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Digoxin and the other digitalis glycosides have direct effects on cardiac muscle and affect the electrical conduction in the heart. Digoxin increases cardiac output by increasing myocardial contractility. It also decreases sympathetic tone and as a result causes increased diuresis and a reduction of edema. The overall result is a reduction in heart size, heart rate, blood volume, and pulmonary and venous pressures.
The electrocardiac effects of digitalis include the
slowing of the conduction velocity at the AV node, and a prolonged effective
refractory period. Other electrocardiographic effects are an increased
PR interval, decreased QT interval, and depression of the ST segment.
Digoxin has traditionally been used in the treatment of congestive heart failure in both dogs and cats. It is usually used in conjunction with other medications including diuretics, and angiotensin converting enzyme (ACE) inhibitors.
Although digoxin has improved and probably prolonged
the quality of life for many veterinary cardiac patients, it remains a
drug that requires careful monitoring. It has a narrow margin of safety,
and there is significant variation in absorption between patients. Cats
in particular have a high incidence of toxicity although toxicity also
occurs in approximately 25% of dogs. The absorption of digoxin may be
affected by food, and by the formulation of the medication. It is usually
given on an empty stomach and the dose may need readjustment with any
change in formulation or manufacturer. Cats frequently dislike the taste
of digoxin elixir, and some investigation of flavoring alternatives is
helpful. Digoxin dosing should be based on lean body weight with adjustment
for ascites and fat. Digoxin is excreted by the kidneys and the dose may
need to be adjusted downward for animals with renal disease. Serum digoxin
levels should be followed carefully in both dogs and cats particularly
when starting on treatment. With the advent of newer cardiac medications,
some veterinary cardiologists are less likely to use digoxin as a first
line drug.
• The most prominent adverse side effects may be due to digitalis toxicity. They may include both cardiac and GI symptoms. The cardiac symptoms can resemble worsening of the underlying heart disease making it difficult to differentiate. It may be necessary to monitor serum digoxin levels and possibly discontinue therapy while determining the underlying cause.
• Adverse cardiac effects include many different arrhythmias: heart block (complete and incomplete), paroxysmal atrial or ventricular tachycardias, multifocal premature ventricular contractions, bigeminy, and ST segment changes.
• Gastrointestinal side effects include anorexia, nausea, and diarrhea.
• Other side effects include central nervous system disturbances,
unsteady gait, and depression. Collie breeds may be more sensitive to
CNS effects.
• Digoxin should not be used in animals with ventricular fibrillation, or those with digitalis intoxication.
• Digoxin may be used with caution in the following conditions: heart failure with glomerulonephritis, idiopathic subaortic stenosis, acute myocardial infarction, myocarditis, myxedema, chronic obstructive pericarditis, ventricular tachycardias, premature ventricular contraction, incomplete AV block, severe pulmonary disease or hypoxia and carotid sinus sensitivity.
• Digoxin is not generally recommended for use in cats with feline hypertrophic cardiomyopathy.
• Digoxin may be used in some patients with bradycardia or complete AV block provided that the block was not caused by digoxin.
• Animals in atrial fibrillation should be taken off digoxin at least 1-2 days prior to attempts at electrical conversion.
• Animals with renal or thyroid disease may require more frequent monitoring of electrolytes and digoxin levels. Animals with serum electrolyte abnormalities, particularly hypokalemia, may require smaller doses and should be monitored very carefully.
• There are many important drug interactions for digoxin. Additional information should be sought when using digoxin with the following drugs:
• Drugs that decrease digoxin absorption: antacids, cimetidine, metoclopramide, oral neomycin, some chemotherapy agents and penicillamine.
• Drugs that increase digoxin levels, or slow the elimination of digoxin: diazepam, quinidine, anticholinergics, succinycholine, verapamil, tetracycline, and erythromycin.
• Drugs that decrease serum potassium may predispose patients to digoxin toxicity: diuretics, amphotericin B, corticosteroids, ACTH, some laxatives, glucagon, dextrose or dextrose/insulin infusion, sodium polystyrene sulfonate.
• Thyroid replacement therapy and spironolactone may affect digoxin
levels.
• Symptoms of chronic toxicity are discussed under side effects. The drug may need to be temporarily stopped while re-evaluating the dose. Drugs used to treat digitalis-induced arrhythmias include lidocaine, phenytoin and atropine.
• Acute toxicity due to ingestion may be treated by stomach emptying and repeated use of activated charcoal.
Dr.
Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania.
She has a master's degree in animal science from the University of Delaware
and graduated from the University of Pennsylvania School of Veterinary Medicine
in 1982.
She began to develop her interest in client education and medical writing 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at www.exclusivelyequine.com
The information contained on this site
is general in nature and is intended for use as an informational aid. It does
not cover all possible uses, actions, precautions, side effects, or interactions
of the products shown, nor is the information intended as medical advice or
diagnosis for individual health problems or for making an evaluation as to the
risks and benefits of using a particular product. You should consult your doctor
about diagnosis and treatment of any health problems. Information and statements
have not been evaluated by the Food and Drug Administration ("FDA"),
nor has the FDA approved the products to diagnose, cure or prevent disease.
Wedgewood compounded veterinary preparations are not intended for use in food and food-producing animals.
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