Background--At the mitral annulus-aorta (MA-Ao) junction, the left atrium is continuous through the subaortic curtain with the musculature of the anterior mitral leaflet. Under experimental conditions, this region can generate abnormal electrical activity. In patients with left atrial tachycardia, we investigated whether this region could be the source of this arrhythmia.

Methods and Results--In 10 (28%) of 35 consecutive patients with left atrial tachycardia, the arrhythmia originated from the MA-Ao junction. Sustained, self-limited episodes of atrial tachycardia (cycle length, 340±56 ms; duration, 125±69 seconds) were repeatedly induced. Prematurity of the extrastimulus and time to first atrial tachycardia complex were directly correlated (R=0.66; P<0.001). During tachycardia, bipolar electrograms at the earliest site preceded onset of the P wave by 44±14 ms and were of longer duration and lower amplitude than those recorded from nearby left atrial sites (52±8 versus 24±4 ms, P<0.001; and 0.53±0.08 versus 3.45±0.96 mV, respectively; P<0.001). Ablation eliminated the tachycardia with no recurrence after a mean follow-up of 24±19 months. A comparative study in mouse embryos demonstrated the presence of the developing specialized conduction system in the MA-Ao region starting at embryonic age 11.5.

Conclusions--The MA-Ao junction can be a frequent source of left atrial tachycardia. This previously unrecognized site of origin may explain why catheter ablation has been less successful in eliminating left versus right atrial tachycardias. Remnants of the developing specialized conduction system could be the underlying substrate of this arrhythmia.