The cause of HVS is unknown, but some persons who are affected appear to have an abnormal respiratory response to stress, sodium, lactate, and other chemical and emotional triggers, which results in excess minute ventilation and hypocarbia. In most patients, the mechanics of breathing are disordered in a characteristic way. When stressed, these patients rely on thoracic breathing rather than diaphragmatic breathing, resulting in a hyperexpanded chest and high residual lung volume. Because of the high residual volume, they are then unable to take a normal tidal volume with the next breath and consequently experience dyspnea. Patients who are treated with breathing retraining, stress reduction therapy, and various medications experience significant reductions in the frequency and the severity of exacerbations.