Digoxin Dosing Calculator

Digoxin inhibits the sodium-potassium ATPase in cardiac myocytes. The resulting rise in intracellular sodium reduces the gradient driving the sodium-calcium exchanger, calcium accumulates in the sarcoplasmic reticulum, and contractility increases. A separate vagomimetic action slows conduction through the atrioventricular node, which is the property exploited for ventricular rate control in atrial fibrillation. Dosing is a two-step calculation because the drug has a very large apparent volume of distribution, several hundred litres, reflecting extensive binding to skeletal muscle ATPase rather than fat. The Jusko-Koup expression used here, 226 plus 298 x CrCl divided by (29.1 plus CrCl), scaled to lean body mass, captures the fact that distribution volume itself contracts as renal function falls. Total body stores are the target concentration multiplied by that volume; the loading dose delivers those stores, split as half then two quarters at six to eight hour intervals, and the maintenance dose replaces only the fraction eliminated each day. Because 60 to 80 percent of digoxin is excreted unchanged by the kidney, half-life stretches from roughly 36 to 48 hours at normal clearance to several days in advanced renal impairment, so steady state may not arrive for two to three weeks and levels drawn earlier mislead. The therapeutic window is narrow and indication-specific: 0.5 to 0.9 ng/mL for heart failure, up to roughly 2.0 ng/mL historically for rate control, with toxicity common above 2.0. Hypokalaemia is the critical modifier, since potassium and digoxin compete for the same pump site and a low potassium produces toxicity at otherwise acceptable concentrations.

Digitalis entered documented medicine through William Withering, a Birmingham physician who investigated a Shropshire herbal remedy for dropsy and published An Account of the Foxglove and Some of Its Medical Uses in 1785. Withering identified Digitalis purpurea leaf as the active ingredient, described dosing to the point of nausea and slowed pulse, and catalogued the toxic effects including disturbed colour vision. Digoxin itself was isolated from the related species Digitalis lanata by Sydney Smith at Burroughs Wellcome in 1930, and its better-behaved absorption made it the dominant cardiac glycoside of the twentieth century. Rational dosing became possible only when concentrations could be measured. Thomas Smith, Vincent Butler and Edgar Haber developed a radioimmunoassay for serum digoxin at Harvard in 1969, revealing how often apparently standard doses were toxic. William Jusko and Jerome Koup published the pharmacokinetic model relating volume of distribution to creatinine clearance in the mid-1970s, which is the basis of this calculation. The same group at Harvard developed digoxin-specific antibody fragments, first used successfully in a poisoned patient in 1976 and now standard antidotal therapy. The final reframing came from the Digitalis Investigation Group trial published in the New England Journal of Medicine in 1997, which randomised about 6,800 patients with heart failure and found no mortality benefit despite reduced hospitalisation; later analyses of those data by Rathore and colleagues linked concentrations at or below 0.9 ng/mL to better outcomes, driving today's much lower targets.