cardiac glycoside
A cardiac glycoside is an ancient heart medicine, originally extracted from the foxglove plant, that makes a weak heart squeeze harder while steadying a racing one. Imagine tightening the grip of a tired hand and slowing a frantic tapping finger at the same time: the heart beats more forcefully yet at a more controlled rate.
Digoxin is the main example. It inhibits the sodium-potassium ATPase pump in heart muscle cells, which indirectly raises intracellular sodium and then intracellular calcium, strengthening contraction (a positive inotropic effect). Separately, it enhances vagal (parasympathetic) tone, slowing conduction through the atrioventricular node, which helps control the ventricular rate in atrial fibrillation. It is used in some patients with heart failure and in rate control of atrial fibrillation.
The catch is a very narrow therapeutic window: the dose that helps and the dose that poisons are uncomfortably close. Digoxin toxicity causes nausea, visual disturbances such as yellow-green halos, confusion and dangerous arrhythmias, and the risk rises sharply when potassium is low (as it can be with diuretics). Digoxin is cleared by the kidneys, so doses are reduced in renal impairment, and blood levels are often monitored.
An elderly patient with atrial fibrillation and heart failure takes a small daily dose of digoxin; when she later becomes dehydrated and her kidney function falls, the drug accumulates and she develops nausea and yellow-tinged vision, signs of toxicity.
A narrow therapeutic window means small changes in kidney function can tip digoxin into toxicity.
Severe digoxin poisoning has a specific antidote: digoxin-specific antibody fragments that bind the drug and pull it off the heart.