The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

1 SETTING RELEASE TIME & PEEP-LOW DURING APRV: A BENCH STUDY

Gary Martin, BS, RRT; Carl Haas, MLS, RRT, FAARC Adult Respiratory Care, University of Michigan Hospital & Health Centers, Ann Arbor MI

BACKGROUND: Controversy exists about the best method of setting the release time (TL) and the low PEEP level (PEEPL) when airway pressure release pressure ventilation (APRV) is used. Two methods have been described: 1) set PEEPL in the traditional manner, using a Best-PEEP methodology and set TL sufficient to just allow full expiration, and 2) set PEEPL to zero and while observing the expiratory flow graphic, adjust TL at a point that expiratory flow is at 25-50% of peak expiratory flow, thereby ensuring auto-PEEP. This study was done to compare the two methods of setting TL and PEEPL on lung mechanics.

METHODS
: An Igmar Medical ASL-5000 Servo Lung was set to a passive two compartment lung model. One lung was adjusted to simulate a normal lung (C=40 mL/cmH2O, R=5 cm H2O/L/sec) and the other ARDS (C=10 mL/cm H2O, R=10 cmH2O/L/sec). A Puritan Bennett 840 ventilator was set to the BiLevel mode with a rate=12, high PEEP=25 cm H2O, and a rise time of 100% for all test conditions. A Bicore Pulmonary Monitor measured peak inspiratory (PIF) and expiratory flow (PEF); the ASL-5000 measured VT and FRC for each lung compartment and mean airway pressure (MAP); and the NPB-840 provided PEEP, total-PEEP, and TL. Four test conditions were studied in a random order: 1) PEEPL =10 cm H2O (set PEEP), TL to allow complete exhalation; 2) PEEPL =0 cm H2O, TL to maintain auto-PEEP =10 cm H2O (AP-10); 3) PEEPL =0 cm H2O, TL to allow expiratory flow to decrease to 50% of PEF (PEF-50%); and 4) PEEPL =0 cm H2O, TL to allow expiratory flow to decrease to 25% of PEF (PEF-25%). The results of 12-15 breaths were averaged for a given run; three runs were conducted for each condition.

RESULTS
: Results are expressed as mean +SD. All comparisons between groups were significant (p<0.0125 with Bonferroni correction) except for: total-PEEP (set PEEP vs AP-10); MAP (set PEEP vs PEF-25% and AP-10 vs PEF-50%); FRC (AP-10 vs PFR-50%) and PIF (set PEEP vs AP-10).

    Auto-PEEP Methods
  set PEEP AP-10 PEF-50% PEF-25%
TL (sec) 1.0 + 0.0 0.38 + 0.0 0.43 + 0.0 0.60 + 0.0
PEEP (cm H2O) 11.0 + 0.0 4.8 + 0.0 4.0 + 0.0 2.3 + 0.1
Total-PEEP (cm H2O) 11.0 + 0.1 10.3 + 0.6 9.0 + 0.1 4.7 + 0.1
MAP (cm H2O) 23 + 0.0 24 + 0.0 23 + 0.0 23 + 0.0
VT, normal lung (mL) 397.2 + 6.4 429.1 + 5.4 455.6 + 4.5 516.0 + 3.4
VT, ARDS lung (mL) 51.6 + 1.0 57.8 + 0.8 62.1 + 0.6 72.9 + 0.6
FRC, normal lung (mL) 159.2 + 2.4 133.0 + 1.0 107.3 + 2.5 50.7 + 1.5
FRC, ARDS lung (mL) 30.0 + 1.0 22.0 + 1.0 19.1 + 0.9 10.3 0.6
PEF (L/min) 67.0 + 0.0 96.0 + 0.0 96.0 0.0 96.0 + 0.0
PIF (L/min) 58.7 + 0.9 60.1 + 0.8 66.3 0.8 76.3 + 0.7

CONCLUSIONS: 1) Setting PEEP to zero is associated with a higher PEF and VT, which may augment CO2 elimination, but it is also associated with a reduced FRC which may be associated with alveolar collapse, reduced oxygenation and alveolar injury. 2) Depending on the level of auto-PEEP (total-PEEP), the MAP may be slightly higher or similar as that with set PEEP. 3) The total-PEEP level can not be estimated from the PEEP display on the ventilator, it must be measured. 4) Clinical studies are necessary to determine the clinical significance of our findings.

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