looking at wikipedia ...
hyperventilation syndrome (HVS) is an disorder that often presents with chest pain and a tingling sensation of the fingertips (paresthesia) and around the mouth, as well as deep and labored breathing (causing hyperventilation), although chronic but subtle hyperventilation can cause these symptoms too.
HVS can be part of a panic attack but, despite all the stigma, most people are not putting on a show but are in true distress.
People with HVS feel like they can't get enough air, but the opposite is actually true: they have too much oxygen and too little carbon dioxide in their blood. The hyperventilation is self-promulgating as rapid breathing causes carbon dioxide (CO2) levels to fall, and respiratory alkalosis (high blood pH) develops. This makes the symptoms worse, which causes the person to try breathing even faster, which further exacerbates the problem.
The respiratory alkalosis leads to changes in the way the nervous system fires and leads to the paresthesia, dizziness, and perceptual changes that often accompany this condition.
therefore, breahting into a paper bag, can help to redress this gaseous balance in the blood xxx
hth xx