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Cardiovascular

Cardiac Glycosides (Digoxin)

High-yield High-alert Verified · Jul 2026

Prototype: digoxin

Digoxin — a positive inotrope with a very narrow margin between the helpful dose and the toxic one.

How it works in the body

The system involved, what goes wrong, and how the drug and body interact.

01 The failing heart — weak pump, fast rhythm

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, the atria fire chaotically at 300–600 impulses a minute and the AV node lets too many through, driving a fast, inefficient ventricular rate.

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.

02 How digoxin intervenes — the sodium-potassium pump

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. A second exchanger (Na⁺/Ca²⁺) then slows down, and calcium accumulates 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 blocks the Na⁺/K⁺ pump (→ ↑ intracellular Ca²⁺, stronger beat) and raises vagal tone (→ slower AV conduction).

03 Why the margin is so narrow — potassium is the key

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 the moment potassium drops. This is the single most important interaction to understand: loop and thiazide diuretics, which are often given alongside digoxin for heart failure, waste potassium and can silently tip a patient into toxicity.

The therapeutic serum range is roughly 0.5–2 ng/mL — and for heart failure, current guidelines aim even lower, around 0.5–0.9 ng/mL. Above that, or with low potassium, the same calcium loading that strengthens the beat begins to trigger dangerous arrhythmias.

Hypokalemia unmasks digoxin toxicity — a critical interaction with diuretics.

Drug names

Generic Brand
digoxin Lanoxin, Digitek, Lanoxin Pediatric

Indications

  • Heart failure with reduced ejection fraction (symptom control; added when other therapy is insufficient)
  • Rate control in atrial fibrillation / atrial flutter (especially with coexisting HF)

Mechanism of action

Inhibits the myocardial Na⁺/K⁺-ATPase, raising intracellular sodium and, via the Na⁺/Ca²⁺ exchanger, intracellular calcium — increasing contractility (positive inotropy). Simultaneously increases vagal tone, slowing SA-node firing and AV-node conduction (negative chronotropy/dromotropy).

In plain terms
It makes each heartbeat stronger while slowing the overall rate — a stronger, steadier pump.

Therapeutic effects — what you'll see working

Judge success by symptoms and rate, not by a single number: improved exercise tolerance and less dyspnea in HF, and a controlled ventricular rate in atrial fibrillation. Because the toxic dose sits so close to the effective one, every therapeutic effect has a toxic mirror image — watch both.

↑ Contractility (positive inotrope) ↓ Ventricular rate in AF ↓ Heart rate
↑ Contractility (positive inotrope)
More intracellular calcium lets the muscle contract more forcefully, improving cardiac output and easing heart-failure symptoms — without raising heart rate the way other inotropes do.
↓ Ventricular rate in AF
Increased vagal tone slows AV-node conduction, so fewer atrial impulses reach the ventricles — bringing a rapid, irregular rate down toward a controlled range.
↓ Heart rate
Slowing the SA node lowers the resting rate. This is also why an apical pulse below 60 bpm is the standard signal to hold the dose — the effect has gone too far.

Adverse effects

Nearly all toxicity is the mechanism overshooting: too much vagal slowing, or too much calcium loading triggering arrhythmias. Toxicity is common because the margin is tiny and is worsened by low potassium, low magnesium, high calcium, and kidney impairment (digoxin is renally cleared).

Caution: Common / early toxicity Hold & notify
Anorexia, nausea, vomiting, fatigue, and visual changes — classically yellow-green halos around lights.
GI upset (anorexia, nausea, vomiting) is often the earliest warning sign of digoxin toxicity — do not dismiss it. The classic visual disturbance is seeing yellow-green halos or blurred/altered color vision. Any of these in a patient on digoxin should prompt a level and a potassium check.
Warning: Serious — dysrhythmias Report immediately
Bradycardia, AV block, and a wide range of dysrhythmias (including PVCs, bigeminy, and life-threatening ventricular arrhythmias).
The calcium loading that strengthens contraction can also make cells fire abnormally, while excess vagal tone blocks conduction — so digoxin can cause almost any dysrhythmia. Toxicity is potentiated by hypokalemia, hypomagnesemia, hypercalcemia, and renal impairment. Severe toxicity is reversed with digoxin immune Fab (DigiFab), which binds and inactivates the drug.

Antidote

Digoxin immune Fab (DigiFab)
Binds and inactivates digoxin; reverses life-threatening toxicity — refractory dysrhythmias, severe bradycardia, or hyperkalemia from acute overdose.

Interactions

Loop & thiazide diuretics drug
Waste potassium — hypokalemia unmasks digoxin toxicity even at a normal serum level.
Amiodarone, verapamil, quinidine drug
Raise serum digoxin — reduce the digoxin dose and monitor levels closely.

Contraindications

The contraindications are the rhythms digoxin would make dangerously worse, plus the electrolyte states that unmask toxicity.

Ventricular fibrillation
Digoxin can worsen ventricular ectopy; it has no role and is dangerous in VF.
2nd/3rd-degree AV block without a pacemaker; sick sinus syndrome
Further slowing of AV conduction on top of an existing block can precipitate complete heart block.
Digoxin toxicity or hypersensitivity
Adding drug to an already-toxic patient risks fatal arrhythmia.
Hypokalemia / hypomagnesemia / hypercalcemia use caution
These electrolyte states dramatically increase digoxin binding and toxicity — correct them before and during therapy.
Renal impairment (dose reduction) use caution
Digoxin is cleared by the kidneys; impaired clearance lets levels accumulate into the toxic range.
The pre-dose check that prevents most digoxin harm.

When to hold

Assess before giving — these findings mean hold the dose and act.

Apical pulse < 60 bpm (adult)
Take the apical pulse for a full minute; hold and notify the prescriber.
Serum potassium low, or digoxin level > 2 ng/mL
Hold and notify — hypokalemia and supratherapeutic levels precipitate toxicity.
Signs of toxicity — anorexia, nausea, visual halos, new dysrhythmia
Hold, obtain a digoxin level and potassium, and notify the prescriber.

Labs & levels

Test Therapeutic / normal Toxic / critical
Digoxin level Draw ≥ 6–8 h after a dose (steady state) Therapeutic 0.5–2 ng/mL (HF target 0.5–0.9) > 2.4 ng/mL
Potassium (K⁺) Before dosing & routinely Normal range 3.5–5.0 mEq/L Low K⁺ potentiates toxicity

Nursing considerations

The RN-specific layer — each action paired with the reason it matters.

Before & during administration
Take an apical pulse for a full minute before every dose; hold and notify the prescriber if < 60 bpm in an adult (< 70 in a child, < 90 in an infant).
Why: Digoxin slows the SA node; dosing an already-bradycardic patient can cause symptomatic bradycardia or heart block. A full minute is needed because the rate may be slow and irregular.
Check the most recent potassium, magnesium, and digoxin level; report K⁺ < 3.5 mEq/L and levels toward/above 2 ng/mL.
Why: Hypokalemia and hypomagnesemia unmask toxicity at "normal" levels; the narrow 0.5–2 ng/mL range means small changes matter.
Treat digoxin as a high-alert medication — verify the dose and use an independent double-check per policy.
Why: The tiny margin between therapeutic and toxic makes dosing errors especially dangerous.
Recognizing & managing toxicity
Suspect toxicity with anorexia, nausea, vomiting, visual halos, confusion, or new dysrhythmias; hold the drug and obtain a level and potassium.
Why: GI and visual symptoms are the classic early clues; catching them before arrhythmias develop prevents harm.
Know that severe toxicity is reversed with digoxin immune Fab (DigiFab).
Why: Fab fragments bind circulating digoxin and are life-saving in refractory arrhythmia or severe hyperkalemia from toxicity.
Patient teaching
Teach patients to check their own pulse, take the dose at the same time daily, and never double up after a missed dose.
Why: Self-monitoring catches bradycardia early, and consistent dosing keeps the level inside the narrow window.
Report nausea, loss of appetite, visual changes, or palpitations promptly, and maintain potassium-rich intake if on a potassium-wasting diuretic.
Why: These are early toxicity signs, and stable potassium is the best defense against unmasking toxicity.

Sources

Educational summary for nursing students. Always verify against current prescribing information and your institution's protocols before administering. Not medical advice.