Method, and system for selective cerebral hypothermia

[0001] This application is entitled to the benefit of provisional application Ser. No. 60/335,177 filed 2001, Nov. 2.

[0002] 1. Field of Invention

[0003] This invention relates to a method, and system for inducing global cerebral hypothermia while maintaining normal core temperature for the prevention of secondary brain injury from stroke, trauma, or surgery.

[0004] 2. Description of Prior Art

[0005] Patients suffering from stroke or head trauma, or have undergone invasive brain or vascular surgery are at risk from secondary brain injury. Secondary brain injury is a result of the innate healing response of the brain to the original insult caused by several not completely understood mechanisms. Regardless of the specific mechanisms involved, the end result is swelling of the brain caused by edema, which can lead to a critical or terminal rise in intra-cranial pressure.

[0006] It has long been known that hypothermia is neuroprotective. Hypothermia has a positive affect on all know mechanisms that lead to secondary brain injury. Hypothermia is routinely used during brain and other invasive surgeries to protect the brain from surgical interruptions in blood flow. Hypothermia has also been shown to be effective in controlling swelling of the brain in trauma and stroke patients.

[0007] The effectiveness of hypothermia is a function of depth and duration; the deeper the hypothermia, and/or the longer it is applied the more neuroprotective it is. However, hypothermia has historically been applied systemically, and the depth and duration of hypothermia is limited by the patient's ability to tolerate the therapy.

[0008] Systemic hypothermia has historically been accomplished by immersion of the patient's body in a cool bath. Today there are several commercial systemic hypothermia systems available. They consist of blankets or pads where cooled water is circulated through channels in the walls of the blanket or pad, and the patient's body is maintained in intimate contact. Medivan Corp. manufactures an example of a modern hypothermia system under the trade name Arctic Sun Cooling System.

[0009] Systemic hypothermia has been demonstrated to be effective in reducing secondary injury from stroke, trauma, and surgery however, there are several drawbacks to this approach: 1) It takes several hours to lower a patient's body to therapeutic temperatures. This delay in achieving therapeutic temperatures allows for the progression of irreversible secondary injury to the brain. 2) The practical therapeutic hypothermic temperature and duration is limited by the ability of the patient to tolerate, or survive the therapy. 3) The side effects of systemic hypothermia are frequent and can be life threatening, especially in frail patients. Side effects include shivering, cardiac arrhythmia and arrest, pneumonia, infections, and coagulation disorders. 4) The target of hypothermia therapy is the brain; therefore inducing hypothermia systemically places the patient at undue risk. 5) During the “critical phase” (rewarming period) of hypothermia treatment, there is no effective way to manage a sudden and critical increase in intra-cranial pressure, since re-cooling the body to reverse the increase in intra-cranial pressure takes several hours. 6) Systemic hypothermia poses significant clinical and logistical patient management issues.

[0010] There are several examples in the art where catheters are constructed with a cooling means, which is placed into the carotid artery to cool the blood entering the head. This offers an advantage over systemic hypothermia, since it provides a means to cool the head to lower temperatures than the rest of the body, but it still results in systemic hypothermia. Also, since the scientific evidence suggests that hypothermia must be maintained for extended periods of time, there is a great risk that clots will form on the catheters and migrate into the brain leading to episodes of stroke.

[0011] Barbut et al. (U.S. Pat. No. 6,217,552) suggests that cerebral hypothermia may be accomplished by placing a catheter in a lateral ventricle of the brain, and a second catheter into the subarachnoid space, and then pumping cooled fluid from the first catheter in the lateral ventricle to the second catheter in the subarachnoid space where the fluid flows from the lateral ventricle, through the third ventricle, and into the subarachnoid space via the cerebral aqueduct. This approach may be effective in inducing cerebral hypothermia in a normal healthy brain; brains that are suffering ischemia or trauma are often swollen. Swelling of the brain compresses the ventricles and cerebral aqueduct which prevents or limits fluid flow between the lateral ventricle, and the subarachnoid space. One significant problem with inducing cerebral hypothermia by cooling the cerebrospinal fluid in the ventricles is that it results in systemic hypothermia due to the fact that the brain is highly vascular, and that up to ⅓ of the body's heat is generated in the brain. Barut does not suggest a means of maintaining normal core temperature while inducing cerebral hypothermia by ventricle cooling.

[0012] Nowhere in the art is it suggested that directly cooling the walls of a ventricle of the brain may induce global cerebral hypothermia and therefore prevent secondary brain injury. Nowhere in the art is it suggested that cerebral hypothermia can be accomplished by removing the cerebrospinal fluid from a brain ventricle, then inflating a balloon within said ventricle, and then cooling the walls of said ventricle with said balloon. Nowhere in the art is a means taught for inducing cerebral hypothermia by ventricle cooling while maintaining normal core temperature.

[0013] Therefore, it is an object of this invention to provide a method and apparatus for preventing secondary brain injury by inducing selective cerebral hypothermia. In accordance with one aspect of this invention, selective brain cooling is accomplished by placing a cooling device(s) into one or more ventricles of the brain and by placing a heating device into the central vascular system, then cooling said ventricle(s) with said cooling device(s) while simultaneously replacing the heat removed from the body by said cooling device(s) with said heating device. In accordance with another aspect of this invention, secondary brain injury is prevented by placement of a cooling balloon catheter into a lateral ventricle of the brain, then removing the cerebrospinal fluid within the lateral ventricle with said cooling balloon catheter, and then cooling the walls of the lateral ventricle with said cooling balloon catheter thereby cooling the brain. In accordance with another aspect of this invention, selective cerebral hypothermia is accomplished by placing a cooling balloon catheter into a lateral ventricle of the brain and a heating catheter into the central vascular system, then removing the cerebrospinal fluid within the lateral ventricle with said cooling balloon catheter, and then cooling the walls of the lateral ventricle with said cooling balloon catheter, where cooling is accomplished by circulating cold fluid through the balloon of said cooling balloon catheter, and simultaneously heating blood in the central vascular system with said heating catheter thereby replacing the heat removed from the body by the cooling balloon catheter. In accordance with another aspect of this invention, selective cerebral hypothermia is accomplished by placing a cooling device into one or more ventricle(s) of the brain and a heating balloon catheter into the central vascular system, then cooling said ventricle(s) with said cooling devices(s), while simultaneously heating blood in the central vascular system where said heating is accomplished by circulating heated fluid through the balloon of said heating balloon catheter thereby replacing the heat removed from the body by the ventricle cooling device(s). In accordance with another aspect of this invention, secondary brain injury is prevented by placement of a balloon catheter into a lateral ventricle of the brain, and a heating catheter into the central vascular system, then removing the cerebrospinal fluid within the lateral ventricle with said balloon catheter, and then cooling the walls of the lateral ventricle to a predetermined temperature for a predetermined time with said balloon catheter, and simultaneously heating blood in the central vascular system with said heating catheter thereby replacing the heat removed from the body by the cooling balloon catheter. In accordance with another aspect of this invention, secondary brain injury is prevented by placement of a balloon catheter into a lateral ventricle of the brain, then removing the cerebrospinal fluid within the lateral ventricle with said balloon catheter, and then cooling the walls of the lateral ventricle with said balloon catheter to a predetermined temperature where then the temperature is increased gradually over a period of time from the initial low temperature, with the period of time being greater than one hour and less than two months and simultaneously heating blood in the central vascular system with said heating catheter thereby replacing the heat removed from the body by said cooling balloon catheter. In accordance with another aspect of this invention, apparatus for inducing selective cerebral hypothermia includes a ventricle cooling device, a central vascular heating device, and a control console that provides a cooling means for said ventricle cooling device and a heating means for said central vascular heating device and a means to control said cooling and said heating. In accordance with another aspect of this invention, apparatus for inducing selective cerebral hypothermia includes a ventricle cooling balloon catheter that is constructed such that when the balloon is inflated within a lateral ventricle, the balloon conforms to the shape of the lateral ventricle without substantially inducing compressive ischemia within the brain tissue surrounding said lateral ventricle. In accordance with another aspect of this invention, apparatus for inducing selective cerebral hypothermia includes a ventricle cooling balloon catheter that is constructed such that when the balloon is inflated within a lateral ventricle the surface of the balloon substantially maintains intimate contact with the walls of said lateral ventricle between the Inferior Horn of said lateral ventricle and the Anterior Horn of said lateral ventricle. In accordance with another aspect of this invention, apparatus for inducing selective cerebral hypothermia includes a ventricle cooling balloon catheter constructed to be inserted into a lateral ventricle where the catheter enters the lateral ventricle in the vicinity of the Anterior Horn of said lateral ventricle and where the distal end of said balloon catheter may then be advance from vicinity of said Anterior Horn to the Inferior Horn of said lateral ventricle, and may due so in a normal ventricle or in a lateral ventricle that is compressed due to brain swelling.

[0014] Accordingly, besides the objects and advantages of the method and apparatus to induce selective cerebral hypothermia to prevent secondary brain injury described in my patent above, several objects and advantages of the present invention are:

• • [0015] (a) to provide selective cerebral hypothermia to a brain at risk of secondary injury to the degree that offers maximum clinical benefit without inducing hypothermia in the rest of the body;
  • [0016] (b) to provide cerebral hypothermia to a brain at risk of secondary injury where the method for inducing hypothermia takes advantage of the fact that the walls of a lateral ventricle can be cooled by a balloon catheter, and brain tissue surrounding said ventricle may be cooled by heat conduction into the walls of said ventricle to the extent that prevents secondary injury.
  • [0017] (c) to provide selective cerebral hypothermia to a brain at risk of secondary injury within a minimal time after patient presentation where therapeutic temperatures are achieved rapidly due to the fact that only the brain is cooled;
  • [0018] (d) to provide selective cerebral hypothermia to a brain at risk of secondary injury where the degree of hypothermia is adjusted according to the physiological response to hypothermia, where the physiological response to hypothermia is a change in intra-cranial pressure;
  • [0019] (e) to provide selective cerebral hypothermia to a brain at risk of secondary injury where the degree of hypothermia is adjusted according to the physiological response to hypothermia, where the physiological response to hypothermia is a change in patient symptoms.
  • [0020] (f) to provide selective cerebral hypothermia to a brain at risk of secondary injury where the degree of hypothermia is adjusted according to the physiological response to hypothermia, where the physiological response to hypothermia is a change in localized blood perfusion;
  • [0021] (g) to provide selective cerebral hypothermia to a brain at risk of secondary injury where the degree of hypothermia is adjusted according to the physiological response to hypothermia, where the physiological response to hypothermia is a change in the size of the volume of infarcted tissue;
  • [0022] (h) to provide selective cerebral hypothermia to a brain at risk of secondary injury where the degree of hypothermia is adjusted according to the physiological response to hypothermia, where the physiological response to hypothermia is a change in blood chemistry.
  • [0023] (i) to provide apparatus for inducing selective cerebral hypothermia to a brain tissue at risk of secondary injury according to the objectives stated above;
  • [0024] (j) to provide a brain cooling system that consists of a ventricle cooling balloon catheter constructed for use in a lateral ventricle, a heating catheter for use in the central vascular system, and a control console;
  • [0025] (k) to provide a brain cooling system that is constructed to directly cool the walls of a lateral ventricle of the brain;
  • [0026] (l) to provide a cooling balloon catheter that is constructed to be placed into a lateral ventricle of the brain using well known surgical methods;
  • [0027] (m) to provide a brain cooling system that is constructed to provide for long term cooling and indwelling;
  • [0028] (n) to provide a brain cooling system that is constructed to provide for a means to sense a response to cooling;
  • [0029] (o) to provide a brain cooling system that is constructed to provide for a means to control the degree of cooling applied to the surrounding brain tissue.

[0036] FIGS. 1A through 1B depict a sagittal section of a human head 1 and the placement of the cooling balloon catheter 2 into a lateral ventricle of the brain 3. FIG. 1A depicts the placement of ventricle access needle 4 by standard ventriculostomy means. FIG. 1B depicts a guide wire 5 placed into lateral ventricle 3 through ventricle access needle 4 where the guide wire enters the Anterior Horn 6 of the ventricle 3 as shown with the distal tip of the guide wire placed into the Inferior Horn 7 of lateral ventricle 3. Fluoroscopic guidance would normally be used to place guide wire 5 into lateral ventricle 3. FIG. 1C depicts the guide wire 5 in operational position after ventricle access needle 4 is removed. FIG. 1D depicts the cooling balloon catheter 2 in operational position after being slid over the guide wire 5. The cerebrospinal fluid is aspirated from the lateral ventricle trough the guide wire lumen 18. FIG. 1E depicts the cooling balloon catheter 2 in operation where catheter balloon 8 is inflated by cooling fluid 9 circulating within the catheter balloon 8. Note: FIG. 1 depicts a normal lateral ventricle, however, in brains which are swollen the lateral ventricle may compressed and contain little or no cerebrospinal fluid. Using a guide wire to access the Inferior Horn from entry in the vicinity of the Anterior Horn facilitates placement of the cooling balloon catheter in a compressed ventricle.

[0037] FIG. 2 depicts in schematic form an embodiment of the system showing the functional components of the control console 10, the cooling balloon catheter 2, intracranial temperature sensor 20, and the interconnects between the control console 10 and the cooling balloon catheter 2. The functional components of the cooling balloon catheter 2 are: elastic balloon 8, catheter shaft 11, temperature sensor 12, fluid inlet 13, fluid outlet 14, distal radiopaque marker 15, proximal radiopaque marker 16, proximal hub 17, guide wire lumen 18, temperature sensor cable 19, cooling fluid inlet tube 21, and cooling fluid outlet tube 22. The functional components of the control console 10 are: cooling fluid reservoir 23, cooling fluid pump 24, fluid cooler 25, system controller 26, and pressure regulator 27. Ventricle cooling is accomplished by pumping fluid, preferably saline at a temperature between 0 degrees centigrade and 37 degrees centigrade through elastic balloon 8 mounted on the distal end of cooling balloon catheter 2. Cooling fluid is supplied to the cooling balloon catheter 2 by system console 10 in a closed loop fashion. Pump 24 pumps cooling fluid from reservoir 23 through fluid cooler 25 into cooling balloon catheter 2 through cooling fluid inlet tube 21. Cooling fluid traverses catheter shaft 11 and enters elastic balloon 8 through fluid inlet 13 and exits elastic balloon 8 at fluid outlet 14 and then traverses catheter shaft 11 and exits cooling balloon catheter through cooling fluid outlet tube 22 and is returned to reservoir 23. Cooling fluid is pumped into elastic balloon 8 at a constant flow rate between 0.1 liters per minute and 1.0 liters per minute. Pressure regulator 27 maintains pressure within the elastic balloon 8. Pressure within elastic balloon 8 is maintained to the minimum value that overcomes the elastic energy within the walls of the balloon so that the balloon expands sufficiently to make intimate contact with the walls of lateral ventricle 3 without dilating lateral ventricle 3 and causing compressive ischemia in brain tissue surrounding lateral ventricle 3. The pressure required to expand elastic balloon 8 is dependent on the balloon material selected and its construction but should be between 10 mmHg and 100 mmHg. Temperature within the elastic balloon 8 is sensed by temperature sensor 12, which is mounted on the surface of catheter shaft 11 under elastic balloon 8. Signals from temperature sensor 12 are sent to system controller 26 via electrical leads 19. System controller 26 adjusts an output of pump 24, and/or the heat removal power of fluid cooler 25 to maintain desired temperature of the cooling fluid within the elastic balloon 8. Intracranial temperature sensor 20 may be optionally used to control the temperature of the cooling fluid in elastic balloon 8, or may be used to monitor intracranial temperature. Distal radiopaque marker 15, and proximal radiopaque marker 16 provide positional indication for fluoroscopic guidance. Guide wire lumen 18 provides for placement of cooling balloon catheter 2 into a lateral ventricle using a guide wire and also provides for aspiration of cerebrospinal fluid from the lateral ventricle by means of luer fitting (not shown) at the proximal end of the lumen 18. Note: FIG. 2 depicts the elastic balloon 8 pressurized and inflated. Elastic balloon 8 will be normally collapsed about catheter shaft 11 when not pressurized.

[0038] FIG. 3 depicts the selective cerebral hypothermia system that includes the ventricle cooling system as previously described with the addition of a means for warming the body core to replace body heat removed by cerebral ventricle cooling in order to maintain normal body temperature during cerebral hypothermia. This embodiment includes a warming balloon catheter 28 which is placed into the inferior vena cava and heats the blood within the vena cava to provide body core warming. In this embodiment the control console 47 contains a means for providing cooling fluid to cooling balloon catheter 2, and a means to provide warming fluid to warming balloon catheter 28, as well as a means to control said cooling and said warming. (Note: The cerebral cooling function and mechanisms were previously described, therefore the following describes only the functionality and mechanisms for body core warming.) Body core warming is accomplished by warming balloon catheter 28 in integral operation with warming components within control console 47. The functional components of warming balloon catheter 28 are: right atrium temperature sensor 29, right atrium radiopaque marker 30, non-compliant balloon 31, fluid inlet 32, temperature sensor 33, fluid outlet 34, distal balloon radiopaque marker 35, proximal balloon radiopaque marker 36, catheter shaft 37, proximal hub 38, temperature sensor leads 39, fluid outlet tube 40, fluid inlet tube 41 and guide wire lumen 42. The functional warming components within control console 47 are: fluid reservoir 43, pump 44, fluid warmer 45, and fluid warming controller 46. Body core warming is accomplished by pumping fluid, preferably saline at a temperature between 37 degrees centigrade and 50 degrees centigrade through non-compliant balloon 31 mounted near the distal end of warming balloon catheter 28. Warming fluid is supplied to the warming balloon catheter 28 by system control console 47 in a closed loop fashion. Pump 44 pumps warming fluid from reservoir 43 through fluid warmer 45 into warming balloon catheter 28 through warming inlet tube 41. Warming fluid traverses catheter shaft 37 and enters non-compliant balloon 31 through fluid inlet 32 and exits non-compliant balloon 31 at fluid outlet 34 and then transverses catheter shaft 37 and exits warming balloon catheter 28 through fluid outlet tube 40 and is returned to reservoir 43. Warming fluid is pumped into non-compliant balloon 31 at a constant pressure sufficient to fully expand non-compliant balloon 31. Warming fluid would normally be supplied at a pressure between 200 mmHg and 500 mmHg. The diameter of the non-compliant balloon is between 6 mm and 10 mm when fully inflated. The non-compliant balloon may be formed from PET polymer. The non-compliant balloon 31 transfers heat from the warming fluid within to blood flowing across the surface of non-compliant balloon 31. Therefore the diameter of the non-compliant balloon when fully inflated is by design significantly smaller then the blood vessel it resides in which in this embodiment is the inferior vena cava. The temperature within the non-compliant balloon is sensed by temperature sensor 33 mounted on the surface of catheter shaft 37. The objective of this invention is to provide cerebral hypothermia to degree that offers maximum clinical benefit while maintaining normal body core temperature. Since the brain is highly vascularized, when the brain is directly cooled as described by this invention, blood entering the brain is warmer than blood leaving the brain. Blood leaving the brain enters the right atrium of the heart though the superior vena cava. Blood is returned to the right atrium of the heart from the lower body through the inferior vena cava. By warming the blood entering the right atrium from the inferior vena cava in a controlled manner, the cooled blood returning from the brain and entering the right atrium from the superior vena cava will be mixed with the warmed blood such that the blood leaving the right atrium and entering the right ventricle will be at normal body temperature. This controlled mixing of warmed blood, and cooled blood is accomplished by positioning warming balloon catheter 28 into the inferior vena cave through femoral vein access such that the non-compliant balloon (warming mechanism) resides in the inferior vena cava and the distal tip of the warming balloon catheter 28 which contains right atrium temperature sensor 29 resides in the right atrium. Signals from right atrium temperature sensor 29 are sent to warming controller 46 via temperature sensor leads 39. The temperature of the fluid in the non-compliant balloon is adjusted by the warming controller to maintain normal body temperature (37 degrees centigrade) in the right atrium as sensed by right atrium temperature sensor 29. Warming balloon catheter is placed into operational position using standard endovasular catheter techniques. Radiopaque markers 30, 35, and 36 provide positional markers for fluoroscopic guidance.

[0039] FIG. 4 depicts the selective cerebral hypothermia system as described in the paragraph above except that body core warming is accomplished with an electrically powered warming catheter 48 instead of warming balloon catheter 28. Function of warming catheter 48 is identical to the function of warming balloon catheter 28. In this embodiment, electrical warming element 49 of warming catheter 48 replaces the warming function of non-compliant balloon 31 of warming balloon catheter 28 as described above. Power is supplied to electrical warming element 49 from warming element power supply 57 of control console 56 via electrical warming element power leads 53. Power to warming element 49 is controlled by system controller 55. Temperature of the surface of the electrical warming element is sensed by temperature sensor 51 mounted on the surface of electrical warming element 49. Signals from temperature sensor 51, and right atrium temperature sensor 50 are sent to system controller 55 of control console 56. Temperature of the surface of warming element 49 is maintained between 37 degrees centigrade and 50 degrees centigrade by system controller 55 to maintain right atrium blood temperature at 37 degrees centigrade as sensed by right atrium temperature 50. Right atrium radiopaque marker 58 is used to guide placement of the distal tip of warming catheter 48 in the right atrium using fluoroscopy.

[0040] FIG. 5 depicts the construction of the cooling balloon catheter 2. Catheter shaft 11 is made of a medical grade thermoplastic material such as nylon. Catheter shaft 11 consists of three sections (see FIG. 6). Catheter shaft 11 has a guide wire lumen that traverses the catheter from the proximal end to the distal end and can accommodate standard guide wires from 0.010 to 0.032 inches in diameter. Catheter shaft 11 has two lumens for cooling fluid circulation and a separate lumen for the leads of temperature sensor 12. Proximal hub 17 provides communication with lumens in catheter shaft 11. Inlet tube 21, guide wire tube 58, and outlet tube 22 are standard medical luer fitting tubes. Temperature sensor leads are sheathed and terminated with a standard electrical connector (not shown) for removable connection to a control console 10, 47, or 56. Proximal hub 17 is formed by standard insert molding technique. Elastic balloon 8 is a medical grade silicone rubber tube with a wall thickness between 0.006 and 0.010 inches. It has a Shore A durometer of between 20 and 25, with an elongation rating between 800 and 1100%. Elastic balloon 8 is bonded to catheter shaft 11 with RTV adhesive with the adhesive joint reinforced by radiopaque markers 15 & 16 as shown. Fluid inlet 13 and fluid outlet 14 are formed by skiving catheter shaft 11 to access respective fluid lumens. The length elastic balloon 8 is between 5 and 10 cm long.

[0041] FIG. 6 depicts cross sections of cooling balloon catheter shaft 11, and warming balloon catheter shaft 37. Construction of both catheters is very similar. FIG. 6A shows section of catheter shaft 11 & 37 proximal to balloons 8 and 31 showing fluid inlet lumen 59, fluid outlet lumen 60, temperature sensor (under balloon) lead lumen 61, and right atrium temperature sensor lead lumen 62 (warming balloon catheter 28 only), and guide wire lumen 18 & 42. FIG. 6B shows a section of catheter shaft 11 & 37 under balloon 8 & 31 showing fluid inlet lumen 59, guide wire lumen 18 & 42, temperature sensor lead lumen 61, and right atrium temperature sensor lead lumen 62. FIG. 6C shows a section of catheter shaft 11 & 37 distal to balloons 8 & 31 showing guide wire lumen 18 & 42, and right atrium temperature sensor lead lumen 62. The three sections of the catheter shaft are thermally welded together to form the integral catheter shaft by standard means.

[0042] Cooling of the brain may be accomplished by withdrawing cerebrospinal fluid from one or more ventricles, cooling the cerebrospinal fluid ex vivo and reintroducing the cooled cerebrospinal fluid back in the ventricle in a continuous or cyclical cycle. The cerebral spinal fluid within one or more ventricles of the brain may be cooled by placing a cooling probe into one or more ventricles where cerebrospinal fluid is cooled by free convection. The brain may be cooled by other means.

[0043] From the description above there are a number of advantages my method and apparatus for inducing selective cerebral hypothermia for the prevention of secondary brain injury provide:

• • [0044] (a) The therapeutic agent (hypothermia) for preventing secondary injury according to this invention is applied directly to the brain.
  • [0045] (b) The therapeutic agent (hypothermia) for preventing secondary injury according to this invention is limited to the brain.
  • [0046] (c) Lower hypothermic temperatures can be practically achieved in the brain than can be achieved by the methods currently described in the art since only the brain is exposed to hypothermia.
  • [0047] (d) Lower hypothermic temperatures can be achieved in the brain than with methods described in the art.
  • [0048] (e) Hypothermic temperatures can be maintained longer in the brain than with methods described in the art.
  • [0049] (f) The degree of hypothermia in the brain can be adjusted according to the physiological response to hypothermia.
  • [0050] (g) Core body temperature may be precisely maintained at normal during cerebral hypothermia.