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Ann Thorac Surg 2001;72:S1069-S1076
© 2001 The Society of Thoracic Surgeons
a Department of Cardiothoracic Surgery, Tel Aviv Sourasky Medical Center, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
b Cardiovascular System Laboratory, Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel
c Department of General Hospital Laboratories , Lady Davis Carmel Medical Center, Bruce Rappaport Faculty of Medicine, Technion-Israel-Institute of Technology, Haifa, Israel
d Department of Anesthesiology , Lady Davis Carmel Medical Center, Bruce Rappaport Faculty of Medicine, Technion-Israel-Institute of Technology, Haifa, Israel
e Department of Pediatrics, Lady Davis Carmel Medical Center, Bruce Rappaport Faculty of Medicine, Technion-Israel-Institute of Technology, Haifa, Israel
Address reprint requests to Dr Nesher, The Department of Cardiothoracic Surgery, Tel Aviv Sourasky Medical Center, 6 Weizman St., Tel-Aviv, 64239, Israel
e-mail: nnesher{at}netvision.net.il
Presented at the Seventh Annual Cardiothoracic Techniques & Technologies Meeting 2001, New Orleans, LA, Jan 24–27, 2001.
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Abstract |
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 Top Abstract IntroductionPatients and methodsResultsCommentAcknowledgmentsReferences |
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Methods. Elective CABG surgery patients were randomized into either Allon thermoregulation (AT, n = 40) or routine thermal care (RTC, n = 20) groups in whom the maintenance of normothermia during the nonbypass phases of the operation was compared. The AT used patients’ rectal temperature as reference data to monitor the maintenance of the water temperature circulating at 37°C in a garment. Rectal temperature, patient hemodynamics, and cardiac-specific troponin I (cTnI) levels were assessed at the induction of anesthesia, 30 minutes into surgery, at discontinuation of bypass, end of surgery, and 2 hours postoperatively.
Results. Body temperature was higher in the AT group compared to the RTC group at all five time points. Cardiac index (CI) (L/min) was higher in the AT group, 2.5 ± 0.5, 2.6 ± 0.5*, 3.2 ± 0.6*, 3.3 ± 0.5*, 3.1 ± 0.7 at the respective time points, compared to the RTC group, 2.3 ± 0.6, 2.1 ± 0.2, 2.6 ± 0.7, 2.7 ± 0.7, 2.7 ± 0.7 (*p < 0.05). Systemic vascular resistance (SVR) (dyne · s)/cm5) was consistently lower in the AT patients. Enzyme levels were elevated in both groups but were less so in the AT patients.
Conclusions. The AT system can efficiently maintain normothermia. The beneficial effects are expressed by reduced SVR, elevated CI, and lower levels of cTnI, which may show a possible attenuation of myocardial injury.
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Introduction |
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Among the numerous methods that have been used to rewarm patients throughout and after cardiac surgery are warming blankets and mattresses, warmed intravenous solutions, aluminum foil space blankets, and radiant and convective heating techniques. Normothermic CPB has been previously proposed; however, evidence exists that indicates a grater possible risk for cerebrovascular event in patients undergoing such strategy. Nevertheless, temperatures tend to drop during the nonbypass phases of surgery and potentially hazardous hypothermia persists when the patient is weaned from the normothermic CPB [5].
We present a novel method of achieving and maintaining perioperative patient thermoregulation (Allon; MTRE Advanced Technologies Ltd, Or-Akiva Industrial Park, Israel) at a level of normothermia. This technique uses continuous readings of the patient’s own body temperature to regulate the degree of external rewarming so that a predesignated core temperature is reached and maintained. The aims of the current study were: (1) to evaluate the efficacy of this technique in maintaining normothermia by observing the perioperative temperature profile; and (2) to compare the clinical hemodynamic benefits of normothermia achieved by using the Allon thermoregulation (AT) system with routine thermal care (RTC), both during and after CABG surgery.
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Patients and methods |
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Exclusion criteria were as follows: (1) known concomitant life-threatening or debilitating disease of noncardiac origin; (2) severe peripheral vascular disease, as defined by a history of intermittent claudication within a walking distance of <100 m; (3) uncontrolled insulin-dependent diabetes mellitus (preoperative fasting glucose levels >250 mg/dL); (4) history of fever or infection within the week before surgery, and (5) clinically significant laboratory abnormalities (ie, creatinine (2.0 mg/dL, total bilirubin (1.5 mg/dL, hemoglobin (10.0 g/100 mL, platelet count (100,000 cells/mL, or white blood cell count of either <3,000 cells/mL or >14,000 cells/mL).
Patients were randomly assigned to one of two groups. The first group consisted of 40 patients designated to be perioperatively warmed using the AT system, and the second consisted of 20 patients who underwent RTC. We deliberately chose this unbalanced cohort based on the authoritative source by Piantadosi [6] ,who recommends "an unequal allocation of subjects, eg, 2:1 in favor of the new treatment."
Anesthetic technique
The anesthetic techniques were the same for all the patients. A quantity of 1 to 3 mg/kg thiopenthal was used for induction, 3 to 5 (g/kg fentanyl and isoflurane were delivered up to 1% from 50% air–O2 mixture for maintenance, and 3 mg/kg propofol was given during CPB. Fentanyl and midazolam were administered through a special circuit for maintaining anesthesia during CPB. Neuromuscular blockade was achieved by 0.1 to 0.15 mg/kg pancuronium bromide, and ventilation was adjusted to maintain normocapnea. The (-stat acid-base management was adopted.
Surgical technique and CPB
Cardiopulmonary bypass was established with a single two-stage, right atrial cannula and conducted with a Univox membrane oxygenator (Jostra Corporation, Irvine, CA) and a Sarns 9000 CPB machine (Terumo Cardiovascular, Ann Arbor, MI). The extracorporeal circuit was primed with 1500 mL Plasmalyte (Baxter, Irvine, CA) and 10 mEq sodium bicarbonate. Active cooling was not carried out. Cardioplegia was established through antegrade priming followed by intermittent, tepid-blood, retrograde intervals. Rectal temperature was allowed to drop during bypass to 31° to 33°C. Nonpulsatile perfusion was used throughout the procedure, with a flow maintained between 2.0 and 2.5 L · min-1 · m-2. Phenylephrine was used as needed to maintain systemic perfusion pressures at 50 to 60 mm Hg. During bypass, hematocrit levels were maintained between 20% and 25%. Intraoperative autotransfusion and postoperative reinfusion of shed blood were not used. Distal anastomoses were constructed during a single period of aortic cross clamping and proximal anastomoses were performed without global ischemia. Rewarming of patients in both study groups commenced during the construction of the last distal anastomoses.
Postoperative management
Upon completion of the operation, the patients were transferred to the intensive care unit (ICU) where they were allowed to wake up when they were hemodynamically stable and blood loss from their chest drains was minimal and stable. If subsequent aggressive behavior was encountered, those patients were tranquilized by short-acting sedative agents (ie, midazolam) and allowed to reawaken later on. A single extubation protocol was used for all patients. They were extubated when they had been adequately rewarmed and oxygenated (arterial oxygen tension >80 mm Hg on an inspired oxygen fraction of 50%). Reexploration for the cause of bleeding was to be performed if blood loss continued at a rate of more than 200 mL/h for several hours or if it was rapid and uncontrolled, thereby compromising the patient’s hemodynamic status.
Thermoregulation and maintenance of normothermia
Operating room (OR) temperature was maintained between 19°C and 21°C during surgery for both groups.
Allon thermoregulation
The Allon technology used in the study group consists of the following elements: (1) a microprocessor-controlled heating/cooling unit; (2) body sensors, ie, core (rectal) and skin thermistors; and (3) a garment that wraps around the patient (Fig 1).
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The garment is composed of a two-layer plastic biocompatible material approved by the United States Food and Drug Administration. The outer layer is made of nonwoven polypropylene material and the inner layer of polyethylene. Water flows between them, transferring controlled heat throughout the operation. This water is the actual heat exchanger, being the only component in direct contact with the patient’s skin surface. It is specially designed to allow exposure of specific areas of the body as dictated by the requirements of CABG surgery while allowing for maximal contact of garment–body surface area. All of its components are disposable materials. The garment is puncture resistant, reusable, and can be sterilized.
Before the induction of anesthesia and until the time of initiation of CPB, the Allon system was set to a rectal temperature of 37°C (pre-CPB period). It was discontinued during the CPB period and reintroduced at the time of rewarming by means of the CPB system (post-CPB period; target temperature 37°C). After surgery, rewarming was carried out for up to 4 hours in the cardiac ICU (target temperature 37°C).
Routine thermal care
All patients in the RTC group were warmed using a combination of water blankets (Thermostat T1000, JMW Medical Systems Ltd, Midlothian, UK) and intravenous fluid warmers (Fenwal model BW5, Deerfield, IL) from the time of induction throughout the entire perioperative period. After termination of surgery, they were covered with convective air warmers (Bair Hugger cardiac blanket, Augustine Medical Inc, Eden Prairie, MN) and warm blankets until the end of the postoperative phase of the analysis.
Measurements
Intraoperative data
Intraoperative variables included the total time in the OR, aortic cross-clamp time, bypass time, postbypass OR time, and the number of grafts applied. Rectal and skin temperatures were assessed throughout the perioperative period using calibrated thermistors (AS3 Datex Instrument Corporation, Helsinki, Finland).
Hemodynamic data
The following parameters were evaluated at 15 minute intervals during the four phases of this study: heart rate, blood pressure (perfusion pressure during CPB), cardiac index (CI), systemic vascular resistance (SVR), pulmonary wedge pressure (PWP) (Swan-Ganz Catheter, Baxter Healthcare Corporation, Irvine, CA), amount of positive inotropic drugs administered (indicated if the CI was <2.5 L · min-1 · m-2 and the patient showed hemodynamic instability after disconnection from CPB), and the amount of vasodilative drugs administered for afterload reduction.
Cardiac-specific troponin I (CTNI)
After we had evaluated the first 30 participants in this investigation (AT = 20, RTC = 10), we conducted an interim analysis of the study variables and carried out hemodynamic studies as well (ie, CI, SVR, and PWP). Due to the unexpected differences between the two groups, we assessed myocardial ischemic injury performed in the remaining patients in the AT (n = 18) and RTC groups (n = 10) by determining cTnI serum levels. Blood samples were collected at the exact time points that the hemodynamic variables were taken: at induction, 30 minutes after commencement of surgery, disconnection from CPB, after chest closure, and 2 hours after the termination of surgery. Blood was kept on ice until serum separation. Serum cTnI mass (physiologic range, < 0.5 ng/mL) was determined by an immunoassay using the Abbott Axsym System (Abbott Park, IL).
Postoperative data
Temperature regulation and hemodynamic variables were evaluated during the immediate postoperative period. The occurrence of perioperative myocardial infarction was evaluated by ECG changes and creatine kinase-MB (CK-MB) elevations. Time to extubation in the ICU as well as acute confusional state, renal, and other hemodynamic complications were also assessed. Special attention was paid to potential related adverse events such as extreme changes in body temperature, skin burns, and external pressure sores. The total length of hospital stay was also recorded.
Data analysis
Temperature differences between groups were assessed using analysis of variance. All other continuous variables were analyzed using the Student’s t test. For categorical data, 
2 analysis and Fisher’s exact test were used. All values are given as means ± 1 standard deviation. For all statistical comparisons, a p value less than 0.05 was considered significant.
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Results |
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TopAbstractIntroductionPatients and methodsResultsCommentAcknowledgmentsReferences |
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The mean body mass index was also similar between the two groups (27.1 ± 3.1 kg/m2 for AT vs 26.5 ± 2.6 kg/m2 for RTC, p = 0.41). A comparable number of patients in each group had a history of diabetes (AT = 7, RTC = 6; p = 0.33), hypertension (AT = 22, RTC = 11; p = 1.00), and smoking (AT = 23, RTC = 8, p = 0.27). Hypercholesterolemia was the only statistically significant risk factor of ischemic heart disease that differed between the two patient populations (AT = 32 and RTC = 10; p = 0.03). Preoperative mean arterial pressure was similar in the AT and RTC groups (94.0 ± 11.8 mm Hg and 93.0 ± 14.3 mm Hg, respectively, p = 0.78), as was heart rate (64.3 ± 10.0 beats/min and 66.3 ± 8.3 beats/min, respectively, p = 0.44) (Table 1).
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Operative data
The total number and type of grafts (conventional nonskeletonized left internal mammary artery or saphenous vein) that were placed were similar in the AT and RTC groups (2.8 ± 0.7 and 2.7 ± 0.5, respectively, p = 0.59). No significant differences were observed in the total OR time (184 ± 39 minutes and 182 ± 24 minutes, respectively, p = 0.84), cross-clamp time (57 ± 14 minutes and 60 ± 11 minutes, respectively, p = 0.28), bypass time (72 ± 24 minutes and 66 ± 14 minutes, respectively, p = 0.37), or postbypass OR time (46 ± 10 minutes and 51 ± 9 minutes, respectively, p = 0.08).
Temperature data
Rectal body temperature was higher in the AT group than in the RTC group at all time points, both intraoperatively and postoperatively (Fig 2, bottom panel).
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Patient hemodynamic status
No differences were observed in the mean arterial pressure and heart rate between the groups throughout the entire study period. The CI was higher and the SVR was lower in the AT group throughout the entire study (Fig 3), although no difference was observed in the dosage of dopamine administered to the AT and RTC groups, both intraoperatively (26.5 ± 27.1 mg and 27.1 ± 18.6 mg, respectively, p = 1.0) and postoperatively (157.1 ± 69.3 mg and 147.4 ± 89.7 mg, respectively, p = 1.0). The amount of vasodilatory drug (ie, nitroglycerin) that had been administered was also similar in the AT and RTC groups during surgery (15.5 ± 17.2 mg and 18.3 ± 17.7 mg, respectively, p = 1.0) and in the postoperative period (88.2 ± 59.9 mg and 102.6 ± 71.1 mg, respectively, p = 1.0).
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None of the patients experienced perioperative myocardial infarction. No significant differences in postoperative complications were observed between the groups. There were no adverse effects related to the AT system.
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Comment |
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TopAbstractIntroductionPatients and methodsResultsCommentAcknowledgmentsReferences |
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The results of the present study indicate AT to be an effective strategy for maintaining perioperative patient normothermia. It achieved an improved hemodynamic state and attenuation of myocardial damage (reflected by reduced levels of cTnI) than RC. These factors are important for all patients who undergo surgery but are especially so for those with ischemic cardiac disease and who undergo CABG surgery.
Normothermia versus hypothermia
Hypothermic CPB is believed to provide some degree of protection against myocardial and cerebral ischemia [1, 3]. Even when normothermic CPB is preferred, passive cooling of the patient still occurs, especially during the nonbypass phases of the operation. This is because of the cool environment in the OR and the low metabolic rate caused by the anesthetic agents.
Exposure of body area surface for prolonged periods of time to cold OR temperatures may induce a number of complications, such as coagulopathies [7, 8], shivering during recovery (which increases oxygen consumption) [9, 10], an increased incidence of wound infections [11], and a left shift of the oxyhemoglobin dissociation curve [12].
In this study, patients provided with AT during the nonbypass phases of the operation and in the ICU maintained normothermia throughout all measured time-points during the perioperative period. Given that significant thermal stress and heat exchange occurs while connected to CPB [2], the proposed mechanism of action of the system presented herein is the prevention of skin-surface cooling during the operation rather than resetting the hypothalamic thermoregulation that had been affected by anesthesia.
Hemodynamic effects
The AT patients maintained a better hemodynamic status throughout the various phases of the perioperative period. There were no differences in the amount of either vasodilatory or vasoactive drugs administered to them and the RTC patients. Therefore, the higher CI and lower SVR in the AT group can be attributed to temperature regulation: eg, skin warming minimized the adrenergic response and reduced the afterload, whereupon an elevated cardiac output was maintained.
Cardiac-specific troponin I measurement
It is currently accepted that mild to moderate hypothermia plays a protective role in preventing myocardial damage by reducing metabolic demands and, as a consequence, oxygen consumption. However, hypothermia has also been shown to initiate profound elevations in serum norepinephrine concentrations, thus elevating SVR (ie, cardiac afterload) [13] and eventually necessitating augmentation of cardiac work by the already-ischemic myocardium in order to provide perfusion to body tissues. It is important to bear in mind that such an adrenergic response begins even before anesthesia induction due to patient stress, and is then augmented because of hypothermia during the initial nonbypass phases of surgery.
Numerous studies have shown that complications can occur when reinstalling blood supply to ischemic regions [14]. Although some degree of injury was observed in both the AT and RTC groups as reflected by the release of cTnI, the revascularized hearts of patients in the AT group exhibited a reduced afterload. Myocardial injury during the phase after discontinuation of CPB and return to spontaneous cardiac activity was therefore less pronounced in the AT group (as assessed by cTnI) than in the comparatively hypothermic RTL patients.
As anticipated by the kinetics of cardiac troponin release from injured tissue [15], we believe that the difference in cTnl between our two study groups in the postbypass and recovery stages probably reflects the ongoing differences in temperature and hemodynamic status during the first hypothermic phase of the operation. As such, our results would indicate that maintaining normothermia at specific intraoperative time points results in less myocardial damage.
Conclusions
In this prospective clinical trial, we demonstrated that patients who are provided with the Allon system who undergo CABG surgery maintain normothermia throughout the entire perioperative course. By averting systemic vasoconstriction, a higher cardiac output was maintained and myocardial damage as assessed by cTnI measurements was attentuated. Finally, maintenance of normothermia shortened the time to extubation during recovery in the ICU. Such beneficial effects may reduce patient morbidity and may be especially advantageous in cardiac patients with low cardiac output states. This approach may also be shown to be beneficial in patients undergoing off-pump cardiac surgery for which a shorter recovery time and hospital stay are anticipated.
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Acknowledgments |
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TopAbstractIntroductionPatients and methodsResultsCommentAcknowledgmentsReferences |
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This article has been cited by other articles:
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  S. R. Insler, M. H. Bakri, F. Nageeb, E. Mascha, T. Mihaljevic, and D. I. Sessler An Evaluation of a Full-Access Underbody Forced-Air Warming System During Near-Normothermic, On-pump Cardiac Surgery Anesth. Analg., March 1, 2008; 106(3): 746 - 750. [Abstract] [Full Text] [PDF]
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 Y J. Woo, P. Atluri, T. J Grand, V. M Hsu, and A. Cheung Active Thermoregulation Improves Outcome of Off-Pump Coronary Artery Bypass Asian Cardiovasc Thorac Ann, June 1, 2005; 13(2): 157 - 160. [Abstract] [Full Text] [PDF]
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p < 0.005 between groups.)
