Cardiac Glycosides (Digoxin)

Cardiac Glycosides

Prototype and Other Medications

The only cardiac glycoside available in the United States is digoxin. Digoxin is made from two types of foxglove plant, digitalis purpurea and digitalis lanata. That is why digoxin is sometimes called digitalis.

Expected Pharmacologic Action

Cardiac glycosides work by inhibiting an enzyme called sodium, potassium-ATPase, which then causes calcium to collect within the cells of the heart. Calcium assists the proteins, actin and myosin, to interact, and this increases cardiac contractility. In general, an increase in contractility also increases the force of contraction in the heart, which is called a positive inotropic effect. Enhanced contractility increases cardiac output, and improved cardiac output increases blood flow to the kidneys. A decrease in angiotensin II in response to the change in arterial pressure in turn decreases vasoconstriction. This allows the kidneys to excrete extra water and sodium. Cardiac glycosides also increase parasympathetic tone, via the vagal nerve, and decrease sympathetic nervous system tone, which causes the heart to be less effective as a pump. The increase in vagal tone slows conduction through the AV node, thus slowing the heart rate.

Adverse Drug Reactions

The most dangerous adverse effects of digoxin are cardiac dysrhythmias, which are especially likely if toxicity occurs. Any dysrhythmia can occur with atrioventricular block, also called AV block, being most common. Early warning signs of toxicity include nausea, anorexia and vomiting and should be taken seriously. Fatigue and visual changes, such as blurred vison, halos around dark objects, or yellow tinge to vison are also experienced by clients taking digoxin. Recent research indicates that digoxin may be more harmful to women than therapeutic, causing increased mortality.

Interventions

When giving cardiac glycosides, it’s important to recognize that vomiting may cause hypokalemia, which increases the risk for digoxin toxicity. Monitor and report gastrointestinal (GI) manifestations as well as CNS effects. Take the client’s apical pulse for a full minute before administering digoxin and withhold the medication if the pulse falls below prescribed parameters, which is usually 60 beats per minute in adults. Monitor digoxin levels frequently, and be sure to monitor serum potassium levels. Be prepared to administer potassium for low or borderline low values. Monitor the cardiac rhythm for changes and treat any dysrhythmias per protocol. For severe digoxin toxicity, digoxin immune Fab (antigen-binding fragments) is administered IV as an antidote to neutralize digoxin.

Administration

Digoxin is available as oral tablets, capsules, and elixir as well as for IV use. Give the oral form with or without food. Tablets may be crushed and mixed with food if necessary. The IV form may be administered directly over at least 5 minutes. Monitor the IV site carefully for infiltration, which can cause tissue damage.

Client Instructions

When giving instructions regarding digoxin, tell clients to report nausea, loss of appetite, vomiting, headache or visual disturbances, and heart palpitations to the provider. Clients need to learn to monitor their pulse rate and report either a decrease or increase outside the prescribed parameters. To decrease the risk of either toxic or low serum levels, instruct clients to take digoxin at the same time each day, and not to skip or double a dose. Since low serum potassium increases the risk for digoxin toxicity, ask clients to report signs of hypokalemia such as muscle weakness, and to recognize situations where they can lose potassium such as vomiting or diarrhea.

Safety Alert

It is critical that clients taking digoxin know the signs and manifestations of digoxin toxicity. Clients may take this medication on a long-term basis, so they will be self-administering it for months to years. You need to instruct clients on how to take their pulse, remind them to always take it prior to their daily dose of digoxin, and what parameters in regard to the rate are acceptable. They need to be aware that the onset of anorexia, nausea and/or vomiting, fatigue, and confusion can indicate that the serum digoxin is above the therapeutic level. A pulse rate that is irregular, if pulse is typically regular, and below 60 beats per minute are also indications of digoxin toxicity. Instruct them to immediately report to the provider any findings that are consistent with digoxin toxicity and anticipate that a serum digoxin level will need to be drawn before they can continue therapy.

Contraindications and Precautions

Cardiac glycosides are contraindicated in clients with uncontrolled ventricular dysrhythmias, AV block, or severe heart disease. They are also contraindicated if the client has digoxin toxicity. Take precautions when giving digoxin to clients with hypokalemia, hypercalcemia or those taking diuretics. Also give digoxin cautiously to infants, children, and older adults who are all at increased risk for digoxin toxicity. Since digoxin has a high risk of toxicity it is no longer considered the medication of choice for treating heart failure.

Interactions

Digoxin interacts with many other medications. Erythromycin and some other antibiotics increase digoxin levels, as do other antidysrhythmics such as verapamil, quinidine, amiodarone, and flecainide. Decrease digoxin dosage if concurrent administration is necessary. Also, be aware that thiazide and loop diuretics increase the risk for digoxin toxicity by decreasing potassium levels. Herbal ginseng increases risk of digoxin toxicity, while St. John’s wort decreases digoxin levels. Note that the above interactions do not comprise a complete list. It is important to check for additional interactions when you give other medications along with digoxin.