June 8, 2011
The use of lasers in veterinary practice continues to expand greatly. Thousands of veterinarians are now using CO2 laser energy in their practices in some capacity.
The implementation of laser wavelengths varies from topical, non-invasive laser irradiation of damaged tissue (high- and low-power therapy lasers) to incisive vaporization of selected tissues (CO2 and diode lasers).
Multiple factors are involved that will ultimately produce a superior surgical outcome and by understanding the variety of factors and components that go into an informed use of laser energy, any type of CO2 laser can be an effective surgical tool. There are four keys to successful use of any laser. Clinicians can maximize laser success by using these four simple keys.
Essential knowledge: Understand the wavelength you are using or don’t use the wavelength.
The majority of clinicians in the United States use CO2 wavelength laser energy produced in the 10,600nm infrared range. The CO2 laser wavelength’s major advantage is that it is absorbed almost completely by the intracellular water present in each cell, causing rapid vaporization of the water and thus the cell.
This allows for a consistent and extremely repeatable vaporization of soft tissue. This also restricts peripheral tissue injury or necrosis to less than 0.01 mm as 99 percent of the photothermal heat is removed in the vapor produced. A reduction in post-operative swelling, bleeding and pain is commonly noted.
Diode wavelength lasers are the second most common laser devices used in clinical veterinary practices. These laser wavelengths are produced in the 670–1064 nm wavelength of the near infrared light spectrum (Figure 1). The 980 nm wavelength is the most commonly used diode wavelength for surgical purposes. Its photons are carried by a fiberoptic quartz fiber to the target tissue. Its slightly higher absorption coefficient for water, hemoglobin and oxy-hemoglobin may make it a more effective cauterization device than a CO2 laser, allowing aggressive tissue de-bulking and resection in vascular tissue.
Varying wavelength penetration and absorption by a variety of chromophores within living cells when the diode laser energy is applied in a true laser-tissue non-contact mode can create a very inconsistent tissue effect from patient to patient. For this reason, the diode laser is used most often in what is called a contact mode during surgical use. This contact mode means that the tip of the fiberoptic quartz fiber is allowed to develop a carbonized surface. The laser photons are absorbed by the carbon surface and its energy transformed into a rapid heating of the tip. Contact mode is not a true laser-tissue interface as CO2 laser energy coming out of a flexible hollow guide or articulated arm.
Power Density (PD) = Watts (W)/cm2
Watts (W) = Joules/sec
Distance from tissue – Diameter of beam – Joules/sec; alters power density, therefore you control the laser power density.
One of the misgivings of CO2 laser energy is that even at inappropriate power density levels, the energy most often produces vaporization “like” effects that appear acceptable to the naked eye. So with just a small increase in your level of knowledge about power density you can provide significantly superior care and get results that truly reflect the advantages of CO2 laser energy (Table 2).
Strive to achieve 4500 power density at the target tissue to produce complete vaporization. The relationship between the number of watts applied, diameter of laser spot size and the total duty cycle (active lasing time in seconds) are critical to power density generated at the target tissue (Table 3) for optimal vaporization potential.
Table 3 can be used for both the hollow wave guided laser devices that have specific diameter tips to attach to the hand piece, or the articulated arm devices that use a focusing hand piece. Newer “dial in” hand pieces that do not require tips are now also available.
The total duty cycle refers to the total time that laser energy is interacting actively with the target tissue. The laser wavelength can be emitted in a continuous 100 percent duty cycle. The CO2 wavelength can be chopped (prevented from being emitted) by a mechanical shutter that physically obstructs the beam. This allows for varying the percentage of total 100 percent duty cycle emitted from 1 percent to 99 percent.
The CO2 wavelength can be pulsed via computer manipulation of the active photon production during the total duty cycle. This allows for more rapid release of finite laser energy bursts into the target tissue. Computer pulsing also allows for the concept of “super-pulsing” a beam to more effectively limit peripheral thermal energy damage to surrounding tissue. By varying the total duty cycle, the diameter of the laser beam at its optimal focal distance and the total watts provided of the CO2 laser beam, a surgeon can be even more specific and exacting when removing targeted tissue with laser energy.
Key III. Laser-Tissue Interaction
Essential knowledge: Time, distance and power are user controlled. The effect of laser energy on tissue is three dimensional.
Understanding the three-dimensional relationship that time, distance and power have on the total laser energy application at the target tissue will maximize a positive outcome (Figure 2).
The laser beam emitted from the end of a laser hand piece is a cone shape with an optimal focal point at a specific distance from the end of a focusing hand piece or guide tip which produces a “top hat” cone. The depth and diameter of either type beam geometry delivered causes the vaporized tissue “laser trough,” which is dependent on the focal point, time, distance and power applied.
So it is these factors that maximize a positive vaporization outcome to the target tissue. A good working knowledge of laser-tissue interaction and experience in maintaining appropriate distance from the tissue and hand speed will maximize effective vaporization and minimize peripheral tissue injury.
Key IV. Lasers As A Tool in Your Surgical Case Management
Essential knowledge: Just because you have a CO2 laser does not mean you have to use it.
There are many variables when considering how to achieve an optimal surgical outcome. A CO2 laser is just one of a number of considerations. Understanding its correct use and capabilities will allow for the rest of the surgical variables to be more easily managed.
There are a number of factors specific to the laser that enhance surgical outcome. Primary among these is the experience and knowledge of the user.
Appropriate surgical technique, tissue preparation and tissue handling are equally important. Appropriate tissue preparation allows for optimal laser tissue interaction. Tissues that are well hydrated enhance the CO2 laser tissue interaction. The use of Lactated Ringers solution as a final rinse over tissue to be lased will optimize laser-tissue interaction.
The use of laser energy often allows for less manipulation of tissues during surgery and can enhance surgical field visualization (less bleeding). Use the laser energy as a dissection tool through tissues allowing for reduced traction and clamping.
Finally, it is important to consider a total pain management scheme that includes CO2 lasers and appropriate medications.
Peter Eeg, BSc., DVM., CVLF, is owner and director of the Poolesville Veterinary Clinic LLC in Poolesville, Md. He is co-author of “Veterinary Laser Surgery, A Practical Guide” (Wiley-Blackwell, 2006) and consultant for Cutting Edge Technologies.
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