Page 70 - Veterinary Laser Therapy in Small Animal Practice

56 Veterinary Laser Therapy in Small Animal Practice

• How long it takes to treat an area: the higher the reach deeper with a higher photon density, we need
power, the shorter the time a treatment will take (W higher power.
= J/s). You will first decide the dose you want, calcu- • How warm the surface gets: this does not only
late the treatment area, and then decide the power depend on power itself, but on power density.
you want to work with; this will determine the time Using 4 W of power with a 1 cm spot hand-piece
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spent on the session. Remember the power used for feels very different to the patient than using the same
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this calculation is the AVERAGE power, not the power over a 10 cm spot. In the first case, power
peak power. density would be 4 W/cm , and in the second, 0.4
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Many devices will have preprogrammed treat- W/cm . Of course, power and its density are not the
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ments. In some of them, preset parameters will only factors affecting how warm it gets: if we move
coincide with what you want. But manufacturers’ the hand-piece instead of having it over a fixed spot,
guides are not always clear. For instance, the laser the tissue will have time to cool off. And the faster
device may have a program that just indicates treat- we move, the less warm it will get.
ment for a 10 cm wound takes 1 minute. That
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information is NOT enough to go on: if the device Most patients will feel uncomfortable over 2–3 W/
is working at 1 W of average power, and we deliver cm and some even over 0.5 W/cm , especially if
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the treatment just over the wound, having it on for there is an open wound. If the patient’s skin/hair is
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a minute (60 s) will deliver 60 J in total, or 6 J/cm dark, the melanin will absorb more radiation and
over the wound area. Will that be enough? Many it will become warm before a light-haired patient
times it will. But if 1 W is the peak power because (especially with lower wavelengths).
you are working with modulated frequency (Hz),
you could actually be using 0.02 W, for instance, and • LT treatments should both be safe and comfort-
the amount of energy used would be 1.2 J in total, able; of course we have to avoid thermal injuries,
or 0.24 J/cm , which will be an insufficient dose for but we don’t even want to get to the point where the
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most conditions. patient starts feeling uncomfortable. So this comfort
threshold can change according to the patient, its
• What density of photons penetrates the surface: own skin temperature, its darkness of coat, and the
higher power means more photons are deliv- condition. For instance, a hot spot or a first-degree
ered per second, and we need a certain amount of skin burn will tolerate less power density than a
photons to elicit a clinical response. So, in order to chronic ulcer.

Calculating your treatment in four steps
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1. Decide the dose (J/cm ) you will deliver, based on reference values and clinical progression of that particular
case.
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2. Estimate or measure the treatment area (cm ).
3. Multiply those two values to calculate how many J you want to deliver in that session, in that area (J/cm ×
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cm = J).
4. The average power you work with (W) will then determine how long it takes to deliver those J (time in s =
J/W).
DO NOT select a treatment time and just deliver a preset protocol, as some devices prompt you to do.

Example
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1. You have decided to apply 2 J/cm to a large wound over the lumbar area.
2. The wound and its 5 cm margins cover a surface of 24 × 20 cm (480 cm ).
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3. Therefore, you want to deliver a total amount of energy of about 960 J in that session.
4. If you work with an average power of 2 W, it will take you 480 s to deliver that energy, which is a bit less
than 7 min.

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