In medicine, hyperventilation, also known as tachypnea or hyperpnea, is the state of breathing faster or deeper than necessary, and thereby reducing the carbon dioxide concentration of the blood below normal. This causes various symptoms such as numbness or tingling in the hands, feet and lips, lightheadedness, dizziness headache, chest pain and sometimes fainting.
Stress or anxiety commonly cause hyperventilation; this is known as hyperventilation syndrome.
Hyperventilation can be brought about voluntarily, by taking excessive deep breaths. It can also result from inflating numerous party balloons by mouth. Hyperventilation also occurs as a consequence of various lung diseases, head injury or stroke, or when the body lacks oxygen (hypoxia), for instance in high altitude or as a result of anaemia.
Lastly, in the case of metabolic acidosis, the body uses hyperventilation to counter the increased acidity of the blood.
In normal breathing, both the depth and frequency of breaths is varied by the neural system in order to balance the flow of oxygen into the body with the flow of carbon dioxide out of the body. The gasses in the alveoli of the lungs are nearly in equilibrium with the gasses in the blood. Normally, less than 10% of the gas in the alveoli is replaced each breath. Deeper or quicker breaths exchange more of the alvolar gas with air and have the net effect of drawing more carbon dioxide out of the body, since the carbon dioxide concentration in normal air is very low.
The resulting low concentration of carbon dioxide in the blood is known as hypocapnia. Since carbon dioxide is held in the blood mostly in the form of carbonic acid, hypocapnia results in the blood becoming alkaline, i.e. the blood pH value rises. (In the normal person, this alkalosis would automatically be countered by reduced breathing, but for various reasons this doesn't happen when the causes of hyperventilation mentioned above are present.)
Reduced carbon dioxide concentration causes the blood vessels in the brain to constrict, resulting in reduced blood flow to the brain. It also affects the nerves, and can cause heart arrhythmias.
The common treatment of breathing into a paper bag is no longer recommended by physicians and nurses, as it can cause the carbon dioxide level to rise too rapidly. Instead, the treatment centers around relieving the underlying condition, such as anxiety.
An acute attack of hyperventilation can be treated by breathing through a piece of garden-hose of about 20 cm. In this way the carbon dioxide level is quickly normalised.