The Science Journal of the American Association for Respiratory Care
Background: The MNPB 840 and Drager E2 ventilators incorporate open expiratory valve or ?free breathing? systems. This allows a patient's spontaneous inspiratory and expiratory efforts to be superimposed upon pressure-regulated breaths. Minimizing dynamic gas trapping and imposed expiratory work requires a responsive, low resistance expiratory valve. We evaluated expiratory valve performance during ?free breathingi? modes and with increasing levels of CPAP
Methods: A Hamilton Veolar set in volume ventilation with a modified sine wave, rate = 34, Vt =300 mL, and I:E = 1:2, simulated spontaneous breathing on a two compartment lung model. Compliance was set 30 mL/cm H2O with minimal airways resistance. Expiratory valve resistance was evaluated with determinations of: maximum expiratory pressure (MEP), peak expiratory flow (PEF), peak expiratory resistance (PER) and expiratory retard (ER). Pressure, flow, and time were measured from scalar waveforms with a Bicore CP-100 monitor. PER was calculated as MEP/PEF. ER was defined as the time from peak expiratory pressure to baseline pressure. Measurements were made on both ventilators during simulated spontaneous breathing on CPAP (0, 5, 10, and 20 cmH2O) and during MNPB 840 Pressure Controled SIMV (PC-SIMV), Drager E2 Airway Pressure Release Ventilation (APRV) and Pressure Controled Ventilation Plus (PCV+) modes. Peak inspiratory pressure was set at 20 with 5 cmH2O PEEP, rate = 6, and cycle ratio = 4:1 during PC-SIMV, APRV, and PCV+. Values from 4 breaths for MEP, PEF, PER and ER were averaged at each CPAP level and are reported as the pooled mean for each ventilator. Values from 3 - 4 superimposed spontaneous breaths at 20 cmH2O during PC-SIMV, APRV, and PCV+ were averaged.
Results: PER of the MNPB 840 remains constant during varying levels of CPAP but increases when the open expiratory valve ?free breathing? feature is active. In contrast, PER across the Drager E2 expiratory valve decreases as the CPAP level is increased. PER was 12.4, 10.9, 10.1, and 8.4 cmH2O/L/sec at CPAP of 0, 5, 10, and 20 cmH2O respectively. PEF was highest during ?free breathing? in both APRV and PCV+ when compared to all other test conditions.
|840 CPAP||E-2 CPAP||840 PC-SIMV||E-2 APRV and PCV+|
Conclusion: Expiratory valve resistance differs between the two ventilators during ?free breathing? modes and CPAP. At high respiratory rates this may affect dynamic gas trapping and expiratory work of breathing.