Page 25 - Veterinary Laser Therapy in Small Animal Practice

Blood and light 11

ice pack, and spatial being hot here and cold there, right
next to it. Both of these effects are present in the act of While this is not usually necessary in practice, you
laser therapy. really want to use it in cases of severe
As we’ve discussed, a beam of light is made up of lots edema. Consider the lymphosomes or
and lots of tiny photons that are absorbed at various rates lymphatic territories involved. [20] For
at various depths within the tissue. Microscopically, instance, if you have severe edema in a
that means a lot of “hots” and “colds” throughout the tarsus, treat the inguinal and popliteal areas first.
tissue, which force blood through different pathways.
And though we talk about arteries, veins, and capillar-
ies as little tubes, the majority of the blood flow in the 2.3 Push vs. pull
body is more akin to “seepage” through bulk tissue. This
kind of fluid motion is even more susceptible to slight I use the word “push” for effect, but in fact this is only
pressure differences, without the fixed boundaries that part of the truth (the lesser part). Once blood leaves the
restrict blood flow in the vessels. arteries and enters the capillaries, there are a combi-
And temporally, you are consistently moving the nation of forces (beyond systolic blood pressure) that
path of light throughout a therapy session. In contrast govern how well it perfuses into the tissue. Some of the
to the slow results obtained using conduction, as seen forces push blood out of the capillaries: arterial pres-
with the ice pack, the energy of light is converted into sure pushes outward and tissue oncotic pressure pulls
heat in the tissue almost instantly. So you are con- blood into the tissues. Opposing these are forces keep-
tinually producing lots of tiny “hots,” whose heat very ing the blood in the capillaries: capillary oncotic pres-
quickly dissipates on a microscopic scale. Moving the sure is holding the water inside while venal pressure is
beam away from a site, however quickly, will cause a pulling the blood through the capillaries back toward
hot–cold pressure wave that gently guides blood to, the heart. These forces are in virtual equilibrium and so
through, and from the area. the perfusion rate of blood is relatively constant over
This principle is demonstrated by an old anecdote time, even though it is highly skewed depending how
from the early days of laser use in podiatry. Physicians far along a capillary you are: the closer to the artery, the
were attempting to use laser to treat gout, which is basi- greater the blood perfusion; the closer to the vein, the
cally an overabundance of uric acid in the foot. They less perfusion, since most of the blood is being sucked
began by simply treating the foot, but their patients back to the heart.
complained of even more intense pain after treatment. This is fairly intuitive, but what may not be is the fact
Some doctors were discouraged; others were curious, as that venal pressure plays a much bigger role in capillary
was I. Before long, we explored the actual mechanisms pressure than does arterial pressure: five times bigger,
of treatment and realized that in order for laser to even in fact. [21] This means that even a fractional change in
have a chance of helping these patients, it would have venal pressure can result in a significant change in the
to get the uric acid out of the foot and back through the blood perfusion along a capillary. It is important to keep
lymphatic system to be excreted out of the body. Well, this in mind as you deliver therapy: it is more impact-
you wouldn’t be able to do that by just treating the foot. ful to “pull” blood away from the area (or at least clear
You’d first have to open up some pathways for the uric the way downstream) than it is to try to push blood
acid to leave. So we began treating down the back of the through it.
knee during the first stages of therapy, then focusing
on the foot, then finishing back up the leg, and voilà, 2.4 Vasodilation: nitric oxide rearing
no more pain. We first opened the lymphatic pathways, its head again
then pushed (against gravity) all that waste-filled blood
back to the center of the body, where there was a much We discussed earlier how light helps to produce nitric
greater blood flow and where it could be scrubbed by oxide (NO): by directly enhancing the nitrosyl reduc-
the liver, kidneys, and lungs. Simply creating a series tion process to produce more NO, and by indirectly
of temperature gradients along the path of the blood forcing the cell to make better use of its O , which often
2
circulation lead to enough pressure differences to be has to compete against NO for some local binding
clinically impactful. effect. In all cases, this means an increased abundance

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