Atrial flutter

Atrial flutter is the term given to one of the four types of supraventricular tachycardia; in it, atrial activation occurs as a consequence of a continuous ‘short circuit’: a defined and fixed anatomical route, resulting in a fairly uniform atrial rate, and uniform atrial flutter waves on the ECG. The ventricles are not a part of this arrhythmia circuit, and ventricular activation is variable, dependent on atrioventricular (AV) nodal conduction. Given that the atrial rate is essentially uniform (e.g. 300 min−1), ventricular activation tends to be regular (i.e. 150 min−1, 100 min−1, 75 min−1, etc., if the atrial rate is 300 mins−1), or regularly irregular if changes are occurring in the fraction of conducted impulses to the ventricles. When AV nodal conduction permits only 4:1 conduction or less, atrial flutter is usually obvious, but when ventricular rates are higher (150 min−1 or more) the flutter waves can be obscured by the QRS complexes, making diagnosis more difficult. Atrial flutter is of two types, typical and atypical. Typical atrial flutter is a right atrial tachycardia, with electrical activation proceeding around the tricuspid valve annulus. This arrhythmia is dependent on a zone of slow electrical conduction through the cavotricuspid isthmus (the tissue lying between the origin of the inferior vena cava and the posterior tricuspid valve). The resulting circuit can be either anticlockwise (activation proceeds up the inter-atrial septum, across the atrial roof, down the free wall, and then through the cavotricuspid isthmus to the basal septum) or clockwise (down the inter-atrial septum and around the circuit in the opposite direction). Anticlockwise typical atrial flutter is more common. Atypical atrial flutter refers to all other atrial flutters, and this includes other right atrial flutters (e.g. pericristal flutter), left atrial flutters, post-ablation or post-surgical flutters, and pulmonary vein flutters. The feature common to all types of flutter and which differentiates flutter from other types of supraventricular tachycardia is the presence of a macro-re-entrant anatomical circuit around which the electrical impulse travels continuously and repeatedly, thereby generating the flutter. Even though typical atrial flutter has a fairly obvious and specific appearance on the ECG, atypical flutters do not, and often it is only possible to differentiate atypical flutter from atrial tachycardias by invasive electrophysiology studies, as the ECG alone may be insufficient.