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 (Wi) 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, Wi during T-piece breathing was evaluated.

Measurements And Results: Wi was determined using a modified Campbell's diagram. Total inspiratory work (Wi), work against flow-resistive resistance (W(ires)), work against elastic resistance (Wiel), work imposed by the ventilator system (W(imp)), dynamic intrinsic positive end-expiratory pressure (PEEPidyn), 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 Wi and its components W(ires) and Wiel significantly compared to the corresponding ZEEP settings. This was due mainly to a significant decrease in PEEPidyn when external PEEP was applied. Flow-by imposed less Wi 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 Wi 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 PEEPidyn. 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.