Digoxin-d is intended for use as an internal standard for the quantification of digoxin by GC- or LC-MS. Digoxin is a cardiac glycoside and metabolite of digitoxin that binds to and inhibits the Na/K-ATPase in cardiac tissues in an ATP- and Mg-dependent manner. This inhibition results in loss of the transmembrane Na gradient, which decreases activity of the Na/Ca exchanger, increasing intracellular Ca levels, inotropy, and cardiac force. It increases activity of mitochondrial ATPase and actomyosin ATPase in rat hearts, which is directly correlated with increased myofibrillar contractile strength. In vivo, digoxin also decreases right atrial pressure and increases cardiac output in a canine model of congestive heart failure produced by pulmonary artery constriction. Formulations containing digoxin have been used to treat atrial fibrillation.