The Why's and
Wherefore's of "Hot Spots" on the Radioaerosol
Inhalation Lung Scan
by Ross Potter, Technical Director, Medi/Nuclear Corp.,
Inc.
Historically,
one of the overriding problems with performance and
interpretation of the Radioaerosol Inhalation Lung Scan
has been 'hot spots'. What are they? What causes them?
What do we do about them?
It
has long been thought that 'hot spots' were the result of
clumping of particles, particles that were either too
large or which were sticking together. We would suggest
that neither answer is appropriate.
At
Medi/Nuclear Corporation we have been manufacturing radioaerosol
inhalation devices for use in Nuclear Medicine for over 10
years. In addition to manufacturing the units, we also
have the ability to do our own in-house testing of the
units for particle size and distribution as well as
nebulizer generation rate. We, like everyone else, depend
on some of the major medical centers to provide clinical
results.
Because
of the hands-on nature of our research and our involvement
in the papers presenting the results, we have come to the
conclusion that particle size, largely, determines the
differential deposition in either the upper or lower
respiratory tract. 'Hot spots,' rather than being related
to particle size, are almost entirely a function of
velocity.
This
velocity comes about either intrinsically or
extrinsically. Intrinsically, it results from conditions
within the patient's lungs. This might be mucous plugs,
tumor invasion or a narrowing of the passages by any
number of disease states. Extrinsically, the cause is
uncontrolled deep breathing or tachypnea. In the first
instance, as the aerosol passes a narrowing of the
breathing passage, it accelerates. Immediately upon
passing the narrowing, it decelerates. Upon decelerating,
an eddy current is created which deposits the aerosol on
the distal side of the narrowing. In the second instance,
an uncontrolled deep breath is by its very nature a high
velocity breath. In this case the aerosol will be
deposited anywhere a bend is too sharp for the particle's
own inertia to allow it to turn. This is usually in the
throat or at the bifurcation.
If
the trachea is visualized, it generally indicates the
presence of excessively large particles, and if these
particles are sufficiently large or represent a
sufficiently large portion of the particle size
distribution, peripheral penetration will be impaired.
Based
on the above observations and hypotheses, we set out to
redesign our nebulizer to both produce very small
particles and to compact the particle size distribution.
The results have been presented in papers from
Cedars-Sinai Medical Center in Los Angeles. We feel we
have now produced the two best aerosol delivery systems
currently available, the Aero/Vent+ and the Insta/Vent.
Either unit will provide particles almost entirely below 1
micron, and the Insta/Vent will deliver them to the
patient in significantly less time.
