In theory, there could be several possible effects of induced bronchoconstriction on the dead space. Firstly, narrowing of the airways could be predicted to decrease airway volume and hence, the anatomic dead space. The present results show that this does not occur: absolute VDan did not alter significantly (Table 1). The reason for this may lie in the fact that there was a significant elevation in FRC following bronchoconstriction, which would act to increase VDan. It is probable that the opposing effects on VDan of bronchoconstriction and increase in lung volume canceled each other and resulted in a net absence of significant change in VDan. These results are in accord with a previous study on chronic asthma subjects in whom the VDan was within predicted normal values. comments
Similarly, it would be predicted that VDphys would increase with an increase in FRC; however, VDphys actually decreased following bronchoconstriction, and this was associated with a decrease in Vt, which is known to reduce VDphys. It is likely, therefore, that had Vt not changed, the VDphys would have increased. Thus, the change in ventilatory pattern following histamine inhalation limited the increases in VDan and VDphys that would otherwise have occurred. It could be argued that a different breathing pattern, with large tidal volumes and the same or lower respiratory frequency, would be more advantageous, since, in normal subjects, while the absolute values of VDan and VDphys increase with this pattern, the ratio Vd/Vt greatly decreases, resulting in higher Pa02 and lower PaC02. However, there may be two reasons why this pattern may be disadvantageous during induced bronchoconstriction in asthmatic subjects: first, the associated increase in FRC may, by increasing VDphys to an even greater extent, result in no decrease in Vd/Vt. Second, the increase in work of breathing associated with the increase in ventilation inherent in this ventilatory pattern could be more dyspnogenic as well as increasing the oxygen uptake of the respiratory muscles.