Hyperventilation refers to overbreathing, in which ventilation exceeds the metabolic demand, and its related physiological consequences. Excessive breathing can cause dizziness, lightheadedness, weakness, shortness of breath, a sense of unsteadiness, muscle spasms in the hands and feet, and a tingling feeling around the mouth and fingertips. All of these symptoms are the result of abnormally low levels of carbon dioxide in the blood caused by overbreathing. The term hyperventilation syndrome (HVS) is sometimes used to describe the effects of hyperventilation observed in an emergency department setting. Many acute (sudden onset) cases of hyperventilation arise from panic, anxiety, and other emotional conditions. Hyperventilation, particularly chronic hyperventilation (that persists over time), can also be due to a range of medical conditions. The opposite of hyperventilation is termed hypoventilation (underventilation).