The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

EVALUATION OF AEROSOL GENERATED BY A LARGE VOLUME HEART NEBULIZER POWERED BY HELIOX



 Kristin Smith AS, RRT, Steve Williams RRT, Tim Cox BS, RRT, James H. Hertzog, MD, AI duPont Hospital for Children, Tom Blackson BS, RRT, Christiana Care Health System, DE

Background:
Helium and oxygen (heliox) mixtures are often used in the emergency room as an adjunct to standard treatment of acute asthma exacerbations. This has been proposed to decrease the patient's work of breathing long enough for bronchodilators to begin to exert their effect. In some situations, continuous nebulization may be preferable. Combining these therapies is currently being studied.

Purpose: To evaluate, gravimetrically, the output and distribution of aerosol to three strategically placed filters, within the tubing, mask, and exhalation ports of a large volume continuous nebulizer powered with heliox 70/30.

Methods & Materials: We created a model to simulate a 5 year old in respiratory distress using a test lung (Michigan Instruments, Grand Rapids, MI) and an LTV-1000. The LTV-1000 was adjusted so that an independent monitor (CO2SMO, Novametrix, Wallingford, CT) on the patient side of the test lung displayed a tidal volume of 275 ml, a respiratory rate of 35 breaths per minute with an Inspiratory to Expiratory ratio of 1:1.5, and a peak inspiratory flow of 26 liters per minute (L/min). The mask was mounted on a plastic surface and the HEART large volume continuous nebulization kit (Westmed, Tucson, AZ) was set up according to the manufacturer's recommendations. Aerosol was collected using filters (Intersurgical, Liverpool, NY) placed between the mask and test lung (A) and at both mask exhalation ports (B and C). The nebulizer, tubing, mask, and filters were weighed pre and post nebulization. The nebulizer was filled with 50 ml of solution (47 ml normal saline solution/ 3 ml albuterol) for a 15 mg output in one hour and was run until nebulization was complete (30 seconds after sputtering started). Three control evaluations with 15 L/min of oxygen were used for comparison with three evaluations each of 24 L/min and 20 L/min actual flow of heliox. The average and standard deviation of each set was used for comparison. Results are reported as a gravimetric percent of the total fill volume weight.

RESULTS:
Filter A, which represents the patient in this model, collected 9.56% (+/-0.58) of the 50 ml of solution with 24 L/min of heliox flow. At a heliox flow of 20 L/min, Filter A collected 10.35% (+/-0.93), which is comparable to the 11.07% (+/-0.52) in the control. Filters B and C, which collected aerosol that would be lost through the mask exhalation ports, collected slightly more (16.64 and 17.02% vs. 16.18%) with heliox than with oxygen. Less aerosol was deposited in the tubing (10.05 and 8.80% vs. 14.34%) and mask (0.57 and 0.71% vs. 1.00%) with both heliox flows when compared with the control.

CONCLUSION:
Although heliox has been shown to improve deposition of aerosol particles, it must be taken into consideration that the nebulizer becomes less efficient, as evidenced by an increased residual volume and decreased aerosol to Filter A. Of the heliox flows studied, 20 L/min deposited the most aerosol in Filter A, representing what the patient in this model would have inhaled. The 20 L/min chosen in this study was based upon previous work by Hess et al. in which the MiniHEART continuous nebulizer was studied using heliox flow rates approximately 33% to 50% greater than the manufacturer recommends for oxygen source gas. Mask leak, which must be reduced to maintain heliox's therapeutic effect, is difficult to control with this particular mask, mainly because of the large exhalation ports. Additional studies are needed to determine the mass median aerodynamic diameter of the aerosol particles generated by heliox powered nebulizers in order to approximate deposition of these particles clinically.

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