Thermal Effect of Holmium Laser Lithotripsy Under Ureteroscopy
Ureteral calculus is a common urological condition that has seen significant advancements in treatment methods over the years. One of the primary techniques employed in recent times is ureteroscopy combined with holmium laser lithotripsy. This method has gained popularity due to its effectiveness in fragmenting and removing stones. However, with its widespread use, there has been an increasing incidence of postoperative complications such as ureteral stricture or even occlusion. This has led clinicians to investigate the underlying causes, with the thermal effects of the holmium laser during the procedure being a focal point of concern.
The holmium laser operates as a long-wavelength pulsed laser, utilizing both optomechanical/photoacoustic and photothermal mechanisms to crush calculi, with the latter being the dominant mechanism. While in vitro studies have confirmed that the holmium laser can increase the temperature of the water in the working area, these studies often fail to replicate the exact conditions of an actual surgical procedure. Therefore, this study aimed to monitor the temperature changes of the lavage fluid in the operative field during real-time holmium laser lithotripsy under ureteroscopy, providing valuable insights into the etiology of postoperative ureteral stricture.
The study was conducted in accordance with the Declaration of Helsinki and received ethical approval from the Ethics Committee of Third Xiangya Hospital of Central South University. Patients suffering from ureteral calculi and scheduled for holmium laser lithotripsy under standard ureteroscopy were selected for the study. The equipment used included an ureteroscope, a holmium laser machine, an endoscope perfusion pump, a thermometer, a thermocouple temperature measurement wire, and a laptop computer for data recording.
The procedure began with combined spinal and epidural anesthesia, followed by the patient being placed in the lithotomy position. After routine disinfection and draping, an ureteroscope was inserted into the ipsilateral ureter under the guidance of a safe guidewire, reaching the location of the calculi. A temperature measurement guidewire was inserted into the ureteroscope, with its tip extending slightly beyond the end of the ureteroscope. The temperature was measured at a frequency of once per second, and the data were transmitted to a computer in real time. The calculi were then cleared using a 550-mm optical fiber inserted through another operation channel of the ureteroscope, with the power settings determined by the surgeon based on the surgical situation. After the calculi were crushed, a double J tube was indwelled for catheterization.
The study included 27 patients with ureterolithiasis, resulting in 30 sets of data. Among these, one patient had bilateral ureteral calculi, one had a giant calculus in the lower ureter, and one had multiple ureterolithiasis, contributing two datasets each. The patient cohort consisted of 16 males and 14 females, with 17 lesions on the left side and 13 on the right side. The mean age of the patients was 47.4 years, and the mean calculi diameter was 14.16 mm. There were 14 cases of incarcerated calculi and 16 cases of nonincarcerated calculi.
The initial temperature of the lavage solution was recorded at 25.41°C, which was the temperature of the operating room. The results showed that the highest temperatures in all 30 sets of data exceeded 43°C, with 19 sets (63.3%) reaching peak temperatures higher than 56°C. The temperature generally increased with higher laser power settings. Notably, the mean intraoperative temperature of the lavage solution in the incarcerated calculi group was significantly higher than that in the nonincarcerated calculi group, with a statistically significant difference.
During the follow-up period, one patient was lost to follow-up, and seven patients reported symptoms of low back pain. Among these, four patients had intraoperative lavage solution temperatures exceeding the 56°C threshold. Three of these patients were diagnosed with hydronephrosis via color Doppler ultrasonography, suggesting possible ureteral obstruction. The intraoperative perfusion temperatures in these three patients all exceeded 56°C, indicating a potential link between high temperatures and postoperative complications.
The study highlighted that the holmium laser’s thermal effect could lead to a significant increase in the temperature of the lavage solution during ureteroscopy. This temperature rise was more pronounced with higher laser power settings and in cases of incarcerated calculi. The findings suggest that the thermal effect of the holmium laser could be a contributing factor to ureteral thermal injury and subsequent stricture formation.
The study also discussed the mechanisms behind the thermal effects of the holmium laser. The laser emits high-energy pulse waves that can cause an increase in ambient temperature during operation. In vitro experiments have shown that the holmium laser can generate extremely high temperatures, even exceeding 1400°C in some cases. However, during actual surgery, the wavelength of the holmium laser is very close to the absorption peak of water, meaning that most of the laser energy is absorbed by the lavage solution. The operative field space between the end of the ureteroscope and the ureteral calculus is very small, often less than 1 mL in volume. In such a confined space, repetitive excitation of the holmium laser can cause the water temperature to increase sharply, potentially leading to thermal damage to the ureteral wall.
Previous in vitro studies have confirmed that the holmium laser can increase the temperature of the lavage solution during excitation. However, these studies often did not account for the continuous perfusion of physiological saline during actual surgery, which can remove some of the heat and reduce the temperature increase. Studies have shown that sufficient drainage can maintain a relatively stable temperature, even at higher laser power settings. Conversely, lower perfusion speeds can lead to significant temperature increases, even at lower power settings.
The study also referenced the “time–temperature relationship” for tissue thermal damage, which states that thermal damage occurs when biological tissue is exposed to temperatures above 43°C for a prolonged period. For every 1°C increase above this threshold, the time required to cause damage decreases by half. At 56°C, tissue damage can occur in just one second. This relationship was used as a criterion for judging whether there was ureteral thermal injury during the study.
The findings of this study have important implications for clinical practice. The thermal effect of the holmium laser during ureteroscopy can lead to significant increases in the temperature of the lavage solution, particularly with higher power settings and in cases of incarcerated calculi. This temperature increase can exceed the threshold for tissue damage, potentially leading to postoperative ureteral stricture. Therefore, clinicians should be aware of the thermal effects of the holmium laser and take steps to minimize the risk of thermal injury during surgery.
In conclusion, this study provides valuable insights into the thermal effects of holmium laser lithotripsy during ureteroscopy. The findings highlight the importance of monitoring and managing the temperature of the lavage solution during surgery to minimize the risk of ureteral thermal injury and subsequent complications. Further research, including animal experiments and pathological studies, is needed to confirm the extent of ureteral wall damage caused by the thermal effect of the holmium laser and to develop strategies for mitigating these risks in clinical practice.
doi.org/10.1097/CM9.0000000000000300
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