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Figure 4 (A) Spatiotemporal loops are unaffected by atrio:ventricular (A:V) ratio, illustrated by here by isthmus-dependent atrial flutter (IDAFL) with regular and variable A:V conduction in the same patient. Loops from each electrocardiogram (ECG) segment reach (1,1) and lie along the line of identity, and spectra are analogous. (B) Concealed entrainment in IDAFL (top) and non-isthmus-dependent atrial flutter (NIDAFL), show F-wave and electrogram similarity between pacing and tachycardia, post-pacing interval equal to atrial flutter cycle length (CL), and stimulus-electrogram equal to electrogram-electrogram intervals. CS 9–10, CSp = proximal coronary sinus; CS 3–4, CSM = mid-coronary sinus. (C) Clockwise IDAFL, shows a dominant spectral peak (14.01 dB) and consistent loops (R = 1.42 ± 0.11; XY, YZ shown). (D) NIDAFL with variable ECG spatial loops and marked intracardiac CL variability; ablated in low lateral right atrium. CS-D = distal coronary sinus; RF = mapping catheter. (E) Counterclockwise mitral annulus NIDAFL. The ECG was interpreted as NIDAFL or atrial fibrillation. Dominant spectral peak (8.91 dB) yet variable loops (R = 0.94 ± 0.13) suggested NIDAFL. Bi-atrial electroanatomic maps showed counterclockwise re-entry around the mitral annulus, with passive counterclockwise tricuspid annulus (TA) activation (left anterior oblique projection), that was entrained then ablated from left lower pulmonary vein to mitral annulus. Standard deviation of CL was 9.9 ms (distal CS) and 13.5 ms (low lateral right atrial). (F) Similar ECGs but dissimilar atrial flutter mechanisms. (Left) Dominant peak (19.44 dB) and consistent loops (R = 1.56 ± 0.02) confirmed counterclockwise IDAFL; TA-inferior vena cava isthmus ablation caused bidirectional block. (Right) Separately induced atrial flutter looked similar on ECG, yet varying loops (R = 1.11 ± 0.11) suggested NIDAFL. Macro-re-entry was entrained in the lateral right atrium and ablated using drag-line to superior vena cava. CS = coronary sinus; FFT = Fast Fourier Transform; RF = mapping catheter. (C) Clockwise IDAFL, shows a dominant spectral peak (14.01 dB) and consistent loops (R = 1.42 ± 0.11; XY, YZ shown). (D) NIDAFL with variable ECG spatial loops and marked intracardiac CL variability; ablated in low lateral right atrium. CS-D = distal coronary sinus; RF = mapping catheter. (E) Counterclockwise mitral annulus NIDAFL. The ECG was interpreted as NIDAFL or atrial fibrillation. Dominant spectral peak (8.91 dB) yet variable loops (R = 0.94 ± 0.13) suggested NIDAFL. Bi-atrial electroanatomic maps showed counterclockwise re-entry around the mitral annulus, with passive counterclockwise tricuspid annulus (TA) activation (left anterior oblique projection), that was entrained then ablated from left lower pulmonary vein to mitral annulus. Standard deviation of CL was 9.9 ms (distal CS) and 13.5 ms (low lateral right atrial). (F) Similar ECGs but dissimilar atrial flutter mechanisms. (Left) Dominant peak (19.44 dB) and consistent loops (R = 1.56 ± 0.02) confirmed counterclockwise IDAFL; TA-inferior vena cava isthmus ablation caused bidirectional block. (Right) Separately induced atrial flutter looked similar on ECG, yet varying loops (R = 1.11 ± 0.11) suggested NIDAFL. Macro-re-entry was entrained in the lateral right atrium and ablated using drag-line to superior vena cava. CS = coronary sinus; FFT = Fast Fourier Transform; RF = mapping catheter.