Abstract

The lability index was examined in 45 normal subjects between 17 and 45 years and also in six patients with asthma, five with a history of asthma in childhood but no clinical attacks, and three with hay fever. The measurements were repeated after β blockade with propranolol. Fifteen per cent of the normal subjects exhibited a degree of lability after propranolol which has been previously found only in asthma. It is suggested that a constrictor mechanism exists in normal subjects on exercise but that β receptor activity protects them from bronchiolar constriction; post-exercise bronchoconstriction in the asthmatic may be explained in terms of this mechanism. It is also suggested that a proportion of `normal' subjects are potentially more labile than the remainder of the population and that it is these who may develop clinical asthma if they also happen to be atopic subjects.