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Case A 37-year-old asymptomatic male physician from Mexico without any previous medical problems was referred after routine physical examination for evaluation of atrial fibrillation with a slow ventricular rate. Arrhythmia duration was unknown. He stated that bradycardia had been consistently noted on previous clinical examinations and had been attributed to competitive long-distance running since childhood. Heart rates in the “40s” had been noted since age 10 to 11 years when he had been informed of a “heart murmur.” Subsequent echocardiographic examination had been normal. The patient denied any family history of sudden death or arrhythmias. He had no siblings and no children. His prior records (and those of his parents) were unavailable. Physical examination was normal except for a regular bradycardia of 45 beats/min. Twelve-lead ECG showed fixed R-R intervals. No clear P-wave activity was identified. However, there was small-amplitude baseline electric activity of debatable origin, interpreted variously by experienced electrocardiographers as standstill, artifact, or atrial fibrillation waves (Figure 1A). A transthoracic echocardiogram showed normal ventricular volume and function (left ventricular end-diastolic diameter, 52 mm; left ventricle end-systolic diameter, 34 mm) and mild tricuspid regurgitation with estimated pulmonary artery systolic pressure of 40 mm Hg. Both atria were severely enlarged (left atrial volume index, 58 mL; right atrial area, 41 cm). Doppler echocardiographic examination of both tricuspid and mitral atrioventricular (AV) inflow showed complete absence of A waves, indicating lack of left and right atrial contractile activity (Figure 2). A transesophageal echocardiogram did not reveal any evidence of an intracardiac shunt. A Bruce protocol exercise treadmill test was stopped at 10 minutes and 26 s because of patient shortness of breath. Heart rate accelerated gradually to a maximum of 131 beats/min and always remained regular. Baseline atrial activity (if any) was not interpretable because of exercise artifact. Isolated premature ventricular contractions were observed during exercise. A 24-hour Holter monitor showed regular rhythm (mean heart rate, 47 beats/ min; range, 35–95 beats/min; maximum R-R interval, 1.8 s) with 56 ventricular bigeminal cycles. The only symptom during recording periods was an abnormal lateral chest sensation that corresponded with heart rates between 30 and 40 beats/min. The patient underwent an external cardioversion without appreciable change in heart rate (44 beats/min), although electrocardiographic intervals between QRS-T complexes became completely isoelectric for several minutes before small-amplitude deflections resumed (Figure 1B). At electrophysiology study, the first electrode catheter was placed in the CS and demonstrated regular, rapid atrial activity at a cycle length of 274 ms, suggestive of atrial flutter (Figure 3A). In striking contrast, a subsequently placed duodecapolar electrode catheter sited conventionally to map the right atrial free wall and cavotricuspid annulus recorded no electric activity, despite excellent tissue contact. This remained unchanged by deflecting the catheter around the right atrium to include posterior locations or by varying its vertical tilt. Point-by-point electroanatomical mapping (CARTO) confirmed absolute lack of recordable electrograms in the free wall (Figure 4A through 4C). Low-voltage activity was observed in the septum at and above the ostium of the CS. Normal CS electrograms were recorded in the body of the CS but diminished distally. Exploration of the anticipated region of the sinus node (filter settings, 0.05–500 Hz) revealed slow ( 30 beats/min) electric activity in a discrete location without corresponding local atrial depolarization (Figure 4D). Atrial flutter was mapped to a region of the proximal CS and its roof. Fractionated local electrograms were recorded at its junction with the inferior midseptum. Flutter was not pace terminable. Radiofrequency energy applied to the proximal CS slowed tachycardia and rendered it pace terminable. On termination, the His bundle escape rhythm was clear (Figure 3C). The His-ventricular interval (50 ms) was normal. An irregular, dissociated slow atrial escape emerged from the distal CS. No AV or VA conduction was observed with CS or

Congenital Sick Sinus Syndrome With Atrial Inexcitability and Coronary Sinus Flutter