Usually, the pain is centered below the left breast regardless of the person's position or physical activity. The shortness of breath results from fluid seeping into the air spaces of the lung, a condition called pulmonary congestion or pulmonary edema. Ultimately, this process is similar to drowning. In the early stages of heart failure, a person may be short of breath only during physical exertion. As heart failure worsens, shortness of breath occurs with less and less activity, until it occurs at rest.
People are short of breath mostly when they lie down because fluid seeps throughout the lung tissue.
When they sit up, gravity causes fluid to collect at the base of the lungs, which is less of a problem. Any disorder that upsets the normal, delicate balance between oxygen supply and oxygen requirement, such as inadequate oxygen-carrying capacity of the blood from anemia or increased overall metabolism in the body from an overactive thyroid, can make a person short of breath. When the heart pumps inefficiently, blood flow to the muscles may be inadequate during exercise, causing the person to feel weak and tired. Symptoms are often subtle. People compensate by gradually diminishing activity, or they may blame the symptoms on increasing age.