Patent Publication Number: US-2019167122-A1

Title: Sensor system for endovascular pulsation balloon

Description:
TECHNICAL FIELD 
     This disclosure relates to medical devices and, in particular, to sensors for endovascular assemblies for improving vascular compliance of a vessel. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Endovascular devices are commonly used in vascular passages when the vascular passageway becomes stiff and loses compliance. When a vascular passageway loses compliance due to, for example, age, congestive heart failure, or atherosclerosis, the vascular passageway stiffens and loses compliance, causing the heart to exert more force to effect the same volume of blood into the vascular passage. It is desirable to have a sensor system associated with endovascular devices to monitor blood flow within the vascular passageway and provide information to adjust parameters of implanted endovascular devices. 
     SUMMARY 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     In one embodiment, a system for monitoring the flow of blood within an endoluminal passage is provided comprising a medical device and a sensor. The medical device is implantable within the endoluminal passage and includes a shaft and an expandable segment coupled to the shaft. The expandable segment is movable between a first state and a second state. Movement of the expandable segment is responsive to a change in the fluid pressure external to the expandable segment. The sensor is positioned on the medical device and is adapted to collect information associated with the state of the expandable segment. 
     In another embodiment, a system for monitoring the flow of blood within an endoluminal passage is provided including a medical device and a plurality of sensors. The medical device is implantable within an endoluminal passage and includes a shaft and a plurality of expandable segments coupled to the shaft. The plurality of expandable segments are arranged linearly along the shaft. Each of the expandable segments is movable between a first state and a second state. Movement of the expandable segments is responsive to a change in fluid pressure external to the expandable segment. The plurality of sensors are positioned on the medical device and are configured to collect information associated with the states of the plurality of expandable segments. 
     In yet another embodiment, a method of determining a characteristic of the flow of blood within an endoluminal passage is provided, including implanting a medical device within an endoluminal passage and transmitting a signal generated by a sensor. The medical device includes a shaft, an expandable segment coupled to the shaft, and a sensor positioned on the medical device. The expandable segment is movable between an expanded state and a compressed state. Movement of the expandable segment is responsive to a change in the flow of blood within the endoluminal passage over the expandable segment. The signal is generated by the sensor responsive to the flow of blood in the endoluminal passage over the medical device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views. 
         FIG. 1  illustrates a cross-sectional view of an aortic passageway including a first example of a medical device; 
         FIG. 2  illustrates a cross-sectional view of a second example of the medical device; 
         FIG. 3  illustrates a cross-sectional view of a first example of the expandable segment; 
         FIG. 4  illustrates a cross-sectional view of a second example of an expandable segment; 
         FIG. 5  illustrates a cross-sectional view of a third example of an expandable segment; 
         FIG. 6  illustrates a cross-sectional view of a fourth example of an expandable segment; 
         FIG. 7  illustrates a cross-sectional view of a fifth example of the expandable segment; 
         FIG. 8  illustrates a cross-sectional view of a sixth example of the expandable segment; 
         FIG. 9  illustrates a cross-sectional view of a seventh example of the expandable segment; 
         FIG. 10  illustrates a cross-sectional view of a eighth example of the expandable segment; 
         FIG. 11  illustrates a flow diagram of operations to operate a medical device to reduce cardiac back pressure. 
     
    
    
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     In some examples, a medical device may be placed in the vascular passageway and manually inflated and deflated using a pump which coordinates inflation with the heartbeat of the patient. This configuration can be useful to assist the heart when the patient is stationary, such as when they are asleep or interred at a treatment facility. However, such a device cannot be used without the pumping assembly and therefore restricts the movement of the patient. 
     In treating particularly serious conditions, such as when a medical device is placed in a non-compliant aorta, frequent or constant monitoring of the blood flow within the endovascular passage may be necessary to ensure the health of the patient. 
     A system for monitoring the flow of blood within an endoluminal passage is provided comprising a medical device and a sensor. The medical device is implantable within the endoluminal passage and includes a shaft and an expandable segment coupled to the shaft. The expandable segment is movable between a first state and a second state. Movement of the expandable segment is responsive to a change in the fluid pressure external to the expandable segment. The sensor is positioned on the medical device and is adapted to collect information associated with the state of the expandable segment. 
     One interesting feature of the systems and methods described below may be that the expandable segment and a reservoir portion are configured to provide a passive gradient of flow through the medical device, passively working with the heartbeat of the patient to inflate and deflate the expandable segment with the proper timing, thereby reducing stress on the heart muscles. The lack of an active pump may grant increased freedom of movement to a patient utilizing the medical device. 
     Another interesting feature of the systems and methods described below may be that the sensor on the medical device may be able to detect and transmit information to the patient and/or physician indicating the status of the blood flow within the endoluminal passage. If the blood flow is non-optimal, the patient may be informed as quickly as possible, allowing the operating parameters of the medical device to be quickly adjusted and avoiding harm to the patient. Furthermore, other aspects of the medical care of the patient may be altered in response to information gathered in the operation of the medical device within the endoluminal passage, such as prescription or alteration of drugs to avoid heart failure. 
     Yet another interesting feature of the systems and method described below may be that the sensor on the medical device may require no power source located outside the body to gather and transmit data. Such a configuration may grant significant mobility to the patient. In some situations, current may be selectively induced through the sensor remotely from outside the body, ensuring that the patient need not be connected to any wires in order to gather and transmit data. 
       FIG. 1  illustrates a medical device  10  positioned in an endoluminal passage  14  of a patient  12 . The endoluminal passage  14  may be any passageway within the body of the patient  12  which is normally compliant to expansion and contraction under healthy conditions. Examples of the endovascular passage  14  may include the aorta, the common iliac arteries, or the subclavian arteries. For example,  FIG. 1  illustrates that the medical device  10  may be positioned in the aorta to reduce the effects of vascular non-compliance and reduce the stress on the left ventricle  16  of the heart to pump blood through the body. 
     The medical device  10  may include a shaft ( 46  in  FIG. 2 ), a reservoir portion  26 , an expandable segment  20 , and electrodes  36 . Examples of the shaft  46  may include any object which may be arranged longitudinally within the endoluminal passage  14  and which provides a structure on which to mount the expandable segment  20 . Examples of the shaft  46  may include a catheter, a sheath, or a wire guide. The shaft  46  may be tubular or solid. 
     The expandable segment  20  may be any portion of the medical device  10  which inflates and deflates to control blood flow in the endoluminal passage  14  to increase blood pressure within the endoluminal passage  14 . Examples of the expandable segment  20  may include a balloon, a bladder, or a sac. The expandable segment  20  may extend longitudinally along the shaft  46  from a first end  22  to a second end  24 . As shown in  FIG. 1 , the first end  22  of the expandable segment  20  may be positioned closer to the left ventricle  16  of the heart (sometime described as being in a more “proximal”, “upstream”, or “cranial” position) compared to the second end  24 . The second end  24  of the expandable segment  20  may be positioned closer to the iliac arteries (sometimes described as being in a more “distal”, “downstream”, or “caudal” position) than the first end  22 . The expandable segment  20  may be made of any material which can inflate and deflate responsive to blood flow and prevent fluid from inside the expandable segment  20  from leaking into the endoluminal passage  14 , such as rubber or a polymer. The expandable segment  20  may be inflated with fluids such as saline or carbon dioxide. 
     The reservoir portion  26  may be any component capable of receiving and storing fluid outside of the expandable segment  20 . Examples of the reservoir portion  26  may include a tube, a sac, an inflatable balloon, or some other container. The reservoir portion  26  may receive fluid from the expandable segment  20  from a tether  42  extending between the expandable segment  20  and the reservoir portion  26 . The tether  42  may be any object which couples together and provides fluid communication between the reservoir portion  26  and the expandable segment  20 . Examples of the tether  42  may include a tube, a catheter, or a sheath. The tether  42  may be an extension of the shaft  46 . 
     The reservoir portion  26 , expandable segment  20 , and tether  42  may form a closed fluid system. All of the reservoir portion  26 , expandable segment  20 , and tether  42  may be filled with a fluid having a density which is less than the density of blood within the endoluminal passage  14 . This closed fluid system provides a pumping mechanism when blood flows through the endoluminal passage  14 . While the expandable segment  20  is inflated within the endoluminal passage  14 , the cross-sectional area that it takes up within the endoluminal passage  14  simulates the changing size of a healthy, compliant endovascular passage  14  and increases the resting pressure of blood within the endoluminal passage  14 . The pumping of blood into the endovascular passage  14  by the left ventricle  16  may create a pressure wave which travels downstream from the left ventricle  16 . Within this pressure wave, blood flowing through the endoluminal passage  14  may apply increased pressure sequentially from the first end  22  to the second end  24  of the expandable segment  20 . Blood flow over a portion of the expandable segment  20  may deflate a portion of the expandable segment  20 , causing fluid within the expandable segment  20  to flow into the reservoir portion  26 . As the fluid enters the reservoir portion  26 , the pressure within the reservoir portion  26  may increase. The increased pressure within the reservoir portion  26  may provide a passive force on the fluid to flow back into the expandable segment  20  as the pressure wave travels past the expandable segment  20  and the pressure of the blood flow diminishes over the expandable segment  20 . 
     The reservoir portion  26  may include a container  28  which may have a fixed volume or which may be inflatable. The container  28  may be made of any material capable of retaining the fluid delivered from the expandable segment  20 , such as rubber or a polymer. The reservoir portion  26  may be located within the endoluminal passage  14 , inside the body of the patient  12  but outside the endoluminal passage  14 , or outside the body of the patient  12  altogether. In some embodiments, the reservoir portion  26  may be located within or directly alongside the expandable segment  20 . In such arrangements, a tether  42  may not be needed. In some embodiments, the reservoir portion  26  may include a port  58  coupled to the container  28 . The port  58  may extend through the skin  18  of the patient  12 . The port  58  may be used to add fluid to the closed fluid system of the expandable segment  20 , the reservoir portion  26 , and the tether  42 , or may be used to remove fluid from the closed fluid system. Adjusting the amount of fluid within the closed fluid system may control the range pressures within the expandable segment  20  and the range of inflation or deflation that the expandable segment  20  experiences. 
     The medical device  10  may also include a mounting element  40  which fixes the position of the expandable segment  20  within the endoluminal passage  14 . The mounting element  40  may be any object which is expandable to grip the walls of the endoluminal passage  14 . Examples of the mounting element  440  may include a self-expanding stent portion or barbs. The mounting element  40  may be arranged anywhere along the length of the shaft  46  or expandable segment  20 , such as, for example, at the first end  22  and the second end  24  of the expandable segment  20 . In some embodiments, the expandable segment  20  may include a mounting element  40  proximate to the first end  22  of the expandable segment  20  to prevent downstream movement of expandable segment  20  as blood flows through the endoluminal passage  14 . In other embodiments, the expandable segment  20  may include an additional mounting element  40  proximate the second end  24  of the expandable segment  20  to better fix the position of the expandable segment  20  within the endoluminal passage  14 . 
     The electrodes  36  may be located anywhere along the medical device  10  within the endoluminal passage  14 . For example, the electrodes  36  may be located on the surface ( 52  in  FIG. 2 ) of the expandable segment  20  or may be located on the shaft  46  at the first end  22  and the second end  24  of the expandable segment  20 . The electrodes  36  may be any devices which, when an electrical current is applied to them, generates an electromagnetic field between them. Examples of the electrodes  36  may be polarizable electrodes or non-polarizable electrodes. The electrodes  36  may be made of any material suitable to create an electrical potential across the electrodes  36  and which is stable and resistant to corrosion, such as carbon, platinum, or platinum iridium. The material of the surface  52  of the expandable segment  20  and the fluid within the expandable segment  20  may both have higher impedances than the blood within the endoluminal passage  14 . Therefore, the lower impedance to current between the electrodes  36  may indicate that the portion of the expandable segment  20  between the two electrodes  36  is deflated or deflating. Similar, an increase in the impedance between the two electrodes  36  may indicate that the portion of the expandable segment  20  between the two electrodes  36  is inflated or inflating. 
     In order to operate, the electrodes  36  may require an electrical current source. The electrical current source may be any device which is configured to provide a flow of electrons to the electrodes  36 . The electrical current source may provide an alternating current or a direct current. In some embodiments, the electric current should not result in heating or stimulation of tissue of the patient  12 . For example, an alternating current at a frequency of about 1 kHz may be used to prevent electrical stimulation of tissue. Examples of the electrical current source may be a battery ( 44  in  FIG. 2 ), or a magnetically activated induction coil  30 . As shown in  FIG. 1 , the induction coil  30  may be located close to the skin  18  of the patient  12 . A physician or operator may induce a current into the induction coil  30  across the skin  18  without puncturing the skin  18  to activate the electrodes  36  and determine the impedance in the circuit. 
     The induction coil  30  may be electrically coupled to the electrodes by a pair of wires  32 ,  34  which form a circuit with the electrodes  36 . The first wire  32  may deliver current the induction coil  30  to the first electrode  36  and the second wire  34  may return current from the second electrode  36 . The wires  32 ,  34  may be made of any material suitable to conduct electricity, such as copper or gold. In some embodiments, the induction coil  30  may be coupled to the reservoir portion  26 . In such embodiments, the wires  32 ,  34 , may run through or along the reservoir portion  26  and tether  42  to reach the shaft  46 . The wires  32 ,  34  may run along or through the shaft  46  to reach the electrodes  36 . 
     Other sensors may be used instead of or alongside the electrodes  36  to sense the inflated state of the expandable segment  20 . 
       FIG. 2  illustrates the medical device  10  having a plurality of expandable segments  20  arranged linearly along the shaft  46 . Each of the plurality of expandable segments  20  may be moveable between an inflated and deflated state, responsive to a change in the fluid pressure external to the expandable segment  20 . The expandable segments  20  are arranged such that as a pressure wave in the blood flow travels through the endoluminal passage  14 , the each expandable segments  20  will sequentially deflate and then reinflate as the pressure wave passes. Sequential deflation of the expandable segments  20  may maintain a more even resting blood pressure within the endoluminal passage  14 , further decreasing the pumping strain on the left ventricle  16 . 
     As shown in  FIG. 2 , in some embodiments, electrodes  36  may be arranged longitudinally along the shaft  46  of the medical device  10 , positioned on the shaft  46  between each of the expandable segments  20 . In such a configuration, impedance between each pair of electrodes  36  may be measured, indicating when each of the expandable segments  20  is deflated and reinflated. Information related to the inflation and deflation of the expandable segments  20  may be useful in ensuring that the medical device  10  is operating effectively. Furthermore, if one of the expandable segments  20  tears or forms a leak, the change in impedance may be readily apparent to an operator. 
     In some embodiments, the electrical current source for the electrodes  36  may be the battery  44 , as shown in  FIG. 2 . The battery  44  may be positioned anywhere along the medical device  10 , such as alongside the shaft  46 , expandable segments  20 , alongside reservoir portion  26 , inside the container  28 , or external to the body  12 . If the amount of potential electrical current of the battery  44  exceeds the amount of current required to operate the medical device  10  for its entire expected use, the battery  44  may be changed periodically. In such a configuration, the battery  44  may be located close to the skin  18  or to a port  58 , to allow easy replacement or charging of the battery  44 . The battery  44  may be coupled in parallel or in sequence to the plurality of electrodes  36  through the first wire  32  and the second wire  34 . 
     The medical device  10  may further include one or more piezoelectric sensors  48 . The piezoelectric sensor  48  may be any material which may release an electric charge in response to mechanical stress. Examples of the piezoelectric sensor  48  may include a ceramic material, a crystal, or a biological material. The piezoelectric sensor  48  may be positioned on the expandable segment  20  to detect whether the expandable segment  20  is inflating or deflating. For example, the piezoelectric sensor  48  may be bonded to the surface  52  of the expandable segment  20  exposed to the endoluminal passage  14  or within the interior  38  of the expandable segment  20 . Alternatively, the piezoelectric sensor  48  may be embedded within the surface  52  of the expandable segment  20 . When the expandable segment  20  inflates or deflates, the surface  52  of the expandable segment  20  may expand or contract, causing the piezoelectric sensor  48  to also expand or contract. Compression or expansion of the piezoelectric sensor  48  may release a charge which can be used to power a transmitter  50  electrically coupled to the piezoelectric sensor  48  to transmit a signal. The signal generated by the piezoelectric sensor  48  may be received by a receiver (not shown) to indicate to the operator the status of the expandable segment  20 . The transmitter  50  may be physically coupled to the piezoelectric sensor  48  or may be positioned elsewhere in the medical device, such as the shaft  46 , and only electrically coupled the piezoelectric sensor  48 . The transmitter  50  may be electrically powered only by the piezoelectric sensor  48  or may be powered by a different electrical current source, such as the battery  44 . 
     As shown in  FIG. 2 , the medical device  10  may contain a plurality of piezoelectric sensors  48  in a variety of configurations. The piezoelectric sensors  48  may be spaced such that each expandable segment  20  contains one piezoelectric sensor  48 . Alternatively, one of the expandable segments  20  may contain multiple piezoelectric sensors  48  arranged longitudinally or radially about the expandable segment  20 . 
       FIG. 3  illustrates an alternative arrangement of the electrodes  36  on the expandable segment  20 . On some embodiments, the electrodes  36  may be radially offset on the surface  52  of the expandable segment  20 . In such a configuration, the impedance between the electrodes  36  may decrease as the expandable segment  20  deflates, filling the space around the expandable segment  20  with increased electrically conductive blood and bringing the electrodes  36  physically closer together. 
       FIG. 4  illustrates an alternative arrangement of the electrodes  36  in to an electrode array  62 . The electrode array  62  may be any combination of three or more electrodes arranged on the medical device  10 . In some embodiments, the electrode array  62  may be a tetrapolar system of electrodes  36 , as shown in  FIG. 4 . The electrode array  62  may include a first pair  59  of electrodes  36  to pass current through the blood within the endoluminal passage  14 , and a second pair  60  of electrodes  36  to detect the potential across the blood. The second pair  60  of the electrodes  36  may be partially or entirely arranged in between the first pair  59  of the electrodes  36 . 
       FIG. 5  illustrates the expandable segment  20  having an inductor capacitor (L-C) sensor  54  positioned on the expandable segment  20 . The inductor capacitor sensor  54  may be any device which contains at least two parallel coil separated by a dielectric. The inductor capacitor sensor  54  may be positioned on the expandable segment  20  to detect whether the expandable segment  20  is inflating or deflating. For example, the inductor capacitor sensor  54  may be bonded to the surface  52  of the expandable segment  20  exposed to the endoluminal passage  14  or within the interior  38  of the inductor capacitor sensor  54 . Alternatively, the inductor capacitor sensor  54  may be embedded within the surface  52  material of the expandable segment  20 . The electrical combination within the inductor capacitor sensor  54  may create a resonant circuit that resonates at a particular frequency. As shown in  FIG. 3 , the coils of the inductor capacitor sensor  54  may be arranged parallel to the surface  52  of the expandable segment  20 . When the expandable segment  20  inflates or deflates, the surface  52  of the expandable segment  20  may expand or contract, causing the coils of the inductor capacitor sensor  54  to also expand or contract. Compression or expansion of the coils of the inductor capacitor sensor  54  may increase or decrease the capacitance of the inductor capacitor sensor  54  and may therefore change the resonant frequency of the inductor capacitor sensor  54 . An RF signal may be applied to the inductor capacitor sensor  54  from inside or outside the endoluminal passage  14  to cause the inductor capacitor sensor  54  to echo back a detectable signal. From this signal, the capacitance of the inductor capacitor sensor  54  may be calculated, indicating the inflation or deflation of the expandable segment  20 . 
     Alternatively, in some embodiments, the inductor capacitor sensor  54  may be electrically coupled to a transmitter  50  to transmit a signal indicating the capacitance of the inductor capacitor sensor  54 . The signal generated by the inductor capacitor sensor  54  may be received by the receiver to indicate to the operator the status of the expandable segment  20 . The transmitter  50  may be physically coupled to the inductor capacitor sensor  54  or may be positioned elsewhere in the medical device, such as the shaft  46 , and only electrically coupled to the inductor capacitor sensor  54 . The transmitter  50  may be electrically powered by a different electrical current source, such as the battery  44 . 
     As shown in  FIG. 6 , the coils of the inductor capacitor sensor  54  may be arranged such that, a first portion of the inductor capacitor sensor  54  may be coupled to the surface  52  of the expandable segment  20 , and a second portion of the inductor capacitor sensor  54  may be coupled to some other component of the medical device  10 , such as the shaft  46 . In such a configuration, when the expandable segment  20  inflates, the coils of the inductor capacitor sensor  54  may expand, changing the inductance of the inductor capacitor sensor  54 . Alternatively, when the expandable segment  20  deflates, the coils of the inductor capacitor sensor  54  may compress, changing the inductance of the inductor capacitor sensor  54 . 
     The electrical current generated within the inductor capacitor sensor  54  may be generated from a variety of sources. For example, a stationary magnet may be arranged within the medical device  10  near the coils of the inductor capacitor sensor  54  to allow expansion or compression of the coils within the magnetic field of the magnet to generate the electrical current. Alternatively, a rotating or otherwise moving magnet could generate a magnetic field which could induce the electrical current within the inductor capacitor sensor  54 . The electrical current could also be provided to the inductor capacitor sensor  54  directly through a power source such as the battery  44 . 
       FIG. 7  illustrates the expandable segment having a plurality of markings  66 . In some embodiments, the inflation or deflation of the expandable segment  20  may be indicated by the markings  66 . The markings  66  may be any feature on the expandable segment  20  which allows non-invasive visualization of the expandable segment  20  from outside the body  12  of the patient. For example, the markings  66  may comprise indentations on the surface  52  of the expandable segment  20  which have a shape, such as a rectangle or triangle, which reflects acoustic energy differently than the surface  52  of the expandable segment  20 . Such indentation markings  66  would be visible under ultrasonic visualization to determine the inflated or deflated state of the expandable segment  20 . The markings  20  may be placed at a point on the expandable segment  20  which, while inflated, extends furthest from the shaft  46  of the medical device  10 . Alternatively or in addition, markings  66  may be spread across the length of the surface  52  of the expandable segment  20 . 
     Another possible embodiment may include metallic markings  66  coupled to the surface  52  of the expandable segment  20 . The metallic markings  66  may be located on the outside of the surface  52 , within the interior  38  of the expandable segment  20 , or embedded within the surface  52  of the expandable segment. The metallic markings  66  may be any metal having a density higher than the other materials of the expandable segment  20 . Metallic markings  66  may be used to determine the state of the expandable segment  20  under a variety of visualization techniques including ultrasound and x-ray. Non-ferrous metals may be used in the metal markings  66  to allow visualization under magnetic resonance imaging. 
     The medical device may also include shaft markings  64  on the surface of or imbedded within the shaft  46  of the medical device  10 . The shaft markings  64  may be of the same type as the markings  66  on the expandable segment  20 . When visualized, a distance  68  between the shaft markings  64  and the markings  66  on the expandable segment  20  may be used to measure the inflated or deflated state of the expandable segment  20 . 
       FIG. 8  illustrates the expandable segment  20  having a plurality of accelerometers  70 ,  80 . A first accelerometer  70  may be arranged on the surface  52  of the expandable segment  20 . The accelerometer  70  may be external to the surface  52  of the expandable segment  20 , embedded within the surface  52  of the expandable segment, or arranged on the interior  38  of the expandable segment  20 . The first accelerometer  70  may be placed at a point on the expandable segment  20  which, while inflated, extends furthest from the shaft  46  of the medical device  10 . The accelerometer  70  may be arranged to measure the acceleration of the surface  52  of the expandable segment which is transverse to an axis  82  defined by the shaft  46  of the medical device  10 . The first accelerometer  70  may be able to transmit information about the acceleration of the surface  52  of the expandable segment  20  to determine the state of the expandable segment  20 . For example, if the first accelerometer  70  indicates that acceleration outward from the shaft  46  of the expandable segment  20 , the operator may determine that the expandable segment  20  is inflating. After accelerating, if the first accelerometer  70  indicates that acceleration has ceased, the operator may determine that the expandable segment  20  is fully inflated. 
     The medical device  10  may include a second accelerometer  80  on the shaft  46  of the medical device  10 . The second accelerometer  80  may be in the interior  38  of the expandable segment  20  on the outer surface of the shaft  46 , or may be embedded within the shaft  46 . The second accelerometer  80  may be arranged to measure acceleration of the shaft  46  which is transverse to the axis  82  defined by the shaft  46 . The second accelerometer  80  may be used in conjunction with the first accelerometer  70  to provide a shaft-stabilized indication of the inflation or deflation of the expandable segment  20 . For example, the medical device  10  as a whole, and the shaft  46  in particular, may move transversely within the endoluminal passage  14  due to blood flowing over the medical device  10  or movement of the patient. The acceleration information provided by the second accelerometer  80  coupled to the shaft  46  may be used as a control in determining the inflation or deflation of the expandable segment  20 . For example, the transverse acceleration information provided by the second accelerometer  80  may be subtracted from the acceleration information provided by the first accelerometer  70 . For improved accuracy, the first accelerometer  70  and the second accelerometer  80  may be transversely aligned with respect to each other. 
       FIG. 9  illustrates the expandable segment  20  including a pair of capacitor elements  72 . The capacitor elements  72  may be any electrically conductive material which is capable of storing an electrical charge. Examples of the capacitor elements  72  may include plates or discs and may be made from materials such as copper, gold, or another metal. A first capacitor element  72  may be positioned on the surface  52  of the expandable segment  20 , within the interior  38  of the expandable segment  20 . A second capacitor element  72  may be positioned on the shaft  42 , transversely aligned with the first capacitor element  72  and spaced apart from the first capacitor element  72 . The fluid, such as air or carbon dioxide, within the interior  38  of the expandable segment may serve as a dielectric between the capacitor elements  72 . The capacitance of the circuit containing the capacitor elements  72  may vary as the expandable segment  20  inflates and deflates, causing the spacing between the capacitor elements  72  to increase and decrease. Accordingly, the varying capacitance of the circuit may be measured and transmitted to determine the inflated or deflated state of the expandable segment  20 . 
       FIG. 10  illustrates the expandable segment  20  including a light emitting source  74  and a light sensor  76 . The light emitting source  74  may be any device which emits electromagnetic radiation, such as radio waves, infrared light, visible light, or ultra-violet radiation. Examples of the light emitting source  74  may be a light emitting diode (LED) or a laser. The light sensor  76  may be any device which may receive light emitted by the light emitting source  74 . Examples of the light sensor  76  may include an optical sensor or a mirror. In some embodiments the light emitting source  74  may be arranged within the interior  38  of the expandable segment  20  and positioned on one of the surface  52  of the expandable segment  20  or on the shaft  46 . The light sensor  76  may also be arranged within the interior  38  of the expandable segment  20  and positioned on the other of the surface  52  of the expandable segment  20  and the shaft  46 . The light sensor  76  may be transversely aligned with the light emitting source  74 . 
     In some embodiments, the light emitting source  74  may periodically or continuously emit light  78 . The light  78  travels from the light emitting source  74  to the light sensor  76 . A circuit containing the light emitting source  74  and the light sensor  76  may be used to measure the time elapsed between when the light  78  was emitted from the light emitting source  74  and when the light  78  was received by the light sensor  76 . When the expandable segment  20  is fully inflated, the time necessary for the light  78  to travel from the light emitting source  74  to the light sensor  76  may be greater than the time necessary for the light  78  to travel from the light emitting source  74  to the light sensor  76  when the expandable segment  20  is deflated. 
     In another embodiment, the light sensor  76  may be a reflective material such as a mirror. In such a configuration, the light sensor  76  may not be connected to an electrical circuit. The light emitting source  74  may also have a sensor to detect when light is received. The light emitting source  74  may periodically or continuously emit light  78 . The light  78  may travel through the interior  38  of the expandable segment  20  to reach the reflective light sensor  76 . Upon reaching the reflective light sensor  76 , the light  78  may reflect against the light sensor  76  and travel back to the light emitting source  74 . The light emitting source  74  may receive the light  78  and calculated the travel time between the light emitting source  74  emitting the light  78  and the light emitting source  74  receiving the light  78 . This information may be used to determine the degree of inflation of the expandable segment  20  and transmit this information outside the body  12 . 
       FIG. 11  illustrates a flow diagram of operations to utilize the medical device  10  to reduce cardiac back pressure. The operations ( 100 ) may include fewer, additional, or different operations than illustrated in  FIG. 6 . Alternatively, or in addition, the operations ( 100 ) may be performed in a different order than illustrated. 
     Initially, the method of operations ( 100 ) may include implanting the medical device  10  within the endovascular passageway  14  ( 102 ). The medical device  10  may include the shaft  46 , the expandable segment  20  coupled to the shaft  46 , and the sensor  36 ,  48 ,  54  coupled to the medical device  10 . The expandable segment  20  may be movable between an expanded state and a compressed state, responsive to a change in the blood flow within the endoluminal passage  14  over the expandable segment  20 . The method of operations ( 100 ) also may include transmitting a signal generated by the sensor  36 ,  48 ,  54  ( 104 ). 
     The method ( 100 ) may also include electrically charging a circuit including the sensor  36 ,  48 ,  54 , where the sensor  36 ,  48 ,  54  are the electrodes  36  positioned on the surface  52  of the expandable segment  20 . The electrodes  36  may longitudinally or radially spaced apart on the surface of the expandable segment  20 . The inflated or deflated state of the expandable segment  20  may be determined by measuring the impedance between the electrodes  36 . 
     In some embodiments, the method ( 100 ) may also include imaging the endoluminal passage  14  before implanting the medical device  10  to determine a responsiveness of the endoluminal passage  14  to blood flow created by the left ventricle  16 . By determining the responsiveness of the endoluminal passage  14 , the sensor  36 ,  48 ,  54  may be calibrated to the specific endoluminal passage  14  such that when the medical device is implanted, the medical device  10  accurately measures the blood flow within the endoluminal passage and the state of the expandable segment  20 . 
     In some embodiments, the method ( 100 ) may also include adjusting the fluid pressure of the medical device  10  based on the signal generated by the sensor  36 ,  48 ,  54 . If the measurement indicates that the expandable segment  20  is not sufficiently deflating, fluid may be released from the medical device  10  through the port  58  to allow for adequate deflation. Alternatively, where the measurement indicates that the expandable segment  20  is not fully inflating at the resting pressure of the endoluminal passage  14 , fluid may be added to the medical device  10  through the port  58  to allow for adequate inflation. 
     Additionally, the method ( 100 ) may also include adjusting a medication dose of a patient based upon information received by the sensor  36 ,  48 ,  54 . For example, impending heart failure of a patient may be detected by changes in the fluid flowing over the medical device  10  positioned in the endoluminal passage  14 . 
     In addition to the advantages that have been described, it is also possible that there are still other advantages that are not currently recognized but which may become apparent at a later time. While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.