Introduction
This topic explains how two of the most common patient complaints, dyspnea (breathlessness) and fatigue (abnormal tiredness), arise. Both are subjective symptoms with many possible causes, so a good answer is mechanism-based: what the sensation is, which pathways generate it, and how those pathways change in the main disease groups. Heart failure, anemia and lung disease are covered in other topics and are referred to here only as examples of mechanisms.
Definitions
Dyspnea is a subjective sensation of uncomfortable breathing that is out of proportion to the effort being made. It is not the same as tachypnea or hypoxemia, which are objective signs that may or may not be present. Fatigue is a subjective feeling of lack of energy and inability to sustain physical or mental effort that is not relieved by ordinary rest. Physiological fatigue follows exertion and resolves with rest; pathological fatigue is disproportionate, persistent, or present at rest.