In vivo assessment of shock-wave pressures. Implication for biliary lithotripsy

During extracorporeal shock-wave lithotripsy, the pressure profile, which is generated by the lithotriptor, determines the risk of tissue damage. In the present study, the pressure distribution of a lithotriptor (Lithostar; Siemens A.G., Erlangen, Federal Republic of Germany) was investigated in 10 pigs, five of which had gallstones surgically implanted into the gallbladder. The in vivo values were compared with in vitro data. Measurements were carried out along the shock-wave transmission path at the focus within the gallbladder, the adjacent liver, the diaphragmatic surface of the right lung, and the shock-wave exit site from the skin. Interposition of ribs did not cause a significant decrease in focal positive pressure. However, a gallstone positioned in the focus caused a 30%-65% reduction in pressure, recorded immediately behind the stone. Pressures obtained in vivo were always 15%-25% lower than those measured in vitro. The spatial distributions of the positive pressure in vivo and in vitro were almost identical. There was a high correlation between the pressures in vitro and in vivo (r = 0.88; P less than or equal to 0.01). This justifies assessment of shock-wave energies generated during biliary lithotripsy by extrapolation of in vitro data. It is concluded that it is possible to characterize different lithotriptors by in vitro pressure profile measurements.