Hyperventilation is a condition where the lungs have too low a concentration of carbon dioxide because they are expelling carbon dioxide faster than the body can produce it.  This in turn results in there being too little carbon dioxide in the bloodstream, which the body compensates for by making the blood more alkaline.  This in turn sets off several other possible reactions including dizziness, tingling in the lips, hands or feet, weakness, fainting or even seizures.

Hyperventilation can be induced voluntarily (and often is by person diving deep underwater) or can be an involuntary response.  A common cause is severe anxiety or stress.

Treatment should focus on the underlying cause.  For example, if it is caused by hypoxia, oxygen should be administered. If it is caused by anxiety, counselling is usually effective.

Hyperventilation at Wikipedia