Digoxin

Digoxin Effect

Digoxin can affect the EKG when levels reach toxicity, though it commonly causes EKG changes at therapeutic levels as well ('digoxin effect'). It is important to remember these these effects and toxicity can be produced by other cardiac glycosides (ie foxglove, lily of the valley). Digoxin decreases the refractory period of the atria and ventricles (increasing their automaticity), leading to a shortened QT interval, ST changes, and T wave abnormalities. The ST changes are usually a scooping ST depression into the T wave. This is sometimes referred to as Salvador Dali's mustache. Although one cannot always distinguish these ST depression from those of ischemia, the scooping appearance is more typical of digoxin effect and more likely to be a manifestation of that. Furthermore, digoxin tends to produce a shorter QT, whereas ischemia tends to produce a more prolonged QT. The most common T wave abnormality is a biphasic T wave with an initial negative deflection. It also causes increased vagal effects at the AV node, which manifests as a prolonged PR interval. These changes all fall into the digoxin effect, which are not signs of toxicity.

Source: LITFL

Examples:

Classic digoxin effect

Note the rate-controlled atrial fibrillation with scooping ST segment into a down-up biphasic T wave and short QT

Source: LITFL

Digoxin Toxicity

Digoxin toxicity is usually from the manifestations of the increased automaticity in the atria and ventricles, as well as the increased vagal effects at the AV node. The most frequent abnormalities are frequent PVC's or an accelerated junctional rhythm. Toxicity may also include AV blocks, atrial arrhythmias (ie atrial fibrillation, flutter, ectopic atrial) with slow ventricular response, VT, and 'regularized atrial fibrillation'. Atrial fibrillation with a slow ventricular response on an EKG should initiate an immediate concern for digoxin toxicity. Regularized atrial fibrillation is when there is atrial fibrillation with a regular ventricular response; this occurs due to 3* AV block from the digoxin toxicity. In other words, there is an atrial fibrillation with 3* AV block and an escape rhythm producing the QRS complexes. Sometimes, patients have alternating LBBB and RBBB. The pathognomonic finding is bidirectional VT. This is VT with beat-to-beat alteration in the QRS that is usually around 180*. This occurs due to competing ventricular pacemakers (one in the left ventricle, one in the right ventricle).

Examples: