Understanding the mechanism of TMR starts with understanding the laser tissue interaction. While numerous devices (24,25), including ultrasound (26), cryoablation (27), radio frequency (28,29), heated needles (30,31), as well as the aforementioned hollow and solid needles have been used; none have engendered the same response that is seen with a laser. Additionally, numerous wavelengths of laser light have also been employed (32,36). Only CO2 and Ho:YAG are used clinically for TMR. The result of any laser tissue interaction is dependent on both laser and tissue variables (35-37). CO2has a wavelength of 10,600 nm, whereas Ho:YAG has a wavelength of 2120 nm. These infrared wavelengths are primarily absorbed in water and therefore rely on thermal energy to ablate tissue. One significant difference, however, is that the Ho:YAG laser is pulsed and the arrival of two successive pulses must be separated by time to allow for thermal dissipation, otherwise the accumulated heat will cause the tissue to explode under pressure. Such explosions create acoustic waves, which travel along the planes of lower resistance between muscle fibers and cause structural trauma as well as thermocoagulation (38). The standard operating parameters for the Ho:YAG laser are pulse energies of 1-2 Js and 6-8 W/pulse. The energy is delivered at a rate of 5 pulses/s through a flexible 1-mm optical fiber. It takes approx 20 pulses to create a transmural channel. Despite the low energy level and short pulse duration, very high levels of peak power are delivered to the tissue so that with each pulse there is an explosion (Fig. 4). Additionally, the fiber is advanced manually through the myocardium, and it is therefore impossible to know whether the channel is being created by the kinetic energy delivered via the mechanical effects of the fiber or whether there has been enough time for thermal dissipation prior to the next pulse.

In contrast, the CO2was used at an energy level of 20-30 J/pulse with a pulse duration of 25-40 ms. At this level the laser photons do not cause explosive ablation and the extent of structural damage is limited. Additionally, a transmural channel can be created with a single pulse (Fig. 4). Confirmation of this transmurality is obtained by observing the vaporization of blood within the ventricle using TEE.

Finally, the CO2 laser is synchronized to fire on the r wave, and with its short pulse duration arrhythmic complications are minimized. The Ho:YAG device is unsynchronized and, because of the motion of the fiber through the myocardium over several cardiac cycles, is more prone to ventricular arrhythmias.

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