Gas in gallstones: quantitative determinations and possible effects on fragmentation by shock waves

The presence of gas in a gallstone can profoundly affect the ability of shock waves to fragment the stone by various mechanisms. In the present study, the aim was to determine the gas content of gallstones and determine if increasing the gas content of the stone affected the outcome of lithotripsy. Thirty human gallstones, transferred directly from gallbladder bile into sterile saline, were studied. The initial gas content of all stones was determined by differential weighing under saline before and after degassing. Eighteen gallstones were pairs; each pair was from a single patient and of similar size and composition. One gallstone of each of the pairs was exposed to air and the other was kept under saline. Then each of the paired gallstones was subjected to 1000 pulses at power level 3 (highest) in a Diasonics Therasonic lithotripter (Diasonics, Milpitas, CA). The volume of gas present at the beginning of the experiment in all groups of stones was 2.4 +/- 2.1 mm3, and 27% of all gallstones tested contained measurable amounts of gas initially. There was no significant difference in the volume of gas present in the paired stones at the beginning of the experiment (group A, 1 +/- 0.4 mm3; group B, 2 +/- 1 mm3). After group A stones were exposed to air, the gas content was significantly higher (36 +/- 18 mm3) than in the paired group B stones stored under saline (3 +/- 2 mm3; P less than 0.05). Stones exposed to air fragmented more easily than stones stored under saline. The mean number of pulses required to cause initial fragmentation was significantly lower in the group of stones exposed to air (22 +/- 7) compared with those stored under saline (610 +/- 139; P less than 0.05). The fragments were smaller and more numerous in the group of stones exposed to air than in those stored under saline. It is concluded that gas is present in some gallstones and that the efficacy of lithotripsy increases with increasing stone gas content. Our data suggest that alterations in the physical characteristics of gallstones can have profound effects on the outcome of lithotripsy.