The Importance of Digoxin

• It is important to understand the pharmacokinetics, uses, drug interactions, contraindications, and adverse effects of digoxin in order to be aware of how to safely administer the medication.
• Digoxin comes in various forms and dosages, and one must understand these in order to know which one should be used in specific situations.
• The nurse has an important role to play in assessment and patient education regarding the use of digoxin.

Digoxin is a cardiac glycoside derived from digitalis that exists naturally in foxglove plants. Clinically, it is classified as an antiarrhythmic drug that assists with making the heart beat stronger and with a more regular rhythm. It inhibits sodium potassium-activated adenosine triphosphatase – an enzyme that regulates the amount of sodium and potassium at the cellular level that results in increased sodium and potassium levels.

Digoxin promotes the movement of calcium from extracellular to intracellular cytoplasm, strengthens the myocardial contraction, and stimulates the release or block of the reuptake of norepinephrine at the adrenergic nerve terminal.  It boosts intracellular calcium at the cell membrane and leads to stronger heart contractility, which makes it useful in heart failure. Digoxin shows antiarrhythmic effects by acting on the central nervous system, enhancing vagal tone, and slowing contractions through the sinoatrial and atrioventricular nodes.

Pharmacokinetics

Digoxin is readily absorbed from the gastrointestinal tract and widely distributed in the body.  It has a protein binding capacity of 30%. Digoxin is metabolized partially in the liver and excreted in the urine.

When given through an oral route, the onset of action occurs in 30 minutes to 2 hours, the peak occurs in 2 to 8 hours, and the duration of action is 3 to 4 days. When given through an intravenous route, the onset of action occurs in 5 to 30 minutes, the peak occurs in 1 to 4 hours, and the duration of action is 3 to 4 days.

The intestinal absorption of digoxin varies.  Capsules are absorbed most efficiently, followed by elixir and then tablets.  It is distributed widely throughout the body.  The highest concentration goes to the heart, liver, and kidneys and it is poorly bound to the plasma proteins. The half- life of digoxin is 36 to 48 hours and this may be increased in patients with impaired renal function or the elderly.

Uses/Drug Interactions

Uses

Digoxin is used in conditions such as the following:

• Treatment of mild to moderate heart failure
• To control ventricular response rate in patients with chronic atrial fibrillation
• Off-label use in fetal tachycardia and to decrease ventricular rate in supraventricular tachyarrhythmias

Drug Interactions

Drug interactions that can occur with digoxin therapy are as follows:

• The therapeutic effects can be reduced by rifampin, barbiturates, cholestyramine, antacids, kaolin, pectin, sulfasalazine, neomycin, and metoclopramide.
• Calcium preparations, quinidine, verapamil, cyclosporine, tetracycline, nefazodone, clarithromycin, propafenone, amiodarone, spironolactone, hydroxychloroquine, erythromycin, itraconazole, and omeprazole can increase the risk of digoxin toxicity. Exercise caution when any of these drugs are given in combination.
• Amphotericin B, potassium-wasting diuretics, and steroids, when given with digoxin, can cause hypokalemia and result in digoxin toxicity. Exercise caution and monitor serum potassium levels.
• Calcium channel blockers and beta blockers when given with this drug can result in excessively slow heart rate and arrhythmias.
• Thyroid drugs, such as levothyroxine and neuromuscular drugs such as succinylcholine when given with digoxin can increase the risk of arrhythmias.
• Herbal preparations such as St. John’s wort and ginseng can increase the serum concentration of digoxin and lead to toxicity.
• Licorice may increase the adverse effects of digoxin.
• Meals high in fiber or pectin may decrease the absorption of digoxin.

Contradictions, Adverse Effects, and Side Effects

Contraindications

Digoxin use is contraindicated under the following conditions:

• If the patient is hypersensitive to digoxin
• Ventricular fibrillation

Administer with extreme caution in cases such as:

• Renal impairment
• Sinus nodal disease
• Acute myocardial infarction
• Second or third degree heart block (unless functioning pacemaker)
• Concurrent use of strong inducers or inhibitors of P-glycoprotein, such as cyclosporine .
• Hyperthyroidism
• Hypothyroidism
• Hypokalemia
• Hypocalcemia

Adverse Effects

Digoxin is a narrow therapeutic index drug, which means it has a very low margin of safety. A small amount can be lethal for the patient, leading to severe adverse effects.

To prevent digoxin toxicity, it should be prescribed individually from patient to patient, depending on the serum’s concentration.  The therapeutic concentration level is 0.8–2 ng/ml.  A concentration greater than this value is considered a toxic serum level.

The adverse reactions or signs and symptoms of digoxin toxicity are as follows:

• Gastrointestinal disturbances such as anorexia, nausea, vomiting, abdominal pain, and diarrhea
• Headache
• Mental changes such as irritability and depression
• Neurological changes such as fatigue, headache, depression, weakness, drowsiness, confusion, and nightmares
• Ocular disturbances such as photophobia, light flashes, halos around bright objects, and yellow or green color perception
• Sinus bradycardia, AV block, ventricular arrhythmias, or complete heart block
• Facial pain
• Personality change
• Insomnia

The antidote for digoxin toxicity is digoxin immune Fab – an antigen-binding fragment (Fab) derived from specific anti-digoxin antibodies.  Its dose depends on the amount of digoxin ingested and the serum’s concentration level.

Side Effects

The most common side effects of digoxin therapy are dizziness, headache, diarrhea, rash, and visual disturbances.

Forms and Dosages of Digoxin

Forms

The available forms and dosages of digoxin are listed below:

• Oral solution – 50 mcg/ml
• Tablets – 62.5 mcg, 125 mcg, 187.5 mcg, and 250 mcg
• Injectable solution – 100 mcg/ml and 250 mcg/ml

For oral administration, it can be given without food.  Tablets can be crushed.

Intramuscular digoxin is rarely used, produces severe local irritation, and leads to erratic absorption.  However, if no other route is possible, it can be given deep into the muscle followed by a massage.  Be sure to not give more than 2 ml at one site.

For intravenous administration, give undiluted or dilute with at least a 4-fold volume of sterile water for injection or D5W.  Less dilution may lead to the precipitate formation.  Use the dilution immediately and administer slowly over 5 minutes.

Dosages

It is important to keep in mind that the loading dose of digoxin is not recommended in heart failure.

The loading dose of digoxin is as follows:

• For adults and elderly: 75 to 1.5mg orally or 0.5 to 1mg IV
• For children 10 years and older: 10 to 15 mcg/kg or 8 to 12 mcg/kg IV
• For children 5 to 9 years: 20 to 35 mcg/kg or 15 to 30 mcg/kg IV
• For children 2 to 4 years: 30 to 40 mcg/kg or 25 to 35 mcg/kg IV
• For children 1 to 23 months: 35 to 60 mcg/kg or 30 to 50 mcg/kg IV
• For full-term neonates: 25 to 35 mcg/kg or 20 to 30 mcg/kg IV
• For premature neonates: 20 to 30 mcg/kg or 15 to 25 mcg/kg IV

The maintenance dose of digoxin is as follows:

• For preterm infant: 5 to 7.5 mcg/kg orally or 4 to 6 mcg/kg IV or IM
• For full-term infant: 8 to 10 mcg/kg orally or 5 to 8 mcg/kg IV or IM
• For 1 month to 2 years child: 10 to 15 mcg/kg orally or 9 to 15 mcg/kg IV or IM
• For 2 to 5 years child: 8 to 10 mcg/kg orally or 6 to 9 mcg/kg IV or IM
• For 5 to 10 years child: 5 to 10 mcg/kg orally or 4 to 8 mcg/kg IV or IM
• For adults greater than 10 to 18: 4 to 5.1 mcg/kg once a day orally as tablets, 3 to 4.5 mcg/kg once a day orally as an oral solution, or  2.4 to 3.6 mcg/kg once a day IV

In the elderly, avoid doses greater than 0.125 mg/day due to decreased renal clearance.

Intramuscular administration of digoxin should be avoided because it can cause pain and muscle necrosis at the injection site.  If it has to be given intramuscularly, do not inject more than 500 mcg of the injectable formulation at a single site.

In patients whose lean weight is an abnormally small fraction of total body mass, such as in edema or obesity, reduce the dosage.  Decrease the loading dose by 50% in patients with end-stage renal disease.

In patients with renal impairment, dosage adjustment is based on creatinine clearance. The dosage of digoxin may need to be adjusted every 2 weeks, considering its therapeutic effect, toxicity, and serum levels.

Nurse’s Role in Digoxin and Patient Education

It is important for nurses to understand the vital role they play in the administration and management of digoxin and the important nursing areas that must be considered.

The following nursing processes are appropriate for patients undergoing digoxin therapy.

Assessment

• Before starting the digoxin therapy, take a complete patient history.
• Before each dose, monitor the patient’s apical pulse for one minute to check the drug’s effectiveness.
• Evaluate the electrocardiogram if ordered and regularly assess the patient’s cardiopulmonary status for signs of improvement.
• Obtain serum digoxin level after 8 hours of oral drug administration. Therapeutic blood levels range from 0.5 to 2.0 ng/ml.
• Be vigilant for any adverse drug reactions or drug interactions.
• Closely monitor the serum potassium levels with digoxin therapy, as hypokalemia can increase the risk of digoxin toxicity.

Key Nursing Diagnoses

• Decreased cardiac output due to the existing condition
• Risk for injury with digoxin therapy due to possible adverse effects and digoxin toxicity.
• Knowledge deficit regarding medication for both patient and/or caregiver.

Planning Outcome Goals

• Improved cardiac output from patient as evidenced by stable vital signs, level of consciousness, and urine output.
• Minimize the risk for injury, with reduced chances of digoxin toxicity and possible adverse drug reactions.
• The patient and caregiver will demonstrate and verbalize correct information about the administration and toxicity of digoxin.

Evaluation

• The patient maintains adequate cardiac output.
• The patient has no evidence of digoxin toxicity.
• The patient and his family verbalize and demonstrate an understanding of drug therapy.

Patient Teaching

Review the following points of digoxin therapy with the patient and/or caregiver:

• Digoxin is given to normalize one’s heart rate and to relieve ankle swelling, fatigue, and shortness of breath – symptoms that correlate with heart problems.
• Take as the healthcare provider has prescribed at the same time every day. Do not miss a dose.  If a dose is missed, do not double the dose.
• Avoid taking over-the-counter medications or herbal remedies with digoxin therapy without consulting with healthcare provider first.
• With digoxin therapy, one will need various blood tests, physical examinations, and ECGs done from time to time to determine if a dose adjustment is needed.
• If any changes are noted in heart rate or rhythm, nausea, vomiting, or vision problems, visit the healthcare provider as soon as possible.
• Limit salt intake and be sure to get enough potassium. Follow the dietary guidelines that the healthcare provider recommends.  Do not take salt substitutes such as potassium chloride without consulting with the healthcare provider first.
• Be sure to use the same brand and type of digoxin since all forms and concentrations are different and are not interchangeable.
• Before each dose, check one’s pulse. If it is less than 60 beats per minute, hold the medication and consult with the prescriber.
• Digoxin tablets can be crushed and can be taken with or without food.
• Digoxin can be taken in a liquid dosage form but must be measured accurately.

The Bottom Line

Digoxin is a cardiac glycoside used in patients with heart failure and atrial fibrillation.  It has a narrow therapeutic index that increases the risk of adverse drug reactions and toxicity; therefore one must be careful when making any dose adjustments of digoxin. Inform the patient to urgently report if any adverse reactions occur.  Encourage the patient to attend follow-up visits in order for effective monitoring of serum digoxin levels to be maintained.

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