Factors affecting oxygen saturation during methacholine challenge in a mixed population.

Bronchial challenge with methacholine or histamine is associated with a reduction in arterial oxygen tension, which can be appreciable. In this study, oxygen saturation was monitored during methacholine challenge in subjects with and without respiratory disease, over a wide age range, in order to identify factors predicting a large fall in saturation during the challenge. Two hundred and twenty subjects aged 24-86 years were included, comprising 15 healthy adult volunteers, and 205 adults from a random sample of the local adult population taking part in a survey of bronchial responsiveness. Subjects with ischaemic heart disease or baseline FEV1 < 60% predicted were excluded. Methacholine challenge was performed by the Newcastle Dosimeter technique; oxygen saturation (SaO2) was monitored using a pulse oximeter and finger probe. Of the 220 subjects, 27% were current smokers and 39.5% were ex-smokers; 26% reported asthma or bronchitis. Mean baseline FEV1 was 100% predicted; mean baseline saturation was 97%. Mean fall in saturation was 3.2% (range 0-17.5%). Multiple regression analysis revealed that fall in saturation during methacholine challenge is related to baseline FEV1, baseline SaO2, log of total methacholine dose inhaled, and fall in FEV1 during challenge. Change in saturation was not related to subject age, smoking history, reported asthma or bronchitis, or the presence of respiratory symptoms. Methacholine challenge produces a significant fall in oxygen saturation, but this is not greater in subjects who are old or have low baseline saturation.