Increased resting bronchial tone in normal subjects acclimatised to altitude

BACKGROUND: Normal subjects frequently experience troublesome respiratory symptoms when acclimatised to altitude. Bronchial hyperresponsiveness (BHR) and full and partial flow-volume loops were measured before and after ascent to 5000 m altitude to determine if there are changes in resting bronchial tone and BHR that might explain the symptoms. METHODS: BHR to histamine was measured using a turbine spirometer to record partial and full flow-volume curves and expressed as log dose slopes. Twenty one subjects were tested at sea level and after acclimatisation at 5000 m altitude. RESULTS: No significant change in log dose slope measurements of forced expiratory volume in 1 second occurred after acclimatisation, and the maximal expiratory flow with 30% of forced vital capacity remaining (MEF(30%)) rose on the full loop and fell on the partial loop. Their ratio (full divided by partial) rose on average by 0.28 (95% confidence limits 0.14 to 0.42) from the mean (SD) sea level value of 0.87 (0.20). CONCLUSIONS: There is no increase in BHR in normal subjects acclimatised to altitude but an increase in resting bronchial tone occurs that could be released by deep inspiration. This may be the result of increased cholinergic tone.