A variety of circuits have been described, including a path around the mitral annulus (perimi- tral flutter), re-entry involving the left atrial roof, and re-entry around regions of scarring in the right or left atrium. Non isthmus-dependent atrial flutters often occur in patients with atrial scarring from prior heart surgery or ablation but also may occur in any form of heart disease or may be idiopathic.134,140,319 Non isthmus-dependent atrial flutters can coexist with CTI- dependent flutter or involve the presence of multiple atrial re- The reentrant circuits are classified as either macroreentrant AT (large; often several centimeters or longer in diameter) or microreentrant AT ( 2 cm in diameter), which may be indistinguishable from focal AT.321 In the presence of substantial atrial disease, prior surgery, or prior radiofrequency catheter ablation, the ECG flutter- wave morphology is not a reliable predictor of whether the flutter circuit involves the CTI. Although an ECG with a typical flutter appearance has good predictive value for CTI- dependent flutter in a patient who has not undergone prior catheter ablation of AF, the ECG appearance is less useful in predicting the flutter circuit in a patient who has previously undergone AF ablation.322 325 The presence of a positive or biphasic (but dominantly positive) deflection in V1, accom- panied by deflections in other leads inconsistent with typical counterclockwise atrial flutter, suggests the presence of an atypical flutter (Figure 17). Definitive diagnosis requires EP study and intracardiac mapping.326 Catheter ablation of non CTI-dependent flutter requires more extensive mapping than does ablation of CTI-dependent flutter, and success rates are lower (Table 8). The location of the circuit determines ablation approach and risks. 