One foxglove molecule blocks one pump. Calcium climbs, the heart squeezes harder, the vagus slows the gate. The whole exam lives in the gap between a normal digitalis effect and a lethal toxicity.
Opening Case
Two patients on digoxin. One has scooped ST segments and feels fine. The other is vomiting, seeing yellow, and the monitor shows atrial tachycardia with AV block. The level is nearly identical.
What separates an expected effect from a medical emergency, and why does a low potassium turn one into the other?
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Overview
Digoxin at a Glance
A cardiac glycoside from foxglove (Digitalis). It does two separate jobs: it makes the heart squeeze harder, and it slows the electrical gate. Both come from one enzyme block.
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Mechanism: Directly inhibits the Na-K-ATPase. Intracellular sodium rises, which slows the sodium-calcium exchanger, so intracellular calcium rises. More calcium means stronger contraction (positive inotropy).
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Vagal brake: Increases parasympathetic (vagal) tone. This slows conduction at the AV node and depresses the SA node, lowering the ventricular rate.
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Clinical use: Heart failure with reduced ejection fraction (symptom control, not mortality benefit) and rate control in atrial fibrillation, useful when a patient is also hypotensive because it does not drop blood pressure.
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Danger: Narrow therapeutic index. Cleared by the kidney. Hypokalemia potentiates it, and quinidine, amiodarone, and verapamil raise its level. The antidote is digoxin-specific antibody fragments.
The Cascade
Why Blocking One Pump Raises Calcium
The inotropy is indirect. Digoxin never touches calcium. Step through the chain and watch how a sodium problem becomes a calcium payoff.
Step 1. Digoxin binds and blocks the Na-K-ATPase. Normally this pump throws 3 sodium out and pulls 2 potassium in. With it blocked, sodium starts backing up inside the cell.
The ECG
Effect Versus Toxicity
This is the single highest-yield trap. The scooped ST is not toxicity. Learn to hold the two apart before the rhythm ever changes.
The digitalis effect is what therapeutic digoxin does to repolarization: a downsloping, scooped ("Salvador Dali mustache") ST depression, a shortened QT, and often a flattened T wave. The patient feels fine and the level is in range. It needs no treatment and it is not a reason to stop the drug.
Digoxin toxicity is a different animal. It is a clinical picture (nausea, vomiting, anorexia, confusion, yellow-green vision) plus rhythm disturbances. The most classic tested rhythm is atrial tachycardia with AV block, because digoxin both increases automaticity and slows AV conduction at once. Bidirectional ventricular tachycardia is nearly pathognomonic. In acute overdose, potassium leaks out of cells (the pump that normally pulls it in is blocked), so the serum potassium climbs, and a high potassium marks a poor prognosis.
Feature
Digitalis effect
Digoxin toxicity
Symptoms
None
Nausea, vomiting, yellow-green vision, confusion
Serum level
In therapeutic range
Often high, but can be normal if potassium is low
ST segment
Scooped, downsloping depression
May look similar; the rhythm is the tell
Rhythm
Controlled, stable
Atrial tachycardia with AV block, bidirectional VT
Potassium
Normal
High in acute overdose (poor prognosis)
Action
Continue, reassure
Correct potassium, consider antibody fragments
Tap the table to reveal. Test yourself first: what changes between the two columns?
Toxicity Drivers
What Pushes a Safe Level Into Trouble
Digoxin has almost no margin. Three forces move a patient across the line: bad kidneys, low potassium, and a handful of drugs.
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Renal failure: Digoxin is cleared by the kidney. When the kidney slows, the drug accumulates. An elderly patient with a rising creatinine on a stable dose can drift into toxicity with no dose change.
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Hypokalemia: Potassium and digoxin compete for the same site on the pump. Low potassium leaves the site open, so more digoxin binds and toxicity appears at an otherwise safe level. Loop and thiazide diuretics are the classic setup.
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Interacting drugs: Quinidine, amiodarone, and verapamil raise the digoxin level (reduced clearance and displacement from tissue binding). Starting one of these on a stable patient can double the level within days.
Drill: tap a finding, then tap whether it is an expected effect or true toxicity that demands action.
Tap a finding above, then choose a bucket.
Clinical Pathway
The Toxicity Discriminator
A young adult is brought in after swallowing a bottle of a relative's digoxin. Heart rate 38, hypotensive. The monitor shows high-grade AV block. Serum potassium is 6.8 mEq/L and the level is 9 ng/mL. Pick the most appropriate next step in management.
Acute digoxin overdose with hyperkalemia and an unstable bradyarrhythmia. What do you reach for?
Correct. Antibody fragments bind free digoxin and pull it off the pump. That reverses the arrhythmia and, as the pump comes back online, drives potassium back into cells, so the hyperkalemia corrects itself. This is the definitive move for a life-threatening acute overdose.
Dangerous. In digoxin toxicity the myocyte is already calcium-overloaded. Adding intravenous calcium risks the classic stone heart of an inexcitable, contracted myocardium. Fix the digoxin, not the potassium number in isolation.
Backwards. This patient is already hyperkalemic from the acute overdose. Loading more potassium makes it worse. Potassium repletion is for chronic toxicity with a low potassium, not acute overdose with a high one.
Not the fix. Chasing the rate with a beta-agonist ignores the cause and can provoke ventricular arrhythmias in a digoxin-poisoned heart. The antidote reverses the whole picture at once.
Visual Atlas
See It
The plant it comes from, the rhythm it controls, and the electrolyte tracings that decide the story. Tap any image to enlarge.
Foxglove (Digitalis purpurea)The source plant. Cardiac glycosides were first extracted from its leaves for dropsy.
Atrial fibrillationDigoxin slows AV conduction via vagal tone to control the ventricular rate here.
HypokalemiaLow potassium (U waves, flat T) opens the pump site and potentiates digoxin toxicity.
HyperkalemiaAcute overdose blocks the pump so potassium climbs. A high potassium marks poor prognosis.
Walkthrough
Board-Style Walkthrough
Original board-style vignettes. Shuffled, never-repeat. Clues glow after you answer. Right-click or long-press a choice to cross it out; double-tap to highlight.
Exam tools: right-click or long-press to cross out a choice, double-click or double-tap to highlight. Tools lock once you answer.
References
Katzung, Basic and Clinical Pharmacology. Goodman and Gilman, The Pharmacological Basis of Therapeutics. Lilly, Pathophysiology of Heart Disease. Images: Wikimedia Commons.
