AIM - AIR 3. 2. 2 - Hyperventilation

Hyperventilation is breathing at a faster and/or deeper rate than the body requires for
good oxygenation at the existing work level. Normally the rate of breathing is
controlled by the amount of carbon dioxide in the lungs and in the blood. In
hyperventilation carbon dioxide is blown-off and this leads to changes in the acidity of
the blood which causes symptoms. The arteries to the brain constrict, reducing the
blood supply. Pilots may notice slight dizziness, a feeling of coldness, a sensation like
a tight band around the head, and pins and needles in the hands and feet. Paradoxically
they will often feel they cannot get enough air. Continued hyperventilation may cause
unconsciousness.

Hyperventilation most commonly occurs in association with anxiety, fear, or during
intense concentration on a difficult task such as performing a complicated instrument
procedure. The symptoms, particularly the shortness of breath, are not unlike those of
hypoxia, so rather than trying to make the diagnosis, the following procedure should
be taken:

(a)     Breathe oxygen, if available, at 100% and, if on a pressure demand system,
         "press-to-test". After 3 or 4 breaths of oxygen the symptoms will improve
         markedly if hypoxia is the cause.

(b)     If the symptoms persist, consciously slow the rate of breathing to 10-12 breaths
         per minute and do not breathe deeply. Keep the respiratory rate slow until the
         symptoms disappear and then resume a normal breathing pattern. [If below
         8 000 feet ASL (2 500 m) hypoxia is unlikely to be the cause of the problem].