To observe, using a prospectively designed study, the effect of type of oral anticoagulant on intra-procedural heparin requirements in patients undergoing Atrial Fibrillation ablation and to assess whether ACT assay accurately reflects heparin anti coagulation effect.

Atrial fibrillation (AF) is the most common arrhythmia affecting the US population and accounts for 15% of strokes worldwide. Radiofrequency ablation has become a frequently used therapy for treatment of afib after failure of at least one anti-arrhythmic drug. Pulmonary Vein Isolation (PVI) remains the cornerstone of AF ablation and requires transseptal puncture and placement of catheters in the left atrium which can be thrombogenic and cause stroke. This is avoided by intra-procedural infusion of heparin and the anti-coagulation effect is monitored using Activated Clotting Time (ACT). With the approval of Novel Oral Anti-Coagulants (NOAC), increasing number of patients are undergoing AF ablation on these medications. It has been observed that patients on NOACs require much larger doses of heparin and take longer time to reach therapeutic ACT. Consequently, patients are at higher risk for thromboembolism and stroke. On the other hand, higher doses of heparin can expose patients to excessive bleeding complications. The investigators seek to explain the mechanism of "heparin resistance" in such a patient population and to develop a protocol that can achieve therapeutic anticoagulation quicker.

Inclusion Criteria:

• All adults, male and female, aged 18 and above.
• Patients with paroxysmal, persistent or chronic AF
• All patients must be on therapeutic doses of Coumadin or one of the NOACs for at least 1 month prior to the procedure.

Exclusion Criteria:

• Patients with known primary or secondary coagulopathy (such as Hemophilia, vWF deficiency, active malignancy, ATIII deficiency, Factor V Leiden deficiency, Hx of recurrent DVT/PE)
• Patients with hypoalbuminemia, cirrhosis
• chronic LMWH therapy,
• ESRD on HD, and severely impaired kidney function with CKD stage IV
• BMI >35
• prosthetic heart valves and
• advanced liver disease
• previous procedural complications such as tamponade

All adults, male and female, aged 18 and above, who have been referred to the EP lab for elective PVI (Pulmonary Vein Isolation).