Aerosol Metered Dose
Inhaler - Testing
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Double-blind Study
Drugs for treating respiratory and nasal disorders are
frequently administered in aerosol formulations through the
mouth or nose by using metered dose inhalers. Patients often
rely on medication delivered by this system for rapid treatment
of respiratory disorders which are debilitating and, in some
cases, even life threatening. Therefore, it is essential that the
prescribed dose of aerosol medication delivered to the patient
consistently. Thus, testing of metered dose inhaler units for
proper drug delivery by the metering dose valve is a part of the
manufacturer's quality assurance procedure.

One conventional method to test delivery of metered dose
inhalers consists of taring each metered dose inhaler and
measuring the weight lost after the delivery of one dose. This
method is accurate and adequate for testing a small number of
samples. However, it slows down the process for a high-speed
production and packaging of metered dose inhalers.

Another common method of testing drug delivery utilizes
indirect pressure decay after activation of each metered dose
inhaler. This method does not render a direct mass
measurement, but rather an approximation based upon the
force exerted indirectly by the superheated vapor on a
pressure transducer. As a consequence, this method is unable
to detect those metered dose inhalers marginally out of
tolerance.

A newer method developed by Glaxo Inc. researchers (US
Patent 5,261,538) comprises passing the mass over a heat
loss measuring device, calculating the amount of heat loss
accompanying the vaporization of the mass and correlating the
heat loss to measurement of heat loss of known liquid aerosol
mass made with the same device.

When a liquid is released from a sealed container, the liquid
extracts heat from its environment, e.g., the surrounding air, to
evaporate. The extraction of heat from the liquid's surrounding
results in a lowering of the temperature of those surroundings.
When a volatile liquid, e.g. propellant 12 with boiling point -
29.8 degree C, is released into the air as a fine spray at
ambient temperature, the evaporation is very rapid and the
consequential cooling of the air is observable even to the
touch.  When the valve of a propelled aerosol dispenser is
activated, the released propellant very rapidly evaporates to
produce the aerosol, i.e., a fine suspension of the aerosol
formulation in air and vaporized propellant. The rapid
evaporation of the propellant extracts heat from the
surrounding air producing a significant cooling effect.

Thus, Glaxo researchers measured a mass of a volatile liquid
by discharging that volatile liquid into a constant temperature
and constant flow air stream and onto the probe of a heat loss
measuring device placed downstream from the point of
discharge. The instrument sensed the cooling effect of the
vaporization of the liquid as a transient loss of heat from the
area surrounding the probe and recorded as a transient
reduction of temperature of the air stream. The researchers
repeated the procedure with two or more different, known
masses of the same liquid (as a standard for comparison).
Thus, they determined the mass of the sample by correlating
the reduction in temperature resulting from its discharge with
reduction in temperature caused by discharge of the samples
of known mass.

                         
Aerosol
               metered dose inhaler
         Testing of Metered Dose Inhaler

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