Digoxin Dosing Calculator
Calculate digoxin loading and maintenance doses from weight, renal function, and lean body mass.
Formula
Vd = (226 + 298 x CrCl/(29.1 + CrCl)) x LBM/70 | Loading = Css x Vd | Maintenance = TBS x Daily Loss%
Volume of distribution (Vd) is calculated using the Jusko-Koup equation adjusted for lean body mass. Total body stores (TBS) equal the desired concentration times Vd. Loading dose equals TBS adjusted for bioavailability (70% oral, 100% IV). Maintenance dose replaces the daily elimination fraction. Creatinine clearance is estimated via Cockcroft-Gault.
Worked Examples
Example 1: Atrial Fibrillation Rate Control Dosing
Problem: A 65-year-old male (70 kg, 170 cm) with atrial fibrillation needs digoxin for rate control. Serum creatinine: 1.2 mg/dL. Target level: 1.0 ng/mL.
Solution: LBM (male) = 50 + 2.3 x ((170/2.54) - 60) = 50 + 2.3 x 6.93 = 65.9 kg\nCrCl = ((140-65) x 70) / (72 x 1.2) = 5250/86.4 = 60.8 mL/min\nVd = (226 + 298 x 60.8/(29.1+60.8)) x 65.9/70 = (226 + 201.5) x 0.941 = 402.5 L\nTBS = 1.0 x 402.5 = 402.5 mcg\nLoading IV = 0.403 mg (give 0.2 mg, then 0.1 mg x2)\nDaily Loss = ~25%\nMaintenance oral = ~0.18 mg/day\nNearest tablet: 0.1875 mg (3/4 of 0.25 mg tab)
Result: Loading: 0.4 mg IV divided | Maintenance: 0.1875 mg oral daily | Predicted SS: ~1.0 ng/mL
Example 2: Heart Failure Patient with Renal Impairment
Problem: A 78-year-old female (55 kg, 157 cm) with heart failure. Serum creatinine: 1.8 mg/dL. Target level: 0.7 ng/mL.
Solution: LBM (female) = 45.5 + 2.3 x ((157/2.54) - 60) = 45.5 + 2.3 x 1.8 = 49.6 kg\nCrCl = ((140-78) x 55) / (72 x 1.8) x 0.85 = 3410/129.6 x 0.85 = 22.4 mL/min\nVd = (226 + 298 x 22.4/(29.1+22.4)) x 49.6/70 = (226 + 129.6) x 0.709 = 252.0 L\nTBS = 0.7 x 252.0 = 176.4 mcg\nLoading IV = 0.176 mg\nReduced CrCl means slower elimination: ~15% daily loss\nMaintenance = 0.176 x 0.15 / 0.7 = 0.038 mg oral = 0.0625 mg every other day
Result: Loading: 0.175 mg IV divided | Maintenance: 0.0625 mg oral every other day | Monitor levels closely
Frequently Asked Questions
How is the digoxin loading dose calculated from patient parameters?
The digoxin loading dose is calculated by first determining the volume of distribution, which depends on lean body mass and renal function. The volume of distribution in liters is estimated using the Jusko-Koup equation: Vd = (226 + 298 x CrCl / (29.1 + CrCl)) adjusted for lean body mass. The total body stores needed are then calculated by multiplying the desired serum concentration in nanograms per milliliter by the volume of distribution in liters. For IV administration, the loading dose equals the total body stores since bioavailability is 100 percent. For oral tablets, the dose is divided by 0.7 to account for 70 percent bioavailability. The loading dose is typically administered in divided doses: half the total dose initially, then one quarter at 6 to 8 hours, and the final quarter at 6 to 8 hours later, to reduce the risk of toxicity from rapid administration.
Why is lean body mass used instead of total body weight for digoxin dosing?
Digoxin is highly lipophobic and distributes primarily into lean tissue, skeletal muscle, and organs rather than adipose tissue. Using total body weight in obese patients would overestimate the volume of distribution and result in supratherapeutic doses that increase toxicity risk. Lean body mass provides a more accurate estimate of the tissue compartment where digoxin actually distributes. The Devine formula is commonly used to estimate lean body mass: for males it equals 50 kg plus 2.3 kg per inch over 5 feet, and for females it equals 45.5 kg plus 2.3 kg per inch over 5 feet. In significantly obese patients where total body weight exceeds 120 percent of ideal body weight, an adjusted body weight using 40 percent of the excess weight is used for creatinine clearance estimation while lean body mass is used for volume of distribution calculations.
What is the therapeutic range for digoxin and why does it differ by indication?
The therapeutic range for digoxin varies based on the clinical indication. For heart failure, the DIG trial and subsequent analyses demonstrated that serum digoxin concentrations of 0.5 to 0.9 ng/mL provide optimal benefit with reduced mortality, while levels above 1.0 ng/mL were associated with increased mortality despite symptom improvement. For atrial fibrillation rate control, higher levels between 0.8 and 2.0 ng/mL may be needed to achieve adequate ventricular rate reduction, though current guidelines recommend targeting the lower end of this range. The narrow therapeutic index of digoxin means that the toxic concentration of approximately 2.0 ng/mL is only about twice the lower therapeutic level. This narrow margin makes careful dosing, monitoring, and awareness of drug interactions and electrolyte abnormalities essential for safe digoxin use.
How does renal function affect digoxin dosing and elimination?
Renal function is the single most important determinant of digoxin maintenance dosing because approximately 60 to 80 percent of digoxin is eliminated unchanged by the kidneys through glomerular filtration and tubular secretion. The remaining 20 to 40 percent undergoes hepatic metabolism and biliary excretion. In patients with reduced creatinine clearance, digoxin elimination is significantly prolonged, leading to accumulation and potential toxicity if doses are not reduced. For a patient with normal renal function and a creatinine clearance of 100 mL/min, the digoxin half-life is approximately 36 to 48 hours. In a patient with severe renal impairment and creatinine clearance of 20 mL/min, the half-life may extend to 4 to 6 days. Digoxin Dosing Calculator uses the Cockcroft-Gault equation to estimate creatinine clearance and adjusts both the volume of distribution and daily maintenance dose accordingly.
What is the Cockcroft-Gault equation and why is it used for digoxin dosing?
The Cockcroft-Gault equation estimates creatinine clearance from serum creatinine, age, weight, and sex: CrCl = ((140 minus age) times weight in kg) divided by (72 times serum creatinine in mg/dL), multiplied by 0.85 for females. Despite the availability of newer GFR estimation equations like CKD-EPI, the Cockcroft-Gault equation remains the standard for drug dosing because most pharmacokinetic studies that established drug dosing guidelines used this equation. It provides creatinine clearance rather than GFR, and these values are not interchangeable. Creatinine clearance overestimates GFR because creatinine is both filtered and secreted by the tubules. For digoxin specifically, creatinine clearance correlates well with actual digoxin renal clearance. When using this equation, the weight input should be adjusted body weight for obese patients and actual body weight for non-obese patients.
What are the signs and symptoms of digoxin toxicity?
Digoxin toxicity manifests across multiple organ systems with a wide range of symptoms. Cardiac symptoms are the most dangerous and include virtually any arrhythmia, with the most characteristic being paroxysmal atrial tachycardia with AV block, bidirectional ventricular tachycardia, and new onset of irregular rhythm in a previously regular pattern. Gastrointestinal symptoms include nausea, vomiting, anorexia, and abdominal pain, often occurring as early warning signs before cardiac toxicity. Neurological symptoms include confusion, drowsiness, dizziness, headache, and the classic visual disturbance of yellow-green halos around lights called xanthopsia. Risk factors that predispose to toxicity at therapeutic levels include hypokalemia, hypomagnesemia, hypercalcemia, hypothyroidism, advanced age, renal dysfunction, and drug interactions with amiodarone, verapamil, and quinidine.