ECG Arrhythmias: Step 1 review notes and self-test
Notes for the ECG Arrhythmias rapid review: the clue that separates each of its 6 confusion pairs, with timestamps and a self-test.
October 21, 2026
This video covers atrial flutter and fibrillation, AV blocks, WPW, torsades, polymorphic ventricular tachycardia, and potassium-related ECG changes. Watch the video, then use these notes to test yourself.
Atrial flutter vs Atrial fibrillation
You separate them by checking whether atrial activity is organized. Flutter has repeating sawtooth flutter waves, while fibrillation has no discrete P waves and an irregularly irregular ventricular rhythm. Flutter with variable block can be irregular, so do not call it AF before looking for repeating flutter waves.
Atrial flutter vs Atrial fibrillation Flutter has organized sawtooth atrial activity; fibrillation has no organized P/flutter waves and is irregularly irregular. Open the comparison and video →Wenckebach vs Mobitz II
You identify Wenckebach by progressive PR lengthening before a dropped QRS, while Mobitz II has fixed PR intervals before a sudden dropped QRS. Dropped QRS complexes alone do not mean Mobitz II, so compare three PR intervals leading into the drop. Any progressive lengthening, even by 20 to 40 ms, indicates Wenckebach.
Wenckebach vs Mobitz II Wenckebach (Mobitz I): progressive PR lengthening before a dropped beat, usually benign; Mobitz II: fixed PR with a sudden drop, needs pacing. Open the comparison and video →Lower-grade AV block vs High-grade AV block
You identify first-degree block by a prolonged PR interval without dropped beats and Mobitz I by progressive PR lengthening before a dropped beat. Mobitz II has fixed PR intervals with dropped beats, while third-degree block has AV dissociation. A 2:1 block can hide PR behavior, so do not name the Mobitz type without enough conducted beats.
Lower-grade AV block vs High-grade AV block First-degree delays, Wenckebach lengthens then drops, Mobitz II drops without lengthening, third-degree dissociates. Open the comparison and video →WPW pattern vs Atrial fibrillation with WPW
You identify the WPW pattern at baseline by a short PR interval, delta wave, and widened QRS. Atrial fibrillation with WPW is an irregular wide-complex tachycardia caused by rapid conduction through the accessory pathway. Do not treat it like ordinary AF because blocking the AV node can favor accessory pathway conduction.
WPW pattern vs Atrial fibrillation with WPW WPW pattern is baseline delta wave; AF with WPW is irregular wide tachycardia where AV nodal blockers can be dangerous. Open the comparison and video →Torsades de pointes vs Other polymorphic ventricular tachycardia
You call polymorphic ventricular tachycardia torsades only when the baseline QT is prolonged. Polymorphic ventricular tachycardia with a normal QT is not torsades and is often driven by acute ischemia. Twisting QRS amplitude around the baseline alone does not establish torsades.
Torsades de pointes vs Other polymorphic ventricular tachycardia Torsades is polymorphic VT plus prolonged QT; polymorphic VT with normal QT is not torsades. Open the comparison and video →Hyperkalemia ECG vs Hypokalemia ECG
You separate them by reading the T-QRS-U sequence. Hyperkalemia causes tall peaked T waves that can progress to QRS widening, while hypokalemia causes flattened T waves with prominent U waves. Both can cause weakness and arrhythmias, so do not rely on symptoms alone.
Hyperkalemia ECG vs Hypokalemia ECG Hyperkalemia: peaked T waves progressing to a widened QRS; hypokalemia: flattened T waves with prominent U waves. Open the comparison and video →Quick self-test
Name the diagnosis from the clue, then tap to check.
Mobitz I second-degree block with progressive PR interval prolongation before a dropped QRS.
Wenckebach · Wenckebach vs Mobitz II
Flattened T waves, ST depression, prominent U waves, and apparent QT/QU prolongation.
Hypokalemia ECG · Hyperkalemia ECG vs Hypokalemia ECG
Dangerous irregular wide-complex tachycardia in WPW when AF conducts rapidly through the accessory pathway.
Atrial fibrillation with WPW · WPW pattern vs Atrial fibrillation with WPW
Second-degree block with constant PR intervals before sudden dropped QRS complexes and higher progression risk.
Mobitz II · Wenckebach vs Mobitz II
Baseline preexcitation: short PR interval, delta wave, and widened QRS from an accessory pathway.
WPW pattern · WPW pattern vs Atrial fibrillation with WPW
Polymorphic VT with prolonged QT and twisting QRS amplitude around the baseline, often from drugs or low potassium/magnesium.
Torsades de pointes · Torsades de pointes vs Other polymorphic ventricular tachycardia
Tall peaked T waves can progress to PR prolongation, P-wave flattening/disappearance, QRS widening, sine-wave rhythm, ventricular arrhythmia, and arrest.
Hyperkalemia ECG · Hyperkalemia ECG vs Hypokalemia ECG
Keep going
Every pair in this review has a free page in the library with a side-by-side comparison and a practice question. Work through the ones you missed in the self-test first.