The most commonly used preparation of digitalis is digoxin,which is obtained from the leaves of Digitalis lanata, a common flowering plant called "foxglove." Digitalis compounds are potent inhibitors of cellular Na ⁺ /K ⁺ -ATPase. The words digitalis and digoxin are widely used interchangeably.

While taking digoxin, it is very important that an appropriate dose be utilized as digoxin has a narrow therapeutic range (goal 0.8-2.0 ng/mL in most patients) and will usually be ineffective (<0.8) or toxic (>2.0) outside of these ranges.

The formula provided can be utilized in 2 settings:

• Calculate how much digoxin to administer to load a patient currently not on digoxin with to achieve a desired serum level of digoxin
• Calculate how much digoxin a patient took in overdose based upon their actual serum level to then determine how much digoxin FAB to administer (each vial binds ~0.5 mg of digoxin)

The formula:

DBL = Digoxin Conc*5.6*Weight/1000

• DBL = Digitalis Body Load in mg
  
• Digoxin Conc = Digoxin Concentration in ng/ml
• The 5.6 is a volume of distribution correction factor in liters/kg
• Weight is in kg

An additional factor if orally dosed is the bioavailability of digoxin administered orally, which is ~0.8.  So 1 mg of oral digoxin will have ~0.8 mg absorbed.

References

DigiFab ^TM (Digoxin Immune FAB (Ovine) FDA label.  Available at http://www.fda.gov/CBER/label/digpro083101LB.pdf (last accessed 6 July 2008)

Jelliffe RW. An improved method of digoxin therapy. Ann Intern Med.1968;69(4):703-17.

Jelliffe RW. Digitalis therapy: simple formulas to plan and adjust dosage regimens. MD Comput. 1984;2:36-43.

Jelliffe RW, et al. Reduction of digitalis toxicity by computer-assisted glycoside dosage regimes. Ann Intern Med 1972;77: 891-906.

Hurstig MJ, et al. Determination of Free Digoxin Concentrations in Serum for Monitoring Fab Treatment of Digoxin Overdose. Clin Chem. 1987;33(9):1652-5.