Experimental study of irrigation rate, ODC and intrarenal temperature in superpulse fiber thulium laser lithotripsy

doi:10.21203/rs.3.rs-4724781/v1

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Experimental study of irrigation rate, ODC and intrarenal temperature in superpulse fiber thulium laser lithotripsy

https://doi.org/10.21203/rs.3.rs-4724781/v1

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The present study aimed to examine the relationship of irrigation velocity, operator duty cycle (ODC), and intrarenal temperature during extracorporeal shockwave lithotripsy with a superpulse fiber thulium laser. Calcium oxalate monohydrate stones were implanted in the renal pelvis of fresh pig kidneys. Puncture technology was used to place a temperature probe accurately in the renal parenchyma about 2 mm away from the stone. To simulate the temperature at which a human body is exposed, that is, 37°C, the experiment was executed in an equilibrated laboratory at a constant temperature of 25°C with 60% humidity. The power of the laser varied between 10W and 30W; that of the irrigation varied from 10 to 30 ml/min. The time that the laser was applied was set at 120 s. Changes in the temperature were recorded online. 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. More specifically, 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. The ratio of laser power to that of irrigation speed should be 1:1; hence, damage or injury to kidney tissue can be efficiently prevented from thermal changes.

superpulse fiber thulium laser

operator duty cycle (ODC)

irrigation rate

Intrarenal temperature

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, medical guidelines recommend the use of holmium lasers not only for home but also for treatment abroad 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. The experimental study results showed that changes in irrigation rates greatly influenced the temperature in the kidney at laser powers between 10 W to 30 W. In clear terms, at a laser power of 20 W and the irrigation level of 10 ml/min, there is a significant increase in temperature within the kidney as the ODC starts to rise. In fact, a decrease in the ODC ratio can bring about a remarkable reduction in intrarenal temperature elevation. The application of 1:1 lithotripsy power and irrigation rate will prevent thermal injury to the renal tissue tremendously.

In vitro modeling

Preparation of the required apparatus and materials was done prior to the experiment. These were fresh isolated porcine kidneys, a disposable electronic ureteroscope (F7.4 at the proximal end, F8.6 at the distal end), an F11/13 ureteroscope sheath, a superpulse thulium laser device with a 200 µm diameter laser fiber, a multi-channel real-time thermometer, a precision laboratory water pump, a thermostatic water bath, saline solution, and a variety of other surgical instruments. The kidney was mounted onto the experimental frame and a temperature probe placed into the renal parenchyma, about 2 mm away from the stone, using the puncture technique. A clinical specimen of calcium oxalate monohydrate stone (CT value: 890 ± 123 HU) was placed in the renal pelvis, and the ureteroscope sheath was inserted. The disposable electronic ureteroscope was then passed through the sheath of the ureteroscope to enable real-time visualization and manipulation. All of these were further submersed into a thermostatic water bath very close to the temperature of the human body at 37°C. The continuity of water inside and around the kidney was maintained by a precision laboratory water pump along with normal saline. On the other hand, the super-pulse thulium laser device was turned on and lithotripsy procedure performed using a 200 micrometer laser fiber. The process was visualized and captured real-time, and the heat changes in the kidney were monitored.

Experimental procedures

The laboratory has a stable temperature maintained at 25°C, and relative humidity is 60%. The laser lithotripsy power was set to 10W (1.0J×10Hz, 0.1J×100Hz), 15W (1.5J×10Hz, 0.15J×100Hz), 20W (2.0J×10Hz, 0.1J×200Hz), 25W (2.5J×10Hz, 0.1J×250Hz), and 30W (1.0J×10Hz, 0. The experiment was done using a 1J×300Hz apparatus, with a irrigation rate of 10. A disposable electronic F8.6 ureteroscope and an F11/13 ureteral sheath were used. Strategies were set up for five ODC modes: ODC50% (excitation with the laser for 10 seconds and off for 10 seconds), ODC60% (excitation with the laser for 12 seconds and off for 8 seconds), ODC70% (excitation with the laser for 14 seconds and off for 6 seconds), and ODC80% (excitation with the laser for 16 seconds and off for 4 seconds) protocols were used. The ODC90% strategy was therefore such that the laser was on for 18 seconds and off for 2 seconds. So in the ODC100% strategy, the laser was permanently on such that there was no off period. Each of this was repeated for 20 seconds, which made up one operational cycle. This cycle was repeated six times to create a group of experiments. The total number of each parameter group was repeated five times for each group to assure data correctness and repeatability.

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.

Changes in intrarenal temperature during continuous laser lithotripsy

From that case, it was observed that within the kidney, under a continuous laser excitation time of 120 s, there was a major significant interaction effect between the different lithotripsy powers and irrigation rates for the temperature. The temperature specifically in this case of stone-crushing power of 10 W in the kidney stabilized at 35°C, 32.7°C, 30.6°C, 29.0°C, and 28.1°C. The corresponding irrigation rates were 10 ml/min, 15 ml/min, 20 ml/min, 25 ml/min, and 30 ml/min. Of course, none of these was above the safe limit, as can be seen in Fig. 2. For a irrigation of 10 ml/min, temperature still rose in the kidney at a stone-crushing power of 15 W. The temperature stabilized at 37.5°C, 34.2°C, 31.8°C, and 30.3°C at irrigation rates of 15 ml/min, 20 ml/min, 25 ml/min, and 30 ml/min respectively (Fig. 3). Again, irrigation over this temperature range resulted in steady states at 37.5°C and 34.2°C for irrigation rates of. At 20 W of stone-crushing power, the temperature rose greatly inside the kidney, but it plateaued at irrigation rates of 10 ml/min and 15 ml/min, with the mean irrigation values shown to be 37.6°C, 34.8°C, and 32.9°C at 20 ml/min, 25 ml/min, and 30 ml/min of irrigation rates, respectively (Fig. 4). During the stone-crushing power of 25 W, the temperature of the kidney continued to increase with irrigation rates of 10 ml/min, 15 ml/min, and 20 ml/min, but for the irrigation rates of 25 ml/min and 30 ml/min, the temperature of the kidney was maintained at 37.6°C and 35.8°C, respectively (Fig. 5). At a stone-crushing power of 30 W, temperature continued to rise within the kidney at irrigation rates of 10 ml/min, 15 ml/min, 20 ml/min, and 25 ml/min while remaining constant at 37.8°C at 30 ml/min (Fig. 6). Interesting to note that, none of these temperatures crossed the safety threshold.

Changes in intrarenal temperature under different ODC ratios

Based on the 20 W power and a irrigation velocity of 10 ml/min, in a cycle of 2 minutes, six different steady states were found for a change in kidney temperature. The results showed that an increase in the ODC ratio was accompanied by increasing kidney temperature during all of the steady-state periods. Initial steady-state temperatures were 33.4 ± 1.58°C for the ODC 50% condition and 34.0 ± 1.91°C for ODC 60%. By temperature, it read 34 ± 1.91°C; at 70% ODC ratio, the temperature is recorded as: The average was 34.8 ± 1.24°C; at 80% ODC ratio, the average came out to be: The temperature was recorded at 35.7 ± 1.03°C; at ODC of 90%. The average was 36.4 ± 0.49°C (Table 1). The kidney temperature resulted in some definite value of steady state and was constant, showing no temperature increase during the process of time under 70% and less ODC maintenance. However, with more than 80% of the ODC, a kidney temperature increase to the second point of steady state can be seen in Fig. 7. The safe threshold of time for reaching the renal temperature was 76.4 ± 1.14 s at an ODC ratio of 80%, 43.6 ± 1.52 s at 90%, and 38.0 ± 1.58 s at 100% (F = 1058.10, p < 0.01; Table 2).

The tremendous progress in medical science and technology has seen revolutionary changes in high-power laser technology. The super-pulsed fiber thulium laser technology, with a wavelength of 1940 nm, is closer to the absorption peak of water than the conventional holmium laser at 2100 nm[1, 5]. This positioning confers many technological advantages to the thulium laser: low energy threshold and efficiency in crushing stones. It has a high pulse frequency with low pulse energy, leading to small fragments during lithotripsy. This is of advantage for the complete powderization of the stones[4]. Besides, super-pulse thulium fiber laser characterizes a smaller fiber core diameter, adjustable pulse shape, and adjustable pulse width duration. These features do not just reduce the recoil force which is exerted by the stones while lithotripsy but also significantly increase the accuracy and safety of the procedure[1, 2]. The use of flexible ureteroscopes introduces the operability and flexibility of the procedure.

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[6]. 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 extracorporeal shock wave lithotripsy, the irrigation rate plays a very important factor in determining the temperature of the tissue[7]. 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[8]. 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[9–11]. 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. 10W lithotripsy power and 10ml/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 15W 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. [12], 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 50–100%. 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. [13], 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 70%. On the other hand, steady-state renal temperature increased to an increasing value after an ODC higher than 80%, and this may reflect increasing heat accumulation effect. It was also observed that a marked reduction in ODC, above an ODC of about 80%, 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. [14]. 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 extracorporeal shock wave lithotripsy can have the effect of substantially reducing the rise in renal temperature. By adjusting the power in direct proportion to the lithotripsy and the irrigation rate in a 1:1 ratio, the renal temperature can be effectively kept within a safe range so as not to generate thermal damage to the tissue of the kidney. This may provide a new safe and effective method for clinical application in surgery.

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.

Funding

There was no source of funding for this project.

Author Contribution

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

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_ODC ^S−S	First time	Second time	Third time	Fourth time	Fifth time	Sixth time
50%	33.4 ± 1.58	36.2 ± 0.34	39.2 ± 0.42	40 ± 0.83	40 ± 0.63	40 ± 0.84
60%	34.0 ± 1.91	37 ± 1.29	39.5 ± 1.34	41 ± 1.01	41 ± 0.68	41 ± 0.57
70%	34.8 ± 1.24	37.7 ± 0.59	40.5 ± 0.39	42 ± 0.82	42 ± 0.47	42 ± 1.01
80%	35.7 ± 1.03	40 ± 1.25	42.7 ± 1.14	44.1 ± 0.53	45.8 ± 0.53	46.5 ± 1.47
90%	36.4 ± 0.49	39.6 ± 1.16	42 ± 1.32	43.9 ± 1.18	46.2 ± 1.43	47.4 ± 1.58

Table.1 The laser gravel power is set to 20W and the irrigation speed is 10ml/min, the steady-state temperature in the kidney under the working cycle (ODC) of different agents (℃)

_Time ^ODC	80%	90%	100%	F	p
Time to reach the safety threshold	76.4 ± 1.14	43.6 ± 1.52	38.0 ± 1.58	1058.10	<0.01

Table.2 The time required for different proportions of ODC to reach the safety threshold(s)

No competing interests reported.

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You are reading this latest preprint version

Experimental study of irrigation rate, ODC and intrarenal temperature in superpulse fiber thulium laser lithotripsy

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

In vitro modeling

Experimental procedures

Data collection and analysis

Results

Changes in intrarenal temperature during continuous laser lithotripsy

Changes in intrarenal temperature under different ODC ratios

Discussion

Conclusion

Declarations

Funding

Author Contribution

References

Tables

Additional Declarations

Status:

Version 1