Circadian rhythm of peak expiratory flow in children passively exposed and not exposed to cigarette smoke.

BACKGROUND Because airway calibre shows a circadian rhythm and since exposure to passive smoking reduced lung function this study was undertaken to investigate whether passive smoking affects the circadian rhythm of peak expiratory flow (PEF) in schoolchildren. METHODS Twenty schoolchildren (12 boys and 8 girls, aged 10-11 years) exposed to passive smoking were matched for sex, age, and height with 20 children who had not been exposed to cigarette smoke. Exposure to passive smoking was assessed by questionnaire and by urinary cotinine concentrations. A portable spirometer was used to measure PEF at 16:00, 20:00, 22:00, 06:00, 08:00, and 12:00 hours on a consecutive Saturday and Sunday. The circadian changes in PEF were measured by the cosinor method. RESULTS Both groups showed diurnal fluctuation in PEF values with a noticeable circadian rhythm. PEF peaks were the same in the two groups and occurred around 15:00 hours. The cosinor mean was approximately 10% lower in children exposed to passive smoking and the amplitude was approximately 60% higher than in the unexposed children. CONCLUSION Passive smoking in children is associated with a reduction in the cosinor mean and an increase in the amplitude of the normal circadian rhythm of airway calibre. This increased PEF rhythm amplitude may be considered as an early indication of airway obstruction.