Digitalis glycosides (cardiac glycosides) are a class of medications derived from the foxglove plant, such as digoxin.
Digitalis glycosides are a class of cardiac glycosides — most commonly represented by digoxin — that exert their effects through inhibition of the Na⁺/K⁺-ATPase (sodium pump), with indications for mild-to-moderate heart failure and ventricular rate control in chronic atrial fibrillation.
They are primarily used to treat heart failure and manage atrial fibrillation by increasing the force of heart contractions while slowing down the heart rate.
They improve cardiac output by increasing the strength of heart muscle contractions, though they are rarely used as a first-line treatment today.
Atrial Fibrillation: They slow electrical signals in the AV node to help control rapid, uncoordinated heart rates.
Mechanism: They work by inhibiting the cellular sodium-potassium ATPase pump, which increases intracellular calcium, promoting stronger muscle contractions.
Narrow Therapeutic Window: The dose required for therapeutic benefit is very close to the dose that can cause toxicity.
Blood levels are carefully monitored.
Symptoms of toxicity can include severe nausea, vomiting, confusion, visual disturbances (e.g., blurred or yellow-tinted vision), and dangerous cardiac arrhythmias.
Digoxin vs. Digitoxin: Digoxin is the most commonly prescribed glycoside medication in the United States.
Digitoxin is a longer-acting alternative commonly used in other parts of the world.
Mechanism of Action: All digitalis glycoside effects are mediated through Na⁺/K⁺-ATPase inhibition:
Cardiac effects: Inhibition of the sodium pump in myocytes reduces the transmembrane Na⁺ gradient, decreasing Na⁺/Ca²⁺ exchanger activity and raising intracellular Ca²⁺.
This increased Ca²⁺ enhances excitation-contraction coupling, producing a positive inotropic effect.
Recent work has further clarified that digoxin’s inotropic action critically depends on Na⁺-dependent inactivation of the Na⁺/Ca²⁺ exchanger (NCX1).
Neurohormonal effects: Inhibition of Na⁺/K⁺-ATPase in noncardiac tissues sensitizes baroreceptors, reducing sympathetic outflow, enhances vagal tone, parasympathetic stimulation, and reduces renal sodium reabsorption with subsequent suppression of the renin-angiotensin-aldosterone system (RAAS).
These neurohormonal effects may be as clinically important as the inotropic action.
Electrophysiological effects: Slows sinus rate by negative chronotropy, prolongs AV nodal conduction, and increases automaticity.
Clinical Indications and Guideline Recommendations:
Heart failure (HFrEF): considered to decrease HF hospitalizations in patients who remain symptomatic despite optimized guideline-directed medical therapy.
The landmark DIG trial showed digoxin reduced hospitalizations but had no effect on mortality.
Treatment with digitals glycosides is associated with lower risk of composite of cardiovascular death or first worsening heart failure event in patients with heart failure, and moderate reduce ejection fraction or heart failure with reduce the ejection fraction, mainly through a lower risk of worsening heart failure events.
Digitalis glycosides may be used as additional medical therapy to reduce worsening heart failure events, in patients with mildly reduced ejection fraction or reduced ejection fraction.
Atrial fibrillation: Digoxin is used for long-term rate control, particularly in combination with beta-blockers.
The RATE-AF trial showed comparable quality-of-life outcomes between digoxin and bisoprolol, with fewer adverse effects in the digoxin group.
AGS Beers Criteria recommend avoiding digoxin as first-line therapy for AF, favoring beta-blockers or nondihydropyridine calcium channel blockers.
Dosing and Therapeutic Monitoring
Digoxin has a narrow therapeutic index — toxic levels are only slightly above therapeutic levels.
Maintenance dose: Typically 0.125–0.25 mg daily; lower doses (0.125 mg daily or every other day) for patients >70 years, with impaired renal function, or low lean body mass.
Target serum concentration: 0.5–0.9 ng/mL is associated with the best benefit-to-risk ratio.
Concentrations ≥1.2 ng/mL are associated with significantly increased mortality risk.
Levels >2 ng/mL are associated with overt toxicity.
Loading dose: If rapid titration is desired, half the total loading dose is given initially, then one-quarter every 6–8 hours for two additional doses.
Loading doses are generally unnecessary for HF management.
Adverse Effects and Toxicity
The overall incidence of adverse reactions is reported at 5–20%, with 15–20% of events considered serious:
Cardiac (~50% of adverse events): Ectopic and re-entrant arrhythmias, AV block, ventricular tachycardia, ventricular fibrillation.
Gastrointestinal (~25%): Anorexia, nausea, vomiting, abdominal pain.
Neurological (~25%): Visual disturbances (classically yellow-green halos), confusion, disorientation, delirium.
Other: Gynecomastia with prolonged use, thrombocytopenia (rare)
Risk factors for toxicity include hypokalemia, hypomagnesemia, hypothyroidism, renal impairment, advanced age, and low lean body mass.
Multiple drug interactions raise digoxin levels, including amiodarone, verapamil, quinidine, clarithromycin, dronedarone, cyclosporine, and itraconazole.
Digoxin-specific antibody fragments (digoxin-Fab) are the definitive treatment for life-threatening cardiac glycoside poisoning, including cardiac arrest.
Calcium administration in the setting of digoxin-related hyperkalemia remains controversial due to concern for myocardial tetany.