The Science Journal of the American Association for Respiratory Care

2011 OPEN FORUM Abstracts


Joseph Orr, Lara M. Brewer; Anesthesiology, bioengineering laboratory, University of Utah, Salt Lake City, UT

Background: Volumetric capnometry combines the flow and CO2 signals to report the parameters in addition to end-tidal CO2 including airway dead space and CO2 excretion (VCO2). Accurate VCO2 measurement requires that the flow and CO2 signals be exactly aligned in time and that both signals have sufficient frequency response times. Precise measurement of VCO2 is needed for accurate calculation of parameters derived from VCO2. For example, cardiac output measurement using the partial CO2 rebreathing method relies on accurate VCO2 measurement. Accurate VCO2 measurement is difficult to achieve because the accuracy is affected by the lung compliance, airway dead space (before and after the sensors), breath rate and tidal volume. Methods: We tested the accuracy of a new volumetric capnometry system (NM3, Philips, Carlsbad CA) using a bench lung model. We ventilated a test lung (TTL, Michigan Instruments) using a Siemens 900C ventilator. The test lung was modified to include a mixing fan within the lung. CO2 was infused into the test lung at flow rates between 100 and 400 ml/min at 50 ml/min increments using a digitally controlled mass flow controller (Alicat model 1-SLPM-D, Alicat Scientific, Tucson AZ). We repeated the tests under nine conditions of varying lung compliance, respiratory rate, tidal volume and added dead volume. The volumetric capnometry flow and CO2 sensors were placed between the test lung and the ventilator Y adaptor. Simulated lung compliance was selected by adjusting the spring on the test lung. Dead volume was simulated by adding sections of expandable tubing between the lung and sensors, or between the sensors and the Y adaptor. Results: The table shows the average percent error in the measured VCO2 at each simulated VCO2 and lung test condition. The average error across all the tests was -0.07 percent of reading with a standard deviation of the error of 0.95 percent of reading. The highest percent error (5.2%) in measured VCO2 was observed when the simulated VCO2 was very low and the tidal volume was very large. Conclusion: The combined on-airway flow and CO2 sensors offer a significant advantage over side-sampling systems. Extremes of airway pressure and flow did not result in error caused by dyssynchrony between the flow and CO2 signals. Furthermore, the step responses of both signals were adequately fast and matched to provide excellent VCO2 accuracy across a wide range of simulated VCO2 values and simulated conditions.
Sponsored Research - Research Funding and test instrument were provided by Philips/Respironics