In this clinical investigation, the safety, feasibility and performance of the novel EnSite™ HD Grid Catheter mapping system for advanced high-density three-dimensional mapping will be studied in patients undergoing catheter ablation procedures for the treatment of non-paroxysmal atrial fibrillation (AF) or left atrial tachycardia (AT).

Catheter ablation for non-paroxysmal AF is more complex as triggers, if present at all, are not immediately apparent, but the abnormal atrial substrates are the likely predominant mechanisms. Additional catheter ablation strategies targeting for atrial substrate modification have been introduced. The common strategies involve either application of empirical linear lesion sets in the atrial areas or ablation of atrial areas with complex fractionated atrial electrograms in addition to pulmonary vein isolation. Nevertheless, the corresponding success rates in long-term sinus rhythm maintenance are modest.

Non-paroxysmal AF and left AT are characterized by fast and regular atrial activities, complex fractionated atrial electrograms, consistent direction of wave front propagation, and low peak-to-peak voltage. Advanced high-density three-dimensional catheter mapping strategies to target the evaluation of these characteristics and mechanisms responsible for the AF and left AT, identification of atrial areas with low peak-to-peak voltages and determination of the "critical" atrial targets for catheter ablation during the procedures would be essential.

In this clinical investigation, the safety, feasibility and performance of the novel EnSite™ HD Grid Catheter mapping system for advanced high-density three-dimensional mapping will be studied in patients undergoing catheter ablation procedures for the treatment of non-paroxysmal AF or left AT.

Inclusion Criteria:

1. Presence of non-paroxysmal atrial fibrillation (AF) or left atrial tachycardia (AT) referred for catheter ablation
2. Age of 18 years of age or older at time of Enrollment
3. On continuous anticoagulation (INR 2-3) for >4 weeks prior to the ablation
4. Able and willing to provide written informed consent to participate in this clinical investigation

Exclusion Criteria:

1. Secondary atrial fibrillation (AF)
2. Presence of a prosthetic valve(s) or hemodynamically significant valvular heart disease as determined by Study Investigator
3. Active systemic infection (e.g. sepsis)
4. Presence of left atrial thrombus (i.e., positive TEE) or myxoma, or interatrial baffle or patch via the transseptal approach
5. Contraindication to systemic anticoagulation (i.e., heparin, warfarin, or a direct thrombin inhibitor)
6. History of cerebrovascular accidents (Stroke, TIA)
7. Previous myocardial infarction, unstable angina pectoris or coronary artery by-pass <180 days at Enrollment or cardiovascular intervention expected in the 180 days post-Enrollment
8. Left atrial size >55mm
9. NYHA (New York Heart Association Classification) functional class III or IV heart failure
10. Left ventricular ejection fraction <35%
11. Uncontrolled Hyperthyroidism
12. Pregnant or of childbearing potential and not using adequate contraceptive methods or nursing
13. Participating in another clinical investigation that may confound the results of this clinical investigation
14. Life expectancy less than 12 months, as determined by Study Investigator
15. Severe clinical condition (e.g. active carcinoma) that, in the opinion of the Study Investigator, excludes the participation in the study