The Science Journal of the American Association for Respiratory Care

2007 OPEN FORUM Abstracts

DOES EXHALED CO2 REBREATHING OCCUR WITH THE MEDICATOR® MÖBIUS™ CONTINUOUS AEROSOL/HELIOX DELIVERY SYSTEM?

M. McPeck1


Background: The Medicator Möbius is a deliberately low-flow continuous aerosol delivery system, utilizing a portless mask as the patient interface. It is operated by 100% O2 or by a mixture of O2 + 80/20 heliox (HeO2). Maximum operating flowrate is 8 L/min in order to conserve HeO2 and minimize the waste of both medication and driving gas typical of conventional high-flow systems. STUDY QUESTION: Does the low-flow paradigm and portless mask predispose the Möbius to allowing exhaled CO2 to accumulate in the mask or delivery tubing and contribute to CO2 rebreathing?

Methods:
I performed 2 end-tidal CO2 analysis studies on each of 5 adult volunteer subjects (office workers) who wore a small bore nasal sampling cannula and deliberately breathed only through their nose as instructed. End-tidal CO2 (PetCO2) was measured with a Novametrix Model 715 “Tidal Wave SP” Capnography/Oximetry monitor in sidestream mode for CO2. For their first study, subjects breathed room air to establish baseline PetCO2; and for their second study a few minutes later, they breathed 80/20 HeO2 at 8 L/min through the adult size portless mask of an operating Möbius device. Subjects were unable to see the monitor during testing and were closely observed to ascertain they were not inadvertently mouth-breathing and that their breathing pattern appeared relaxed and normal. The monitor displayed a digital readout corresponding to the PetCO2 value in mmHg and also displayed a time-based exhaled CO2 tracing (capnogram), which was used to determine when the subject was breathing properly and had reached a “steady state” (no variation in PetCO2).

Results:
The data from the 5 test subjects is shown in the table. The mean baseline PetCO2 averaged 34.6 ±0.9 (SD) mmHg. When breathing on the Möbius system with heliox and a total gas flow of 8 L/min, the average PetCO2 was essentially unchanged at 34.3 ±0.9 (SD) mmHg.

Conclusion:
There is essentially no difference in end-tidal CO2 measurements between baseline room air breathing and breathing on the Möbius device. This small limited study on normal adult volunteers suggests that there is no rebreathing of exhaled carbon dioxide with the Möbius device despite the fact that the operating flowrate is only 8 L/min. The unidirectional flow apparently keeps the mask and patient tee flushed with fresh gas so that exhaled CO2 does not have an opportunity to accumulate.

PetCO2 (mmHg) in 5 Volunteer Subjects
Subject: 1 2 3 4 5 Mean ±SD
Baseline: 35 36 34 35 34 34.6 0.9
Möbius: 33 35 34 35 35 34.4 0.9



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