Type of laser and temperature

In another study, Molina et al. [2] investigated the temperature profiles of two laser systems, the 120 W Ho: YAG (model P120, Lumenis Ltd., Israel) and the 60 W SPTF (SOLTIVETM, Olympus, MA), during ureteral lithotripsy using an ex vivo porcine kidney and ureter model. The irrigation was conducted with saline at room temperature (27 °C) using a manual pumping system. The authors reported that both laser systems' temperature profiles remained below the threshold for potential cellular injury (43 °C) during the experiments. Moreover, at dusting settings (0.3 J, 70 Hz, Long Pulse), the Ho: YAG laser's median temperature increase was higher than that of the SPTF laser (40.6 °C vs. 35.8 °C, respectively, p = 0.064).

Belle et al. [6] investigated the temperature differences between thulium fiber laser (TFL) and Ho: YAG lasers during ureteral stone lithotripsy. TFL was found to produce the highest temperature at all laser settings of 3.6 W, 10 W, and 30 W compared to the 30 W Medilas H Solvo HL (Dornier, Munich, Germany) and 100 W Empower HL (Olympus, MA) lasers. There was no significant difference between the TFL (40.93 °C) and the Empower HL (41.17 °C, p > 0.05) at 20 W. At 30 W, the TFL produced a significantly high maximum temperature of 44.37 °C and exceeded the threshold.

Hardy et al. [10] compared the effects of Ho: YAG and TFL on temperatures in an in vitro ureteral model. They used Ho: YAG laser at 600 mJ, 350 μs, 6 Hz, and TFL at 35 mJ, 500 μs, 150–500 Hz settings. Saline irrigation flow rates were 22.7 ml/min for TFL and 13.7 ml/min for Holmium laser. They found a mean peak irrigation fluid temperature of 24 ± 1 °C for Holmium and 33 ± 3 °C, 33 ± 7 °C, and 39 ± 6 °C for TFL at pulse rates of 150, 300, and 500 Hz, respectively. The researchers concluded that to avoid excessive thermal rise and provide a sufficient safety margin, TFL lithotripsy should be performed with pulse rates under 500 Hz and/or with increased irrigation rates.

In another experimental study, Sierra et al. [15] compared the safety of Ho: YAG and TFL in an in vivo porcine model and in vitro ureteral model. The temperature reached 40.2 °C with TFL and 41 °C with Ho: YAG laser at max 20W power with gradually decreased irrigation (40 cmH2O continuous).

Similarly, Taratkin et al. [16] investigated the temperature profile between Ho: YAG laser and a novel TFL during laser lithotripsy in an in vitro model. The researchers reported that both lasers yielded similar temperature increases, and the amount of energy used was equivalent to the amount of heat introduced into the system. Temperature increase at 20W without irrigation was 10.9 ± 0.5 °C for Ho: YAG and 11.0 ± 0.5 °C for super-pulsed TFL.

Laser settings, irrigation flow rate and temperature

In their study, Wollin et al. [1] examined the impact of laser settings and irrigation flow rates on ureteral temperature during ureterolithotripsy in an in vitro setting. Specifically, they utilized a 200 μm laser with varying pulse energies and frequencies (0.2 J/50 Hz, 0.6 J/6 Hz, 0.8 J/8 Hz, 1 J/10 Hz, and 1 J/20 Hz) for a duration of 60 s within a tubing system that allowed for specified room temperature flow rates (100, 50, and 0 mL/minute). Their findings indicated that at an irrigation rate of 100 mL/min, only the highest laser setting (1 J/20 Hz) produced a maximum temperature of 30.7C, which was deemed clinically insignificant. However, with no irrigation fluid flow, all maximum temperatures exceeded the threshold for cellular thermal injury (43 °C). Furthermore, at the highest laser setting (1 J/20 Hz) with no irrigation, the maximum temperature exceeded 100 °C in their study [1].

Molina et al. [3] investigated the temperature profile of Ho: YAG laser lithotripsy in the urinary tract of Ovis Aries (sheep) with and without irrigation. They reported that the temperature rise was significantly higher in the intact ureteral model without irrigation compared to with irrigation, with temperatures of 49.5 ± 2.3 °C versus 37.4 ± 2.5 °C, respectively.

Aldoukhi et al. [5] investigated the caliceal fluid temperature during high-power Ho: YAG lithotripsy in an in vitro porcine model. They used the Ho: YAG laser at a 0.5 J × 80 Hz = 40 W setting with high, medium, or no irrigation. The peak temperatures for no, medium, and high irrigation were 84.8 °C, 63.9 °C, and 43.6 °C, respectively.

Hein et al. [11] utilized an in vitro model to simulate the renal pelvis and assess the changes in temperature during Ho: YAG lithotripsy. The results indicated that without irrigation, laser application led to a rapid temperature rise of up to Δ28 K, increasing to 68 °C at 100 W. However, higher irrigation rates resulted in a lower temperature increase. At the highest irrigation rates of 100 ml/min and the highest laser power setting (100 W), the temperature increased by only 5 K.

Teng et al. [17] conducted a study to assess the changes in irrigation fluid temperature in renal calyces during Ho: YAG lithotripsy using flexible URS in a real-life scenario. The authors found that at a power setting of 1 J/20 Hz and an irrigation flow rate of 15 ml/min, the temperature rise was significantly higher than other groups. Moreover, the temperature increase was significantly higher in groups with lower irrigation flow rates at the same laser settings. The time required to reach a temperature of 43 °C at 1 J/20 Hz was significantly shorter than that at 0.5 J/20 Hz when there was no irrigation.

In another study, Winship et al. [19] investigated the temperature rise in a benchtop ureteral model using both flexible and semi-rigid ureteroscopes with Ho: YAG laser. Laser energy was delivered at various settings, and with irrigation at 100 mm Hg using the semi-rigid scope, 1 J/20 Hz was the only laser setting to produce a temperature rise over 6 °C. The temperature returned to close to baseline levels within 2 s after laser cessation.

Aldoukhi et al. [4] investigated the effect of pedal activation on fluid temperature and thermal profiles in an in vitro experimental caliceal model using the Ho: YAG laser (pulse120; Lumenis). They reported that longer pedal activation times led to higher peak temperatures and thermal effects. The thermal injury threshold (43 °C) was reached in 9 s when 40 W was applied at 50% operator duty cycle (ODC = lasing time/lithotripsy time) with laser activation patterns of 30 s on/off and 15 s on/off.

Wriedt et al. [20] evaluated tissue heating with magnetic resonance imaging in an ex vivo porcine kidney model using different lithotripsy parameters. The authors reported that an irrigation rate of at least 70 ml/min is necessary to avoid exceeding 120 min CEM43 when using a laser application of 30 W for 10 s. Additionally, they observed a focal temperature rise on the calyx wall in experiments with human stones, which they attributed to non-elimination of heated fragmented stone pieces from the surgical area in the kidney.

Pelvicalyceal volume and temperature

Khajeh et al. [13] studied the relationship between pelvicalyceal volume and temperature rise during Ho: YAG laser lithotripsy. The findings showed that the temperature elevation and thermal dose from laser activation were inversely related to the fluid volume in each model and the irrigation rate. The safe temperatures below the threshold of tissue injury were obtained at an irrigation rate of 40 mL/min during 1 min of continuous laser activation in all models [13].

Types of pulse modulation and temperature

In a study by Peteinaris et al. [14], the effects of the newly introduced Holmium pulse modulation system MOSES were compared with conventional pulse delivery technology on irrigation fluid temperature (IFT) during flexible ureteroscopy in a live-anesthetized porcine model. The study considered the threshold for a dangerous IFT as 54 °C. The results revealed that both the MOSES and conventional laser activation at 60 W led to a significant temperature rise, exceeding the safety threshold of 54 °C in approximately 10 s and reaching a hazardous temperature of 66.4 °C in 18 s under gravity irrigation. However, the IFT did not exceed 54 °C at any settings with manual pump irrigation.

Winship et al. [18] investigated the effect of pulse type of Ho: YAG (short pulse (SP), long pulse (LP), MOSES contact (Mc), MOSES distance (MD)) on irrigation fluid in an experimental ureteral model. The authors reported that LP produced the greatest temperature change from baseline at a 1 J/20 Hz setting, and thermal dose exceeded the injury threshold for all pulse types in < 3 s of activation at the same setting.

Formula to suggest temperature rise

In a separate study, Hein et al. [12] examined the thermal effects of Ho: YAG laser during retrograde intrarenal surgery and percutaneous nephrolithotomy in an ex vivo porcine kidney model. The researchers suggested a formula to calculate temperature changes with irrigation rates higher than 30 ml/min: ΔT = 15 K × (power [W]/ irrigation [ml/min]) based on their results.

Lastly, Williams et al. [21] developed a mathematical model to predict laser-induced temperature alterations in a kidney during lithotripsy. The model was based on renal volume, irrigation flow rate, irrigation fluid temperature, and laser power. The researchers cautioned clinicians to use room temperature irrigation fluids for irrigation and avoid extensive laser usage. They also suggested altering the irrigation fluid rate by adjusting the inflow (i.e., pressured irrigation fluid) or outflow (i.e., usage of UAS).

Possible preventive measures for temperature rise

In their study, Dau et al. [7] investigated the effect of chilled irrigation on caliceal fluid temperature and time to reach the thermal injury threshold in an in vitro model. The experiments utilized a 120 W Ho:YAG laser (pulse120; Lumenis, CA) at a 0.5 J · 80 Hz (40 W) setting in short pulse mode for 60 s. The thermal injury threshold was reached in 28 s with room temperature (19 °C) irrigation and in 33 s with chilled irrigation (1 °C) with a flow rate of 8 mL/min. With 12 mL/min irrigation, the threshold was reached in 46 s with room temperature irrigation, but it was not reached with chilled irrigation. In another study, they evaluated and compared the thermal effects of chilled irrigation (4 °C–1 °C), room temperature (RT) (20 °C–1 °C), and warm (WM) irrigation (37 °C–1 °C) during ureteroscopy with laser activation in an in vivo porcine model. The researchers concluded that irrigation with chilled saline during ureteroscopic laser lithotripsy decelerates temperature rise, decreases peak temperature, and lengthens the time to thermal injury compared with irrigation with room temperature or warm saline solutions [7].

In their study, Gallegos et al. [8] reported that using a ureteral access sheath (UAS) provides lower intrarenal temperatures regardless of laser configuration and irrigation solution height. The rise in temperature obtained per minute with an irrigation height of 50 cm H2O, an energy of 1 J, and a frequency of 15 Hz was higher when UAS was not utilized (5.8 °C versus 3.8 °C) [8]. Similarly, Noureldin et al. [9] investigated the effects of irrigation rates and UAS size on intrarenal temperature during flexible ureteroscopy in a live-anesthetized porcine model. They found that fURS under gravity irrigation (the fluid bag at 1 m over the operation table) without UAS was associated with hazardous intrarenal temperatures (54 °C) even at a laser power as low as 20 W for as short as 20 s of laser activation. With pump irrigation, if the laser was activated at the highest power setting (60 W) for 60 s, the intrarenal temperature remained on the safe side, even without the use of UAS [9] (Table 1).

Table 1 Descriptives and significant results of studies on effect of lasers on intrarenal temperatures