ECG Rhythms
Comprehensive guides to normal and abnormal ECG rhythms — learn to identify, interpret, and understand the clinical significance of each rhythm.
Normal Sinus Rhythm
Normal sinus rhythm is the normal heart rhythm originating from the sinoatrial node with a regular rate and normal conduction.
60–100 bpm (human), 60–140 bpm (canine)Sinus Bradycardia
Sinus bradycardia is a regular rhythm originating from the SA node with a rate below the normal lower limit.
< 60 bpm (human), < 60 bpm (large dogs)Sinus Tachycardia
Sinus tachycardia is a regular rhythm originating from the SA node with a rate above the normal upper limit.
> 100 bpm (human), > 160 bpm (canine)Atrial Fibrillation
Atrial fibrillation is an irregularly irregular rhythm characterized by chaotic atrial electrical activity and absent organized P waves.
Variable; uncontrolled 100–180 bpm, controlled 60–100 bpmAtrial Flutter
Atrial flutter is a rapid, organized atrial rhythm with a characteristic sawtooth pattern, typically conducting to the ventricles in a fixed or variable ratio.
Atrial rate ~300 bpm; ventricular rate depends on conduction ratio (e.g., 150 bpm with 2:1 block)Premature Atrial Complexes
Premature atrial complexes are early beats originating from an ectopic focus in the atria, occurring before the next expected sinus beat.
Underlying rate varies; PACs cause momentary irregularityPremature Ventricular Complexes
Premature ventricular complexes are early, wide QRS beats originating from an ectopic focus in the ventricles.
Underlying rate varies; PVCs interrupt regular rhythmSupraventricular Tachycardia
Supraventricular tachycardia is a rapid, regular tachycardia originating above the ventricles, typically involving the AV node in a re-entrant circuit.
150–250 bpmVentricular Tachycardia
Ventricular tachycardia is a rapid rhythm originating from the ventricles, characterized by wide QRS complexes at a rate of 100 bpm or greater.
100–250 bpmVentricular Fibrillation
Ventricular fibrillation is a chaotic, disorganized electrical activity in the ventricles resulting in no effective cardiac output — a cardiac arrest rhythm.
No measurable rate; chaotic electrical activityTorsades de Pointes
Torsades de Pointes is a specific form of polymorphic ventricular tachycardia associated with prolonged QT interval, characterized by QRS complexes that appear to twist around the baseline.
150–300 bpm during the arrhythmiaFirst-Degree AV Block
First-degree AV block is a conduction delay (not a true block) with a prolonged PR interval where every atrial impulse still reaches the ventricles.
Normal (depends on underlying sinus rate)Second-Degree AV Block Type I (Wenckebach)
Wenckebach is a progressive lengthening of the PR interval until a P wave fails to conduct, producing a dropped QRS.
Usually normal or slightly slowSecond-Degree AV Block Type II (Mobitz II)
Mobitz Type II is an intermittent failure of AV conduction with a constant PR interval — P waves are suddenly dropped without preceding PR prolongation.
Normal or slow depending on conduction ratioThird-Degree AV Block
Third-degree AV block is complete failure of conduction between the atria and ventricles, with independent atrial and ventricular rhythms.
Atrial rate normal (60–100 bpm); ventricular escape rate 20–60 bpm depending on level of blockRight Bundle Branch Block
Right bundle branch block is a conduction delay in the right bundle branch, causing delayed right ventricular depolarization and a characteristic RSR' pattern in V1.
Normal (depends on underlying rhythm)Left Bundle Branch Block
Left bundle branch block is a conduction delay in the left bundle branch system, causing delayed left ventricular depolarization with a broad, notched QRS.
Normal (depends on underlying rhythm)Wolff-Parkinson-White Syndrome
WPW is a pre-excitation syndrome caused by an accessory pathway (Bundle of Kent) that bypasses the AV node, producing early ventricular activation.
Normal at baseline; SVT episodes 150–250 bpmJunctional Rhythm
Junctional rhythm originates from the AV junction when the SA node fails to pace or its impulses are blocked, producing a narrow complex escape rhythm.
40–60 bpm (junctional escape); 60–100 bpm (accelerated junctional); > 100 bpm (junctional tachycardia)Wandering Atrial Pacemaker
Wandering atrial pacemaker is a rhythm where the dominant pacemaker shifts between the SA node and other atrial foci, producing at least 3 different P wave morphologies.
< 100 bpmMultifocal Atrial Tachycardia
Multifocal atrial tachycardia is an irregular tachycardia with at least 3 different P wave morphologies and a rate > 100 bpm, commonly associated with severe pulmonary disease.
> 100 bpm (typically 100–150 bpm)Left Anterior Fascicular Block
Left anterior fascicular block is a conduction delay in the anterior fascicle of the left bundle branch, causing left axis deviation.
Normal (depends on underlying rhythm)Left Posterior Fascicular Block
Left posterior fascicular block is a conduction delay in the posterior fascicle of the left bundle branch, causing right axis deviation.
NormalIdioventricular Rhythm
Idioventricular rhythm is a slow, wide complex rhythm originating from a ventricular escape pacemaker, typically at 20–40 bpm.
20–40 bpm (escape); 40–100 bpm (accelerated idioventricular rhythm, AIVR)Asystole
Asystole is the absence of any electrical activity in the heart, presenting as a flatline on ECG — a cardiac arrest rhythm.
0 bpm (no cardiac activity)