A hiatal hernia occurs when a portion of the stomach protrudes above the diaphragm (the diaphragm is the large, sheet-like muscle which should separate the contents of the chest from the contents of the abdomen). The majority of hiatal hernias (90%) are of a type causing reflux, which occurs when the acidic contents of the stomach wash up the esophagus (the esophagus is the tube which should only carry swallowed substances down into the stomach). Presence of these acidic contents burn the esophagus, resulting in the symptom commonly referred to as heartburn. Most of these types of hernias do not require surgical repair. Symptoms are helped by various medications which decrease the acidity of the stomach contents, and thwarting the effects of gravity which can exacerbate the problem at night (patients should go to sleep propped up on an angle by a sufficient number of pillows). This other type of hiatal hernia more frequently requires surgical repair, because its complications include hemorrhage (massive bleeding), incarceration, and strangulation (which can result in death of stomach tissue). Furthermore, in this more serious type of hiatal hernia, other abdominal contents (intestine, spleen) may also protrude into the chest cavity, and pressure from crowding in the chest cavity can result in heart problems.