RESPIRATORY SENSATIONS IN SUBJECTS WHO LACK A VENTILATORY RESPONSE TOCO2

An urge to breathe is perceived during breath hold and hypercapnia (te rmed 'air hunger') and during heavy exercise (often termed 'shortness of breath'). To better understand the neural mechanisms responsible fo r these sensations we studied five patients (8-17 years old) with cong enital central hypoventilation syndrome (CCHS) who lack ventilatory re sponse to CO2. CCHS patients reported no respiratory discomfort during CO2 inhalation or during maximal breath hold which was of much longer duration than age-matched controls. However, all 3 CCHS patients who exercised heavily reported some sensations akin to shortness of breath (they increased breathing nearly as much as controls). Our results ar e consistent with two possibilities. First, the air hunger of hypercap nia and breath hold is caused by projection to the forebrain of respir atory chemoreceptor afferents which bypass the respiratory centers, wh ile exercise shortness of breath is caused by direct projections of li mb afferents or locomotory center activity. Second, air hunger and sho rtness of breath share the same origin - projection of increased brain stem respiratory center motor activity (corollary discharge) to the f orebrain.