An in vitro study of irrigation rate, operator duty cycle and intrarenal temperature in superpulse fiber thulium laser lithotripsy

Abstract

Objective

The present study aimed to examine the relationship between irrigation velocity, operator duty cycle (ODC), and intrarenal temperature during retrograde intrarenal surgery with a superpulse fiber thulium laser.

Methods

Place the stones into the fresh isolated porcine kidneys, use puncture needle to place the temperature probe 2 mm around the stones, and place the pressure probes in the upper calyx, lower calyx, and renal pelvis. Place the entire setup in a 37 °C constant temperature water bath to simulate the human body environment. The laser power varies between 10 and 30 W, and the irrigation speed is 10-30 ml/min. Additionally, at a laser power of 20 W and an irrigation speed of 10 ml/min, different On-Duty Cycles (ODC) are set. Monitor the changes in temperature and pressure.

Results

A direct proportionality of temperature in the kidney to the rate of irrigation has been reported between 10 W and 30 W laser powers. The percentage ratio of the rate of irrigation and power in the laser is 1:1, which can keep the temperature in the kidney at a safe level. At a laser power of 20 W and irrigation of 10 ml/min, the temperature inside the kidney increases sharply with the increase in ODC. By decreasing the ratio of ODC, the increase of temperature inside the kidney can be brought to a great reduction.

Conclusion

Maintaining a 1:1 ratio between laser power and irrigation speed can effectively prevent thermal damage or injury to kidney tissue.Additionally, by adjusting the On-Duty Cycle (ODC) ratio, the intrarenal temperature can also be reduced.

Introduction

The continuous development of medical technology has resulted in the continued improvement of laser lithotripsy, which is now a mature treatment technique. Superpulsed fiber thulium laser with a 1940-nm wavelength has come to be one of the important candidates for lithotripsy. Due to the proximity of this wavelength to the absorption peak of water, the absorption coefficient of that wavelength in aqueous solutions is markedly higher than the conventional holmium laser at 2100 nm. This gives this wavelength a substantial edge over others in lithotripsy efficiency [1, 2]. Today, holmium laser lithotripsy is the preferred method for (flexible) URS, because of the wide range of applications and good outcomes in the procedure of lithotripsy [3]. What is more important is that besides showing stone-crushing efficiency superior to that with the holmium laser, the super-pulsed fiber thulium laser also represents a new breakthrough in the treatment of the urinary tract stone, which features lower stone recoil force and thinner fiber diameter [4]. These features would thus indicate that the super-pulsed fiber thulium laser could be well applied and developed in the field of urinary stone treatment. The development of lithotripsy technology always attaches importance to the safety issue of laser lithotripsy as much as the stone comminution effect does. This work is aimed to optimize the temperature control and the irrigation flow rate during the process of laser lithotripsy to accomplish safe and effective therapy.

Materials and methods

In vitro modeling

This study utilized fresh ex vivo porcine kidneys, sourced under conditions that strictly adhered to ethical protocols, with all pigs being humanely euthanized. The research was conducted with the approval of the Ethics Review Committee and was granted the corresponding ethical approval number [IACUC-202306-P-001].The calculi specimens were obtained from our calculi specimen library, all of which underwent compositional analysis and were confirmed to be calcium oxalate monohydrate stones (CT value: 890 ± 123 HU). The study employed a range of equipment, including a single-use electronic ureteroscope with an outer diameter of F8.6 (ZebraScope Anhui Happiness Factory Medical Equipment Co., Ltd), a ureteral access sheath (F11/13 Laikai Medical Devices (Beijing) Co., Ltd), a super-pulsed thulium fiber laser with a 200 μm laser fiber (A-ONE Laikai Medical Devices (Beijing) Co., Ltd) , a multi-channel real-time thermometer (NF4000P-8 Guangzhou Ruimanting Instrument Factory) , a precision constant-flow experimental pump (BT-100 Hebei Rongbai Electric Appliance Factory) , High-precision digital pressure gauge (DT-1 Shanghai Ruxiang Instrument Factory) , a temperature-controlled water bath, physiological saline, and other essential surgical instruments.

The procedure involved the careful placement of the calculi into the renal pelvis, with temperature probes inserted using a puncture needle, positioned approximately 2 mm from the calculi within the renal pelvis. Ureteral access sheath was carefully inserted and secured to the ureter with ligatures. The pressure transducer is placed in the upper calyx, lower calyx, and renal pelvis using the same puncture needle. The entire setup was immersed in a temperature-controlled water bath, with the temperature maintained at 37 °C to simulate the human body’s environmental temperature.

Experimental procedures

The laboratory temperature is set at 25 °C. The laser lithotripsy power is set at 10 W (1.0 J × 10 Hz, 0.1 J × 100 Hz), 15 W (1.5 J × 10 Hz, 0.15 J × 100 Hz), 20 W (2.0 J × 10 Hz, 0.1 J × 200 Hz), 25 W (2.5 J × 10 Hz, 0.1 J × 250 Hz), and 30 W (1.0 J × 10 Hz, 0.1 J × 300 Hz). The irrigation speed is set at 10, 15, 20, 25, and 30 ml/min (the irrigation fluid is physiological saline, and the temperature is 25 °C). The continuous laser excitation time is 120 s.

For the laser lithotripsy power of 20 W (2.0 J × 10 Hz) and the irrigation speed of 10 ml/min (the irrigation fluid is physiological saline, and the temperature is 25 °C), each 20 s constitute an operational cycle. The laser operates in the ODC mode, with 10 different ODC modes set to simulate various surgical strategies: ODC10%: The laser is activated for 2 s and then turned off for 18 s. ODC20%: Activated for 4 s, then off for 16 s. ODC30%: Activated for 6 s, then off for 14 s. ODC40%: Activated for 8 s, then off for 12 s. ODC50%: Activated for 10 s, then off for 10 s. ODC60%: Activated for 12 s, then off for 8 s. ODC70%: Activated for 14 s, then off for 6 s. ODC80%: Activated for 16 s, then off for 4 s. ODC90%: Activated for 18 s, then off for 2 s. ODC100%: The laser is continuously activated for 20 s without an off cycle. Each cycle is repeated 6 times to form a large group experiment.

Both of the temperature and pressure probes monitor the parameter in real-time at a frequency of once per second and records changes. The above experiments are repeated 5 times for each set of parameters, and a new kidney is used for each repetition.

Data collection and analysis

The experiments have been conducted at least in quintuplicate, and the data are given as averages. Data were graphed using ggplot2 in R version 4.3.3. Values are presented as mean ± standard deviation. Statistical analysis was conducted by paired t-test between two groups and nonparametric rank sum tests using multiple samples for multiple groups. The significant difference was set at p < 0.05.

Results

Changes in intrarenal temperature during continuous laser lithotripsy

At a lithotripsy power of 10 W, with irrigation speeds of 10 ml/min, 15 ml/min, 20 ml/min, 25 ml/min, and 30 ml/min, the intrarenal temperature reached a steady state with steady-state temperatures of 38.6 °C, 35 °C, 33.1 °C, 31.7 °C, and 30.7 °C, respectively, none of which exceeded the safety threshold (Fig. 1). At a lithotripsy power of 15 W, the intrarenal temperature continued to rise with an irrigation speed of 10 ml/min. However, at irrigation speeds of 15 ml/min, 20 ml/min, 25 ml/min, and 30 ml/min, the intrarenal temperature reached a steady state with steady-state temperatures of 38.4 °C, 35.9 °C, 34.7 °C, and 34.4 °C, respectively, none of which exceeded the safety threshold (Fig. 2). At a lithotripsy power of 20 W, the intrarenal temperature continued to rise with irrigation speeds of 10 ml/min and 15 ml/min. However, at irrigation speeds of 20 ml/min, 25 ml/min, and 30 ml/min, the intrarenal temperature reached a steady state with steady-state temperatures of 38.7 °C, 36.3 °C, and 34.2 °C, respectively, none of which exceeded the safety threshold (Fig. 3). At a lithotripsy power of 25 W, the intrarenal temperature continued to rise with irrigation speeds of 10 ml/min, 15 ml/min, and 20 ml/min. However, at irrigation speeds of 25 ml/min and 30 ml/min, the intrarenal temperature reached a steady state with steady-state temperatures of 38.3 °C and 35.7 °C, respectively, neither of which exceeded the safety threshold (Fig. 4). At a lithotripsy power of 30 W, the intrarenal temperature continued to rise with irrigation speeds of 10 ml/min, 15 ml/min, 20 ml/min, and 25 ml/min. However, at an irrigation speed of 30 ml/min, the intrarenal temperature reached a steady state with a steady-state temperature of 37.5 °C, which did not exceed the safety threshold (Fig. 5). At different irrigation speeds, the intrarenal pelvic/calyceal pressure did not exceed 30mmH₂O (Table 1).

Fig. 1.

Renal temperature at each irrigation speed at 10 W of gravel power

Fig. 2.

Renal temperature at each irrigation speed at 15 W of gravel power

Fig. 3.

Renal temperature at each irrigation speed at 20 W of gravel power

Fig. 4.

Renal temperature at each irrigation speed at 25 W of gravel power

Fig. 5.

Renal temperature at each irrigation speed at 30 W of gravel power

Table 1.

The intrarenal pressure conditions under various irrigation speeds.（mmH₂O）

Irrigation speed	upper calyx	lower calyx	renal pelvis	F	p
10 ml/min	4.2 ± 0.07	3.85 ± 0.23	4.83 ± 0.09	33.46	<0.01
15 ml/min	4.6 ± 0.19	4.88 ± 0.08	5.48 ± 0.15	28.27	<0.01
20 ml/min	5.96 ± 0.04	6.08 ± 0.15	6.35 ± 0.12	10.05	<0.05
25 ml/min	6.18 ± 0.04	6.50 ± 0.28	9.08 ± 0.15	215.68	<0.01
30 mlmin	7.95 ± 0.54	8.65 ± 0.53	11.55 ± 0.15	54.82	<0.01

Changes in intrarenal temperature under different ODC ratios

When the On-Duty Cycle (ODC) is maintained below 60%, the steady-state intrarenal temperature tends to stabilize after reaching a certain value and does not further increase with the extension of time. In contrast, when the ODC exceeds 70%, the intrarenal temperature during the subsequent steady-state continues to rise over time (Fig. 6). With the increase of ODC, the highest intrarenal temperature shows a significant upward trend: ODC50%: 40.0 ± 0.52 °C; ODC 60%: 41.6 ± 0.38 °C; ODC 70%: 43.4 ± 0.48 °C; ODC 80%: 44.8 ± 0.73 °C; ODC 90%: 46.7 ± 0.54 °C (Table 2). When the ODC ratios are 70%, 80%, 90%, and 100%, respectively, the time for intrarenal temperature to reach the safety threshold temperature is 56.0 ± 1.0s, 51.8 ± 1.48s, 32.0 ± 1.58s, and 20.6 ± 1.14s, respectively (Table 3).

Fig. 6.

Curve of renal temperature during the working cycle (ODC) of different proportions (laser power set to 20 W, irrigation speed is 10 ml/min)

Table 2.

Different On-Duty Cycles (ODC) result in different intrarenal temperatures. (℃)

ODC	50%	60%	70%	80%	90%
Temperatures	40.0 ± 0.52	41.6 ± 0.38	43.4 ± 0.48	44.8 ± 0.73	46.7 ± 0.54

Table 3.

The time required for different proportions of ODC to reach the safety threshold(s)

ODC Time	70%	80%	90%	100%
Time to reach the safety threshold	56.0 ± 1.0	51.8 ± 1.48	32.0 ± 1.58	20.6 ± 1.14

Discussion

Studies conducted earlier have shown that there is a considerable amount of cell necrosis in the human tissue above 43 °C temvperature and cell death further increases to 45 °C [5]. At a temperature of 60 °C, the proteins present in the tissue are irreversibly denatured; therefore, the structure of the tissue is permanently destroyed and its function is lost. This is why, to some extent, this is also considered as the temperature threshold for human tissue at 43 °C. During retrograde intrarenal surgery, the irrigation rate plays a very important factor in determining the temperature of the tissue [6]. The use of high irrigation rates will help avoid increased temperatures inside the kidney. A irrigation rate of 100 ml/min will maintain the temperature so that, even if the laser works continuously for one minute, the temperature does not exceed 38.5 °C [7]. But if the irrigation rate is too high, the intra-renal pressure will be increased, leading to postoperative pain and the patient being more prone to infection [8–10]. On the contrary, an inadequate irrigation rate might end up causing a high intraoperative temperature, leading to thermal injury in the tissues and affecting the visual field with an inability to wash away the stone powder in time, thus causing further damage to the tissues. And therefore, it is very important to measure the irrigation rate reasonably so that a surgical intervention in the patient can be safely made. In this experiment, we made a detailed observation and analysis of the temperature control used in laser lithotripsy. 10 W lithotripsy power and 10 ml/min irrigation rate kept the temperature of the kidney steady at a plateau in the early stage of the experiment, wherein this constant temperature was always below the safety threshold. The power of the laser had been increased to 15 W while proceeding with the experiment, although the irrigation rate remained the same. The temperature plateau reached 44 °C in the kidney, which was beyond the limit for safe human tissue. This damage was then effectively contained by increasing irrigation to a flow of 15 ml/min, such that the temperature was maintained at safe levels to prevent thermal injury. The results of the experiment show that with an increase in power, the temperature in the kidney grows appropriately to this change; conversely, an increase in the irrigation rate results in a decrease of the temperature in the kidney. Our observations correspond to the findings of Winship et al. [11], who conducted their experiments using 3.6 W, 6.4 W, 10 W, 16 W, and 20 W laser power with irrigation pressures of 0, 100, and 200 mmHg. These authors concluded that both the temperature in the kidney and the speed of irrigation determine the conducted temperature: the higher the laser power, the higher the temperature in the kidney; the faster the irrigation rate, the lower the temperature in the kidney [11]. The most striking result from this experiment is that when lithotripsy power and irrigation speed are varied together at a 1:1 ratio, the plateau temperature in the kidney stays below the safety threshold for all conditions. This approach is not only superior to the safety of the procedure but also provides physicians with a new way to adjust operational parameters in maximizing the benefit of lithotripsy while minimizing the possibility of thermal injury to the patient’s kidney.

In our work, the 20 W lithotripter power and an irrigation flow rate of 10 mL/min were considered, paying particular attention to ODC values in a large range from 50 to 100%. This phenomenon is just the same as the classical sawtooth fluctuation wave which is corresponding to the laser periodical activation: the temperature in liquid increases rapidly when the laser is activated and decreases slowly to form a plateau when it is deactivated. The ODC ratio of the kidney showed a significantly raised level at the steady-state. This is similar to what was also observed in Louters et al. [12], so that a higher ODC increases temperature and energy release markedly. The steady-state renal temperature shows a trend of leveling off after attaining some value and does not increase further in case the ODC was kept below 60%. On the other hand, steady-state renal temperature increased to an increasing value after an ODC higher than 70%, and this may reflect increasing heat accumulation effect. It was also observed that a marked reduction in ODC, above an ODC of about 70%, considerably prolonged the time for which renal temperature to reach the safety threshold. This is likely due to the concordance with the results of Wanderling et al. [13]. The current theory is supported by the experimental results, which demonstrate that, over the very fine-tuning of the ODC required to minimize continuous activation time of the laser, the laser may be used for a greater length of time without crossing the safety temperature limit. One interesting point must be considered in this trial.

The experiment used a fixed superpulsed thulium fiber laser, one diameter of the fiber, and a ureteral guide sheath with a flexible ureteroscope. The effect of fiber diameter on temperature and size of the soft sheath is an area for future research. In addition, the fact that this was an in vitro experiment does not bring into play the physiological factors in the changes in temperature, like renal irrigation and urine production. More studies are essential to clarify whether experiments on animals present a solution to this problem. In a nut shell, the outcome of the study has indicated that the decrease in ODC ratio from applying retrograde intrarenal surgery can have the effect of substantially reducing the rise in renal temperature. This may provide a new safe and effective method for clinical application in surgery.

Conclusion

Therefore, reducing the ratio of ODC allows substantial relief in renal temperature elevation during super-pulse optical fiber thulium laser lithotripsy. When gravel power and irrigation speed have a 1:1 adjustment ratio, it can effectively keep renal temperature within safe limits, guarding against thermal injury of kidney tissue.

Author contributions

Ding tianfu.Xu zheng.and Li jianxing. wrote the main manuscript text. Huang zhongyue. Xiao bo. and Huweiguo prepared all figures and table. All authors reviewed the manuscript.

Funding

There was no source of funding for this project.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.