12 patients with Atrial Fibrillation, a kind of irregular heart beat who have undergone Radio frequency (RF) ablation will be asked to swallow the capsule.

As they swallow the capsule, images of the esophagus will be taken to see if the RF ablation caused any form of damage to the esophagus considering it being so close to the heart.

After the capsule has been removed from the mouth, they will be asked about the tolerability of the capsule.

A total of 12 patients with a known diagnosis of Atrial Fibrillation who have previously undergone Atrial RF ablation will be enrolled in the study. Patients will be asked to swallow the OFDI capsule while being unsedated. The capsule is attached to a tether that allows the capsule operator to navigate the capsule as it progresses down the esophagus using natural propulsion called peristalsis as well as to bring it back up to pulled out of the mouth when the procedure is done.

As the capsule progresses down the esophagus, multiple, two dimensional, microscopic cross- sectional images of the esophagus are acquired.

Inclusion Criteria:

• Subjects must be over 18 years of age
• Subjects must be able to give informed consent.
• Subjects must be referred to Massachusetts General Hospital (MGH) endoscopy for RF Ablation treatment of their Atrial Fibrillation.

Exclusion Criteria:

• Subjects with current esophageal strictures and dysphagia
• OR subjects with a prior history of intestinal strictures, prior GI surgery or intestinal Crohn's disease.
• OR subjects with a known history of chronic aspiration.
• OR women who are currently pregnant.