The Science Journal of the American Association for Respiratory Care

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.

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