It’s Getting Hot in There: In Vitro Study on Ureteral Tissue Thermal Profiles During Laser Ureteral Lithotripsy

Abstract

Introduction: The integration of laser technology in urologic interventions, especially ureteral lithotripsy, has greatly advanced the field, with laser lithotripsy becoming the preferred method for treating ureteric stones via ureteroscopy. Recent advancements focus on enhancing power settings and reducing operating times, introducing high-power laser equipment capable of frequencies up to 120 Hz. However, concerns arise regarding thermal injuries to adjacent tissues due to increased energy delivery, potentially causing ureteric strictures. Objective: To explore temperature dynamics during ureteroscopic laser lithotripsy, considering factors like laser power settings and ureteroscope size, to optimize outcomes and mitigate risks for patients. Methods: A simulated in vitro model for ureteroscopic laser lithotripsy was designed with a holmium laser. Measurements of the temperature were recorded using a thermocouple placed at the laser tip at different sizes of ureteroscope (URS 6.0 Fr and URS 7.0 Fr), holmium laser (272 µm and 365 µm), various power settings (5 to 25 Hz; 0.2 to 3.0 J) and activation durations (3 to 30 s). Analysis of the variables associated with temperature change was performed. Results: All of the variables showed rising temperature trends as the laser activation time was prolonged, while ureteroscope size had no significant impact. Smaller laser fibers exhibited lower overall temperature profiles, around 34–35 °C. Notably, power settings significantly influenced temperature, with a substantial rise at 20 W (42.62 °C) and 30 W (40.02 °C). There was a significant rise in temperature as power (J × Hz) increased, where frequency carries a higher effect than energy at the same power setting. Conclusions: The recommendation includes exercising caution with higher power levels, shorter activation times, and preferably using small-caliber laser fibers to maintain lower temperatures.

1. Introduction

Since laser technology has been incorporated, the field of urologic interventions, particularly ureteral lithotripsy, has received revolutionary advances. The compact dimensions of the laser fiber and its capability for high-energy delivery have established laser lithotripsy as the preferred standard for treating ureteric stones through ureteroscopy [1]. Current developments are focused on reaching higher power settings and shorter operating times. The imperative for quicker lithotripsy has prompted the introduction of high-power laser equipment (100 W), enabling frequencies of up to 120 Hz, in contrast to the 20 Hz of low-power laser generators (30 W). Some researchers even advocate for utilizing extreme laser configurations, reaching up to 2 J and 50 Hz during flexible ureteroscopy [2]. While higher power facilitates increased energy delivery, not all the surplus energy is directed toward the stone. A significant portion of the energy disperses and is absorbed by the water, resulting in a substantial temperature increase that may lead to thermal injuries to the adjacent urothelial mucosa.

Advances focus on higher-power lasers for quicker lithotripsy, yet excess energy can cause thermal injuries. Strictures post-laser lithotripsy is linked to tissue damage at 43 °C for 120 min, exacerbated by stone impaction [3], suggesting that higher temperatures require less exposure time to achieve the same effect. Likewise, a rapid temperature increase at the onset of laser lithotripsy can expose tissues to damaging high temperatures, potentially harming the collecting system and ureter [4,5]. The thermal dose is a nonlinear function, so a much shorter time is needed to produce tissue damage with subsequent minimal rise in temperature (e.g., 15 s at 53 °C to produce damage) [6].

Several factors can lead to ureteric stricture after ureteroscopy: the duration of the disease [5], the location and size of the stones [7,8], the existence of embedded stones [9,10], the type of lithotripsy that was selected [9,10], and mechanical damage from ureteroscopic manipulation [11]. Of these many causes, heat damage during lithotripsy stands out as a commonly overlooked yet significant substantial etiological element. To mitigate heat production and limit cellular damage, active measures such as intermittent laser activation—pausing every 10 s to reduce temperature to safer levels—are recommended [12]. Another strategy involves maintaining adequate irrigation during ureteroscopy (URS) through the use of an access sheath, which enhances irritant outflow and reduces intrarenal pressure [13,14].

Drawing insights from previous research and literature, we have incorporated factors known to influence temperature dynamics during ureteroscopic laser lithotripsy. Our objective is to establish how different holmium laser power settings, ureteroscope size, laser fiber diameter, and laser activation times produce an intrarenal temperature profile that is essential for guiding urologists in optimizing outcomes.

2. Methods

An in vitro model of ureteroscopic laser lithotripsy was used to assess the optimum laser setting affecting temperature changes during laser discharge while regulating the temperature of irrigation fluid at 37 °C using a fluid warmer, the ureter internal diameter at 7 mm, and the rate of irrigation fluid at 15 mL/min (by keeping the saline bag height at 100 cm H₂O; Figure 1).

A ligated PTFE tube segment with an internal diameter of 21 Fr was utilized to mimic the human ureter. This PTFE tube was submerged in a 37 °C water bath, simulating physiological conditions to human body temperature. Approximately 1 cm in size, a stone was introduced at the tube’s end. The artificial carbonate apatite stones used throughout the procedure were scanned using a CT scan to choose those with approximately 1000 Hounsfield Units. A semirigid ureteroscope was inserted, and a laser fiber was threaded through the scope’s working channel. Temperature measurements were conducted using a 1 mm diameter K-type thermocouple sensor [UT320D, UNI-T, China] (Figure 1).

Two thermocouples (T1 and T2) were positioned within the PTFE tube—one adjacent to the fiber tip within 1mm (T1) and the other 1–2 mm away from the fiber tip (T2). The initial resting temperature within the PTFE tube was recorded. The thermocouple, positioned alongside the laser tip, was used to document temperature changes. Intracorporeal lithotripsy was simulated using two different sizes of ureteroscope (URS 6.0 Fr and URS 7.0 Fr) and two sizes of the Holmium:yttrium/aluminium/garnet (Ho:YAG) laser (272 µm and 365 µm). Power (watts) = Frequency (Hertz) × Energy (Joule). Various power settings (frequencies ranged from 5 Hz to 25 Hz; energy ranged from 0.2 J to 3.0 J) and activation durations (3 s, 5 s, 10 s, 15 s, 20 s, 25 s, and 30 s) were tested. The laser was activated in a continuous manner for 30 s. Temperature readings for each power and duration combination were taken three times, and the means were calculated for statistical analysis across different setups. The duration required to return from the maximum temperature to the resting temperature was also documented for each setting. These procedures were repeated using different laser power settings and varying sizes of the ureteroscope and laser fiber. No ureteric access sheath was utilized, and irrigation saline was maintained at a constant height and temperature.

Statistical Analysis

Analyses were conducted at a factorial arrangement of different dimensions of the ureteroscope and the laser fiber along with different laser power settings during the laser activation period using SPSS 21.0 software. Analysis of variance (ANOVA) was used to analyze the effects between variables, followed by Duncan post hoc comparisons. Data were analyzed at the 95% confidence level and were considered significant when the p-value was less than 0.05. Results were further computed in the table to study the trend of the temperature of each variable during the laser activation period and were displayed as means ± standard deviation. Principal component analysis (PCA) was carried out to relate overall thermal profiles recorded at multiple variables (GraphPad Prism version 9).

3. Results

3.1. Effects of Different Sizes of Ureteroscope and Laser Fiber on Temperature

There was no significant difference (p > 0.05) observed between the interaction of the two sizes of ureteroscope and two diameters of laser fiber during the activation period. The larger laser fiber exhibited a significantly higher temperature rise at 36.44 °C than a 272 µm laser fiber (35.00 °C) when laser activation time reached 10 s (Figure 2). The temperature continued to increase with an extended activation time, reaching its peak at 37.02 °C for the bigger laser fiber 30 s after laser activation was initiated. Despite both fibers displaying an ascending temperature trend during the activation period, the smaller fiber exhibits a lower thermal profile, ranging between 34–35 °C (Figure 2). Likewise, two distinct sizes of ureteroscope experienced a rise in temperature upon the initiation of laser activation. However, the size of the ureteroscopes did not influence the temperature during laser lithotripsy (). The ureteroscopes of 6.0 Fr and 7.0 Fr exhibited average temperatures of 35.63 °C and 35.83 °C, respectively (Figure 2).

This study revealed a highly significant difference (p ≤ 0.01) in the interaction of power settings across six levels of energy and five levels of frequency. The comparative evaluation of the energy with various configurations resulted in notable temperature increases during laser lithotripsy, reaching the highest temperature at 3.0 J, ranging from 39.31 °C to 42.23 °C (Figure 3). Similar thermal profiles were observed in both power settings, with higher frequency levels during the activation period yielding higher temperatures. The frequency configurations attained the highest temperature at 20 Hz and exhibited a significant decrease at 25 Hz. All recorded temperatures exhibited a decrease of approximately ±5 °C, falling below 36 °C (Figure 3).

In this investigation, the highest temperature, 44.48 °C, was reached at 10 W (0.5 J × 20 Hz). Conversely, at the same power of 10 W but with a different energy setting of 1.0 J × 10 Hz, the temperature recorded a lower value of an average of 36.20 °C. With increased power levels of 20 W (2.0 J × 10 Hz) and 30 W (3.0 J × 10 Hz), temperatures surpassed 40 °C, measuring an average of 42.62 °C and 40.02 °C, respectively (Figure 4). During laser lithotripsy, heat accumulates, and the temperature increases with rising power (J × Hz), as depicted in Figure 4.

3.2. Possible Outcomes of Multiple Variables for Temperature Rise

Multiple variables were clustered with a correlation using a multivariate discriminant analysis. All variables exhibited positive correlations with each other. The loadings indicated that a greater distance from the origin corresponds to a more effective representation of the variable. The analysis of the relationships among various parameters in the study reveals a notable finding: the activation time and temperature demonstrate an angle deviation close to 90°, suggesting a potential orthogonal relationship between these variables. This orthogonality indicates that activation time and temperature may vary independently of one another, implying minimal correlation between them. On the other hand, the orientation of joules and hertz (Hz) in opposite directions suggests a negative correlation, meaning that as one variable increases, the other tends to decrease.

This behavior is graphically represented in Figure 5, illustrating the distinct orientations and relationships among these variables. In the Principal Component Analysis (PCA) study, it becomes evident that the size of the ureteroscope and the laser activation period exhibit relatively lower importance or variability compared to other variables analyzed. This conclusion is drawn from the shorter magnitudes of these vectors in the PCA plot, as shown in Figure 5. Shorter vector lengths indicate a lesser contribution to the overall structure or patterns observed in the dataset, meaning that these variables do not significantly impact the primary components that explain the variance within the data. The PCA results suggest that while ureteroscope size and laser activation period might be considered in the experimental setup, their influence is overshadowed by other factors such as energy output (joules) and frequency (Hz). These latter variables exhibit stronger correlations with the observed outcomes and contribute more significantly to the data’s variance.

4. Discussion

Our study developed an in vitro model to assess the thermal impact of the holmium laser and the causal effect of multiple variables in the context of ureteroscopic laser lithotripsy. All the variables exhibited an increasing trend in thermal effects as the laser activation time extended. Over time, the target material and surrounding tissues may absorb a substantial amount of energy, leading to a reduction in the efficiency of heat dissipation. Furthermore, laser lithotripsy involves the vaporization and ablation of the target material. Prolonged activation time generates energy and heat, which accumulates and contributes to the rise in temperature [11,15].

While the Ho:YAG laser has played a transformative role in urology, it does come with significant limitations. In this study, the larger diameter size of laser fiber (365 µm) exhibits higher thermal profiles, which can be due to several reasons. First, larger diameter fibers have a higher capacity to deliver laser energy. With a larger cross-sectional area, more energy can be transmitted through the fiber, leading to a greater concentration of energy at the target site. Secondly, larger diameter fibers may have less efficient heat dissipation compared to smaller ones. The increased volume of the larger fiber may make it more challenging to dissipate heat effectively, leading to higher temperatures in and around the fiber. Furthermore, employing a larger fiber in the ureteroscope’s working channel decreased the irrigation flow rate, consequently hindering the heat exchange process. Lastly, the spatial distribution of laser energy can vary with fiber diameter. Larger fibers may distribute energy over a larger area, affecting temperature gradients and overall temperature levels [16]. In this investigation, both laser diameter sizes demonstrated average temperatures below 37 °C, making them viable options for laser lithotripsy.

The lack of significance of the size of the URS (ureteroscopic) instrument in the thermal profile could be influenced by other more dominant factors such as laser power settings and laser fiber size. In such cases, the effect of URS size may be overshadowed by these more impactful variables. Additionally, it might be due to the tolerance range for different URS sizes in the thermal profile. In other words, the variations in URS size within the study range might not be substantial enough to cause a significant difference in thermal outcomes.

In laser lithotripsy, one of the key controllable parameters is laser power, measured in watts (W), which dictates the amount of energy delivered in each pulse and is influenced by both frequency and energy settings [13,17]. However, the independent effects of energy and frequency on temperature modulation during lithotripsy remain inadequately understood. The study model highlighted the importance of both joules and hertz, showing that higher energy levels result in significantly increased temperatures compared to lower energy settings [18], a finding consistent with previous research by Gallegos et al. [19]. This underscores the critical need for careful calibration of these variables to prevent excessive thermal buildup, which can lead to tissue damage. Moreover, the effectiveness of laser lithotripsy is determined by the proportion of laser energy that successfully reaches the kidney stone. An in vitro study demonstrated a significant decline in energy delivery efficiency at higher frequencies, with only 52% of emitted pulses reaching the stone at 20 Hz, dropping to 23% at 50 Hz, and a mere 4% at 80 Hz [20]. These findings suggest that high-frequency settings may not always translate to optimal stone fragmentation and highlight the importance of balancing power and frequency to improve treatment outcomes. Our study provides a clearer understanding of how each parameter influences clinical outcomes, helping establish a safety margin in practice. Liang et al. [11] showed through an in vitro model that different operating settings can cause variations in ureteric temperatures during ureteroscopic holmium lithotripsy, advocating for the use of low-power lithotripsy to minimize thermal injury. In support of this approach, Winship et al. [21] demonstrated that strategies such as controlled irrigation, limited activation time, and appropriate intervals between laser pulses effectively reduce the risk of thermal injury in high-energy settings. Together, these findings emphasize the necessity for a balanced and strategic approach to laser settings, ensuring patient safety while maximizing the efficacy of lithotripsy treatments.

The PTFE catheter, known for its corrosion resistance, heat resistance, and pressure resistance, provides a relatively safe option for experimental use. However, it does not accurately simulate a real human ureter, as it lacks muscle tone, which could affect outflow and irrigation rates. Additionally, the PTFE tube remains at room temperature due to the absence of blood perfusion. The use of different thermometers, each with varying sensitivity and precision, may also impact the results. Furthermore, human ureteral diameters vary, but in this experiment, a fixed-diameter tube was used to represent the ureter, which may not accurately reflect these variations. Other factors, such as irrigation rate, periureteral blood flow, stone impaction, and ureteral diameter, could influence intraurethral temperature during lithotripsy in living subjects.

5. Conclusions

In essence, the challenge of uncontrolled temperature increase during laser lithotripsy is complex. It demands meticulous consideration of various parameters, encompassing laser power settings, sizes of the laser fiber and temperature, and the laser activation time. Laser exposure has the potential to inflict irreversible thermal damage to the ureter. Therefore, careful consideration and caution are advised during the clinical application of laser lithotripsy techniques.