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.