ECG Case 054

This is a tricky ECG!

There is evidence of atrial fibrillation, as evidenced by the irregular baseline with fibrillatory waves most prominent in V1-2.

NB. Fibrillatory waves are characteristically seen in V1-2 (which overlie the atria), as opposed to tremor artefact which may be in seen in multiple leads without a predominance for V1-2. 

However, the ventricular rhythm is regular. How can this be? AF is irregular by definition…

This is an example of “regularised AF” due to digoxin toxicity:

• The underlying rhythm is AF, which is being treated with digoxin.
• There is complete heart block, prevent atrial impulses from reaching the ventricles.
• There is an accelerated junctional rhythm maintaining cardiac output.

If this all seems like too much of a coincidence, then consider the pathophysiology of digoxin toxicity…

Mechanisms of Digoxin Toxicity

Digoxin toxicity produces a wide variety of dysrhythmias, due to:

• Increased automaticity of atrial, junctional and ventricular tissues — via actions at the Na/K and Na/Ca exchangers causing increased intracellular calcium and therefore increased spontaneous depolarisation of cardiac pacemaker cells.
• Decreased AV conduction — via increased vagal tone at the AV node.

Digoxin toxicity produces some combination of:

• Increased atrial automaticity — especially atrial tachycardia, but also atrial ectopics, AF, flutter.
• Increased junctional automaticity — especially accelerated junctional rhythms.
• Increased ventricular automaticity — frequent VEBs and bigeminy, polymorphic VT.
• AV blocks — including 1st, 2nd and 3rd degree AV block.

Characteristic ECG patterns include:

• Atrial tachycardia with high-grade AV block (= the classic dig-toxic rhythm).
• Regularised AF =  AF with complete heart block + accelerated junctional escape rhythm, producing a paradoxically regular rhythm.
• Bidirectional VT = polymorphic VT with QRS complexes that alternate between left- and right-axis-deviation, or between LBBB and RBBB morphology.

NB. Digoxin toxicity should not be confused with digoxin effect (= “sagging” ST depression and T-wave inversion in patients on therapeutic doses of digoxin; not predictive of toxicity). 

Clinical Pearls 

• Check for tremor artefact before you start diagnosing regularised AF!
• If the ECG pattern appears genuine and the clinical picture is compatible with digoxin toxicity (GI upset, xanthopsia, current digoxin treatment), then check an urgent digoxin level.