Digoxin has one of the narrowest therapeutic windows of any drug in clinical practice — the difference between a therapeutic and a toxic dose can be vanishingly small, especially in older patients, those with renal impairment, or anyone with low serum potassium.
Key Points — Digoxin Toxicity
- 01 →Digoxin toxicity may occur above the therapeutic range, or earlier in the presence of hypokalaemia.
- 02 →Hypokalaemia below 3.5 mmol/L dramatically sensitises the myocardium to digoxin.
- 03 →Classic signs include nausea, vomiting, anorexia, bradycardia, heart block, and visual disturbance.
- 04 →Any new dysrhythmia in a patient on digoxin should raise concern for toxicity.
- 05 →Withhold the dose immediately, obtain a stat level and electrolytes, and escalate urgently.
- 06 →Severe toxicity may require digoxin-specific antibody fragments.
Why Hypokalaemia Makes It Worse
Digoxin works by inhibiting the Na⁺/K⁺-ATPase pump on cardiac cells, increasing intracellular calcium and enhancing contractility. Potassium and digoxin compete for the same binding site. When serum potassium falls, digoxin binds more avidly and its toxic effects are amplified.
This is why electrolyte monitoring is not optional in any patient on digoxin. If your patient is also receiving loop diuretics, the risk is compounded and demands vigilant, proactive monitoring.
Clinical Warning
If your patient on digoxin develops new bradycardia, heart block, nausea, or visual disturbance, withhold the next dose immediately, take a 12-lead ECG, draw electrolytes and a digoxin level, and escalate now.
Recognising Toxicity — Signs by System
| System | Signs of Digoxin Toxicity |
|---|---|
| GI | Nausea, vomiting, anorexia, abdominal pain |
| Cardiac | Bradycardia, heart block, PAT with block, VT, VF |
| Visual | Yellow-green halos, blurred or altered colour vision |
| Neurological | Confusion, fatigue, dizziness, headache |
Nursing Response: Step by Step
Immediate Actions
Withhold the digoxin dose, apply continuous cardiac monitoring, obtain a 12-lead ECG, and draw blood for serum digoxin level, urea and electrolytes, and magnesium.
Clinical Tip
Serum digoxin levels drawn within 6 hours of the last dose can appear falsely elevated. For a trough level, draw at least 6 hours post-dose and communicate the timing clearly.
Ongoing Management
Correct potassium and magnesium carefully, review renal function, and be prepared for DigiFab in severe toxicity.