When you over-breath the tendency is to hyperventilate. Hyperventilation: "Excessive rate and depth of respiration leading to abnormal loss of carbon dioxide from blood", Merriam-Webster. A normal breath is 12 bpm for men and 14 bpm for women. If you are asthmatic your breath might be two times the normal rate.
As an asthmatic you may take in plenty of oxygen, but your exhaling more carbon dioxide.
When the carbon dioxide is low in the body, the pH of the blood rises. This makes the hemoglobin hold onto the oxygen tighter than usual and the bodies cells can't get as much oxygen. What then can happen is the asthmatic will breath even faster to bring in more oxygen, as they blow off even more carbon dioxide making the problem worse. This is why you hear of people breathing into a paper bag so they can put the carbon dioxide back into the body.
The person that is hyperventilating may experience terror and feelings of losing control which escalates the problem. A lot of times this can happen during an anxiety attack.
My next blog will explain how to help with the hyperventilation issues.