2007 OPEN FORUM Abstracts
BENCH EVALUATION OF AN INDIRECT CALORIMETRY SYSTEM USING ON-AIRWAY SENSORS
J. Orr1, L. Brewer1
Introduction: Indirect calorimetry calculates metabolic rate and calorie consumption from directly measured oxygen uptake (VO2) and carbon dioxide production (VCO2). These systems measure the difference in volume of inspired and expired O2 and CO2 in respiratory gases. Most indirect calorimetry systems designed for use in critical care use paramagnetic oxygen analyzers, which draw a sample through a sampling tube. Side sampling introduces challenges including signal alignment with the flow sensor signal, sampling tube occlusion and others.
Methods: We used a patient simulator based on propane combustion to model oxygen uptake (VO2) and CO2 production (VCO2). We compared the inspired oxygen measurement (FiO2), VCO2 and VO2 as measured by the on-airway system (Respironics, Wallingford, CT) to the standard paramagnetic oxygen sensor-type metabolic analyzer (Deltatrac, Datex). The respiratory quotient (RQ) is the ratio of CO2 production to oxygen consumption. Because the simulator burns propane gas, we know that the true respiratory quotient (RQ) should always be 0.6.
We ventilated the patient simulator using a Siemens 900C ventilator at two simulated metabolic rates using three inspired O2 (FiO2) levels at each simulated metabolic rate. The measured VO2 and VCO2 should have been the same at each simulated metabolic rate regardless of the inspired oxygen concentration. The measurements (FiO2, VCO2, and VO2) were recorded as they were reported in real time. FiO2, VO2 and VCO2 were recorded and compared for both monitors. The VO2 results were also compared to the ideal VO2 measurement calculated using the measured VCO2 and the known RQ of 0.6 for propane gas.
Results: The average difference in CO2 production between the two systems was 0.5 ± 3.9 mL/min. The average difference in percent VCO2 was 0.3± 2.8%. The data plot below shows that the inspired oxygen (FiO2) measurements for the two analyzers compared well (r2 = 0.999). The average oxygen consumption (VO2) error for the Deltatrac monitor was 23 ± 50 ml/minute (8.3 ± 17.8%). The average VO2 error for the on-airway system was -13 ± 2.5 mL/minute (-5.4 ± 1.3%).
Conclusion: Accurate measurement of oxygen consumption is one of the most challenging applications of respiratory oxygen monitoring. Our data show that an on-airway oxygen analyzer can be applied to provide accurate oxygen uptake measurements.