Stability of the circadian alteration in lung function in asthma

Although studies on pharmacologic interventions for nocturnal asthma are increasing, information about the long-term stability of circadian spirometric changes or bronchial responsiveness is not known. This study was undertaken to evaluate these variables in 10 patients with asthma measured quarterly during a 1-year period. We have found that the overnight decrease in peak expiratory flow rate measurements is stable (mean range, 13.8% to 16.4%) during the year, as are the quarterly 4 PM (1600-hour) and 4 AM (0400-hour) FEV1 values. The 4 PM provocative concentration of methacholine that produced a 20% fall in FEV1 (PC20) (range of geometric mean, 0.213 to 0.359 mg/ml) and the 4 AM PC20 (range, 0.057 to 0.152 mg/ml) for the group were also relatively stable. Individual variation was higher for the PC20 values than for the FEV1. We concluded that during a 1-year period, without acute respiratory events, (1) the overnight decrement in peak expiratory flow rate and the 4 AM and 4 PM FEV1 values were constant and (2) bronchial responsiveness demonstrated some individual variability, but for the group, it remained stable.