Cardiac glycosides Cardiac glycosides

The cardiac glycosides are used for their positive inotropic effects by enhancing calcium influx into myocardial cells increasing the force of contraction of the myocardium, and also for their negative chronotropic effect in reducing the rate of myocardial contraction. In elderly people and horses, the main indication for cardiac glycosides is for heart failure in the presence of atrial fibrillation. In cats and dogs the main indication is for supraventricular arrhythmias, or for myocardial failure (i.e. congestive heart failure).

Digoxin is cleared mainly via glomerular filtration in the kidney (half-life 20-35 hours) whereas digitoxin is cleared via the liver (half-life 8-12 hours) thus concomitant organ disease should be considered and screened out before their administration.

In humans digoxin clearance in the elderly is equivalent to the creatinine clearance and its half-life is prolonged in elderly patients. The same is probably true in geriatric dogs and cats, thus sensitivity to digoxin toxicity may be greater in older animals. It has been suggested that the dose of digoxin should be halved if azotaemia is present but a better approach would be to give digitoxin instead.

Digitoxin can be cleared by the liver even in the presence of liver disease

Both digoxin and digitoxin have a narrow therapeutic margin and there are many factors that may increase the sensitivity of a patient to toxic side-effects including: age, hypokalaemia, hypomagnesaemia, hypercalcaemia, acidosis, calcium channel blockers, antibiotics, renal failure, hypothyroidism.

Special care is needed in the administration of these drugs to geriatric patients, and screening for subclinical conditions which might enhance toxic side-effects or alter efficacy is mandatory.

The value of monitoring serum digoxin concentrations has been questioned because of overlap in digoxin concentrations seen in groups of patients with and without toxic side-effects, and also because false elevations may be seen in sera from patients with chronic renal failure or liver disease.


Renal impairment, sinus or AV node disease (arrhythmias).