Under ordinary circumstances, thermoregulatory mechanisms exist in the healthy human body to maintain the body at a constant temperature of about 37° C. (98.6° F.), a condition sometimes referred to as normothermia. To maintain normothermia, the thermoregulatory mechanisms act so that heat lost to the environment is replaced by the same amount of heat generated by metabolic activity in the body.
For various reasons, however, a person may accidentally develop a body temperature that is above or below normal, conditions known as hyperthermia or hypothermia respectively. These conditions have generally been regarded as harmful and patients suffering from either condition have been treated to return them to normothermia by various mechanisms, including application of warming or cooling blankets, administration of hot or cold liquids by mouth, hot or cold liquids infused into the bloodstream, immersion of the patient in hot or cold baths, and directly heating or cooling blood during cardiopulmonary bypass.
Besides treating undesirable hypothermia to reverse the condition and restore normothermia, medical science recognizes that it is sometimes valuable to intentionally induce and maintain regional or whole body hypothermia for therapeutic reasons. The term “whole body hypothermia” refers to the condition where the whole body temperature, usually measured as the core body temperature, is below normothermia. “Regional hypothermia” refers to the condition where target tissue of one region of the body such as the brain or the heart is maintained at a temperature below normothermia. During regional hypothermia, the core body temperature may be normothermic, or may be slightly hypothermic but is generally warmer than the target tissue.
It may be desirable, for example, to induce whole body or regional hypothermia for the purpose of treating, or minimizing the adverse effects of, certain neurological diseases or disorders such as head trauma, spinal trauma and hemorrhagic or ischemic stroke. Additionally, it is sometimes desirable to induce whole body or regional hypothermia for the purpose of facilitating or minimizing adverse effects of certain surgical or interventional procedures such as open heart surgery, aneurysm repair surgeries, endovascular aneurysm repair procedures, spinal surgeries, or other surgeries where blood flow to the brain, spinal cord or vital organs may be interrupted or compromised. Neural tissue such as the brain or spinal cord, is particularly subject to damage by blood deprivation for any reason including ischemic or hemorrhagic stroke, cardiac arrest, intracerebral or intracranial hemorrhage, and head trauma In each of these instances, damage to brain tissue may occur because of brain ischemia, increased intracranial pressure, edema or other processes, often resulting in a loss of cerebral function and permanent neurological deficits. Hypothermia has also been found to be advantageous to protect cardiac muscle tissue during or after ischemia, for example during heart surgery or during or after a myocardial infarct.
Traditional methods inducing and/or maintaining hypothermia include application of surface cooling such as an ice bath or cooling blankets, infusing cold liquid into the vascular system of a patient, or controlling the temperature of a patient's blood during cardiopulmonary bypass. While each of these may be useful in certain settings, they each have significant disadvantages. For example, inducing hypothermia by placing a patient into a cold bath lacks precise control over a patient's core temperature and thus may result in harmful overshoot, which may be difficult if not impossible to reverse with any degree of control. It generally cannot be used in conjunction with surgery because sterility and access to the patient's body may make its use impractical or impossible. Cooling blankets are often too slow to cool the patient, or simply unable to overcome the body's natural ability to generate heat, particularly if the patient is shivering or experiencing vasoconstriction. Even if the patient is anesthetized, or has otherwise had his thermoregulatory responses impaired or eliminated, cooling by means of cooling blankets is still often too slow and inefficient to be useful. Control over the patient's temperature is generally poor, which is particularly dangerous if the patient's own thermoregulatory controls are eliminated or impaired.
Infusion of cold or hot fluid into a patient's bloodstream has also been used to affect the temperature of a patient. However, this procedure is severely limited because of the hazards of fluid loading. Particularly where hypothermia is to be maintained for a long period of time, continuous infusion of sufficient cold liquid to counter the heat generated by ordinary bodily activity creates an unacceptable amount of fluid introduced into the body. In addition, as with the methods described above, control over the patient temperature is limited.
Another method sometimes employed, especially during heart surgery, is cardiopulmonary bypass, where blood is removed from the body, oxygenated and returned to the circulatory system by means of a mechanical pump. While being circulated outside the body, the temperature of the blood may be controlled by directly heating or cooling it and then pumping it back into the body, and in this way the temperature of the entire body of the patient may be controlled. Because of the large volume of blood removed, treated, and pumped back into the body, heating or cooling the body by means of cardiopulmonary bypass is very rapid and may be precisely controlled. However, the use of an external mechanical pump to circulate blood tends to be very destructive of the blood and thus physicians try to minimize the time on which the blood is being subjected to this treatment, preferably to four hours or less. Furthermore, the situations in which the use of this method for temperature control is very limited because of the extremely invasive nature of cardiopulmonary bypass. The patient must be anesthetized, highly trained personnel are required, and the procedure is only available in an operating room or similarly equipped facility.
Intravascular heat exchangers have been developed to control patient temperature for either treating hypothermia or hyperthermia or inducing and maintaining hypothermia The intravascular heat exchanger overcomes many of the shortcomings of the above mentioned methods while permitting the advantageous aspects of controlling patient temperature. The intravascular heat exchanger comprises a catheter in which heat transfer fluid is circulated between an external heat exchanger, such as a solid state thermoelectric plate of one or more Peltier cooling units and a heat transfer region such as a balloon region on the end of the catheter. The heat exchange region is inserted into the vasculature of a patient. The heat transfer fluid exchanges heat with the blood at the heat transfer region to change the temperature of the blood and thus of the patient. The heat transfer fluid is then circulated out of the body and exchanges heat with the external heat exchanger outside the body to add or remove the heat lost or gained from the blood. In this manner the temperature of the blood and ultimately of the patient may be controlled by controlling the temperature of the external heat exchanger.
Some intravascular heat exchange catheters may be designed to affect a small amount of tissue, for example a small bolus of blood in thermodilution catheters (see e.g. Williams, U.S. Pat. No. 4,941,475) or catheters designed to protect or affect the tissue in contact with the catheter (see e.g. Neilson, et al., U.S. Pat. No. 5,733,319). However, intravascular heat exchangers designed to affect whole or regional body temperature may be expected to exchange a significant amount of energy, for example more than 100 watts. This is achieved by maintaining a maximum difference in temperature between the blood and the heat transfer region (ΔT), and flowing a maximum amount of heat exchange fluid through the circuit. A heat exchange fluid that can be maintained between 0° C. and 45° C. is generally preferable, along with a fluid supply system that can supply adequate flow of heat transfer fluid and temperature control of that fluid. Such systems ideally will also have one of more of the following properties: maximum external heat exchange ability, closed circuit for sterility, small volume for precise and rapid control of temperature, a system for pressure regulation to precisely control flow rate, optimal flow rate, disposable features, ease of handling, and reliability.
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