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).