Abstract
BACKGROUND: End tidal inspiratory activity (ETIA) in diaphragm and parasternal intercostal muscles can be evoked in man and in animals by administration of histamine. Exacerbations of asthma and administration of histamine are often accompanied by hyperinflation. The aims of the study were to determine (1) the magnitude of ETIA in response to histamine in man, (2) the relative contributions of chemical and mechanical stimulation of airway receptors to ETIA, and (3) the importance of ETIA to hyperinflation. METHODS: The effects of inhalation of histamine on the electrical activities of the diaphragm and parasternal intercostal muscles measured with surface electrodes were studied in 21 subjects. The experiments were repeated after inhalation of 600 micrograms of salbutamol to prevent histamine induced bronchoconstriction and concomitant mechanical stimulation of airway receptors. Subjects were connected to a closed breathing circuit to measure the changes in functional residual capacity (FRC) for the different experiments. RESULTS: The mean values of histamine induced ETIA were 60.6% and 46.9% of peak inspiratory activities during control conditions for the diaphragm and intercostal muscles, respectively. After salbutamol histamine induced ETIA was reduced to about one quarter of pre-salbutamol values. FRC increased by 427 ml as a result of inhalation of histamine, but after salbutamol this increase was only 53 ml. The data for ETIA and FRC were interpreted as indicating that the contributions of airflow limitation and ETIA to histamine induced hyperinflation are comparable. CONCLUSIONS: Histamine is a forceful stimulus for inducing ETIA. Both chemical and mechanical stimulation of airway receptors contribute to evoke ETIA, of which the contribution of mechanical stimulation is the more important one. ETIA contributes substantially to histamine induced hyperinflation.