6. Excessive lower esophageal acid exposure during pH monitoring (defined as distal esophageal pH < 4 for > 6.0% of the monitoring time) performed after at least 5 days off of PPIs and at least 2 days off of H2 blockers. At least 18 hour of esophageal pH recording will be considered adequate and inclusion will be based on the day (at least 18 hours of valid data) with the highest acid exposure percentage time.