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CARDIOVASCULAR ANESTHESIOLOGY

An Evaluation of a Full-Access Underbody Forced-Air Warming System During Near-Normothermic, On-pump Cardiac Surgery

Steven R. Insler, DO^*, Mohamed H. Bakri, MD, PhD, Fady Nageeb, MD, Edward Mascha, PhD, Tomislav Mihaljevic, MD^||, and Daniel I. Sessler, MD

From the Departments of *Cardiothoracic Anesthesia, Outcomes Research, Quantitative Health Sciences, and ||Cardiovascular Surgery The Cleveland Clinic, Cleveland, Ohio; and Division of Anesthesia, Critical Care, and Comprehensive Pain Management, The Cleveland Clinic, Cleveland, Ohio.

Address correspondence and reprint requests to Daniel I. Sessler, MD, Department of Outcomes Research, Cleveland Clinic Foundation, 9500 Euclid Ave. — P77, Cleveland, OH 44195. Address e-mail to ds{at}or.org or www.or.org.

Abstract

BACKGROUND: A new underbody forced-air warming system is available for use during cardiac surgery. We tested the hypothesis combining underbody forced-air warming with standard thermal management would maintain intraoperative core temperature and reduce core temperature after-drop (largest decrease in core temperature in the 60 min after bypass) in patients undergoing near-normothermic cardiopulmonary bypass (CPB).

METHODS: Patients undergoing routine, nonemergent cardiac surgery were randomly assigned to routine thermal management (fluid warming and passive insulation, n = 30) or routine management supplemented by an active underbody forced-air system (n = 30; Arizant Healthcare Model 635, Eden Prairie, MN). Core body temperature was measured by bladder catheter at 15-min intervals during the perioperative period. Comparisons were made between groups for temperature before, during, and after CPB.

RESULTS: Data from four patients were excluded for cause, leaving 29 patients in the routine management group and 27 patients in the forced-air group. Initial temperatures were similar, but temperatures in the forced-air group were higher than in the routine group at the start of CPB (36.3°C ± 0.6°C vs 35.7°C ± 0.7°C, P = 0.002). There were no differences between groups in the lowest temperatures during CPB (forced air, 35.5°C ± 1.5°C vs routine, 35.3°C ± 1.3°C, P = 0.67); the end of CPB (36.7°C ± 0.4°C vs 36.6°C ± 0.4°C, P > 0.99); or the temperature at departure from the operating room (36.5°C ± 0.4°C vs 36.2°C ± 0.5°C, P = 0.36). After-drop was 0.03°C ± 0.54°C in patients randomized to underbody forced-air warming and 0.21°C ± 0.51°C in those assigned to routine management (P = 0.20).

CONCLUSIONS: Adding an underbody forced-air warming system to the near-normothermic thermal management protocol significantly increased pre-bypass temperature; however, it had no further clinically important effect on core temperature.

Hypothermia during cardiac surgery and in the postoperative period is associated with adverse outcomes, including impaired drug metabolism,1–3 cardiac morbidity,2,4–7 shivering,8–10 impaired immune function,6,11 coagulopathy,11,12 and increased use of hospital resources.1,13 Several clinical studies have demonstrated that maintenance of normothermia during the perioperative period significantly reduces morbidity.10,12,14,15 There have been several methods developed and techniques proposed to maintain normothermia during surgery, including warming patients before induction of anesthesia, conductive warming by water mattresses covering the patient's back and upper extremities, convective warming from forced-air blanket systems, and infusions of warmed liquids.

The effectiveness of any warming system depends on its ability to transfer heat to the patient. Patients undergoing cardiac surgery are often positioned above a circulating-water mattress. Although these systems permit unrestricted access to the anterior surfaces of the patient, they are inefficient warmers and might cause injury because the combination of heat and decreased local perfusion from the patient's own body weight restricts capillary blood flow which can lead to burns and pressure-heat necrosis.16–18 Recently developed systems allow circulating-water garments to cover a larger surface area of the body and thus transfer more heat than traditional water mattresses that only heat the back and legs. Additionally, there are circulating-water systems using highly efficient "energy transfer" pads that transfer far more heat per square meter of body surface area than conventional mattresses.19,20

Conventional (over-body) forced-air warming blankets transfer much more heat for a given surface area than circulating-water mattresses,14,16 although less than circulating-water garments and energy transfer pads.19 Convective warming is relatively inefficient on a per-area basis; but anterior forced-air covers contact a large surface area and thus transfer considerable heat. As a result, they have become routine for noncardiac surgery.

The difficulty with thermal management during cardiac surgery, though, is that much of the anterior surface must be left exposed, especially when vein harvesting from the legs is required. An underbody forced-air warming system has recently been developed (Arizant Healthcare Model 635 Full-Access Underbody blanket, Eden Prairie, MN) that is placed on the operating room (OR) table surface and contacts the posterior surface of the patient. The design of this system prevents heated air from making contact with regions of the body that are under the most pressure from bony prominences (i.e., scapulae, sacrum, and occiput). The patient's body weight effectively stops airflow to the areas of concern. However, the efficacy of this system has yet to be formally evaluated. We tested the hypothesis combining underbody forced-air warming with a near-normothermia thermal management protocol would better maintain intraoperative bladder temperature in patients undergoing cardiopulmonary bypass (CPB), and reduce after-drop following surgery.

METHODS

With approval from the IRB at the Cleveland Clinic, Cleveland, Ohio, and after receiving written informed consent, 60 consecutive patients scheduled for elective cardiac surgery using CPB were enrolled between April and July 2006. Exclusion criteria included emergency procedures, serious skin disease, infection or sepsis, presence of an intraaortic balloon pump, obesity (body mass index 35 kg/m²), fever within 24 h before surgery, and planned circulatory arrest.

Routine heat conservation methods were applied in both groups that included warming IV fluids to 38°C, a steri-drape cardiovascular sheet (Cardiovascular Sheet with Ioban; 3M Corporation, Minneapolis, MN), and a circulating-water mattress positioned beneath the pad of the OR table (Medisearch HF7010; Gaymar Industries, Buffalo, NY). The circulating-water mattress was set to 37°C.

Patients were randomly assigned to routine thermal management alone (n = 30) or the combination of routine thermal management supplemented by forced-air warming (n = 30). Randomization was based on computer-generated codes maintained in sequentially numbered opaque envelopes that were opened just before the patients were transferred to the OR. Patients randomized to supplemental forced-air warming were positioned directly on the surface of a forced-air warming blanket connected to a warming unit (Bair Hugger Model 635 Full Access Underbody blanket and Model 750 base unit, Arizant Healthcare, Eden Prairie, MN). The warming unit was set to the highest temperature (43°C). The forced-air warming unit was deactivated whereas the aorta was cross-clamped to allow for mild temperature drift and then reactivated when aortic flow was re-established.

Anesthetic management was left to the discretion of the attending anesthesiologist. General anesthesia was induced with either etomidate or thiopental followed by fentanyl, and muscle relaxation with either succinylcholine followed by pancuronium or pancuronium alone. Anesthesia was maintained with fentanyl, midazolam, isoflurane (maximum 1% inhaled in 50% oxygen/50% nitrogen), and pancuronium to maintain one twitch in response to supramaximal train-of-four stimulation. Neither techniques designed to prewarm the patient nor presurgical vasodilators were used. CPB was conducted under near-normothermic conditions with perfusate temperatures kept at approximately 35°C. Patients were rewarmed to a bladder temperature of 37°C bladder before separation from CPB.

Measurements
Morphometric and demographic characteristics were recorded. Ambient temperature in the OR was recorded on patient arrival. The time when the forced-air warming system was activated and deactivated was also recorded. Core body temperature was measured via a bladder catheter equipped with a temperature probe. Esophageal and pulmonary artery temperatures were also measured. Temperatures were recorded every 15 min and at specific events, which included arrival in the OR, immediately after induction of anesthesia, initiation and termination of CPB, departure from the OR, arrival in the intensive care unit (ICU), and at the time of tracheal extubation. After-drop was calculated for each patient as the difference between the last temperature on CPB and the lowest recorded temperature within 60 min after the end of CPB.

Statistical Methods
Results are presented as mean ± sd for normally distributed variables and as median (quartiles) for non-normally distributed variables. The baseline variables in the forced-air and standard warming groups were examined for clinically important differences. Repeated measures analysis of variance (ANOVA) with a compound symmetry correlation structure was used to assess the effects of randomized group (active vs standard), time point (induction, CPB start, CPB end, OR departure, and tracheal extubation), and the group-by-time interaction on temperature. The Tukey–Kramer multiple comparison procedure was used to assess the group effect at each time in the presence of a significant interaction to maintain a family-wise Type I error rate of 5%. Reported P values for the five temperature time points are thus the Tukey–Kramer adjusted P values, and noted so in the Results as "adjusted P value." Group comparisons on secondary outcomes of interest such as temperature after-drop or lowest temperature during CPB were made using either two-sample t-tests or Wilcoxon Rank-Sum tests, as appropriate, and using a 0.05 significance criterion.

Thirty patients per group provided 80% power at the 0.05 significance level to detect a 1°C core-temperature difference between the two treatment groups, assuming a standard deviation of 1°C and using a two-sided, two-sample t-test. Our estimates are conservative based on similar studies where standard deviation ranges from 0.5 to 1.0°C.21,22

RESULTS

Four patients were excluded from the analysis because of unplanned circulatory arrest (one patient from the standard care group and two from the forced-air group) and one patient in the forced-air group had surgery cancelled due to failed fiberoptic tracheal intubation.

Morphometric and demographic characteristics of the patients in the two treatment groups were similar (Table 1). Although the length of the various stages of surgery varied considerably among patients, there was no difference between the treatment groups in the duration of surgery. There were more valve surgeries in the forced-air group but the number of isolated coronary artery bypass graft (CABG) or CABG plus any other procedure was similar between groups. Ambient OR temperature was 17.2°C ± 1.1°C throughout the procedure.

The effect of forced-air versus standard care on temperature was not consistent across the time points assessed, as revealed in a significant group-by-time interaction (P = 0.003) in our repeated measures ANOVA (Fig. 1). The forced-air and standard care groups had essentially the same temperature at anesthesia induction (35.9°C ± 0.5°C vs 36.0°C ± 0.7°C, adjusted P > 0.99) as noted on the pulmonary artery catheter. The forced-air group had a higher temperature before the start of CPB (36.3°C ± 0.6°C vs 35.7°C ± 0.5°C, adjusted P = 0.002). However, the mean core temperature at the end of CPB was similar between groups (forced-air group, 36.7°C ± 0.4°C vs standard care group, 36.6°C ± 0.4°C, adjusted P > 0.99). The lowest temperature during CPB was 35.5°C ± 1.5°C in the forced-air group and 35.3°C ± 1.3°C in the standard care group (P = 0.67). There was no difference in temperature between the groups upon departure from OR (forced-air group, 36.5°C ± 0.4°C vs standard care, 36.2°C ± 0.5°C, adjusted P = 0.36) or at tracheal extubation (37.2°C ± 0.59°C vs 37.2°C ± 0.66°C, adjusted P > 0.99). After-drop was 0.03°C ± 0.54°C in patients in the forced-air group compared with 0.21°C ± 0.51°C in the standard care group (P = 0.20).

View larger version (18K): [in this window] [in a new window]   Figure 1. The effect of forced-air underbody warming versus routine temperature management on patient temperature during near-normothermic cardiopulmonary bypass. The group-by-time interaction was statistically significant (P = 0.003). P values are adjusted for multiple comparisons (Tukey–Kramer). Asterisk (*) indicates P value statistically significant (<0.05). Data are presented as mean ± sd.

A secondary analysis comparing the randomized groups on the five primary outcome temperature measurements was made adjusting for type of surgery, given that in Table 1 there appears to be clinical imbalance on that variable. Results were nearly identical to the main analysis results, and the P value for type of surgery in the model was nonsignificant at P = 0.48.

DISCUSSION

There are essentially two strategies for minimizing heat loss and maintaining normothermia in patients during CPB: passive and active warming. Passive warming methods use insulation to reduce loss of metabolic heat whereas active methods require energy-using heat transfer devices to substantially reduce heat loss to the environment or even increase heat transfer across the skin surface into the patient.

Passive methods are relatively easy to use but are usually insufficient for surgical patients. These materials include blankets, paper drapes, and plastic wraps that are used as insulators. A single layer of any of these materials reduces regional heat loss by about 30%.23,24 Forced-air is the most commonly used perioperative warming technique, since it is inexpensive, easy to use, remarkably safe, and highly effective. These systems work best when a considerable fraction of the anterior body surface is available for heat transfer and when there is a sufficient temperature gradient between the patient's skin and the air expelled by the blanket. Typically, they reduce or eliminate both convective and radiant heat loss during surgery and may even provide a net transfer of heat across the entire skin surface.25

An interesting observation of our study is that patients undergoing near-normothermic thermal management did not become hypothermic upon separation from CPB or before leaving the OR. Furthermore, there was only a trivial amount of temperature after-drop, even in patients who were not in the actively warmed group (approximately 0.2°C). Typically, incomplete rewarming of peripheral tissues after CPB manifests as a large core-to-peripheral temperature gradient. For example, Rajek et al. found that the core-to-peripheral tissue temperature gradient at the end of rewarming after CPB at 31°C was 3.5°C ± 1.8°C and 4.6°C ± 1.9°C after CPB at 27°C.26 This results in a rapid temperature after-drop after surgery as heat from the core tissues redistributes to peripheral tissues.26 We speculate that maintaining perfusate temperature at 35°C during CPB in our study allowed for enough warming of the peripheral tissues to minimize core-to-peripheral tissue gradient and heat redistribution.

After-drop has been defined as the decrease in core temperature after discontinuation of CPB.26 It has been our experience that the most critical period of after-drop is within the first 60 min after CPB. Minimal after-drop in this study indicates that the core-to-peripheral tissue temperature gradient was small at the end of CPB. Although we did not measure peripheral tissue temperature, it is likely that it would be only slightly less than perfusate temperature (35°C). However, core temperature in both groups was near 36.6°C at the end of bypass, thus leaving a gradient not much exceeding 1.5°C, which is roughly the normal gradient unanesthetized humans maintain in a typical hospital environment. (Some thermal gradient is necessary; otherwise metabolic heat would accumulate in the core rather than being dissipated to peripheral tissues and subsequently to the environment.) Critically, for the hypothesis of this article, it is also unsurprising that the underbody blanket was only modestly effective under these conditions. Lacking a substantial temperature gradient, one would not expect to see an appreciable treatment effect when warming patients who were already warm.

A limitation of our study is that the effect of the underbody blanket would likely be more apparent in patients undergoing CPB at lower body temperatures than used in our protocol. Forced-air warming significantly increased core temperature before CPB during a period when the body was not actively warmed by extracorporeal circulation. Additionally, our study was not blinded which might have introduced bias in our results.

In summary, the use of an underbody forced-air warming system in patients undergoing near-normothermic thermal management during CPB significantly increased temperature before CPB but not after surgery.

ACKNOWLEDGMENTS

The authors thank Nancy Alsip, PhD, (University of Louisville) for editing the manuscript.

Footnotes

Accepted for publication November 8, 2007.

Supported by Arizant Medical, Inc. (Eden Prairie, MN), NIH Grant GM 061655 (Bethesda, MD), and the Joseph Drown Foundation (Los Angeles, CA). Mallinckrodt Anesthesiology Products, Inc. (St. Louis, MO) donated the thermocouples we used.

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