The relationship between M W/FEVj and reactivity of asthmatic airways may account for the presence of abnormal MW/FEVj ratios in some but not all asthmatic subjects; in this study, MW/FEVj ratios were clearly lower and SGawDI/SGawMW^ ratios were clearly higher in those asthmatic subjects with greater methacholine responsiveness. Worsening pulmonary function has also been associated with increased responsiveness to pharmacologic bronchoconstrictors, although that analysis is complicated by the effects of airway geometry on methacholine responses. However, based on the lack of correlation between MW/ FEVi and expiratory flow rates, lower FEVi values did not account for lower MW/FEVj ratios; the same was true for specific conductance. canadian good neighbor pharmacy
The explanation for the correlation between age and MW/FEV! ratios (Fig 1) is unclear. It is known that FEVX decreases as a function of age, primarily because of loss of lung elastic recoil. It is also known that the loss of lung recoil (Pst[L]) is greater at high than at low lung volumes. Since FEVx maneuvers are initiated at total lung capacity whereas MW maneuvers are performed at lower lung volumes, loss of Pst(L) (with age) might be relatively more important on forced expiratory than on MW maneuvers. In addition, in contrast to FEVx, MW is influenced by both inspiratory and expiratory events. Since Pst(L) is an important determinant of expiratory but not of inspiratory flow, loss of lung recoil with age might favor higher ratios of MW/FEVX.
In conclusion, MW/FEVx ratios were lower in asthmatic than in normal subjects; this was true in some asthmatic subjects whose pulmonary function results were otherwise within the predicted normal range. Low MW/FEY^ values may indicate airway hyperreactivity (which can be confirmed by methacholine bronchoprovocation testing). These results should be considered during the interpretation of pulmonary function testing; this is especially pertinent to disability evaluations, a situation in which the MW/ FEVj ratio has been advocated as an index of patient effort.