Despite its name, heartburn has nothing to do with the heart. Symptoms, however, are similar to those of a heart attack or heart disease. Heartburn is an irritation of the esophagus -- the tube that connects your throat and stomach. It's caused by stomach acid. One feels a burning discomfort in your upper belly or below your breastbone. Heartburn symptoms can start up because of a problem with a muscular valve called the lower esophageal sphincter (LES) (WebMD Medical Reference, 2017).