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INTRODUCTION
DISCUSSION & SUMMARY
The Omron MicroAir nebulizer (Figure 1, opposite) has been recently
developed and its design refined to facilitate hand-held drug aerosol
delivery of aqueous aerosol using the latest ultrasonic technology. The
MicroAir is designed to transform aqueous drug solutions and
suspensions into an aerosol mist in two modes: continuous aerosol
generation or manual intermittent (to synchronize with inhalation).
The MicroAir employs a mesh having a lot of fine nozzles and horn
oscillator that combine to release a fairly fine liquid aerosol with
relatively low power consumption.
We report an investigation to determine the aerosol output and aerosol
size from this unit using testing methods embodied in the new
European Standard [1].
1. The 24 MicroAir nebulizers assessed had notable variation in both
aerosol output and aerosol size in the pairs of data obtained for each
unit (Table, Figures 2 & 3).
2. The MicroAir units n=1-12 had a markedly slower rate of aerosol
output (1 minute output) than units n=13-24 (Figure 2)
RESULTS
units 1-12
units 13-24
98 µl
156 µl
(19)
(24)
920 µl
879 µl
83.4
(80.7)
3.19 µm
4.75 µm
(0.32)
(0.48)
aerosol output 1st minut mean µl
(std dev)
Selection of test nebulizers: 24 MicroAir units were supplied by
Omron Japan in early 2002 for testing. These were thought to have
aerosol output rates in two distinct categories; low output (units n=112), and high output (U22 units n=13-24). Low output units ( n=1-12)
have larger diameter of nozzles in a mesh plate , and high out put units
(n=13-24) have smaller diameter of nozzles in a mesh plate.
aerosol output total mean µl
(std dev)
aerosol size mean (MMAD) µm
(std dev)
5. Regressing Total Aerosol Output Vs MMAD indicates little or no
relationship – i.e. total aerosol output is independant of aerosol size
(Figure 4).
Fig 2 Aerosol Output from Omron MicroAir
Aerosol output 1 minute µg
Aerosol Output Total ug
1200
Fig 3 Aerosol Size from Omron MicroAir
6
800
600
MMAD µm
Aerosol Output (µg )
1000
400
6. Regressing Aerosol Output Rate Vs MMAD indicates a clear and
direct relationship (Figure 5), ie.e. lower rates of aerosol output
related to lower aerosol sizes, and vice versa. This relationship is
thought due to evaporation of aerosol solvent as the ’inhaled’
ambient air absorbs aerosol solvent (i.e. water) as it mixes with
nebulized aerosol. The amount of solvent absorbed is a constant
defined by airflow rate and ambient conditions (oC & %RH). The
lower the rate of aerosol output, the greater the influence of
evaporative solvent loss on the remaining aerosol particles. Hence,
the more drastic reduction in aerosol size. This effect has been
observed in other systems (personal observation).
7. By changing the diameter of nozzles selectively within the MicroAir,
MMAD of the released aerosol can be controlled.
5
4
3
200
0
0
2
4
6
8
10
12
14
16
18
20
22
2
24
0
MicroAir unit number
2
4
6
8
10 12 14 16 18 20 22 24
MicroAir unit number
Fig 4 MicroAir Total Output Vs MMAD
Fig 5 MicroAir Aerosol Output Rate Vs MMAD
1500
y = -36.336x + 1045.5
Aerosol Output Rate
µg/min
aerosol output µg
Aerosol Size. Each MicroAir unit was tested as per methods described
in the European Standard [1]. Each unit was filled with 2.5 ml of
salbutamol and connected to an 8 stage low flow cascade impactor,
Marple Series 298X (Westech Instruments Ltd) set to 2.0 L/min flow.
The cascade drew the 2 L/min flow at right angles from an overall flow
rate of 15 L/min was induced by use of a vacuum pump to simulate
patient inhalation. After sampling for 2 minutes, nebulization was
suspended and the cascade impaction filters removed for quantification
of salbutamol by HPLC. Aerosol size was determined in duplicate from
each device.
4. The MicroAir units 1-12 had a lower MMAD compared to units 1324 (Table & Figure 3).
The main findings are presented in the Table and Figures below.
METHOD
Aerosol Output. Each U22 unit tested according to method specified
in the European Standard [1]. Each unit was filled with 2.5 ml of
salbutamol, connected to a breath simulation device (Figure 2) set at
500ml sinus flow at 15 cycles per minute. Aerosol ‘inhaled’ was
collected on a low resistance high capacity 3M filtrete elesctrostatic
filter. After collection for 1 minute of nebulization, the collection filter
was replaced, and aerosol collection continued to completion (selfindicated by the MicroAir device upon completion of nebulization).
Each of these 2 aerosol inhalation filters were then removed for later
quantification of salbutamol by HPLC. Aerosol output was measure
from each device in duplicate.
3. There was no difference in total aerosol output between units 1-12
and units 13-24 (Figure 2).
2
R = 0.1477
1000
500
0
2
3
4
MMAD µm
5
6
250
REFERENCES
y = 32.533x - 2.8478
200
2
R = 0.6111
1. prEN13544-1. European Standard 13544-1. Nebulizing
systems and their components.
150
100
50
0
2
3
4
MMAD µm
5
6