Hyperventilation

In medicine, hyperventilation (or hyperpnea) is the state of breathing faster or deeper (hyper) 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, slurred speech and sometimes fainting.

The related symptom tachypnea (or "tachypnoea") (Greek: "rapid breathing") is characterized by rapid breathing and is not identical with hyperventilation - tachypnea may be necessary for a sufficient gas-exchange of the body, for example after exercise, in which case it is not hyperventilation.

Causes
Stress or anxiety commonly cause hyperventilation; this is known as hyperventilation syndrome. Hyperventilation can be brought about voluntarily, by taking excessive deep breaths. Hyperventilation also occurs as a consequence of various lung diseases, head injury or stroke (central neurogenic hyperventilation, apneustic respirations, ataxic respiration, Cheyne-Stokes respirations or Biot's respiration), or when the body lacks oxygen (hypoxia), for instance in high altitude or as a result of anaemia. It may also occur as a result of sepsis, and is usually a sign of the beginning of refractory sepsis. Lastly, in the case of metabolic acidosis, the body uses hyperventilation to counter the increased acidity of the blood; this is known as Kussmaul breathing.

Mechanism
In normal breathing, both the depth and frequency of breaths is varied by the neural system in order to supply appropriate levels of oxygen to the body's tissues. This is mainly done by measuring the carbon dioxide content of the blood; normally, a high carbon dioxide concentration signals a low oxygen concentration, as we breathe in oxygen and breathe out carbon dioxide at the same time, and the body's cells use oxygen to burn fuel molecules to carbon dioxide.

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.)

If carbon dioxide levels are high, the body assumes that oxygen levels are low, and accordingly the brain's blood vessels dilate, to assure sufficient blood flow and supply of oxygen. Conversely, low carbon dioxide levels (e.g. from hyperventilation) cause the brain's blood vessels to constrict, resulting in reduced blood flow to the brain and lightheadedness. Doctors sometimes artificially induce hyperventilation after head injury to reduce the pressure in the skull, though the treatment has obvious risks (Stocchetti 2005).

The high pH value resulting from hyperventilation also reduces the level of available calcium (hypocalcemia), which affects the nerves and causes the numbness or tingling of the hands.

Treatment
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.