Kidney stones are common on Earth, and space travel only exacerbates the risk. Over 15 years of National Institutes of Health (NIH) and National Space Biomedical Research Institute (NSBRI) funded research have developed burst wave lithotripsy (BWL), which comminutes stones faster and more completely than shock wave lithotripsy (SWL) in a bench top water bath. The University of Washington licensed the technology to a commercial partner, and NSBRI funded the BWL development for integration into NASA’s Exploration Medical Capability (ExMC)’s Flexible Ultrasound (FUS) system. However, neither SWL nor BWL comminutes stones as well in a confined space, such as the kidney, and both perform even less well when the stone is obstructing and surrounded by tissue. In addition, cavitation is acutely sensitive to carbon dioxide, which is 30 times more soluble than oxygen in water, and carbon dioxide levels can be elevated 20 times in NASA vehicles. Carbon dioxide dissolves bubble nuclei and quenches cavitation collapse, has been associated with visual impairment and intracranial pressure (VIIP), and suppresses kidney stone detection. This study sought to discover effective BWL exposures to counteract the effects of confinement and carbon dioxide.
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Five farm-bred pigs were implanted with ex vivo human calcium oxalate monohydrate kidney stones (approximately 4-mm diameter) that had been stored in water for at least a week and known to display the twinkling artifact in vitro via retrograde ureteroscopy. A baseline twinkling was collected at least 15 minutes before the anesthetic gas carrier was changed in four of the pigs from oxygen to elevated carbon dioxide in compressed air; the fifth pig was maintained on oxygen for more than one hour to serve as a control. The four treated pigs continued to breathe the increased carbon dioxide levels for two hours or until the oxygen saturation levels dropped below 45%, at which point the anesthetic gas carrier was switched back to oxygen. After about thirty minutes on oxygen, the pigs were again exposed to the elevated carbon dioxide levels until the oxygen saturation levels dropped below 45% at which time the pigs were once again returned to oxygen. Blood and urine samples were collected every thirty minutes and immediately before changing the anesthetic carrier gas.
A mannequin phantom was developed for testing and as a continued training and research tool. Outputs were developed and measured to effectively comminute human kidney stones in the confinement of the kidney and ureter phantoms and in animal models breathing elevated carbon dioxide. Investigators developed and tested an ultrasound phantom and surrogate testing capabilities in the ground analog environment of an elevated CO2 atmosphere to enhance the utility of therapeutic ultrasound for kidney stone comminution. These validated outputs have been added to the BWL system integrated with NASA's flexible ultrasound system (FUS).
Simon JC, Wang YN, Cunitz BW, Thiel J, Starr F, Liu Z, and Bailey MR. Effect of carbon dioxide on the twinkling artifact in ultrasound imaging of kidney stones: A pilot study. Ultrasound in Medicine and Biology.
2017. May; 43(5):877-83. [DOI]
May PC, Kreider W, Maxwell AD, Wang YN, Cunitz BW, Blomgren PM, Johnson C, Park JSH, Bailey MR, Lee D, Harper JD, and Sorensen MD. Detection and evaluation of renal injury in burst wave lithotripsy using ultrasound and magnetic resonance imaging. Journal of Endourology.
2017. August; 31(8):786-92. [DOI]