In this randomized control trial, we investigated the effect of two different uses of underbody FAW blankets in preventing IPH in patients undergoing arthroscopic shoulder surgery. Our study showed that the use of a FAW blanket placed over the body was superior to the use of that placed under the body in preventing IPH. Although the CBT was significantly different between the two groups at all time points from 45 minutes of using FAW onward, the difference never exceeded 0.6 ℃, the null hypothesis could not be rejected. The difference in CBT was not clinically significant. Nevertheless, during the postoperative phase, 57.4% of the patients in the UB group suffered hypothermia but only 32.7% of the patients in the OB group suffered hypothermia.
Patients undergoing arthroscopic shoulder surgery are more prone to suffering IPH than those undergoing other orthopedic surgeries. This is due to the use of large quantities of irrigation solution to ensure adequate intra-articular visualization. The shoulder is located close to the thorax and great vessels, and the room-temperature irrigation solution passing through the operative region can remove core body heat, which may lead to a considerable decrease in CBT. Although previous studies showed that IPH in arthroscopic shoulder surgery could be prevented with the use of warm irrigation solution[20, 21], other similar studies found no difference between the use of room-temperature irrigation solution and warm irrigation solution[7, 22]. Warm irrigation solution facilitates vasodilatation and bleeding during arthroscopy, which leads to surgical field of view impairment and prolonged duration of surgery. In consideration of these factors, room-temperature irrigation solution has always been used for arthroscopic shoulder surgery in our hospital.
According to the NICE clinical guidelines, the patient's temperature should be measured at 30 min intervals from the start of anesthesia induction to the end of surgery. CBT measurements are considered to be more reliable than peripheral area temperature measurements because they are not influenced by ambient temperature or any other external factors. ARON recommends using one consistent site and method to monitor CBT throughout the perioperative phase when clinically feasible. However, it is difficult to monitor CBT continuously and accurately. The most accurate CBT measurement sites commonly used in the clinic (such as the oesophagus, pulmonary artery, and nasopharynx) are also the most invasive. Therefore, CBT measurement is usually performed after general anesthesia. The iThermonitor accurately estimates CBT by measuring axillary temperature[17, 18]. Moreover, its noninvasive and portable features allowed us to continuously and accurately monitor CBT during the study.
IPH should be prevented or treated by appropriate methods throughout the perioperative period. Among all warming methods, FAW has been considered to be the most effective[24, 7]. It protects patients from IPH not only through heat conduction, but also by preventing radiant and convective heat loss. Before the study, we tested the effectiveness of the underbody FAW unit used in this study. We included 20 patients and randomly divided them into a control group and a FAW group (10 per group). The patients in the control group did not receive any active body warming methods, and the FAW group used the FAW unit, with the FAW blanker placed under the patients. The results of our pilot trial showed that the body temperature of patients after using the FAW unit for 90 minutes was significantly higher: 35.9℃ ± 0.4℃ in the FAW group versus 35.4℃ ± 0.4℃ in the control group, which was consistent with previous literature reports. The FAW unit we used in this study has exhibited great potential in the prevention of IPH.
Both underbody and over-body FAW blankets can be used for heat maintenance during arthroscopic shoulder surgery. The underbody blanket can be placed on the operating table in advance and is suitable for nearly all types of surgery and patient position. Moreover, it exhibited adequate effects in preventing IPH[25, 26]. These advantages allow anesthesiologists and operating room nurses to focus on the patient and warming as soon as the patient arrives in the operating room, thus making it increasingly popular in clinical practice. In this study, we placed full access underbody FAW blankets in 2 different positions (under and over the patient's body) during arthroscopic shoulder surgery, and investigated the effects of the 2 different uses. Our results showed that FAW blankets placed over patients have more advantages in preventing IPH than those placed under patients. This is a contradiction to the findings in the previous studies of Gulia et al.  and Alparslan et al., as they found that an underbody FAW blanket can be as effective as both over-body and upper body blankets in preventing IPH in lower abdominal surgery. A possible reason is that the over-body and upper body blankets may not be able to cover the large body surface area involved the surgical site in lower abdominal surgery. However, in arthroscopic shoulder surgery, nearly two-thirds of the body surface area can be covered and heated with a full body blanket, thus placing the underbody blanket over patients may enhance its warming effects. In addition, when the blanket was placed under the body of the patients, the patient’s natural pressure points and the positioning devices used for stabilizing the lateral decubitus position may compress the underbody blanket, and prevent heat transfer.
It should be noted that this study has limitations. We only compared the warming effects of two different placements of full access underbody FAW blankets. Over-body and upper body FAW blankets were not used in our study, although the upper body blanket is preferred in clinics and has been validated to be more effective than lower body blankets in patients in the lateral decubitus position. However, the full access blanket can cover a much greater area than the upper body blanket in arthroscopic shoulder surgery, and its warming effects are better in theory. The difference in warming effects between placing over-body blankets and underbody blankets over patients in arthroscopic shoulder surgery will be investigated in a future study. Additionally, we did not warm the IV fluids, but we restricted fluid administration during the operation, which may have minimized bias. Finally, to standardize the warming protocol and reduce the impact of airflow caused by FAW on disinfection of the surgical area, we did not initiate FAW as soon as the patients arrived in the operating room. This may have weakened the warming effects of FAW.
In conclusion, CBT was significantly better with the use of underbody FAW blankets placed over patients than with them placed under patients. Although the differences in CBT never reached the level of predefined clinical significance (0.6℃), the incidence of postoperative hypothermia in the OB group was much lower than that in the UB group. Therefore, placing underbody FAW blankets over patients is recommended for the prevention of IPH in patients undergoing arthroscopic shoulder surgery.