Digitalis pharmacokinetics and therapy with respect to impaired renal function

The various cardiac glycosides differ significantly in their retention as a result of renal failure. In the case of digoxin, digitoxin, and strophanthin the retention is directly related to the normal renal clearance of these cardiac glycosides: Strophanthin has the highest clearance and the most marked prolongation of pharmacological action in renal failure, whereas digitoxin shows the lowest renal clearance and even in uremic patients a total elimination comparable to normal subjects as a result of increased hepatic clearance; digoxin takes an intermediate position. The quantity of a cardiac glycoside and its metabolites excreted by the kidneys depends, besides the renal clearance, on the plasma concentration which increases considerably during the first days after onset of treatment. From the daily dose approximately 90% of strophanthin, 70% of digoxin, 50% of digitoxin plus metabolites are excreted by normal kidneys under steady-state conditions. The efficiency of hemodialysis in the elimination of cardiac glycosides is low (3-5%) if estimated in relation to a single dose injected before dialysis and high (30-50%) if estimated in relation to the excretory capacity of normal kidneys during a period corresponding to the duration of a dialysis. During hemodialysis the plasma concentration of digoxin decreases as rapidly as in patients with normal renal function. Beside the efficiency of dialysis this finding may be explained by the decrease in the apparent volume of distribution of cardiac glycosides in patients with advanced renal failure; a reduced tissue protein binding seems likely to be the main reason for these changes in chronic renal insufficiency. A reduced volume of distribution and a reduced myocardial sensitivity are the main reasons for a very low predictability of the necessary individual maintenance dose of cardiac glycosides from the creatinine clearance. In patients with advanced renal insufficiency the tolerance to cardiac glycosides is reduced with respect to the daily dose, but it is rather increased in relation to the plasma concentration required to maintain the positive inotropic effect. The combination of hyperkalemia, hypermagnesemia, bypocalcemia and acidosis which is found almost exclusively with chronic renal failure, may explain the reduced myocardial sensitivity. Dosage regimens based on the measurement of creatinine-clearance are of little help in "effective digitalisation". Serial measurements of steady-state plasma concentration of cardiac glycosides may be the only way to reduce the risk of under- and overtreatment in patients with impaired renal function.