Esophageal impedance-pH monitoring will be performed in all participating subjects using an ambulatory multichannel intraluminal impedance (MII) and pH monitoring system (Sleuth; Sandhill Scientific, Inc. Higland Ranch, CO, USA). During the test, each subject will eat three standard meals, while the beginning and ending times of every meal will be indicated in a diary. Subjects will also be instructed to press the event marker button on the data logger whenever they experience chest pain. Data recording will be concluded after 24 hours, when patients will return to the endoscopy unit for the catheter removal.