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A
BENCH EVALUATION OF ALVEOLAR PRESSURE CHANGES DURING ENDOTRACHEAL
SUCTION.
Steve Y. Wong, MD,
Guilherme Schettino, MD, Robert Kacmarek PhD RRT FAARC,
Dean Hess PhD RRT FAARC. Massachusetts General Hospital and Harvard
Medical School, Boston MA.
Closed suction may
avoid alveolar derecruitment (Intensive Care Med 2001; 27:648).
However, one study reported negative airway pressures using this
procedure (Acta Anaesthesiol Scand 2001;45:167), which we reasoned
may result from inadequate ventilator flow to replenish that withdrawn
by the suction catheter. We studied the relationship between alveolar
pressure and ventilator modes during open and closed suctioning.
Method: A Michigan
Instruments TTL simulated a resistance of 5 cm H2O/L/s
and compliance of 40 mL/cm H2O (C40/R5), or a resistance
of 20 cm H2O/L/s and compliance of 80 mL/cm H2O
(C80/R20). An 8 mm endotracheal tube size was used. A Puritan-Bennett
7200 ventilator was used with volume control at a flow of 30 L/min
(VCV 30), volume control at 60 L/min (VCV 60), or pressure control
(PCV). A VT of 500 mL and rate of 15/min were used
(I:E ratio 1:3 for PCV). Trigger sensitivity was ?1 cm H2O
or flow trigger of 3 L/min (base flow 10 L/min or 20 L/min). PEEP
of 5 and 10 cm H2O were used. Experiments were performed
with a 14 French catheter (Ballard Trach Care), which was advanced
to the distal end of the endotracheal tube. Suction pressure of
120 mm Hg or 180 mm Hg was applied for 10 s. Pressure was recorded
from the entrance to the bellows (Ventrak). The experiment was
also performed with the ventilator Y-piece disconnected from the
inline suction catheter to simulate open suctioning. Pressure
drop below PEEP was measured post-hoc from the pressure waveform.
Results: During
closed suction, the pressure drop below the set PEEP in the bellows
was not affected significantly by PEEP level (P=0.32), trigger
method (P=0.51), or suction pressure (P=0.31). Pressure drop was
significantly affected by resistance/compliance setting (P<0.001)
and flow setting (P<0.001) as shown in the figure below. With
open suction, the pressure drop was ?8 cm H2O.
Conclusions: When flow from the ventilator was adequate (PCV
or VCV 60), alveolar pressure drop below PEEP was small. When
PEEP is used, this should rarely result in sub-atmospheric pressure
in the lung. When open suction is used, which results in no PEEP,
the alveolar pressure drop is always sub-ambient. These data suggest
that closed suction may useful as a technique to avoid alveolar
derecruitment during the suction procedure.
OF-01-141
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