Cough Frequency and Cough-Receptor Sensitivity are Increased in Man at Altitude

Abstract

1. Travellers to high altitude often complain of paroxysmal cough, which has not been previously investigated. We recorded overnight cough frequency and cough-receptor sensitivity to inhaled citric acid in a group of climbers travelling to 5300 m or higher. 2. Cough frequency, monitored in ten subjects, increased from a median of 0 coughs at sea level (range 0-1) to 5 coughs at 5000 m (range 0-13) and to over 60 coughs in subjects ascending to 7000 m. Citric acid cough threshold, measured in 42 subjects, was unchanged on arrival at 5300 m compared with sea level (geometric mean difference 1.26, 95% confidence intervals 0.84-1.89, P = 0.25), but was significantly reduced after 6 days, or more, at altitude compared with sea level (geometric mean difference 2.2, 95% confidence intervals 1.54-3.15, P = 0.0002). Cough threshold was not related to symptoms of acute mountain sickness, oxygen saturation, carbon dioxide tension or lung function. 3. These results indicate an increase in cough and cough-receptor sensitivity after some days at altitude. This may be due to respiratory tract damage from breathing cold dry air at increased ventilatory rates. Other explanations, such as sub-clinical pulmonary oedema or an effect on the cough centre of acclimatization to altitude, cannot be excluded.

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

“…The authors have previously [6] demonstrated a significant increase in the frequency of nocturnal cough and a decrease in the CACT in a group of 42 subjects ascending to Everest Base Camp at 5,300 m in the Nepalese Himalaya. The changes in NCF were most obvious in a subgroup of three climbers ascending to 7,000 m or above.…”

“…Using data from a previous study [6], it was estimated that nine subjects would be needed to demonstrate a significant fall in the cough threshold with a study power of 80% [37]. It was only possible to accommodate eight subjects in the chamber, and a number of data points were missed owing to either subject illness or logistical problems.…”

“…Solutions of increasing concentrations of citric acid were inhaled via an ultrasonic nebulizer (Sonix 2000; Clement Clarke International, Harlow, UK) during a slow vital capacity inspiration over 5 s as previously described [6]. Citric acid was chosen as it is less likely to cause bronchoconstriction than other tussive agents such as capsaicin and is well tolerated.…”

“…Numerous anecdotal reports exist of paroxysmal cough in climbers and travellers to high altitude [1±3] which may be severe enough to cause rib fractures [2,3]. The cause of this cough is not known, but has been attributed to the inspiration of cold, dry air, acute mountain sickness (AMS), high altitude pulmonary oedema (HAPO), bronchoconstriction or respiratory tract infection [4,5].In the first systematic study of cough at high altitude [6], an increase in cough frequency and cough receptor sensitivity was reported in a group of subjects ascending to Mount Everest Base Camp in Nepal at an altitude of 5,300 m. However, because of the nature of the study, subjects at Base Camp were unavoidably exposed to cold, dry air, which has been shown to cause cough [7]. The aim of this study was therefore to measure cough frequency and cough receptor sensitivity in a group of subjects making a simulated ascent of Mount Everest (8,848 m) in a hypobaric chamber in which the temperature and humidity were controlled within normal sea level limits, and also to study the effects of extreme hypobaric hypoxia on cough.…”

“…In the first systematic study of cough at high altitude [6], an increase in cough frequency and cough receptor sensitivity was reported in a group of subjects ascending to Mount Everest Base Camp in Nepal at an altitude of 5,300 m. However, because of the nature of the study, subjects at Base Camp were unavoidably exposed to cold, dry air, which has been shown to cause cough [7]. The aim of this study was therefore to measure cough frequency and cough receptor sensitivity in a group of subjects making a simulated ascent of Mount Everest (8,848 m) in a hypobaric chamber in which the temperature and humidity were controlled within normal sea level limits, and also to study the effects of extreme hypobaric hypoxia on cough.…”