Prevention of intraoperative hypothermia is an important aspect of patient management in off-pump coronary artery bypass surgery (OPCAB). In addition to the core-to-peripheral redistribution of the body heat, convective losses from the large body surface area exposed and low cardiac operating room (OR) temperature are responsible for the development of hypothermia in cardiac patients. Potential adverse effects of intraoperative hypothermia include impaired coagulation profile [1–3], which leads to excessive bleeding and transfusion, and cardiac arrhythmia, which may deteriorate hemodynamic stability in patients with already depressed cardiac function such as coronary artery obstructive disease. Among the warming devices, skin surface warming with a forced-air warming device is a simple, safe, and efficient means of preventing hypothermia and its use is recommended during major surgical procedures [4,5]. However, the device was not frequently used during cardiac surgery because the body surface needed to be covered by the device was too little during surgery. And also, intraoperative skin surface warming cannot raise the core temperature drift occurring during the first hour of anesthesia called redistribution hypothermia. However, when skin surface warming is started before the anesthesia, sufficient body surface can be covered and redistribution hypothermia can be prevented. There was a study reporting that warming the patients before the anesthesia for 1 h avoided hypothermia during the first 2 h of surgery due to the prevention of core-to-peripheral redistribution in elective abdominal surgery [6]. This study was designed to evaluate the effect of preoperative warming using forced-air warming device on redistribution hypothermia during the first 90 min of anesthesia induction in patients undergoing OPCAB. Since delaying anesthesia induction for prewarming is impractical in most institutions, patients were prewarmed for the time required to prepare necessary monitoring before anesthesia induction. This study followed Institutional Review Board Guidelines and written informed consent was obtained from all patients included in this study. Forty patients undergoing OPCAB were randomized to either the prewarming group (n = 20) or the control group (n = 20) using sealed envelope system. Patients were excluded if they had clinically significant peripheral vascular disease, a history of fever within a week before surgery, any skin lesion, or history of hypersensitivity to skin contact devices. Patients having reduced left ventricular function (ejection fraction < 40%), single coronary artery disease, and history of neurologic disease were also excluded. A warming mattress with circulating water at 38 °C was applied to the patients in both groups. On arrival in the operating room, patients in control group were covered with two cotton blankets and patients in prewarming group were warmed with a Bair Hugger forced-air heater (model 200 blower, full-body cover, Augustine Medical, Eden Prairie, MN, USA) with the blower set at medium (≈40 °C). Patients were covered from the trunk to legs, but the arms were exposed for monitoring. Prewarming time was not decided to prevent delay in induction. Patients were prewarmed for the time required to attach monitoring equipment including arterial line and pulmonary artery catheter (Swan-Ganz, CCO combo, Baxter Healthcare Co., Irvine, CA, USA) for measurement of continuous cardiac output (CO) and core temperature of the patients. Skin temperature probe was attached to the right index finger of the patients. Forced-air warming was discontinued immediately after anesthetic induction and patients subsequently were fully exposed to the ambient environment. After induction, heat-and-moisture exchanging filters were used in all patients. Therefore, thermal management after induction of anesthesia always was the same regardless of the preinduction treatment during the study period. OR temperature was maintained around 20 °C. A standardized anesthetic including intravenous midazolam (0.03–0.05 mg/kg), sufentanil (1.0–3.0 μg/kg), and rocuronium (1 mg/kg) for induction was used in all patients. Anesthesia was maintained with 0.2–0.5% of isoflurane and continuous intravenous infusion of sufentanil at 0.5–1.5 μg/kg/min. Ventilation was maintained with an oxygen–air mixture (FiO2 0.6) to maintain end-tidal CO2 at 35–38 mmHg. Skin temperature measurement was started on arrival in the OR and all other temperature and hemodynamic variables were recorded just before the induction of anesthesia (Tpre = baseline), at 30 min, 60 min, and 90 min after the induction (T30, T60, and T90, respectively). Hemodynamic variables including heart rate, mean arterial pressure (MAP), central venous pressure (CVP), mean pulmonary artery pressure, and CO were recorded and cardiac index and systemic vascular resistance index were calculated according to standard formulae. Statistical analysis was performed with SPSS 11.5 (SPSS 11.5 for Windows. SPSS Inc., Chicago, IL, USA). Data are expressed as mean ± standard deviation or number of patients. Comparisons of continuous data within the groups such as temperature and hemodynamic variables were performed using repeated measures of ANOVA except the comparison between skin temperatures at arrival in the OR and at Tpre, which were compared using t-test. Student's t-test, Fisher's exact test or chi-square test were used to compare variables between the groups where appropriate. P-values < 0.05 were considered statistically significant. Demographic data are presented in Table 1. The two groups were comparable in patient characteristics. There were no differences between groups in preoperative OR temperature (20.0 ± 0.9 °C and 20.2 ± 0.9 °C) and skin temperature on arrival in the OR (28.2 ± 3.2 °C and 29.4 ± 2.2 °C in control and prewarming group, respectively). Active warming duration was 49.7 ± 9.9 min. No patient in prewarming group complained of thermal discomfort. Demographic data Temperature variables are listed in Table 2. The skin temperature, core temperature, and the difference between the two temperatures at Tpre were similar between the groups. The skin temperature of patients in prewarming group was significantly increased after prewarming period (P = 0.016). Although the decrease in core temperature within the group after induction was statistically significant in both groups, the core temperature was statistically higher in prewarming group when compared to that in control group at T30, T60, and T90. The difference between skin and core temperature decreased significantly at T90 in control group but not in prewarming group. Changes in skin and core temperature Core temperature of six (30%) and seven patients (35%) in control group was reduced below 35 °C at T60 and T90, respectively, whereas core temperature of only one patient (5%) in prewarming group was reduced at T90 (P = 0.02). Hemodynamic variables are listed in Table 3. There were no statistically significant differences in hemodynamic variables between the two groups at Tpre except for MAP and CVP. The changes in hemodynamic variables were sporadic within the group as well as between the groups. Hemodynamic changes In this study, active warming using forced air blanket before the induction of anesthesia reduced the incidence and degree of redistribution hypothermia in patients undergoing OPCAB. Although Artic Sun®, a new temperature management system, was reported to be effective in reducing intraoperative hypothermia in the absence of any other temperature modulating techniques during OPCAB, its cost prevents general use for now [7,8]. On the other hand, forced air blanket used in this study was simple, cost effective, and time saving since it did not delay the anesthesia induction. Hypothermia was defined when the core temperature fell below 35.0 °C in this study because intraoperative core hypothermia (0.5–1.5 °C) developing immediate after induction of anesthesia results largely from an internal core-to-peripheral redistribution of body heat due to anesthesia-induced vasodilation and reduced thermoregulatory vasoconstriction [9]. As with most non-cardiac settings, avoidance of intraoperative hypothermia in patients undergoing OPCAB has become standard care in many cardiac settings. A recent study supports this reporting that a strict thermoregulation attenuated myocardial injury during coronary artery bypass graft surgery [10]. Since large body surface area is exposed and considerable blood loss is also expected due to heparin use during OPCAB, maintaining body temperature can be a challenge. It is difficult to treat redistribution hypothermia both because the internal flow of heat is large and the heat applied to the skin surface requires considerable time to reach the core thermal compartment [11]. However, prewarming can prevent redistribution hypothermia. In a previous study, intraoperative hypothermia was minimized by 2 h of active skin surface warming before the induction of general anesthesia in volunteers using a forced-air warming device [12]. However, such prolonged prewarming cannot be applied in most hospitals because it is impractical and patients may complain thermal discomfort. In this study, it took 49.7 ± 9.9 min to attach monitors and to insert arterial line and pulmonary artery catheter for measurement of baseline hemodynamic variables. This duration of prewarming was not enough to completely prevent redistribution hypothermia but the increase in peripheral tissue heat content did lessen the temperature gradient between the two compartments. Since preinduction preparation takes long in cardiac surgery, actively prewarming the patient with forced-air warming device before the induction is effective to lessen the degree of redistribution hypothermia in patients undergoing OPCAB. Core temperature of the patients in this study was 36.7 °C at baseline in both groups and it decreased after the induction for 90 min in both groups but the extent of the decrease was significantly less in prewarming group compared to that in control group. Core temperature decreased below 35 °C from T60 to T90 in over 30% of patients in control group, whereas it did in only one patient in prewarming group at T90. Previous studies regarding skin surface warming to prevent hypothermia reported the duration to be at least 1 h [12–14]. Camus et al. [14] reported that in order to prevent decrease in core temperature below 36 °C, preinduction skin surface warming for at least 1 h was required to raise the skin temperature more than 2 °C. The skin temperature in prewarming group in this study was raised about 1.1 °C with 50 min of skin surface warming, which was not enough to completely prevent the decrease in core temperature. This may explain why there were no clinically significant and consistent hemodynamic changes associated with prewarming in this study. In addition, since surgery was well under way by the time temperatures were measured, it is difficult to draw any conclusions about the true significance of hemodynamic changes. The average rates of core temperature change during the first 1 h after the induction in other studies using prewarming and in this study were 0.6–1.1 °C/h and 0.9 °C/h, respectively [12–14]. Considering the cold environment and large body surface exposure during OPCAB, the increase in peripheral heat content must have been significant during prewarming period in this study. The fact that significant decrease in difference between the core and skin temperature occurred in control group but not in prewarming group in this study also supports the effectiveness of skin surface warming for less than an hour on prevention of redistribution hypothermia after induction of anesthesia. In this study, the core temperature was not measured at arrival in the OR because it was measured with pulmonary artery catheter. This would not have affected the result of the study since core temperature is maintained constant when the patients are awake regardless of cutaneous warming [12,15]. In a quantitative study of systemic heat balance and regional body heat distribution in volunteers, it was indicated that after 1 h of anesthesia, core temperature decreased by 1.6 °C, with redistribution contributing 81% to the decrease [16]. Since contribution of redistribution to intraoperative hypothermia is reduced during the subsequent 2 h of anesthesia, the temperature was measured for 90 min in this study. After 90 min, aggressive temperature management was applied to patients whose temperature fell below 35.0 °C using fluid warming devices and/or sterile forced air blanket on lower extremities after the venous harvest. This temperature management strategy decreased the temperature differences between the two groups after the study period. Further evaluation is needed to find out if the beneficial effect of prewarming extends throughout the surgery as well as its impact on the outcome. The body surface area, extreme age, incision size, and the presence of neuropathy are known risk factors for intraoperative hypothermia [17–19]. Since there was no difference in demographic data and patients with neuropathy are excluded in this study, these factors might not have influenced the result. In conclusion, active warming using forced air blanket during patient preparation before induction of anesthesia reduced the incidence and degree of redistribution hypothermia in patients undergoing OPCAB. It is a simple and effective method for temperature management with reasonable cost, which does not delay the induction of anesthesia and surgery.
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