Effect of physical activity on the day-to-day variation in serum digoxin concentration

Physical exercise has been found to increase digoxin binding in working skeletal muscle along with a concomitant decrease in serum digoxin concentration. In a recent study on healthy volunteers, moderate physical activity during maintenance digoxin treatment was shown to decrease the renal excretion of digoxin secondary to this redistribution of the drug, thereby affecting the body content of digoxin. In the present study the influence of changes in everyday physical activities, carried out during a 10-h period after ingestion of the daily maintenance digoxin dose, on the steady-state serum digoxin concentration (24 h after the last dose) was studied in 10 digoxin-treated outpatients (61-81 years of age). Compared to normal daily activity, complete bed rest for 10 h after ingestion of the maintenance dose did not affect the steady-state serum digoxin concentration. The lack of such an influence may be explained either by a low degree of everyday physical activity in the investigated patients or to a compensatory increase in the renal excretion of digoxin during the night preceding the serum digoxin measurement. Thus, standardization of physical activity 1-2 h before blood sampling is adequate when analysing the serum digoxin concentration in elderly outpatients.