Significance of effect of beta blockade on ventilatory function in normal and asthmatic subjects.

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.