Application Of Drug Therapy In Managing Heart Failure Patients (Cardiac Glycosides)
Digoxin is the most popular and widely used cardiac glycoside in use, though several other preparations are available, e.g, Lanatoside-C, oubain, gitalin, etc. They are used only for restricted indications.
Digoxin is available in the form of tablets of 0.0625mg, 0.125mg and 0.25mg and as an alcoholic solution (0.25mg/ml) for intravenous injection. Effect of the oral dose begins to manifest in 30-45 minutes. The drug is cumulative on repeated administration. The digitalising dose is the total dose of digoxin required to produce the optimum therapeutic effect. Following a full digitalising dose maximal action is seen in 3-5 hours and some activity may persist for up to 2 to 6 days. The drug can be given orally as a rapid loading dose or as repeated smaller doses. Rapid digitalisation is done by giving 0.5 to 0.75 mg initially followed by 0.25 to 0.5 mg at 6 to 8 hours intervals. In the majority of patients, the digitalising dose is 2 to 2.5mg. In slow digitalisation, 0.25 mg is given once or twice a day and the serum level reaches the desired maximum in 5 to 7 days. After initial digitalisation, the serum level is kept up by maintenance therapy given in a dose of 0.125 to 0.25 mg daily for five to six days weekly.
Intravenous digoxin is given in emergencies such as acute left ventricular failure or when the drug is not tolerated by the oral route. Dose of 0.5 to 1mg should be given slowly (over 3-5 min). After a loading dose, the effect starts within minutes, but the full effect is seen in 2 to 3 hours. Repetition of dose is considered after 4 to 6 hours. Clinical effects of digoxin are:
- reduction in heart rate
- improvement in cardiac output; and
- clearance of edema.
Diuresis results from improvement in cardiac function and a direct renal effect.
Tablets of digoxin
Indications, contraindications & Toxicity
Indications for digoxin: In several clinical conditions it is a very effective and useful drug. These conditions are: atrial fibrillation with rapid ventricular rate and acute or chronic low output cardiac failure where there is no mechanical obstruction to blood flow. This drug is less effective in high output failure states.
Contraindications for digoxin: Acute myocardia infarction, acute myocarditis, second degree or unstable heart block, ventricular tachycardia and frequent ventricular ectopics are contraindications. Full intravenous dose should not be given to patients who have received digoxin therapy within the preceding two weeks. Patients with contraindications for digoxin should be managed with diuretics and other inotropic agents.
Toxic effects: The therapeutic ration of digoxin is small, so that the clinically effective and toxic doses overlap. Therefore even though digoxin is a very effective and useful drug, the serious toxic effects make it dangerous as well. Elderly subjects and those with renal impairment are particularly at risk. Hypokalemia aggravates the toxic effects of digitalis. Mild toxic effects include nausea, vomiting, headache, drowsiness, visual disturbances and disorientation. Cardiac toxicity manifests as the drug accumulates. Early signs are ventricular actopics and pulsus begeminus (due to the occurrence of alternate ectopic beats).
More serious arrhythmias are paroxysmal atrial tachycardia with block (PATB), different grades of atrioventricular blocks, runs of ventricular extrasystoles, ventricular tachycardia and ventricular fibrillation. these may prove fatal and deaths due to digitalis toxicity are not incommon, unless proper care is taken. The toxic effects correlate with serum levels of digoxin.
The bioavailability of digoxin from different preparations is very variable. Therefore in all modern hospitals, serum digoxin levels are monitored (2 ng/ml and above are toxic). Where facilities for serum levels are not available, regular ECG monitoring should be undertaken. Digitalis produces diagnostic changes in the ECG.
Treatment of digoxin toxicity: At the first signs of cardiac toxicity, the drug should be withdrawn and potassium chloride should be administered orally. Frequent ventricular ectopics, PATB and ventricular tachycardia may have to be treated with propanolol or phenytoin. At present digoxin antibodies have been employed with benefit to manage severe grades of toxicity.
© 2013 Funom Theophilus Makama