Mechanical load and inspiratory muscle action during induced asthma

To examine the relationship between inspiratory mechanical load in asthma and the pattern of respiratory muscle recruitment, we studied lung and chest wall mechanics in 7 asymptomatic asthmatics in whom progressive bronchoconstriction was induced with inhaled aerosolized histamine. A fall in the FEV1 to 49.5 +/- 3.9% of the control value (mean +/- 1 SE) was associated with a 10.7-fold increase in the inspiratory work rate of the inspiratory muscles from 6.7 +/- 1.6 to 71.4 +/- 11.4 Joules/min. Elastic work accounted for 69% of the total work during the control period and 57% at the maximal level of bronchoconstriction studied. The net pressure-time product for the inspiratory muscles, measured over 1 min, rose fivefold from 245 +/- 33 cmH2O.s to a maximum of 1,211 +/- 107 cmH2O.s, indicating a relatively greater increase in the recruitment of the intercostal/accessory muscles of inspiration. The abdominal muscles, which were recruited during bronchoconstriction, relaxed during inspiration, and permitted outward movement of the abdominal wall with progressively smaller net increases in Pga. We concluded that during induced asthma (1) the increase in inspiratory muscle work was largely the result of hyperinflation, (2) the recruitment of the intercostal/accessory muscles exceeded that of the diaphragm, and (3) the combined action of the intercostal/accessory and abdominal muscles favored the diaphragm.