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 different CPAP trigger modes in the Bennett 7200a ventilator (demand-valve and 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-pulmonary trauma or post-operative stay in the ICU, and six patients recovering from acute respiratory failure due to exacerbation of chronic obstructive pulmonary disease (COPD), breathing either FB-CPAP or DV-CPAP with the Bennett 7200a ventilator.Interventions The patients studied 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 l/min and flow trigger of 2 l/min). Furthermore, W_i during T-piece breathing was evaluated.Measurements and results W_i was determined using a modified Campbell's diagram. Total inspiratory work (W_i), work against flow-resistive resistance (W_ires), work against elastic resistance (W_iel), work imposed by the ventilator system (W_imp), dynamic intrinsic positive end-expiratory pressure (PEEP_idyn), airway pressure decrease during beginning inspiration (P_aw) and spirometric parameters were measured. In the NL group, only minor, clinically irrelevant changes in the measured variables were detected. In the COPD group, in contrast, PEEP reduced W_i and its components W_ires and W_iel significantly compared to the corresponding ZEEP settings. This was due mainly to a significant decrease in PEEP_idyn when external PEEP was applied. Flow-by imposed less W_i on the COPD patients during PEEP than did demand-valve CPAP. Differences in W_imp between the flow-by and demand-valve trigger models were significant for both groups. However, in relation to W_i these differences were small.Conclusion We conclude that the application of low-level external 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 small amount of pressure support.Supported by Deutsche Forschungsgemeinschaft SFB 330 Organprotektion Project B17