Cough frequency and cough receptor sensitivity to citric acid challenge during a simulated ascent to extreme altitude

The aim of this study was to determine the frequency of cough and the citri c acid cough threshold during hypobaric hypoxia under controlled environmen tal conditions. Subjects were studied during Operation Everest 3, Eight subjects ascended t o a simulated altitude of 8,848 m over 31 days in a hypobaric chamber. Freq uency of nocturnal cough was measured using voice-activated tape recorders, and cough threshold by inhalation of increasing concentrations of citric a cid aerosol, Spirometry was performed before and after each test. Subjects recorded symptoms of acute mountain sickness and arterial oxygen saturation daily. Air temperature and humidity were controlled during the operation. Cough frequency increased with increasing altitude, from a median of 0 coug hs (range 0-4) at sea level to 15 coughs (range 3-32) at a simulated altitu de of 8,000 m. Cough threshold was unchanged on arrival at 5,000 m compared to sea level (geometric mean difference (GMD) 1.0, 95% confidence interval s (CI) 0.5-2.1, p=0.5), but fell on arrival at 8,000 m compared to sea leve l (GMD 3.3, 95% CI 1.1-10.3, p=0.043). There was no relationship between co ugh threshold and symptoms of acute mountain sickness, oxygen saturation or forced expiratory volume in one second. Temperature and humidity in the ch amber were controlled between 18-24 degrees C and 30-60%, respectively. These results confirm an increase in cough frequency and cough receptor sen sitivity associated with hypobaric hypoxia, and refute the hypothesis that high altitude cough is due to the inhalation of cold, dry air, The small sa mple size makes further conclusions difficult, and the cause of altitude-re lated cough remains unclear.