Abstract:
Methods are provided for partial aortic occlusion for cerebral perfusion augmentation in patients suffering from global or focal cerebral ischemia. The descending aorta is accessed. A device is then located downstream from the takeoff of the brachiocephalic artery. The device is operated to at least partially obstruct blood flow in the aorta during systole and diastole. A physiologic parameter can be measured. The device can then be adjusted to modify the degree of obstruction based on the measured physiologic parameter.

Description:
[0001]     This is a continuation of U.S. application Ser. No. 10/947,808, filed Sep. 22, 2004, which is a continuation of U.S. application Ser. No. 10/411,743, filed Apr. 11, 2003, which is a continuation of U.S. application Ser. No. 09/531,443, filed Mar. 20, 2000, now U.S. Pat. No. 6,635,046, which is a divisional of U.S. application Ser. No. 09/260,371, filed Mar. 1, 1999, now U.S. Pat. No. 6,231,551. All of the above patents and applications are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to medical devices. More particularly, the invention relates to methods and devices for augmenting blood flow to a patient&#39;s vasculature. More particularly, the invention relates to apparatus and methods which provide partial obstruction (“coarctation”) to aortic blood flow to augment cerebral perfusion in patients with global or focal ischemia. The devices and methods also provide mechanisms for continuous constriction and variable blood flow through the aorta.  
       BACKGROUND OF THE INVENTION  
       [0003]     Patients experiencing cerebral ischemia often suffer from disabilities ranging from transient neurological deficit to irreversible damage (stroke) or death. Cerebral ischemia, i.e., reduction or cessation of blood flow to the central nervous system, can be characterized as either global or focal. Global cerebral ischemia refers to reduction of blood flow within the cerebral vasculature resulting from systemic circulatory failure caused by, e.g., shock, cardiac failure, or cardiac arrest. Shock is the state in which failure of the circulatory system to maintain adequate cellular perfusion results in reduction of oxygen and nutrients to tissues. Within minutes of circulatory failure, tissues become ischemic, particularly in the heart and brain.  
         [0004]     The most common form of shock is cardiogenic shock, which results from severe depression of cardiac performance. The most frequent cause of cardiogenic shock is myocardial infarction with loss of substantial muscle mass. Pump failure can also result from acute myocarditis or from depression of myocardial contractility following cardiac arrest or prolonged cardiopulmonary bypass. Mechanical abnormalities, such as severe valvular stenosis, massive aortic or mitral regurgitation, acutely acquired ventricular septal defects, can also cause cardiogenic shock by reducing cardiac output. Additional causes of cardiogenic shock include cardiac arrhythmia, such as ventricular fibrillation.  
         [0005]     Treatment of global cerebral ischemia involves treating the source of the systemic circulatory failure and ensuring adequate perfusion to the central nervous system. For example, treatment of cardiogenic shock due to prolonged cardiopulmonary bypass consists of cardiovascular support with the combination of inotropic agents such as dopamine, dobutamine, or amrinone and intra-aortic balloon counterpulsation. Vasoconstrictors, such as norepinephrine, are also administered systemically to maintain systolic blood pressure (at approximately above 80 mmHg). Unfortunately, these agents produce a pressure at the expense of flow, particularly blood flow to small vessels such as the renal arteries. The use of the vasoconstrictors is, therefore, associated with significant side effects, such as acute renal failure.  
         [0006]     Focal cerebral ischemia refers to cessation or reduction of blood flow within the cerebral vasculature resulting from a partial or complete occlusion in the intracranial or extracranial cerebral arteries. Such occlusion typically results in stroke, a syndrome characterized by the acute onset of a neurological deficit that persists for at least 24 hours, reflecting focal involvement of the central nervous system and is the result of a disturbance of the cerebral circulation. Other causes of focal cerebral ischemia include vasospasm due to subarachnoid hemorrhage or iatrogenic intervention.  
         [0007]     Traditionally, emergent management of acute ischemic stroke consists of mainly general supportive care, e.g. hydration, monitoring neurological status, blood pressure control, and/or anti-platelet or anti-coagulation therapy. Heparin has been administered to stroke patients with limited and inconsistent effectiveness. In some circumstances, the ischemia resolves itself over a period of time due to the fact that some thrombi get absorbed into the circulation, or fragment and travel distally over a period of a few days. In June 1996, the Food and Drug Administration approved the use of tissue plasminogen activator (t-PA) or Activase®, for treating acute stroke. However, treatment with systemic t-PA is associated with increased risk of intracerebral hemorrhage and other hemorrhagic complications. Aside from the administration of thrombolytic agents and heparin, there are no therapeutic options currently on the market for patients suffering from occlusion focal cerebral ischemia. Vasospasm may be partially responsive to vasodilating agents. The newly developing field of neurovascular surgery, which involves placing minimally invasive devices within the carotid arteries to physically remove the offending lesion may provide a therapeutic option for these patients in the future, although this kind of manipulation may lead to vasospasm itself.  
         [0008]     In both global and focal ischemia, patients develop neurologic deficits due to the reduction in cerebral blood flow. Treatments should include measures to increase blood flow to the cerebral vasculature to maintain viability of neural tissue, thereby increasing the length of time available for interventional treatment and minimizing neurologic deficit while waiting for resolution of the ischemia. Augmenting blood flow to the cerebral vasculature is not only useful in treating cerebral ischemia, but may also be useful during interventional procedures, such as carotid angioplasty, stenting or endarterectomy, which might otherwise result in focal cerebral ischemia, and also cardiac procedures which may result in global cerebral ischemia, such as cardiac catheterization, electrophysiologic studies, and angioplasty.  
         [0009]     New devices and methods are thus needed for augmentation of cerebral blood flow in treating patients with either global or focal ischemia caused by reduced perfusion, thereby minimizing neurologic deficits.  
       SUMMARY OF THE INVENTION  
       [0010]     The invention provides vascular constriction devices and methods for augmenting blood flow to a patient&#39;s cerebral vasculature, including the carotid and vertebral arteries. The devices constructed according to the present invention comprise a constricting mechanism distally mounted on a catheter for delivery to a vessel, such as the aorta. The constrictor is collapsed to facilitate insertion into and removal from the vessel, and expanded during use to restrict blood flow. When expanded, the constrictor has a maximum periphery that conforms to the inner wall of the vessel, thereby providing a sealed contact between it and the vessel wall. The constrictor typically has a blood conduit allowing blood flow from a location upstream to a location downstream. The devices further include a variable flow mechanism in operative association with the blood conduit, thereby allowing blood flow through the conduit to be adjusted and controlled. The devices can optionally include a manometer and/or pressure limiter to provide feedback to the variable flow mechanism for precise control of the upstream and downstream blood pressure. Other medical devices, such as an infusion, atherectomy, angioplasty, hypothermia catheters or devices (selective cerebral hypothermia with or without systemic hypothermia, and typically hypothermia will be combined with measures to increase perfusion to overcome the decreased cerebral blood flow caused by the hypothermia, such that hypothermia and coarctation are complimentary), or electrophysiologic study (EPS) catheter, can be introduced through the constrictor to insert in the vessel to provide therapeutic interventions at any site rostrally.  
         [0011]     In a preferred embodiment, the expandable constrictor comprises an outer conical shell and an inner conical shell. Each shell has an apex and an open base to receive blood flow. One or a plurality of ports traverses the walls of the two conical shells. Blood flows through the open base and through the ports. The inner shell can be rotated relative to the outer shell so that the ports align or misalign with the ports in the outer shell to allow variable blood flow past the occluder, thereby providing adjustable and controlled flow. The inner shell is rotated by a rotating mechanism, e.g., a torque cable disposed within the elongate tube and coupled to the inner shell. The constrictor can be expanded by, e.g., a resilient pre-shaped ring, graduated rings, or a beveled lip formed at the base of the shell, and collapsed by, e.g., pull wires distally affixed to the occluder or a guide sheath.  
         [0012]     In another embodiment, the outer conical shell includes a plurality of resilient flaps, which are pivotally affixed to the base or the apex and can be displaced to variably control blood flow through the conduit. The flaps can be displaced by a plurality of pull wires affixed to the flaps.  
         [0013]     In still another embodiment, the constrictor comprises a first cylindrical balloon mounted to a distal end of the catheter, and a second toroidal balloon disposed about the cylindrical balloon. The chamber of the first balloon communicates with an inflation lumen. Blood flow occurs through the cylindrical balloon and through the center of the toroidal balloon. The toroidal balloon is expanded by inflation through a second and independent inflation lumen to reduce blood flow through the cylindrical balloon. In this manner, the first balloon provides an inflatable sleeve and the second toroidal balloon provides variable control of blood flow through the sleeve. Other embodiments include an expandable sleeve (not a balloon) surrounded by a toroidal balloon for adjustably constricting the flow of blood through the cylindrical sleeve.  
         [0014]     In a preferred method, the occlusion devices described above are inserted into the descending aorta through an incision on a peripheral artery, such as the femoral, subclavian, axillary or radial artery, in a patient suffering from global or focal cerebral ischemia, during cardiac surgery (including any operation on the heart, with or without CPB), or during aortic surgery (during circulatory arrest, as for aortic arch surgery, repair of an abdominal aortic aneurysm, or thoracic aneurysm repair, to reduce perfusion and the amount of blood loss in the operating field). The devices can be introduced over a guide wire. With assistance of transesophageal echocardiography (TEE), transthoracic echocardiography (TTE), intravascular ultrasound (IVUS), aortic arch cutaneous ultrasound, or angiogram, the constrictor is positioned downstream from the takeoff of the brachiocephalic artery and upstream from the renal arteries. The constrictor is expanded to partially occlude blood flow in the aorta and maintained during systole, during diastole, or during systole and diastole. The constrictor preferably achieves continuous apposition to the wall of the vessel, resulting in fewer emboli dislodgment. The pressure limiter, connected to the rotary unit and the pressure monitor, prevents the upstream and downstream blood pressure from exceeding, respectively, a set maximum and minimum pressure differential.  
         [0015]     Flow rates can be varied within one cardiac cycle (e.g., 80% during systole, 20% during diastole, or 70% during systole, 30% during diastole), and every few cycles or seconds (e.g., 80% for 6 cycles, 20% for 2 cycles, or 70% for 5 cycles, 10% for 1 cycle). In certain cases it may be preferred to cycle to cycle between lesser and greater occlusion so that the brain does not autoregulate. This ensures constant and continued increased cerebral perfusion. In this manner, blood in the descending aorta is diverted to the cerebral vasculature, thereby increasing cerebral perfusion and minimizing neurological deficits. By selectively increasing cerebral blood flow, the use of systemically administered vasoconstrictors or inotropic agents to treat shock may be reduced or eliminated.  
         [0016]     In another method, in patients anticipating a major cardiothoracic surgery, such as abdominal aortic aneurysm repair, the device is introduced and deployed approximately 24 hours prior to surgery, thereby inducing mild artificial spinal ischemia. This induces endogenous neuroprotective agents to be released by the spinal cord and/or brain in response to the ischemia, thereby protecting the tissue from ischemic insult of surgery. This technique is known as“conditioning”. The devices are inserted into the descending aorta. To induce spinal ischemia, the constrictor is positioned downstream from the takeoff of the brachiocephalic artery and upstream from the renal artery and expanded to partially occlude blood flow in the aorta, resulting in reduction of blood flow to the spinal cord. A similar technique may be employed to condition the brain to stimulate production of neuroprotective agents. To induce cerebral ischemia, the constrictor is positioned upstream from the takeoff of the innominate artery, or between the innominate artery and the left common carotid artery.  
         [0017]     Prolonged hypertension often causes ischemic damage to the kidneys. In still another method, the partial occlusion devices are introduced peripherally and positioned in the renal arteries to reduce blood pressure to the renal vasculature, thereby minimizing damage to the kidneys that might otherwise result from hypertension.  
         [0018]     It will be understood that there are many advantages in using the partial aortic occlusion devices and methods disclosed herein. For example, the devices can be used (1) to provide variable partial occlusion of a vessel; (2) to augment and maintain cerebral perfusion in patients suffering from global or focal ischemia; (3) to condition the brain or spinal cord to secrete neuroprotective agents prior to a major surgery which will necessitate reduced cerebral or spinal perfusion; (4) to prolong the therapeutic window in global or focal ischemia; (5) to accommodate other medical devices, such as an atherectomy catheter; (6) prophylactically by an interventional radiologist, neuroradiologist, or cardiologist in an angiogram or fluoroscopy suite; (7) for prevention of cerebral ischemia in patients undergoing procedures, such as coronary catheterization or surgery, where cardiac output might fall as a result of arrhythmia, myocardial infarction or failure; (8) to treat shock, thereby eliminating or reducing the use of systemic vasoconstrictors; and (8) to prevent renal damage in hypertensives. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  illustrates a patient&#39;s systemic arterial circulation relevant to the present invention.  
         [0020]      FIG. 2  illustrates an embodiment of the devices constructed according to the present invention for providing partial occlusion of a vessel.  
         [0021]      FIG. 3  illustrates a constrictor of the device depicted in  FIG. 2 .  
         [0022]      FIG. 4A  illustrates an outer conical shell employed in the constrictor of  FIG. 3 .  
         [0023]      FIG. 4B  illustrates an inner conical shell employed in the constrictor of  FIG. 3 .  
         [0024]      FIG. 5  illustrates an alternative embodiment of the constrictors of  FIG. 3  having elongate rectangular ports.  
         [0025]      FIG. 6  illustrates another embodiment of the occluder having a beveled lip.  
         [0026]      FIG. 7  illustrates another embodiment of the occluder having a plurality of graduated rings.  
         [0027]      FIG. 8  illustrates complete misalignment of the ports on the outer and inner conical shells.  
         [0028]      FIG. 9  illustrates partial alignment of the ports on the outer and inner conical shells.  
         [0029]      FIG. 10  illustrates complete alignment of the ports on the outer and inner conical shells.  
         [0030]      FIG. 11  illustrates another embodiment of the device for providing partial occlusion of a vessel.  
         [0031]      FIG. 12  illustrates another embodiment of the constrictor employed in the device of  FIG. 11 .  
         [0032]      FIG. 13A  illustrates a frontal view of the constrictor of  FIG. 12  having a plurality of preformed flaps extending perpendicular to the longitudinal axis of the constrictor.  
         [0033]      FIG. 13B  illustrates a frontal view of the flaps of  FIG. 13A  under an external force.  
         [0034]      FIG. 13C  illustrates a frontal view of the constrictor of  FIG. 12  having a plurality of preformed flaps extending parallel to the longitudinal axis of the constrictor.  
         [0035]      FIG. 13D  illustrates a frontal view of the flaps of  FIG. 13C  under an external force.  
         [0036]      FIG. 14  illustrates another embodiment of the occluder having flaps included in the collar of the outer conical shell.  
         [0037]      FIG. 15  illustrates still another embodiment of the device for providing partial occlusion of a vessel.  
         [0038]      FIG. 16  illustrates an embodiment of the constrictor employed in the device of  FIG. 15 .  
         [0039]      FIG. 17  illustrates the constrictor of  FIG. 16 , having an inflated ring-shaped balloon for reducing blood flow through a blood conduit.  
         [0040]      FIG. 18  illustrates the occluder of  FIG. 16 , having a deflated ring-shaped balloon.  
         [0041]      FIG. 19  illustrates a suction/atherectomy catheter introduced through the constrictor of  FIG. 16 .  
         [0042]      FIG. 20  illustrates a perfusion and an EPS catheter introduced through the constrictor of  FIG. 16 .  
         [0043]      FIG. 21A  illustrates the constrictor of  FIG. 3  inserted in the aorta downstream from the left subclavian artery and partially occluding aortic blood flow.  
         [0044]      FIG. 21B  illustrates the constrictor of  FIG. 14  inserted in the aorta downstream from the left subclavian artery and partially occluding aortic blood flow.  
         [0045]      FIG. 22  illustrates the constrictor of  FIG. 3  inserted in the aorta downstream from the right brachiocephalic artery and partially occluding aortic blood flow.  
         [0046]      FIG. 23  illustrates a suction/atherectomy catheter introduced through the constrictor of  FIG. 3  and inserted in the left carotid artery proximal to a thromboembolic occlusion.  
         [0047]      FIG. 24  illustrates the constrictor of  FIG. 3  inserted in the aorta upstream from the lumbar or lumbar or spinal arteries.  
         [0048]      FIG. 25  illustrates the constrictor of  FIG. 3  inserted in the renal arteries. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0049]     The devices and methods disclosed herein are most useful in treating patients suffering from global cerebral ischemia due to systemic circulatory failure, and focal cerebral ischemia due to thromboembolic occlusion of the cerebral vasculature. However, it will be understood that the devices and methods can be used in other medical conditions, such as hypertension and spinal cord conditioning.  
         [0050]     Systemic arterial circulation relevant to the methods of the present invention is described in  FIG. 1 . During systole, oxygenated blood leaving heart  8  enters aorta  10 , which includes ascending aorta  12 , aortic arch  14 , and descending aorta  22 . The aortic arch gives rise to brachiocephalic trunk  16 , left common carotid artery  18 , and left subclavian artery  20 . The brachiocephalic trunk branches into right common carotid artery  24  and right subclavian artery  26 . The right and left subclavian arteries, respectively, give rise to right vertebral artery  28  and left vertebral artery  34 . The descending aorta gives rise to a multitude of arteries, including lumbar (i.e., spinal) arteries  38 , which perfuse the spinal cord, renal arteries  40 , which perfuse the kidneys, and femoral arteries  42 , which perfuse the lower extremities.  
         [0051]      FIG. 2  depicts occlusion catheter  100  for use in the methods described herein. The device includes elongate catheter  102 , distally mounted expandable constrictor, i.e., occluder,  104  having distal opening  124  and variable flow mechanism  108 . The constrictor, when expanded, has maximum periphery  110 , which conforms to the inner wall of a vessel to form a secure seal with the vascular wall, such that blood flow through the vessel can be effectively controlled. Opening  124  receives blood from distal the constrictor and controls the passage of blood proximal the constrictor. Variable flow mechanism  108 , connected to rotary unit  150 , operates the constrictor, thereby controlling (1) the flow rate through the occlusion, and (2) upstream blood pressure. Preferably, the device includes manometer  112 , which is connected to pressure monitor  156  and pressure limiter  114 . Rotary unit  150  receives blood pressure measurements from the manometer. Pressure limiter  114 , connected to the rotary unit and the pressure monitor, prevents the upstream and downstream blood pressure from exceeding, respectively, a set maximum and minimum pressure differential. A proximal end of the catheter is equipped with adapter  103 , from which pull wires  132  can be manipulated for collapsing the occluder and to which the rotary unit, pressure monitor, and/or pressure limiter can be connected.  
         [0052]     Referring to  FIG. 3 , the occlusion device comprises catheter  102  and constrictor  104 . The catheter is constructed from a biocompatible and flexible material, e.g., polyurethane, polyvinyl chloride, polyethylene, nylon, etc. The catheter includes lumen  116  through which various operative elements pass. Alternatively, the catheter may include more than one lumen to support various operative elements. The catheter also includes proximal adapter  103  (see  FIG. 2 ), which provides an interface between the catheter and the various instruments received by the catheter. The occluding mechanism consists of outer conical shell  118  and inner conical shell  136 , each having a distal open base and a proximal apex. Pre-shaped ring  130  is affixed to base  120  of the outer shell to facilitate expansion of the constrictor. The ring is formed of a resilient material, capable of expanding the occluder to achieve a maximum periphery, which is defined by the outer circumference of the ring. Ring  130 , may, in certain embodiments, further include an anchoring mechanism, such as hooks, bonded to the outer circumference of the ring. Expansion of the ring causes the grasping structure to engage the surface of the vessel wall, thereby securing the occluder and preventing displacement in the vessel due to force exerted by blood flow. In other embodiments, the anchoring is provided by an adhesive strip, vacuum, or merely by frictional engagement of the vessel lumen by the ring.  
         [0053]     The constrictor can be collapsed to facilitate insertion into and removal from a vessel. A plurality of pull wires  132  ( FIG. 2 ) are disposed within torque cable  148 , and are distally connected to base  120  of outer shell  118  and proximally passes through adapter  103 . The constrictor is collapsed by applying a tensile force on wires  132 , using torque cable  148  to provide leverage to the pull wires, thereby drawing the circumference of the open base  120  towards its center and collapsing the occluder. A guide sheath (not shown) can be alternatively used to collapse the constrictor. Using this technique, the guide sheath would cover the constrictor and be withdrawn to release the constrictor and advanced to collapse the constrictor.  
         [0054]     Opening  124  is formed in base  138  and  120  of the respective inner and outer conical shells to provide an inlet for blood flow. Conical interior  106  communicates with ports  128  of the outer shell. When the constrictor is deployed, blood flows into opening  124 , through interior  106 , and exits through ports  128 . The occluding mechanism comprises inner conical shell . 136  (partially shown in phantom in  FIG. 3 ), which is rotatably disposed within outer shell  118  as shown in  FIGS. 8, 9 , and  10 . The inner shell can be rotated relative to the outer shell through torque cable  148 , which is disposed in lumen  116  of catheter  102 .  
         [0055]     Manometer  112  comprises upstream pressure tube  152  and downstream pressure tube  154 , both connected proximally to a pressure monitor to provide respective blood pressure measurements upstream and downstream the constrictor. The upstream pressure tube extends distal to opening  124 , or may be attached to the inner shell. The downstream pressure tube extends through an orifice in the catheter proximal to the constrictor. The upstream and downstream blood pressure measurements are recorded and displayed by the pressure monitor at a proximal end of the catheter. A pressure limiter, programmed with a maximum pressure threshold to limit the upstream blood pressure and a minimum pressure threshold to limit the downstream blood pressure, is connected to the pressure monitor to receive pressure measurements therefrom, and transmits information to a rotary unit. The limiter thereby prevents the rotary unit from rotating the inner shell relative to the outer shell in a manner that would cause the upstream blood pressure to exceed the maximum threshold, or the downstream blood pressure to fall below the minimum threshold. Without the rotary unit, torque cable  148  can also be manually rotated to obtain desired upstream and downstream blood pressures. An audible alarm may be incorporated into the pressure limiter to sound when blood pressures exceeds the thresholds. The pressure limiter may further comprise an interlocking device. The interlocking device, in operative association with upstream and downstream tubes  152  and  154 , can lock inner shell  136  with respect to outer shell  118  as blood pressures approach the set thresholds. It should be noted that although the rotary unit, pressure monitor, and pressure limiter are shown as separate units, they may be incorporated into an integral unit.  
         [0056]     Referring to  FIGS. 4A and 4B , the expanded constrictor comprises outer conical shell  118  having base  120  and apex  122 , and inner conical shell  136  having base  138  and apex  140 . The constrictor is preferably composed of a biocompatible material coated with heparin to prevent blood clotting. The conical shape of the expanded constrictor minimizes turbulence caused by placement of the occluder in the vessel. The outer and inner shells include 2, 3, 4, 5, 6, or any other number of ports  128  and  144 , respectively, in communication with the conical interior to permit blood flow through the occluder. The inner shell can be rotated relative to the outer shell, so that ports  144  communicate with ports  128 . Apices  122  and  140  of the respective outer and inner shells further comprise collar  126  and  142 . The collars may include engaging threads, so that collar  142  can be inserted and secured into collar  126 , and bonded to a distal end of the torque cable, such that the inner shell is coupled to and rotates with the torque cable. A rotary unit, preferably including a stepper motor (not shown), may be mechanically coupled to a proximal end of the torque cable to provide precise rotational position of the inner shell relative to the outer shell, thereby providing variable flow through the occluder.  
         [0057]     Instead of having the circular ports in the inner and outer shells as depicted in  FIGS. 4A and 4B , the constrictor may include 2, 3, 4, 5, 6, or any other number of ports having other suitable geometric shapes.  FIG. 5  depicts constrictor  104  having a plurality of ports constructed as elongate rectangular slots  175 .  
         [0058]      FIG. 6  depicts another embodiment of the constrictor, which comprises beveled lip  140  having distal end  142  and proximal end  141 . The proximal end is affixed to base  120  of the outer conical shell. The proximal end has a larger diameter than the distal end and is everted to prevent the constrictor from being displaced in the direction of blood flow, thereby securing the constrictor in the vessel.  
         [0059]     Still another embodiment of the occluder may includes 1, 2, 3, 4, 5, or any other number of graduated inflatable rings. In  FIG. 7 , ring  151  is affixed to the base of the conical shell. Ring  153 , having the smallest inflated diameter, is attached to ring  152 , which is then attached to ring  151 , having the largest inflatable diameter. The fully inflated rings will have a thickness of approximately 2 to 3 millimeters. Similar to the beveled lip of  FIG. 8 , the rings prevent the outer conical shell from being displaced in the direction of blood flow, thereby securing the constrictor in the vessel.  
         [0060]     The flow rate of blood through the constrictor can be easily controlled by rotating inner conical shell  136  (shown with dotted lines) relative to outer conical shell  118  as depicted in  FIGS. 8, 9 , and  10 . In  FIG. 8 , the inner shell is rotated so that ports  144  and  128  are completely misaligned, thereby achieving no flow through the ports and complete vascular occlusion distally. As the inner shell is rotated clockwise relative to the second shell in  FIG. 9 , ports  144  on the inner shell become partially aligned with ports  128  on the outer shell, thereby achieving partial flow through the ports and partial vascular occlusion. In  FIG. 10 , with continuing clockwise rotation of the inner shell, ports  144  become completely aligned with ports  128 , thereby achieving maximum flow through the ports. To provide a broader and more predictable range of blood flow through the conduit, the ports of the inner and outer shells are preferably of equal size and number such that they may align with each other.  
         [0061]      FIG. 11  depicts another embodiment of the occlusion device for partial occlusion of blood flow in a vessel. Device  200  comprises elongate catheter  202 , distally mounted expandable constrictor  204  with maximum periphery  210 , opening  224 , and variable flow mechanism  208  operatively associated with the constrictor. The catheter includes adapter  203  at its proximal end. Preferably, the device includes manometer  212  and pressure limiter  214 , and pressure monitor  240 . The pressure monitor records and displays blood pressure data received from the manometer. Longitudinal positioning unit  208 , receiving signals from pressure limiter  214 , and controls variable flow mechanism  208  to provide variable blood flow through the constrictor.  
         [0062]     Referring to  FIG. 12 , catheter  202  includes lumen  216 . Constrictor  204  comprises hollow conical shell  218  having base  220  and apex  222 . The inner circumference of the base forms opening  224 , which provides a distal inlet for blood flow through the constrictor. The inner circumference of apex  222  forms collar  228  with proximal opening  226 , which provide an outlet for blood flow through the constrictor. The conical interior, disposed within shell  218 , communicates with opening  224  distally and opening  226  proximally. When the base of the constrictor is positioned upstream in a vessel, blood flows into opening  224 , through the conical interior, and exits downstream through opening  226 . The catheter is bonded to collar  228  about a portion of its inner circumference. The constrictor is expanded by operation of ring  230 , a beveled lip, or a series of graduated toroidal balloons as described above. The constrictor is collapsed and may be delivered to a vessel location by using a guide sheath.  
         [0063]     The manometer comprises upstream pressure tube  236  and downstream pressure tube  238 , which are disposed in lumen  216  of the catheter and connected proximally to a pressure monitor. The upstream pressure tube extends distal from the constrictor or may be bonded to the inner surface of the conical shell, thereby providing upstream blood pressure measurement. The downstream pressure tube extends through an orifice in the catheter proximal to the constrictor, thereby providing downstream blood pressure measurement.  
         [0064]     The variable flow mechanism comprises a plurality of flaps  230  pivotally affixed to base  220 . The flaps are preferably made of a resilient material, such as Nitinol, to resist movement caused by blood flow through the conduit. A plurality of pull wires  232 , disposed through lumen  216 , are distally connected to flaps  230 , such that applying a tensile force to the wires pivotally displaces flaps  230  from their preformed position. Three of the flaps (shown in dotted lines) are displaced inward. Releasing the wires allows the resilient flaps to relax and return to their preformed position. The pull wires are coupled proximally to the longitudinal positioning unit, which provides precise displacement of the flaps relative to opening  224 . Alternatively, wires  232  can be manually tensed to operate the flaps. The pressure limiter receives pressure measurements from the pressure monitor and transmits signals to the longitudinal positioning unit to prevent the upstream and downstream blood pressures from exceeding the set thresholds.  
         [0065]      FIGS. 13A, 13B ,  13 C, and  13 D depict frontal views of the constrictor having flaps in various positions for controlling blood flow. In  FIG. 13A , preformed flaps  230  extend radially inward toward the longitudinal axis of the catheter, as in the absence of a displacing force, i.e., an external force other than that created by blood flow. When the constrictor is positioned in the descending aorta, for example, the size of opening  224  and blood flow through the opening is minimized, thereby providing maximal aortic occlusion. In the presence of a displacing force, such as pulling the wires to displace flaps  230  from their preformed position as depicted in  FIG. 13B , the size of aperture  224  and blood flow through the conduit increases, thereby providing partial aortic occlusion.  
         [0066]     Alternatively, preformed flaps  230  extend parallel to the longitudinal axis of opening  224  in the absence of a displacing force as depicted in  FIG. 13C . The size of opening  224  and blood flow through the conduit are maximized, thereby providing minimal blood flow occlusion. In the presence of a displacing force, flaps  230  are pivotally displaced from their preformed position as depicted in  FIG. 13D . The size of opening  224  and blood flow through the opening are minimized, thereby providing maximal blood flow occlusion. Thus, by pivotally displacing flaps  230  relative to opening  224 , the size of the opening and flow rate through the constrictor is controlled to provide variable vessel occlusion.  
         [0067]     The constrictor shown in  FIG. 12  can be alternatively mounted on catheter  202 , such that base  220  is proximal to apex  222  as shown in  FIG. 14A . In this embodiment, flaps  230  are formed on open apex  222 . When constrictor  204  is inserted downstream in the aorta, for example, pressure tube  238  extends distally from opening  226  to provide downstream blood pressure measurements, whereas pressure tube  236  extends proximally through an orifice in the catheter to provide upstream blood pressure measurements.  
         [0068]     In  FIG. 15 , another embodiment of the device comprises catheter  302 , a distally mounted occluder  304  with maximum periphery  310 , blood passage  306  disposed within the constrictor, and variable flow mechanism  308  in operative association with the blood conduit. Inflation device  334  communicates with the constrictor, and inflation device  338  communicates with the variable flow mechanism. The device preferably includes proximal adapter  303 , manometer  312 , and pressure limiter  314 . Pressure monitor  312  records and displays blood pressure data from the manometer. The pressure limiter is connected to the pressure monitor and to an interlocking valve on inflation device  338 , such that the blood pressure upstream and downstream the constrictor can be controlled to prevent from exceeding set thresholds.  
         [0069]     Referring to  FIG. 16 , constrictor  304  is mounted to a distal end of catheter  302  having lumen  316 . The constrictor comprises a sleeve or cylindrical balloon  318  having outer wall  320  and inner wall  322 , which enclose chamber  323 . The cylindrical balloon has first end  324  with opening  328  and second end  326  with opening  330 . Catheter  302  is bonded to inner wall  322  of the cylindrical balloon. Inflation tube  332 , housed within lumen  316  of the catheter, communicates distally with the cylindrical balloon and proximally with a syringe or other inflation device. The cylindrical balloon can be expanded or collapsed by injecting or removing air, saline, or other medium. Occlusion is provided by toroidal balloon  334  disposed about the outer or inner surface of sleeve  318  and communicating with inflation tube  336  and a syringe. The inflation device may include an interlocking valve to prevent unintended deflation.  
         [0070]     Lumen  306  communicates with opening  328  distally and opening  328  proximally. When deployed in a vessel, blood flows through lumen  306  and exits downstream opening  330 . The constrictor may further include an anchoring structure, shown in  FIG. 16  as rings  333 , which are disposed about outer wall  320  of the cylindrical sleeve and define maximum periphery  310  of the occluder.  
         [0071]     Manometer  312  comprises upstream pressure tube  340  and downstream pressure tube  342 , which are operatively connected proximally to a pressure monitor. Pressure tube  340  is bonded to the lumen of the cylindrical balloon and extends distal to provide upstream blood pressure measurements, while tube  342  emerges from the catheter proximal the occluder to provide downstream blood pressure measurements.  
         [0072]     In  FIG. 17 , fluid is injected to expand balloon  334 , thereby constricting sleeve  318 . As a result, blood flow is constricted. In  FIG. 18 , balloon deflation allows sleeve  318  to revert back to its pre-shaped geometry, increasing blood flow therethrough. Thus, balloon  334  can be inflated and deflated to vary the cross-sectional diameter of lumen  306  to vary flow rate.  
         [0073]     The occlusion devices described herein can be employed with a variety of therapeutic catheters to treat vascular abnormalities. For example, as depicted in  FIG. 19 , suction/atherectomy catheter  402  can be inserted through lumen  306 , such that the suction/atherectomy catheter is independently movable relative to occlusive device  300 . Catheter  402  includes elongate tube  404  and distally located aspiration port  406 , cutting device  408 , and balloon  410  for removing thromboembolic material in a vessel.  
         [0074]     In  FIG. 20 , infusion catheter  502  and EPS catheter  504  are inserted through opening  206  of occlusion device  200 , such that catheter  502  and  504  are independently movable relative to occlusion device  200 . The infusion catheter, which includes elongate tube  506 , distally located perfusion port  508 , and expandable balloon  510 , can be used to remove thromboembolic material in a vessel. EPS catheter  504 , which includes elongate tube  512  and distally located ablation device  514 , may be used to map out or ablate an extra conduction pathway in the myocardial tissue, e.g., in patients suffering from Wolff-Parkinson-White syndrome. The occlusion device, capable of augmenting cerebral perfusion, is therefore useful not only in facilitating definitive treatment but also in cerebral ischemia prevention during EPS and other cardiac interventions or cardiac surgery, such as coronary catheterization, where sudden fall in cerebral blood flow may occur due to arrhythmia, myocardial infarction, or congestive heart failure.  
         [0075]     Referring to  FIG. 21A , occlusion device  100  described above can be used to partially occlude blood flow in aorta  10  of a patient suffering from global cerebral ischemia due to, e.g., septic shock, congestive heart failure, or cardiac arrest. Constrictor  104  can be introduced in its collapsed geometry through an incision on a peripheral artery, such as the femoral, subclavian, axillary, or radial artery, into the patient&#39;s aorta. A guide wire may first be introduced over a needle, and the collapsed constrictor is then passed over the guide wire and the needle to position distal to the takeoff of left subclavian artery  20  in the descending aorta. The constrictor is expanded, such that maximum periphery  110  of the occluder, formed by expandable ring  130 , sealingly contacts the inner aortic wall. The position and orientation of the collapsed or expanded device can be checked by TEE, TTE, aortic arch cutaneous ultrasound in the emergency room, or IVUS and angiography in the angiogram suite.  
         [0076]     The expanded constrictor is maintained during systole, during diastole, or during systole and diastole, during which blood distal to the brachiocephalic artery is forced to pass through opening  106 , thereby providing a continuous partial occlusion of aortic blood flow. Alternatively, partial occlusion of aortic blood flow can be intermittent. As a result, blood flow to the descending aorta is partially diverted to brachiocephalic artery  16 , left subclavian artery  20 , and left carotid artery  18 , thereby augmenting blood flow to the cerebral vasculature. In treating global ischemia, such as in shock, cerebral perfusion is increased by increasing blood flow through both carotid and vertebral arteries. Additionally, blood flow to the aorta is partially diverted to the coronary arteries by using the occlusion device, thereby augmenting flow to the coronary arteries. Using the partial occlusion methods during systemic circulatory failure may, therefore, improve cardiac performance and organ perfusion. By selectively increasing cerebral and coronary blood flow in this manner, the dosage of commonly used systemic vasoconstrictors, such as dopamine and norepinephrine, may be reduced or eliminated.  
         [0077]     Alternatively, the device of  FIG. 14 , much like the device used to extinguish the flame of a candle, can be introduced through an incision on left subclavian artery  36  as depicted in  FIG. 21B . Constrictor  204  is inserted in aorta  22  distal to the takeoff of the left subclavian artery to provide partial, variable, and/or continuous aortic occlusion and is advanced antegrade into the descending aorta. This device is particularly useful in situations where peripheral incision can not be made on the femoral arteries due to arteriosclerosis, thrombosis, aneurysm, or stenosis.  
         [0078]     The devices and methods described in  FIGS. 21A and 21B  are useful in treating stroke patients within few minutes of stroke symptom, and the treatment can be continued up to 96 hours or more. For example, in treating focal ischemia due to a thromboembolic occlusion in the right internal carotid artery the constrictor may be position distal to the takeoff of the left subclavian. As a result, blood flow is diverted to brachiocephalic artery  16  and left CCA to augment both ipsilateral and contralateral collateral circulation by reversing direction of flow across the Circle of Willis, i.e., increasing flow in the right external carotid artery and left common carotid artery. The collateral cerebral circulation is further described in details in co-pending U.S. application Ser. No. [Lyon &amp; Lyon Docket No. 239/096], incorporated herein by reference.  
         [0079]     In treating focal ischemia due to a thromboembolic occlusion in the left internal carotid artery, for example, the constrictor can be positioned proximal to the takeoff of left carotid artery  18  and distal to the takeoff of brachiocephalic artery  16  as shown in  FIG. 22 . Contralateral collateral enhancement is provided by increasing flow through the brachiocephalic artery, thereby reversing blood flow in the right posterior communicating artery, right PCA, left posterior communicating artery  68  and anterior communicating artery, resulting in increased perfusion to the ischemic area distal to the occlusion and minimizing neurological deficits. Alternatively, the constrictor may be positioned distal to the takeoff of the left subclavian artery to provide both ipsilateral and contralateral collateral augmentation. Ipsilateral circulation is enhanced by increasing flow through the left external carotid artery and reversing flow along the left ophthalmic artery, both of which contribute to increased flow in the left ICA distal to the occlusion.  
         [0080]     As a result of partially occluding aortic blood flow, blood pressure distal to the aortic occlusion may decrease, and this may result in a reduction in renal output. Blood pressure proximal the aortic occlusion will increase and may result in excessive rostral hypertension. The blood pressures, measured by the manometer, are monitored continuously, and based on this information the occlusion is adjusted to avoid peripheral organ damage. After resolution of the cerebral ischemia, the constrictor is collapsed and removed, thereby removing the aortic occlusion and restoring normal blood flow in the aorta.  
         [0081]     In  FIG. 23 , constrictor  304  is inserted in aorta  10  and can be used to remove thromboembolic material  72  from left common carotid artery  18 , while augmenting and maintaining cerebral perfusion distal to the occluding lesion. The occluder may be introduced through a guide sheath until it is positioned distal to left subclavian artery  20 . In emergency situations, the constrictor can be inserted through a femoral incision in the emergency room, and atherectomy/suction catheter  402  can be inserted through the constrictor under angioscopic vision in the angiogram suite after the patient is stabilized hemodynamically. The atherectomy/suction catheter, which includes expandable balloon  410 , distal aspiration port  406 , and atherectomy device  408 , is introduced through opening  306  until its distal end is positioned in left common carotid artery  18  proximal to the thromboembolic occlusion.  
         [0082]     Constrictor  304  is then expanded to partially occlude aortic blood flow, thereby increasing perfusion to the ischemic region distal to the occluding lesion by enhancing ipsilateral collateral flow through left external carotid artery  46  and left vertebral artery  34  and contralateral collateral flow to right carotid artery  24  and right vertebral artery  28 . The variable flow mechanism of constrictor  304  can be adjusted to control blood flow to the cerebral vasculature and the blood pressure. Balloon  410  of catheter  402  is expanded in the left common carotid artery, thereby creating a closed chamber between constrictor  410  and the thromboembolic occlusion. Suction can be applied to aspiration port  406  to create a negative pressure in the closed chamber, thereby increasing the pressure differential across the thromboembolic occlusion, which may dislodge the occluding lesion onto the aspiration port and remove the occluding lesion. Thromboembolic material  72  may be further removed by atherectomy device  408 . The methods herein can also be used to remove thromboembolic occlusion in the vertebral artery. The occlusion device  304 , therefore, not only augments cerebral perfusion in patients suffering from focal stroke or global ischemia, but also maintains cerebral perfusion while waiting for invasive or noninvasive intervention. The devices and methods of using atherectomy/suction catheter  102  are further described in copending U.S. application Ser. No. [Lyon &amp; Lyon Docket No. 239/096], incorporated herein by reference.  
         [0083]     During abdominal aortic aneurysm (AAA) surgery, lumbar or spinal arteries, which provide blood supply to the spinal cord, are often dissected away from the diseased abdominal aorta, resulting in reduction of blood flow to the spinal cord. The devices herein disclosed may be used to condition the spinal cord prior to AAA repair, thereby reducing the damage resulting from spinal ischemia during surgery. In  FIG. 24 , constrictor  104  is inserted in aorta  10  and expanded preferably distal to left subclavian artery  20  and proximal to lumbar arteries  38 . As a result, blood flow to the lumbar or spinal arteries is reduced. When this device is used in patients anticipating a major thoracoabdominal surgery, such as AAA repair, approximately 24 hours prior to surgery, blood flow to the lumbar arteries can be intentionally reduced to induce mild spinal ischemia, thereby conditioning the spinal cord to produce neuroprotective agents which may protect the spinal cord from more significant ischemic insult during surgery.  
         [0084]     In hypertension, end organ damage often results, e.g., cardiac, renal, and cerebral ischemia and infarction. The devices and methods herein may be employed in hypertension to protect the kidneys from ischemic insult. In  FIG. 25 , constrictors  104 , which can be introduced through a femoral artery, are inserted in right renal artery  80  and left renal artery  82 . The constrictors are expanded to partially occlude blood flow from descending aorta  10  to the renal arteries, thereby reducing blood pressure distal to the occlusion. The constrictors can be deployed for the duration of any systemic hypertensive condition, thereby protecting the kidneys from damage that might otherwise be caused by the hypertension.  
         [0085]     The length of the catheter will generally be between 20 to 150 centimeters, preferably approximately between 30 and 100 centimeters. The inner diameter of the catheter will generally be between 0.2 and 0.6 centimeters, preferably approximately 0.4 centimeters. The diameter of the base of the outer conical shell will generally be between 0.3 and 3.0 centimeters, preferably approximately 0.5 and 2.0 centimeters. The diameter of the inflated balloon occluder will generally be between 0.3 and 3.0 centimeters, preferably approximately 0.5 and 2.0 centimeters. The ports of the inner and outer conical shells will generally have a diameter of between 1 to 6 millimeters, preferably approximately 3 to 4 millimeters. The foregoing ranges are set forth solely for the purpose of illustrating typical device dimensions. The actual dimensions of a device constructed according to the principles of the present invention may obviously vary outside of the listed ranges without departing from those basic principles.  
         [0086]     Although the foregoing invention has, for the purposes of clarity and understanding, been described in some detail by way of illustration and example, it will be obvious that certain changes and modifications may be practiced which will still fall within the scope of the appended claims.