Air Pockets Trapped During Routine Coupling in Dry Head Lithotripsy Can Significantly Decrease the Delivery of Shock Wave Energy

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Abstract
Purpose: Current lithotriptors use a dry treatment head that must be coupled to the patient with gel or oil. We determined how the quality of coupling affects stone breakage under conditions that simulated patient treatment. Materials and Methods: Experiments were performed with a Dornier® DoLi-50 electromagnetic lithotriptor. The test tank had a clear Mylar® membrane for coupling with the treatment head water cushion. Thus, air pockets trapped at the coupling interface could be photographed for quantitation. Coupling efficiency was assessed using a fiberoptic hydrophone and different coupling regimes were tested for the effect on gypsum stone breakage. Results: The quality of coupling was variable with air pockets covering 1.5% to 19% of the coupling area, resulting in a mean decrease in shock wave amplitude of approximately 20%. Breaking and reestablishing contact, as when a patient is repositioned during treatment, decreased acoustic pressure almost 32%, representing a 57% decrease in acoustic energy transmission. Stone breakage was also decreased when air was trapped in coupling and only 2% coverage by air pockets decreased stone breakage by 20% to 40%. Conclusions: These in vitro results suggest that coupling in lithotripsy can pose a significant barrier to the transmission of shock wave energy to the patient. Stone breakage was sensitive to air pockets at the coupling interface. Recoupling was particularly disruptive, suggesting that repositioning the patient could substantially degrade coupling quality. It seems reasonable that variability in the quality of coupling could contribute to variability in clinical outcomes.