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Diagnose and prevent digoxin toxicity; prevent underdosage; monitor therapeutic drug level; prevention and therapy of cardiac arrhythmias.
Digoxin should not be confused with digitoxin. The elimination half-life in normal subjects is 37 hours. Patients with renal failure may have an endogenous digoxin-like material in their serum which makes digoxin measurements unreliable.
Be sure the patient is not on digitoxin instead of digoxin. Digitoxin is also an active component of digitalis leaf. The 2013 ACCF/HCA Guidelines for Management of Heart Failure suggest a therapeutic range of 0.5 -0.9 ng/mL for digoxin. Ninety percent of nontoxic patients have levels ≤2.0 ng/mL, 87% of toxic patients have levels >2.0 ng/mL. Levels >3.0 ng/mL in adults are strongly suggestive of overdosage. However, digitalis levels must always be interpreted in light of clinical and chemical data. Older, smaller patients require less digoxin. Proportionally lower loading doses are advocated in the elderly.1 The primary cause of digoxin toxicity in the aged is decreased renal function. Maintenance doses should be adjusted to the glomerular filtration rate.1 Renal failure, hypercalcemia, alkalosis, myxedema, hypomagnesemia, recent MI and other acute heart disease, hypokalemia, and hypoxia may increase sensitivity to the toxic effects of digoxin.
Quinidine may cause elevation of digoxin level by decreasing its excretion.2,3 It is recommended that serum digoxin concentration be measured before initiation of quinidine therapy and again in four to six days.
When confronted with unexpectedly low digoxin levels, consider thyroid disease, malabsorption, cholestyramine, colestipol, kaolin, pectin, neomycin, sulfasalazine, anticholinergic drug effects, and reduced intestinal blood flow from mesenteric arteriosclerosis. Consider as well congestive failure when low digoxin levels are encountered.
Patients with digitalis resistance may require larger doses and higher than usual serum levels (eg, patients with hyperthyroidism).
The probability that a patient will take a drug exactly as the physician has prescribed it has been shown to be hardly better than half. The probability is less among elderly patients getting a large number of medications. Measure trough, because of variability of peak interval.
FAB fragments of digoxin-specific sheep antibodies are available for the treatment of digoxin toxicities but should be limited to potentially life-threatening overdoses.
Compounds with “digoxin-like” immunoreactivity are present in a variety of clinical states associated with salt and fluid retention (eg, renal failure, pregnancy third trimester, congestive heart failure) and are also present during the first two weeks of neonatal life. These compounds (DLF−digoxin-like factors, etc) cross react with digoxin-specific immunoassays and give falsely elevated plasma digoxin levels. Laboratories must evaluate new antibody preparations for cross reactivity with the factors.
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