(UroToday.com) Dr. William Roberts continued the morning plenary session with a presentation on laser lithotripsy and whether temperature matters. Dr. Roberts highlighted that with the introduction of newer laser technologies our abilities to ablate larger stones and greater volumes of tissue has increased but with this comes increased temperature and heat that is generated within the irrigation fluid. The heating of the fluid leads to secondary heating of surrounding tissues and structures.
Dr. Roberts outlines a porcine in-vivo study whereby ureteroscopy was performed on pigs. Thermocouples were placed around the ureteroscope, and an incision was made into the collecting system and temperatures were measured. 40 Watts of power were delivered for 1 minute at varying irrigation rates. With no flow temperature elevations were from 35-40 C, at medium flow 30 C and even at 40 mL/min considered to be high flow there were elevations up to 15 C. The kidneys were then bivalved which revealed classic thermal injury with discoloration of the collecting system, a whitish area of damage to the infundibula and reddish discoloration of surrounding papillae.
Dr. Roberts then presented a human study by Wang et. al from China in which thermocouples were placed in humans during ureteroscopy. They demonstrated that almost all patients experienced temperatures of 43 C and some patients even up to 56 C despite varying power settings. In this study powers of 20 Watts were used within the kidney.
In 2021, Olympus issued a recall with device corrections for laser settings for their new Thulium laser fiber after reports of ureteral injury following reports from FDA, Health Canada and other authorities. The question arises what temperature is too hot? The biologic effect of cell death and tissue injury depends on thermal dose. Thermal dose includes temperature and time at that temperature to determine the potential for cellular injury. Cell death occurs when tissue is maintained at 43 C for 120 minutes. This time drastically is reduced as the temperature increases with cell death occurring in 0.9 minutes when the temperature is 56 C.
The factors that affect thermal dose are the energy applied and heat capacity of the system. The power applied is energy per unit time. The time of pedal activation has a profound effect on temperatures that are achieved and overall thermal dose. Another factor is heat capacity which is how readily the system can absorb the heat before the thermal threshold is reached. This includes the irrigation rate and the irrigation temperature plus the volume of fluid.
Strategies that can be used to mitigate these effects are controlling the activation time of the pedal and when operating at above 20 Watts it is reasonable to consider 5 seconds as a good length of time not to exceed with each fire of the laser. Being aware of the volume in which the lithotripsy is occurring, plus increasing the rate of irrigation and not warming the fluid.
Dr. Roberts concludes with the question, does temperature matter? And yes, it does. An important concept is the concept of thermal dose. Thermal dose is the quantitative metric by which we can define and mitigate the thermal effects of laser lithotripsy.
Presented by: William W. Roberts, MD, University of Michigan
Written by: Sohrab Naushad Ali, MD, MSc, FRCSC, Assistant Clinical Professor, Department of Urology, University of California Irvine, @sohrabnaushad on Twitter during the 39th World Congress of Endo urology and Uro-Technology (WCET), Oct 1 - 4, 2022, San Diego, California.