Effects of physical activity and immobilization on plasma digoxin concentration and renal digoxin clearance

Plasma digoxin concentration and renal digoxin clearance were determined during 2 hr of normal physical activity and during 2 hr of complete immobilization in eight healthy subjects on steady-state digoxin dosing. Mean plasma digoxin concentration rose from 0.64 +/- 0.13 ng/ml during physical activity to 1.04 +/- 0.19 ng/ml (63%) after 2 hr of rest. Resumption of physical activity resulted in gradual decline of plasma digoxin, and subsequent strenuous exercise reduced the value to preimmobilization level. Mean renal digoxin clearance was reduced from 168.4 +/- 18.7 ml/min during physical activity to 137.2 +/- 32.7 ml/min during rest whereas creatinine clearance was unchanged. The rise in plasma digoxin during rest is presumed to be due to changes in the binding of the drug to tissues such as skeletal muscles. Our findings indicate that attention should be given to the state of physical activity when kinetic studies are performed or when digoxin therapy is monitored by means of plasma digoxin analysis.