Page 116 - Veterinary Laser Therapy in Small Animal Practice

102 Veterinary Laser Therapy in Small Animal Practice

guesstimates based on the “typical shoulder,” for stimulus to trigger a chain of chemical signaling reac-
example, to determine an average dose to be delivered tions or get more nutrients into the cells or simply as a
to where we expect the pathology to originate. general physical catalyst (as in acupuncture) to initiate
But when it comes to frequency, depending on how a physiological response.
we dial the “knob,” we are able to expose the entire But in either case, whether we are talking saturation
treatment volume to a given pulse frequency if we or secondary response, there is a period of time after
think that frequency might hit a “sweet spot” in the which the local environment changes because of this
clinical-benefit-effect curve. heat. The way to mitigate this, and potentially get the
most out of the therapy, is to allow time in between
pulses for the body to dissipate this heat.
This theory is most popular because more of Tiina
IMPORTANT: This book is all about fine-tuning [268]
the bits of laser therapy technique you can control. Karu’s work illuminates the mechanism. What was
If you take nothing else from this section, it should so special about these different pulse frequencies? For
be that you as the clinician CAN make a difference a given pulse there are two main components: the
in the treatment outcomes of your patients, pulse width/duration (i.e. how long the beam is on) and
depending on your choice of pulse frequencies that the “dark” period in between (i.e. how long between
you point at your target tissue. pulses). When her team varied the pulse width from
5 to 1000 ms, they saw a fairly flat behavior, not much
difference in the measured effect (cell attachment in
this case).

8.2 Understanding the WHY But when they varied the dark period between
pulses for this cell line, they saw a significant (almost a
Being good scientists (again why I love their work so FACTOR OF TWO) increase in the effect between its
much), Karu and her team attempted to get down to the peak and its baseline. Again, we very much care about
why, and from what I’ve read they have done the best factors of two. So why the big jump?
job in simplifying the explanation. And, conveniently The hypothesis, which is substantially more than
for me as I’ve been writing my sections of this book, the just a hypothesis by now, was that this particular cell
reason boils down to heat, a topic that has come up at line’s thermal dissipation constant was right in line
least four times so far (I’ve lost count at this point, so with the peak dark period. In other words, if there was
apologies for yet another quick summary here). not enough time between pulses for the heat from that
Laser absorption results in a very small heat conver- pulse to be dissipated, then the physiological effect
sion/transfer. The body is quite good at dissipating heat, wasn’t as strong as it could be, probably because of
within certain extremes. And that dissipation takes this saturation. And if that were the end of the story,
time, where the amount of time is determined by the there would just be a lower threshold for the dark
amount of heat and the size of the “thing” dissipating period such that the effect after that was the same (i.e.
it; i.e. whether we are talking about bulk tissue or indi- as long as you waited longer than that threshold for
vidual cells. Creating time gaps between when the laser the next pulse, it didn’t matter how much longer you
is on (i.e. pulsing) allows time for the cells to dissipate waited).
heat. Going back to the macro-example, this is akin to But that’s not what they saw as they dug deeper.
using an ice-pack (or heating pad) for 20 minutes on, It turns out that the curve went back down AFTER
then 20 minutes off, then repeat. the peak. In other words, if there was too much time
While the laser is on, heat is accumulating, and there between pulses, so that the heat was completely gone
is some threshold where the body simply saturates. One from the previous pulse, then there was only a more
could make the case that if there is too much accumu- modest increase in effect. Within a “sweet spot” though,
lation, beyond mere saturation, the body will attempt a heightened effect (remember, a factor of two increase)
to take countermeasures to maintain homeostasis, was achieved. This means there is some natural thermo-
and therefore secrete some enzymes or make its cells’ relaxing resonance of cells we can exploit to get the
membranes more permeable. Some theories of laser most out of laser therapy … if we make the right guesses
mechanisms of action say this can be exploited as the about what we are treating.

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