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Anesth Analg 2002;95:1752-1756
© 2002 International Anesthesia Research Society

NEUROSURGICAL ANESTHESIA

Initial Experience with a Novel Heat-Exchanging Catheter in Neurosurgical Patients

Anthony G. Doufas, MD PhD*, Ozan Akça, MD*, Atul Barry, MD{dagger}, David A. Petrusca, MD{ddagger}, Mohammad-Irfan Suleman, MD*, Nobutada Morioka, MD*, John J. Guarnaschelli, MD{ddagger}, and Daniel I. Sessler, MD*§

*Outcomes Research® Institute and Department of Anesthesiology, University of Louisville; {dagger}Department of Anesthesiology, Jewish Hospital Health Care Services, Louisville; {ddagger}Neurosurgical Group of Greater Louisville and Southern Indiana, Louisville, Kentucky; and §Ludwig Boltzmann Institute, University of Vienna, Austria

Address correspondence and reprint requests to Anthony G. Doufas, MD, PhD, Department of Anesthesiology, University of Louisville Hospital, 530 South Jackson St., Louisville, KY 40202. Address e-mail to agdoufas{at}Louisville.edu


   Abstract
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 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Even mild hypothermia provides marked protection against cerebral ischemia in animal models. Hypothermia may be of therapeutic value during neurosurgical procedures. However, current cooling systems often fail to induce sufficient hypothermia before the dura is opened. Furthermore, they usually fail to restore normothermia by the end of surgery, thus delaying extubation. We evaluated a new internal heat-exchanging catheter. Eight ASA physical status II–IV patients (29–72 yr) undergoing craniotomy were enrolled. After the induction of general anesthesia, we introduced the SetPoint® catheter into the inferior vena cava via a femoral vein. The target core body temperature was 34°C–34.5°C. After reaching the target, core temperature was maintained until the dura was closed. Target core temperature was then set to 37.0°C, and the patient was rewarmed as quickly as possible. Seven patients had a tumor resection, and one had an aneurysm clipped. The core-cooling rate was 3.9°C ± 1.6°C/h, and the rewarming rate was 2.0°C ± 0.5°C/h; core temperature was 35.9°C ± 0.2°C by the end of surgery. Patients were subsequently kept normothermic for 3 h before the catheter was removed. No thrombus or other particulate material was identified on the extracted catheters. None of the patients suffered any complications that could be attributed to the SetPoint® system or thermal management.

IMPLICATIONS: Because current systems for inducing therapeutic hypothermia are too slow, we tested an internal counter-current thermal management system during hypothermic neurosurgery. The SetPoint® catheter cooled at 3.9°C ± 1.6°C/h and rewarmed at 2.0°C ± 0.5°C/h. Catheter-based internal thermal management thus seems to be rapid and effective.


   Introduction
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 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Considerable animal data indicate that even mild hypothermia provides substantial protection against cerebral ischemic or hypoxic insult (1,2). Limited human data suggest that therapeutic hypothermia reduces intracranial pressure (3) and is relatively safe (4,5). Hypothermia was reported to be an effective treatment for traumatic brain injury (6), although a subsequent study failed to confirm any benefit (7). Recently, two randomized clinical trials demonstrated that mild therapeutic hypothermia improves neurologic outcomes in survivors of out-of-hospital cardiac arrest (8,9). Hypothermia also seems to improve recovery from catastrophic strokes resulting from middle cerebral artery occlusion (10).

Although benefits of mild intraoperative hypothermia have yet to be demonstrated in neurosurgical patients, mild hypothermia is often used during cerebral aneurysm surgery (5,11,12) because the animal data are overwhelming. A large, randomized prospective clinical trial (Intraoperative Hypothermia for Aneurysm Surgery Trial, Part 2) to evaluate the effect of mild intraoperative hypothermia on neurological outcome after cerebral aneurysm surgery is underway (13). Intraoperative cooling and rewarming are usually induced using surface methods (14), but the limitations of these traditional systems have been repeatedly demonstrated (5,11,14,15) and are almost invariably associated with an imprecise timing of the hypothermic effect.

An alternative method of controlling core temperature is counter-current heat exchange (16). Counter-current heat exchange is routinely used during cardiopulmonary bypass and has also been used to rapidly rewarm trauma victims via temporary arteriovenous fistulae (17). Recently, an internal heat-exchanging catheter was developed to facilitate thermal management of patients for therapeutic hypothermia. A major potential advantage of this system is that heat is directly removed from or added to the thermal core, thus bypassing the heat sink and insulating effects of peripheral tissues. Therefore, we quantified the rate at which this new catheter cools and rewarms neurosurgical patients of normal body habitus.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
The SetPoint® endovascular temperature management system (Radiant Medical, Inc, Redwood City, CA) couples a central-venous heat exchange catheter with a microprocessor-driven controller. The catheter is positioned in the inferior vena cava just beneath the diaphragm (Fig. 1). Cool or warm saline is circulated through the closed loop formed by the catheter and cassette. As blood flows over the catheter, it is cooled or warmed by the saline flowing in the opposite direction inside. Maximum capacity of the system is achieved using the maximum flow rate for the circulating saline with an inflow temperature of 2°C during cooling and 45°C during rewarming. The controller uses the actual patient temperature, sensed by a pair of esophageal probes, to control the temperature of the catheter and thus drive the patient’s temperature to a designated target temperature within the range of 32°C–37°C. The controller is also equipped with a redundant safety system that shuts down and warns the user of patient overheating or overcooling, saline leakage, temperature sensor failure, and electrical or mechanical malfunction.



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  		Figure 1. The SetPoint® system couples a central-venous heat-exchange catheter with a microprocessor-driven controller. The catheter is positioned in the inferior vena cava just beneath the diaphragm. Cool or warm saline is circulated through the closed loop formed by the catheter and cassette (see text for details).

 
The SetPoint® catheter is single-use, heparin-coated, and consists of a triple-lobed, helically wound balloon mounted on the distal portion of a multilumen shaft. The catheter is inserted via the femoral vein through a 10F hemostatic introducer sheath. The catheter has a fluid supply lumen, a fluid return lumen, a guide wire port at the proximal end of the catheter, and a guide wire lumen that allows for distal infusions. Once expanded, the balloon portion of the catheter has a diameter of 9 mm and a length of 25 cm. The catheter has a radiopaque marker mounted at the distal and proximal end of the balloon portion of the catheter; the distal end has a nontraumatic soft tip. The fluid supply and fluid return lumens of the catheter are connected to the cassette via insulated extension lines 2 m in length.

The single-use SetPoint® cassette consists of a thin-walled heat-exchange membrane joined to a pump housing. The pump-housing portion of the cassette has an IV spike and tubing to connect the cassette to a source of sterile saline (250-mL IV bag) after the air has been eliminated from the latter. It also has fluid supply and return lumen ports that attach to the insulated extension lines. The cassette includes a gear that couples to the controller’s integral drive unit that drives the pump head in the cassette. The pump continuously circulates the saline. The cassette can be removed from the portable control console allowing the catheter to remain in the patient to facilitate moving the patient to another location where temperature management can continue using the same or another control console.

With approval of the University Human Studies Committee and written informed consent, we evaluated eight neurosurgical patients undergoing hypothermic craniotomy for tumor resection or aneurysm clipping at University or Jewish Hospitals of the Louisville Medical Center, KY. Patients were excluded from the study if they were <18 or older than 80 yr old, if they had sustained traumatic injuries (direct head or spinal cord injury), if they had a history of coagulopathy or thromboembolic events, if postoperative leg exercise or early ambulation would be contraindicated, if they had an active infection at the time of operation, if they were <1.5 m tall, if they had an inferior vena cava filter in place, if they had a psychiatric history, or had an ASA physical status more than IV.

The anesthetic protocol was similar for all patients, and the ambient temperature in the operating room was kept near 22°C. After the induction of general anesthesia, the SetPoint® catheter was introduced into the inferior vena cava via a femoral vein. Care was taken to position the catheter tip below the end of the xiphoid process. We achieved this by measuring the distance from the insertion point on the patient’s groin to the tip of the xiphoid process before the catheter was inserted. The target core body temperature was set to 34°C–34.5°C on the controller, and the system was activated at maximum capacity. A pair of distal esophageal thermistors, which relay core temperature information to the controller, was also inserted. After reaching the target core temperature, temperature was maintained until the dura was closed. The SetPoint® target temperature was then increased to 37.0°C, and the patients were rewarmed at the maximum rate. Postoperatively, patients were kept normothermic for 3 h before the catheter was removed.

A third esophageal probe was inserted into the distal esophagus to provide a temperature reading independent from the SetPoint® system. This thermocouple, which has an accuracy of near 0.1°C, was connected to a Mon-a-Therm 6510 thermometer (Tyco-Mallinckrodt, St Louis, MO). This thermometer also has an accuracy of near 0.1°C. Demographic and morphometric characteristics of the patients were recorded. Temperatures from the SetPoint® and the independent esophageal thermometer were recorded at 5-min intervals throughout the study. The catheter was thoroughly inspected after removal for any thrombus or adherent particulate material.

Height (m) and weight (kg) were used to calculate the body mass index (BMI = Wt/Ht2) in individual patients. The temperature from the independent esophageal probe was used to calculate individual cooling and rewarming rates using linear regression. The slopes of the individual regressions during cooling and rewarming were averaged among the participating patients. Results are presented as mean ± SD.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
The participants were 51 ± 18 yr old; 4 were women. Seven had tumors resected, and one had an aneurysm clipped. BMI was 29 ± 4 kg/m2. Ambient temperature during the study averaged 22.0°C ± 1.0°C.

Because the SetPoint® system is programmed to slow the cooling rate when the patient body temperature is within 0.3°C of the target, we excluded measurements within 0.3°C of the target temperature from the cooling rate analysis (Fig. 2). Initial core temperature (immediately after the induction of anesthesia) was 36.0°C ± 0.5°C. Cooling in individual patients was highly linear and occurred at a rate of 3.9°C ± 1.6°C/h, r2 = 0.95 ± 0.04. Once the target temperature was reached, core temperature was maintained within a few tenths of a degree throughout surgery. Rewarming was also highly linear and occurred at a rate of 2.0°C ± 0.5°C/h, r2 = 0.94 ± 0.03. Consequently, core temperature was 35.9°C ± 0.2°C by the end of surgery. No hypothermic overshoot was observed.



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  		Figure 2. Cooling and rewarming rates for individual patients and group means (with 95% confidence levels).

 
In one patient, with a target core body temperature set to 34°C, the operation finished earlier than originally anticipated. Rewarming thus started at 35°C in this case. In another patient, active cooling was started at an initial intraoperative temperature that was only 0.3°C higher than the target. The overall temperature curve for the remaining patients is shown in Figure 3.



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  		Figure 3. Average (± SD) intraoperative esophageal temperatures (Tesoph) during the cooling, temperature maintenance, and rewarming periods. Time zero identified the beginning of each thermal management period; the duration of these periods differed in individual patients depending on the duration of surgery and other factors. The regression lines (dashed lines) shown on the graph have been obtained from the average core temperatures during the cooling/rewarming periods. They thus differ from the averages of the individual regression lines reported in the Results section.

 
No thrombus or any particulate material was found on the extracted catheters. None of the patients suffered any complications that could be attributed to the SetPoint® system or thermal management.


   Discussion
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 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Perioperative thermal manipulations are usually directed at the skin surface. However, the induction of core hypothermia using surface cooling is a relatively slow process. One limitation is that forced-air cooling systems transfer relatively little heat (14,18). A second limitation is that skin and peripheral tissue temperature must be reduced considerably before core temperature decreases much. Because heat only flows down a temperature gradient, peripheral tissue temperatures well below 34°C are required before core temperature can even approach that value. This of course limits the rate at which core temperature can be therapeutically reduced.

Postoperative vasoconstriction markedly impairs heat transfer from the skin surface to the core (14,19), whereas peripheral-to-core heat transfer is relatively facile with or without arteriovenous shunt vasoconstriction during anesthesia (18). Intraoperative vasoconstriction nonetheless restricts the rate at which surface cooling reduces core temperature. For example, it takes 1.7 ± 0.4 hours to reduce core temperature to 34°C with forced-air cooling in vasodilated neurosurgical patients; when patients are vasoconstricted, it requires even longer (2.3 ± 0.6 hours) (20). In contrast, our patients required only approximately one hour to reach 34°C. This difference is clinically important because the counter-current system cools quickly enough to reach therapeutic brain temperatures before the dura is opened. Although, an obvious limitation of our study is that we simply evaluated internal catheter-based thermal manipulation without any direct comparison to existing methods.

An additional problem with surface cooling and the consequent large core-to-peripheral tissue temperature gradients is that it promotes a hypothermic overshoot (14) that has been problematic when applied in acute ischemic stroke patients (21). Despite the rapid thermal manipulations induced in our patients, we did not observe a hypothermic overshoot. These data thus suggest that computer-controlled internal cooling is an efficient and accurate way to manipulate the thermal state of the body core.

The heat-exchange catheter allowed us to cool our patients to the target temperature before the dura was opened in most cases and yet rewarm to near-normothermic temperatures by the end of anesthesia. Our patients were thus extubated and protected from the hypothermia-related complications during the immediate postoperative period (22,23).

The rate of rewarming was slower than the cooling rate probably because of the larger thermal gradient between the saline perfusing the catheter and the patient’s temperature during cooling. Much faster rates of rewarming have been achieved in many animal models (24–27) without any demonstrable neurological risk. In patients with cerebral aneurysm surgery, for example, cardiopulmonary bypass was used to induce deep hypothermic circulatory arrest and produce rewarming rates as high as 18°C/h (28). The maximum saline inflow temperature provided by the SetPoint® controller is 45°C; this value is considered to be safe because rapid rewarming using central IV infusion of 65°C fluid was not associated with endothelial or blood component injury (24–26,29).

A study by Baumgardner et al. (30) suggested that IV infusion of ice-cold fluid reduces core temperature far more than expected if the cold load was evenly distributed between core and peripheral tissues. A follow-up study confirmed that central-venous infusion of cold (4°C) fluid decreases core temperature more than would be expected were the reduction in body heat content proportionately distributed (31). This isolation between the core and peripheral thermal compartments not only facilitates core cooling, but also enhances postinfusion spontaneous rewarming that results from constraint of metabolic heat to the core thermal compartment. Infusion-based cooling is an effective method of rapidly inducing therapeutic hypothermia. However, this type of cooling requires the administration of larger volumes of fluid than are optimum for patients undergoing neurological surgery or having limited cardiac function. The internal heat-exchange catheter that we evaluated produced comparable core cooling rates without any vascular volume expansion.

Complications of acute vascular access are generally of two types: those related to catheter insertion (hematoma, puncture, perforation, vascular complications, or hemorrhage) and those related to the presence of an indwelling venous line (thrombosis and infection) (32). We did not observe any complications related to the counter-current heat-exchanging catheter or its insertion. But an obvious limitation of our study is that it was powered to evaluate efficacy rather than safety.

Our results indicate that the SetPoint® internal heat-exchanging catheter is an efficient and accurate target-controlled temperature management system. The system seems to be an effective means of inducing stable mild hypothermia and rewarming to stable normothermia within the time frame of an intracranial neurosurgical procedure.


   Acknowledgments
 
Supported, in part, by Radiant Medical, Inc (Redwood City, CA), National Institutes of Health Grant GM 58273 (Bethesda, MD), The Joseph Drown Foundation (Los Angeles, CA), and the Commonwealth of Kentucky Research Challenge Trust Fund (Louisville, KY).


   Footnotes
 
Presented, in part, at the 2001 Annual Meeting of the American Society of Anesthesiologists, New Orleans, October 13–17.

Tyco-Mallinckrodt, Inc (St Louis, MO) donated the thermocouples that we used. Dr Sessler has a personal financial interest in Radiant Medical, but none of the other authors does; he was not involved in patient recruitment, data acquisition, or analysis of the results.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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Accepted for publication August 28, 2002.




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Lippincott, Williams & Wilkins 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 HighWire Press