[Respiratory mechanics of chronic obstructive lung disease in acute respiratory insufficiency].

Respiratory mechanics abnormalities in patients with chronic obstructive pulmonary disease (COPD) in acute respiratory failure (ARF) consist of the followings : 1) expiratory flow limitation, 2) marked increase in airway resistance, 3) dynamic hyperinflation. As a results, both resistive and elastic loads to the respiratory muscles are increased. These abnormalities, which are already present in stable COPD patients, are considerably more marked in ARF. Our contribution was to systematically describe the passive mechanical properties of lung and chest wall of COPD patients tracheally intubated, mechanically ventilated, sedated-paralyzed for ARF. Mechanical properties, i.e. resistances and elastances, were obtained from the rapid end-inspiratory airway occlusion technique during constant-flow inflation. This method allows to partition the resistance into its two components, namely the interrupter resistance, which reflects airway resistance, and additional tissue resistance, which pertains to time constant unequalities and/or viscoelastic behavior. We also determined the static and dynamic elastances of both lung and chest wall. Static intrinsic positive end-expiratory pressure (PEEPi) was obtained from end-expiratory airway occlusion. By changing, for one breath, inflation flow, at constant volume, and inflation volume, at constant flow, we investigated the time, and hence the frequency dependence of resistance and elastance. In addition, we divided the inspiratory work of breathing into its four components which are the PEEPi component, the static work, the purely resistive work and the additional work. Finally we compared our results with those of normal anesthetized and paralyzed subjects. We found that airway resistance was markedly higher in COPD, as were also the additional resistance and the dynamic elastance of the lung. Additional resistance and dynamic elastance of lung and chest wall exhibited a marked frequency dependence in COPD. Shortening the inspiratory time could result not only to reduce the hyperinflation but also to increase expiratory flow through the increased dynamic pulmonary elastance. The inspiratory work was twice higher in COPD than in normals because of the PEEPi and the resistive components. Due to their flow and volume dependence, the results of resistances and elastances should be standardized.