Ann Thorac Surg 2009;87:284-288. doi:10.1016/j.athoracsur.2008.08.067
© 2009 The Society of Thoracic Surgeons
New Technology
Intraoperative Temperature Control Using the Thermogard System During Off-pump Coronary Artery Bypass Grafting
Gary S. Allen, MD*
Department of Cardiac Surgery, Memorial Regional Hospital, Hollywood, Florida
Accepted for publication August 20, 2008.
* Address correspondence to Dr Allen, 1150 N 35th Ave, Ste 440, Hollywood, FL 33021 (Email: gaallen{at}mhs.net).
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Abstract
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Purpose: Normothermia during off-pump coronary bypass (OPCAB) grafting reduces metabolic derangements and contributes to improved clinical outcomes. Thus study examined the feasibility and efficacy of intraoperative temperature control using a novel endovascular heating system during OPCAB.
Description: Thirty-eight consecutive patients undergoing OPCAB were prospectively randomized to receive conventional warming (elevated room temperature, warmed intravenous fluids, warming blanket) or the Thermogard system (Alsius Corp, Irvine, CA). The triple-lumen temperature control Icy catheter (Alsius Corp) was inserted percutaneously into the inferior vena cava through common femoral vein. The catheter was removed after all wounds were closed. Temperature measurements (bladder, nasopharyngeal, and blood) were recorded at 5-minute intervals and compared between groups.
Evaluation: Patient demographics did not significantly differ between groups. The 17 Thermogard patients warmed at a significantly faster rate than the 21 control patients (0.28° vs 0.11°C/h, p = 0.03). Furthermore, Thermogard patients received more bypass grafts (3.4 ± 0.6 vs 2.6 ± 0.9, p < 0.001) and less intraoperative fluids (1557.0 ± 547.7 vs 2012.3 ± 723.1 mL, p = 0.02) despite longer operative times (150.3 ± 123.4 vs 108.1 ± 43.7 min; p = 0.12). All catheters were placed successfully on the first attempt, and there were no device-related complications.
Conclusions: Endovascular warming is safe, simple to use, and obviates the need for uncomfortably warm operating room temperatures. The Thermogard system compared favorably with conventional methods for warming during OPCAB.
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Technology
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Unintentional hypothermia (core temperature < 36°C) commonly occurs during coronary artery bypass grafting (CABG) that is done off-pump (OPCAB) and results from a combination of anesthetic-impaired thermoregulation and direct heat loss through the sternotomy. Volatile anesthetics inhibit tonic thermoregulatory vasoconstriction, leading to peripheral shunting of blood and a rapid redistribution hypothermia (1° to 1.5°C) within the first hour of anesthesia [1]. The anesthetic-mediated rapid temperature drop is followed by a more linear reduction that plateaus during the operation after 3 to 4 hours [1].
The consequences of even mild hypothermia are generally not well appreciated. Coagulation is impaired by mild hypothermia primarily by way of a cold-induced defect in platelet function [2]. Wound infection and delayed wound healing are increased both by direct immune function impairment and thermoregulatory vasoconstriction, which decreases oxygen delivery to the wound [3]. Hypothermia is physiologically stressful (increased heart rate, blood pressure, and plasma catecholamine levels) and uncomfortable for patients [4]. Related shivering is a potentially serious complication, which in extreme cases can double oxygen consumption [4]. Drug metabolism is also reduced and may significantly prolong postanesthetic recovery [5].
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Technique
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Strategies to maintain normothermia in the operating room have centered around three avenues, each with significant drawbacks: (1) elevated room temperature of 22°C or more, (2) warmed intravenous fluids and inhaled gases, and (3) surface warming (convective or conductive, or both). Elevated room temperatures are uncomfortable for the operating staff, especially when noted that temperatures at the operating field are 2° to 4°C warmer than surrounding areas. Warmed fluids have never been shown to be effective in patient warming. Surface warmers are frequently cumbersome to apply and may limit access to the extremities. The purpose of this study was to determine the safety and efficacy of intravascular warming using the Thermogard System (Alsius Corp, Irvine CA) compared with conventional warming methods during OPCAB.
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Clinical Experience
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Patient Selection
In accordance with our Institutional Review Board approved study, patients undergoing OPCAB between June 2006 and February 2007 were assessed for study eligibility. All patients were aged older than 18 years, underwent CABG only, and were operated on electively. Exclusion criteria were (1) acute myocardial infarctions, (2) cardiogenic shock, (3) inferior vena cava filters, and (4) history of bleeding disorder. All patients were operated on by a single surgeon with an OPCAB experience exceeding 600 cases. Among the 51 patients initially assessed, 13 were excluded (Fig 1). Four patients refused to participate, 3 had inferior vena cava filters, and 4 had histories of abnormal bleeding. Two patients randomized to the Thermogard group were excluded from final analysis because one was converted to traditional on-bypass CABG, and the other was determined to need mitral valve repair after consent was signed.
Procedure
Each patient was intubated with a single-lumen endotracheal tube and received a standardized anesthetic regimen of dexmedetomidine (0.2 to 0.7 µg/kg/h), fentanyl (5 to 10 µg/kg), and pancuronium (0.01 mg/kg) with desflurane (0.5% to 2.0%). Intraoperative transesophageal echocardiographic monitoring was used in 3 Thermogard patients and in 4 control patients. All patients underwent placement of pulmonary artery catheters, which supplied both hemodynamic and blood temperature data. Nasopharyngeal and bladder temperature monitoring was established shortly after endotracheal intubation and correlated closely with blood temperature. Composite temperature monitoring (blood, bladder, and nasopharyngeal) was recorded at 5-minute intervals from patient entry to patient exit from the operating room. The bladder temperature monitor was used as the input source for Thermogard warming.
Heparin was administered to achieve an activated clotting time (ACT) exceeding 200 seconds, generally between 4000 and 7000 U. Protamine (50 to 75 mg) was administered until baseline ACT was achieved. All operations were performed with the Acrobat stabilization system (Marquet Corp, San Jose, CA). Procedure length was defined as the time between skin incision and when all wounds were closed.
The ambient operating room temperature was preset at 23°C in the control group and 19°C in the Thermogard group. Control patients received warmed (41°C) intravenous fluids (Bair Hugger Fluid Warmer Model 24100, Arizant Inc, Edem Prairie, MN), a convective forced air warming system (Bair Hugger Model 505, Arizant Inc), and were placed on a warming (40°C) gel pad (Moel OTM1, Inithirm Plc, Totherham, UK). After Thermogard patients were prepared and draped, the Icy catheter (Alsius Corp) was placed in the right common femoral vein using a Seldinger technique. The Icy catheter is a 9.3F 45-cm triple-lumen central line with 3 balloon chambers that circulate heated sterile saline. The control module (Fig 2) target temperature was set to 37°C. The maximal incoming temperature is 42°C, with an average temperature of 38.5°C.
The randomization protocol consisted of the operator opening a sequential sealed envelope after informed consent was obtained. Data were analyzed using StatView 5.0 statistical software (SAS Institute, Cary, NC) and are expressed as mean ± SD or as a percentage of patients. Continuous variables were analyzed using a t test. Categoric variables were analyzed using the Fisher exact test. Statistical significance is considered for p < 0.05.
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Results
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Control patients were a mean age of 63.9 ± 9.4 years (range, 42 to 81 years) compared with 66.0 ± 11.6 years (range, 45 to 78 years) for Thermogard patients (Table 1). The hospital's routine preoperative hemoglobin A1C screening resulted in a new in-hospital diagnosis of diabetes mellitus in an additional 6 control patients, for a total of 10 (47.6%) and 7 Thermogard patients, for a total of 11 (64.7%; p = 0.34). Significant left main coronary artery disease was present in 4 control patients (19.0%) and 5 Thermogard patients (29.4%, p = 0.31). Although chronic renal insufficiency was not uncommon, occurring in 2 control patients (14.3%) vs 3 Thermogard patients (17.6%; p = 0.44), only one control patient was dialysis dependent. Two patients in the Thermogard group refused blood products preoperatively.
Thermogard patients generally started at lower and finished at higher core temperatures than control patients (Table 2). Thermogard patients warmed at more than 2.5 times the rate of control patients (0.28°C/h vs 0.11°C/h, p = 0.03). Most of this benefit occurred during the first 30 minutes of the operation (Fig 3). After the initial steep assent, a more gradual assent was observed shortly after sternotomy (Fig 3). Approximately 20 minutes after sternotomy, the Thermogard patients experienced a brief temperature drop, with full recovery within about 10 minutes of sternal closure. Control patients warmed slowly and did not experience a similar temperature drop after sternotomy.
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Fig 3. Characteristic mean temperature patterns of control (diamonds) vs Thermogard groups (squares) during the sternotomy. Sternotomy occurred at roughly equal times for both groups, but sternal closure occurred earlier in the control group, consistent with fewer bypass grafts performed.
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Despite having similar coronary artery disease patterns, Thermogard patients received significantly more bypass grafts compared with the controls (3.4 ± 0.6 vs 2.6 ± 0.9, p = 0.02). Consistent with more bypasses, the Thermogard patients experienced longer operating times. Interestingly, the higher degree of revascularization and operating times did not lead to increased administration of intravenous fluids during the operation; in fact, Thermogard patients received significantly less intraoperative fluids (1557 ± 547 vs 2012 ± 723 mL, p = 0.02). The intensive care unit length of stay was approximately one-half day shorter for the Thermogard patients but did not quite achieve statistical significance.
Only one control patient (5%) experienced a superficial sternal wound infection, which resolved after a combination of intravenous and oral antibiotic therapy. Neither group experienced a death or return to the operating room for reexploration. Blood or blood product transfusions, or both, occurred in 5 control patients (36%) and 4 Thermogard patients (24%; p = 0.34).
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Comment
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In contrast with on-pump CABG, there is no active warming element in OPCAB procedures to control patient temperature. The challenge to keep patients warm falls upon the operating room staff, anesthesiologist, and surgeon. Strategies to achieve normothermia are generally nonstandardized, passive (ie, elevated operating room temperature and warmed intravenous fluids and ventilation gases), and variably successful. Conductive warming devices have recently been introduced to improve patient warming [6]. The clinical efficacy of surface warming, however, remains in question and has not been widely embraced.
Examining the efficacy of warmed IV fluids in OPCAB surgery, Jeong and colleagues [7] needed to maintain a room temperature of 25°C to prevent hypothermia. Ambient temperatures as high as 28°C have been reported in other series to achieve similar results [8]. Operating room environments in this temperature range are generally not well tolerated by the operating team, especially considering that temperatures at the operating table are about 3°C warmer than ambient temperature (institution observational experience). Although 19°C was arbitrarily chosen as a comfortable temperature for the study group, our institutional experience of more than 400 OPCAB-Thermogard operations has demonstrated satisfactory temperature control with room temperatures as low as 15°C.
Heated fluids are simple to administer but are limited as a primary modality in patient warming. Warmed fluids cannot safely exceed 41°C due to the risk of skin burns at the delivery site and risk of blood protein denaturation. A relatively small volume (1 to 3 L) of fluid at that temperature is too small to obtain a significant effect [9]. In the present study, Thermogard patients received approximately 0.5 L less fluids compared with control patients. This is especially notable given that the study patients underwent more extensive grafting and longer operative times. Less hemodilution may have a clinically positive impact on blood transfusions, extubation times, and postoperative atrial fibrillation, especially with longer operations. The thermal capacity of air is small and less than 10% of metabolically produced heat is lost through the respiratory tract [10]. Similar to heated fluids, airway heating has a minimal influence on the core temperature.
The most common perianesthetic warming system is forced air. Their usefulness in OPCAB is limited because the anteriorly placed blankets cannot cover the chest and cannot be placed on the legs until vein harvesting is completed. Conductive warming devices have been presented as solutions for temperature management during OPCAB [10]. Although attractive in theory, both commercially available systems have considerable limitations. Each system requires a large surface area of the thorax or lower extremities, or both, to be covered, thus making saphenous vein harvest more difficult. The Icy catheter inserted easily, resulted in no device-related complications, and added a central venous access port. The Thermogard system can be easily transported with the patient to the recovery area to reduce temperature after-drop.
Thermogard patients warmed approximately two-and-a-half times faster than controls. This difference was largely reflected within the first 30 minutes after device insertion. Rapid warming is especially helpful for patients coming to the operating room with core temperatures of less than 36°C. Because the patients were prospectively randomized and displayed similar coronary artery disease patterns, it is unclear why the study patients received significantly more bypass grafts. Given the warming characteristics of both groups, we surmise that either a larger cohort or longer operative times, or both, may further distinguish the efficacy of the Thermogard system.
Intravascular warming is safe, easy to deliver, and compares favorably with traditional warming. Device ease of use and portability make it an attractive option as a primary perianesthetic warming system. The ability to manipulate the ambient temperature in the operating room according to the surgeon's preference, without concern for patient safety, is notable. Future warming catheters will be smaller, more powerful, and allow for application at multiple sites (ie, subclavian and internal jugular). Larger studies may determine the clinical effect of normothermia in OPCAB.
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Disclosures and Freedom of Investigation
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The Thermogard system and Icy catheters were purchased at full cost to the author's institution. The author had full control in the patient selection, operative technique, study design, data analysis, and preparation of the article.
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Acknowledgments
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This study is partially supported by a grant from Alsius Corporation Irvine, California.
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Footnotes
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Disclaimer The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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References
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- Michelson AD, MacGregor H, Barnard MR, Kestin AS, Rohrer MJ, Valeri RC. Reversible inhibition of human platelet activation by hypothermia in vivo and in vitro Thromb Haemost 1994;71:633-640.[Medline]
- Kurz A, Sessler DI, Lenhardt RA. Study of wound infections and temperature group: perioperative normothermia to reduce incidence of surgical wound infection and shorten hospitalization N Engl J Med 1996;334:1209-1215.[Medline]
- Frank SM, Fleisher LA, Breslow MJ, et al. Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events: a randomized clinical trial JAMA 1997;277:1127-1134.[Medline]
- Hemmerling TM, Prieto I, Choiniere J, Basile F, Fortier JD. Ultra-fast-track anesthesia in off-pump coronary artery bypass grafting: a prospective audit comparing opioid-based anesthesia vs thoracic epidural based anesthesia Can J Anesthes 2004;51:163-168.
- Stanley TO, Grocot HP, Phillips-Bute B, et al. Preliminary evaluation of the Artic Sun temperature-controlling system during off-pump coronary artery bypass surgery Ann Thorac Surgery 2003;75:1140-1144.[Abstract/Free Full Text]
- Jeong SM, Hahm KD, Jeong YB, Yang HS, Choi IC. Warming of intravenous fluids prevents hypothermia during off-pump coronary artery bypass graft surgery J Cardiothorac Vasc Anesthes 2008;22;:67-70.[Medline]
- Clark JA, Bar-Yosef S, Anderson A, Newman MF, Landolfo K, Grocott HP. Postoperative hyperthermia following off-pump coronary artery bypass surgery J Cardiothorac Vasc Anesthes 2005;19:426-429.[Medline]
- Chassot PG, van der Linden P, Zaugg M, et al. Off-pump coronary artery bypass surgery: physiology and anesthetic management Br J Anaesth 2004;92:400-413.[Abstract/Free Full Text]
- Ginsberg S, Solina A, Papp D, et al. A prospective comparison of three heat preservation methods for patients undergoing hypothermic cardiopulmonary bypass J Cardiothorac Vasc Anesth 2000;14:501-505.[Medline]
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