Page 66 - Veterinary Laser Therapy in Small Animal Practice

52 Veterinary Laser Therapy in Small Animal Practice

the delivered dose was identical. Your new pulse may How much though? Table 6.2 shows the integrated
not be short enough to produce an acoustic wave, or power densities (a quantity we will cover soon) for
there may not be sufficient dark time before the next some relevant wavelength ranges as well as the result-
pulse to allow the redox reaction to reset. That said, it ant accumulated doses of each that we are exposed to
may coincide with some different reaction rate or reso- per hour of sunshine. You can see that you are exposed
nant effect. Or your changes might have been too subtle to 33 mW/cm of light in the “therapeutic window”
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to make a difference. The point is, the pulse structure (the region in the near infrared between the peaks of
has at least some impact on the biological effect; if not, melanin and water where the greatest penetration into
why are we pulsing at all? Chapter 8 will answer that in the body occurs) from the sun (Fig. 6.9). Some dose
much more detail. prescriptions you’ll find in the coming chapters are
But an equally important question is the timescale attainable by sitting in the sun for an hour or two at this
of treatment, both within and between treatments. exposure rate. Does that mean that the clinical effects
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Should a treatment that aims to deliver 2 J/cm of light would be the same if you received all that light in a
take 2 minutes or 20 minutes or 2 hours? Should those few seconds or minutes? Again, who knows, but this
treatments be repeated every hour or every day or every element of time (i.e. determining the minimum thresh-
week? Or does it matter at all; i.e. will all of these situ- old of light per time that can stimulate a clinical effect)
ations produce the same clinical effect as long as they plays a crucial role and should influence your decision-
deliver the same total dose? These are ultimately clini- making process when it comes to selecting a light
cal questions that I have no business answering, but source, as well as choosing a protocol once you have
there are some useful limits governed by concepts in done so. In any case, if you are delivering light therapy
my realm that we can cover before turning the question at even close to that low a power density (and therefore
over to the clinician in the next few chapters. over the lengthy treatment sessions you’d need at that
The first limit is on the short side. Remember, there power to deliver a therapeutic dose), then you’d prob-
is a maximum power density that can be delivered to ably be better off prescribing walks in the park rather
the skin before inflicting pain. This number depends than laser therapy.
somewhat on wavelength, since melanin absorption in Then there is the idea of energy storage in cells.
the skin is the primary tissue heating mechanism and One idea behind the “lasting” effect of a laser therapy
since melanin’s absorption falls sharply from its peak session is that the cells produce enough energy during
in the ultraviolet (UV) and visible ranges (an evolution- treatment to be used later. Leaving aside the biological
ary advantage that protects us from the sun’s ionizing processes in between, as well as fats in adipose tissue,
radiation) to virtually zero in the mid-infrared. But excess cellular energy can be stored in the form of gly-
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for reference, about 1–2 W/cm of 800 nm, continu- cogen, which is synthesized when blood glucose and
ous wave light will begin to hurt after a few seconds, insulin levels are high, and which can be useful to the
and about 2–5 W/cm will inflict immediate pain. So cells as energy for up to 12 hours. So if we indeed stim-
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you can’t deliver too much energy at a time before heat ulate cellular metabolism and enhanced production
comes into play. Then, there is the issue of area cover- of adenosine triphosphate (ATP), and if we think this
age. Whatever your dose prescription, odds are you’ll leads to the storing of energy, then the timescale for
want to deliver that dose uniformly to every part of the effectiveness is about daily, or half that. Keep in mind
treatment area. And you can only move your hand so though, that many laser treatments of acute injuries
fast when trying to be predictable and repeatable, so aim to actually “fix” what is “broken” and so the effects
you can’t deliver 2 J/cm to an entire hip in 10 seconds, can last far longer than this.
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even if your laser is capable of delivering that much So while reciprocity does not seem to hold across
output light. the board, there are certain windows within the broad
On the high side, a clean example is to look at the treatment spectrum where you can be reasonably con-
sun. Figure 6.9 shows the solar irradiance data recorded fident that slight tweaks (because a pet owner couldn’t
by the National Renewable Energy Laboratory. [195] get to the clinic that morning or because you have the
Surprising to some is the fact that the majority of the chance to deliver a second treatment since the animal is
radiation we experience from the sun is not in the UV boarded overnight) will not severely impact your stand-
or visible spectrum, but rather in the infrared range. ard expectation of therapeutic outcome.

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