3. Patients with prior history of esophageal surgery (e.g. Nissen fundoplication, partial resection, dilation, stent placement), known anatomic abnormality of the esophagus (e.g. stricture, Schatzki's ring, Zenker's diverticulum, malignancy, varices, ulcers), and/or known functional abnormality of the esophagus (e.g. achalasia, dysmotility).