A New Method for Determining Gastric Acid Output Using a Wireless Capsule

Background: \- Acid-related gastrointestinal disorders are numerous and may require chronic treatment and maintenance, but they are often difficult to diagnose and treat. Gastric analysis is a critical tool for diagnosing and treating acid-related disorders; however, at present it requires a technically difficult procedure, using a nasogastric tube, that is costly, invasive, and uncomfortable. Recently a new tool, the SmartPill, has been developed that can measure acid levels, temperature, and pressure throughout the stomach, small intestine, and colon by swallowing a small capsule. Researchers are interested in comparing current methods of measuring stomach acid with the SmartPill by studying both healthy volunteers and individuals with acid-related gastrointestinal disorders. Objectives: \- To evaluate the usefulness and accuracy of the SmartPill for gastric analysis, compared with current procedures. Eligibility: * Phase 1A/1B: Healthy volunteers between 18 and 60 years of age. * Phase 2: Individuals at least 18 years of age who have been diagnosed with Zollinger-Ellison Syndrome or acid hypersecretion. * Phase 3: Individuals at least 18 years of age who have been diagnosed with refractory gastroesophageal reflux disease, peptic ulcer disease, or idiopathic gastric hypersecretion. Design: * This study involves a screening visit, a 3-day inpatient study visit, and possible additional outpatient visits. * Participants will be screened with a medical history and physical examination, as well as blood and urine samples. * Phase 1A/1B: Participants will have a 3-day inpatient study visit with the following procedures: * Imaging study of the stomach (simultaneous gastric emptying scintigraphy), followed by a special diet for the next day s test. * Gastric analysis using a nasogastric tube to collect stomach acid secretions. * SmartPill study, in which participants will ingest a SmartPill and wear a monitor for 24 to 48 hours to collect readings. * Participants i