Circadian rhythm of peak expiratory flow in asthmatic and normal children

A study was undertaken to examine the circadian rhythm of peak flow rate in asthmatic and normal children in the community by means of cosinor analysis. An initial study of 12 matched pairs of asthmatic and normal children was used to determine the mean amplitude of peak expiratory flow (PEF) variability in the two groups (7.8% and 4.4%) and the number of subjects required to show a significant difference. On the basis of this study 37 community based children with asthma diagnosed by questionnaire and 40 control subjects measured PEF four times daily for 14 days. Cosinor analysis of the data produced a significant fit in 20 asthmatic and 18 control children. A small but significant difference in amplitude was observed between the asthmatic (6.2%) and the control (4.2%) children. There was no significant phase difference between the rhythms in the two groups. Cosinor analysis explained 14% of PEF variance. It did not provide a reproducible estimate of phase between week 1 and week 2; the acrophase changed by more than one hour in 26 of 37 asthmatic children. The cosinor model may be inappropriate for the investigation of low amplitude circadian rhythms, especially when measurements are made infrequently.