2007 OPEN FORUM Abstracts
EVALUATION OF AN ON-AIRWAY OXYGEN UPTAKE MEASUREMENT SYSTEM IN VOLUNTEERS
J. Orr1, L. Brewer1, T. Anderson1
Oxygen uptake measurements (VO2) provide the basis for indirect calorimetry calculations in critical care patients. Indirect calorimetry has application in determining feeding requirements as well as in assessment of sepsis and other disease states. Oxygen uptake is the breath-to-breath difference between volume of inspired and expired oxygen. In an on-airway system, this measurement requires precise alignment of the flow and oxygen signals. Conventional calorimetry systems use a mixing chamber to make the signal alignment less critical. A mixing chamber makes the system bulky and requires separate measurement sites for inspired oxygen and exhaust gases. Newer systems use side-sample gas analysis, which is susceptible to sample tube obstruction and signal alignment difficulties. We evaluated a prototype system that uses on-airway sensors to acquire the O2, CO2 and flow signals needed for indirect calorimetry (Respironics, Wallingford, CT). The calorimetry values from the on-airway sensor were compared with those from the gold standard Deltatrac (Datex, Helsinki, Finland) calorimeter.
A gas blender was used to create FiO2 conditions of 21% and 40% O2. One-way respiratory valves were used so that the Deltatrac system correctly sampled FiO2 and analyzed only the exhaled gas. Data were collected simultaneously as nine healthy volunteers breathed through both the on-airway system and the Deltatrac indirect calorimeters (Deltatrac). Each volunteer breathed into the apparatus until steady state conditions were achieved. Average steady state oxygen uptake measurements from the two monitors were compared.
Oxygen uptake measurements ranged from 166 to 425 mL/min. The average difference between the two systems for inspired oxygen of 21% was -9.4 ± 20 mL/min or -2.7 ± 5.3%. With FiO2 of 40%, the average difference was 7.9 ± 16.6 mL/min or -2.7 ± 6.0%. The figure shows the on-airway measurements plotted against the corresponding reference VO2 measurements. The correlation coefficient is r2 = 0.96.
This preliminary study shows that accurate oxygen uptake measurements are possible using the on-airway integrated oxygen, carbon dioxide, and flow sensors. If future testing continues to show positive results, this type of system could make indirect calorimetry convenient and reliable in a critical care setting.