2006 OPEN FORUM Abstracts
A Method for Measuring the Impact of a Hyperbaric Environment on Ventilator Readings of Flow and Exhaled Volume
Robert Hase, MS RRT-NPS, Darla
Myron, RN, CHRN, Douglas Ross, RN, BSN, ACHRN, CWCN, Claude Wreford-Brown RN, ACHRN,
Virginia Mason Medical Center, Seattle, Washington
Background: Boyle's
Law states that, assuming constant temperature, gas volumes will contract as
pressure increases. Ambient pressure changes are relatively slight across most
geographic locations where mechanical ventilators are used. Typically,
mechanical ventilators require only periodic recalibration, and the miniscule
effects of moment-by-moment barometric pressure differences are ignored. In a
hyperbaric environment, however, ambient pressure increases as much as 600%
during treatment, and the ambient pressure frequently if not constantly is in flux
while descending to and ascending from treatment depth. Observing the effect of
these pressure changes on the performance of mechanical ventilators employed
during hyperbaric treatment is confounded by the inability to accurately
measure using machine-integrated volume measuring devices. Calibration of
machine-based flow transducers during hyperbaric treatment is impractical. We devised a simple method where a correction
nomogram can be easily determined so that correction factors for any dive depth
can be applied to a mechanical ventilator's exhaled volume measurements. As a
result, actual exhaled volume at any depth can be calculated, with the
corrected exhaled volumes then used as the basis for ventilator adjustments to
achieve desired target blood gas values.
Methods: Our
facility uses Seimens Servo 900C ventilators to ventilate patients in the
hyperbaric chamber. We use two ventilator configurations: one remains as a
whole unit connected via a through-hull to a 12 volt power source that remains
outside the multi-place chamber. The second configuration separates the control
head from the pneumatic system, connecting the two via through-hull cabling.
The second inside/outside configuration was used for our study, but either
machine could have been used, and presumably any other machine could be
validated the same way. We set the ventilator up with a hyperbaric nurse inside
the chamber operating a one liter calibration syringe attached to the patient
circuit at the wye. The machine was set to CPAP mode with 3 cm H2O CPAP, and
alarms were dialed off. The nurse inside the chamber operated the calibration
syringe by completely pulling the piston out then completely pushing the piston
in, followed by a several second pause to act as a manual breathing simulator
with a known one liter volume. Ten baseline exhaled tidal volume readings were
taken for each pull of the piston with the chamber at ambient pressure. The
chamber was then pressurized to a depth of 165 feet salt water and another 10
"breaths" were measured. This process was repeated to collect volume readings
over ascending depths of 132, 99, 66, 60, 45, 33, and 15 feet of salt water. All
data collected was entered into a Microsoft Excel spreadsheet. Averages were
calculated at each depth, and then a single regression slope was calculated using
the Excel "SLOPE" function.
Results: Data
points for volumes recorded varied no more than 6% among all breaths at any
given depth. Plotted average volumes
were nearly straight-line before applying regression to derive true straight-line
slope. The resultant slope allows calculation of correction factors at
infinitely variable depth levels, and a graph was plotted from the equation
using each whole atmosphere of depth as well as two additional common treatment
depths as a clinical reference. This process proved to be easily accomplished
with simple equipment readily available in most hospitals, and can be used to
derive correction factors for any volume-measuring device under hyperbaric
conditions.