Influence of external loading and assisted ventilation on chest wall mechanical properties

Instantaneous respiratory effective elastance (E t eff ) and tissue resistance (R t ) may be measured by studying the relationship between flow at the airway opening and at the chest during forced oscillations; using that method it has been shown that R t varies little during breathing while E t eff presents much larger phasic variations than can be explained by the curvature of the static pressure-volume curve ( Tomalak et al., 1997 ). The aim of this study was to test the hypothesis that the large variations of E t eff were related to the activity of respiratory muscles. For this, we studied in six healthy subjects the changes in E t eff and R t induced by inspiratory and expiratory elastic loading (IEL and EEL, respectively), resistive loading (REL) and assisted ventilation (ASV) while keeping lung volume, ventilation and breathing pattern as constant as possible. IEL and EEL predominantly increased E t eff during the inspiratory and expiratory phases, respectively; in contrast, ASV decreased E t eff during inspiration and almost completely abolished its positive volume dependence. The changes of R t usually paralleled those of E t eff . We conclude that respiratory muscle activity is responsible for most of the variations of E t eff during spontaneous breathing.