Effect of heliox on end-tidal CO2 measurement in healthy adults

Therapeutic gases and other modalities delivered by inhalation may affect the accuracy of capnographic measurements in 2 ways. First is the specificity of the measurement of CO2 within the device, and second is the dilution effect of supplemental gases in the ambient air during CO2 sampling by the device. Our goal was to determine if variables such as inhaled gas composition, gas flows delivered via non-rebreather mask, and mouth open or closed affect measurements of end-tidal CO2 pressure (PETCO2) measured with the Capnostream 20 capnograph.

We measured PETCO2 and breathing frequency by capnography in 20 adult normal subjects, with coaching to maintain respiratory frequency between 10 and 20 breaths/min. SpO2 was monitored to detect hypoxemia. A 6 min wash-out period occurred between each 6 min level of testing.

A mixed models analysis revealed that the mean ± SD PETCO2 for all subjects and flows while breathing heliox (37 ± 5 mm Hg) was not different (P = .50) from the value while breathing room air (36 ± 5 mm Hg). Repeated measurements with given subjects over 6 min periods of breathing spontaneously 0 L/min, with 10 L/min, and with 15 L/min of either air or heliox showed no difference in PETCO2 related to flow: P = .97 for 0 L/min vs 10 L/min, P = .87 for 0 L/min vs. 15 L/min.

In normal subjects, PETCO2 measurements with the Capnostream 20 were not affected by heliox or gas flow at 10 or 15 L/min through a non-rebreathing mask.