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Comparison of warming methods for core temperature preservation during total knee arthroplasty using a pneumatic tourniquet

Background: It is important to ensure that patients are normothermic during surgery. In total knee arthroplasty, the pneumatic tourniquet affects body temperature. We compared the ability of two warming devices to preserve core temperature in patients using a lower limb tourniquet under general anesthesia. Methods: We included 132 patients with American Society of Anesthesiologists physical status I – II who were scheduled to undergo total knee arthroplasty. The patients were randomly divided into four groups (n = 33): group 1, without any heating method; group 2, with fluid warming; group 3, with forced-air warming; and group 4, with a combination of the two heating methods. After the induction of anesthesia, the esophageal and urinary bladder temperatures were monitored and recorded every 5 min before tourniquet deflation and every 1 min after tourniquet deflation. Results: Before tourniquet deflation, compared with group 1, the odds ratios of groups 3 and 4 were less than 1. After tourniquet deflation, compared with group 1, the odds ratios of all groups using warming devices were less than 1. In particular, group 4 showed the largest hypothermia-preventive effect among the four groups. There was a significant correlation between esophageal temperature and bladder temperature before and after tourniquet deflation. Conclusions: After tourniquet deflation, a combination of a fluid warmer and forced-air warmer is the most effective method to prevent hypothermia, although either a fluid warmer or forced-air warmer alone could help to prevent hypothermia. Urinary bladder temperature changes correlate well with esophageal temperature changes throughout this operation. (Anesth Pain Med 2016; 11: 91-98)


INTRODUCTION
The pneumatic tourniquet was introduced in 1904 by Harvey Cushing to decrease bleeding and ensure the cleanliness of the operative field in limb surgery. It is still commonly used.
However, this procedure may lead to intraoperative hemodynamic changes and may induce metabolic changes in the limb where the pneumatic tourniquet is applied, such as decreases in pH and PO2 and increases in PCO2, K + , and lactate [1]. Moreover, inflation and deflation of the pneumatic tourniquet, in conjunction with anesthesia, causes changes in the core temperature [2][3][4]. When the tourniquet is inflated, the blood pressure and heart rate increase, leading to an increase in body metabolism and heat production. However, because the heat cannot be redistributed to the distal part of the limb beyond the tourniquet, the core temperature increases [1,5]. Tourniquet inflation also interrupts perfusion of the distal part of the limb, resulting in an ischemic state. Consequently, energy sources for cellular metabolism become deficient and the temperature of the distal part of the limb decreases through exposure to the temperature of the room. After the tourniquet is deflated, warmer blood from the body core starts to perfuse to the distal part beyond the tourniquet, and cooler blood from the distal part of the limb flows into the systemic circulation, causing the core temperature to decrease [4][5][6]. Mild perioperative hypothermia can even lead to more serious complications, such as wound infection, prolongation of postoperative care unit and hospital stays, severe cardiac complications and ventricular tachycardia, coagulopathy, shivering, and reduced immune function. Therefore, continuous monitoring of the core body temperature and careful management of the patient are required [7][8][9][10][11].
To prevent hypothermia, many anesthesiologists use tools such as forced-air warmers, fluid warmers, and respiratory gas humidifiers. Of these devices, forced-air warmers and fluid warmers are considered the most effective [12][13][14]. The aim of our present study was to compare the hypothermia-preventive effects of forced-air and fluid warmers in patients undergoing unilateral total knee replacement arthroplasty under general anesthesia in whom a rapid decrease in core body temperature is expected after tourniquet deflation. In addition, the effect of the combination of the two devices was evaluated. The relationship between esophageal temperature and urinary bladder temperature during tourniquet deflation was also analyzed.  During the operation, lactated Ringer's solution was infused by considering the compensatory intravascular volume expansion caused by anesthesia, deficits, losses, and maintenance according to the "4-2-1 rule" [15]. Heart rate, blood pressure, electrocardiography, oxygen saturation, and the core body temperature were continuously monitored. When the systolic blood pressure decreased below 20% of the initial value or dropped below 80 mmHg, 10 mg of ephedrine or 100 g of phenylephrine was injected, depending on the heart rate value.

MATERIALS AND METHODS
The ambient temperature of the OR was measured at a Blood pressure, heart rate, oxygen saturation, and end-tidal

RESULTS
A total of 132 patients (122 women, 10 men) were enrolled in this study. These individuals were scheduled to receive a unilateral total knee arthroplasty with a tourniquet under general anesthesia. Thirty-three patients were allocated to each of the four groups. The general characteristics of each group are listed in Table 1 Table 1 except for sex for the "before tourniquet deflation" period showed two differences from the univariate analysis. First, hypothermia prevention was not significant in group 2. Second, hypothermia prevention was significantly better in group 4    more effective in maintaining normothermia than group 3. The results from the multivariate analysis adjusted for "before tourniquet deflation" information and the variables in Table 1except for sex, BMI, tourniquet time, and total urine outputwere similar to those of the univariate analysis ( Table 3).
The Pearson's correlation coefficient was used to analyze the potential correlation between bladder and esophageal temperatures.
During tourniquet inflation, this coefficient displayed a highly statistically significant association (r = 0.791, Fig. 2). During tourniquet deflation, the coefficient was also considered to indicate a highly statistically significant association (r = 0.909, Fig. 3).

DISCUSSION
In our current study, a body core temperature below 36 o C was defined as hypothermia. In order to compare the ability of two warming devices to prevent hypothermia during total knee arthroplasty under general anesthesia, we compared the ratio of hypothermia based on the esophageal temperature among the control, fluid-warmed, and forced-air-warmed groups, and a combination of the two warming device groups.
Sessler [5] described a 1 o C-1.5 o C drop occurring in body temperature within 1 h after induction of general anesthesia to Note: Group 1, control group without any warming method; group 2, fluid-warmed group; group 3, forced-air-warmed group; group 4, warmed using both warming methods. *Adjusted for the "before tourniquet deflation" period information and the variables in Table 1 except for sex, BMI, tourniquet time, and total urine output.

Fig. 2.
Pearson's correlation between esophageal temperature and bladder temperature before tourniquet deflation. The Pearson's correlation coefficient was calculated as 0.791. The previous study by Kim et al. [16] showed that forced-air warmers can effectively maintain a normothermic core body temperature during unilateral total knee arthroplasty under general anesthesia. After deflation of the tourniquet, the forced-air warmer group showed a lower body temperature reduction than the group not using intraoperative warming devices. In our present study, we further compared fluid warming with forced-air warming and tested the potential advantage of using a combination of warming devices. Our findings were consistent with those of the previous study of Kim and colleagues because we also found that forced-air warming could effectively maintain normothermia after tourniquet deflation. In addition, we found that a combination of fluid warming and forced-air warming provided further benefit and that fluid warming alone was sufficient for preserving body temperature after tourniquet deflation.
A remarkable outcome of our present analyses was that the results depended on the tourniquet inflation phase (before vs. after). Before pneumatic tourniquet deflation, forced-air warming and a combination of forced-air and fluid warming effectively prevented hypothermia and forced-air warming alone was more effective than the combination. On the other hand, after tourniquet deflation, all warming devices effectively maintained patients in the normothermic range and their combination was the best method for preventing hypothermia.
These results may be explained by differences in the flow rate of fluid. Horowitz et al. [17] reported that the "Ranger", which is a type of dry heat blood/fluid warmer similar to the We also investigated the ability of a bladder thermometer to measure core body temperature during total knee arthroplasty.
All organs produce heat, but at different rates. Generally, the venous blood temperature reflects the mean organ temperature.
Therefore, pulmonary artery catheters are the most accurate way to measure core temperature [17]. However, this approach is invasive, so the core temperature is usually measured at the distal esophagus, tympanic membrane, or larynx [18]. An esophageal thermometer is economical and accurate but difficult to use with monitored anesthesia care (MAC) or after surgery. In contrast, a bladder thermometer can be used with MAC or after surgery, but the accuracy varies according to urine output [14]. The results obtained using a bladder thermometer are different from those of an esophageal thermometer, but there was a strong correlation between the two locations at all intervals measured. Cork et al. [19] suggest that bladder temperature is as accurate a source for core temperature measurement as the esophagus, rectum, larynx, or the tympanic membrane. Lefrant et al. [20] have shown that the temperature of the pulmonary artery is consistent with the temperature measured in the bladder and esophagus, but not at the rectum, axilla, or inguinal area. In our current study, the esophageal temperature was significantly correlated with the bladder temperature. Hence, a bladder thermometer can accurately and efficiently measure core body temperature. However, Fallis [21] suggested that the bladder temperature does not reflect rapid changes in the core temperature. Additionally, Sato et al. [22] [14,26]. We thus ensured that the OR temperature was relatively low and, although the body temperature of the experimental groups were higher than those of the control group before tourniquet deflation, the body temperature of the control group was also maintained relatively well near the baseline until tourniquet deflation.
Second, there were a few significant differences in the general characteristics of the patients among the groups. The BMI of group 4 was lower than that of the other three groups. Fernandes et al. [27] reported that the average core body temperature is maintained during the perioperative period at a higher level with air warmers in obese patients than in normal patients. Not only does thermal conduction occur more slowly in obese patients, but also the heat is produced to a greater degree by sympathetic nervous system activation through leptin secreted from fat cells [28,29]. However, as the body temperature of the combined warmed group was better maintained than that of other groups, this may have had a minor effect on the study results. The other general characteristic that was significantly different among the groups was the total amount of crystalloid infused throughout the anesthesia. The amount of crystalloid infused of group 2 was significantly lower than that of other groups. The total amount of crystalloid infused may be correlated with flow rate, so it may affect the results, as mentioned above.
Third, the use of phenylephrine may have affected the results. Ikeda et al. [30] studied how phenylephrine infusion affects the core body temperature under general anesthesia. In that study, phenylephrine infusion reduced the magnitude of redistribution hypothermia. However, no patient needed phenylephrine injection to maintain a stable blood pressure in our current study series. In addition, it is considered that a single bolus injection of phenylephrine has no important effect on the redistribution of heat.
Fourth, no analysis of postoperative complications was performed in our present study. The major rationale for preventing perioperative hypothermia is to reduce complications and to improve outcomes [7][8][9][10][11]. Further analysis of the frequency of complications, including shivering, may have shown the actual benefit of the warming devices in clinical outcomes.
In conclusion, the use of fluid warmer or forced-air warmer devices effectively preserves the core body temperature after tourniquet deflation. The combination of the two warming devices leads to better normothermia preservation than use of a fluid warmer or forced-air warmer alone. Urinary bladder temperature changes strongly correlate with esophageal temperature changes throughout the operation, indicating that this noninvasive monitoring site may be useful.