Shortness of breath is most often a symptom of cardiac or respiratory (lung) disorders. Heart failure and coronary artery disease commonly present with breathlessness as a common symptom.
Patients with heart failure may experience shortness of breath with exertion, or when lying flat. They may also experience an accumulation of fluid in the ankles and lower legs as well as general fatigue.
Coronary artery disease can manifest as shortness of breath on exertion, with breathlessness in place of typical chest pain symptoms.
Shortness of breath is almost always a sign of a significant medical problem, and should always be evaluated by a doctor.