When dyspnea occurs in a patient with no lung abnormalities, it is called hyperventilation syndrome. Hyperventilation is defined as overbreathing to the point that the level of arterial carbon dioxide falls below normal. This may be an indicator of major, life-threatening illness. For example, a patient with diabetes who has very high blood glucose levels, a patient who has taken an overdose of aspirin, or a patient with a severe infection is likely to hyperventilate. In these patients, rapid, deep breathing is the body's attempt to stay alive. The body is trying to compensate for acidosis, the buildup of excess acid in the blood or body tissues that results from the primary illness. Because carbon dioxide, mixed with water in the bloodstream, can add to the blood's acidity, lowering the level of carbon dioxide helps to compensate for the other acids.

Similarly, in an otherwise healthy person, blood acidity can be diminished by excessive breathing, because it "blows off" too much carbon dioxide. The result is a relative lack of acids. The resulting condition, alkalosis, is the buildup of excess base (lack of acids) in the body fluids.