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CARDIOVASCULAR ANESTHESIA

A Randomized Controlled Trial of the Arctic Sun® Temperature Management System Versus Conventional Methods for Preventing Hypothermia During Off-Pump Cardiac Surgery

From the Department of Anesthesiology and Surgery, Duke University Medical Center, Durham, North Carolina

Address correspondence and reprint requests to Hilary P. Grocott, MD, Associate Professor of Anesthesiology, Department of Anesthesiology, Duke University Medical Center, Box 3094, Durham, NC 27710. Address email to h.grocott{at}duke.edu

	Abstract

Top Abstract Introduction Methods Results Discussion Appendix References

 
In this trial we compared the hypothermia avoidance abilities of the Arctic Sun® Temperature Management System (a servo-regulated system that circulates temperature-controlled water through unique energy transfer pads adherent to the patient’s body) with conventional temperature control methods. Patients undergoing off-pump coronary artery bypass (OPCAB) surgery were randomized to either the Arctic Sun System alone (AS group) or conventional methods (control group; increased room temperature, heated IV fluids, convective forced air warming system) for the prevention of hypothermia (defined by a temperature <36°C). The AS group had nasopharyngeal temperature servo-regulated to a target of 36.8°C. Temperature was recorded throughout the operative period and comparisons were made between groups for both the time and area under the curve (AUC) for a temperature <36°C (AUC<36°C). Twenty-nine patients (AS group = 14, control group = 15) were studied. The AS group had significantly less hypothermia than the control group, both for duration of time <36°C (2.5 [0–22] min, median [interquartile range] AS group versus 118 [49–192] min, control group; P = 0.0008) as well as for AUC<36°C (0.3 [0–2.2] °C x min, AS group versus 17.1 [3.6–173.4] °C x min, control group; P = 0.002). The Arctic Sun Temperature Management System significantly reduced intraoperative hypothermia during OPCAB surgery. Importantly, this was achieved in the absence of any other temperature modulating techniques, including the use of IV fluid warming or increases in the ambient operating room temperature.

IMPLICATIONS: The Arctic Sun® Temperature Management System was more effective than conventional methods in preventing hypothermia during off-pump coronary artery bypass graft surgery.

	Introduction

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Just as efforts are made to prevent hypothermia in most noncardiac surgery settings (1), its avoidance during off-pump coronary artery bypass (OPCAB) surgery has been adopted as the standard of care in many cardiac centers. Although the optimal temperature management strategy during OPCAB surgery has not been clearly established, there are several potential advantages to avoiding hypothermia in the perioperative setting (2). These advantages include earlier time to extubation (3), improved coagulation profiles with decreased blood loss (4–7), and decreased infection rates attributable to the avoidance of hypothermia-induced impairment in immune function (8–11). Arguably, there may be some advantages to transient hypothermia (such as neuroprotection during times of significant, and often frequent, hemodynamic compromise) (12), but the inability to quickly return to normothermia prevents this strategy from being used to its full advantage.

There are several strategies that can be used to reduce intraoperative hypothermia. In addition to increasing the ambient operating room (OR) temperature, the use of convective forced air warmers is commonplace. These are frequently used in combination with aggressive warming of IV fluids. The applicability of convective warming during OPCAB surgery, however, is limited because of the inability to adequately expose sufficient body surface to the warmer to achieve efficacy. As a result, hypothermia is a common occurrence though relatively understudied in the OPCAB setting; improved strategies are needed to decrease its incidence.

The purpose of this trial was to compare the hypothermia-avoidance abilities of the Arctic Sun® Temperature Management System (13), a servo-regulated system that circulates temperature-controlled water through unique energy transfer pads adherent to the patient’s body, with conventional temperature control methods.

	Methods

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After IRB approval and written informed consent, patients 21 yr of age or older, ASA physical status III-IV, undergoing OPCAB surgery were randomized to either the Arctic Sun system (AS group) or conventional methods (control group) for the prevention of intraoperative hypothermia. Patients were excluded if they had any skin diseases, current irritation, lacerations or previous burns of the skin of the back and legs, or any known hypersensitivity to skin contact devices.

A standardized anesthetic including midazolam (0.02–0.1 mg/kg IV), fentanyl (5–10 µg/kg IV), and pancuronium (0.01 mg/kg IV) with isoflurane (0.5%–1.0% IV) was used in all patients. Nasopharyngeal (NP) temperature was continuously measured (Mallinkrodt Inc., St. Louis, MO) and recorded every minute on the automated anesthesia record system (SATURN^TM; North American Draeger, Telford, PA). Procedure length was defined as the time from the first recorded temperature in the OR (within minutes of anesthesia induction and intubation) until the patient was transferred to the intensive care unit (ICU). The OPCAB procedures used either the Octopus^TM (Medtronic, Inc., Minneapolis, MN) myocardial stabilization device or the CTS system (Genzyme, Boston, MA) along with placement of an ascending aorta partial occlusion clamp for proximal vein graft anastomoses.

In the AS group, 3 single-use disposable energy transfer pads were placed on the posterior portions of the torso and legs (covering approximately 25% of the body surface area) of subjects before induction of anesthesia. After induction of anesthesia, the pads were connected to the control module that circulates temperature-controlled water to maintain a preset target temperature. Immediately after placement of the NP temperature probe (after induction and intubation), the device began servo-regulating the temperature to 36.8°C. The energy transfer pads incorporate a biocompatible and highly conductive hydrogel material (containing 50% water) that tightly adheres to the patient’s skin. It is designed to simulate water immersion, thereby providing effective heat transfer to and from the patient. The pads differ from conventional water mattresses that only provide heat transfer in areas of direct skin contact that represent only a small portion of the complete mattress surface. In contrast, the entire Arctic Sun pad adheres to skin thereby optimizing heat transfer. No other warming strategies were used in the AS group, and the room temperature was 16°C–18°C.

Temperature in the control group was managed according to standardized institutional practice with a combination of warming IV fluids (to 42°C) (Hotline^TM; SIMS Inc., Rockland MD), increasing the OR temperature to 24°C–28°C (the range varied according to the temperature settings chosen to balance the needs of the study and the comfort of the OR staff), and use of a convective forced air warming system (set at 43°C) with a U-shaped blanket (Progressive Dynamics Medical, Marshall, MI) positioned along side the latent aspects of the upper and lower limits under the surgical drapes. No other insulation was added to this blanket except that provided by the single layer surgical drape. In addition, no heating or humidification of the ventilator circuit was used. The operating table in both groups had a standard vinyl covered foam mattress, covered by a bedsheet on which the patients were positioned. The patient heads remained uncovered throughout. The transit time from the OR to the ICU was approximately 3–5 min during which the patient was covered with a single bedsheet.

Hypothermia was defined a priori as a NP temperature <36°C and quantified primarily by measuring the area under the curve for a temperature <36°C (AUC < 36°C) for each subject and secondarily as the time (minutes) that the temperature was <36°C. Mean temperature and the temperature before leaving the operating room were also compared. Data were not collected from any other temperature sites. At the end of the case, and after removal of both the electrocautery pads and energy transfer pads, the skin was inspected for any evidence of irritation or damage.

Patient demographics were compared between groups using Student’s t-test for continuous variables and Fisher’s exact test for categorical variables. Hypothermia was defined in 2 ways: AUC <36°C and time <36°C. The primary hypothesis, the comparison of hypothermia measures between the two treatment groups, was tested using a Wilcoxon’s ranked-sum test. However, as both the AUC <36°C and time <36°C are related to the overall duration of the procedure, as confirmatory analyses, linear regression analyses were also performed to account for the variable case duration. The dependent variable of the first linear regression analysis was AUC <36°C; the predictor of primary interest was treatment group with patient age and length of procedure included as covariates. A second linear regression was performed with time <36°C as the dependent variable, including the same predictors in the model. P < 0.05 was considered significant. All analyses were conducted with SAS version 6.03 (SAS Institute, Cary, NC).

	Results

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Twenty-nine patients (AS group = 14, control group = 15) were studied. Patient demographics are presented in Table 1. There were no differences between groups with respect to age, body mass index, gender, or procedure length. No skin damage or irritation was reported in any of the patients. The overall temperature patterns in both groups are represented in Figure 1. The mean overall intraoperative temperature was 36.3°C ± 0.3°C in the AS group and 35.8°C ± 0.5°C in the control group (P = 0.009; Fig. 1). The temperature on admission to the ICU was 36.5°C ± 0.3°C and 36.1°C ± 0.8°C in the AS and control groups, respectively (P = 0.07).

View larger version (16K): [in this window] [in a new window]   Figure 1. The temperatures between groups at various time points during and after off-pump coronary artery bypass surgery are depicted. The mean overall temperature was higher in the Arctic Sun® group versus controls (P = 0.009). ICU = intensive care unit.

View larger version (12K): [in this window] [in a new window]   Figure 2. Using the area under the curve to integrate the time and magnitude of the nasopharyngeal temperature < 36°C, demonstrated that the Arctic Sun® group had less hypothermia during off-pump coronary artery bypass surgery (P = 0.002).

View larger version (12K): [in this window] [in a new window]   Figure 3. Compared with conventionally treated patients, the Arctic Sun® group spent significantly less time with a nasopharyngeal temperature < 36°C during off-pump coronary artery bypass surgery (P = 0.0008).

	Discussion

Top Abstract Introduction Methods Results Discussion Appendix References

Little, if any, outcome data relating to perioperative temperature in the OPCAB setting are available. As a result, the optimal temperature during and after surgery is not known. It may be advantageous to have hypothermia during part or all of the intraoperative time period. For example, neuroprotection does occur with hypothermia (12,21) and OPCAB surgery often leads to hemodynamically unstable time periods when the brain may be at risk of hypoperfusion. Balancing the risks and benefits of hypothermia needs to be further addressed, but current practices, right or wrong, are frequently ineffective at preventing hypothermia, thus necessitating the study of improved techniques.

Although not extensively studied, there are several potential disadvantages of hypothermia in OPCAB surgery. Hypothermia delays extubation time (3). In addition, hypothermia-induced shivering increases metabolic rate of myocardial oxygen consumption (17). As residual areas of myocardium may remain at risk for ischemia in OPCAB patients (who are often incompletely revascularized), these increases in oxygen consumption may lead to myocardial ischemia (18,19). Coagulation profiles may also be impaired, leading to excessive bleeding and increased transfusion requirements (4–7). Also, hypothermia can alter immune function by impairing white blood cell abilities, thereby increasing the risk of perioperative infection (8–11).

Although of lesser clinical significance, comfort of the operating room staff is also an issue. Increases to the ambient operating room temperature, in an attempt to minimize patient heat loss, may lead to significant caregiver discomfort. The use of a system that avoids the need for increased operating room temperatures would be a significant advantage. Indeed, the AS group in the present study not only avoided OR temperature increases, but also maintained normothermia in the absence of any other temperature control strategies or devices.

In this study, the Arctic Sun® system was more effective at preventing hypothermia compared with conventional methods. The reasons for this difference likely relate to the larger body surface area covered by the pads compared with the U-shaped forced air convective blanket that was used and also the increased efficiency of the Arctic Sun® system. Efficiency of the heating system can be assessed by measuring the heat exchange coefficient of the system. The heat exchange coefficient represents the rate of heat exchange divided both by the area exposed as well as the temperature gradient between the heat exchange surfaces. The Arctic Sun® system has been measured to have a heat exchange coefficient five times that of conventional forced air systems (information on file with Medivance Inc.).

Although the control group in this study received conventional warming therapy, the efficacy of convective forced-air warming devices has not been specifically evaluated in the OPCAB setting. Its relative effectiveness should be balanced against theoretical risk. One might question the safety of creating a potential current of air being blown from a relatively nonsterile area beneath the drapes towards the sterile surgical field. Servo-regulated systems such as the one used herein are not without potential risks. For example, if there is an interruption in the feedback loop, which might occur if the NP temperature probe became dislodged, false information might be fed back into the system. If the probe falsely reads a low temperature, excessive warming could theoretically result. The system used herein does, however, have several built-in safety features, including an alarm and water flow stop if the patient temperature decreases to less than 32°C. However, it is very important to ensure that the temperature probes are adequately secured and their positions confirmed intermittently. Of note, the NP site seems an ideal location, as it is easily accessible and very well approximates brain temperature (20), which is particularly important in the setting of cardiac surgery where brain injury can occur.

There were several limitations to this study. This was a single-blinded study in that the caregivers were not blinded to the Arctic Sun® use. It is possible that this may have biased the subsequent treatment of patients. Patients in the control group followed strict institutional intraoperative practices that, if not practiced, may have served to increase the amount of hypothermia and increased differences between groups. Another potential limitation was our definition of hypothermia (<36°C). We believe, however, that this is a meaningful definition, as patients at our center would have been actively warmed until >36°C before discontinuing CPB in conventional CABG patients, and after OPCAB, and would not be extubated until reaching that temperature in the ICU. Finally, no cost comparisons were made between these different groups, partly as the cost of the equipment and disposable pads/blankets used in the various groups differs considerably among hospitals depending on factors such as contracts and individual volume usage. Overall, however, it is estimated that the Arctic Sun® would likely incur a larger direct cost than the conventional group. Whether these increased costs would be offset by efficiencies or improved outcomes resulting from better perioperative temperature management remains the object for future study.

In summary, the Arctic Sun® Temperature Management System significantly reduced intraoperative hypothermia during OPCAB surgery. Importantly, this occurred in the absence of any other temperature modulating techniques, including the use of fluid warming or increases in the ambient operating room temperature.

	Appendix

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Neurologic Outcome Research Group of the Duke Heart Center
Director: Mark F. Newman, MD; Co-Director: James A. Blumenthal, PhD.

Anesthesiology.
Fiona M. Clements, MD, Norbert de Bruijn, MD, Katherine Grichnik, MD, Hilary P. Grocott, MD, Steven E. Hill, MD, Joseph P. Mathew, MD, J. G. Reves, MD, Debra A. Schwinn, MD, Mark Stafford Smith, MD, David Warner, MD, G. Burkhard Mackensen, MD, Timothy Stanley, MD, Jerry L. Kirchner, BS, Aimee M. Butler, MS, Vincent E. Gaver, BA, Wayne Cohen, MPH, Bonita L. Funk, RN, E. D. Derilus, BS, Deborah Manning, BS, Scott Lee, BS, Jonathan Williams, BS, Melanie Tirronen, BS, Erich Lauff, BA, Shonna Campbell, BS, Keinya Lee, BS, William D. White, MPH, and Barbara Phillips-Bute, PhD.

Behavioral Medicine.
James A. Blumenthal, PhD, Michael A. Babyak, PhD, and Parinda Khatri, PhD.

Neurology.
Carmelo Graffagnino, MD, Daniel T. Laskowitz, MD, Ann M Saunders, PhD, and Warren J. Strittmatter, MD.

Surgery.
Robert W. Anderson, MD, Thomas A. D’Amico, MD, R. Duane Davis, MD, Donald D. Glower, MD, R. David Harpole, MD, James Jaggers, MD, Robert H. Jones, MD, Kevin Landolfo, MD, Carmelo Milano, MD, Peter K. Smith, MD, and Walter G. Wolfe, MD.

	Acknowledgments

 
Supported by Medivance® Inc, Louisville, Colorado. Drs. Grocott and Newman have received honoraria as consultants to Medivance Inc. but retain no other financial interest.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999