Asian Cardiovasc Thorac Ann 2005;13:157-160
© 2005 Asia Publishing EXchange Ltd
Active Thermoregulation Improves Outcome of Off-Pump Coronary Artery Bypass
Y Joseph Woo, MD,
Pavan Atluri, MD,
Todd J Grand, BS,
Vivian M Hsu, BA,
Albert Cheung, MD1
Division of Cardiothoracic Surgery, Department of Surgery
1 Division of Cardiovascular Anesthesia, Department of Anesthesia, University of Pennsylvania School of Medicine, Philadelphia, USA
For reprint information contact: Y Joseph Woo, MD Tel: 1 215 662 2956 Fax: 1 215 349 5798 Email: wooy{at}uphs.upenn.edu, Division of Cardiothoracic Surgery, Department of Surgery, University of Pennsylvania, Silverstein 4, 3400 Spruce St., Philadelphia, PA 19104, USA.
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ABSTRACT
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During off-pump coronary artery bypass grafting, hypothermia increases vasoconstriction, myocardial afterload, coagulopathy and postoperative bleeding. Traditional thermoregulatory techniques do not maintain core body temperature intraoperatively. The efficacy of a commercially available, computer-controlled, water-circulating, dorsal surface, active warming system for thermoregulatory control was evaluated. All patients who underwent non-emergency off-pump coronary bypass grafting by a single surgeon in a 1-year period were studied: the thermoregulation device was used in 50 cases and unavailable for use in 19. The patients who underwent active thermoregulation demonstrated significantly improved core body temperatures compared to the controls: lowest intraoperative, 35.8°C ± 0.1°C vs. 35.0°C ± 0.2°C; immediately postoperative, 36.5°C ± 0.1°C vs. 35.6°C ± 0.2°C; and 1-hour postoperative, 36.6°C ± 0.1°C vs. 35.9°C ± 0.2°C. Thermoregulated patients had significantly reduced 24-hour chest tube drainage (764 ± 38 vs. 1227 ± 183 mL), packed red blood cell transfusions (1.4 ± 0.2 vs. 3.3 ± 0.7 units), time to extubation (6.8 ± 0.5 vs. 11.4 ± 2.3 hours), intensive care unit stay (1.3 ± 0.1 vs. 2.0 ± 0.3 days), and hospital stay (4.3 ± 0.1 vs. 5.1 ± 0.3 days).
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INTRODUCTION
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With the increased application of off-pump coronary artery bypass (OPCAB), the challenges of intraoperative thermoregulation, previously not seen with the cardiopulmonary bypass (CPB) circuit, have been recognized. Often, the mediastinum, both hemithoraces, the lower extremities, and possibly the upper extremities, need to remain exposed during surgery, leaving a substantial area of the body surface subject to conductive heat loss. Furthermore, general anesthesia induces hypothermia, typically resulting in a drop of 1°C to 3°C in core temperature.1 Raising ambient room temperature and warming intravenous fluids and ventilator gases have limited efficacy in maintaining normothermia. Warm air circulating blankets are cumbersome and pose a theoretical risk of blowing non-sterile air around the operative field. Selective hot air heating of the head is not ideal as it results in increased cerebral temperatures relative to the body. Active warming of the body allows for tight regulation of core body temperature while maintaining a comfortable operating room environment. Previous studies have demonstrated a significant difference in core body temperature with active thermoregulation, but effects on patient outcomes have not been assessed.2,3 A commercially available, computer-controlled, water-circulating, dorsal surface, active warming system was utilized during OPCAB in this study, and for the first time, improvements in clinically measurable patient outcomes were documented.
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PATIENTS AND METHODS
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All 69 consecutive patients who underwent non-emergency primary OPCAB via a median sternotomy by a single surgeon during a one-year period were retrospectively analyzed. Patients randomly received the thermoregulatory device, depending upon device availability, which was primarily influenced by other cardiac surgical procedures scheduled that day. Thus, the 50 thermoregulated and 19 control patients were concurrently intermingled. All patients received standard core body temperature maintenance measures, such as warming of intravenous fluids, ventilator gases, and the operating room. The commercially available active warming device consisted of an adherent gel-containing pad placed on the posterior thorax through which warm water up to 42°C was circulated under negative pressure and computer-controlled via feedback from a core temperature probe (Arctic Sun; Medivance, Louisville, CO, USA). A core temperature of 37°C was targeted. Core temperatures were measured by either bladder temperature probes or pulmonary artery catheter temperature probes.
Both groups of patients exhibited similar preoperative characteristics (Table 1
). Postoperative care was standardized. The team, consisting of the cardiothoracic surgeon, cardiothoracic anesthesiologist, surgical residents, and nurse practitioners, was blinded to whether an active warming device was used intraoperatively. Specific criteria for extubation consisted of neurologic recovery with the ability to maintain and protect a patent airway, adequate reversal of muscle paralysis with recovery of respiratory muscle strength as determined by the Tobin index, adequate ventilation and oxygenation as determined by arterial blood gas analysis, hemodynamic stability, and absence of significant mediastinal hemorrhage likely to require surgical reexploration. Specific criteria for transfer from the intensive care unit (ICU) comprised a stable neurologic examination, hemodynamic stability, absence of heart block requiring external pacing, minimal chest tube output, absence of vasoactive medication infusion, and a patent airway with adequate spontaneous ventilation and oxygenation. Blood product transfusion was reserved for hemoglobin levels < 8.0 g·dL–1, active mediastinal hemorrhage, or hemodynamic instability. Multiple perioperative parameters were retrospectively compared between the two groups and analyzed by the Mann-Whitney U test using 95% confidence intervals.
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RESULTS
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Patients underwent an equivalent extent of revascularization: the number of grafts per patient was 2.63 ± 0.21 in the control group, and 2.98 ± 0.11 in the thermoregulated group (p > 0.05). Thermoregulated patients began OPCAB at the same core temperature as control patients, but remained at a more physiologic temperature intra- and postoperatively (Table 2). Control and thermoregulated patients had equivalent preoperative hemoglobin levels. Notably, thermoregulated patients bled less postoperatively and required fewer transfusions (Table 3). At the time of hospital discharge, hemoglobin levels were similar in both groups. The thermoregulated patients were extubated more quickly, transferred from the ICU sooner, and discharged from the hospital earlier (Table 4). In either group, there were no conversions to on-pump operations. There was no major perioperative morbidity including cerebrovascular accident, myocardial infarction, mediastinal hemorrhage requiring operative reexploration, respiratory failure requiring reintubation, renal failure, or sternal wound infection. There was no 30-day postoperative mortality.
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DISCUSSION
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Hypothermia has adverse hematologic and physiologic consequences leading to diminished cardiac function, coagulopathy, blood loss, and prolonged postoperative recovery. There is a significant association between hypothermia and increased systemic vascular resistance and lower cardiac output, ultimately resulting in increased cardiac injury as measured by cardiac-specific troponin I levels.2 Increasing core body temperature improves cardiac output and minimizes cardiac injury.2,4,5 This study showed that aggressive maintenance of more physiologic temperatures during OPCAB reduced postoperative bleeding, transfusion requirements, mechanical ventilatory time, and length of ICU and hospital stay. Although not directly investigated in this initial report, these results may be due in part to the improvement in cardiac afterload and blood component function. These effects may become even more pronounced in patients requiring extensive off-pump revascularization or reoperations.
Hypothermia has a profound effect on the coagulation cascade. Porcine models have demonstrated hypothermia-associated prolonged prothrombin and activated partial thromboplastin times due to impaired enzyme kinetics of clotting factors, which correct with a return to normothermia.6 Hypothermia inhibits thrombin and thromboxane-induced platelet aggregation, surface expression of P-selectin, and expression of the glycoprotein Ib-IX complex.7 This spectrum of abnormalities results in prolonged bleeding time that is reversible on return to normothermia.7 The trauma population has provided significant insight into the mechanism of hypothermia-induced coagulopathy. The triad of hypothermia, coagulopathy, and acidosis is a well-documented phenomenon. Core temperature of less than 34°C, acidosis, and hypotension are independent risk factors for coagulopathy.8 Hypothermia results in a statistically significant slowing of enzymatic activity and decreased platelet function, each independently lengthening the coagulation cascade.9 Prolonged postoperative hypothermia is associated with extended periods of coagulopathy, increased blood loss, and greater transfusion requirements. Studies of coagulopathy in patients undergoing on-pump coronary artery bypass grafting have shown a significant increase in the utilization of all blood products with decreased core body temperature.10 In practice, adequate resuscitation requires not only the repletion of clotting factors but also the normalization of core body temperature for optimal coagulation.11
Hypothermia also prolongs the time to extubation, postoperative recovery, and ICU discharge. A recent retrospective review compared patients undergoing traditional OPCAB with those on "ultra-fast track anesthetic", a technique to counter hypothermia that includes the use of intravenous fluid warmers, pre-warmed skin preparation, a circulating water warmer blanket, humidified inspired gases, a forced air warmer, and elevated operating room temperature.12 In contrast to patients receiving standard anesthesia, those who experienced the thermoregulatory maneuvers underwent successful extubation in the operating room.12 The combination of active rewarming and an altered anesthetic regimen to offset hypothermia gave a median extubation time of 1.5 hours, allowing for a lower nurse-to-patient ratio and decreased ICU monitoring.12 Early extubation has been associated with improved intrapulmonary shunt fraction and reduced ICU and hospital stay, without an increase in morbidity.13
In the high-risk cardiac population undergoing peripheral vascular, abdominal or thoracic surgery, hypothermia increases the risk of significant vasoconstriction with increases in afterload, ischemic electrocardiographic changes, ventricular tachycardias, and other morbid cardiac events.14 These changes are consequences of both an enhanced adrenergic response associated with hypothermia and increased metabolic demands associated with internal thermoregulatory actions such as shivering. An enhanced adrenergic response results in elevated norepinephrine levels, higher systemic arterial blood pressure, and increased peripheral vasoconstriction, all variables that increase myocardial afterload and cardiovascular morbidity in hypothermic post-cardiac surgery patients.15 At the cellular level, hypothermia produces exponential decreases in intrinsic enzymatic activity leading to ionic dysregulation; for example, Na+/K+ ATPase dysfunction disrupts Na+/Ca2+ exchange, resulting in altered intracellular ion concentrations, disturbed membrane potentials, and massive cellular hypercalcemia.16
Although this study is somewhat limited by the retrospective nature of its design, it was found that the application of active thermoregulation during OPCAB equates with an approximate difference of 1°C in core temperature, one or more units of packed red blood cell transfusion, and 1 day of hospitalization. The preservation of core body temperature is consistent with previously published studies, which unfortunately were limited by lack of analysis of patient outcomes.2,7 Although not directly examined in this study, the combination of fewer transfusions and shorter length of hospital stay should make intraoperative active thermoregulation highly cost effective. Given these preliminary findings, use of an active warming system has become the standard in our practice for all OPCAB cases. Furthermore, utilization of this device has become routine in our patients undergoing non-CPB procedures such as lung transplantation, partial CPB cases such as thoracoabdominal aortic repairs, robotic procedures, operations involving profound hypothermic circulatory arrest, and CPB cases requiring prolonged pre- and post-CPB periods.
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ABSTRACT
INTRODUCTION
