The dose of Digoxin for each patient has to be tailored individually according to age, lean bodyweight and renal function. Suggested doses are intended only as an initial guide.
The difference in bioavailability between injectable Digoxin and oral formulations must be considered when changing from one dosage form to another. For example, if patients are switched from oral to the I.V. formulation the dosage should be reduced by approximately 33%.
Adults and children over 10 years of age: Parenteral Loading:
Note: For use in patients who have not been given cardiac glycosides within the preceding two weeks.
The loading of parenteral Digoxin is 500 to 1000 micrograms (0.5 to 1.0 mg) depending on age, lean body weight and renal function.
The loading dose should be administered in divided doses with approximately half the total dose given as the first dose and further fractions of the total dose given at intervals of 4 to 8 hours. An assessment of clinical response should be performed before giving each additional dose. Each dose should be given by intravenous infusion over 10 to 20 minutes.
Maintenance Dose: (See equation.)
Click on icon to see table/diagram/image
The maintenance dosage should be based upon the percentage of the peak body stores lost each day through elimination. The following formula has had wide clinical use: C
cr is creatinine clearance corrected to 70 kg body weight or 1.73 m
2 body surface area. If only serum creatinine (S
cr) concentrations are available, a C
cr (corrected to 70 kg bodyweight) may be estimated in men as: (See equation.)
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NOTE: Where serum creatinine values are obtained in micromol/L these may be converted to mg/100 mL (mg %) as follows: Where 113.12 is the molecular weight of creatinine.
For women, this result should be multiplied by 0.85.
NOTE: These formulae cannot be used for creatinine clearance in children.
In practice, this will mean that most patients will be maintained on 0.125 to 0.25 mg digoxin daily; however, in those who show increased sensitivity to the adverse effects of digoxin, a dosage of 62.5 microgram (0.0625 mg) daily or less may suffice. Conversely, some patients may require a higher dose.
Neonates, infants and children up to 10 years of age (if cardiac glycosides have not been given in the preceding two weeks): In the newborn, particularly in the premature infant, renal clearance of digoxin is diminished and suitable dose reductions must be observed, over and above general dosage instructions.
Beyond the immediate newborn period, children generally require proportionally larger doses than adults on the basis of body weight or body surface area, as indicated in the schedule as follows.
Children over 10 years of age require adult dosages in proportion to their body weight.
Parenteral Loading: The parenteral loading dose in the above groups should be administered in accordance with the following schedule: Preterm neonates: <1.5 kg: 20 microgram/kg over 24 hours.
Preterm neonates 1.5 kg to 2.5 kg: 30 microgram/kg over 24 hours.
Term neonates to 2 years: 35 microgram/kg over 24 hours.
2 to 5 years: 35 microgram/kg over 24 hours.
5 to 10 years: 25 microgram/kg over 24 hours.
The loading dose should be administered in divided doses with approximately half the total dose given as the first dose and further fractions of the total dose given at intervals of 4 to 8 hours. An assessment of clinical response should be performed before giving each additional dose. Each dose should be given by intravenous infusion over 10 to 20 minutes.
If cardiac glycosides have been given in the two weeks preceding commencement of Digoxin therapy, it should be anticipated that optimum loading doses of Digoxin will be less than those recommended as previously mentioned.
Maintenance Dose: The maintenance dose should be administered in accordance with the following schedule: Preterm neonates: daily dose = 20% of 24-hour loading dose (intravenous or oral).
Term neonates and children up to 10 years: daily dose = 25% of 24-hour loading dose (intravenous or oral).
These dosage schedules are meant as guidelines and careful clinical observation and monitoring of serum digoxin levels should be used as a basis for adjustment of dosage in these paediatric patient groups.
If cardiac glycosides have been given in the two weeks preceding commencement of Digoxin therapy, it should be anticipated that optimum loading doses of Digoxin will be less than those recommended as previously mentioned.
Use in the elderly: The tendency to impaired renal function and low lean body mass in the elderly influences the pharmacokinetics of Digoxin such that high serum digoxin levels and associated toxicity can occur quite readily, unless doses of Digoxin lower than those in non-elderly patients are used. Serum digoxin levels should be checked regularly and hypokalaemia avoided.
Dose recommendations in renal disorder or with diuretic therapy: See Precautions.
Monitoring:
Serum concentrations of digoxin may be expressed in conventional units of nanogram/mL (ng/mL) or SI Units of nanomol/L (nmol/L). To convert ng/mL to nmol/L, multiply ng/mL by 1.28.
The serum concentration of digoxin can be determined by radioimmunoassay. Blood should be taken 6 hours or more after the last dose of Digoxin. Several post hoc analyses of heart failure patients in the Digitalis Investigation Group trial suggest that the optimal trough digoxin serum level may be 0.5 ng/mL (0.64 nanomol/L) to 1.0 ng/mL (1.28 nanomol/L).
Digoxin toxicity is more commonly associated with serum digoxin concentration greater than 2 ng/mL. However, toxicity may occur with lower digoxin serum concentrations. In deciding whether a patient's symptoms are due to digoxin, the patient's clinical state together with the serum potassium level and thyroid function are important factors.
Other glycosides, including metabolites of digoxin, can interfere with the assays that are available and one should always be wary of values which do not seem commensurate with the clinical state of the patient.