The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

COMPARISON OF TWO METHODS FOR MEASURING NEBULIZER OUTPUT

Ravi Tandon MD, Michael McPeck RRT and Gerald C Smaldone MD PhD. Departments of Medicine and Respiratory Care, University Hospital, SUNY at Stony Brook, (supported by UH operational grant #371318).

The measurement of output of SVNs has often been conducted by gravimetric methods, i.e., by weighing the nebulizers before and after nebulization. Newer techniques determine the aerosol output directly by measuring aerosolized drug or radioactive tracer (^{99m}Tc) particles captured on absolute filters. Because of evaporation, the equivalence of these methods is a uncertain. The aim of this study was to determine if the gravimetric method is a close measure of aerosol production under different conditions of aerosol generation (e.g., nebulizer brand, flowrate, pressure, volume fill and concentration of solution). Methods: We measured the aerosol output of 9 commercially available SVNs simultaneously using gravimetrics and absolute filters under standard conditions (2.5 mL volume fill, normal saline diluent, flowrate of 6 L/min and pressure of 10.5 - 19 psig). Each data point represented 2 minute serial runs to the end point of dryness. The change in the weight of the nebulizer and radioactivity captured on the filter were expressed as % of the total originally in the nebulizer. Next, additional gravimetric and filter data were collected as each of the standard conditions was varied individually for each nebulizer. Results: Total weight loss for 9 SVNs ranged from 25.0 to 64.6% and total aerosol captured varied from 12.6 to 38.8%. While different, the % weight loss and % aerosol captured were closely correlated (y= -0.519+ 0.612x, r^{2} = 0.922, p=0.0001). Weight loss exceeded the corresponding aerosol captured by 35-40%. Change in volume fill and concentration did not affect this relationship (p=0.88 and 0.36, respectively). However, changing flowrate from 6 to 10 L/min significantly affected the relationship (y= -3.703 + 0.829x), but both techniques still had good correlation (r^{2} = 0.934, p=0.04). Conclusion: The gravimetric method of assessing nebulizer output over-estimates the actual aerosol output by about 35-40%, presumably because of the loss of solvent by evaporation during nebulization. However, the relationship between the methods is predictable and appears unaffected by changing the brand of nebulizer, fill volume or drug concentration. Flowrate alone significantly changed the correlation. The gravimetric method can be used as a simple and convenient method for predicting aerosol output in SVNs under a wide range of experimental conditions.

Reference: OF-96-174

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