Digoxin is a cardiac glycoside , a class of naturally derived compounds originally extracted from the foxglove plant ( Digitalis purpurea ) . It is primarily used in the management of atrial arrhythmias and heart failure due to its positive inotropic and vagomimetic effects .
Examples of Cardiac Glycosides
- Digoxin – the most commonly used in clinical practice
- Digitoxin
- Ouabain
High-Yield Fact : All cardiac glycosides share a similar mechanism of action - inhibition of the Na⁺/K⁺-ATPase pump , which indirectly increases intracellular calcium to enhance myocardial contractility.
Mechanism of Action
In Heart Failure (Positive Inotropic Effect)
- Na⁺/K⁺-ATPase Inhibition : Digoxin inhibits the sodium-potassium pump in cardiac myocytes.
- ↑ Intracellular Na⁺ : This reduces the sodium gradient across the sarcolemma.
- ↓ Na⁺/Ca²⁺ Exchange : Less sodium entry = less calcium extrusion.
- ↑ Intracellular Ca²⁺ : Calcium is sequestered in the sarcoplasmic reticulum and released during contraction.
- ↑ Actin-Myosin Interaction : Enhanced cardiac contractility (positive inotropy).
In Atrial Arrhythmias (Negative Chronotropic and Dromotropic Effects)
- Vagal Stimulation : Digoxin enhances parasympathetic (vagal) tone.
- ↓ SA Node Firing : Reduces heart rate (negative chronotropy).
- ↓ AV Node Conduction : Prevents rapid ventricular rates in atrial fibrillation/flutter (negative dromotropy).
Used mainly to control ventricular rate in atrial fibrillation/flutter, especially in patients with concurrent heart failure.
Pharmacokinetics
- Routes of Administration : Oral and intravenous (IV)
- Onset :
- Oral: ~2 hours
- IV: ~30 minutes
- Half-life : ~36–48 hours (prolonged in renal impairment)
- Volume of Distribution : High (binds extensively to tissues)
- Metabolism & Clearance : Primarily renal excretion
Digitalization (Loading Dose)
Due to slow onset and long half-life, loading doses are used to rapidly achieve therapeutic plasma concentrations.
Therapeutic Range and Monitoring
- Therapeutic Plasma Concentration : 0.5 – 1.5 ng/mL
- Toxic Level : > 2.0 ng/mL
Always monitor renal function and serum electrolytes (K⁺, Mg²⁺, Ca²⁺) in patients on digoxin to avoid toxicity.
Toxicity (Digitalis Toxicity)
Risk Factors
- Renal impairment
- Hypokalemia, hypomagnesemia
- Drug interactions
- Elderly patients
Early Signs
- Anorexia
- Nausea, vomiting
- Fatigue
- Visual disturbances (e.g., yellow-green halos)
ECG Changes
- Premature ventricular contractions (PVCs)
- AV block
- Bradycardia
- ST depression with scooped appearance (“digitalis effect”)
Severe Toxicity
- Life-threatening arrhythmias (e.g., ventricular tachycardia/fibrillation)
- Confusion, delirium
Management
- Stop Digoxin
- Correct electrolytes (especially potassium)
- Administer Digoxin-specific antibody fragments (Digibind or DigiFab)
- Class IB antiarrhythmics (e.g., lidocaine or phenytoin) in case of ventricular arrhythmias
Digibind is used in severe toxicity , especially when arrhythmias are unresponsive to supportive therapy.
Drug Interactions
| Drug/Class | Effect |
|---|---|
| Amiodarone | ↑ Digoxin levels (↓ clearance) |
| Verapamil, Diltiazem | ↑ Plasma levels (↓ renal excretion) |
| Quinidine, Propafenone | Compete for binding sites, ↑ Digoxin |
| Diuretics (loop, thiazide) | Cause hypokalemia , ↑ toxicity risk |
| NSAIDs | Reduce renal blood flow, ↓ clearance |
| Tolvaptan | Alters Na⁺ balance, ↑ Digoxin effect |
Clinical Uses of Digoxin
| Condition | Role of Digoxin |
|---|---|
| Atrial Fibrillation/Flutter | Rate control (esp. in HF) |
| Chronic Heart Failure | Enhances contractility in reduced ejection fraction HF (HFrEF) |
| Paroxysmal Supraventricular Tachycardia (PSVT) | Less commonly used, vagomimetic effect helps AV nodal suppression |
Note : Not typically used in acute decompensated heart failure due to delayed onset of action.
Contraindications
- Ventricular fibrillation
- AV block without pacemaker
- Acute myocardial infarction (caution)
- Hypokalemia, hypomagnesemia (predispose to toxicity)
