Important findings of the present study were that the incidences of postoperative fever and shivering decreased significantly as the temperature of the irrigation fluid was increased from 17 °C to 37 °C. Postoperative WBC count, serum PCT level and incidence of suspected infection were comparable between groups. Although there was one case of flash pulmonary edema and one case of bleeding in 37 °C group, both complications resolved after therapy. Our novel findings suggest that isothermic irrigation during flexible ureteroscopic holmium laser lithotripsy can reduce the incidence of postoperative fever and shivering. However, further studies are required to establish the optimal temperature for the irrigation fluid.
There are many potential causes of fever after ureteroscopic holmium laser lithotripsy, including ureteral obstruction by stone fragments, urinary tract infection, intraoperative backflow and extravasation of urine due to prolonged high-pressure irrigation, intraoperative and postoperative bleeding, and postoperative backflow of urine in the bladder and ureter due to poor drainage via the catheter . Furthermore, hypothermia and shivering can occur due to the large amounts of intravenous and irrigation fluids administered during surgery and anesthesia , particularly given that a substantial amount of irrigation fluid is absorbed by the patient [26, 27]. The present study investigated the effects of varying the temperature of the irrigation fluid on core body temperature during surgery and the incidence of postoperative fever while minimizing the influence of possible confounding factors. For example, all procedures were performed by the same experienced surgical team, irrigation fluid pressure was standardized and maintained below 40 mmHg by suspending the liquid bag at the same height, and patients with preoperative urinary tract infection or postoperative ureteral stent displacement were excluded from the analysis.
In this study, core body temperature at the end of the procedure was significantly lower in 17℃ group than in the other two groups, indicating that the use of irrigation fluid at the lower temperature (17℃) contributed to the fall in body temperature during surgery. By contrast, core body temperature at the end of the operation did not differ significantly between 37℃ group and 27℃ group, indicating that additional heating of the irrigation fluid from 27℃ to 37℃ was not beneficial for maintaining body temperature during surgery. This may have been due to the energy generated by the holmium laser during the operation, which increased the local temperature and heated the irrigation fluid .
The main finding of this study was that the incidence of postoperative fever (core body temperature > 37.5℃ during the 48 hours after surgery) was significantly lower in 27℃ group than in 17℃ group and significantly lower in 37℃ group than in both the other groups. However, there were no significant differences between the three groups in the postoperative WBC count, serum PCT level or incidence of suspected infection (core body temperature > 38.5℃ and PCT > 0.5 µg/L) . The above results suggest that the differences in the incidences of fever between groups were not due to differences in the incidences of infection, further implying that the temperature of the irrigation fluid has little influence on the incidence of postoperative urinary tract infection. The use of hypothermic irrigation fluid can cause intraoperative hypothermia, and the risk of postoperative shivering is increased when core body temperature drops below 36℃ . A recent meta-analysis of 28 randomized controlled trials found that the fall in body temperature during surgical treatment of benign prostatic hypertrophy and the incidence of postoperative shivering were lower for warm irrigation fluid than for room-temperature irrigation fluid . Therefore, our findings are in good agreement with the results of the above meta-analysis  and with other studies evaluating the effects of irrigation fluid temperature on body temperature [32, 33]. Our observation that the incidence of postoperative fever increased as the irrigation fluid temperature was decreased (from 37℃ to 17℃) might be explained by a compensatory reaction of the body to hypothermia at the end of surgery, with the shivering mechanism possibly contributing to this.
The incidence of postoperative fever in this study was relatively high in comparison to that reported by previous studies, ranging from 13.5–38.9%. This may have been due to the wide definition of fever used in this study, since the patient was classified as having fever even if only one temperature reading was above 37.5℃ during the 48 hours after surgery. Ten patients (9.2%) had a postoperative body temperature higher than 38.5℃ after the operation: 5 of these patients improved after symptomatic treatment without the use of antibiotics, and the remaining 5 patients were diagnosed with urinary tract infection. The incidence of urinary tract infection in our study (4.6%) was higher than rates of 1–1.8% reported in the literature [10, 11], and we speculate that there may be three reasons for this. First, our study had a small sample size. Second, stone burden was larger in our study than in the previous studies, which may have resulted in more bacteria being released from the stone during the process of laser lithotripsy. Third, our study included more cases with renal calculi, whereas the prior studies mainly included patients with ureteral calculi.
There was one case of flash pulmonary edema in 37℃ group. It has been reported that elevating the temperature of the irrigation fluid increases the amount that is absorbed by the body . Therefore, for the same irrigation pressure and duration of surgery, it would be expected that patients in 37℃ group would have absorbed a greater volume of fluid than patients in the other groups. As a result, high-temperature irrigation fluid is more likely to cause volume overload and lead to pulmonary edema than low-temperature irrigation fluid, especially in elderly patients. However, the risk of irrigation fluid absorption during flexible ureteroscopy is lower than that during TURP or PCNL because flexible ureteroscopy causes far less tissue injury than TURP and uses a smaller perfusion pressure than PCNL . However, there remains a risk of increased fluid absorption for warmed irrigation fluid, which needs further study. Therefore, decisions as to whether or not to warm the irrigation fluid and the degree of warming necessitate full consideration of factors such as the predicted duration of surgery and cardiovascular risk of the patient, among others.
In addition, there was one case of postoperative bleeding in 37℃ group, but the bleeding stopped after irrigation for 24 hours using a three-cavity catheter. By contrast, there were no cases of bleeding in the other two groups, raising the possibility that warming the irrigation fluid might increase the risk of bleeding. Cao et al. reported that the amount of blood lost during TURP did not differ significantly between warmed and room-temperature irrigation fluid . However, Kati et al. found that the amount of blood loss during PCNL was significantly lower for patients irrigated with fluid at room temperature . It will be necessary to further study the effects of irrigation fluid temperature on bleeding risk. Although the risk of bleeding is very low for most patients undergoing flexible ureteroscopy, it is higher for patients with coagulation dysfunction or on long-term oral anticoagulants.
One limitation of this study is that there was no calculation of the amount of irrigation fluid absorbed, although it should be noted that the methods used for this are not always very accurate . In addition, patient comfort was not evaluated in our study.