WCE 2019: Defining Thermally Safe Laser Lithotripsy Power and Irrigation Parameters: In Vitro Model

Abu Dhabi, United Arab Emirates (UroToday.com) The collaboration between the University of Michigan and the University of Washington focused on developing safety parameters of thermal thresholds using in-vitro ureteral, calyceal, and renal pelvis models. In this study, the in-vitro models simulating ureter, renal calyx, and renal pelvis placed in a water bath maintained at 37°C (figure 1).  A 242 µm laser fiber (Flexiva, Boston Scientific) was inserted through a ureteroscope (Lithovue, Boston Scientific). Real-time temperature was recorded by a wire thermocouple placed at the level of the ureteroscope with laser activation for 60 seconds. Trials were conducted at strategically selected power levels (2.5 – 50W) and irrigation rates (0 – 30 ml/min). The irrigation used was at room temperature. The thermal dose for each trial was calculated based on Dewey and Sapareto t43 methodology with thermal dose > 120 equivalent minutes considered to result in thermal tissue injury (figure 2). A parameter safety boundary was established by plotting the maximal safe power level for each irrigation rate.

Figure 1.

wce 2019 in vitro models

Figure 2.

WCE 2019 thermal dose calc

They found that a linear safety boundary/threshold in each scenario. The renal pelvis was able to tolerate the highest laser power while the renal calyx had the lowest tolerance of power. Figure 3 depicts the calyceal model and the linear relationship. The red circle represents exceeding the thermal dose threshold 100% of times while the green circles represent no instance of excess thermal dose. The yellow circles represent a fraction of trials that had a toxic thermal dose. The examples given show the linear relationship of the amount of energy used correlating with the rate of irrigation fluid. The specific example given to further highlight this point is: when using a laser setting between 15 – 30W an irrigation rate between 10 – 20 ml/min is needed.

Figure 3.

WCE 2019 calyceal model

This study highlights through a benchtop study the safety thresholds of thermal dose with laser lithotripsy. The renal pelvis has a higher thermal dose tolerance than the ureter or calyx.

Presented by: Ali Aldoukhi, MBBS, MS, Research Fellow, Department of Urology, University of Michigan, Ann Arbor, Michigan