Abstract:
A cardiopulmonary bypass patient is precooled using an indwelling catheter. Cardiopulmonary bypass is initiated when a target temperature or range are achieved, as determined by automatic temperature feedback provided to a control module. The patient may also be rewarmed at a controlled rate during or after termination of cardiopulmonary bypass such that faster and safer termination is realized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to cardiopulmonary bypass procedures, and more particularly, to controlling patient temperature in conjunction with said procedures. 
     2. Description of Related Art 
     Cardiopulmonary bypass surgery (CPB) is one of the most common surgical procedures performed in the United States. During CPB, the heart is stopped and the blood which normally returns to the right side of the heart passes through a pump and oxygenating system and is returned to the aorta, thereby bypassing the heart and lungs. 
     Primary goals of cardiopulmonary bypass for heart surgery are to provide life-support functions, a motionless, decompressed heart, and a dry, bloodless field of view for the surgeon. In a basic heart-lung life-support system oxygen-poor blood is diverted from the venous circulation of the patient and is transported to the heart-lung machine where reoxygenation occurs, carbon dioxide is discarded and heat regulation (warming or cooling) is accomplished. This processed blood is then returned (perfused) into the patient&#39;s arterial circulation for distribution throughout the entire body to nourish and maintain viability of the vital organs. 
     Although a common procedure (in excess of 400,000 open heart procedures per annum are conducted in North America) and although tremendous strides have been made so that open heart surgery is safer for patients, the procedure is not without its dangers. Further, while the vast majority of patients have marked improvement in their cardiac functional status following their procedure, of concern is the potential for damage to other organ systems which can result from the CPB procedure. 
     Particularly, time on bypass is positively and independently correlated to adverse outcome of CPB, and reducing pump time is a clinically meaningful measure of device performance. Neurocognitive deficits are associated with CPB and attributed to emboli in the arterial circulation inevitably associated with arterial cannulation, surgical procedures and large, complex extracorporeal devices. 
     Neurologic and other embolic related sequelae are typical for surgery where CPB is used. The reason for these problems is that emboli from various sources are launched into the arterial circulation as a result of extracorporeal circulation and procedures inside the heart. These emboli are in the arterial circulation and pass into the major organ systems throughout the body, without benefit of capture by the lungs. Emboli larger that blood cells (8-15 microns) lodge in the arterioles and capillaries and cause ischemic areas corresponding to the areas perfused by the occluded blood vessel. 
     Conventionally, various means are employed to either prevent the formation and release of emboli into the arterial blood circulation or filter or trap blood-borne emboli prior to infusion into the patient&#39;s arterial circulation. Examples of filters and traps are screen or depth type filters in the extracorporeal blood circuit. These filters or traps may be in reservoirs, integral to blood gas exchange devices (oxygenators), cardiotomy reservoirs, and arterial line filters. Antithrombotic coatings may be applied to extracorporeal devices and cannulae to prevent thromboemboli. Carbon dioxide flushes may be used to displace air (carbon dioxide is much more soluble in blood than air) from extracorporeal circuits and reduce the potential for air bubbles. Not withstanding the above measures, emboli and the associated neurologic sequelae are a feature of CPB. 
     Filtration methods for removing emboli from blood are limited by the cellular nature of blood and the blood&#39;s propensity to form thrombi when exposed to artificial surfaces and/or shear forces. As a filter&#39;s pore size approaches that of the blood cells (8-15 microns) the pressures needed to achieve sufficient flow are increased or the area of the filter must be increased to impractically large size. Additionally, as the shear forces adjacent to artificial surfaces increase, platelet aggregates and/or fibrin thrombi formation ensue on the downstream side of the filter and, paradoxically, create blood emboli. 
     Actions and manipulations of the patient and equipment for CPB cause emboli in the arterial blood flow. It has not been possible to practically eliminate all the emboli so caused. For example, insertions of the venous and arterial cannulae cause small pieces of cut or torn tissue to enter the blood. Cardiotomy suction blood (typically filtered and returned to the CPB circuit) has air, fat and tissue emboli that can only be partially filtered (for reasons previously mentioned) out of the blood before going into the arterial circulation. As part of CPB, large clamps are applied and released at various times to stop/start blood flow in major blood vessels and this action causes damage to the blood vessel lumen and creates stagnant, clot prone areas near the clamp. Subsequent movement and eventual release of clamps has been shown to launch measurable emboli into the arterial (including cerebral) circulation. 
     Cooling the patient is routinely employed in CPB. This is accomplished by heat exchangers in the extracorporeal circuit. The benefits of cooling to protect the patient from ischemic insult are well recognized. However, conventionally the patient is at normal temperature at the time that CPB is initiated and no cooling protection is afforded until after the initial embolic insult. 10-20 minutes may be required after the start of CPB to reach the desired hypothermic temperature. Significant emboli (tissue, particulate, air and thrombus) are released at the precise time that CPB is initiated and the patient has not yet cooled below the normothermic range (36.0-37.5° C.). 
     Cooling also provides the benefit of increasing the margin of safety in case of equipment failure, whereby patient metabolism is reduced by the cooled blood being reintroduced into the body, in turn reducing the body&#39;s need for oxygen and the tolerance for its deprivation in the event of such failure. Specifically, an 8-10% decrease in oxygen consumption is correlated with each degree (Celcius) drop in body temperature. Thus it is not uncommon to lower patient core body temperature to about 32° C., or even lower, during CPB. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to methods and systems for controlling patient temperature during cardiopulmonary bypass surgery. In one embodiment, a patient undergoing cardiopulmonary bypass surgery is precooled using an indwelling catheter inserted into the central venous system of the patient. The indwelling catheter operates to cool the patient, lowering core body temperature before cardiopulmonary bypass is initiated. A pre-determined patient target temperature can be set so that cooling is terminated or patient target temperature is automatically maintained when the target temperature is reached. 
     In a second embodiment, the indwelling catheter can be used to control the rate of patient rewarming once the cardiopulmonary bypass surgery is near or at completion. Controlled rewarming can be encompassed by applying heat to the patient&#39;s blood flow using the indwelling catheter. Additionally, the caregiver can control the rate at which the patient is rewarmed by selecting the pump speed and bath temperature of the temperature control module. 
     It is the object of the present invention to provide a systemic heat exchange method and system that are effective, are easy to use and require minimal added work for medical personnel. Additional objects and advantages of the invention will be set forth in part in the description which follows, and may be obvious from the description or learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the method acts, instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
     FIG. 1 is a schematic diagram showing temperature control in accordance with the invention; 
     FIG. 2 a  is a perspective view of a catheter in accordance with the invention; 
     FIG. 2 b  is a perspective view of another catheter in accordance with the invention; 
     FIG. 2 c  is a cross-sectional view taken along line  6 — 6  of FIG. 2 b;    
     FIG. 3 is a is a cross-sectional view taken along line  3 — 3  of FIG. 2 a;  and 
     FIG. 4 is a sectional view of a catheter balloon in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an arrangement in accordance with the present invention. A patient  20  is schematically depicted undergoing cardiopulmonary bypass (CPB) surgery using a CPB device  26 , wherein patient blood is conveyed to and from the CPB device via a tube set  28 . During CPB, device  26  operates to provide life-support functions, a motionless, decompressed heart, and a dry, bloodless field of view for the surgeon. In a basic heart-lung life-support system oxygen-poor blood is diverted from the venous circulation of the patient  20  and is transported to CPB device  26  where reoxygenation occurs, carbon dioxide is discarded and heat regulation (warming or cooling) is accomplished. This processed blood is then returned (perfused) into the patient&#39;s arterial circulation for distribution throughout the entire body to nourish and maintain viability of the vital organs. 
     FIG. 1 also shows a catheter  30  implanted in patient  20 . Catheter  30 , discussed in greater detail with reference to FIG. 2 below, is implanted in the patient and serves to remove heat from, or add heat to, the patient&#39;s blood flow in accordance with a temperature control device  40 . Catheter  30  is in thermal communication with control unit  40  which regulates the core body temperature of patient  20  by controlling the temperature of the catheter. Thermal communication between catheter  30  and control unit  40  can be effected in a variety of ways. Preferably, a heat exchange fluid circuit  43  is used, wherein fluid is circulated through catheter  30  in a closed loop which includes intake and outflow tubes  22  and  24 , respectively. Pump  41  provides the force for circulating the heat exchange fluid. The heat exchange fluid in fluid circuit  43  is in heat exchange relationship with a water bath  42  of control unit  40 . The temperature of water bath  42  is controlled in accordance with an output from a temperature control module  44 , which module receives patient core body temperature information from a probe  46 . Control unit  40  contains a cooler (not shown) for cooling water bath  42 . Control unit  40  may also contain a heater (not shown) for heating the water bath  42 . Heating may also be accomplished using an electrical resistance heating element (not shown) or other means disposed on catheter  30 . 
     Although depicted as occupying a location in the patient which is different from that of catheter  30 , probe  46  may in fact be disposed on the catheter and therefore lie in the same location—that is, it may be disposed within a region of venous blood circulation, the preferred location of catheter  30  as discussed in further detail below. It is preferred, however, that probe  46  be of the bladder, esophagial, rectal or tympanic type. 
     Temperature control module  44  preferably includes a programmable processor (not shown) which receives input from an operator through an input device such as keypad  48 . Using keypad  48 , the operator can input various parameters for the operation of control unit  40 . One such parameter is a targeted patient core body temperature level, which, in a precooling operation, may be about 32° C. To achieve this core body temperature, water bath  42  operates to cool the circulating fluid to about 4° C. 
     FIGS. 2 a  and  3  show in greater detail an exemplary arrangement of catheter  30  in accordance with the invention, with FIG. 3 being a cross-sectional view taken along lines  3 — 3  in FIG. 2 a.  Catheter  30  is an essentially tubular structure of about 8.5 French diameter. Catheter  30  includes a heat exchange region, such as one or more exterior balloons  32  in fluid communication with internal lumens  31  and  35  formed in the catheter. At the proximal end of catheter  30 , lumens  31  and  35 , along with a central lumen  33 , extend into tubes  51 ,  53  and  55 , which tubes are provided with appropriate fittings  52 ,  54  and  56  for connection to suitable devices (not shown). A suture anchor  57 , from which tubes  51 ,  53 , and  55  emerge, may be provided for anchoring catheter  30  to the patient  20  during operation. One or more radiopaque markers (not shown) may also be provided to aid with catheter visualization, or the tubular structure of the catheter, which is made of biocompatible material such as biocompatible polyurethane, may be impregnated with radiopaque material, such as barium sulfate. Depth markers (not shown) may also be provided to aid in insertion and manipulation. 
     Central lumen  33  provides a conduit for passage of a guidewire (not shown) through catheter  30 . The guidewire, which is typically about 0.032 to 0.035 inches in diameter, may be used as in conventional practice to guide the catheter  30  through the patient&#39;s body to the appropriate location during initial introduction of the catheter. To that end, central lumen  33  communicates with the exterior of catheter  30  through fitting  54  at the proximal portion, and through aperture  58  at the distal portion, or tip  59 , of catheter  30 . Central lumen  33  may also be used to provide a conduit for passage of infusate to the body, or for removal of fluid such as blood therefrom. 
     Side lumens  31  and  35  are contiguous with fluid circuit  43  (FIG.  1 ), providing fluid flow paths for heat exchange fluid to circulate in catheter  30 . Lumen  31  is an inflow lumen, extending through tube  51  to communicate with tube  22  of circuit  43 . Lumen  35  is an outlflow lumen, extending through tube  55  to communicate with tube  24  of circuit  43 . Thus fluid in circuit  43  enters catheter  30  through lumen  31  and exits catheter  30  through lumen  35 . 
     With reference to FIG. 4, balloons  32  are formed exteriorly of catheter  30  and each comprise a tubular sheet of pliant material  34 , such as extruded polymer, which is sealed at both ends against the exterior body structure of catheter  30 , such that a cavity  36  bounded by the catheter and the tubular sheet of pliant material is formed. Inflow lumen  31  communicates with cavity  36  through a supply port  37 , whence heat exchange fluid enters balloon  32  and causes the balloon to inflate. The fluid circulates through balloon  32 , and exits at return port  39  into outflow lumen  35 . When inflated, the diameter of each of balloons  32  expands to about 5-8 mm. 
     In another embodiment of a catheter  150 , as shown in FIGS. 2 b  and  2   c,  at least two central venous components can be in communication with the catheter  150  for undertaking central venous functions in addition to controlling the temperature of the patient. These functions include and are not limited to drug infusion, blood extraction and blood pressure monitoring. For instance, a blood monitor  60  can communicate with the catheter  150  via a line  62  to monitor blood pressure or withdraw blood from the central venous system of the patient. Also, a drug source such as a syringe  64  can engage the catheter  150  via a connector with line  66  for infusing drugs or other medicament into the patient. The components  154 ,  60  and  64  can all be connected to the catheter  150  via a proximal connector hub  68  of the catheter  150 . The hub  68  can be formed with a suture anchor  70  or other anchor structure such as tape for providing a means to fasten the catheter  150  to the skin of the patient for long-term use. Also, a guide wire lumen tube  72  may be engaged with the hub  68  and extend therethrough to a guidewire lumen. 
     Turning to the catheter  150 , a preferably plastic, flexible catheter elongate body  74  extends distally away from the hub  68 . The body  74  is biocompatible, and can be coated with an anti-microbial agent and with an anti-clotting agent such as heparin. The body  74  can be a unitary piece of hollow plastic or it can be made of more than one coaxial tube. Distally bonded to a portion or the body  74  is a comparatively more rigid frusto-conical shaped guide head  76 , an open distal end of which can establish a distal infusion port  77 . 
     A flexible, collapsible, helical-shaped heat exchange elongate element  78  surrounds the body  74 . The heat exchange element  36  can be made of a plurality of discrete turns that are formed separately from each other and then joined together to communicate with each other. However, in a more preferred embodiment more easily fabricated, the elongate element  78  is a single, unitary tube made of very thin catheter balloon material that extends from a first end  80  to a second end  82  and the element  78  includes a heat transfer lumen extending longitudinally therethrough. The heat transfer lumen is in fluid communication with an input lumen  84  which is in turn in communication with the supply line  56 . The heat transfer lumen of the element  78  is also in communication at the second end  82  with an output lumen  86  communicating with the return line  58 . The elongate element  78  is in communication with the output lumen  86  at the second end  82 . Thus, working fluid flows distally through the input lumen  84 , into the helical transfer lumen of the elongate element  78 , and then proximally back through the element  78  and the output lumen  86 . In a separate embodiment, the working fluid flows proximally through the input lumen  84 , into the helical transfer lumen of the elongate element  78 , and then distally back through the element  78  and the output lumen  86 . 
     In addition to the input lumen  84  and output lumen  86 , the catheter  150  may have two or more infusion lumens which may be operated simultaneously with the control of the patient&#39;s temperature. Specifically, the first infusion lumen  88  terminates at a medial outlet port  90  and a second infusion lumen  92  terminates at a separate outlet port  94 . Both lumens  88  and  92  are separated from the heat transfer fluid and both extend to the hub  68 . A guide wire tube  96  communicates with the tube  72  extends to the distal port  78 . These several passages provide communication for the introduction of medicine, the sampling of blood, the sensing of temperature and other purposes requiring access into the body passageway. The ports are shown separated to preclude mixing of drugs in the blood stream. In another embodiment, port  94  is distally located from the elongated element  78 . 
     Looking specifically to the elongate element  78 , a plurality of turns  98  are shown to define the helix which extends longitudinally of the elongate body  74 . The turns  98  are bonded along a fraction of the length of each turn at locations  100  and are otherwise displaced from the body  74 . This allows body fluid flow between the turns  98  and the body  74 . Again, the turns  98  are of thin-walled, flexible material. The material need only retain the working fluid and may collapse under fluid pressure of the body fluid when the heat transfer lumen is at atmospheric pressure. 
     In accordance with the invention, a patient about to undergo cardiopulmonary bypass is precooled such that the patient&#39;s core body temperature is lowered in advance of bypass. First, catheter  30  is implanted into the patient. A preferred location is the central venous system, in order to maximize heat exchange with the patient&#39;s blood by exposing a volumetrically significant amount of blood to the catheter, and particularly, to balloons  32  thereof. Access to the central venous system can be gained through the subclavian or jugular veins, into the superior vena cava, or through the femoral vein into the inferior vena cava. 
     The indwelling catheter  30  operates to cool the patient, lowering core body temperature before cardiopulmonary bypass is initiated. A predetermined target temperature or temperature range, and possibly a cooling rate, are set by an operator, who inputs the temperature or temperature range to temperature control module  44  of control unit  40  using keypad  48 . Temperature control module  44  monitors patient core body temperature using feedback from probe  46 , and automatically adjusts the temperature in water bath  42 , and in the circulating cooling fluid, to thereby conform to the target temperature or range. It will be noted that the speed of pump  41  can additionally or alternatively be controlled in order to adjust patient core body temperature. 
     Typically, precooling is conducted in the operating room and can commence about 15 minutes to one hour before cardiopulmonary bypass, which is typically the length of time required to bring core body temperature down to a target temperature or range of about 32° C. to 34° C. It is envisioned, however, that precooling can take place in the field, using portable equipment, particularly in the event of an emergency. 
     It is preferred that the precooling using catheter  30  occur before extracorporeal circulation is initiated. Thus it is contemplated that the precooling, preferably to the target temperature or range, take place before blood pumps (not shown) in CPB device  26  are turned on and the process of bypassing the patient&#39;s heart and lung functions takes place. It may also be appropriate to conduct precooling even earlier, such as before cannulation of the patient in preparation for bypass. The details of the precooling operation will of course be dictated by the particular circumstances, based on factors such as patient condition, location, etc. 
     An exemplary precooling process in accordance with the invention may take place as follows: 
     1. Patient begins anesthesia/ventilator in the operating room; 
     2. Catheter  30  is inserted into inferior vena cava of patient via the femoral vein, and cooling, preferably at the maximum rate wherein heat exchange fluid temperature of about 4° C. is used, commences in the direction of the target temperature or range; 
     3. Following, or in parallel to, active cooling using catheter  30 , the patient&#39;s chest is opened and all the normal activities in preparation for normal cannulation for heart/lung bypass are conducted; 
     4. If needed, appropriate shivering control measures, including Demorol,™ can be used; and 
     5. Cooling can be stopped when patient core body temperature reaches the target temperature or range, or when normal heart/lung bypass commences, preferably whichever occurs first. 
     The invention is also directed to providing post CPB temperature control. Termination of CPB involves various procedures and associated risks. Some of these procedures are release of the cross clamp (not shown) used in diverting the patient&#39;s blood flow, and performing the gradual “weaning off” process. Weaning off of bypass involves gradually restoring normal heart function, by flushing the heart and washing off the potassium used to stop beating, and by rewarming the heart and subsequently discontinuing the pumping function of CPB device  26 . In accordance with the invention, some or all of these procedures are performed at a temperature of about 32° C. to 34° C., facilitated by the use of indwelling catheter  30  and control unit  40  since at this juncture CPB bypass will have been or will be in the process of termination. Importantly, this ensures that the patient is at a temperature that is neuroprotective when the inevitable embolic shower associated with release of the cross clamp and weaning off bypass occur. 
     Further in accordance with the invention, catheter  30  and control unit  40  are used to provide a controlled rate of patient rewarming, for instance retarding this rate as desired, again to prolong the effects of cooling and maximize their benefits depending on the circumstances. Rewarming rate can be selectable such that operator enters this rate into temperature control module  44  of control unit  40 , using keypad  48 . This rate would preferably govern the rate of change of cooling, and particularly, the decrease in cooling, of heat exchange fluid in fluid circuit  43  and catheter  30  by water bath  42 . 
     Controlled rewarming in accordance with the invention can also encompass applying heat to the patient&#39;s blood flow using catheter  30 . This would of course accelerate rewarming and would save time by allowing removal of CPB and performance of post CPB procedures, such as closing the chest, in parallel with warming by catheter  30 . 
     In accordance with the invention, the catheter  30  can be provided with instructions for use with precooling or rewarming of a cardiopulmonary patient as described above. In this manner, the catheter can be vended as a kit of parts which may include these instructions, along with for example the attendant tubing sets, fittings, and possibly, the control module  40  and other componentry necessary for practice of the invention. Although the precooling and rewarming processes have been described using catheter  30 , these processes can just as well be implemented using catheter  150  as described herein. 
     The above are exemplary modes of carrying out the invention and are not intended to be limiting. It will be apparent to those of ordinary skill in the art that modifications thereto can be made without departure from the spirit and scope of the invention as set forth in the following claims.