2007 OPEN FORUM Abstracts
ROOM AIR ENTRAINMENT THRESHOLD OF THE MEDICATOR MÖBIUS.
M. McPeck1
Background: The Healthline Medicator Möbius is a low-flow continuous
aerosol delivery system operated either by O2 or by a mixture of O2 + 80/20
HeO2. Inspired mixtures of 80/20, 70/30 or 60/40 HeO2 are achieved by adjusting
the ratio of 80/20 HeO2 flow vs. the flow of O2, keeping a total flow of 8 L/min
to minimize waste of gas. STUDY QUESTION: Does the low-flow paradigm predispose
the Möbius to room air entrainment (RAE) that would dilute the He portion
of the HeO2 mixture to unacceptable levels?
Methods: Testing conditions included tidal volumes (VT) of 200, 300,
400 & 500 mL with respiratory frequencies (f) of 10, 20, 25, 30, 35, 40 & 45
BPM resulting in minute volumes (Vmin) of 2.0 to 22.5 L/min, generated by a
sine wave piston ventilator at I:E of 1:1. The portless mask was removed from
the patient tee of the Möbius and a 6 inch corrugated tube was used to
connect the piston ventilator to the mask (patient) position on the patient
tee. The Möbius was operated with an 80/20 HeO2 mixture powering the 90
mL capacity nebulizer at a flow of 8 L/min. FIHe and FIO2 was measured by continuously
pumping gas from the "airway opening" position in the test setup through a HeO2
analyzer (thermal conductivity He analyzer + galvanic O2 fuel cell). Withdrawn
gas was returned to the circuit to prevent a net loss of gas that might have
triggered RAE. The nebulizer was operated dry to avoid introducing moisture
into the HeO2 analyzer which would have adversely affected the accuracy or stability
of the readings.
Results: The Vmin:FIHe relationship shows that there is no significant
change in FIHe at either the 200 or 300 mL VT across the entire range of f and
resulting Vmins tested. At a 400 mL VT, FIHe began to decrease only when Vmin
exceeded 12.0 L/min and f exceeded 30. At a typical normal VT of 500 mL, FIHe
began to decrease at a Vmin of 12.5 L/min which corresponds to a f of 25 BPM.
Conclusion: This data indicates that the threshold for clinically significant
air entrainment with the Medicator Möbius is at a VT of 400 mL and a f
of at least 30 BPM (Vmin = 12 L/min). If attempting clinically to administer
80/20 HeO2, FIHe would fall to 73% because of RAE at Vmins as high as 18 L/min,
which is probably not attainable by severe acute asthmatics. Therefore, the
Möbius can be used in acute asthmatics with tidal volumes as high as 200–400
mL with no clinically significant RAE, even at breathing rates up to 30 BPM.
FIHe related to Minute Volume