Digoxin

Digoxin is prescribed to treat:

• Heart failure and atrial fibrillation • Various related conditions in the cardiovascular category • Associated symptoms and complications

It is an important medication that helps manage these conditions effectively.

Digoxin (Lanoxin) is a cardiac glycoside used for rate control in atrial fibrillation and for symptom control in heart failure with reduced ejection fraction. It is among the oldest drugs still in regular use, derived from foxglove, and it has the narrowest therapeutic window of anything commonly prescribed in outpatient cardiology.

Mechanism of Action

Digoxin inhibits the sodium-potassium ATPase in cardiac myocytes. Sodium accumulates intracellularly, which reduces the gradient driving the sodium-calcium exchanger, so less calcium leaves the cell. The resulting rise in intracellular calcium increases the force of contraction — the positive inotropic effect.

Separately and more importantly for its main modern use, digoxin increases vagal tone at the atrioventricular node, slowing conduction and reducing the ventricular rate in atrial fibrillation. This vagal mechanism is the key to understanding its limitations: it works well at rest, when vagal tone is naturally high, and poorly during exertion, when sympathetic drive overrides it. A patient rate-controlled on digoxin at rest may still have a rapid response on climbing stairs.

Potassium is central to digoxin safety because digoxin and potassium compete for the same binding site on the sodium-potassium ATPase. Hypokalemia increases digoxin binding and therefore its effect, so a patient with a therapeutic digoxin level can become toxic simply by becoming hypokalemic — the reason diuretics such as furosemide are so often implicated in digoxin toxicity.

Clinical Use

Digoxin's position has narrowed considerably. For rate control in atrial fibrillation, beta blockers such as metoprolol and non-dihydropyridine calcium channel blockers such as diltiazem are preferred, because they control rate during activity as well as at rest. Digoxin is added when those are insufficient or not tolerated, and it is genuinely useful in sedentary patients and in those with hypotension, where it does not lower blood pressure as the alternatives do.

In heart failure with reduced ejection fraction, digoxin reduces hospitalisation but not mortality, and it sits well below the four foundational therapies — a beta blocker such as carvedilol, a renin-angiotensin agent, an SGLT2 inhibitor such as empagliflozin, and a mineralocorticoid antagonist such as spironolactone. It is considered for persistent symptoms despite optimal therapy.

Target levels for heart failure are lower than were historically used. Higher levels were associated with worse outcomes, and modern practice aims at the low end of the range. A patient on a dose set decades ago may be sitting above where current evidence suggests they should be. The heart failure early signs article covers presentation, and our cardiovascular team manages these regimens.

Toxicity

Digoxin toxicity is common enough, and its presentation distinctive enough, to deserve its own understanding.

Gastrointestinal symptoms — anorexia, nausea, vomiting — are often first. Neurologic features include confusion, weakness, and lethargy, which in an older patient are readily attributed to age, infection, or dementia rather than to the drug. The classic visual disturbance is a yellow-green tinge to vision or halos around lights, which is uncommon but close to pathognomonic when present.

Cardiac manifestations range widely and can include almost any arrhythmia. The combination of increased automaticity with atrioventricular block is characteristic.

The most important practical point is that toxicity occurs at apparently therapeutic levels when potassium is low, magnesium is low, or kidney function has declined. Checking a level in isolation, without electrolytes and creatinine, misses the majority of the picture. Any acute illness causing dehydration, vomiting, or diarrhoea can precipitate toxicity in a previously stable patient.

Numerous drugs raise digoxin levels, including amiodarone, verapamil, and clarithromycin, generally through P-glycoprotein inhibition. A new prescription is a common trigger. The MedlinePlus digoxin entry lists the interaction profile.

Monitoring and Follow-Up

Digoxin levels are checked at steady state, at least six to eight hours after a dose — a level drawn too soon reflects distribution rather than tissue concentration and reads falsely high, which leads to unnecessary dose reduction.

Levels are not checked routinely in a stable patient. They are checked when toxicity is suspected, after a dose change, after a change in kidney function, and when an interacting drug is started.

Potassium, magnesium, and creatinine matter as much as the level itself and should be checked alongside it. Kidney function governs clearance, and digoxin doses that were appropriate for years become excessive as eGFR declines with age — a slow, silent drift that is a common route to toxicity in older patients.

Heart rate and rhythm are assessed clinically, with an ECG where toxicity is suspected. The American Heart Association provides patient background on rate control.

Special Populations

In older adults, reduced kidney function, lower lean body mass, polypharmacy, and atypical presentation of toxicity all combine to make digoxin considerably more hazardous. Lower doses are appropriate, and periodically asking whether it is still needed is worthwhile.

In kidney impairment, dose reduction is essential and levels should be monitored more closely. Digoxin is contraindicated in ventricular fibrillation and used cautiously in Wolff-Parkinson-White syndrome with atrial fibrillation, where slowing AV nodal conduction can paradoxically accelerate conduction down an accessory pathway. In pregnancy it is used when indicated and is considered acceptable. Digoxin immune Fab is available as a specific antidote for severe toxicity.

When to Contact Your Doctor

Report nausea, vomiting, loss of appetite, or unexplained weakness and confusion — in an older patient these are more likely to be digoxin toxicity than the drug's reputation suggests. Report visual changes, particularly a yellow or green tinge, halos, or blurred vision.

Report palpitations, a very slow or irregular pulse, dizziness, or fainting. Any illness causing vomiting, diarrhoea, or reduced fluid intake is a reason to make contact, since dehydration and electrolyte loss precipitate toxicity quickly. Always check before starting a new medication, including antibiotics.

To review your digoxin level, your kidney function and electrolytes, or whether digoxin is still the right choice, contact us or schedule a visit.