestations include:

Gastrointestinal symptoms — nausea, vomiting, diarrhea.
Visual disturbances — a specific symptom called xanthopsia (altered color vision with objects appearing yellowish-green).
Neurological and psychiatric disorders — headaches, sleep disturbances, psychomotor agitation, hallucinations.

What ECG changes, aside from arrhythmias, are characteristic of cardiac glycoside action and overdose?

In addition to arrhythmogenic effects, the therapeutic and toxic actions of cardiac glycosides are characterized by the following ECG changes:

ST-segment shift — characteristic "scooped" (reverse tick) depression.
T-wave changes — decreased amplitude.
Q-T interval changes — shortening.

What is the most frequent cause of death in digitalis toxicity?

Ventricular fibrillation with a fiber contraction rate of 450–600 per minute, which rapidly transitions to asystole.

What is the mechanism of action of unithiol in this toxicity?

Unithiol acts as a sulfhydryl group donor, forms disulfide bridges with the glycoside, and frees the intrinsic thiol groups of $Na^+,K^+$-ATPase, restoring its activity.

Which diuretics promote arrhythmias when taking cardiac glycosides?

Loop and thiazide diuretics, because they induce hypokalemia and hypomagnesemia.

Which drug is the agent of choice for treating AV block?

Atropine, which blocks $M_2$ muscarinic receptors and eliminates the inhibitory effect of the vagus nerve on the atrioventricular node.

Go deeper

Pharmacokinetic aspects of cardiac glycoside tissue accumulation
Role of ionic imbalance (hypokalemia and intracellular hypercalcemia) in enhancing automaticity
Pharmacological properties of class IB sodium channel blockers (phenytoin, lidocaine) in arrhythmias
Chelating agents and calcium ion binding in systemic circulation
Influence of the vagus nerve (n. vagus) and negative dromotropic effects on cardiac conduction