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Digoxin

Digoxin is a cardiac glycoside extracted from the foxglove plant, digitalis. It is widely used in the treatment of various heart conditions, namely atrial fibrillation, atrial flutter and congestive heart failure that cannot be controlled by other medication. more...

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The main effects of digoxin are on the heart, its extracardiac effects are responsible for most of the side effects, i.e. nausea, vomiting, diarrhea and confusion.

Its main cardiac effects are:

A decrease of conduction of electrical impulses through the AV node, making it a commonly used drug in controlling the heart rate during atrial fibrillation or atrial flutter.
An increase of force of contraction via inhibition of the Na⁺/K⁺ ATPase pump (see below).

Mechanism of action

Digoxin binds to a site on the extracellular aspect of the α-subunit of the Na⁺/K⁺ ATPase pump in the membranes of heart cells (myocytes). This causes an increase in the level of sodium ions in the myocytes, which then leads to a rise in the level of calcium ions. The proposed mechanism is the following: inhibition of the Na⁺/K⁺ pump leads to increased Na⁺ levels, which in turn slows down the extrusion of Ca²⁺ via the Na⁺/Ca²⁺ exchange pump. Increased amounts of Ca²⁺ are then stored in the sarcoplasmic reticulum and released by each action potential, which is unchanged by digoxin. This is a different mechanism from that of catecholamines.

Digoxin also increases vagal activity via its central action on the central nervous system, thus decreasing the conduction of electrical impulses through the AV node. This is important for its clinical use in different arrhythmias (see below).

Clinical use

Today, the most common indications for digoxin are probably atrial fibrillation and atrial flutter with rapid ventricular response. High ventricular rate leads to insufficient diastolic filling time. By slowing down the conduction in the AV node and increasing its refractory period, digoxin can reduce the ventricular rate. The arrhythmia itself is not affected, but the pumping function of the heart improves owing to improved filling.

The use of digoxin in congestive heart failure during sinus rhythm is controversial. In theory the increased force of contraction should lead to improved pumping function of the heart, but its effect on prognosis is disputable and digoxin is no longer the first choice for congestive heart failure. However, it can still be useful in patients who remain symptomatic despite proper diuretic and ACE inhibitor treatment.

Digoxin is usually given by mouth, but can also be given by IV injection in urgent situations (the IV injection should be slow, heart rhythm should be monitored). The half life is about 36 hours, digoxin is given once daily, usually in 125μg or 250μg dosing. In patients with decreased kidney function the half life is considerably longer, calling for a reduction in dosing or a switch to a different glycoside (digitoxin).

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Optimal digoxin range for men is 0.5 to 0.8 ng/mL - Patient Oriented Evidence That Matters: practice recommendations from key studies
From Journal of Family Practice, 5/1/03 by Jeffrey Rosenberg

Rathore SS, Curtis JP, Wang Y, Bristow MR, Krumholz, HM. Association of serum digoxin concentration and outcomes in patients with heart failure. JAMA 2003; 289:871-878.

* PRACTICE RECOMMENDATIONS

The optimal serum drug concentration for digoxin among men in sinus rhythm with stable heart failure is 0.5 to 0.8 ng/mL. This range is associated with decreased risk of hospitalization and mortality compared with placebo. Higher levels are associated with either no reduction, or an increased risk of hospitalization and mortality compared with placebo.

* BACKGROUND

The Digitalis Investigation Group (DIG) trial of patients with heart failure found no effect on mortality from digoxin therapy at serum concentrations of 0.5 to 2.0 ng/mL, but did find that hospitalization due to worsening heart failure was modestly reduced. The original investigators did not evaluate the relative efficacy of different concentrations.

Other studies have demonstrated an association between serum drug concentrations <1.0 ng/mL and improved left ventricular function, but have not shown beneficial effects on neurohormonal function, hemodynamic profiles, or exercise tolerance. The authors of this study obtained a public use copy of the DIG data to evaluate the association of various serum concentrations on risk of mortality and hospitalization among men with heart failure and left ventricular dysfunction.

* POPULATION STUDIED

This reanalysis of data from the DIG trial included 5281 men with stable heart failure recruited from 229 cardiology and 73 primary care sites in the United States and Canada from 1991 to 1995. All participants were in sinus rhythm, had stable and clinically confirmed heart failure, and had a left ventricular ejection fraction of less than 45% as determined by radionuclide, contrast angiography, or 2-dimensional echocardiography.

Important exclusion criteria included age <21 years, recent myocardial infarction or interventional cardiac procedures, unstable angina, and significant renal dysfunction (serum creatinine >3 mg/dL).

Study participants were all men, with mean age of 63 years; 13% of the subjects were of African descent. The majority of the patients were New York Heart Association Class II or III and had >4 signs or symptoms of heart failure. Other medications known to benefit heart failure, such as angiotensin-converting enzyme inhibitors and diuretics, were continued. Patients using digoxin at the time of enrollment could be allocated to drug or placebo.

* STUDY DESIGN AND VALIDITY

This study is a post-analysis of data collected from the DIG study. The DIG study was a double-blind, placebo-controlled study of digoxin in the treatment of heart failure. Digoxin dosing was based on a published algorithm; serum digoxin levels were drawn 1 month after randomization. The present study is a subgroup analysis of men based on serum drug concentrations at 1 month of 0.5-0.8 ng/mL, 0.9-1.1 ng/mL, or [greater than or equal to] 1.2 ng/mL. These ranges have been used in previous studies of digoxin in the treatment of heart failure.

The study was sufficiently well done to draw conclusions about use of digoxin in treatment of heart failure. Patients and data investigators were blinded with respect to treatment, and cause of death was determined without knowledge of assigned treatment group. Digoxin levels were reported from the 1 month visit only. No subsequent levels were documented.

Generalizability to the general population is limited, as only men with heart failure in sinus rhythm and a left ventricular ejection fraction <45% were evaluated. Concealment of allocation occurred, but method of randomization and intention-to-treat analysis were not reported.

* OUTCOMES MEASURED

The primary outcome measure is all-cause mortality at follow-up (mean 37 months, range 24-48 months). Secondary endpoints include death due to cardiovascular causes, death due to worsening congestive heart failure, and hospitalization for worsening heart failure.

* RESULTS

There was no overall difference in all-cause mortality for patients assigned to placebo as compared with those treated with digoxin (36.2% placebo vs. 36.6% digoxin, P not significant). When separated by serum concentration, however, lower doses of digoxin affected mortality. Patients with serum concentration of 0.5 to 0.8 ng/mL had a 6.3% lower rate (29.9% vs. 36.2%; 95% confidence interval [CI], 2.1%-10.5%; number needed to treat [NNT]= 15) of all-cause mortality and 5.9% lower rate (61.9% vs. 67.8%; 95% CI, 1.5%-10.2%; NNT=17) of hospitalizations compared with placebo.

Secondary endpoints, including cardiovascular mortality and mortality due to worsening heart failure, also showed benefit with lower serum drug concentrations. Serum concentrations of 0.9-1.1 ng/mL have similar rates of primary and secondary endpoints as placebo. Patients with serum concentrations of [greater than or equal to] 1.2 ng/mL had higher all-cause and cardiovascular mortality compared with patients assigned to placebo.

Jeffrey Rosenberg, MD, and Carol Federiuk, MD, PhD, Department of Family Medicine, Thomas Jefferson University, Philadelphia, Pa. E-mail: drjsrosenberg@yahoo.com.

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