Overview

Not uncommon because of digoxin's narrow therapeutic index and prevalence of renal impairment in elderly patients with cardiac disease.

Signalment

• Dog and cat
• More common in geriatric patients

Signs

Causes & Risk Factors

• Renal disease-impairs digoxin elimination.
• Chronic pulmonary disease-results in hypoxia and acid–base disturbances.
• Obesity-if dosage not calculated on lean body weight.
• Hypokalemia, hypercalcemia, hypomagnesemia, and hypoxia predispose to arrhythmias.
• Drugs and conditions that alter digoxin metabolism or elimination (e.g., quinidine and hypothyroidism).
• Rapid IV digitalization.
• Overdosage or accidental ingestion of owner's medication.
• Administration of diuretic leading to hypokalemia.

Differential Diagnosis

• Arrhythmias and conduction disturbances-may reflect structural heart disease, not digoxin toxicity.
• Anorexia-common in animals with heart failure.

CBC/Biochemistry/Urinalysis

Animals with hypokalemia, hypercalcemia, hypomagnesemia, and renal failure are predisposed to toxicity.

Other Laboratory Tests

• Consider checking thyroid status.
• Obtain digoxin serum concentration 8–10 hours after an oral dose-therapeutic range is 0.5–1.5 ng/mL. A recent study in humans found digoxin levels greater than 1 mg/ml were associated with increased mortality; not all patients with concentrations >1.5 ng/mL have signs of toxicity; some with values in the normal range have signs of toxicity, especially if hypokalemic. To minimize the risk of toxicity, I aim to achieve a digoxin level between 0.5 and 1 ng/mL.

Imaging

N/A

Diagnostic Procedures

Drug(s)

• Treat clinically important bradyarrhythmias with atropine or a temporary transvenous pacemaker.
• Treat clinically important ventricular arrhythmias with lidocaine or phenytoin; phenytoin also reverses high-degree AV block.
• Digoxin-binding antibodies (e.g., Digibind) rapidly drop digoxin concentration in critically ill animals; the use of these products is limited in veterinary practice by their exorbitant cost.
• Thyroxin supplementation if hypothyroidism confirmed.

Contraindications/Possible Interactions

• Avoid or discontinue drugs that slow digoxin elimination or metabolism (e.g., quinidine, verapamil, and diltiazem).
• Avoid drugs that could worsen conduction disturbances (e.g., beta-blockers and calcium channel blockers).
• Class 1A antiarrhythmic drugs (e.g., quinidine and procainamide) may worsen AV block.

See Also

• Atrioventricular Block, Complete (Third Degree)
• Atrioventricular Block, First Degree
• Atrioventricular Block, Second Degree-Mobitz I
• Atrioventricular Block, Second Degree-Mobitz II
• Ventricular Tachycardia

Abbreviations

• AV = atrioventricular
• ECG = electrocardiogram