EFFECT OF LOW-LEVEL PEEP ON INSPIRATORY WORK OF BREATHING IN INTUBATED PATIENTS, BOTH WITH HEALTHY LUNGS AND WITH COPD

Objective: Evaluation of low-level PEEP (5 cm H2O) and the two differe nt CPAP trigger modes in the Bennett 7200a ventilator (demand-valve an d flow-by trigger modes) on inspiratory work of breathing (W-i) during the weaning phase. Design: Prospective controlled study. Setting: The intensive care unit of a university hospital. Patients. Six intubated patients with normal lung function (NL), ventilated because of non-pu lmonary trauma or post-operative stay in the ICU, and six patients rec overing from acute respiratory failure due to exacerbation of chronic obstructive pulmonary disease (COPD), breathing either FB-CPAP or DV-C PAP with the Bennett 7200a ventilator. Interventions: The patients stu died were breathing with zero end-expiratory pressure (ZEEP), as well as CPAP of 5 cm H2O (PEEP), with the following respiratory modes: the demand-valve trigger mode, pressure support of 5 cm H2O, and the flow- by trigger mode (base flow of 20 1/min and flow trigger of 2 1/min). F urthermore, W-i during T-piece breathing was evaluated. Measurements a nd results. Wi was determined using a modified Campbell's diagram. Tot al inspiratory work (W-i), work against flow-resistive resistance (Wir es), work against elastic resistance (W-iel), work imposed by the vent ilator system (W-imp), dynamic intrinsic positive end-expiratory press ure (PEEP(idyn)), airway pressure decrease during beginning inspiratio n (P-aw) and spirometric parameters were measured. In the NL group, on ly minor, clinically irrelevant changes in the measured variables were detected. In the COPD group, in contrast, PEEP reduced W-i and its co mponents W-ires and W-iel significantly compared to the corresponding ZEEP settings. This was due mainly to a significant decrease in PEEP(i dyn) when external PEEP was applied. Flow-by imposed less W-i on the C OPD patients during PEEP than did demand-valve CPAP. Differences in W- imp between the flow-by and demand-valve trigger models were significa nt for both groups. However, in relation to W-i these differences were small. Conclusion. We conclude that the application of low-level exte rnal PEEP benefits COPD patients because it reduces inspiratory work, mainly by lowering the inspiratory threshold represented by PEEP(idyn) Differences between the trigger modes of the ventilator used in this study were small and can be compensated for by the application of a sm all amount of pressure support.