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The CO2 laser, using gas (CO2) as its medium, emits coherent light at a wavelength of 10,600 nm. At this infrared wavelength, water is the tissue chromophore. The water in skin is located both intracellularly and extracellularly, and with sufficient heating, vaporization occurs, leading to tissue coagulation and ablation. This ablation leads to immediate, visible tissue contraction. Months later, histologic evidence of new collagen and elastin formation is seen. CO2 laser resurfacing has been used for photoaging, including lentigos, fine and moderate rhytides, rhinophyma, dyschromias and acne scarring.
The first CO2 laser to be introduced emitted light at 10,600 nm in a continuous fashion. Continuous wave (CW) lasers apply heat during the entire time the laser is in operation mode. The amount of heat generated will depend on how quickly the handpiece is moved and how many passes are performed. Results were variable, with a high rate of complications, especially in new users, which led to changes in the CO2 laser delivery system.
Simply changing the delivery of the same wavelength of light has resulted in a system that has been used successfully in dermatology for years. In the 1990s, two new CO2 systems were developed to shorten the exposure time of the skin to this infrared light (the pulsed CO2 and the scanning spot CW CO2).
The goal with any laser system is to avoid collateral damage when treating the target. This is done by keeping the exposure time to less than the thermal relaxation time of the target, preventing excessive spread of heat to surrounding tissues. Exposure times less than or equal to one millisecond have proven to be much safer with the CO2. The pulsed CO2 laser does just that. It delivers pulses of light of duration less than one millisecond. The high-energy short-pulsed system uses a flash of energy that is short enough in duration to prevent heat from dissipating into surrounding tissue. The scanned carbon dioxide laser system uses a computer-controlled mirror to rapidly scan an area in less time than is needed to expel heat into the bordering tissue. The scanning CO2 laser uses the continuous wave laser, but delivers spots with each exposure time less than one millisecond. Depth of tissue injury is more controlled between 20 and 120 microns per pass. This reduces the potential for scarring.