85-year old patient presenting with nausea, vomiting and visual disturbance. Looks clinically dehydrated. Describe the ECG.
Describe and interpret this ECG
ECG ANSWER and INTERPRETATION
- 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).
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).