Arrhythmias originating from the SA node, atria, or AV junction are categorized as supraventricular arrhythmias because the impulses come from above the ventricles. Impulses that originate from the ventricles are categorized as ventricular arrhythmias. 

The width (duration) of the QRS complex differentiates supraventricular rhythms from ventricular rhythms. In the normal heart, an impulse from the SA node reaches the ventricle in < 120 milliseconds if it travels through the heart’s normal conduction pathway. All other pathways take > 120 milliseconds. 

A QRS complex of less than or equal to 120 milliseconds is supraventricular in origin. 

Supraventricular arrhythmias include:

• Sinus arrhythmia
• Sinus rhythm with premature atrial contractions
• Sinus or another rhythm with variable atrioventricular block
• Multifocal atrial rhythm or wandering atrial pacemaker when the heart rate is < 100 bpm, or multifocal atrial tachycardia when the heart rate is > 100 bpm
• Atrial fibrillation

The most common cause of a wide QRS complex is ventricular in origin and may be due to an abnormal focus in the ventricles that takes over as the pacemaker of the heart. However, a wide QRS complex > 120 milliseconds (0.12 seconds) does not always mean that an arrhythmia is ventricular in origin. A supraventricular impulse may be impeded or slowed in the bundle branches, or a ventricle may be in a refractory state in which a supraventricular impulse cannot follow. 

• A narrow QRS complex (< 120 milliseconds) is supraventricular in origin.
• A wide QRS complex (> 120 milliseconds) may be either supraventricular or ventricular in origin.