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Cardiac Glycosides

*Glycosida cardiaca*

For medical students2 min readUpdated 2026-10-10

Cardiac glycosides are a group of plant-derived medications with potent positive inotropic effects. They increase myocardial contractility, decrease heart rate, and are primarily used in the management of heart failure.

Main EffectPositive inotropic effect (increased myocardial contractility)
Primary TargetInhibition of sarcolemmal $Na^+,K^+$-ATPase
Plant SourcesFoxglove (*Digitalis*), Strophanthus, Lily of the valley (*Convallaria*)
Onset of Action2–10 minutes (ouabain) to 30–120 minutes (digoxin)

Origin and Classification

Cardiac glycosides are derived from medicinal plants. Modern medical practice utilizes preparations extracted from three main groups:

  1. Foxglove glycosides (Digitalis) — include digoxin, acetyldigoxin B, and lanatoside C.
  2. Strophanthus glycosides (Strophanthus) — represented by ouabain (strophanthin G).
  3. Lily of the valley glycosides (Convallaria) — contain corglycone and convallatoxin.

Digitoxin and older lily of the valley preparations are largely obsolete in modern therapeutics.

Molecular Mechanism of Action

The therapeutic action is based on altering ion transport across the sarcolemma of cardiomyocytes.

Excess calcium is sequestered into the sarcoplasmic reticulum and released upon excitation, producing a powerful myocardial contraction.

Primary Pharmacological Effects

The effects of cardiac glycosides on the heart are classically divided into four main properties:

Reduced heart rate and conduction velocity are largely mediated by vagotonia (increased vagal tone).

Clinical Use and Adverse Effects

Cardiac glycosides are prescribed for acute and chronic heart failure, as well as for rate control in atrial fibrillation with rapid ventricular response.

The choice of agent depends on the desired onset of action:

Overdose causes dangerous rhythm and conduction disturbances (ventricular extrasystoles, AV blocks), as well as extracardiac symptoms such as gastrointestinal distress, xanthopsia (yellow-green vision), and neurological disturbances.

Mnemonic

H-K-K: Foxglove (Nap*h*erstyanka in Russian), Potassium down, Calcium up — strong contraction!

Frequently asked questions

Which specific rhythm and conduction disturbances are characteristic of digitalis toxicity?

Digitalis toxicity induces various arrhythmias due to altered automaticity and suppressed impulse conduction:

  • Ventricular extrasystoles — resulting from enhanced ventricular automaticity.
  • Atrioventricular (AV) block — resulting from the negative dromotropic effect.
  • Ventricular paroxysmal tachycardia — consecutive occurrence of 5 or more ventricular ectopic beats.
  • Ventricular flutter and fibrillation — terminal arrhythmias.
  • Non-paroxysmal junctional tachycardia — often seen with chronic toxicity.

At high doses, atrial fibrillation may transition to flutter or an independent (idioventricular) rhythm.

What are the specific gastrointestinal and neurological manifestations of cardiac glycoside overdose?

Systemic extracardiac manifestations of overdose include:

  • Gastrointestinal disturbances — anorexia, nausea, vomiting, and diarrhea.
  • Neurological and psychiatric symptoms — headache, sleep disturbances, fatigue, confusion, and hallucinations.
  • Visual disturbances — xanthopsia (yellow-green color vision).
What are the absolute and relative contraindications to cardiac glycosides?

While absolute versus relative categorizations vary, cardiac glycosides are contraindicated or used with extreme caution in:

  • Atrioventricular (AV) block (second- or third-degree).
  • Hypertrophic cardiomyopathy — contraindicated in patients with normal sinus rhythm due to worsening left ventricular outflow obstruction and proarrhythmic risk.
  • Wolff-Parkinson-White (WPW) syndrome — dangerous in the presence of atrial fibrillation, as enhanced AV nodal blocking can promote conduction down the accessory pathway, leading to ventricular fibrillation.
Which drugs are used as antidotes in cardiac glycoside poisoning?

Specific antidote therapy aims to bind the toxin or restore enzyme activity:

  • Digoxin-specific Fab antibody fragments (e.g., Digibind) — monoclonal antibodies that bind free glycoside molecules into inactive complexes.
  • Unitiol (dimercaprol derivative) — a sulfhydryl group donor that binds to the toxin and frees up thiol groups on the $Na^+,K^+$-ATPase, restoring its transport function.
What ECG changes are produced by therapeutic doses of cardiac glycosides?

Therapeutic administration of cardiac glycosides characteristically causes:

  • ST segment — sagging, "scooped" or "crescentic" downsloping depression.
  • T wave — flattening or inversion.
  • QT interval — shortening.

With higher or toxic doses, ventricular ectopy, non-paroxysmal junctional tachycardia, and conduction blocks may appear.

What is the primary pharmacological effect of cardiac glycosides?

The primary effect is a positive inotropic effect, which increases myocardial contractility by elevating cytosolic calcium ion concentrations within cardiomyocytes.

What is the difference between the pharmacokinetic roles of the glycone and aglycone moieties?

The sugar portion (glycone) determines water solubility, absorption, tissue accumulation, and excretion, whereas the non-sugar portion (aglycone or genin) is responsible for the specific pharmacodynamic properties and mechanism of action.

What causes xanthopsia during digitalis toxicity, and how does it manifest?

Xanthopsia is a specific symptom of digitalis toxicity characterized by altered color vision where surrounding objects appear tinted yellow or green.

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