The CTI is wider in the lateral portion and thinner in the central portion. The width and muscle thickness of CTI are variable, from several millimeters to around 3 cm in width and depth of 1 cm roughly. The CTI is bounded anteriorly by the tricuspid annulus and posteriorly by the ostium of the IVC and the eustachian ridge. However, the flutter wave morphology might change in the presence of underlying atrial disease, prior surgery, or previous ablation which makes the flutter wave morphology not a reliable indicator of AFL type. Clockwise AFL is observed in only 10% of clinical cases. It is also known as “common AFL” or “CTI-dependent AFL.” When the circuit rotates in the opposite direction, it is referred to as clockwise (CW) typical AFL or reverse typical AFL ( Figure 1). According to the new classification, typical AFL is a macroreentrant atrial tachycardia that usually proceeds up the atrial septum (counterclockwise or CCW), down the lateral atrial wall, and through the CTI between the tricuspid valve annulus and inferior vena cava (IVC). The most practical classification is based on isthmus versus non-isthmus dependency ( Diagram 1). In contrast, focal atrial tachycardia (AT) is a rapid abnormal atrial rhythm originating from a “point source” with a baseline between P waves on ECG. AFL is defined as abnormal atrial activity inside a reentrant circuit with a diameter more than 2 cm 2 at a high rate of 240–320 bpm which makes a continuous oscillation without an isoelectric baseline.
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