Cardiac Glycosides (Digoxin)

Two problems bring digoxin to the bedside. In heart failure with reduced ejection fraction, the ventricle contracts too weakly to move enough blood forward. In atrial fibrillation, too many atrial impulses pass through the AV node, driving a fast ventricular rate that needs to be controlled.

Digoxin — a plant extract from foxglove (Digitalis) used for over two centuries — is unusual because it treats both at once: it makes each beat stronger and the overall rhythm slower. That combination is why it survives in an era of newer drugs, but its narrow safety margin makes it an ISMP high-alert medication.

Inside each heart-muscle cell, a pump called the Na⁺/K⁺-ATPase constantly pushes sodium out and potassium in. Digoxin blocks that pump, so sodium backs up inside the cell. Through a second exchanger (Na⁺/Ca²⁺), the extra sodium drives calcium to accumulate inside the cell. More intracellular calcium means a stronger contraction — the positive inotropic effect.

Separately, digoxin increases vagal (parasympathetic) tone to the heart. That slows the SA node and, crucially, slows conduction through the AV node — reducing how many atrial impulses reach the ventricles in atrial fibrillation. So one drug delivers both a stronger squeeze and a slower, more controlled rate.

Digoxin and potassium compete for the same binding site on the Na⁺/K⁺ pump. When potassium is low (hypokalemia), digoxin binds more easily — so a "normal" digoxin level can become toxic when potassium drops. This is a key interaction to understand: loop and thiazide diuretics, which are often given alongside digoxin for heart failure, cause hypokalemia and raise the risk of digoxin-induced arrhythmias.

StatPearls and Open RN give a therapeutic serum range of 0.8–2 ng/mL; the LANOXIN label links levels below 0.5 ng/mL with reduced efficacy and levels above 2 ng/mL with more toxicity and no added benefit. In heart failure the goal is lower, about 0.5–0.9 ng/mL: levels in that range have been linked to better prognosis and higher levels to more deaths, and in a Digitalis Investigation Group (DIG) trial analysis of men with heart failure and left ventricular dysfunction, 0.5–0.8 ng/mL carried lower mortality than placebo while 1.2 ng/mL or more raised it. For atrial-fibrillation rate control there is no single target — suggested upper limits range from below 0.9 to 1.2 ng/mL. Above 2 ng/mL, or with low potassium, the same calcium loading that strengthens the beat begins to trigger dangerous arrhythmias.