The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

Comparison of Physiologic Dead Space Calculation Using Volumetric Capnography on the Drager XL Ventilator vs Metabolic Analyzer Measurements

Mark Siobal BS RRT, Julin Tang MD MS, San Francisco General Hospital ,

 Department of Anesthesia and Perioperative Care, University of California San Francisco

Background: Calculation of physiologic dead space (VD/VT) requires exhaled gas collection and analysis, use of a metabolic analyzer, or use of a volumetric capnography monitor. The Drager XL Ventilator (Drager Medical Inc., Telford PA) is equipped with integrated volumetric CO2 monitoring and calculates minute CO2 production (VCO2). This enables calculation of VD/VT using ventilator measurements of VCO2 and exhaled minute ventilation (VE). By dividing the VCO2 by the VE corrected for compression volume, the fraction of exhaled CO2 (FeCO2) can be calculated. The partial pressure of mean expired CO2 (PeCO2) can then be calculated by multiplying FeCO2 by the barometric pressure minus water vapor pressure. PeCO2 is then used to calculate VD/VT using the Enghoff modification of the Bohr equation. We calculated VD/VT  directly from the Drager XL Ventilator measurements and compared it to metabolic analyzer measurements.

Method: A Metascope metabolic analyzer (Cybermedic, Louisville, CO) was used to measure FeCO2 on nine ICU patients receiving mechanical ventilation on Drager XL Ventilator. The metabolic analyzer, ventilator CO2 and expiratory flow sensors were calibrated using standard operating procedure, and the circuit was checked for leaks. After a 20-minute warm-up and a 10-minute measurement period, FeCO2 from the metabolic analyzer, and PaCO2 obtained by arterial blood gas was used to calculate VD/VT corrected for circuit compression volume. Ventilator trend data stored every 15 seconds for VCO2 and VE averaged over the last 5 minutes of the metabolic analyzer measurement period was recorded and used to calculate FeCO2 and VD/VT corrected for circuit compression volume. Ten measurements of VD/VT using both methods were compared by linear regression. The study was approved by the UCSF Committee on Human Research.

Results: There was a strong correlation between ventilator and metabolic analyzer measurements of VCO2 (r2 = 0.98), FeCO2 (r2 = 0.90), calculated PeCO2 (r2 = 0.96), and VDVT  (r2 = 0.96).

Conclusion: Physiologic dead space can be accurately calculated directly from the Drager XL Ventilator volumetric capnography measurements without use of a metabolic analyzer.


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