Copper vapor laser fragmentation of gallstones: In vitro measurements of wall heat transmission

Abstract

Laser fragmentation is a promising new modality in management of retained CBD stones. Recent reports demonstrate the feasibility of lasers for this, but few studies have evaluated their safety (e.g., thermal injury may occur at 43°C). This study was conducted to measure heat transmission from lased bilirubinate and mixed stones to a simulated CBD wall. Four welded thermocouples were passed to the inside wall of 6-mm polyvinyl tubing 90° apart to surround the lumen stone. The thermocouples were interfaced to a computer and temperatures were recorded every 270 msec. The tubing was submerged in a 37°C water bath for all lasing work. A copper vapor laser (wavelength, 510 nm; 5.6 W; 5 kHz; pulse length, 30 ns) was attached to a 650-μm quartz fiber. A stone was “impacted” in the tubing and the laser fiber was pushed against the stone while making multiple passes to fragment it. Thirty mixed gallstones (mean size, 6.9 × 5.1 mm) and 20 bilirubinate gallstones (mean size, 7.1 × 5.2 mm) were fragmented during the study. Maximum temperature ( T max), duration of T max ( T max D ), interval to stone piercing ( T ip), and interval to fragmentation ( T iF) were measured and comparisons were carried out with the SPSS statistical package using the t test procedure. The T max generated during fragmentation of bilirubinate stones (43.4 ± 1.7°C) was significantly less ( P < 0.002) than the T max for mixed stones (54.0 ± 2.7°C) but both T max values represented potentially injurious temperature levels. While less heat was generated during bilirubinate stone fragmentation, T iP and T iF were significantly longer ( P < 0.05) for the bilirubinate stones (14.2 ± 2.2 and 21.3 ± 2.4 sec, respectively) than for the mixed stones (4.6 ± 1.0 and 15.9 ± 1.0 sec, respectively). The mean recorded T max during fragmentation of the two types of gallstones in this study suggests the possibility of CBD thermal injury. The relative greater “hardness” of the bilirubinate stones probably accounted for the longer T iP and T iF when compared to mixed stones. Careful in vivo and clinical studies need to be done to establish the safety of this modality for surgeons.