The Science Journal of the American Association for Respiratory Care

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.

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