MISCELLANEOUS Effect of Preoperative Forced-Air Warming on Hypothermia in Elderly Patients Undergoing Transurethral Resection of the Prostate Youn Yi Jo, Young Jin Chang, Yong Beom Kim, Sehwan Lee, Hyun Jeong Kwak* Purpose: Elderly patients under spinal anesthesia are vulnerable to hypothermia, leading to increased morbidity. The aim of this study was to investigate the effects of preoperative forced-air warming on perioperative hypo- thermia and shivering in elderly patients undergoing transurethral resection of the prostate (TURP) under spinal anesthesia. Materials and Methods: Patients (> 65-year-old) scheduled for TURP under spinal anesthesia were randomly assigned to receive preoperative forced-air skin warming for 20 min (the pre-warmed group, n = 25) or not (con- trol group, n = 25). Core temperatures were measured at 15-min intervals after spinal anesthesia, and intra- and post-operative shivering were also assessed. Results: Incidences of intraoperative hypothermia (< 36ºC) in the pre-warmed and control groups were not signif- icantly different (10/25 [40%] vs. 15/24 [62.5%], P = .259). However, severities of hypothermia were significantly different (P = .019). No patient in the pre-warmed group showed moderate or profound hypothermia, whereas of patients in control group 21% and 13% did so, respectively. Conclusion: This study demonstrated that a brief period of preoperative forced-air warming did not completely prevent intraoperative hypothermia or shivering, but it could significantly reduce its severity in elderly male pa- tients under spinal anesthesia. Keywords: anesthesia; spinal; body temperature; hypothermia; etiology; prevention & control; transurethral re- section of prostate; adverse effects. INTRODUCTION Spinal anesthesia significantly impairs thermoregula-tion by inhibiting vasomotor and shivering respons- es(1) and the redistribution of body heat,(2) and predis- poses patients to perioperative hypothermia. Because elderly patients have a reduced shivering threshold,(3) they are at greatest risk of hypothermia,(4) which can lead to serious clinical complications such as, myocar- dial ischemia,(5) blood loss,(6) and surgical wound in- fection.(7) Thus, it is important to monitor and control body temperature and prevent hypothermia in elderly patients under spinal anesthesia. In addition, shivering is a major problem for surgeons during transurethral resection of the prostate (TURP). Shivering may inter- fere with visual field for resectable prostate tissue and increase the risk of injury to the urethra, bladder, and rectum during the surgery. Hypothermia frequently occurs during TURP, because cold bladder irrigation fluid is an important source of heat loss and decreases core body temperature by 1-2°C. (8) In a previous study, isothermic irrigation fluid was found to significantly reduce body temperature more so than room temperature irrigation fluid.(9) However, it can difficult to warm a large volume of irrigation fluid. On the other hand, preoperative skin surface warming using a forced-air warmer for 2 h has been reported to reduce the temperature differential between the core and periphery and heat redistribution during epidural anesthesia in healthy young volunteers not undergoing surgery,(10) and in another study, pre-operative warming for 10 or 20 min reduced shivering and largely prevent- ed hypothermia during general anesthesia in healthy adult patients.(11) A recent meta-analysis also showed that prewarming patients with forced-air warmer could effectively reduce the peri-operative hypothermia.(12) We hypothesized that preoperative surface warming us- ing a forced-air warmer before spinal anesthesia might reduce perioperative hypothermia and shivering in el- derly patients. Thus, the aim of this prospective study was to investigate the effects of preoperative forced-air warming on perioperative hypothermia and shivering in elderly male patients undergoing TURP under spinal anesthesia. MATERIALS AND METHODS Study Population After obtaining approval by the institutional review board of Gachon University Gil Medical Center, writ- ten informed consent was obtained from all enrolled pa- tients. Fifty male patients aged over 65 years of Ameri- Department of Anesthesiology and Pain Medicine, Gachon University, Gil Medical Center, Incheon, South Korea. *Correspondence: Department of Anesthesiology and Pain Medicine, Gachon University, Gil Medical Center, 1198 Guwol-dong, Namdong-gu, Incheon 405-760, South Korea. Tel: +82 32 4603637. Fax: +82 32 4696319. E-mail: hyun615@gilhospital.com. Received March 2015 & Accepted September 2015 Miscellaneous 2366 can Society of Anesthesiologists physical status I-II and scheduled to undergo elective TURP were enrolled in this prospective randomized study. The exclusion crite- ria were a pre-anesthetic tympanic membrane tempera- ture of > 37.5ºC or < 36ºC, uncontrolled hypertension or diabetes mellitus, and a condition requiring fluid restriction, such as, end-stage renal disease, peripheral vascular disease, uncompensated heart failure, or pro- gressive respiratory disease. Patients were not pre-med- icated. Procedure On arrival at the pre-anesthetic care unit, standard mon- itors were applied and tympanic temperature was meas- ured using an infrared tympanic thermometer (Ther- moScan IRT 1020; Braun, Germany). All patients were placed under a forced-air cover and pre-warming was performed using a forced-air warmer (WarmTouch; Mallinckrodt Medical, St Louis, MO, USA). Patients were randomized to receive forced-air pre-warming at 38ºC for 20 min (the pre-warmed group, n = 25) or not (control group, n = 25) (Figure 1). In the pre-anesthetic care unit, tympanic temperature was measured imme- diately after arrival in the pre-anesthetic care unit and 10 and 20 mins later (Pre-T0, Pre-T10 and Pre-T20, re- spectively). All patients received 8-10 mL/kg/h of plas- ma solution for 20 min as pre-hydration, and ambient temperature in the pre-anesthetic care unit was main- tained at 21-23ºC. After a 20-min stay in the pre-anesthetic care unit, pa- tients were transferred to the operating room. Room temperature was set at 24-25ºC, and warming mattress containing circulating water at 36ºC was applied on the operating table. Spinal anesthesia was performed in the lateral decubitus position using 0.5% hyperbaric bupi- vacaine (10-12 mg intrathecally) by an anesthesiologist unaware of group identities. When systolic blood pres- sure fell to 80% below the baseline value or to lower than 90 mmHg, phenylephrine (50 μg) or ephedrine (5 mg) was given at 2 min intervals. Room temperature plasma solution was infused at a constant rate of 6 mL/ kg/h. All patients were covered with one layer of surgi- cal drapes over chest, thigh, and calves during TURP. Mean arterial pressure, heart rate, patient discomfort to temperature and the occurrence of shivering and of hypothermia were recorded immediately after arrival in the operation room. Hypothermia was defined as a core temperature of < 36ºC. Tympanic temperature was measured at 15-min intervals from spinal anesthesia in the operating room (T0) to 60 min (T60) and from arrival (Post-T0) to 60 min after arrival (Post-T60) in the post-anesthetic care unit. Irrigation fluid for TURP was not warmed. When tympanic temperature fell be- low 36.0ºC or a patient asked for warming, forced-air warming was supplied regardless of group identity. Statistical Analysis The sample size of 25 patients per each group was cal- culated using power analysis based on the findings of a previous study,(13) in which the overall incidence of hypothermia after neuroaxial anesthesia was found to be 77%. To detect a mean intergroup difference in the incidence of hypothermia, 23 subjects were required with type I error (an α error of 0.05) and type II error (a β error of 0.2), and to account for possible losses, we included 25 patients per group. Statistical Package for the Social Science (SPSS Inc, Chicago, Illinois, USA) version 17.0 was used for the analysis. Results are expressed as numbers of patients or as mean ± SD or median (interquartile range). The independent t-test or Table 1. Demographics and perioperative data of study subjects. Variables Pre-warmed (n = 25) Control (n = 24) P Value Age (years) 73 ± 6 72 ± 4 .461 Weight (kg) 63 ± 8 65 ± 6 .464 Height (cm) 165 ± 5 167 ± 4 .282 Sensory block level T8 (T6-T10) T8 (T6-T9) .185 Operation time (min) 63 ± 22 55 ± 24 .360 Total irrigation fluid (mL) 8000 (3000-13500) 8000 (3000-10000) 1.00 Total infused fluid (mL) 400 (375-425) 400 (300-500) .335 Data are expressed as mean ± SD or medians (interquartile ranges). Figure 1. Flow CONSORT diagram Preoperative Forced-Air Warming for Elderly Patients-Jo et al. Vol 12 No 05 September-October 2015 2367 the chi-square test was used, as appropriate, to compare variables between the groups, and repeated measures ANOVA was used to compare changes in core tem- perature between the pre-warmed and control groups. P values of less than .05 were considered statistically significant. RESULTS Twenty-five patients were initially enrolled in each group, but one patient in the control group was exclud- ed from the analysis, because the anesthetic technique was changed to general anesthesia (Figure 1). Patient characteristics and peri-operative data are presented in Table 1. No significant differences were observed be- tween the two groups in terms of sensory block level, volume of irrigation fluid, or total amount of intrave- nous fluid infused during TURP. The incidences of intraoperative hypothermia (< 36ºC) in the pre-warmed and control groups were not statisti- cally significant (10/25 [40%] vs. 15/24 [62.5%], P = .259). However, severities of hypothermia were signifi- cantly different (P = .019); no patient in the pre-warmed group showed moderate or profound hypothermia, whereas of patients in control group 21% and 13% did so, respectively. Frequencies of rescue warming dur- ing and after surgery, and the incidences of intra- and postoperative shivering were similar in the two groups (Table 2). Figure 2 illustrates observed changes in core temper- ature during the perioperative period. During pre- and postoperative periods, the changes in core temperature between the groups were not significantly different (both P values > .05). During the intraoperative period, a significant decrease in core temperature (P < .001) was observed in both groups, but these changes were not statistically significant (P = .763). Mean core tem- perature was significantly higher in the pre-warmed group at Pre-T20 and at T0 (immediately after spinal anesthesia). DISCUSSION In the present study, preoperative forced-air warming for 20 min did not completely prevent intraoperative hypothermia or shivering, but did significantly reduce Table 2. Incidences of hypothermia and shivering during the peri-operative period. Variables Pre-warmed (n = 25) Control (n = 24) P Value Intra-operative period (in OR) Normothermia ( ≥ 36.0oC) 15 (60) 10 (41) .259 Hypothermia (< 36.0oC) 10 (40) 14 (59) Severity of hypothermia .019 Mild (35.5-35.9oC ) 10 (40) 6 (25) Moderate (35.0-35.4oC ) 0 (0) 5 (21) Profound (34.5-34.9oC ) 0 (0) 3 (13) Shivering 5 (20) 8 (33) .345 Needing rescue warming 3 (12) 3 (13) .888 Post-operative period (in PACU) Hypothermia (< 36.0oC) on arrival 10 (40) 13 (54) .321 Shivering 2 (8) 1 (4) .485 Needing rescue warming 7 (28) 6 (25) .564 Time required to normothermia (min) 44 ± 23 52 ± 13 .347 Abbreviations: OR, operating room; PACU, post-anesthetic care unit. Data are expressed as mean ± SD or numbers of patients (%). Figure 2. Peri-operative changes in core temperature in patients who received forced-air warming for 20 min (pre-warmed group, ○) or not (control group, ●) during transurethral resection of the prostate. Error bars represent standard deviations. Pre-T0-20, from arrival to 20 min stay in the pre-anesthetic care unit; T0-60, immediately to 60 min after spinal anesthesia; post-T0-60, from arrival to 60 min stay in the post-anesthetic care unit. A significant decrease in core temperature was observed during the intraoperative period (P < .001). Core temperature changes were not statistically significantly different in two study groups (P = .763). *P < .05, control group vs. pre-warmed group. Preoperative Forced-Air Warming for Elderly Patients-Jo et al. Miscellaneous 2368 its severity in elderly male patients undergoing TURP under spinal anesthesia. Several authors have reported that preoperative warming using a forced-air warm- er, reduces the risk of core hypothermia and prevents post-anesthesia shivering after general anesthesia or an epidural block,(10,14) and suggested that effective skin surface warming helpfully increases body heat content and reduces the risk of redistribution hypothermia asso- ciated with anesthesia. A previous analysis of 19 stud- ies with total 1451 patients suggested that as a single strategy, preoperative forced air warming had signifi- cant benefits than other warming methods.(15) However, studies on pre-warming prior to spinal anes- thesia, which might prevent hypothermia or shivering during procedures requiring large volumes of cold ir- rigation are lacking. Although during neuroaxial block (spinal/epidural anesthesia), heat loss from superficial tissue to the environment is less than during general anesthesia, due to a smaller temperature gradient, the temperature difference between core and superficial tis- sues is greater during neuroaxial block, and thus, core temperatures could fall due to heat redistribution.(10) Accordingly, skin surface warming should theoretically reduce the core to superficial tissue temperature gradi- ent and possibly prevent redistribution hypothermia.(10) Kim and colleagues(16) demonstrated that skin surface warming during anesthetic preparation could signifi- cantly reduce the difference between core and skin tem- peratures in patients undergoing coronary artery bypass graft. In the present study, skin surface warming for 20 min significantly reduced the severity of hypothermia, but not its incidence. We believe this is probably due to age-related decreases in thermoregulatory functions, such as, vasoconstriction and shivering. In the elderly, reduced norepinephrine release and the down-regula- tion of α-adrenoreceptors impair vasomotor response to cold. Furthermore, loss of lean body mass due to aging reduces shivering, and thus, metabolic heat gen- eration.(17) In fact, with the exception of block level, an advanced age is the most significant predictor of core hypothermia during spinal anesthesia.(4) In the present study, we enrolled patients aged over 65 years old in view of the fact that the vasoconstrictive threshold is about 0.8oC lower in those aged 60-80 years than in those aged 30-50 years (35.0oC vs 35.8oC).(18) Because of these thermoregulatory changes in elderly patients, pre-operative skin surface warming for 20 min is prob- ably not sufficient to maintain a core body temperature of > 36oC during TURP in the present study. Furthermore, spinal anesthesia per se alters afferent thermal inputs and impairs thermoregulatory respons- es, and thresholds of shivering and vasoconstriction decrease by 0.5-0.9oC during spinal anesthesia.(1,19) In this study, we observed median maximal falls in core temperatures of 0.4oC and 0.5oC in the pre-warmed and control groups, respectively. Furthermore, presumably because spinal anesthesia might decrease thermoregu- latory thresholds further in the elderly, observed inci- dences of post-anesthesia shivering were relatively low (25% and 33% in the pre-warmed and control groups, respectively) as compared with those of intra-operative hypothermia (40% and 62.5% in the pre-warmed and control groups, respectively ). The restriction of the study population to elderly males is the main limitation of the present study. Thermal re- sponses to exogenous and endogenous heat losses dif- fer between the sexes, because body surface to body mass ratios, subcutaneous fat contents, and exercise capacities differ.(20) Furthermore, in a recent study, it was found that the incidence of postoperative shiver- ing was higher in elderly females than in elderly males. (21) Thus, our results cannot be generalized to elderly females, and further study is needed to elucidate the effect of pre-operative forced-air warming on hypother- mia or shivering in female patients. Another limitation in this study is that tympanic temperature alone might not guarantee the accurate core body temperature. Gil- bert and colleagues(22) demonstrated that additional use of digital oral thermometer to tympanic thermometer could provide valid patients’ temperature. However, another previous study has reported that oral or tym- panic temperature frequently over- or underestimate the rectal temperature.(23) Additionally, if we have measure the skin temperatures on limbs and trunk in addition to core temperature, we could estimate mean body tem- perature and calculate total heat body content, based on previously reported formula.(24) However, we did not measure the skin temperatures, because applying forced air warming device would interfere with the accurate measurement of skin temperatures. Meanwhile, we could calculate mean energy input in the pre-warmed group, since the heat capacity of human body is 0.812 kcal/kg oC,(25) which means that to change the body tem- perature of a 60 kg human by oC, 49 kcal is needed. In this study, mean patient weight was 63 kg, mean eleva- tion in core temperature in the pre-warmed group was 0.2°C, and mean energy input in the pre-warmed group was about 10 kcal for 20 mins. CONCLUSIONS In conclusion, in elderly male patients during TURP un- der spinal anesthesia, pre-operative forced-air warming for 20 min significantly reduced the severity but not the incidence of intraoperative hypothermia. We suggest that only short-time skin surface warming could reduce somewhat the severity of redistribution hypothermia in elderly male patients. CONFLICT OF INTEREST None declared. REFERENCES 1. Ozaki M, Kurz A, Sessler DI, et al. Thermoregulatory thresholds during epidural and spinal anesthesia. Anesthesiology. 1994;81:282-8. 2. Matsukawa T, Sessler DI, Christensen R, Ozaki M, Schroeder M. Heat flow and distribution during epidural anesthesia. Anesthesiology. 1995;83:961-7. 3. Vassilieff N, Rosencher N, Sessler DI, Conseiller C. Shivering threshold during spinal anesthesia is reduced in elderly patients. Anesthesiology. 1995;83:1162-6. 4. Frank SM, El-Rahmany HK, Cattaneo CG, Conseiller C. Predictors of hypothermia during spinal anesthesia. 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