THE EFFECTS OF APPLIED VS AUTO-PEEP ON LOCAL LUNG UNIT PRESSURE AND VOLUME IN A 4-UNIT LUNG MODEL

Background: The application of positive end-expiratory pressure (PEEP) and maintenance of increased mean airway pressure (MAP) has been asso ciated with improved oxygenation in adult respiratory distress syndrom e. Recently, attention has been directed toward elevating MAP by estab lishing auto-PEEP when ventilating with an inverse inspiratory to expi ratory ratio in opposition to applied PEEP. We theorized that FRC dist ribution and local lung unit end-expiratory pressure (EEP) would be di fferent 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) a nd evaluated local lung unit EEP and end expiratory lung volume (EELV) . Results: During all trials with applied PEEP, local lung unit EEP wa s 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 w ith the longest TC (slow lung unit) had an EEP of 15.8 cm H2O, while t he 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 mt vs 918 with applied PEEP (p<0.01), whereas the fast lung unit's EELV increase wit h auto-PEEP was 142 mt compared with 212 mt with applied PEEP (p<0.01) . Conclusion: Comparing equal levels of auto-PEEP with applied PEEP, a greater maldistribution of local lung unit EEP and EELV was establish ed with the auto-PEEP. During auto-PEEP, the greatest EEP and EELV occ urred in the slow lung unit, and the lowest EEP and EELV developed in the fast lung unit.