Background: The application of positive end-expiratory pressure (PEEP) and maintenance of increased mean airway pressure (MAP) has been associated with improved oxygenation in adult respiratory distress syndrome. Recently, attention has been directed toward elevating MAP by establishing auto-PEEP when ventilating with an inverse inspiratory to expiratory ratio in opposition to applied PEEP. We theorized that FRC distribution and local lung unit end-expiratory pressure (EEP) would be different when equal levels of PEEP were established by applying PEEP or by producing auto-PEEP.
Methods: Using a four-chamber lung model with each chamber having a different time constant (TC), we applied equal levels of applied PEEP (I:E ratio 1:3) and auto-PEEP (I:E ratio 3:1) and evaluated local lung unit EEP and end expiratory lung volume (EELV).
Results: During all trials with applied PEEP, local lung unit EEP was equal to applied PEEP, whereas during auto-PEEP local EEP differed (p < 0.01). At a tracheal auto-PEEP level of 12.7 cm H2O, the lung unit with the longest TC (slow lung unit) had an EEP of 15.8 cm H2O, while the shortest TC unit (fast lung unit) had an EEP of 10.1 cm H2O (p < 0.01). Similarly, local EELVs were more maldistributed with auto-PEEP than with applied PEEP. At a tracheal PEEP level of 12.7 cm H2O, the EELV increase in the slow lung unit with auto-PEEP was 1,054 mL vs 918 with applied PEEP (p < 0.01), whereas the fast lung unit's EELV increase with auto-PEEP was 142 mL compared with 212 mL with applied PEEP (p < 0.01).
Conclusion: Comparing equal levels of the auto-PEEP with applied PEEP, a greater maldistribution of local lung unit EEP and EELV was established with the auto-PEEP. During auto-PEEP, the greatest EEP and EELV occurred in the slow lung unit, and the lowest EEP and EELV developed in the fast lung unit.