Hyperventilation is over-breathing or breathing in excess of metabolic requirements. It usually involves rapid high thoracic rather than diaphragmatic breathing. Typically it occurs during an asthma attack or when a predisposed person is feeling anxious or in a state of shock such as following surgery or severe injury (see also ‘Asthma’). From a clinical perspective it is important to distinguish between acute or transient over-breathing and chronic or persistent hyperventilation. It is the latter type that has generated a lot of interest especially in connection with anxiety disorders. Controversially the chronic form has even been given the status of a syndrome, namely hyperventilation syndrome or HVS. Attempts have been made to attribute it as a cause of various disorders and panic disorder in particular. Clinical accounts, together with proposed treatments, have appeared for more than 100 years. This history provides a good example of the interaction of physical (e.g. asthma) and mental (e.g. anxiety) factors and the inherent difficulty of differentiating between cause and effect.

Over-breathing occurs as part of the classic ‘fight or flight’ response as the body involuntarily prepares for action (see ‘Stress and health’). Such breathing soon removes sufficient carbon dioxide from the lungs to lead to a significant fall in the blood level of carbon dioxide, a state known as ‘hypocapnia’. There is also a loss of carbonic acid that leads to buffer depletion as the body compensates.