Holmium:YAG lithotripsy:: Photothermal mechanism

Objective: A series of experiments were conducted to test the hypothesis that the mechanism of holmium:YAG lithotripsy is photothermal, Methods and Results: To show that holmium:YAG lithotripsy requires direct absorption of optical energy, stone loss was compared for 150 J Ho:YAG lithotripsy of calcium oxalate monohydrate (COM) stones for hydrated stones irradiated in water (17 +/- 3 mg) and hydrated stones irradiated in air (25 +/- 9 mg) v dehydrated stones irradiated in air (40 +/- 12 mg) (P < 0.01), To show that Ho:YAG lithotripsy occurs prior to vapor bubble collapse, the dynamics of lithotripsy in water and vapor bubble formation were documented with video flash photography. Holmium:YAG lithotripsy began at 60 mu sec, prior to vapor bubble collapse. To show that Ho:YAG lithotripsy is fundamentally related to stone temperature, cystine, and COM mass loss was compared for stones initially at room temperature (similar to 23 degrees C) v frozen stones ablated within 2 minutes after removal from the freezer. Cystine and COM mass losses were greater for stones starting at room temperature than cold (P less than or equal to 0.05), To show that Ho:YAG lithotripsy involves a thermochemical reaction, composition analysis was done before and after lithotripsy, Postlithotripsy, COM yielded calcium carbonate; cystine yielded cysteine and free sulfur; calcium hydrogen phosphate dihydrate yielded calcium pyrophosphate; magnesium ammonium phosphate yielded ammonium carbonate and magnesium carbonate; and uric acid yielded cyanide. To show that Ho:YAG lithotripsy does not create significant shockwaves, pressure transients were measured during lithotripsy using needle hydrophones. Peak pressures were <2 bars. Conclusion: The primary mechanism of Ho:YAG lithotripsy is photothermal, There are no significant photoacoustic effects.