The risk and benefits of experimental cardiovascular therapies

308 nm XeCl excimer lasers have been developed to permit long pulse operation. The 135 to 200 ns pulse duration permits effective fiberoptic transmission which makes this wavelength practical for percutaneous (interventional) medical applications. Defining optical operating parameters in part depends on the desired procedure. In general, ablation of cardiovascular tissues (muscle, atheroma and fibrous tissue can be performed effectively at 35 to 50 mJ/mm². However, ablation of calcified tissue requires energies of 80 to 100 mJ/mm² and higher repetition rates (40 to 80 Hz). At these higher energies and repetition rates more thermal damage is observed. For clinical applications the fiberoptic delivery system plays a major role in the feasibility, safety and efficacy of procedures. Lack of beam homogeneity produces both hot spots and dead space within the ablative field. Hot spots can lead to tissue tearing and unwanted thermal damage. Tissue tearing is produced by a combination of the gas generated during ablation, the shockwaves produced during bubble collapse and the shape of and force applied to the delivery system. At 308 nm the intensity of the shockwaves is a function of the fluence, the environment, pressure within the bubble and tissue characteristics. Using PDVF transducers we have measured these shockwaves at 7-10 Atm in albumen solutions to as high as 70-80 Atm in solutions containing X-ray contrast agents during ablation of vascular tissue or myocardium. Thermal damage can result from multiple factors including irradiation at subablative thresholds, repetition rates and fluences which exceed the thermal relaxation time of the tissue and fiberoptic configurations which confine ablation products to a cavity in front of the fiberoptics