Half-Life of Digoxin in Patients with Normal Renal Function
The half-life of digoxin in patients with normal renal function is 1.5 to 2.0 days (36-48 hours). 1
Pharmacokinetic Properties of Digoxin
Absorption and Distribution
Digoxin has variable bioavailability depending on the formulation:
- Tablets: 60-80% bioavailability
- Elixir: 70-85% bioavailability
- Solution in capsules: 90-100% bioavailability
- IV: 100% bioavailability 1
Following administration, digoxin undergoes:
- Initial 6-8 hour tissue distribution phase
- Followed by a more gradual decline in serum concentration 1
Digoxin is concentrated in tissues with a large apparent volume of distribution
Approximately 25% of digoxin in plasma is bound to protein 1
Distribution correlates best with lean body weight rather than total body weight 1
Metabolism and Elimination
- Only a small percentage (16%) of digoxin is metabolized
- Metabolism is not dependent on the cytochrome P-450 system 1
- Elimination follows first-order kinetics (amount eliminated is proportional to total body content)
- 50-70% of digoxin is excreted unchanged in the urine in patients with normal renal function 1
- Renal excretion is proportional to glomerular filtration rate and largely independent of urine flow 1
Factors Affecting Digoxin Half-Life
Renal Function
- The clearance of digoxin correlates primarily with renal function (creatinine clearance)
- In patients with anuria, half-life is prolonged to 3.5 to 5 days 1
- Dose adjustments are necessary in patients with renal impairment
Drug Interactions
- Certain drugs may increase plasma digoxin levels:
- Amiodarone
- Diltiazem
- Verapamil
- Certain antibiotics
- Quinidine 2
Clinical Implications
- Steady state may take longer to achieve in patients with renal impairment 2
- Therapeutic serum concentration should be between 0.6 and 1.2 ng/mL 2
- Digoxin is not effectively removed by dialysis, exchange transfusion, or cardiopulmonary bypass 1
Monitoring Considerations
- The digoxin concentration should be checked early during chronic therapy in patients with normal renal function 2
- Signs of toxicity include confusion, nausea, anorexia, and disturbance of color vision 2
- In case of toxicity, digoxin-specific Fab antibody fragments may be considered for ventricular arrhythmias 2
Dosing Implications
- Starting dose: loading doses generally not required in stable patients with sinus rhythm
- Maintenance dose: 0.25 mg daily in adults with normal renal function
- Reduced doses (0.125 or 0.0625 mg daily) should be used in elderly patients and those with renal impairment 2
Understanding the half-life of digoxin is crucial for appropriate dosing and monitoring, especially when adjusting for renal function or when managing potential drug interactions.