The lability index has been used to classify asthmatic subjects into three physiologically distinct groups and provide indices of the fixed and labile components of airways resistance. This classification was used to examine two series of asthmatics (A and B) in order to determine the effect of age on bronchial lability. The mean follow-up period was five years in series A and 18 years in series B. In series A the lability index had been measured previously and changes in value over a period of years were observed. Series B had been carefully assessed on clinical grounds in childhood, and from the case histories obtained then it was possible to determine the severity of the original symptoms and to compare them with the situation at follow-up when the lability index was measured.
Results show that the labile component decreases with age but remains persistently abnormal in some subjects despite the absence of symptoms of asthma. The fixed component remains or increases with age. Thus highly labile asthmatic subjects (called group 2) have the best chance of improvement, while those subjects in whom the fixed component of airways resistance predominates (called group 3) are likely to remain in this group and have persistent symptoms.
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