Abstract
The metabolic half-times and excretion rates of labeled cardiac glycosides are related to pharmacologic or therapeutic activity. Digoxin, a short-acting glycoside, is excreted primarily in the urine and has a serum half-time of 34hours. This function is related primarily to digoxin excretion. Digitoxin, a long-acting glycoside, has a serum half-time of 50+ hours and a physiologic half-time of 51/2 days. It is excreted primarily in the urine as metabolites of the parent compound. Renal failure compromises digoxin excretion and prolongs the digoxin serum half-time and thereby the pharmacologic activity of the drug. As excretion is related to creatinine clearance, dosage should be reduced in proportion to reduction in creatinine clearance in renal insufficiency. Hyper-thyroid patients have lower digoxin serum levels and hypothyroid patients higher digoxin serum levels when compared to euthyroid patients. This finding confirms the clinical impression of digitalis resistance in hyperthyoidism and sensitivity in myxe-dema. Appropriate alterations in digoxin dosage in thyroid disease are justified. Patients with liver disease and cor pulmonale have normal digoxin serum levels, half-times and excretion rates. Increased digitalis sensitivity in these patients is related to other factors. Knowledge of the serum half-time, route of excretion, and rate of excretion of the cardiac glycosides is now available. Application of this information to the patient receiving digitalis assists in avoiding over- and under-digitalization—both common clinical problems. If doubt exists regarding digitalis dosage, smaller, rather than larger, doses are suggested to avoid toxicity.
Published Version
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