Hyperventilation (deriving from HYPER = TOO MUCH and VENTILATION = AIR MOVEMENT) refers to a type of breathing which is too deep and too rapid.
Such breathing results in :
1) too much oxygen
2) too little carbon dioxide
entering the blood stream.
Indeed, severe hyperventilation can result in the amount of carbon dioxide in the blood stream falling by 50℅ within sixty seconds.
Why is a reduction of the amount of carbon dioxide in the blood undesirable?