Response of normal infants to inhaled histamine.

The age at which nonspecific bronchial hyperresponsiveness (BHR) is first seen in humans is unknown, though both genetic and environmental factors have been implicated in its development. The current study aimed to establish whether BHR to histamine can be demonstrated in normal infants. Twelve infants, mean age of 7.8 months (range, 3 to 18 months), were studied. None had any history of a previous significant respiratory illness. Respiratory function was monitored using the maximal flow at function residual capacity (VmaxFRC) obtained with the forced expiratory flow-volume technique. Histamine was inhaled in doubling concentrations from 0.125 to 8 g.L-1. A greater than 30% fall in VmaxFRC was considered a response. All infants responded to histamine, the geometric mean concentration for their response being 1.4 g.L-1. Associated transient changes for the group were an increase in respiratory rate (p less than 0.02) and a fall in SaO2 (p less than 0.001). Forced expiratory flow-volume curves were concave in all infants after the last dose of histamine. We speculate that humans are born with "bronchial hyperresponsiveness" and that genetic or environmental factors determine which infants lose it thereafter.