Background: Prior studies have reported that focal atrial tachycardias (AT) originate more frequently from the right atrium than the left atrium with an average ratio of 66% to 33%.
Objective: We hypothesized that whenever early activation is recorded near the septum, detailed mapping of both atria may uncover a left atrial origin more frequently than previously reported.
Methods: We analyzed 135 consecutive patients (57 males; age 50±18 years) that underwent catheter ablation of focal AT, defined as a discrete source of activation with centrifugal spread. Thirteen patients had a prior ablation attempt. Ten patients had incessant tachycardia and had developed a tachycardia-induced cardiomyopathy. The other patients had normal LV function (EF 58±7%).
Results: In 135 patients we found 148 different ATs. Seventy-four tachycardias (50%) originated from the left atrium (Table). The ATs originated from a septal or para-septal region in 56 (38%) of patients. Right septal/paraseptal ATs (n=21) originated from the coronary sinus ostium (10), fast pathway area (5), slow pathway area (2), Tendon of Todaro (2), and fossa ovalis (2). Left septal/paraseptal ATs (n=35) originated from the mitral annulus-aorta junction (anterior leaflet-non coronary cusp) (17), right sided pulmonary vein (15), and fossa ovalis (3).
Conclusion: Septal or paraseptal sources account for 38% of all focal ATs and more frequently originate in the left atrium (63%) than in the right atrium (37%). This finding underscores the value of mapping both atria when early activation is found near the septum.

Anatomical Distribution of Focal Atrial Tachycardia

Right atrium	74	Left atrium	74
Crista terminalis/sinus venosa	31	Left pulmonary veins	20
Right septa/paraseptal	22	Left septal/paraseptal	35
Tricuspid annulus	13	Mitral annulus	13
Right atrial appendage	5	Posterior wall or roof	4
Superior vena cava	3	left atrial appendage	2