Digoxin dosages

R4. Redfors, A., Plasma digoxin concentration—its relation to digoxin dosage and clinical effects in patients with atrial fibrillation. Brit. Heart J. 34, 383-391 (1972). 

Before discharge, review the digoxin dosages carefully with the patient, and make sure he or she knows how to take the drug as prescribed... 

Digoxin is not extensively metabolized in humans almost two thirds is excreted unchanged by the kidneys. Its renal clearance is proportional to creatinine clearance and the half-life is 36-40 hours in patients with normal renal function. Equations and nomograms are available for adjusting digoxin dosage in patients with renal impairment. 

Spironolactone inhibits the active tubular secretion of digoxin by about 25% and in some cases digoxin dosages may have to be reduced (295). 

Doherty, J. E. et al., A multicenter evaluation of the absolute bioavaUability of digoxin dosage forms, Curr. Ther. Res., 35 301-306, 1984. 

Itraconazole causes a marked increase in serum digoxin levels. Toxicity may occur unless the digoxin dosage is suitably reduced. Theoretically, itraconazole might also oppose the positive inotropic effects of digoxin. 

Information seems limited to the studies cited. Nevertheless, the effects of concurrent use should be monitored very closely, and the digoxin dosage should be adjusted as necessary. 

A very well established interaetion of clinical importance. Monitor the ef-feets ofeoneurrent use and reduce the digoxin dosage appropriately in order to avoid toxieity. Most patients appear to be affected and dosage reductions in the range 15 to 70% were found necessary in one of the studies cited. The data available suggest that the extent of the rise may possibly depend on the propafenone serum concentration rather than on its dose. ... 

An established interaction of clinical importance but only moderately documented. Monitor the effects of concurrent use (e.g. for bradycardia) and reduce the digoxin dosage where necessary. Some patients may have a substantial increase in serum digoxin levels whereas others will have only a small or moderate rise. There appear to he no case reports of digoxin toxicity arising from this interaction. 

Another report describes 2 patients undergoing renal dialysis whose digoxin dosage needed to be doubled while they were taking rifampicin, and similarly reduced when the rifampicin was withdrawn. This confirms an earlier report. ... 

Information seems to be limited to this study in healthy subjects but it suggests that the digoxin dosage should he reduced if valspodar is given. An initial 50% reduction has been suggested. 

It is not known whether these changes would be sustained during chronic concurrent use, or the extent to which the digoxin dosage might need to be increased. More study is needed to find out if this interaction is of practical importance. 

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