The Science Journal of the American Association for Respiratory Care

1995 OPEN FORUM Abstracts

Air Entrainment Properties Of Three Brands Of Mdi Spacer Systems

Edwin M. Lybarger, RRT, CPFT, RCP, El Dorado Hospital & Medical Center, Tucson, Az

An important aspect of MDI spacer performance is its air entrainment characteristics. When a MDI canister is actuated, a medicinal aerosol cloud fills the chamber of the spacer. As the patient inhales, they take in air that flows through the holding chamber, thus inhaling the suspended medication. Entrained air is that air that enters the spacer system without moving through the holding chamber. This entrainment typically occurs at the mouth piece and the MDI canister stem. As the patient inhales, the entrained air mixes with the air that has moved through the spacer holding chamber at the time of inhalation. This mixing results in a net dilution of the medication in the volume of air inhaled by the patient. METHOD: Ten devices each, for three brands of spacer systems were tested in order to determine the percentage of air entrainment. These brands include Baxter's Hand-Held MediSpacer, Diemolding Healthcare Division's Ace Aerosol Cloud Enhancer, and Monaghan's Aerochamber. To test entrainment, the mouth of the spacer system is connected to a vacuum pump. The end of the spacer is connected via tubing to a long cylinder of known radius and length. Knowing the radius, the cross section perpendicular to the length is determined. The vacuum pump is used to create a flow rate of 30 Liters per minute. The flow rate entering the end of the spacer system is measured by periodically dipping the cylinder into a container of soapy solution. Bubbles are formed across the cross section of the cylinder. These bubbles are timed as they traverse up the cylinder. The velocity of the bubbles are determined. The velocity of the bubbles multiplied by the cross section of the tubing is used to determine the flow rate for the end of the spacer system (measured intake flow rate). Any flow rate that enters the spacer other than at the end of the spacer is the entrainment flow rate. Thus the 30 Liter per minute flow rate will equal the measured intake flow rate plus the entrainment flow rate. The soap bubble flow rate measurement system has minuscule flow resistance (less than 1mm water pressure resistance at 30 L/min). Thus the spacer system is not disturbed by the flow rate measurement made.

Results: There was a significant difference in the air entrainment flow rates for the three brands of spacers tested (Ace: 19.1±.6 L/min, Aerochamber: 12.0±.6 L/min, Hand-Held MediSpacer: 6.5±.7 L/min : Average ± standard deviation; \alpha=.001, p < 2E-10).


The Hand Held MediSpacer exhibits the least amount of air entrainment (leakage) with the highest amount of air flow thru the chamber. Therefore having the lowest amount of dilution of medication per volume of air inhaled.



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