Mechanism of Action and Indications

Digitalis refers to a class of cardioactive drugs called glycosides which exert both mechanical and electrical effects on the heart. The most commonly used digitalis preparation in current practice is digoxin.

The mechanical action of digitalis glycosides is to increase the strength of myocardial contractions in selected patients with dilated hearts and systolic chronic heart failure (CHF). Their therapeutic electrical effects relate primarily to decreasing automaticity and conductivity in the sinoatrial (SA) and atrioventricular (AV) nodes, in large part by increasing parasympathetic (vagal) tone. Consequently, digitalis is used for controlling the ventricular response in atrial fibrillation (AF) and atrial flutter, which are characterized by the excessive transmission of stimuli from the atria to the ventricles through the AV junction.

Since a number of more efficacious and safer medications have become available, the use of digitalis is mostly limited to the patients with AF, systolic CHF, and hypotension who cannot tolerate beta blockers or calcium channel blockers, or it is used as an adjunct to therapy. More rarely, digoxin is still used in the treatment of certain reentrant types of paroxysmal supraventricular tachycardia (PSVT), for example, during pregnancy, when other drugs might be contraindicated.

Although digitalis has indications in the treatment of certain forms of chronic systolic heart failure and selected supraventricular arrhythmias, it also has a relatively low margin of safety. The difference between therapeutic and toxic concentrations is narrow.

Digitalis Toxicity Vs. Digitalis Effect

Digitalis toxicity refers to the arrhythmias and conduction disturbances (as well as the toxic systemic effects described later) produced by this class of drug. Digitalis toxicity should not be confused with digitalis effect. Digitalis effect (Figs. 18-1 and 18-2) refers to the distinct scooping (“thumbprinting” sign) of the ST-T complex (associated with shortening of the QT interval) typically seen in patients taking digitalis glycosides.

Figure 18-1 Characteristic scooping of the ST-T complex produced by digitalis. (Not all patients taking digitalis exhibit these changes.)

Figure 18-2 The characteristic scooping of the ST-T complex produced by digitalis is best seen in leads V₅ and V₆. (Low voltage is also present, with total QRS amplitude of 5 mm or less in all six limb leads.)

Note: The presence of digitalis effect, by itself, does not imply digitalis toxicity and may be seen with therapeutic drug concentrations. However, most patients with digitalis toxicity manifest ST-T changes of digitalis effect on their ECG.

Symptoms and Signs of Digitalis Toxicity

Digitalis toxicity can produce general systemic symptoms as well as specific cardiac arrhythmias and conduction disturbances. Common extracardiac symptoms include weakness, anorexia, nausea, and vomiting. Visual effects (altered color perception) and mental changes may occur.

As a general clinical rule virtually any arrhythmia and all degrees of AV heart block can be produced by digitalis. However, a number of arrhythmias and conduction disturbances are commonly seen with digitalis toxicity (Box 18-1). In some cases, combinations of arrhythmias will occur, such as AF with a relatively slow ventricular response and frequent ventricular ectopy (Fig. 18-3).

BOX 18-1 Arrhythmias and Conduction Disturbances Caused by Digitalis Toxicity

Bradycardias

Sinus, including sinoatrial (SA) block

Junctional rhythms^∗

Atrial fibrillation (or flutter) with a slow/regularized response

Tachycardias

Accelerated junctional rhythms

Atrial tachycardia with block

Frequent ventricular ectopy, including ventricular bigeminy and multiform ventricular premature beats (VPBs)

Ventricular tachycardia/ventricular fibrillation

AV Conduction Delays and Related Disturbances

Prolonged PR interval (first-degree atrioventricular [AV] block)

Second-degree AV block (AV Wenckebach, but not Mobitz II block)

Third-degree AV block/AV dissociation

^∗ Two classes of junctional (nodal) rhythms may occur: (1) a typical junctional escape rhythm with a rate of 60 beats/min or less, and (2) an accelerated junctional rhythm (also called nonparoxysmal junctional tachycardia

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