METHODS AND SYSTEMS FOR TREATING ISCHEMIA

Abstract: Methods for treating total and partial occlusions employ a perfusion conduit which is penetrated through the occlusive material. Oxygenated blood or other medium is then perfused through the conduit in a controlled manner, preferably at a controlled pressure below the arterial pressure, to maintain oxygenation and relieve ischemia in tissue distal to the occlusion. Optionally, the occlusion may be treated while perfusion is maintained, typically by introducing a thrombolytic or other agent into the occlusive material using the perfusion conduit. Such methods are particularly suitable for treating acute stroke to prevent irreversible damage to the cerebral tissue.

Description: METHODS AND SYSTEMS FOR TREATING ISCHEMIA The present application is a continuation-in-part of Application Serial No. 09/018,214, filed of February 3, 1998, the full disclosure of which is incorporated herein by reference. BACKGROUND OF THE INVENTION Field of Invention The present invention relates generally to medical devices and methods. More particularly, the present invention relates to catheters, systems, kits, and methods for treating ischemia, such as intracerebral ischemia associated with stroke. Hemodynamically significant restriction of arterial blood flow can lead to oxygen deprivation in tissue, referred to as ischemia, and can quickly lead to cell death and organ dysfunction. The brain is the organ most sensitive to ischemia, followed by the heart, the abdominal organs, and the extremities. The brain will usually not tolerate ischemia for very long without massive neuron death (stroke). When treating ischemic events in the brain, it is imperative to restore blood flow quickly and safely. The most common causes of acute arterial ischemia in the cerebrovasculature are thrombosis and embolus. Thrombus usually forms at the site of a pre-existing atherosclerotic lesion and can cause an acute occlusion. Atherosclerosis can occur at any location within the arteries that deliver blood from the heart to the brain, but the most common locations of significant atherosclerosis are the cervical carotid artery at the carotid bifurcation, the proximal middle cerebral artery, and the vertebrobasilar arterial system. Clinically significant atherosclerosis also can occur in other intracerebral vessels. Emboli are formed when previously stable thrombus or atheroma is released into the blood stream and becomes lodged in smaller blood vessels. Emboli can originate from atherosclerotic lesions and from within the cardiac chambers. They can cause acute obstructions of blood vessels, resulting in tissue hypoxia and neuron death. Further obstruction can also occur distally to the embolus due to secondary inflammatory responses and other reactions. Transient ischemic attacks (TIA's) occur with temporary and intermittent obstructions, allowing for neuron recovery. Stroke occurs with longer term obstruction to blood flow. Acute arterial ischemia can also result from vasospasm, as a delayed response to intracranial hemorrhage, and may be iatrogenic, as when intracranial blood vessels are temporarily occluded during neurosurgery. Other causes include head trauma, inflammation, and infection. Traditional therapy of acute stroke has been limited to the delivery of supportive measures. Newer treatments for stroke attempt to relieve or bypass vessel occlusion before neuron death occurs. In the life threatening emergency of acute stroke, there is a time-limited window of opportunity for treatment after the onset of symptoms. After this treatment window has closed, there is minimal opportunity for recovery of neuronal function. For all these reasons, physicians have emphasized the early treatment of patients, usually within six hours of the onset of symptoms, and on relieving the obstruction emergently. When the obstruction is relieved, the ischemic bed will be exposed to normal arterial pressure. As will be discussed below, the inventor believes that reperfusion syndrome may result from prematurely exposing the previously ischemic bed to normal arterial pressure. Reperfusion syndrome may result from disordered function of the capillary endothelium and the blood-brain barrier in the previously ischemic vascular bed. A number of techniques have been proposed which employ site-specific administration of thrombolytic drugs and/or mechanical means, laser or ultrasound energy sources to remove thrombus. Angioplasty, atherectomy and stent placement are employed to relieve atherosclerotic stenoses. These methods all require positioning catheter based devices at or near the site of the arterial obstruction. The primary objective is to restore blood flow as quickly as possible. Such devices, however, require significant time to position and use. There are also risks of damaging the obstructed artery, of dislodging and embolizing blood thrombus or atherosclerotic plaque, of inducing intracerebral hemorrhage or other serious complications. Directed thrombolysis using currently available catheters and guidewires often takes many hours to complete. While excellent technical results are feasible, many patients cannot tolerate the wait and their condition can deteriorate during the procedure. Moreover, the thrombolytic agents themselves may have deleterious biologic side effects. Surgical bypass does not work as well as standard medical therapy in preventing stroke recurrence and is only rarely performed. New classes of "neuroprotectant" agents and "angiogenesis promoters" are being developed and tested. These drugs may extend the effective therapeutic window for stroke therapy and permit better long term outcomes. Their use, however, may require novel delivery systems and often require that the patient be stabilized and ischemia relieved in order to obtain a lasting clinical improvement. For these reasons, it would be desirable to provide improved methods and apparatus for treating acute ischemic conditions, particularly stroke. It would be further desirable if such methods and apparatus were also useful for treating chronic ischemia in other portions of a patient's vasculature, including the coronary vasculature and the peripheral and mesenteric vasculature. The methods and apparatus should be capable of rapidly reestablishing blood flow, at controlled pressures, at a rate sufficient to relieve ischemia distal to the occlusion, and may be adaptable for use in an emergency situation (i.e., outside the hospital) and/or within a hospital environment. The methods and apparatus should provide for control over the rate of flow, pressure in the ischemic bed and/or cessation of flow to the ischemic region in order to avoid reperfusion injury. In addition to relieving ischemia, the methods and devices of the present invention may also provide access and support for performing other therapeutic interventions to treat the occlusion, including both drug interventions and mechanical interventions. The methods and devices should be adaptable to use access routes of a type which are familiar to interventionalists so as to permit rapid and wide spread adoption. At least some of these objectives will be met by different aspects of the present invention. Description of the Background Art U.S. Patent No. 5,149,321 describes an emergency system for infusing an oxygenated medium into the cerebral vasculature in patients following a heart attack. Active perfusion through coronary angioplasty catheters is described in a number of patents and published applications, including U.S. Patent Nos. 5,106,363; 5,158,540; 5,186,713; and 5,407,424; Canadian Patent 1,322,315; and WO 97/19713. The latter describes perfusion of an oxygenated medium through a guidewire. Perfusion and/or infusion catheters and systems are described in a number of patents, including U.S. Patent Nos. 5,584,804; 5,090,960; 4,611,094; 4,666,426; 4,921,483; 5,643,228; 5,451,207; 5,425,723; 5,462,523; 5,531,715; 5,403,274; 5,184,627; 5,066,282; 4,850,969; 4,804,358; 4,468,216; and WO 92/20398. U.S. Patent No. 5,090,960 describes a passive perfusion catheter having spaced-apart balloons and a suction tube for recirculating a thrombolytic agent. SUMMARY OF THE INVENTION The present invention provides methods, apparatus, and kits for treating patients suffering from ischemia resulting from the partial or total obstruction of a blood vessel. Usually, the obstructions will be high-grade blockages, e.g., those which result in greater than 75% flow reduction, but in some instances they may be of a lower grade, e.g., ulcerated lesions. As used hereinafter, the terms "obstruction/' "occlusion," and "blockage" will be used generally interchangeably and will refer to both total obstructions where substantially all flow through a blood vessel is stopped as well as to partial obstructions where flow through the blood vessel remains, although at a lower rate than if the obstruction were absent. Preferred use of the present invention is for the treatment of patients suffering from acute stroke resulting from a sudden, catastrophic blockage of a cerebral artery. The invention may also be used to minimize or prevent ischemia during other conditions which result in blocked points or segments in the cerebral arterial vasculature, such as iatrogenic occlusion of an artery, e.g., during neurosurgery, or to relieve vasospasm induced ischemia. The present invention, however, will also be useful for treating acute blockages in other portions of the vasculature as well as for treating chronic occlusions in the cerebral, cardiac, peripheral, mesenteric, and other vasculature. Optionally, the methods of the present invention may be used to facilitate dissolving or removing the primary obstruction responsible for the ischemia, e.g., by drug delivery, mechanical intervention, or the like, while perfusion is maintained to relieve the ischemia. Methods according to the present invention comprise penetrating a perfusion conduit through the blockage and subsequently pumping an oxygenated medium through the conduit at a rate or pressure sufficient to relieve ischemia downstream from the blockage. The oxygenated medium is preferably blood taken from the patient being treated. In some instances, however, it will be possible to use other oxygenated media, such as perfluorocarbons or other synthetic blood substitutes. In a preferred aspect of the present invention, the pumping step comprises drawing oxygenated blood from the patient, and pumping the blood back through the conduit at a controlled pressure and/or rate, typically a pressure within the range from 50 mmHg to 400 mmHg, preferably at a mean arterial pressure in the range from 50 mmHg to 150 mmHg, and at a rate in the range from 30 cc/min to 360 cc/min, usually from 30 cc/min to 240 cc/min, and preferably from 30 cc/min to 180 cc/min, for the cerebral vasculature. Usually, pressure and flow rate will both be monitored. The blood flow system preferably keeps the pressure at or below 400 mmHg, 350 mmHg, or 300 mmHg. Pressure is preferably monitored using one or more pressure sensing element(s) on the catheter which may be disposed distal and/or proximal to the obstruction where the blood or other oxygenated medium is being released. Flow rate may easily be monitored on the pumping unit in a conventional manner or may be monitored by a separate control unit. Conveniently, the blood may be withdrawn through a sheath which is used for percutaneously introducing the perfusion conduit. It will usually be desirable to control the pressure and/or flow rate of the oxygenated medium being delivered distally to the occlusion. Usually, the delivered pressure of the oxygenated medium should be maintained below the local peak systolic pressure and/or mean arterial blood pressure of the vasculature at a location proximal to the occlusion. It will generally be undesirable to expose the vasculature distal to the occlusion to a pressure above that to which it has been exposed prior to the occlusion. Pressure control of the delivered oxygenated medium will, of course, depend on the manner in which the medium is being delivered. In instances where the oxygenated medium is blood which is being passively perfused past the occlusion, the delivered pressure will be limited to well below the inlet pressure, which is typically the local pressure in the artery immediately proximal to the occlusion. Pressure control may be necessary, however, when the oxygenated medium or blood is being actively pumped. In such cases, the pump may have a generally continuous (non- pulsatile) output or in some cases may have a pulsatile output, e.g., being pulsed to mimic coronary output. In the case of a continuous pump output, it is preferred that the pressure in the vascular bed immediately distal to the occlusion be maintained below the mean arterial pressure usually being below 150 mmHg, often being below 100 mmHg. In the case of a pulsatile pump output, the peak pressure should be maintained below the peak systolic pressure upstream of the occlusion, typically being below 200 mmHg, usually being below 150 mmHg. Pressure control of the oxygenated medium being delivered downstream of the occlusion is preferably achieved using a digital or analog feedback control apparatus where the pressure and/or flow output of the pump is regulated based on a measured pressure and/or flow value. The pressure value may be measured directly or indirectly. For example, the pressure downstream of the occlusion may be measured indirectly through the perfusion conduit. A separate pressure lumen may be provided in the perfusion conduit and a pressure measurement transducer located at the proximal end of the conduit. Pressure sensed by a distal port of the pressure measuring conduit will then be transmitted through the conduit to the transducer. Pressure transducers are a preferred pressure sensor for measuring pressure in the vasculature distal to the occlusion. The pressure sensors may be mounted near the distal tip of the perfusion conduit itself or could be mounted on a separate guidewire or other structure which crosses the occlusion with the perfusion conduit. The pressure signals generated by the transducers are transmitted through electrically conductive elements, such as wires, to the proximal end of the perfusion conduit where they are connected to a pressure monitor connected to or integral with the controller. The pump output can then be controlled based on conventional control algorithms, such as proportional control algorithms, derivative control algorithms, integral control algorithms, or combinations thereof. In one embodiment of the present invention, the pressure sensor is spaced from the perfusion outlets so that fluid flow forces do not affect the pressure measurements. Actual manipulation of the pressure and/or flow provided by a circulating pump can be effected in a variety of ways. In the case of centrifugal pumps, the flow can be measured at the pump output and the pressure can be measured in any of the ways set forth above. Control of both the flow rate and the pressure can be achieved by appropriately changing the pump speed and downstream flow resistance, where the latter can be manipulated using a control valve. Suitable flow control algorithms are well described in the patent and technical literature. Control of peristaltic and other positive displacement pumps is achieved in a slightly different way. Flow volume from a positive displacement pump is a linear function of the pump speed and thus may be controlled simply by varying the pump speed. Pressure output from the positive displacement pump, in contrast, will be dependent on flow resistance downstream from the pump. In order to provide for control of the output pressure from the pump (which is necessary to control the pressure downstream of the occlusion), a pressure control system may be provided. Typically, the pressure control system may comprise a by-pass flow loop from the pump output back to the pump inlet. By then controlling the amount of blood output which is by-passed back to the inlet, that pressure can be manipulated. Typically, a flow control valve can be used to adjust the by-pass flow in order to achieve the target pressure control point downstream of the obstruction. Suitable flow and pressure control algorithms for positive displacement pumps, such as roller pumps, are well described in the patent and technical literature. In addition to controlling pressure and/or flow rates, the systems of the present invention can provide control for a number of other parameters, such as partial oxygen pressure (pO2) in the perfused blood, partial carbon dioxide pressure (pCO2) in the perfused blood, pH in the perfused blood, temperature of the perfused blood, metabolite concentrations, and the like. Both pO2 and pCO2 can be controlled using the oxygenator in the system, as described in more detail below. The pH can be controlled by introducing appropriate physiologically acceptable pH modifier(s), such as buffer and bicarbonate solutions and the like. Temperature is controlled by providing appropriate heat exchange capabilities in the extracorporeal pumping system. The temperature will usually be decreased in order to further inhibit tissue damage from the ischemic conditions, but could be elevated for other purposes. Suitable sensors and devices for measuring each of the parameters are commercially available, and suitable control systems can be provided as separate analog units or as part of a digital controller for the entire system, such as a desk or lap top computer which is specially programmed to handle the monitoring and control functions as described in this application. Concentration and/or physiologic activity of certain formed cellular elements, such as white blood cell or platelets, can be selectively controlled with suitable control systems and devices A particular advantage of the present invention lies in the ability to lessen or eliminate reperfusion injury which can result from the rapid restoration of full blood flow and pressure to ischemic tissue. As described above, the use of thrombolytics and other prior treatments can cause the abrupt removal of an obstruction causing rapid infusion of blood into the ischemic tissue downstream of the occlusion. It is believed that such rapid restoration of full blood flow and pressure, typically at normal physiologic pressures, can result in further damage to the leaky capillary beds and dysfunctional blood-brain barrier which results from the prior ischemic condition. The present invention allows for a controlled reperfusion of the ischemic tissue where blood can initially be released downstream of the obstruction at relatively low pressures and/or flow rates. That is, it will be desirable to initiate the flow of blood or other oxygenated medium slowly and allow the flow rate and pressure to achieve their target values over time. For example, when actively pumping the oxygenated medium, the pumping rate can be initiated at a very low level, typically less than 30 cc/min, often less than 10 cc/min, and sometimes beginning at essentially no flow and can then be increased in a linear or non-linear manner until reaching the target value. Rates of increase can be from 1 cc/min/min to 360 cc/min/min, usually being from 5 cc/min/min to 120 cc/min/min. Alternatively, the flow of blood or other oxygenated medium can be regulated based on pressure as mentioned above. For example, flow can begin with a pressure in the previously ischemic bed no greater than 10 mmHg, typically from 10 mmHg to 70 mmHg. The pressure can then be gradually increased, typically at a rate in the range from 5-100 mmHg over 2, 8 or even 48 hours. In some instances, it may be desirable to employ blood or other oxygenated medium that has been superoxygenated, i.e., carrying more oxygen per ml than normally oxygenated blood. While pumping will usually be required to achieve and/or maintain adequate perfusion, in some instances passive perfusion may be sufficient. In particular, perfusion of the smaller arteries within the cerebral vasculature can sometimes be provided using a perfusion conduit having inlet ports or apertures on a proximal portion of the conduit and outlet ports or apertures on a distal portion of the conduit. By then positioning the inlet and outlet ports on the proximal and distal sides of the obstruction, respectively, the natural pressure differential in the vasculature will be sufficient to perfuse blood through the conduit lumen past the obstruction. Usually, the inlet ports on the perfusion conduit will be located at a location as close to the proximal side of the occlusion as possible in order to minimize the length of perfusion lumen through which the blood will have to flow. In some instances, however, it may be necessary to position the inlet ports sufficiently proximal to the occlusion so that they lie in a relatively patent arterial lumen to supply the necessary blood flow and pressure. The cross-sectional area of the perfusion lumen will be maintained as large as possible from the point of the inlet ports to the outlet ports. In this way, flow resistance is minimized and flow rate maximized to take full advantage of the natural pressure differential which exists. While perfusion is maintained through the perfusion conduit, treatment of the blood vessel blockage may be effected in a variety of ways. For example, thrombolytic, anticoagulant and/or anti-restenotic agents, such as tissue plasminogen activator (tPA), streptokinase, urokinase, heparin, or the like, may be administered to the patient locally (usually through the perfusion catheter) or systemically. In a preferred aspect of the present invention, such thrombolytic and/or anticoagulant agents may be administered locally to the arterial blockage, preferably through a lumen in the perfusion catheter itself. Such local administration can be directly into the thrombus, e.g., through side infusion ports which are positioned within the thrombus while the perfusion port(s) are positioned distal to the thrombus. Optionally, a portion of the blood which is being perfused could be added back to or otherwise combined with thrombolytic and/or anticoagulant agent(s) being administered through the catheter. The addition of blood to certain thrombolytic agents will act to catalyze the desired thrombolytic activity. The availability of the patient blood being perfused greatly facilitates such addition. It would also be possible to deliver the agent(s) through the same lumen and distal port(s) as the blood being pumped back through the perfusion lumen so that the agents are delivered distally of the catheter. The latter situation may be used advantageously with neuroprotective agents, vasodilators, antispasmotic drugs, angiogenesis promoters, as well as thrombolytics, anticoagulants, and anti-restenotic agents, and the like. The two approaches, of course, may be combined so that one or more agents, such as thrombolytic agents, are delivered directly into the thrombus while neuroprotective or other agents are delivered distally to the thrombus. Moreover, such delivery routes can also be employed simultaneously with systemic delivery of drugs or other agents to the patient. Alternatively or additionally, mechanical interventions may be performed while the vasculature is being perfused according to the present invention. For example, a perfusion conduit may have a very low profile and be used as a guide element to introduce an interventional catheter, such as an angioplasty catheter, an atherectomy catheter, a stent-placement catheter, thrombus dissolution device, or the like. The perfusion of the oxygenated medium may be performed for a relatively short time in order to relieve ischemia (which may be advantageous because of damaged capillaries and/or blood-brain barrier) while other interventional steps are being taken, or may be performed for a much longer time either in anticipation of other interventional steps and/or while other long-term interventions are being performed. In particular, when thrombolytic and/or anticoagulant agents are being used to treat the primary blockage, the perfusion can be continued until the blockage is substantially relieved, typically for at least thirty minutes, often for four to eight hours, or even 2-3 days. In other instances, perfusion can be maintained for much longer periods, e.g., more than one week, more than two weeks, more than a month, or even longer. In some cases, it may even be desirable to maintain perfusion and placement of the perfusion conduit for an extended period of time with the patient having a portable or implantable pump coupled to the conduit. The pump may also have a reservoir for delivery of therapeutic agents and may be implanted or carried on a belt or the like. The ability of the present invention to provide for gradual or controlled restoration of physiologic blood perfusion pressures and flow rates is a particular advantage when subsequent interventional steps would otherwise result in abrupt restoration of blood flow. As described above, abrupt restoration of blood flow can cause or contribute to reperfusion injuries. By providing for controlled restoration of blood flow prior to such interventional steps, the ischemic tissue can be conditioned to tolerate physiologic blood flow rates and pressures prior to full restoration by dissolution or other removal of the occlusion. Such gradual restoration of blood flow from very low levels to physiologic flow rates can typically be achieved over time periods in the range from one minute, an hour or even up to 48 hours or longer. Perfusion at controlled pressure and/or flow rate may last typically in the range of 30 minutes to 2 hours, more typically 30 mintues to 9 hours. It will be desirable, for example, to initiate perfusion through the perfusion conduits of the present invention at mean arterial pressures downstream of the occlusion which are no greater than 25- 50% of normal with typical pressures being 20-40 mmHg. The blood flow rates 10 which correspond to such pressures will depend largely on the nature of the vasculature into which the blood is being perfused and may be less than 200 ml/min, less than 150 ml/min and even less than 100 ml/min. In addition to delivering oxygen to the ischemic region distal to the primary occlusion, the blood or other oxygenated medium may carry other treatment agents, including thrombolytic agents, anticoagulant agents, tissue preservative agents, and the like. Moreover, in order to further preserve the cerebral tissue distal to the blockage, the oxygenated medium may be cooled to below body temperature, e.g., to a temperature in the range from 2°C to 36°C, typically from 25°C to 36°C, in order to cool and preserve the tissue. Cooling may be effected externally as part of the extracorporeal pumping system and/or may be effected using a thermoelectric or Joule-Thomson expansion cooler on the catheter itself. Patients suffering from ischemia resulting from acute or chronic occlusion in the cerebral vasculature may be treated according to the preferred methods described below. A perfusion conduit is introduced to the patient's vasculature, and a distal port on the conduit is guided through the occlusion in the cerebral vasculature. Blood, optionally oxygenated and/or superoxygenated, is obtained from the patient and perfused back to the patient through the distal port on the conduit past the occlusion at a rate sufficient to relieve the ischemia. The oxygenated blood may be arterial blood which may be returned to the patient without further oxygenation. Alternatively, arterial or venous blood can be oxygenated in suitable apparatus external to the patient and returned to the patient. External oxygenation allows the blood to be " superoxygenated," i.e., oxygenated at higher levels than would normally be available from arterial blood. Usually, the method further comprises delivering a therapeutic agent to the patient while the perfusing step is continued, usually being a thrombolytic agent which is delivered through the conduit directly to the vascular occlusion. The occlusion is usually in either a carotid artery, vertebral artery, proximal subclavian artery, brachiocephalic artery, or an intracerebral artery, and the conduit is usually introduced via the femoral artery in a conventional intravascular approach, typically being positioned over a guidewire which is first used to cross the occlusion. Alternatively, the conduit may be introduced through the axillary or brachial arteries, also in a conventional manner. 11 Apparatus according to the present invention comprises perfusion/infusion catheters which include a catheter body having a proximal end and a distal end. The catheter body has at least a perfusion lumen and may have other lumens. The catheter may be tapered or may have a constant cross-sectional shape. The catheter may be formed as a single, multi-lumen or single-lumen extrusion or the lumens may be formed as separate tubes. When formed as separate tubes, the tubes may be fixed relative to each other or may be provided with appropriate sliding seals to permit them to slide relative to each other. Additional lumens and/or tubes may also be provided for purposes discussed in more detail below. Often, although not always, the catheters will be free from external dilatation balloons or other external structure which could complicate penetration of the distal end of the catheter through an obstruction. A first embodiment of the catheter is characterized by a large diameter proximal section and a small diameter distal section, where at least two isolated lumens extend from the proximal end of the catheter body through both sections to near the distal end of the catheter body. One of the lumens will extend entirely through the catheter body and usually have side ports over a distal length thereof. The other lumen will usually terminate some distance proximal of the distal tip of the catheter body and will also usually have side ports over a distal length thereof. The proximal section has an outer diameter in the range from 1 mm to 3 mm, usually from 1.5 mm to 2.5 mm, and typically from 1.5 mm to 2 mm, and the distal section has an outer diameter in the range from 0.5 mm to 2 mm. preferably from 0.5 mm to 1.5 mm. The first isolated lumen which extends entirely through the catheter body will usually be tapered, i.e., have a larger diameter over a proximal length thereof than over a distal length thereof. Usually, the first isolated lumen will have an inner diameter in the range from 0.75 mm to 1.25 mm in the proximal section, more usually being from 0.9 mm to 1 J mm in the proximal section, and an inner diameter in the range from 0.25 mm to 1 mm in the distal section, usually being from 0.3 mm to 0.75 mm in the distal section. The second isolated lumen will usually be disposed annularly about the first isolated lumen and will have an inner diameter in the range from 0.9 mm to 2.9 mm in the proximal section, usually from 1.4 mm to 1.9 mm in the proximal section, and an inner diameter in the range from 0.4 mm to 1.9 mm in the distal section, usually in the range from 0.5 mm to 1.5 mm in the distal section. The second, 12 outer annular lumen will typically terminate from 5 cm to 25 cm from the distal end of the catheter body. The catheter may also have a larger flow conduit for achieving higher flow rates. For example, the inner diameter of the first lumen may be 1.5-3.0 mm in the proximal section and 1.0-2.0 mm in the distal section. The second lumen has an inner diameter which is preferably 0.25-1.0 mm larger than the outer diameter of the first lumen. The wall thickness of the first lumen is preferably between 0.07-0.20 mm. If the catheter has a straight instead of tapered configuration the inner diameter of the first lumen is preferably 1.5-2.5 mm. The catheter of the present invention may, of course, have any other suitable tapered shape or may have a constant cross-sectional profile. For example, in another preferred embodiment, the first catheter has the perfusion lumen, and in a specific embodiment no other fluid lumens. Such a catheter has a small, flexible construction which can be passed through tortuous vessels. Other catheters may be advanced over the perfusion catheter to remove or displace the obstruction as discussed below. The catheters may be another fluid perfusion catheter for delivery of thrombolytic agents or may be an obstruction removal catheter which removes the obstruction with mechanical action or with an ultrasound transducer, RF electrode or a laser. Apparatus according to the present invention further comprises systems including a perfusion/infusion catheter as set forth above in combination with a sheath for percutaneously introducing the perfusion/infusion catheter and a pump for receiving blood from the sheath and delivering blood back to the catheter. Optionally, an infusion device may be provided in the system for infusing a drug to a lumen of the perfusion/infusion catheter. Preferably, the systems will include control apparatus for controlling blood infusion pressures, blood infusion flow rates, pO2, pCO2, pH, temperature, and/or other parameters of the blood/oxygenated medium being perfused back to the patient. The present invention still further comprises kits, including a perfusion catheter and instructions for use setting forth a method for penetrating the catheter through a blockage in a patient's vasculature and thereafter perfusing an oxygenated medium through the conduit to relieve ischemia. Kits will usually further comprise a container, such as a pouch, tray, box, tube, or the like, which contains the catheter as well as the instructions for use. Optionally, the instructions for use set forth on a separate instructional sheet within the package, but alternatively could be 13 printed in whole or in part on the packaging itself. Optionally, other system components useful for performing the methods of the present invention could be provided within the kit, including guidewires, introductory sheaths, guiding catheters, and the like. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A-1C illustrate an exemplary protocol for treating a total occlusion in a blood vessel according to the method of the present invention. Fig. 2 illustrates an exemplary system for treating a total occlusion within a patient's cerebral vasculature according to the present invention. Fig. 3 is a cross-sectional view taken along line 3-3 of Fig. 2. Fig. 4 is a cross-sectional view taken along line 4-4 of Fig. 2. Fig. 5 is a cross-sectional view taken along line 5-5 of Fig. 2. Fig. 6 is a cross-sectional view taken along line 6-6 of Fig. 2. Fig. 7 illustrates a protocol using the system of Fig. 2 for treating a cerebral occlusion according to the present invention. Fig. 8 is a detailed view of the catheter used for treating the occlusion in the protocol of Fig. 7. Fig. 9 illustrates a kit including components according to the present invention. Fig. 10 illustrates an alternative embodiment of a perfusion conduit constructed in accordance with the principles of the present invention. Fig. 1 1 illustrates yet a further embodiment of a perfusion conduit constructed in accordance with the principles of the present invention. Fig. 12 illustrates yet another exemplary embodiment of a perfusion conduit constructed in accordance with the principles of the present invention. Fig. 13 illustrates another perfusion catheter with a second catheter advanced over the perfusion catheter. Fig. 14 illustrates a perfusion used in connection with the catheters of Fig. 13. Fig. 15 illustrates another perfusion catheter having a balloon inflated by fluid infused through the fluid lumen; Fig. 16 illustrates a still another perfusion catheter having a balloon with an inflation lumen. 14 Fig. 17 illustrates a perfusion catheter with a stent delivery catheter advanced over the perfusion catheter. Fig. 18 illustrates a perfusion catheter with a balloon catheter advanced over the perfusion catheter. DESCRIPTION OF THE SPECIFIC EMBODIMENTS The general principles of the present invention for treating partial and total occlusions within a patient's vasculature will be described in connection with Figs. lA-lC. A blood vessel BV which is usually an artery, more usually a cerebral artery, such as a carotid artery, vertebral artery, or an intracerebral artery, is obstructed by a total occlusion TO. The occlusion may result from thrombosis at a pre-existing atherosclerotic lesion or may result from the shedding of an embolus from an artery which flows distally to the particular vessel in which the occlusion occurs. Usually, the occlusion will occur abruptly and the sudden loss of perfusion through the blood vessel distal to the total occlusion TO will place the patient at great risk of neuron death. As discussed above in the Background section, it is usually necessary to reestablish perfusion within a matter of hours in order to avoid significant tissue damage or death, particularly in the case of strokes. While six hours is often considered a maximum delay, earlier treatment is much more desirable. The present invention provides a method for very quickly reestablishing perfusion through the total occlusion TO in a controlled manner. Such perfusion is established using a perfusion conduit 10 (Fig. 1C through which oxygenated blood or an oxygenated synthetic medium, such as a perfluorocarbon oxygen carrier, is actively pumped back through a lumen of the catheter from a source 12. Usually, the conduit will include side perfusion ports 14 near its distal end 16 in order to less traumatically disperse the perfused fluid. Optionally, proximal portions of the conduit 10 (not shown) may have enlarged lumen diameters in order to reduce flow resistance and shear forces to further reduce or prevent hemolysis. It will be appreciated that while the distal portion of the conduit 10 will usually have a relatively low profile to access small diameter blood vessels, the proximal portions can be made significantly larger to improve the hemodynamic flow and handling characteristics and reduce hemolysis. Optionally, the conduit 10 will be introduced over a conventional guidewire GW which may be initially used to cross the total occlusion TO, as shown in Fig. IB. 15 In other instances, however, the perfusion conduit 10 may be adapted so that it is able to cross the total occlusion TO without the use of a conventional guidewire. In some cases, the perfusion conduit may be in the form of a guidewire, e.g., a tapered guidewire, which is suitable for both guiding through the vasculature to the site of the total or partial occlusion as well as crossing the occlusion. The perfusion conduit 10 may be introduced from any normal intravascular introduction site, e.g., through the femoral artery using the Seldinger technique. Alternatively, the infusion conduit can be introduced through the axillary and other arteries. A system 20 suitable for treating occlusions within the cerebral vasculature is illustrated in Figs. 2-6. The system 20 includes a perfusion conduit in the form of intravascular catheter 22. The catheter 22 comprises a catheter body 24 having a distal end 26 and a proximal end 28. The catheter body 24 comprises a pair of coaxial tubular elements, including an outer tube 30 and an inner tube 32. Proximal hub 34 comprises a first port 36 which is fluidly coupled to an interior lumen of the inner tube 32 and a second port 38 which is fluidly coupled to an annular lumen between the exterior surface of outer tube 32 and the interior of tube 30. Proximal port 40 (typically a hemostasis valve) also communicates with the lumen of the inner tubular member 32 and is suitable for intravascular positioning of the catheter 22 over a guidewire. The system usually further includes a guiding catheter 50 having dimensions and characteristics suitable for introducing the catheter 22 to the desired intravascular target site. Although illustrated as having a straight configuration, the guiding catheter 50 will often have a preformed, curved tip selected specifically to reach the intravascular target site, and the guiding catheter could further be reinforced (e.g., braided), have a variable stiffness over its length, have a variable diameter, or the like. The system 20 will usually still further comprise a sheath 60 which is used to percutaneously access the vasculature at the introductory site, e.g., in the femoral artery. The sheath 60 has a proximal hub 61 including at least one side arm 62. The hub 61 receives the catheter 22 therethrough and will include a mechanism for maintaining hemostasis about the catheter. The side arm 62 permits withdrawal of blood for oxygenation and return to the patient according to the present invention. Other side arm(s) may be provided for removal of blood (optionally combined with 16 drugs being delivered back to the patient), for infusing agents through the sheath 60, or for other purposes. Entry of blood into the lumen of the sheath is optionally facilitated by side ports 64 formed over at least a distal portion of the sheath. The catheter body 24 is tapered in the distal direction, i.e., the diameter is larger near the proximal end 28 than at the distal end 26. As illustrated in Figs. 2-6, the outer tube 30 has a large diameter proximal section (observed in Fig. 3) and a smaller diameter distal section (observed in Figs. 4 and 5). Similarly, the inner tube 32 has a large diameter proximal section (shown in Fig. 3) and a smaller diameter distal section (shown in Figs. 4-6). The particular outer diameters and inner lumen diameters of both the outer tube 30 and inner tube 32 are within the ranges set forth above. Since the distal terminii of the outer tube 30 and inner tube 32 are staggered, the catheter body 24 is tapered in three stages, with a first diameter reduction occurring at location 33 (Fig. 2) where the diameter of the outer tubular member 30 is reduced from the diameter shown in Fig. 3 to the diameter shown in Fig. 4. The second diameter reduction occurs at location 35 where the outer tubular member 30 terminates, leaving the outer surface of the inner tubular member 32 to define the catheter body. Such tapered configurations are preferred since they maximize the cross- sectional area of the flow lumens over the length of the catheter to reduce flow resistance for both the blood (or other oxygenated medium) and the drug to be delivered. As can be seen in Fig. 3, lumen 70 of the inner tubular member 32 which carries the blood is maximized until the diameter is reduced near the distal end of the catheter, as shown in Fig. 4. Similarly, the annular lumen 72 which carries the drug is maximized over the proximal portion before it is reduced after the transition at location 33. Maintaining the larger diameters and lumen areas is desirable in order to decrease flow resistance and shear forces to reduce or eliminate hemolysis as the blood is introduced through the entire catheter length. Similarly, a reduction in flow resistance to the drug being introduced facilitates drug delivery during the procedure. Side wall penetrations 80 are provided in a distal portion 26 of the outer tubular member 30, as best seen in Figs. 2 and 5. The penetrations 80 will be useful for delivering a therapeutic agent through port 38 in order to treat the primary occlusion, as described in more detail hereinafter. Similarly, ports 90 may be formed over at least a distal portion of the inner tubular member 32 which extends beyond the distal end of the outer tubular member 17 30. The penetrations 90 will be available to release blood or other oxygenated medium that is being perfused back to the patient through port 36 and the continuous lumen of the tube 32. Note that while the lumen 70 of tube 32 will be available for introduction of the catheter 22 over a guidewire, the guidewire may be at least partially withdrawn from the lumen 70 in order to further decrease blood flow resistance as it is perfused back to the patient. Optionally, the catheter 22 may comprise at least one pressure sensing element 96 disposed at a location near where the blood or other oxygenated medium is returned to the blood vessel. Preferably, the pressure sensing element 96 may be a piezoelectric or other solid state pressure sensing device and will be connected through the hub 34 by a pair of wires 97 which may be connected to conventional electronic devices for measuring pressure. Thus, pressure may be measured and used for controlling rate and/or pressure of blood or other oxygenated medium pumped back to the patient using conventional analog or digital control circuitry. A pressure control point will be selected, usually within the ranges set forth above, and the rate or pressure of oxygenated medium being pumped back through the catheter 22 will be controlled to maintain the control point. Conventional control algorithms, such as proportional, derivative, integral, and combinations thereof, may be employed for maintaining the desired control point. In some instances, it will be desirable to provide at least a second pressure sensing element 98 which will be located proximal to the obstruction when the catheter is in use. For example, the pressure sensing element 98 may be near the location 35 where the outer tubular member 30 terminates. The sensor 98 will permit monitoring of the pressure in the vasculature proximal of the occlusion, which pressure will usually approximate that of the vasculature in the region of the occlusion prior to an acute occlusion event. This pressure, in turn, may be utilized as a target pressure for the blood or other oxygenated medium which is being perfused distal to the occlusion. That is, it may be desirable to treat the measured "background" pressure as a maximum desirable pressure for perfusion in order to prevent injury to the vasculature distal to the occlusion. Referring now to Fig. 7, use of the system 20 for treating the cerebral vasculature of a patient P will be described. Access to the target cerebral artery is established using the sheath 60 in a conventional manner. The guiding catheter 50 is 18 then introduced through the sheath 60 and establishes a protected access lumen to a location within the cerebral vasculature. The catheter 22 is then introduced through the guiding catheter to the target site within the cerebral vasculature, typically over a guidewire (not illustrated). Conveniently, the catheters will be partly radiopaque and/or radiopaque markers 92 (Fig. 2) will be provided at the distal tip of the catheter as well as on either side of the drug ports 80 so that the catheter 22 may be properly positioned under fluoroscopic guidance relative to the obstruction being treated. After the tip 26 of the catheter 22 is penetrated through the occlusion TO (Fig. 8) the penetrations 80 are preferably located within the occlusive material in order to deliver the thrombolytic or other agent to the material. The distal portion of the catheter, ' including ports 90, in contrast, are located beyond the occlusive material in order to provide the desired blood perfusion. Blood flow is immediately established using an external pump 100 which receives blood from the port 62 of access sheath 60 and returns the oxygenated blood to the catheter 22 through port 36. Any suitable therapeutic agent, such as a thrombolytic agent, may be introduced through port 38 from a source 102. Any other suitable drugs may also be delivered from the source 102 and through the port 38. Optionally, the blood may be cooled before, during, or after it has passed through the pump unit 100. Still further optionally, the blood may be oxygenated or superoxygenated using an oxygen-saturated bubble chamber or conventional cardiopulmonary bypass oxygenators ORS. In some instances, it may be desirable to combine the thrombolytic agent with a portion of the recirculating blood before infusing the thrombolytic agent/blood back through the port 38. Optionally, the pump unit 100 may be controlled by an analog or digital control unit 110 (Fig. 7). The control unit 110 will receive various input control parameters 112, typically including at least oxygenated medium flow rate and pressure. Other control parameters, such as pO2, pCO2, pH, temperature, and the like, may also be input into the control unit 110. In turn, the control unit will provide a control output 114, typically at least to the pump unit 100 to control output flow and pressure, as described above. If control of other parameters is desirable, other capabilities may be added, such as the ability to control the degree of oxygenation in the medium supplied by source 102, the ability to add pH modifiers, such as buffers, bicarbonate, and the like, to the oxygenated medium, the ability to control a heat exchanger located in the blood flow circuit, and the like. The source 102 may 19 provide any of the various drugs or therapeutic agents described herein for delivery through the ports. Kits according to the present invention are illustrated in Fig. 9. The kit will include a perfusion conduit, such as perfusion conduit 10, as well as instructions for use 120. The catheter and instructions for use will usually be combined within a suitable container, such as a pouch, tray, box, tube, or the like. The catheter and possibly other components of the system (such as guide catheters, sheaths, thrombolytic or other therapeutic agents, disposable cartridges for pump/oxygenation systems, or the like) will optionally be included and/or sterilized within the packaging. The instructions for use may be on a separate sheet of paper or may be printed in whole or in part on the packaging materials. Referring now to Fig. 10, a perfusion conduit 200 includes an inner tube 202 and outer tube 204. The inner tube has perfusion ports 206 formed in its side wall over a portion of the distal end, and the outer tube 204 has perfusion ports 208 formed over a portion of its distal end. The perfusion conduit 200 differs from catheter 22 primarily in that the inner tubular member 202 is able to slide axially relative to the outer tubular member 204. A sliding seal 210, typically an O-ring or similar passive seal, is provided to maintain pressure within the lumen of outer tubular member 204 so that thrombolytic and other drugs can be delivered without excessive loss through the distal tip. Some loss of the agent, however, will usually be acceptable so that the seal need not be completely tight. If a more positive seal is desired, an inflatable balloon 211 (shown in broken line) may be provided in addition to or in place of the sliding seal 210. Use of the balloon 211 is advantageous in that it permits higher infusion pressures without leakage from the distal end of the outer tube 204, but disadvantageous in that it limits the range of axial placement of the outer tube 204 relative to the inner tube 202. Use of the inner tube 202 for perfusing blood or other oxygenated medium therethrough will generally be as described with the prior embodiments. Radiopaque markers 212 and 214 on the inner tube 202 will be positioned distally of the occlusion to assure that the perfusion ports 206 will release the delivered blood with minimal resistance. Radiopaque markers 216 and 218 on outer tube 208, in contrast, will be positioned so that the infusion ports 208 lie generally within the occluded region. Optionally, the balloon 212 will be inflated to both lock the inner and outer tubes relative to each other and to provide a positive seal 20 at the distal end of the outer tube, and the thrombolytic or other therapeutic agent will then be delivered through the lumen of the outer tube into the occlusive material, such as thrombus. Referring now to Fig. 11 , a perfusion conduit 300 also includes an inner tube 302 and an outer tube 304. The inner and outer tubes are slidable relative to each other, and a sliding seal 310 is provided at the distal end of the outer tube 304. The perfusion conduit 300, in contrast to prior embodiments, is not intended to deliver a therapeutic agent. Instead, it is intended only to perfuse blood or other oxygenated medium therethrough. The lumen 312 within the outer tube 304 is intended for passing the blood or other oxygenated medium to near the distal end of the conduit 300. The inner tube 302 then receives the blood or other oxygenated medium through ports 314 which permit the medium to flow from lumen 312 into the interior lumen of the tube 302. An enlarged portion 316 of the tube 302 is provided in order to prevent axial advancement of the tube so that the ports 314 cannot extend outside of the outer tube 304. Alternatively or additionally, an inflatable balloon 316 may be provided in order to both prevent excess axial advancement of the inner tube 302 and provide a more positive seal. Usually, since the blood will be perfused at lower pressures than might be used for drug delivery, use of the balloon 316 for isolation will often not be necessary. The perfusion conduit 300 can thus provided reduced flow resistance for the blood or other oxygenated medium being returned to the patient through the conduit. Additionally, the ability to slide the outer tube 304 relative to the inner tube 302 helps the tubes be properly positioned relative to each other depending on the circumstances of the patient being treated. Referring now to Fig. 12, a perfusion conduit 400 intended for passive perfusion, i.e., without active pumping, is illustrated. The catheter 400 usually comprises a single extrusion having a proximal section 402 with an enlarged diameter and a distal section 404 with a reduced diameter. The proximal and distal diameters will generally be in the ranges set forth above. Blood inlet ports 408 are provided on the catheter near its proximal end while blood outflow ports 410 are provided near the distal end. The relative positions of the inflow ports 408 and outflow ports 410 allow the perfusion conduit 400 to be introduced to a patient so that the inflow ports are proximal to the occlusion while the outflow ports 410 are distal to the occlusion. The inflow ports 408 are usually relatively near to the distal end of the proximal section 21 402 having the enlarged diameter in order to decrease the overall flow resistance between the inflow ports 408 and outflow ports 410. Generally, however, the inflow ports 408 will be positioned so that they will lie proximally of the occlusion so that the occluding material does not block blood flow into the inflow ports. In some instances, they will be spaced proximally of the transition 412 from large diameter to small diameter by a distance in the range from 1 cm to 15 cm, usually from 2 cm to 10 cm, to assure proper placement in the vasculature. The inflow ports 408 are thus able to receive blood and pass the blood distally through the large diameter section with minimum pressure drop. A pressure drop through the narrow diameter section 404 will be greater, in many instances the total pressure drop of the conduit 400 will be sufficiently low so that adequate blood perfusion can be maintained to relieve patient ischemia. Optionally, the conduit 400 could have a slidable structure, as shown in conduit 300 of Fig. 11, but such structure will increase the flow resistance and will not be preferred in all instances. The conduit 400 preferably has a ID of 0.5 mm to 1.8 mm, more preferably 0.75 to 1.5 mm, between the inflow and outflow ports. Referring to Figs. 13 and 14, another catheter 500 is shown which has a perfusion conduit 502. The catheter 500 has a rounded, atraumatic distal end 504 which is preferably guided through the vasculature over a guidewire which is advanced ahead of the catheter 500. The perfusion conduit 502 may have any of the shapes and sizes discussed herein and preferably has a cross-sectional size of 0.77 to 7.1 mm squared, more preferably 1.7 to 2.9 mm squared, along a distal portion 506 of the catheter 500. In order to maintain adequate flow rates at acceptable pressures, the cross-sectional size is preferably at least 1.7, more preferably at least 3.0 and most preferably at least 4.2 mm squared along the distal portion 506. The distal portion 506 extends for a length of at least 5, 10, 15, 20 or 25 cm from distal end 507 or from the most proximal outlet 518. The catheter 500 and conduit 502 are sized large enough to provide sufficient blood flow rates while blood pressure is within allowable limits to prevent hemolysis. Specifically, the conduit 502 is sized so that the pressure of oxygenated blood in the catheter is 0-400 mmHg, more preferably 20-350 mmHg, at blood flow rates of at least 30, 80, 120 or 160 ml/min. Furthermore, the overall length of the catheter 500 is preferably at least 120, 150 or 175 cm depending upon the access site and size of the patient. 22 The overall maximum outer dimension of the catheter 500 shaft along the distal portion 506 is preferably no more than 1 .6 mm, 2.3 mm, or 3.2 mm. The various diameters and dimensions given throughout the application are equally applicable to any other suitable embodiments described herein. For example, all catheter dimensions discussed above are suitable dimensions for catheter 500 and all dimensions for catheter 500 are applicable to other catheters described herein. Although catheter 500 may include additional open lumens, such as balloon inflation, vent or pressure lumens, the catheter 500 preferably includes only the perfusion conduit 502 to minimize the overall size. The catheter 500 may also be a passive inflation catheter such as the passive inflation catheter 400 of Fig. 12. The catheter 500 may include proximal and distal pressure sensors 510, 512 for measuring pressure on both sides of the obstruction. In a preferred embodiment, the catheter 500 has only one pressure sensor 512 and only the perfusion conduit 502. Wires 514 extending through or along shaft are coupled to a pressure monitor 516 which in turn is integral with or coupled to the control unit 110 for controlling the pump 100 in any manner described herein. The distal pressure sensor 512 is preferably positioned a distance A which is at least 0.5 cm more preferably at least lcm, from the most proximal outlet 518 so that pressure measurement is not distorted by flow forces from the fluid perfused through the outlets 518. A heater and/or cooler 517 is also provided for heating or cooling the oxygenated medium. The control unit 1 10 also receives input control parameters 112 with the parameters measured with suitable sensors along the fluid line. The pressure is preferably maintained below normal arterial pressure for a period of time to protect the previously ischemic bed from reperfusion injury. The inventor believes that prematurely exposing the ischemic bed to normal arterial pressure may cause reperfusion injury and that maintaining low pressure for a period of time can minimize or eliminate reperfusion injury. Low pressure in the previously ischemic bed can be maintained by pressure feedback control of the pump 100 as mentioned above. Alternatively, low pressure can be maintained without direct measurement and feedback by simply selecting low perfusion flow rates. A second catheter 520 is slidably coupled to the catheter 500 and is advanced into the vascular system with the catheter 500 guiding the second catheter 520 to the obstruction. The catheter 500 passes through a hemostasis valve 521 in the second 23 catheter 520. The second catheter 520 passes over the catheter 500 but may also have an interlocking relationship with the catheter 500. The second catheter 520 may also be completely independent from the catheter 500 since advancing the second catheter 520 quickly may not be necessary with catheter 500 perfusing and protecting the previously ischemic vascular bed. The second catheter 520 has a lumen 522 defined by the annular space between the catheters 500, 520. The lumen 522 may be used to deliver liquids, including any of the therapeutic agents described herein such as a thrombolytic agent, from a liquid source 524. The second catheter 520 may also be coupled to a vacuum source 526 to vent blood, therapeutic byproducts and emboli through lumen 522. The second catheter 520 may also include an obstruction removal device 528 for removing the obstruction. The obstruction removal device 528 may simply be the distal tip of the catheter 520 which is used to mechanically remove the obstruction. The osbtruction removal device 528 may also be any suitable non-mechanical device such as an ultrasound transducer, an RF electrode, or a laser. Fig. 13 shows the obstruction removal device 528 as an ultrasound transducer coupled to a power source 531 (Fig. 14) with wires 534. The wires 534 may float within lumen 522 or may be embedded in the wall of the catheter 520. If the osbtruction removal device is an RF electrode, a suitable second electrode (not shown) is placed in contact with the patient's body for monopolar RF or on either catheter 500, 520 for bipolar RF. An electrically conductive fluid, such as saline, may be passed through the lumen 522 from the liquid source 524 during activation of the RF electrode for enhanced conduction. Thus, the second catheter 520 is used to remove the obstruction by mechanical disruption, delivery of obstruction removing liquids through the lumen 522 or use of any of the other suitable devices mentioned above. Referring to Fig. 15, another perfusion catheter 600 is shown which has a perfusion conduit 602. The catheter 600 also has an expandable member 604 which is preferably an inflatable balloon 606 but may also be a mechanically actuated device. The expandable member 604 prevents the previously ischemic bed from being exposed to full arterial pressure if the obstruction is cleared prematurely before the perfusion therapy is completed. The balloon 606 may also be used to prevent parts of the obstruction or other emboli from flowing downstream before therapeutic agents or 24 other obstruction removing methods are used to dissolve, destroy, displace or otherwise remove the obstruction. The balloon 606 has an inflation hole 608 leading to the perfusion conduit 602 so that perfusion of fluid through the conduit 602 inflates the balloon 606. An advantage of using the perfusion conduit 602 to inflate the balloon 606 is that a separate inflation lumen is not required which minimizes the size of the catheter 600. Referring to Fig. 16, the perfusion catheter 600 may also include a separate inflation lumen 610 for inflating the balloon 606 so that the balloon 606 may be selectively inflated independent of perfusion. The balloon 606 may also be used for flow- directed placement of the catheter 600. Referring to Fig. 17, a stent delivery catheter 700 is passed over the perfusion catheter 500, which may be any of the perfusion catheters described herein, and a balloon 702 is used to expand a stent 704 and open the artery. Referring to Fig. 18, a balloon catheter 706 having a balloon 708 is advanced over the perfusion catheter 500. The balloon 708 is expanded in the obstruction to displace the obstruction and open the artery. An advantage of the present system is that the perfusion catheter 500 perfuses and protects of the previously ischemic bed while the stent 704 or balloon 708 is positioned and deployed. While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims. 25

Claims: WHAT IS CLAIMED IS: 1. A method for treating a patient having ischemia resulting from a pre-existing blockage in a blood vessel, said method comprising: penetrating a perfusion conduit through the blockage; and pumping an oxygenated medium through the conduit at a controlled rate or pressure sufficient to relieve ischemia. 2. A method as in claim 1, further comprising oxygenating the medium prior to the pumping step. 3. A method as in claim 1 , further comprising cooling the medium prior to the perfusing step. 4. A method as in claim 1, wherein the oxygenated medium is blood obtained from the patient.. 5. A method as in claim 4, wherein the perfusing step comprises drawing blood from the patient and pumping the blood through the conduit. 6. A method as in claim 5, wherein the blood is drawn from a sheath percutaneously deployed with the perfusion conduit. 7. A method as in claim 1, further comprising delivering a therapeutic agent to the blockage and/or distally of the blockage while pumping the oxygenated medium through the conduit. 8. A method as in claim 7, wherein the perfusion and therapeutic agent delivery steps are performed for a time sufficient to substantially reduce the blockage. 9. A method as in claim 8, wherein the steps are performed for at least thirty minutes. 26 10. A method as in claim 1, wherein the steps are performed for at least one week 11. A method as in claim 1, wherein the blood vessel is in the cerebral vasculature. 12. A method as in claim 1, wherein the blood vessel is in the coronary vasculature. 13. A method as in claim 1, wherein the blood vessel is in the peripheral vasculature. 14. A method as in claim 1, wherein the blood vessel is in the mesenteric vasculature. 15. A method as in claim 1, further comprising monitoring pressure at a location where the oxygenated medium is being pumped back into the blood vessel. 16. A method as in claim 15, monitoring pressure at a location proximal to the occlusion. 17. A method as in claim 15, further comprising controlling the rate or pressure of oxygenated medium being pumped in order to maintain pressure of the oxygenated fluid at the location where it is being pumped back into the blood vessel at a control point. 18. A method as in claim 17, wherein the control point is in the range from 50 mmHg to 300 mmHg. 19. A method as in claim 18, wherein the control point is increased over time from a value below the range to a value within the range. 20. A method for treating a patient having ischemia resulting from an occlusion in the cerebral vasculature, said method comprising: 27 percutaneously introducing a conduit into the patient's vasculature; guiding a distal port on the conduit through the occlusion; obtaining oxygenated blood from the patient; and perfusing the blood back to the patient through the distal port of the conduit past the occlusion at a rate sufficient to relive the ischemia. 21. A method as in claim 20, further comprising delivering a therapeutic agent to the patient while the pumping step is continued. 22. A method as in claim 21, wherein the therapeutic agent is delivered locally through the conduit. 23. A method as in claim 22, wherein oxygenated blood is perfused through a first lumen of the conduit and the therapeutic agent is delivered through a second lumen of the conduit. 24. A method as in claim 21, wherein the therapeutic agent is a thrombolytic agent, an anticoagulant, or an anti-restenotic agent delivered directly into the occlusion and/or distally of the occlusion 25. A method as in claim 20, wherein the percutaneously introducing step comprises introducing the conduit via the femoral, axillary, brachial or carotid artery. 26. A method as in claim 20, wherein the occlusion is in an artery selected from the group consisting of the carotid artery, vertebral artery, proximal subclavian artery, brachiocephalic artery, and intracerebral arteries 27. A method as in claim 20, wherein the obtaining step comprises withdrawing oxygenated blood from the same blood vessel into which the conduit has been introduced and the perfusing step comprises flowing the blood back through the conduit. 28 28. A method as in claim 27, wherein the oxygenated blood is withdrawn through a sheath introduced together with the conduit. 29. A method as in claim 28, wherein the sheath is disposed coaxially about the conduit 30. A method as in claim 20, wherein the perfusing step comprises pumping the blood into the conduit at a rate in the range from 30 cc/min to 360 cc/min 31. A method as in claim 30, further comprising monitoring blood pressure at a location where the blood enters the blood vessel 32. A method as in claim 30, further comprising monitoring blood pressure at a location proximal to the occlusion. 33. A method as in claim 31 , further comprising controlling the rate or pressure of blood being pumped in order to maintain pressure of the blood at the location where it enters back into the blood vessel at a control point. 34. A method as in claim 33, wherein the control point is in the range from 50 mmHg to 300 mmHg 35. A method as in claim 34, wherein the control point is increased over time from a value below the range to a value within the range. 36. A method as in claim 20, wherein the obtaining and perfusing steps drawing blood into proximal ports on the conduit and flowing the blood through the conduit in response to a natural pressure drop. 37. A perfusion/infusion catheter comprising a catheter body having a proximal end, a distal end, a large diameter proximal section, and a small diameter distal section, wherein a first lumen extends from the proximal end to near the distal end with the first lumen has side ports over a distal length of the distal section. 29 38. A perfusion/infusion catheter as in claim 37, wherein the proximal section has an outer diameter in the range from 1 mm to 6 mm and the distal section has an outer diameter in the range from 0.2 mm to 3 mm. 39. A perfusion/infusion catheter as in claim 38, wherein the first lumen has a diameter in the range from 0.5 mm to 3 mm in the proximal section and a diameter in the range from 0.1 mm to 2.5 mm in the distal section, and wherein a second lumen is disposed annularly about the first isolated lumen and has a diameter in the range from 0.9 mm to 5.9 mm in the proximal section and a diameter in the range from 0.1 mm to 2.9 mm in the distal section. 40. A perfusion/infusion catheter as in claim 39, wherein the second isolated lumen terminates from 5 cm to 25 cm from the distal end of the first isolated lumen 41. A system comprising: a perfusion/infusion catheter as in claim 1 ; a sheath for percutaneously introducing the perfusion/infusion catheter to a blood vessel and withdrawing blood from the blood vessel while the catheter is present; and a pump assembly for receiving blood from the sheath and delivering blood to a lumen of the perfusion/infusion catheter. 42. A system as in claim 41, further comprising an infusion device for infusing a drug to the other lumen of the perfusion/infusion catheter. 43. A kit comprising: a perfusion conduit; and instructions for use setting forth a method according to claim 1. 44. A perfusion/infusion catheter comprising: an outer tube having a proximal end, a distal end, a lumen therethrough, and infusion ports near the distal end thereof; 30 an inner tube having a distal end, a proximal end, and an outer diameter smaller than the diameter of the lumen of the outer tube, wherein the inner tube is slidably received in the lumen of the outer tube so that the distal end of the inner tube extends distally beyond the distal end of the outer tube; and a sliding seal at the distal end of the outer tube which seals against an outer surface of the inner tube 45. A perfusion/infusion catheter comprising: an outer tube having a proximal end, a distal end, and a lumen therethrough; an inner tube having a proximal end, a distal end, and a lumen therethrough, wherein the inner tube has an outer diameter which is less than the lumen diameter of the outer tube and the inner tube is slidably received in the lumen of the outer tube so that the distal end of the inner tube extends distally beyond the distal end of the outer tube; and a sliding seal at the distal end of the outer tube which seals against an outer surface of the inner tube, wherein inflow ports on the inner tube are positioned proximally of the sliding seal when the inner tube is received in the lumen of the outer tube. 46. A perfusion catheter for perfusing a patient's cerebral vasculature, comprising: a tubular catheter body having a proximal segment, a distal segment, at least one inflow port, and at least one outflow port, and a lumen extending between the at least one inflow port and the at least one outflow port, the at least one inflow port being formed in the proximal segment to permit blood flow into the catheter and the at least one outflow port being positioned along the distal segment to perfuse blood passing into the at least one inflow port and through the lumen so that said blood can passively flow through the distal segment. 47. A perfusion device for perfusing an oxygenated medium into a patient's cerebral vasculature, comprising: a catheter shaft having a distal portion, the distal portion having a length of at least 10 cm from the distal end, the distal portion also having a maximum outer dimension of no more than 3.2 mm; 31 a perfusion lumen extending through the shaft, the perfusion lumen having a cross-sectional area of 0.77 mm squared to 7.1 mm squared over the distal portion of the shaft; and at least one fluid outlet coupled to the lumen for delivering an oxygenated medium into the cerebral vasculature. 48. The perfusion device of claim 47, wherein: the catheter shaft has a maximum outer dimension of no more than 2.4 mm along the distal portion. 49. The perfusion device of claim 47, wherein: the catheter shaft has a maximum outer dimension of no more than 2.0 mm along the distal portion 50. The perfusion device of claim 47, wherein: the distal portion of the catheter shaft is at least 15 cm long. 51. The perfusion device of claim 47, wherein: the cross-sectional area of the perfusion lumen is at least 1.0 mm squared over the distal portion. 52. The perfusion device of claim 47, wherein: the cross-sectional area of the perfusion lumen is at least 1.5 mm squared over the distal portion. 53. The perfusion device of claim 47, wherein: the cross-sectional area of the perfusion lumen is at least 2.0 mm squared over the distal portion. 54. The perfusion device of claim 47, wherein: the cross-sectional area of the perfusion lumen is at least 4.0 mm squared over the distal portion 32 55. The perfusion device of claim 47, further comprising: a pressure transducer positioned along the distal portion of the catheter shaft. 56. The perfusion device of claim 47, further comprising: a balloon attached to the catheter shaft. 57. The perfusion device of claim 47, wherein: the catheter shaft includes only the perfusion lumen and no other lumens. 58. A method of accessing and perfusing a cerebral vascular site in a patient, comprising the steps of: providing a perfusion catheter having a lumen and a distal end; coupling the lumen to a source of oxygenated blood; introducing the perfusion catheter into the patient; advancing the distal end of the catheter to a selected site in the patient's cerebral arterial vasculature; and perfusing oxygenated blood to the cerebral arterial vascular site at a rate of at least 30 ml/min, the catheter having a length of at least 120 cm and the blood pressure is within the range of 20 to 350 mmHg in the catheter. 59. The method of claim 58, wherein: the providing step is carried out with the catheter having a distal portion, the distal portion extending at least 10 cm from the distal end, the catheter having a maximum outer dimension along the distal portion of less than 3.2 mm. 60 The method of claim 58, wherein: The providing step is carried out with the catheter having a length of at least 100 cm. 61. The method of claim 58, wherein: the perfusing step is carried out at a rate of at least 100 ml/min. 33 62. The method of claim 58, wherein: the perfusing step is carried out at a rate of at least 120 ml/min. 63. A method of treating an ischemic bed downstream of an obstruction in a patient's cerebral arterial vasculature, comprising the steps of: introducing a catheter into a patient, the catheter having a distal portion, a distal end and a lumen having at least one outlet; advancing the catheter to an obstruction in an artery in the patient's cerebral vasculature; passing the distal end of the catheter through the obstruction; supplying an oxygenated medium downstream of the obstruction; and maintaining the pressure at the location downstream of the obstruction at a pressure lower than normal for a period of time so that the ischemic bed is not exposed to full arterial pressure. 64. The method of claim 63, wherein: the introducing step is carried out with the catheter having first means for sensing pressure at a position along the distal portion of the catheter; the method further comprising the step of measuring the arterial pressure with the first pressure sensing means; and the maintaining step being carried out by maintaining a selected pressure measured by the first pressure sensing means. 65. The method of claim 64, further comprising: perfusing the oxygenated medium through the lumen in the catheter at a rate to maintain the selected pressure. 66. The method of claim 64, wherein: the introducing step is carried out with the catheter having second means for sensing pressure, the second pressure sensing means being positioned proximal of the first pressure sensing means. 34 67. The method of claim 63, wherein: the passing step is carried out so that the catheter and obstruction maintain occlusion of the artery. 68. The method of claim 63, further comprising the step of: providing an expandable member on the catheter, the expandable member being expandable from a collapsed shape to an expanded shape; expanding the expandable member at a position near or at the obstruction to occlude the artery to prevent the ischemic bed downstream of the occlusion to be exposed to full arterial pressure. 69. The method of claim 63, wherein: the perfusing step is carried out with the period of time being at least 5 minutes. 70. The method of claim 63, wherein: the perfusing step is carried out with the period of time being at least 2 hours. 71. The method of claim 63, wherein: the perfusing step is carried out with the period of time being at least 2 days. 72. The method of claim 63, wherein: the introducing step is carried out with the catheter having a maximum outer dimension of no more than 3.2 mm at a distal portion of the catheter, the distal portion extending for at least 10 cm from the distal end. 73. The method of claim 63, further comprising: advancing an obstruction removal device to the obstruction during the supplying step; and removing the obstruction with the obstruction removal 74. The method of claim 74, wherein: 35 the removing step is carried out with the obstruction removal device being a device selected from the group consisting of a mechanical disruption element, an RF electrode, an ultrasound transducer and a laser. 75. The method of claim 74, wherein: the obstruction removal device passes over the catheter and forms a lumen in a space between the obstruction removal device and the catheter. 76. The method of claim 75, wherein: the lumen is coupled to a source of liquid which is to be delivered to the obstruction. 36

Inventors: LEWIS BRIAN DOUGLAS, LEWIS, BRIAN, DOUGLAS
Applicants: SALIENT INTERVENTIONAL SYSTEMS, SALIENT INTERVENTIONAL SYSTEMS, INC.
Application year: 1999
Main classification: A61M29
IPC codes: A61B17, A61M1, A61M25, A61M29