Lung and chest wall mechanics in normal anaesthetized subjects and in patients with COPD at different PEEP levels

In order to assess the relative contribution of the lung and the chest wall to the derangements of respiratory mechanics in chronic obstructive pulmonary disease (COPD) patients with acute ventilatory failure (AVF), we studied eight COPD patients undergoing controlled mechanical ventilation for AVF and nine normal subjects anaesthetized for surgery as a control group. With the use of the interrupter technique together with the oesophageal balloon technique we measured: static lung and chest wall elastances (Est,L and Est,w, respectively), maximum (RL,max), minimum (RL,min) and additional (ΔRL) lung resistances, additional chest wall resistance (ΔRw) and, in the COPD group, total intrinsic positive end-expiratory pressure (PEEPtot). Measurements were repeated at 0, 5, 10 and 15 cmH₂O of applied positive end-expiratory pressure (PEEP). We found that, in the COPD group: 1) both Est,w and ΔRw were higher than in the normal group; 2) RL,max was markedly increased due to an increase of both RL,min and ΔRL; 3) even low levels of PEEP increased PEEPtot; 4) PEEP did not reduce elastance or total resistance of either the lung or the chest wall. We conclude that chest wall mechanics are abnormal in chronic obstructive pulmonary disease patients with acute ventilatory failure undergoing controlled mechanical ventilation and that positive end-expiratory pressure does not seem to be effective in reducing either elastance or resistance of the lung or chest wall.