Laser operator duty cycle effect on temperature and thermal dose: in-vitro study

Purpose High-power laser lithotripsy can elevate temperature within the urinary collecting system and increase risk of thermal injury. Temperature elevation is dependent on power settings and operator duty cycle (ODC)—the percentage of time the laser pedal is depressed. The objective of this study was to quantify temperature and thermal dose resulting from laser activation at different ODC in an in-vitro model. Methods Holmium laser energy (1800 J) was delivered at 30 W (0.5 J × 60 Hz) to a fluid filled glass bulb. Room temperature irrigation was applied at 8 ml/min. ODC was evaluated in 10% increments from 50–100%. Bulb fluid temperature was recorded and thermal dose calculated. Time to reach threshold of thermal injury and maximal allowable energy were also determined at each ODC. Results Upon laser activation, there was an immediate rise in fluid temperature with a “saw-tooth” oscillation superimposed on the curves for 50–90% ODC corresponding to periodic activation of the laser. Higher ODC resulted in greater maximum temperature and thermal dose, with ODC ≥ 70% exceeding threshold. Use of 50% compared to 60% ODC resulted in a tenfold increase in time required to reach threshold of thermal injury and an eightfold increase in maximal allowable energy. Conclusions Laser activation at higher ODC produced greater fluid temperature and thermal dose. Time to threshold of thermal injury and maximal allowable energy were dramatically higher for 50% compared to 60% ODC at high-power settings. Proper management of laser ODC can enhance patient safety and optimize stone treatment.