While atrial fibrillation represents the most common cause of irregular tachycardia, the differential diagnosis includes several other possibilities. Multifocal atrial tachycardia and any other supraventricular tachycardia (SVT) with variable atrioventricular (AV) conduction (such as atrial tachycardia or atrial flutter with variable AV block) can present as irregularly irregular rhythms. Sinus tachycardia with premature atrial contractions or premature ventricular contractions can also present as an irregular tachycardia. In this case the patient had conduction down 2 separate pathways within the AV node, demonstrating a pattern known as a double ventricular response. The mechanism of this arrhythmia is described in the following paragraphs.
The Irregular Tachycardia That Was Not Atrial Fibrillation—Discussion. Arch Intern Med. 2011;171(22):1986-1988. doi:10.1001/archinternmed.2011.578