Hyperventilation

Overview
In medicine, hyperventilation (or overbreathing) is the state of breathing faster and/or deeper than necessary, thereby reducing the carbon dioxide concentration of the blood below normal.

This is in contrast to hyperpnea, where the increased breathing is required to meet demand, as during and following exercise or when the body lacks oxygen (hypoxia), for instance in high altitude or as a result of anaemia. Hyperpnea may also occur as a result of sepsis, and is usually a sign of the beginning of refractory sepsis.

Hyperventilation can, but does not necessarily cause symptoms such as numbness or tingling in the hands, feet and lips, lightheadedness, dizziness, headache, chest pain, slurred speech and sometimes fainting, particularly when accompanied by the Valsalva maneuver. Sometimes hyperventilation is induced for these same effects. Hyperventilation can sometimes be self induced for moments of needed focus and adrenaline.

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 are causes of hyperventilation; this is known as hyperventilation syndrome.

Hyperventilation can also be brought about voluntarily, by taking many deep breaths. Hyperventilation can also occur as a consequence of various lung diseases, head injury, coffee abuse or stroke (central neurogenic hyperventilation, apneustic respirations, ataxic respiration, Cheyne-Stokes respirations or Biot's respiration).

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 primarily in order to maintain normal amounts of carbon dioxide but also 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 gases in the alveoli of the lungs are nearly in equilibrium with the gases 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 alveolar 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 neural control is not 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. Thus, though it seems counterintuitive, breathing too much can result in a decrease in the oxygen supply to the brain. Doctors sometimes artificially induce hyperventilation after head injury to reduce the pressure in the skull, though the treatment has potential risks.

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. This occurs because alkalinisation of the plasma proteins (mainly albumin) increases their calcium affinity.