The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

EFFECTS ON TIDAL VOLUME WHEN CHANGING FROM A HEAT AND MOISTURE EXCHANGER (HME) TO A CONCHATHERM HEATED HUMDIFIER USING A STANDARD CONCHA-COLUMN

Theodore Stryker, BA, RRT, University of Washington Medical Center, Seattle, WA

Background: Many ventilators do not automatically compensate for compressible volume. The volume lost must be added to the ordered tidal volume. At our institution, HMEs are the primary method of humidification. As a patients condition warrants, active humidity is provided by using the CONCHATHERM system (Hudson RCI, Temecula, CA). We have recently become aware that the addition of necessary hardware (water traps, CONCHA-COLUMN and reservoir) for active humidification changes the tubing compliance and, therefore, the delivered tidal volume. This change in tubing compliance was noted to increase as the water reservoir emptied. We attempted to measure the effects this changing compliance had on a delivered tidal volume.

Methods
: Each of the test ventilators (Servo 300, Maquet, Inc. Bridgewater, NJ) was configured in the same manner in which they would be used with low compliance re-useable HYTREL (Maquet, Inc.) circuits. A CO2SMO Plus Respiratory Profile Monitor (Novametrix Medical Systems Inc., Wallingford, CT) measured pressure and volume at the patient wye. A Servo test lung 190 was used as the test lung. Measurements were taken with an HCH (Humid-Vent Filter Light, Hudson RCI, Temecula, CA) and no water traps. Water traps and CONCHATHERM were then added and measurements obtained with full and empty reservoirs. The circuit temperature was allowed to warm up to 30°C. Tubing compliance factors were calculated by adjusting the tidal volume to obtain a static pressure of 50±cm H2O with no peep and end-inspiratory pause of 1second. Compliance was calculated as Tidal volume/Static pressure.

Results: The tubing compliance factor increased from 1 cc/cm H2O to 3 cc/cm H2O when changing from HCH to CONCHATHERM. Median tidal volume reduction was 16 percent (range 10 to18). When the water reservoir was empty, the tubing compliance factor had increased to 4 or 5 with a median tidal volume reduction of 27 percent (range 21 to 31) as compared to the HCH. As tidal volume decreased, static pressures also decreased.

Conclusions: In using a low compliance circuit, corrected tidal volumes should be recalculated when switching from an HCH to CONCHATHERM active humidification. The use of a standard CONCHA-COLUMN will result in the tidal volumes changing over time. This will also occur on ventilators that automatically compensate for compressible volume. The use of a low compliance CONCHA-COLUMN may result in less variation in tidal volumes.

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