ECG Case 030

Main Abnormalities

• Atrial tachycardia, with regular P waves visible at ~ 160 bpm (many of the P waves are hidden within T waves and VEBs)
• Evidence of high-grade AV block — there is a 4:1 conduction ratio between P waves and QRS complexes, with a QRS rate of ~ 40 bpm
• Frequent ventricular ectopic beats occurring in a pattern of ventricular bigeminy
• Alternating LBBB and RBBB morphology, with the conducted QRS complexes demonstrating RBBB morphology (RSR’ in V1) and the VEBs demonstrating LBBB morphology (dominant S wave in V1)

Diagnosis

The combination of…

• Atrial tachycardia
• Frequent ventricular ectopic beats
• High-grade AV block

… is almost pathognomonic of severe digoxin toxicity.

ECG Features of Digoxin Toxicity

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

• Increased automaticity of atrial 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 therefore usually produces some combination of:

• Increased atrial automaticity — especially atrial tachycardia, but also atrial ectopics, AF, flutter
• 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).