Digoxin, a cardiac glycoside used to treat heart failure and certain arrhythmias, was developed in the late 18th century and became widely used in clinical practice in the 20th century. Its history is marked by its effectiveness in improving cardiac contractility and controlling ventricular rate, but also by the narrow therapeutic index that requires careful dosing and monitoring to avoid toxicity. Digoxin is included in multiple treatment regimens for heart failure and atrial fibrillation. Its development and clinical use have emphasized the importance of therapeutic drug monitoring, especially in patients with renal impairment or those taking interacting medications.
BRAND NAMES
Lanoxin – one of the most widely known digoxin brands
Digox – oral and injectable formulations in some countries.
MECHANISM OF ACTION
Digoxin is a cardiac glycoside that works by inhibiting the sodium-potassium ATPase pump in cardiac cells. This inhibition increases intracellular sodium, which in turn raises intracellular calcium via the sodium-calcium exchanger, enhancing myocardial contractility and producing a positive inotropic effect. Additionally, digoxin slows conduction through the atrioventricular (AV) node and increases vagal tone, resulting in a negative chronotropic effect that helps control heart rate, particularly in atrial fibrillation.
PHARMACOKINETICS
Absorption
Digoxin is well absorbed orally, with bioavailability varying by formulation. Standard tablets and capsules typically achieve 60–80% bioavailability, while liquid and elixir forms can reach up to 90–100%.
Distribution
Digoxin has a large volume of distribution, approximately 4–6 L/kg, reflecting extensive tissue binding, particularly in the heart, kidneys, liver, and skeletal muscle. Only a small fraction of digoxin remains in the plasma, while the majority distributes into tissues where it exerts its therapeutic effects.
Metabolism
Digoxin undergoes minimal hepatic metabolism, with the majority of the drug remaining unchanged. Only a small fraction is metabolized by liver enzymes into inactive metabolites. Because digoxin is largely excreted unchanged by the kidneys, hepatic function has limited impact on overall clearance, but metabolism may slightly vary in patients with severe liver disease.
Elimination
Digoxin is primarily eliminated unchanged by the kidneys through glomerular filtration and tubular secretion, with approximately 50–70% excreted in urine. Its elimination half-life ranges from 36 to 48 hours in patients with normal renal function, but it can be significantly prolonged in renal impairment, increasing the risk of toxicity.
PHARMACODYNAMICS
Digoxin exerts its effects primarily on the heart by increasing myocardial contractility and modulating conduction. By inhibiting the sodium-potassium ATPase pump, it raises intracellular calcium in cardiac cells, producing a positive inotropic effect that enhances cardiac output. Digoxin also slows conduction through the atrioventricular (AV) node and increases vagal tone, resulting in a negative chronotropic effect, which helps control heart rate in atrial fibrillation.
ADMINISTRATION
Digoxin can be administered orally or intravenously depending on the clinical need. Oral forms, including tablets, capsules, and elixirs, are used for chronic management of heart failure and atrial fibrillation and are typically taken once daily at the same time to maintain steady plasma levels. Intravenous administration is reserved for rapid control in acute heart failure or supraventricular tachyarrhythmias and should be given slowly to avoid cardiac complications.
DOSAGE AND STRENGTH
For adults, digoxin is typically administered orally at 0.125–0.25 mg once daily for heart failure or rate control in atrial fibrillation, with dose adjustments based on renal function, age, and body weight. Intravenous administration is used for rapid control in acute settings, usually 0.25 mg IV over 5 minutes, repeated every 6–8 hours if necessary, up to a total loading dose. Oral formulations are available in 0.0625 mg, 0.125 mg, 0.1875 mg, and 0.25 mg tablets, and as an elixir of 0.05 mg/mL.
DRUG INTERACTIONS
Digoxin has multiple drug interactions due to its narrow therapeutic index and renal excretion. Drugs that increase digoxin levels include verapamil, amiodarone, quinidine, and certain macrolide antibiotics, which can increase the risk of toxicity. Conversely, medications such as rifampicin, St. John’s Wort, and certain antacids can decrease digoxin absorption or plasma levels, reducing its effectiveness. Electrolyte disturbances caused by diuretics, particularly hypokalemia, hypomagnesemia, or hypercalcemia, can potentiate digoxin’s effects and increase the risk of arrhythmias.
FOOD INTERACTIONS
Digoxin absorption can be influenced by certain foods and dietary components. High-fiber meals may reduce digoxin absorption, potentially lowering its effectiveness, so it is often recommended to take the medication consistently with regard to meals. Foods or supplements high in bran, psyllium, or certain herbal products may bind digoxin in the gastrointestinal tract, decreasing bioavailability.
CONTRAINDICATIONS
Digoxin is contraindicated in patients with known hypersensitivity to the drug. It should not be used in individuals with ventricular fibrillation or certain types of ventricular tachyarrhythmias unrelated to heart failure. Caution is warranted in patients with severe bradycardia, heart block (without a pacemaker), or sick sinus syndrome, as digoxin can exacerbate conduction abnormalities.
SIDE EFFECTS
Nausea and vomiting
Diarrhea
Loss of appetite
Fatigue and weakness
Dizziness or headache
Bradycardia (slow heart rate)
Arrhythmias (ventricular or atrial ectopy)
Atrioventricular (AV) block
Blurred or yellow vision.
OVER DOSAGE
Digoxin overdose can be life-threatening due to its narrow therapeutic index, primarily causing cardiac arrhythmias such as bradycardia, ventricular tachycardia, or AV block. Non-cardiac symptoms may include nausea, vomiting, diarrhea, confusion, dizziness, and visual disturbances (e.g., blurred or yellow vision). Management is supportive and symptom-directed, including discontinuation of digoxin, correction of electrolyte imbalances (especially potassium and magnesium), and monitoring of cardiac rhythm.
TOXICITY
Digoxin toxicity occurs when plasma levels exceed the therapeutic range, often due to overdose, impaired renal function, drug interactions, or electrolyte imbalances. The most significant manifestations are cardiac, including bradycardia, atrioventricular (AV) block, and life-threatening ventricular arrhythmias. Gastrointestinal symptoms such as nausea, vomiting, and diarrhea are common early signs, while neurological effects may include fatigue, confusion, and visual disturbances like blurred or yellow vision (xanthopsia). Risk is increased in elderly patients, those with renal impairment, and individuals with hypokalemia, hypomagnesemia, or hypercalcemia.
