Cardiac Glycosides (Digitalis Compounds)

General Pharmacology

Cardiac glycosides represent a family of compounds that are derived from the foxglove plant (Digitalis purpurea). The therapeutic benefits of digitalis were first described by William Withering in 1785. Initially, digitalis was used to treat dropsy, which is an old term for edema. Subsequent investigations found that digitalis was most useful for edema that was caused by a weakened heart (i.e., heart failure).

Mechanisms of action

Digitalis compounds are potent inhibitors of cellular Na⁺/K⁺-ATPase. This ion transport system moves sodium ions out of the cell and brings potassium ions into the cell. This transport function is necessary for cell survival because sodium diffusion into the cell and potassium diffusion out of the cell down their concentration gradients would reduce their concentration differences (gradients) across the cell membrane over time. Loss of these ion gradients would lead to cellular depolarization and loss of the negative membrane potential that is required for normal cell function. The Na⁺/K⁺-ATPase also plays an active role in the membrane potential. this pump is electrogenic because it transports 3 sodium ions out of the cell for every 2 potassium ions that enter the cell. This can add several negative millivolts to the membrane potential depending on the activity of the pump.

Cardiac myocytes, as well as many other cells, have a Na⁺-Ca⁺⁺ exchanger (not an active energy-requiring pump) that is essential for maintaining sodium and calcium homeostasis. The exact mechanism by which this exchanger works is unclear.  It is known that calcium and sodium can move in either direction across the sarcolemma. Furthermore, three sodium ions are exchanged for each calcium, therefore an electrogenic potential is generated by this exchanger.  The direction of movement of these ions (either inward or outward) depends upon the membrane potential and the chemical gradient for the ions. We also know that an increase in intracellular sodium concentration competes for calcium through this exchange mechanism leading to an increase in intracellular calcium concentration. As intracellular sodium increases, the concentration gradient driving sodium into the cell across the exchanger is reduced, thereby reducing the activity of the exchanger, which decreases the movement of calcium out of the cell. Therefore, mechanisms that lead to an accumulation of intracellular sodium cause a subsequent accumulation of intracellular calcium because of decreased exchange pump activity.
By inhibiting the Na⁺/K⁺-ATPase, cardiac glycosides cause intracellular sodium concentration to increase. This then leads to an accumulation of intracellular calcium via the Na⁺-Ca⁺⁺ exchange system. In the heart, increased intracellular calcium causes more calcium to be released by the sarcoplasmic reticulum, thereby making more calcium available to bind to troponin-C, which increases contractility (inotropy). Inhibition of the Na⁺/K⁺-ATPase in vascular smooth muscle causes depolarization, which causes smooth muscle contraction and vasoconstriction.
By mechanisms that are not fully understood, digitalis compounds also increase vagal efferent activity to the heart. This parasympathomimetic action of digitalis reduces sinoatrial firing rate (decreases heart rate; negative chronotropy) and reduces conduction velocity of electrical impulses through the atrioventricular node (negative dromotropy).

Pharmacokinetics and toxicity

The long half-life of digitalis compounds necessitates special considerations when dosing. With a half-life of 40 hours, digoxin would require several days of constant dosing to reach steady-state, therapeutic plasma levels (digitoxin with a half-life of 160 hours, would require almost a month!). Therefore, when initiating treatment, a special dosing regimen involving "loading doses" is used to rapidly increase digoxin plasma levels. This process is termed "digitalization." For digoxin, the therapeutic plasma concentration range is 0.5 - 1.5 ng/ml. It is very important that therapeutic plasma levels are not exceeded because digitalis compounds have a relatively narrow therapeutic safety window. Plasma concentrations above 2.0 ng/ml can lead to digitalis toxicity, which is manifested as arrhythmias, some of which may be life-threatening. If toxicity occurs with digoxin, it may take several days for the plasma concentrations to fall to safe levels because of the long half-life. There is available for digoxin toxicity an immune Fab (Digibind) that can be used to rapidly reduce plasma digoxin levels. Potassium supplementation can also reverse the toxic effects of digoxin if the toxicity is related to hypokalemia (see below).

Drug Interactions

Many commonly used drugs interact with digitalis compounds. The Class IA antiarrhythmic, quinidine, competes with digoxin for binding sites and depresses renal clearance of digoxin. These effects increase digoxin levels and can produce toxicity. Similar interactions occur with calcium-channel blockers and nonsteroidal anti-inflammatory drugs. Other drugs that interact with digitalis compounds are amiodarone (Class III antiarrhythmic) and beta-blockers. Diuretics can indirectly interact with digoxin because of their potential for decreasing plasma potassium levels (i.e., producing hypokalemia). Because potassium competes with digoxin for binding sites on the Na⁺/K⁺-ATPase, hypokalemia results in increased digoxin binding and thereby enhances its therapeutic and toxic effects. Hypercalcemia enhances digitalis-induced increases in intracellular calcium, which can lead to calcium overload and increased susceptibility to digitalis-induced arrhythmias. Hypomagnesemia also sensitizes the heart to digitalis-induced arrhythmias.

Therapeutic Uses of Digitalis Compounds

Heart Failure

• ↑ inotropy
• ↑ ejection fraction
• ↓ preload
• ↓ pulmonary congestion/edema

Arrhythmias

• ↓ AV nodal conduction
  (parasympathomimetic effect)
• ↓ ventricular rate in atrial flutter
  and fibrillation

Therapeutic Uses

Heart failure

Digitalis compounds have historically been used in the treatment of chronic heart failure owing to their cardiotonic effect. Although newer and more efficacious treatments for heart failure are available, digitalis compounds are still widely used. Clinical studies in heart failure patients have shown that digoxin, when used in conjunction with diuretics and vasodilators, improves cardiac output and ejection fraction, and reduces filling pressures and pulmonary capillary wedge pressure (this reduces pulmonary congestion and edema); heart rate changes very little. These effects are to be expected for a drug that increases inotropy. Although the direct effect of digoxin on blood vessels is vasoconstriction, when given to patients in heart failure, the systemic vascular resistance falls. This most likely results from the improvement in cardiac output, which leads to withdrawal of compensatory vasoconstrictor mechanisms (e.g., sympathetic adrenergic activity and angiotensin II influences). Digitalis compounds have a small direct diuretic effect on the kidneys, which is beneficial in heart failure patients.

Atrial fibrillation and flutter

Atrial fibrillation and flutter lead to a rapid ventricular rate that can impair ventricular filling (due to decreased filling time) and reduce cardiac output. Furthermore, chronic ventricular tachycardia can lead to heart failure. Digitalis compounds, such as digoxin, are useful for reducing ventricular rate when it is being driven by a high atrial rate. The mechanism of this beneficial effect of digoxin is its ability to activate vagal efferent nerves to the heart (parasympathomimetic effect). Vagal activation can reduce the conduction of electrical impulses within the atrioventricular node to the point where some of the impulses will be blocked. When this occurs, fewer impulses reach the ventricles and ventricular rate falls. Digoxin also increases the effective refractory period within the atrioventricular node.

Specific Drugs

Three different digitalis compounds (cardiac glycosides) are listed in the table below. The compound most commonly used in the U.S. is digoxin. Ouabain is used primarily as a research tool.

Drug	Oral Availability*	Half-life (hours)	Elimination
Digoxin	75%	40	kidneys
Digitoxin	>90%	160	liver
Ouabain	0%	20	kidneys

* percent absorption

Side Effects and Contraindications

The major side effect of digitalis compounds is cardiac arrhythmia, especially atrial tachycardias and atrioventricular block. Digitalis compounds are contraindicated in patients who are hypokalemic, or who have atrioventricular block or Wolff-Parkinson-White (WPW) syndrome. Impaired renal function leads to enhanced plasma levels of digoxin because digoxin is eliminated by the kidneys. Lean, elderly patients are more susceptible to digitalis toxicity because they often have reduced renal function, and their reduced muscle mass increases plasma digoxin levels at a given dose because muscle Na⁺/K⁺-ATPase acts as a large binding reservoir for digitalis.