Chest
Volume 108, Issue 4, October 1995, Pages 1073-1079
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Laboratory and Animal Investigations: Articles
The Effects of Applied vs Auto-PEEP on Local Lung Unit Pressure and Volume in a Four-Unit Lung Model

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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 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.

Section snippets

Lung Model

We developed a four-unit lung model from two training test lungs (model 1600 Dual Adult, Michigan Instruments Inc, Grand Rapids, Mich) connected in parallel (Fig 1). For each of the four lung units, compliance was established by varying the application point of springs along the sides of each unit, while resistance was varied by using endotracheal tube segments of different diameters. Resistance to gas flow for each of the lung units was estimated from the measured resistance of each resistor,

Lung Unit EEP

The EEP of lung units and the total system PEEP are listed in Table 2 for each of the three PEEP levels evaluated. Figure 2 shows the change in pressure vs time of the model trachea and of each lung unit during both 1:3 and 3:1 ventilation. Equal levels of total system PEEP were established for both 1:3 and 3:1 I:E ratio settings during all trials. None of the three trials at the 1:3 I:E ratio produced differences between total system PEEP and the EEP in any of the four lung units. At the 12 cm

Discussion

The effects of auto-PEEP on the distribution of local lung unit EEP and the distribution of EELV are different from those established by applied PEEP. With applied PEEP, local lung unit EEP is equal to the level of PEEP applied at the trachea, whereas with auto-PEEP, large variations in EEP developed both between lung units and between the trachea and specific lung units. Those lung units with the highest compliance, the greatest resistance (slow lung units), or both develop the highest level

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    Supported in part by grants from the Puritan-Bennett Corporation and the Newport Company.

    revision accepted February 16.

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