The Science Journal of the American Association for Respiratory Care

2011 OPEN FORUM Abstracts

A NOVEL METHOD FOR MEASURING CO2 ELIMINATION DURING HIGH FREQUENCY OSCILLATORY VENTILATION.

Craig D. Smallwood1, John Thompson1, Gerhard K. Wolf2,3, John H. Arnold2,3; 1Respiratory Care, Children's Hospital Boston, Boston, MA; 2Anesthesia/Critical Care Medicine, Children's Hospital Boston, Boston, MA; 3Harvard Medical School, Boston, MA

BACKGROUND: The application of volumetric carbon dioxide (VCO2) monitoring is used during conventional ventilation as it provides continuous feedback pertaining to ventilation and lung perfusion. Exhaled gas monitoring is difficult during high frequency oscillatory ventilation (HFOV) due, in part, to rapid and significant changes in gas flow. This study examines the accuracy of a novel method for measuring VCO2 during HFOV in a pediatric lung model. METHODS: A high frequency amplitude attenuation device (HAAD) was attached to the exhalation valve of the Sensormedics 3100A high frequency ventilator. The HAAD works by directing exhaled gas through a 1 liter anesthesia bag which vents to an airway monitor for gas analysis. The HAAD reduces the oscillatory flow variations of HFOV exhaled gas and permits accurate monitoring using conventional flow and carbon dioxide monitoring technology. A bench model was constructed to test the method. A test lung was connected to the ventilator circuit. CO2 production (VCO2prod) was simulated by bleeding various amounts of CO2 (40-300ml/min) into the test lung. VCO2prod flow rate was measured and recorded before it was introduced into the test lung. Partial pressure of mixed expired CO2 (PeCO2), barometric pressure (Pb) and flow were measured using the Philips NM3 airway monitor. CO2 elimination (VCO2elim) was calculated by multiplying the CO2 fraction (PeCO2/Pb) by the total gas flow. Ventilator settings were recorded. A Pearson correlation and Bland-Altman analysis were used to determine the accuracy of VCO2elim compared to the known VCO2prod level. RESULTS: Correlation between VCO2prod and VCO2elim was r2=0.996, P<\<>0.0001. Figure 1 is a plot of VCO2elim versus VCO2prod. The best fit (solid line) and the one-to-one line of identity (dotted line) are shown. Figure 2 is a Bland-Altman plot of VCO2prod and VCO2elim. Bias was 8.1 +/- 12.5 ml/min (mean +/- SD), 95% limits of agreement were -16.5 to 32.7. CONCLUSIONS: The main finding was a statistically significant agreement between a known CO2 production level and CO2 elimination, measured using a novel, non-invasive method during HFOV. The Bland-Altman plot revealed a predictable proportional error and very good accuracy in the pediatric CO2 elimination range (~70-150 ml/min). Clinical application of this method may offer a significant advancement in airway monitoring during HFOV; permitting rapid determination of changes in ventilation and lung perfusion.
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