The prevalence and incidence of kidney stones has increased by more than threefold over the last 50 years. Each year in the United States, more than 500,000 patients undergo treatment of kidney and ureteral stones with ureteroscopic laser lithotripsy.¹ Recent technological advancements, specifically the development of high-power lasers, have enhanced the efficiency of laser lithotripsy and broadened applicability to larger and more complex stones. A recent survey of endourologists revealed that 67% regularly use high-power laser lithotripsy techniques.²

However, laboratory research has shown that delivery of high-power laser energy can substantially elevate fluid temperature in the collecting system and injure adjacent renal parenchymal tissues.³ Unfortunately, ureteral stricture and renal failure have now also been reported in patients following ureteral stone treatment with a recently introduced high-power thulium laser system. Investigation revealed that “using laser power settings exceeding the cooling power of irrigation fluid” likely led to ureteral thermal injuries. This prompted a notice of urgent medical device correction for this laser system⁴ but remains a risk for all laser lithotripsy systems in clinical use. To properly manage this risk, urologists must understand the concept of “thermal dose” and have familiarity with factors that influence laser heating of fluid within the ureter and kidney.

Thermal injury to biological tissues is dependent upon both temperature elevation and exposure time. Dewey and Sapareto developed the metric of thermal dose, reported in “equivalent minutes at 43 °C” (designated as t₄₃), to standardize cumulative temperature exposure for varying temperature curves.⁵ The threshold of thermal injury (commonly defined as 120 equivalent minutes) is reached when tissue is exposed to 43 °C for 120 minutes.^5,6 Greater temperatures have a disproportionate effect on tissue toxicity, so the same thermal dose is achieved with temperature at 44 °C for 60 minutes, 45 °C for 30 minutes, 50 °C for 1 minute, or 56 °C for 1 second. Summation of the equivalent minutes for each portion of a temperature curve provides the overall thermal dose.

Bench studies as well as mathematical simulations have taught us that the fluid in the collecting system can indeed get hot and t₄₃ values can far exceed the 120 equivalent minutes threshold, as depicted in the Figure, which shows laser heating of fluid in the collecting system in response to laser activation in a porcine model. The volume of fluid in which the laser is activated plays an important role—the smaller the volume, the faster and higher the temperature rises. For example, laser activation in a small calyx will induce a greater elevation in fluid temperature than laser activation in the renal pelvis. The irrigation flow rate is also an important factor, with irrigation of more than 40 mL/min generally being protective against laser power settings up to 40 W. However, raising the irrigation rate can increase the intrarenal pressure, especially when a ureteral access sheath is not used. Chilled irrigation, proper control of operator duty cycle (the ratio of on time to lithotripsy time), and the length of pedal activation can also help control temperature elevation and thermal dose.

The risk of thermal injury in the ureter requires additional emphasis. While thermal injury to the kidney may manifest as a focal lesion and create a small or imperceptible reduction in overall renal function, a small injury to the ureter may lead to a stricture that puts the entire kidney at risk. Furthermore, the heat capacity of the ureter is much less than that of the kidney, due to substantially less vascular perfusion than the kidney and limited luminal fluid volume. Despite the commonly held belief that irrigation in the ureter leads to fluid flow up into the kidney, a bench study using a model ureter demonstrated that the volume of total fluid mixing within the ureter is small, less than 1.26 cm³ even with irrigation at 40 mL/min.⁷ These factors indicate that the ureter is more susceptible to thermal injury than the kidney and therefore greater attention needs to be placed on selection of laser and irrigation parameters when treating stones within the ureter. Additionally, the small size of the ureter compared to the renal collecting system increases the risk of direct iatrogenic laser injury to the ureter, which in turn can exacerbate thermal tissue effects.

Further research is needed to understand the patterns of thermal spread into papillary/cortical renal tissue and into the ureteral wall to better predict the extent and severity of thermal tissue injury. Technological development also has a role to play. Inclusion of a thermocouple on the tip of the ureteroscope can provide a measure of luminal temperature and allow calculation of real-time cumulative thermal dose. This process of quantification of relevant parameters and real-time monitoring will facilitate a shift from rote ureteroscopic strategy to feedback driven individualized strategies that account for anatomical variability and optimize laser lithotripsy for each individual patient.