Diurnal rhythms in airway obstruction.

Diurnal patterns of variation in peak expiratory flow rate were studied in patients with all types of airway obstruction. Most regular patterns fall into one of two types: morning dip (M) with the lowest reading in the morning and (P) or double dip with the lowest readings morning and evening. Regular patterns were most frequently seen in asthmatics, but occurred also in bronchitis, particularly those with a history of wheeze. Treatment with bronchodilators reduces the prevalence of all regular patterns but does not abolish them. M pattern is characteristic of asthma but P pattern is the most frequent regular pattern seen in bronchitis. Amplitude of variation is greater in asthmatics than in simple bronchitis. It is reduced by about 20% with bronchodilators. Random observation of magnitude of response of peak flow rate to a bronchodilators is not a good indicator of amplitude of variation over the day. Because of the occurrence of a peak pattern in many patients the magnitude of diurnal variation may be underestimated by twice-daily peak flow rate readings. Treatment with corticosteroids does not appear strongly to influence either characteristic patterns or amplitude of variation.