TITLE: HYPOXIA SUPPRESSES SYMPTOM PERCEPTION IN ASTHMA.

ABSTRACT Any factor that inhibits an asthmatic’s ability to appropriately recognise their symptoms maycontribute to treatment delay, “near miss” events and death during acute severe asthma. Thepurpose of this study was to investigate the effects of two common features of acute severeasthma, hypoxia and hypercapnia, upon respiratory sensation. 16-stable asthmatics were exposedto 3 gas conditions (34-mins each): isocapnic hypoxia (arterial blood O 2 saturation~80%),hypercapnia (increase in end-tidal CO 2 ~5-10 Torr) or isocapnic normoxia on 3 separate days.The perceived magnitude of externally applied resistive loads, measured during each gascondition, was reduced throughout hypoxia compared with normoxia and there was a trend for aprogressive decline during hypercapnia. Within the 15-min post gas inhalation period,methacholine-induced symptoms of difficult breathing, chest tightness and breathlessness,measured using modified Borg scales, were 25-30% lower after hypoxia compared withnormoxia, but were not reduced following hypercapnia. We conclude that 30 minutes ofsustained hypoxia and possibly hypercapnia impair sensations of respiratory load and that theeffects of hypoxia persist for at least 10-minutes after returning to normoxia.