The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

Accuracy of Tidal Volume Measurements in the Presence of Helium/Oxygen Mixtures

James Bement, RRT, Dinesh G. Haryadi, Ph.D.Clinical Research Department, Respironics Hospital Division, Wallingford, CT 06492

Introduction: Helium/oxygen mixtures are used to improve gas exchange in anesthesia and critical care. Respiratory flow and volume measurements needs to be adequately compensated when using helium since it has a lower density when compared with air. The objective of this study was to evaluate the performance of the NICO (Model 7300) Cardiopulmonary Management System while measuring volume of gas mixtures containing heliox in concentrations of 70% helium/30% oxygen and 60% helium/40% oxygen.

Methods: For each concentration of Heliox, a 30-liter non-diffusing gas bag (Hans Rudolf, Inc.) was filled with the gas mixture. The NICO flow/CO2 sensor was connected between a one-liter calibration syringe and the gas bag. The valve of the gas bag was additionally connected to a one-liter test lung (Siemens, Inc.) so that testing could be performed with and without pressure to determine if airway pressure has an effect on gas measurement. The calibration syringe was set to the full one liter, and pumped back and forth on the gas bag, and measurements were recorded using data collection software. The concentration of Heliox was entered into the NICO monitor. The valve was then switched, and the measurements repeated cycling the gas back and forth through the one-liter test lung. The volume of the calibration syringe was decreased in increments of 100 ml and the test repeated until the volume of 100 ml was reached. Average bias was calculated as the difference between the syringe volume and measured volume. Data was analyzed using Bland-Altman Statistics.

Results: The average difference between the tidal volume measurements of the calibrated syringe and the NICO measurements were -0.65% (Bias) ± 1.79% (1 STDEV).

Gas Mixture Bias ± 1 STDEV
60/40 (No pressure) +0.71% ± 0.26%
70/30 (No pressure) -1.83% ± 1.67%
60/40 (With pressure) +0.32% ± 0.63%
70/30 (With pressure) -1.81% ± 2.10%

Based on the above results, the accuracy of the tidal volume measurements using the NICO in the presence of heliox mixtures typically used in clinical settings is within specification.

Reference: Sondergaard S, Karason S, Lundin S, Stenqvist O. Evaluation of a Pitot type spirometer in helium/oxygen mixtures. J Clin Monit Comput. 1998 Aug;14(6):425-31.

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