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Effect of air and heliox as carrier gas on CO2 transport in a model of high-frequency oscillation comparing two oscillators.

OBJECTIVE
To study the effect of carrier gas on CO(2) transport during high-frequency oscillatory ventilation in a closed model.


DESIGN
In vitro model study.


SETTING
Respiratory research laboratory affiliated with a tertiary center for pediatric critical care.


SUBJECT
In vitro, closed-lung model consisting of a glass tube (9.8 x 1000 mm) covered at each end with balloons.


INTERVENTION
Air or heliox (80:20) at constant pressure was oscillated inside the model, comparing the Sensormedics 3100A and Hummingbird BMO-20N oscillators at equal amplitude.


MEASUREMENTS AND MAIN RESULTS
Tracer gas (CO(2)) was injected at one end of the model, and CO(2) concentration was measured at the opposite end. Speed of CO(2) transport was expressed as the time for the CO(2) concentration to reach 63% of the final concentration (the time constant). In room air, using the Hummingbird oscillator and increasing frequency stepwise from 5 to 20 Hz, the time constant decreased from 2813 to 457 secs (p =.05). Using the Sensormedics oscillator in room air at increasing frequency from 5 to 15 Hz, the time constant decreased from 1584 to 551 secs (p =.05). In heliox, using the Hummingbird oscillator, the speed of CO(2) transport increased by 85% (p =.029) at 5 Hz and by 28% (p =.05) at 15 Hz. With the Sensormedics oscillator using heliox, the speed of CO(2) transport increased by 16% at 5 Hz (p =.009) and 52% at 15 Hz (p =.008). Proportionally, the increase in CO(2) transport with heliox was greater at 5 Hz for the Hummingbird oscillator and at 15 Hz for the Sensormedics oscillator.


CONCLUSIONS
In a closed model, we showed that during high-frequency oscillatory ventilation in room air, CO(2) transport increases with increasing frequency for both ventilators. Using heliox as carrier gas significantly augmented CO(2) transport, but the increase is frequency and device dependent. The effect of heliox on oscillator performance and the clinical applicability of our findings require further study.