2004 OPEN FORUM Abstracts
Performance Characteristics of a Small Volume Nebulizer Interfaced with a Dosimeter for Methacholine Challenge Testing.
Carl D. Mottram RRT
RPFT FAARC, S. Michael Witzke CPFT Kaiser Lim MD, Paul D. Scanlon
MD. Mayo Clinic College of Medicine, Rochester, MN 55905.
The
American Thoracic Society’s Guidelines for Methacholine and
Exercise Challenge Testing dosimeter method recommends an aerosol
delivery output of 9 µl (0.009 g) +
10% using a small volume nebulizer (SVN) interfaced with a dosimeter
set at 0.6 second during inhalation. This recommendation is based on
the performance characteristics of the DeVilbiss model 646 SVN and is
intended to standardize the amount of drug delivered during a
methacholine challenge test. Our laboratory studied the performance
of a small volume nebulizer from Hudson RCI MicroMist (Temecula, CA)
to determine it’s performance for diagnostic testing. The
purpose of the study was to define the output characteristics of the
MicroMist SVN using the KoKo dosimeter (Devens, MA). Identify if the
MicroMist has acceptable inter-nebulizer reproducibility. Based on
the output characteristics define a protocol that would meet the ATS
recommended dose per actuation.
Method:
5 MicroMist (MM) SVN’s were assembled using standard laboratory
configuration and filled with 2 ml of methacholine. Each nebulizer
was then weighed using a Mettler Toledo AC100 high precision scale
(Switzerland), which is encased to prevent environmental effects on
the measurement. The dosimeter was set for a 0.6-second actuation per
the ATS recommendation with a driving pressure of 40 psi. After each
actuation the nebulizers were re-weighed and output calculated (fig.
1). The procedure was repeated with 9 new MM SVN’s, 3 SVN’s
with the dosimeter actuation time set at 0.7, 0.8, and 0.9 seconds.
(Fig 2).
Results:
| KoKo Dosimeter/MicroMist | 0.6 seconds (n=5) | 0.7 seconds (n=3) | 0.8 seconds (n=3) | 0.9 seconds (n=3) |
| Initial dose (g): mean (SD) | 0.0022 (0.0006) | 0.0035 (0.0004) | 0.0050 (0.0009) | 0.0060 (0.0001) |
| Subsequent doses (g) (2-5): mean (SD) | 0.0069 (0.0006) | 0.0080 (0.0005) | 0.0096 (0.0007) | 0.0104 (0.0006) |
| Initial vs. Subs. 2-5 | p<.001 | p<.001 | p<.003 | p<.001 |
There is a
reproducible “priming effect” with the first dosimeter
actuation, which is statically significant (p<0.001) from all
subsequent actuations. The dose delivered by the MicroMist SVN using
the standard 0.60-second time is less than the ATS’s recommend
dose of 9 µl (0.009 g). All
actuations are reproducible within 10%.
Conclusions:
To achieve the ATS’s recommend dose per actuation using the
MicroMist SVN the dosimeter should to be set to 0.8 seconds. There is
a priming effect that needs to be considered in the dosing protocol.
Our laboratory performs 2800 methacholine challenge tests per year.
Using the MicroMist nebulizer ($1.11 per unit) versus the Devilbiss
646 ($30.00 per unit) would result in cost savings of $80,000 per
year.