Abstract:
Embodiments of the present disclosure are configured to assess the severity of a blockage in a vessel and, in particular, a stenosis in a blood vessel. In some particular embodiments, the devices, systems, and methods of the present disclosure are configured to collect and wirelessly distribute reliable pressure signals to other devices, and do so in a small, compact device that integrates with existing proximal and distal pressure measurement systems and does not require a separate power source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/745,418, filed Dec. 21, 2012, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to the assessment of vessels and, in particular, the assessment of the severity of a blockage or other restriction to the flow of fluid through a vessel. Aspects of the present disclosure are particularly suited for evaluation of biological vessels in some instances. For example, some particular embodiments of the present disclosure are specifically configured for the evaluation of a stenosis of a human blood vessel. 
       BACKGROUND 
       [0003]    A currently accepted technique for assessing the severity of a stenosis in a blood vessel, including ischemia causing lesions, is fractional flow reserve (FFR). FFR is a calculation of the ratio of a distal pressure measurement (taken on the distal side of the stenosis) relative to a proximal pressure measurement (taken on the proximal side of the stenosis). FFR provides an index of stenosis severity that allows determination as to whether the blockage limits blood flow within the vessel to an extent that treatment is required. The normal value of FFR in a healthy vessel is 1.00, while values less than about 0.80 are generally deemed significant and require treatment. Common treatment options include angioplasty and stenting. 
         [0004]    Coronary blood flow is unique in that it is affected not only by fluctuations in the pressure arising proximally (as in the aorta) but is also simultaneously affected by fluctuations arising distally in the microcirculation. Accordingly, it is not possible to accurately assess the severity of a coronary stenosis by simply measuring the fall in mean or peak pressure across the stenosis because the distal coronary pressure is not purely a residual of the pressure transmitted from the aortic end of the vessel. As a result, for an effective calculation of FFR within the coronary arteries, it is necessary to reduce the vascular resistance within the vessel. Currently, pharmacological hyperemic agents, such as adenosine, are administered to reduce and stabilize the resistance within the coronary arteries. These potent vasodilator agents reduce the dramatic fluctuation in resistance (predominantly by reducing the microcirculation resistance associated with the systolic portion of the heart cycle) to obtain a relatively stable and minimal resistance value. 
         [0005]    However, the administration of hyperemic agents is not always possible or advisable. First, the clinical effort of administering hyperemic agents can be significant. In some countries (particularly the United States), hyperemic agents such as adenosine are expensive, and time consuming to obtain when delivered intravenously (IV). In that regard, IV-delivered adenosine is generally mixed on a case-by-case basis in the hospital pharmacy. It can take a significant amount of time and effort to get the adenosine prepared and delivered to the operating area. These logistic hurdles can impact a physician&#39;s decision to use FFR. Second, some patients have contraindications to the use of hyperemic agents such as asthma, severe COPD, hypotension, bradycardia, low cardiac ejection fraction, recent myocardial infarction, and/or other factors that prevent the administration of hyperemic agents. Third, many patients find the administration of hyperemic agents to be uncomfortable, which is only compounded by the fact that the hyperemic agent may need to be applied multiple times during the course of a procedure to obtain FFR measurements. Fourth, the administration of a hyperemic agent may also require central venous access (e.g., a central venous sheath) that might otherwise be avoided. Finally, not all patients respond as expected to hyperemic agents and, in some instances, it is difficult to identify these patients before administration of the hyperemic agent. 
         [0006]    To obtain FFR measurements or other similar measurements such as an instantaneous wave-free ratio (iFR) measurement, one or more ultra-miniature sensors placed on the distal portion of a flexible device, such as a catheter or guide wire used for catheterization procedures, are utilized to obtain the distal pressure measurement, while a sensor connected to a measurement instrument, often called the hemodynamic system, is utilized to obtain the proximal or aortic pressure measurement. Currently only large expensive systems or a combination of multiple devices connected to the distal pressure wire and the hemodynamic system can calculate and display an FFR measurement. In that regard, to calculate the FFR or iFR these devices require both the aortic or proximal pressure measurement and the coronary artery or distal pressure measurement. Accordingly, these systems require the catheter lab&#39;s hemodynamic system to have a high level analog voltage output. “High level” in this context generally implies 100 mmHg/Volt output. Unfortunately, there are many hemodynamic systems that don&#39;t provide a high level output. As a result, when using these hemodynamic systems, providing an FFR measurement or other measurement that requires an aortic pressure measurement is difficult if not impossible. Further, space in a typical catheter lab is extremely limited. Consequently, devices that are large and located in the catheter lab are disfavored compared to smaller devices, especially if the smaller device can provide much if not all of the functionality of the larger device. As a result, it is highly desirable to have a device that that can collect both distal and aortic pressure measurements and yet is small and lightweight that can sit on, or near, the patient bed and be easily read by the physician. 
         [0007]    Further, most pressure measurement devices require an extra source of power like an AC adapter or wall plug. This adds to wire clutter and available medical grade AC outlets are not often available near the patient bed. In addition, any device that uses AC power must undergo stringent safety precautions to reduce patient risk due to leakage currents. Batteries are another alternative for power. But, batteries must be replaced, disposed of correctly and have a finite shelf life. 
         [0008]    Additionally, many pressure measurement devices transmit received pressure data to one or more other devices via wires that must be routed and accounted for within a catheter lab. These wires add to the wire clutter and therefore increase the risk of accidents. Further, computing systems inside or outside of a catheter lab frequently rely on pressure measurements for the calculation of pressure-based diagnostic characterizations, such as FFR or iFR. For reliable diagnostic characterizations, a strong aortic pressure signal is often needed. Current hemodynamic systems sometimes fail to provide a strong and reliable aortic pressure signal, which may lead to an inaccurate or incomplete patient diagnosis. 
         [0009]    Accordingly, there remains a need for improved devices, systems, and methods for assessing the severity of a blockage in a vessel and, in particular, a stenosis in a blood vessel. In that regard, there remains a need for improved devices, systems, and methods for collecting and distributing reliable pressure signals that have a small, compact size (e.g., suitable for hand-held use), integrate with existing proximal and distal pressure measurement devices, and do not require a separate power source. 
       SUMMARY 
       [0010]    Embodiments of the present disclosure are configured to assess the severity of a blockage in a vessel and, in particular, a stenosis in a blood vessel. In some particular embodiments, the devices, systems, and methods of the present disclosure are configured to collect and wirelessly distribute reliable pressure signals to other devices, and do so in a small, compact device that integrates with existing proximal and distal pressure measurement systems and does not require a separate power source. 
         [0011]    In one embodiment, an interface for intravascular pressure sensing devices is provided. The interface comprises: a distal input configured to receive a distal pressure signal from a distal pressure sensing device; a distal output configured to output the distal pressure signal to a hemodynamic system in a format useable by the hemodynamic system; a proximal input configured to receive a proximal pressure signal from a proximal pressure sensing device; a proximal output configured to output the proximal pressure signal to the hemodynamic system in a format useable by the hemodynamic system; and a wireless transceiver coupled to the distal input, distal output, proximal input, and proximal output, the wireless transceiver being configured to wirelessly transmit the distal pressure and the proximal pressure to a computing system spaced from the interface, wherein the computing system is distinct from the hemodynamic system. In some embodiments, the distal input, distal output, proximal input, proximal output, and wireless transceiver are secured to a housing. Further, in some instances the distal pressure sensing device is a pressure-sensing guidewire and the proximal pressure sensing device is a pressure-sensing catheter configured for use with the hemodynamic system. 
         [0012]    In another embodiment, a system for evaluating a vascular stenosis is provided. The system comprises: a distal pressure sensing device sized and shaped for insertion into human vasculature; a proximal pressure sensing device sized and shaped for insertion into human vasculature; and an interface, where the interface includes: a distal input configured to receive a distal pressure signal from the distal pressure sensing device; a proximal input configured to receive a proximal pressure signal from the proximal pressure sensing device; a proximal output configured to output the proximal pressure signal to a processing system in a format useable by the processing system; and a wireless transceiver coupled to the distal input, distal output, proximal input, and proximal output, the wireless transceiver being configured to wirelessly transmit the distal pressure and the proximal pressure to a computing system spaced from the interface, wherein the computing system is distinct from the processing system. In some instances, the processing system is a hemodynamic system. Further, in some instances the distal input, distal output, proximal input, proximal output, and wireless transceiver are secured to a housing. In some instances, the distal pressure sensing device is a pressure-sensing guidewire and the proximal pressure sensing device is a pressure-sensing catheter configured for use with the processing system. 
         [0013]    Additional aspects, features, and advantages of the present disclosure will become apparent from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Illustrative embodiments of the present disclosure will be described with reference to the accompanying drawings, of which: 
           [0015]      FIG. 1  is a diagrammatic perspective view of a vessel having a stenosis according to an embodiment of the present disclosure. 
           [0016]      FIG. 2  is a diagrammatic, partial cross-sectional perspective view of a portion of the vessel of  FIG. 1  taken along section line  2 - 2  of  FIG. 1 . 
           [0017]      FIG. 3  is a diagrammatic, partial cross-sectional perspective view of the vessel of  FIGS. 1 and 2  with instruments positioned therein according to an embodiment of the present disclosure. 
           [0018]      FIG. 4  is a diagrammatic, schematic view of a system according to an embodiment of the present disclosure. 
           [0019]      FIG. 5  is a diagrammatic, schematic view of an interface device of the system of  FIG. 4  according to an embodiment of the present disclosure. 
           [0020]      FIG. 6  is a diagrammatic, schematic view of a portion of the interface device of  FIG. 5  according to an embodiment of the present disclosure. 
           [0021]      FIG. 7  is a diagrammatic, schematic view of a portion of the interface device of  FIG. 5  similar to that of  FIG. 6 , but illustrating another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It is nevertheless understood that no limitation to the scope of the disclosure is intended. Any alterations and further modifications to the described devices, systems, and methods, and any further application of the principles of the present disclosure are fully contemplated and included within the present disclosure as would normally occur to one skilled in the art to which the disclosure relates. In particular, it is fully contemplated that the features, components, and/or steps described with respect to one embodiment may be combined with the features, components, and/or steps described with respect to other embodiments of the present disclosure. For the sake of brevity, however, the numerous iterations of these combinations will not be described separately. 
         [0023]    Referring to  FIGS. 1 and 2 , shown therein is a vessel  100  having a stenosis according to an embodiment of the present disclosure. In that regard,  FIG. 1  is a diagrammatic perspective view of the vessel  100 , while  FIG. 2  is a partial cross-sectional perspective view of a portion of the vessel  100  taken along section line  2 - 2  of  FIG. 1 . Referring more specifically to  FIG. 1 , the vessel  100  includes a proximal portion  102  and a distal portion  104 . A lumen  106  extends along the length of the vessel  100  between the proximal portion  102  and the distal portion  104 . In that regard, the lumen  106  is configured to allow the flow of fluid through the vessel. In some instances, the vessel  100  is a blood vessel. In some particular instances, the vessel  100  is a coronary artery. In such instances, the lumen  106  is configured to facilitate the flow of blood through the vessel  100 . 
         [0024]    As shown, the vessel  100  includes a stenosis  108  between the proximal portion  102  and the distal portion  104 . Stenosis  108  is generally representative of any blockage or other structural arrangement that results in a restriction to the flow of fluid through the lumen  106  of the vessel  100 . Embodiments of the present disclosure are suitable for use in a wide variety of vascular applications, including without limitation coronary, peripheral (including but not limited to lower limb, carotid, and neurovascular), renal, and/or venous. Where the vessel  100  is a blood vessel, the stenosis  108  may be a result of plaque buildup, including without limitation plaque components such as fibrous, fibro-lipidic (fibro fatty), necrotic core, calcified (dense calcium), blood, fresh thrombus, and mature thrombus. Generally, the composition of the stenosis will depend on the type of vessel being evaluated. In that regard, it is understood that the concepts of the present disclosure are applicable to virtually any type of blockage or other narrowing of a vessel that results in decreased fluid flow. 
         [0025]    Referring more particularly to  FIG. 2 , the lumen  106  of the vessel  100  has a diameter  110  proximal of the stenosis  108  and a diameter  112  distal of the stenosis. In some instances, the diameters  110  and  112  are substantially equal to one another. In that regard, the diameters  110  and  112  are intended to represent healthy portions, or at least healthier portions, of the lumen  106  in comparison to stenosis  108 . Accordingly, these healthier portions of the lumen  106  are illustrated as having a substantially constant cylindrical profile and, as a result, the height or width of the lumen has been referred to as a diameter. However, it is understood that in many instances these portions of the lumen  106  will also have plaque buildup, a non-symmetric profile, and/or other irregularities, but to a lesser extent than stenosis  108  and, therefore, will not have a cylindrical profile. In such instances, the diameters  110  and  112  are understood to be representative of a relative size or cross-sectional area of the lumen and do not imply a circular cross-sectional profile. 
         [0026]    As shown in  FIG. 2 , stenosis  108  includes plaque buildup  114  that narrows the lumen  106  of the vessel  100 . In some instances, the plaque buildup  114  does not have a uniform or symmetrical profile, making angiographic evaluation of such a stenosis unreliable. In the illustrated embodiment, the plaque buildup  114  includes an upper portion  116  and an opposing lower portion  118 . In that regard, the lower portion  118  has an increased thickness relative to the upper portion  116  that results in a non-symmetrical and non-uniform profile relative to the portions of the lumen proximal and distal of the stenosis  108 . As shown, the plaque buildup  114  decreases the available space for fluid to flow through the lumen  106 . In particular, the cross-sectional area of the lumen  106  is decreased by the plaque buildup  114 . At the narrowest point between the upper and lower portions  116 ,  118  the lumen  106  has a height  120 , which is representative of a reduced size or cross-sectional area relative to the diameters  110  and  112  proximal and distal of the stenosis  108 . Note that the stenosis  108 , including plaque buildup  114  is exemplary in nature and should be considered limiting in any way. In that regard, it is understood that the stenosis  108  has other shapes and/or compositions that limit the flow of fluid through the lumen  106  in other instances. While the vessel  100  is illustrated in  FIGS. 1 and 2  as having a single stenosis  108  and the description of the embodiments below is primarily made in the context of a single stenosis, it is nevertheless understood that the devices, systems, and methods described herein have similar application for a vessel having multiple stenosis regions. 
         [0027]    Referring now to  FIG. 3 , the vessel  100  is shown with instruments  130  and  132  positioned therein according to an embodiment of the present disclosure. In general, instruments  130  and  132  may be any form of device, instrument, or probe sized and shaped to be positioned within a vessel. In the illustrated embodiment, instrument  130  is generally representative of a guide wire, while instrument  132  is generally representative of a catheter. In that regard, instrument  130  extends through a central lumen of instrument  132 . However, in other embodiments, the instruments  130  and  132  take other forms. In that regard, the instruments  130  and  132  are of similar form in some embodiments. For example, in some instances, both instruments  130  and  132  are guide wires. In other instances, both instruments  130  and  132  are catheters. On the other hand, the instruments  130  and  132  are of different form in some embodiments, such as the illustrated embodiment, where one of the instruments is a catheter and the other is a guide wire. Further, in some instances, the instruments  130  and  132  are disposed coaxial with one another, as shown in the illustrated embodiment of  FIG. 3 . In other instances, one of the instruments extends through an off-center lumen of the other instrument. In yet other instances, the instruments  130  and  132  extend side-by-side. In some particular embodiments, at least one of the instruments is as a rapid-exchange device, such as a rapid-exchange catheter. In such embodiments, the other instrument is a buddy wire or other device configured to facilitate the introduction and removal of the rapid-exchange device. Further still, in other instances, instead of two separate instruments  130  and  132  a single instrument is utilized. In that regard, the single instrument incorporates aspects of the functionalities (e.g., data acquisition) of both instruments  130  and  132  in some embodiments. 
         [0028]    Instrument  130  is configured to obtain diagnostic information about the vessel  100 . In that regard, the instrument  130  includes one or more sensors, transducers, and/or other monitoring elements configured to obtain the diagnostic information about the vessel. The diagnostic information includes one or more of pressure, flow (velocity), images (including images obtained using ultrasound (e.g., IVUS), OCT, thermal, and/or other imaging techniques), temperature, and/or combinations thereof. The one or more sensors, transducers, and/or other monitoring elements are positioned adjacent a distal portion of the instrument  130  in some instances. In that regard, the one or more sensors, transducers, and/or other monitoring elements are positioned less than 30 cm, less than 10 cm, less than 5 cm, less than 3 cm, less than 2 cm, and/or less than 1 cm from a distal tip  134  of the instrument  130  in some instances. In some instances, at least one of the one or more sensors, transducers, and/or other monitoring elements is positioned at the distal tip of the instrument  130 . 
         [0029]    The instrument  130  includes at least one element configured to monitor pressure within the vessel  100 . The pressure monitoring element can take the form a piezo-resistive pressure sensor, a piezo-electric pressure sensor, a capacitive pressure sensor, an electromagnetic pressure sensor, a fluid column (the fluid column being in communication with a fluid column sensor that is separate from the instrument and/or positioned at a portion of the instrument proximal of the fluid column), an optical pressure sensor, and/or combinations thereof. In some instances, one or more features of the pressure monitoring element are implemented as a solid-state component manufactured using semiconductor and/or other suitable manufacturing techniques. Examples of commercially available guide wire products that include suitable pressure monitoring elements include, without limitation, the PrimeWire PRESTIGE® PLUS pressure guide wire, the PrimeWire PRESTIGE® pressure guide wire, the PrimeWire® pressure guide wire, and the ComboWire® XT pressure and flow guide wire, each available from Volcano Corporation, as well as the PressureWire™ Certus guide wire and the PressureWire™ Aeris guide wire, each available from St. Jude Medical, Inc. Generally, the instrument  130  is sized such that it can be positioned through the stenosis  108  without significantly impacting fluid flow across the stenosis, which would impact the distal pressure reading. Accordingly, in some instances the instrument  130  has an outer diameter of 0.018″ or less. In some embodiments, the instrument  130  has an outer diameter of 0.014″ or less. 
         [0030]    Instrument  132  is also configured to obtain diagnostic information about the vessel  100 . In some instances, instrument  132  is configured to obtain the same diagnostic information as instrument  130 . In other instances, instrument  132  is configured to obtain different diagnostic information than instrument  130 , which may include additional diagnostic information, less diagnostic information, and/or alternative diagnostic information. The diagnostic information obtained by instrument  132  includes one or more of pressure, flow (velocity), images (including images obtained using ultrasound (e.g., IVUS), OCT, thermal, and/or other imaging techniques), temperature, and/or combinations thereof. Instrument  132  includes one or more sensors, transducers, and/or other monitoring elements configured to obtain this diagnostic information. In that regard, the one or more sensors, transducers, and/or other monitoring elements are positioned adjacent a distal portion of the instrument  132  in some instances. In that regard, the one or more sensors, transducers, and/or other monitoring elements are positioned less than 30 cm, less than 10 cm, less than 5 cm, less than 3 cm, less than 2 cm, and/or less than 1 cm from a distal tip  136  of the instrument  132  in some instances. In some instances, at least one of the one or more sensors, transducers, and/or other monitoring elements is positioned at the distal tip of the instrument  132 . 
         [0031]    Similar to instrument  130 , instrument  132  also includes at least one element configured to monitor pressure within the vessel  100 . The pressure monitoring element can take the form a piezo-resistive pressure sensor, a piezo-electric pressure sensor, a capacitive pressure sensor, an electromagnetic pressure sensor, a fluid column (the fluid column being in communication with a fluid column sensor that is separate from the instrument and/or positioned at a portion of the instrument proximal of the fluid column), an optical pressure sensor, and/or combinations thereof. In some instances, one or more features of the pressure monitoring element are implemented as a solid-state component manufactured using semiconductor and/or other suitable manufacturing techniques. Currently available catheter products suitable for use with one or more of Siemens AXIOM Sensis, Mennen Horizon XVu, and Philips Xper IM Physiomonitoring 5 and that include pressure monitoring elements can be utilized for instrument  132  in some instances. 
         [0032]    In accordance with aspects of the present disclosure, at least one of the instruments  130  and  132  is configured to monitor a pressure within the vessel  100  distal of the stenosis  108  and at least one of the instruments  130  and  132  is configured to monitor a pressure within the vessel proximal of the stenosis. In that regard, the instruments  130 ,  132  are sized and shaped to allow positioning of the at least one element configured to monitor pressure within the vessel  100  to be positioned proximal and/or distal of the stenosis  108  as necessary based on the configuration of the devices. In that regard,  FIG. 3  illustrates a position  138  suitable for measuring pressure distal of the stenosis  108 . The position  138  is less than 5 cm, less than 3 cm, less than 2 cm, less than 1 cm, less than 5 mm, and/or less than 2.5 mm from the distal end of the stenosis  108  (as shown in  FIG. 2 ) in some instances.  FIG. 3  also illustrates a plurality of suitable positions for measuring pressure proximal of the stenosis  108 . In that regard, positions  140 ,  142 ,  144 ,  146 , and  148  each represent a position that is suitable for monitoring the pressure proximal of the stenosis in some instances. In that regard, the positions  140 ,  142 ,  144 ,  146 , and  148  are positioned at varying distances from the proximal end of the stenosis  108  ranging from more than 20 cm down to about 5 mm or less. Generally, the proximal pressure measurement will be spaced from the proximal end of the stenosis. Accordingly, in some instances, the proximal pressure measurement is taken at a distance equal to or greater than an inner diameter of the lumen of the vessel from the proximal end of the stenosis. In the context of coronary artery pressure measurements, the proximal pressure measurement is generally taken at a position proximal of the stenosis and distal of the aorta, within a proximal portion of the vessel. However, in some particular instances of coronary artery pressure measurements, the proximal pressure measurement is taken from a location inside the aorta. In other instances, the proximal pressure measurement is taken at the root or ostium of the coronary artery. In some instances, the proximal pressure measurement is referred to as the aortic pressure. 
         [0033]    Referring now to  FIG. 4 , shown therein is a system  150  according to an embodiment of the present disclosure. In that regard,  FIG. 4  is a diagrammatic, schematic view of the system  150 . As shown, the system  150  includes an instrument  152 . In that regard, in some instances instrument  152  is suitable for use as at least one of instruments  130  and  132  discussed above. Accordingly, in some instances the instrument  152  includes features similar to those discussed above with respect to instruments  130  and  132  in some instances. In the illustrated embodiment, the instrument  152  is a guide wire having a distal portion  154  and a housing  156  positioned adjacent the distal portion. In that regard, the housing  156  is spaced approximately 3 cm from a distal tip of the instrument  152 . The housing  156  is configured to house one or more sensors, transducers, and/or other monitoring elements configured to obtain the diagnostic information about the vessel. In the illustrated embodiment, the housing  156  contains at least a pressure sensor configured to monitor a pressure within a lumen in which the instrument  152  is positioned. A shaft  158  extends proximally from the housing  156 . A torque device  160  is positioned over and coupled to a proximal portion of the shaft  158 . A proximal end portion  162  of the instrument  152  is coupled to a connector  164 . A cable  166  extends from connector  164  to a connector  168 . In some instances, connector  168  is configured to be plugged into an interface  170 . In that regard, interface  170  is a patient interface module (PIM) in some instances, but in other instances it may be a hub that routes data signals to various systems and devices. In some instances, the cable  166  is replaced with a wireless connection. In that regard, the interface  170  includes an antenna  171  for wireless data transmissions. It is understood that various communication pathways between the instrument  152  and the interface  170  may be utilized, including physical connections (including electrical, optical, and/or fluid connections), wireless connections, and/or combinations thereof. 
         [0034]    The interface  170  is communicatively coupled to a hemodynamic system  172  via a connection  174 . In some instances, the hemodynamic system  172  is a Siemens AXIOM Sensis, a Mennen Horizon XVu, or a Philips Xper IM Physiomonitoring 5. Together, connector  164 , cable  166 , connector  168 , interface  170 , and connection  174  facilitate communication between the one or more sensors, transducers, and/or other monitoring elements of the instrument  152  and the hemodynamic system  172 . However, this communication pathway is exemplary in nature and should not be considered limiting in any way. In that regard, it is understood that any communication pathway between the instrument  152  and the interface  170  may be utilized, including physical connections (including electrical, optical, and/or fluid connections), wireless connections, and/or combinations thereof. In that regard, the hemodynamic system  172  includes an antenna  173  for wireless data transmissions. Similarly, it is understood that any communication pathway between the interface  170  and the hemodynamic system  172  may be utilized, including physical connections (including electrical, optical, and/or fluid connections), wireless connections, and/or combinations thereof. Accordingly, it is understood that additional components (e.g., connectors, routers, switches, etc.) not illustrated in  FIG. 4  may be included to facilitate communication between the instrument  152 , the interface  170 , and the hemodynamic system  172 . 
         [0035]    In some embodiments, the connection  174  is a wireless connection. In some instances, the connection  174  includes a communication link over a network (e.g., intranet, internet, telecommunications network, and/or other network). In that regard, it is understood that the hemodynamic system  172  is positioned remote from an operating area where the instrument  152  is being used in some instances. Having the connection  174  include a connection over a network can facilitate communication between the instrument  152  and the remote hemodynamic system  172  regardless of whether the hemodynamic system is in an adjacent room, an adjacent building, or in a different state/country. Further, it is understood that the communication pathway between the instrument  152  and the hemodynamic system  172  is a secure connection in some instances. Further still, it is understood that, in some instances, the data communicated over one or more portions of the communication pathway between the instrument  152  and the hemodynamic system  172  is encrypted. 
         [0036]    The system  150  also includes an instrument  175 . In that regard, in some instances instrument  175  is suitable for use as at least one of instruments  130  and  132  discussed above. Accordingly, in some instances the instrument  175  includes features similar to those discussed above with respect to instruments  130  and  132 . In the illustrated embodiment, the instrument  175  is a catheter-type device. In that regard, the instrument  175  includes one or more sensors, transducers, and/or other monitoring elements adjacent a distal portion of the instrument configured to obtain the diagnostic information about the vessel. In the illustrated embodiment, the instrument  175  includes a pressure sensor configured to monitor a pressure within a lumen in which the instrument  175  is positioned. In one particular embodiment, instrument  175  is a pressure-sensing catheter that includes a fluid column extending along its length. In such an embodiment, a hemostasis valve is fluidly coupled to the fluid column of the catheter, a manifold is fluidly coupled to the hemostasis valve, and tubing extends between the components as necessary to fluidly couple the components. In that regard, the fluid column of the catheter is in fluid communication with a pressure sensor via the valve, manifold, and tubing. In some instances, the pressure sensor is part of or in communication with hemodynamic system  172 . In other instances, the pressure sensor is a separate component positioned between the instrument  175  and the interface  170  or between the interface  170  and the hemodynamic system  172 . The instrument  175  is in communication with the interface  170  via connection  177 . The interface  170 , in turn, is communicatively coupled to the computing device  172  via a connection  178 . 
         [0037]    Similar to the connections between instrument  152  and the interface  170  and the hemodynamic system  172 , connections  177  and  178  facilitate communication between the one or more sensors, transducers, and/or other monitoring elements of the instrument  175  and the interface  170  and the hemodynamic system  172 . Again, however, this communication pathway is exemplary in nature and should not be considered limiting in any way. In that regard, it is understood that any communication pathway between the instrument  175  and the interface  170  may be utilized, including physical connections (including electrical, optical, and/or fluid connections), wireless connections, and/or combinations thereof. Similarly, it is understood that any communication pathway between the interface  170  and the hemodynamic system  172  may be utilized, including physical connections (including electrical, optical, and/or fluid connections), wireless connections, and/or combinations thereof. Accordingly, it is understood that additional components (e.g., connectors, routers, switches, etc.) not illustrated in  FIG. 4  may be included to facilitate communication between the instrument  175 , the interface  170 , and the hemodynamic system  172 . 
         [0038]    In some embodiments, the connection  178  is a wireless connection. In some instances, the connection  178  includes a communication link over a network (e.g., intranet, internet, telecommunications network, and/or other network). In that regard, it is understood that the hemodynamic system  172  is positioned remote from an operating area where the instrument  175  is being used in some instances. Having the connection  178  include a connection over a network can facilitate communication between the instrument  175  and the remote hemodynamic system  172  regardless of whether the computing device is in an adjacent room, an adjacent building, or in a different state/country. Further, it is understood that the communication pathway between the instrument  175  and the hemodynamic system  172  is a secure connection in some instances. Further still, it is understood that, in some instances, the data communicated over one or more portions of the communication pathway between the instrument  175  and the hemodynamic system  172  is encrypted. 
         [0039]    It is understood that one or more components of the system  150  are not included, are implemented in a different arrangement/order, and/or are replaced with an alternative device/mechanism in other embodiments of the present disclosure. Alternatively, additional components and/or devices may be implemented into the system. Generally speaking, the communication pathway between either or both of the instruments  152 ,  175  and the hemodynamic system  172  may have no intermediate nodes (i.e., a direct connection), one intermediate node between the instrument and the computing device, or a plurality of intermediate nodes between the instrument and the computing device. 
         [0040]    In some embodiments, the interface  170  includes a wireless transceiver and is configured to wirelessly transmit pressure readings from one or both of the instruments  152  and  175  to other devices in the system  150 , such as a computing device  180 . For example, the interface  170  may wirelessly transmit a distal pressure and/or distal pressure waveform, a proximal (i.e., aortic) pressure and/or proximal pressure waveform, to the computing device  180 . In one embodiment, the computing device  180  is a computer system with the hardware and software to acquire, process, and display multi-modality medical data, but, in other embodiments, the computing device  180  may be any other type of computing system operable to process medical data. For example, in some instances the computing device  180  utilizes the distal pressure and/or distal pressure waveform with the proximal pressure and/or proximal pressure waveform to calculate FFR, calculate iFR, calculate a pressure differential between the proximal and distal pressures, identify a suitable diagnostic window for performing a pressure differential calculation without administering a hyperemic agent to the patient, calculate a pressure differential during the identified diagnostic window, calculate any other medical diagnostic characterization that is influenced by distal pressure and/or proximal (i.e., aortic) pressure, and any combinations thereof. 
         [0041]    In the embodiments in which computing device  180  is a computer workstation, the system includes at least a processor such as a microcontroller or a dedicated central processing unit (CPU), a non-transitory computer-readable storage medium such as a hard drive, random access memory (RAM), and/or optical read only memory (CD-ROM, DVD-ROM, Blu-Ray), a video controller such as a graphics processing unit (GPU), and a network communication device such as an Ethernet controller or a wireless communication transceiver  182 . In some instances, the computing device  180  is portable (e.g., handheld, on a rolling cart, etc.). Further, it is understood that in some instances computing device  180  comprises a plurality of computing devices. In some instances, the medical system  150  is deployed in a catheter lab having a control room, with the computing device  180  being located in the control room or the catheter lab itself. In other embodiments, the computing device  180  may be located elsewhere, such as in a centralized information technology area in a medical facility, or at an off-site location (i.e., in the cloud). 
         [0042]    In some embodiments, the interface  170  itself includes a processor and random access memory and is programmed to execute steps associated with the data acquisition and analysis described herein. In particular, in some embodiments the interface  170  is configured to receive and display pressure readings from one or both of the instruments  152  and  175  and/or calculate (and display) FFR or other pressure differential based on the pressure measurements obtained from the instruments  152  and  175 . Accordingly, it is understood that any steps related to data acquisition, data processing, instrument control, and/or other processing or control aspects of the present disclosure, including those incorporated by reference, may be implemented by the interface  170  using corresponding instructions stored on or in a non-transitory computer readable medium accessible by the computing device. In some embodiments, the interface  170  includes one or more processing and/or signal conditioning features and/or associated components/circuitry as described in U.S. Pat. No. 6,585,660, which is hereby incorporated by reference in its entirety. 
         [0043]    In the embodiments in which the interface  170  includes a wireless transceiver and is also configured to calculate FFR, iFR, or another diagnostic characterization differential based on the pressure measurements obtained from the instruments  152  and  175 , the interface  170  may first calculate the diagnostic characterization and then wirelessly transmit the pre-calculated result to one or more other devices such as the computing device  180  and/or hemodynamic system  172 . 
         [0044]    In the illustrated embodiment of  FIG. 4 , the interface  170  includes a housing  184 . The housing  184  contains the electronic components of the interface  170 . In that regard, exemplary embodiments of electronic component arrangements suitable for interface  170  as described below with respect to  FIGS. 5 and 6 . In some embodiments, the interface  170  is sized to be handheld and/or sized to be positioned on or near a patient bed (e.g., attached to a bed rail or IV pole). In that regard, in some instances the interface  170  is similar in size to the SmartMap® Pressure Instrument available from Volcano Corporation, which has housing dimensions of approximately 15.75 cm (6.3″) wide, 8.853 cm (3.54″) tall, and 4.48 cm (1.79″) deep. Generally, the interface  170  has a width between about 5 cm and about 25 cm, a height between about 5 cm and about 25 cm, and a depth between about 1 cm and about 10 cm. In some instances, the interface  170  also includes a display and one or more virtual or physical buttons configured to facilitate use of the interface. 
         [0045]    Referring now to  FIG. 5 , shown therein is a schematic of the interface  170  according to an exemplary embodiment of the present disclosure. In that regard, the interface  170  includes an input connector  190  for receiving signals from a distal pressure sensing component  191 . Accordingly, in some embodiments with an arrangement similar to that shown in  FIG. 4 , input connector  190  is configured to receive the connector  168  that is in communication with instrument  152 , where distal pressure sensing component  191  is a pressure sensing component of the instrument  152 . The interface  170  also includes an output connector  192  configured to send a distal pressure signal to a distal pressure input  193  of a hemo system or other computing device. Accordingly, in some embodiments with an arrangement similar to that shown in  FIG. 4 , output connector  192  is configured to send the distal pressure signal to an input of the hemodynamic system  172  over connection  174 . In that regard, in some embodiments the distal pressure signal is modulated based on the hemo system&#39;s excitation voltage to provide a low level output of the distal pressure signal to the hemo system. A low level output in this context is typically 5 μV/Vexc/mmHg, where Vexc is the excitation voltage. However, larger or smaller level outputs are used in some instances. 
         [0046]    The interface  170  further includes a wireless transceiver  206  that is configured to wireless transmit medical data provided to the interface  170 . The wireless transceiver  206  may be any type of wireless communications module capable of wirelessly transmitting data to other devices spaced from the interface  170 . For instance, the wireless transceiver  206  may transmit data using IEEE 802.11 Wi-Fi standards, Bluetooth standards, cellular standards (i.e., GSM, CDMA, HSDPA, LTE, etc), Ultra Wide-Band (UWB) standards, wireless FireWire standards, wireless USB standards, and/or any other short-range, medium-range, and/or long-range wireless standards. In the illustrated embodiment of  FIG. 5 , the wireless transceiver  206  receives a distal pressure signal from the input connector  190  and subsequently passes the distal pressure signal to the output connector  192 . Upon receiving the distal pressure signal, the wireless transceiver  206  may wirelessly transmit the distal pressure signal to one or more remote devices such as a computing system configured to calculate FFR, iFR, or another pressure-based diagnostic characterization. The wireless transceiver  206  may transmit the distal pressure signal by itself or it may combine the distal pressure signal with other received medical data such as a proximal pressure signal and transmit the signals together. In one instance, the distal pressure signal is converted from an analog signal to a digital signal with an Analog-to-Digital converter (ADC) prior to reaching the wireless transceiver  206 . In other instance, the wireless transceiver  206  performs the analog-to-digital itself prior to transmitting the distal pressure signal. 
         [0047]    In a medical facility with a plurality of devices wirelessly transmitting medical data, such medical data must be tightly correlated with the patient from whom the medical data was acquired. In that regard, in some embodiments, the wireless transceiver  206  may associate the medical data with identification information (e.g., a unique identifier, patient name, patient social security number, patient date of birth, procedure location, procedure time, practitioner name, etc.) before wirelessly transmitting the medical data to one or more remote devices. For instance, the wireless transceiver  206  may convert received medical data into a plurality of messages (i.e., packets) containing both the medical data and associated identifying information, and then wirelessly transmit the plurality of messages. Additional details of message formats that associate medical data with identifying patient information are described in U.S. Provisional Patent Application No. 61/473,591, filed on Aug. 8, 2011 and titled “DISTRIBUTED MEDICAL SENSING SYSTEM AND METHOD,” which is hereby incorporated by reference in its entirety. Further, in some instances, the wireless transceiver  206  is configured to wirelessly transmit any pressure data over a secure communication link. For instance, the wireless transceiver  206  may transmit over a wireless network secured with Wired Equivalent Privacy (WEP), Wi-Fi Protected Access (WPA), Extensible Authentication Protocol (EAP), or another type of wireless security. Further, the wireless transceiver  206  may encrypt and/or compress any data prior to transmission. 
         [0048]    In some embodiments, the output connector  192  is also used to facilitate energy harvesting from the hemo system or other pressure measuring system. In that regard, to eliminate the need for an additional power supply within the interface  170 , a power extraction circuit  220  extracts a small amount of power from the hemo system&#39;s excitation voltage associated with the distal pressure input  193 . The power extraction circuit  220  converts the extracted energy into the power needed to run the remaining circuitry of the interface  170 . In some instances, the power extraction circuit  220  is configured to be the only power source used to power the components of interface  170 . Since the excitation signal can be AC, positive or negative DC, and/or have various wave form shapes and voltages, the power extraction circuit  220  must be able to accept these and convert to a regulated power supply. In that regard, the voltage extracted from the excitation signal is converted to a regulated Vcc voltage to operate the low power circuitry using a buck or boost regulator depending on the input voltage. A current limiter minimizes distortion to the hemo system&#39;s waveform at the peaks. In some instances, the current is limited to a level below the AAMI transducer limits as to be compatible with most hemo systems. In some instances, the hemo system&#39;s excitation voltage meets the IEC 60601-2-34 standard. In some alternative embodiments, the power extraction circuit  220  is configured to interface with a battery or other rechargeable power supply device that can be utilized to power the components of the interface. In some alternative embodiments, the power extraction circuit  220  is configured to interface with an AC adapter that is to be plugged into a wall outlet in order to provide power to the components of the interface. 
         [0049]    The interface  170  also includes input/output connectors  194  and  195  for interfacing with a proximal pressure measurement system. In some particular embodiments, the input/output connectors  194  and  195  are configured to work with a pressure monitoring device of a hemo system. Generally, the input/output connector  194  is configured to receive signals from a proximal pressure sensing component  196 . In some instances, the proximal pressure sensing component  196  is a transducer configured to detect aortic pressure. Accordingly, in some embodiments with an arrangement similar to that shown in  FIG. 4 , input/output connector  194  is configured to receive signals from instrument  175 , where the proximal pressure sensing component  196  is a pressure sensing component associated with the instrument  175 . The input/output connector  195  is configured to send a proximal pressure signal to a proximal pressure input  197  of a hemo system or other computing device. Accordingly, in some embodiments with an arrangement similar to that shown in  FIG. 4 , the input/output connector  195  is configured to send the proximal pressure signal to an input of hemodynamic system  172  over connection  178 . 
         [0050]    In the illustrated embodiment of  FIG. 5 , conductors  200  and  202  carry an excitation signal to the proximal pressure sensing component  196 . An amplifier  204  is electrically connected to the conductors  200  and  202  as shown. The amplifier  204  is an operational amplifier in some embodiments. In some instances, the excitation signal extracted by amplifier  204  is sent to the wireless transceiver  206  for wireless transmission to remote devices such as the computing device  180  shown in  FIG. 4 . 
         [0051]    In the illustrated embodiment of  FIG. 5 , conductors  208  and  210  carry the proximal pressure signal from the proximal pressure sensing component  196  back to the proximal pressure input  197  of the hemodynamic system. In that regard, an amplifier  212  is electrically connected to the conductors  208  and  210  as shown. The amplifier  212  is an operational amplifier in some embodiments. The amplifier  212  is configured to monitor or sample the proximal pressure signal (i.e., aortic pressure signal) being supplied from the proximal pressure sensing component  196 . The sampled proximal pressure signal is then sent to the wireless transceiver  206  for wireless transmission to remote devices such as the computing device  180  shown in  FIG. 4 . In some instances the sampled proximal pressure signal is converted from an analog signal to a digital signal prior to being provided to the wireless transceiver  206 , but, in other instance, the wireless transceiver  206  performed the analog-to-digital conversion itself. Accordingly, both the excitation signal/voltage sampled from conductors  200  and  202  and the proximal pressure signal sampled from conductors  208  and  210  are fed to the wireless transceiver  206  for transmission. In this manner, an aortic pressure signal received from an aortic pressure sensing transducer may be transmitted directly to a computing device where the aortic pressure signal may be utilized in various calculations. The computing device therefore does not need to rely on receiving the aortic pressure signal from a hemodynamic system, which may provide unreliable data. 
         [0052]    As noted above, the interface  170  is also configured to receive and process distal pressure signals from a distal pressure sensing component  191 . In that regard, in some instances, the wireless transceiver  206  is configured to associate the distal pressure signal with the proximal pressure and wireless transmit them together to a remote device where pressure-based diagnostic characterizations may be calculated. In some instances, a single distal pressure signal and a single proximal pressure signal are each represented by 2 bytes of data. With an example sampling rate of 200 Hz, the wireless transceiver  206  may transmit approximately 800 bytes per second—well within the data transmission rate limits of most wireless communication standards. In other instance, the distal pressure signal and the proximal pressure signal may be represented by different amounts of data and the sampling rate may be different, requiring the wireless transceiver  206  to transmit a greater or smaller amount of data every second. And as described above, the wireless transceiver  206  may associate identifying information with the pressure signals, which would increase the amount of data wirelessly transmitted. 
         [0053]    In some instances, the interface  170  includes a microprocessor that calculates the proximal pressure based on the excitation signal voltage (Vexc), and the proximal pressure sensing component&#39;s output, and the calculated values are provided to the wireless transceiver  206  for transmission. In that regard, the proximal pressure sensing component&#39;s output conforms to the AAMI standard of 5 uV/Vexc/mmHg in some instances. For an AC excitation signal, the microprocessor must measure the proximal pressure signal voltage in synchrony with the excitation waveform. In some instances, rather than the low-level inputs described above, the proximal pressure signal is received by the interface  170  as a high level signal. For example, the proximal pressure signal is a high level signal from a Volcano LoMap (available from Volcano Corporation) or from an external hemo system. 
         [0054]    In some embodiments, the input/output connectors  194  and  195  are also used to facilitate energy harvesting from the hemo system or other pressure measuring system. In that regard, to eliminate the need for an additional power supply within the interface  170 , the power extraction circuit  220  may be connected to conductors  200  and  202  and utilized to extract a small amount of power from the hemo system&#39;s excitation voltage for the proximal pressure sensing component  196  and convert it into the power needed to run the remaining circuitry of the interface  170 , including the wireless transceiver  206 . As noted above, when connected to the proximal pressure sensing side the power extraction circuit  220  is still configured to extract power from the excitation signal sent from the controller/computing device such that the extracted power can be used to power the components of interface  170 . Since the excitation signal can be AC, positive or negative DC, and/or have various wave form shapes and voltages, the power extraction circuit  220  must be able to accept these and convert to a regulated power supply without distorting the waveform that continues on to the proximal pressure sensing component  196 . This is necessary to avoid affecting the pressure measurements obtained by the proximal pressure sensing component  196 . The voltage extracted from the excitation signal is converted to a regulated Vcc voltage to operate the low power circuitry using a buck or boost regulator depending on the input voltage. 
         [0055]    In some instances, a signal conditioning portion  216  of the interface  170  is in communication with input  190  that receives the distal pressure signal. Referring now to  FIG. 6 , shown therein is a schematic of a portion the interface  170  according to an exemplary embodiment of the present disclosure. In particular,  FIG. 6  shows a schematic of an exemplary embodiment of signal conditioning portion  216  of the interface  170 . In that regard, the signal condition portion  216  is configured to condition signals received from the distal pressure measurement device. The signal conditioning portion  216  provides the excitation and amplification required for the distal pressure measurement device&#39;s pressure sensors, Ra and Rb, which collective form distal pressure sensing component  191  in some instances. 
         [0056]    Calibration coefficients provided by the distal pressure measurement device utilizing an EPROM in the device connector, for example, are read to adjust the gain, offset, and temperature sensitivity for the device. The read values are used to adjust the three Digital to Analog Converters (DACs)  224 ,  226 , and  228 , in the distal pressure front end circuitry  216  that control the gain, offset, and temperature (TC) compensation, respectively. The distal pressure signal is then digitized with an Analog to Digital Converter  230 , ADC, and sent to the wireless transceiver  206 . The wireless transceiver  206  can then wirelessly transmit the distal pressure to a remote device for further calculations. For example, a computing device that wirelessly receives the transmitted distal pressure and/or distal pressure waveform and the proximal pressure and/or proximal pressure waveform may use them to calculate FFR, calculate iFR, calculate a pressure differential between the proximal and distal pressures, identify a suitable diagnostic window for performing a pressure differential calculation without administering a hyperemic agent to the patient, calculate a pressure differential during the identified diagnostic window, and/or combinations thereof. 
         [0057]    Referring now to  FIG. 7 , shown therein is a schematic of a portion the interface  170  according to another exemplary embodiment of the present disclosure. In particular,  FIG. 7  shows a schematic of an exemplary embodiment of signal conditioning portion  216 ′ of the interface  170 . In that regard, the signal condition portion  216 ′ is configured to condition signals received from the distal pressure measurement device. The signal conditioning portion  216 ′ provides the excitation and amplification required for the distal pressure measurement device&#39;s pressure sensors, Ra and Rb, which collective form distal pressure sensing component  191  in some instances. The distal pressure signal from the pressure sensors, Ra and Rb, is digitized with a two-channel Analog to Digital Converter  230 , ADC, and sent to the wireless transceiver  206  transmission. In the embodiments in which the interface  170  includes a microprocessor, calibration coefficients provided by the distal pressure measurement device utilizing an EPROM  222  in the device connector, for example, are read by the microprocessor to adjust the gain, offset, and/or temperature sensitivity for the device. The read values are used by the microprocessor to control the gain, offset, and/or temperature compensation. Firmware within the microprocessor is utilized to control these parameters in some instances. The microprocessor can then utilize the distal pressure or the distal pressure waveform for additional calculations. For example, in some instances the microprocessor utilizes the distal pressure and/or distal pressure waveform with the proximal pressure and/or proximal pressure waveform to calculate FFR, calculate a pressure differential between the proximal and distal pressures, identify a suitable diagnostic window for performing a pressure differential calculation without administering a hyperemic agent to the patient, calculate a pressure differential during the identified diagnostic window, and/or combinations thereof. Any of these calculations may be provided to the wireless transceiver  206  for wireless transmission to one or more remote devices. Accordingly, the signal conditioning portion  216 ′ of  FIG. 7  provides similar functionality to the signal conditioning device  216  of  FIG. 6 , but without the need for the three Digital to Analog Converters (DACs)  224 ,  226 , and  228 . 
         [0058]    Referring again to  FIG. 5 , the interface  170  is also configured to output the distal pressure signal to an input  193  of a hemodynamic system. In that regard, the wireless transceiver  206  or a separate microprocessor within the interface  170  provides a digitized signal to an additional set of DACs in the distal output circuitry  218  that modulate the excitation of the hemo system to provide a proportional distal waveform of the distal pressure voltage back to the hemo system through output  192 . In some embodiments, the scaled voltage returned is the same as a standard proximal pressure transducer, 5 uV/Vexc/mmHg, per the AAMI standards. The output stage  218  modulates the external excitation of the hemo system to provide a duplicate wave shape, or a DC voltage, scaled to 5 μV/Vexc/mmHg, per AAMI standards for aortic transducers of the distal pressure for the hemo system. Accordingly, by outputting the distal pressure signal through output  192  and the proximal pressure signal through output  196 , both proximal and distal pressures can then be observed on the hemo system&#39;s display using the hemo system&#39;s standard low-level inputs. 
         [0059]    As noted above, in some instances in which the interface  170  includes a microprocessor either integrated into or distinct from the wireless transceiver  206 , the interface uses the proximal and distal pressure data received from the instruments to calculate and display information that can be useful in the evaluation of the vessel and, in particular, evaluation of a stenosis of the vessel. In some instances, the interface is configured to calculate and display FFR. For an FFR measurement, the microprocessor first normalizes the distal pressure to the aortic pressure. The distal and aortic pressures will become disparaging as the distal pressure wire crosses the arterial lesion. The peak difference between the two pressures is captured automatically, or with a manual button press, and an FFR calculation started. The resultant number is shown on the display. The peak difference is typically measured during hyperemia with the use of drugs like adenosine 
         [0060]    In some embodiments the interface  170  includes external user-controlled buttons. In one particular embodiment, one of the buttons causes the microprocessor to ‘normalize’ the distal pressure measurement to the proximal pressure measurement. This is typically performed with the pressure sensing components  191  and  196  positioned in close proximity to one another within the patient such that they are subjected to similar pressures. In some instances, this calibration is performed proximal of the lesion and before the distal pressure sensing component  191  is advanced distally beyond the lesion. After the distal pressure sensing component  191  is placed beyond the suspect lesion actuation of another button causes the microprocessor to calculate the ratio of the distal pressure to the proximal pressure, which provides an FFR value or pressure differential. In that regard, in some implementations the button is pressed by a user at a precise moment during hyperemia based on observation of the proximal and distal waveforms, which may be displayed on a separate device (e.g., a display of the hemo system) or displayed on a display of interface  170 . Alternatively, the determination of the appropriate moment for the FFR calculation can be done automatically by the microprocessor. In that regard, in some instances the FFR calculation is performed at a point coinciding with the peak difference between the distal and proximal (aortic) pressures. In some embodiments, a pressure differential is calculated during a diagnostic window without application of a hyperemic agent, as discussed below. In such embodiments, the pressure measurements and/or the pressure differential may be displayed continuously. 
         [0061]    In that regard, in some instances the interface  170  is configured to provide pressure measurements and/or pressure differentials based on evaluation techniques as described in one or more of UK Patent Application Publication No. GB 2479340 A, filed Mar. 10, 2010 and titled “METHOD AND APPARATUS FOR THE MEASUREMENT OF A FLUID FLOW RESTRICTION IN A VESSEL”, UK Patent Application No. GB 1100137.7, filed Jan. 6, 2011 and titled “APPARATUS AND METHOD OF ASSESSING A NARROWING IN A FLUID FILLED TUBE”, U.S. Provisional Patent Application No. 61/525,739, filed on Aug. 20, 2011 and titled “Devices, Systems and Methods for Assessing a Vessel,” and U.S. Provisional Patent Application No. 61/525,736, filed on Aug. 20, 2011 and titled “Devices, Systems, and Methods for Visually Depicting a Vessel and Evaluating Treatment Options,” each of which is hereby incorporated by reference in its entirety. 
         [0062]    In some embodiments, the interface  170  is utilized to calculate and display FFR in a traditional FFR procedure where the patient is administered a hyperemic agent. In other embodiments, the interface  170  is utilized to calculate a pressure differential similar to FFR (i.e., the ratio of distal pressure to proximal pressure) but without the use of a hyperemic agent. In that regard, a suitable diagnostic window for making such calculations must be determined and/or identified to have a useful measurement. The diagnostic window for evaluating differential pressure across a stenosis without the use of a hyperemic agent in accordance with the present disclosure may be identified based on characteristics and/or components of one or more of proximal pressure measurements, distal pressure measurements, proximal velocity measurements, distal velocity measurements, ECG waveforms, and/or other identifiable and/or measurable aspects of vessel performance. In that regard, various signal processing and/or computational techniques can be applied to the characteristics and/or components of one or more of proximal pressure measurements, distal pressure measurements, proximal velocity measurements, distal velocity measurements, ECG waveforms, and/or other identifiable and/or measurable aspects of vessel performance to identify a suitable diagnostic window. 
         [0063]    In some embodiments, the determination of the diagnostic window and/or the calculation of the pressure differential are performed in approximately real time or live to identify the diagnostic window and calculate the pressure differential. In that regard, calculating the pressure differential in “real time” or “live” within the context of the present disclosure is understood to encompass calculations that occur within 10 seconds of data acquisition. It is recognized, however, that often “real time” or “live” calculations are performed within 1 second of data acquisition. In some instances, the “real time” or “live” calculations are performed concurrent with data acquisition. In some instances the calculations are performed by a processor in the delays between data acquisitions. For example, if data is acquired from the pressure sensing devices for 1 ms every 5 ms, then in the 4 ms between data acquisitions the processor can perform the calculations. It is understood that these timings are for example only and that data acquisition rates, processing times, and/or other parameters surrounding the calculations will vary. In other embodiments, the pressure differential calculation is performed 10 or more seconds after data acquisition. For example, in some embodiments, the data utilized to identify the diagnostic window and/or calculate the pressure differential are stored for later analysis. 
         [0064]    In some instances, the diagnostic window is selected by identifying a portion of the cardiac cycle corresponding to a time period in which the change in velocity (i.e., dU) fluctuates around zero. Time periods where the change in velocity is relatively constant and approximately zero (i.e., the speed of the fluid flow is stabilized) are suitable diagnostic windows for evaluating a pressure differential across a stenosis of a vessel without the use of a hyperemic agent in accordance with the present disclosure. In that regard, in a fluid flow system, the separated forward and backward generated pressures are defined by: 
         [0000]    
       
         
           
             
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         [0065]    where dP is the differential of pressure, p is the density of the fluid within the vessel, c is the wave speed, and dU is the differential of flow velocity. However, where the flow velocity of the fluid is substantially constant, dU is approximately zero and the separated forward and backward generated pressures are defined by: 
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         [0066]    In other words, during the time periods where dU is approximately zero, the forward and backward generated pressures are defined solely by changes in pressure. 
         [0067]    Accordingly, during such time periods the severity of a stenosis within the vessel can be evaluated based on pressure measurements taken proximal and distal of the stenosis. In that regard, by comparing the forward and/or backward generated pressure distal of a stenosis to the forward and/or backward generated pressure proximal of the stenosis, an evaluation of the severity of the stenosis can be made. For example, the forward-generated pressure differential can be calculated as 
         [0000]    
       
         
           
             
               
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         [0000]    while the backward-generated pressure differential can be calculated as 
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         [0068]    In the context of the coronary arteries, a forward-generated pressure differential is utilized to evaluate a stenosis in some instances. In that regard, the forward-generated pressure differential is calculated based on proximally originating (i.e., originating from the aorta) separated forward pressure waves and/or reflections of the proximally originating separated forward pressure waves from vascular structures distal of the aorta in some instances. In other instances, a backward-generated pressure differential is utilized in the context of the coronary arteries to evaluate a stenosis. In that regard, the backward-generated pressure differential is calculated based on distally originating (i.e., originating from the microvasculature) separated backward pressure waves and/or reflections of the distally originating separated backward pressure waves from vascular structures proximal of the microvasculature. 
         [0069]    In yet other instances, a pressure wave is introduced into the vessel by an instrument or medical device. In that regard, the instrument or medical device is utilized to generate a proximally originating forward pressure wave, a distally originating backward pressure wave, and/or combinations thereof for use in evaluating the severity of the stenosis. For example, in some embodiments an instrument having a movable membrane is positioned within the vessel. The movable membrane of the instrument is then activated to cause movement of the membrane and generation of a corresponding pressure wave within the fluid of the vessel. Based on the configuration of the instrument, position of the membrane within the vessel, and/or the orientation of the membrane within the vessel the generated pressure wave(s) will be directed distally, proximally, and/or both. Pressure measurements based on the generated pressure wave(s) can then be analyzed to determine the severity of the stenosis. 
         [0070]    There are a variety of signal processing techniques that can be utilized to identify time periods where the change in velocity is relatively constant and approximately zero, including using a differential, first derivative, second derivative, and/or third derivative of the velocity measurement are utilized. For example, identifying time periods during the cardiac cycle where the first derivative of velocity is relatively constant and approximately zero allows the localization of time periods where velocity is relatively constant. Further, identifying time periods during the cardiac cycle where the second derivative of velocity is relatively constant and approximately zero allows the localization of a time period where acceleration is relatively constant and near zero, but not necessarily zero. 
         [0071]    While examples of specific techniques for selecting a suitable diagnostic window have been described above, it is understood that these are exemplary and that other techniques may be utilized. In that regard, it is understood that the diagnostic window is determined using one or more techniques selected from: identifying a feature of a waveform or other data feature and selecting a starting point relative to the identified feature (e.g., before, after, or simultaneous with the feature); identifying a feature of a waveform or other data feature and selecting an ending point relative to the identified feature (e.g., before, after, or simultaneous with the feature); identifying a feature of a waveform or other data feature and selecting a starting point and an ending point relative to the identified feature; identifying a starting point and identifying an ending point based on the starting point; and identifying an ending point and indentifying a starting point based on the ending point. Additional details of techniques for selecting a suitable diagnostic window are described in U.S. Provisional Patent Application No. 61/525,739, filed on Aug. 20, 2011 and titled “Devices, Systems and Methods for Assessing a Vessel,” which is hereby incorporated by reference in its entirety. In that regard, it is understood that the interface  170  may be programmed to determine one or more diagnostic windows based on the techniques described in the present disclosure, including those incorporated by reference, and/or include one or more hardware features configured to identify one or more diagnostic windows based on the techniques described in the present disclosure, including those incorporated by reference. 
         [0072]    Further, for a variety of reasons the proximal pressure measurements and the distal pressure measurements received by the interface  170  are not temporally aligned in some instances. For example, during data acquisition, there will often be a delay between the distal pressure measurement signals and the proximal pressure measurement signals due to hardware signal handling differences between the instrument(s) utilized to obtain the measurements. In that regard, the differences can come from physical sources (such as cable length and/or varying electronics) and/or can be due to signal processing differences (such as filtering techniques). The resulting delay between the signals is between about 5 ms and about 150 ms in some instances. Because individual cardiac cycles may last between about 500 ms and about 1000 ms and the diagnostic window may be a small percentage of the total length of the cardiac cycle, longer delays between the proximal and distal pressure measurement signals can have a significant impact on alignment of the pressure data for calculating a pressure differential for a desired diastolic window of a cardiac cycle. 
         [0073]    As a result, in some instances, it is necessary to shift one of the proximal and distal pressures relative to the other of the distal and proximal pressures in order to temporally align the pressure measurements. For example, a portion of the distal pressure measurement or proximal pressure measurement may be shifted to be temporally aligned with a corresponding portion of the proximal pressure measurement or distal pressure measurement, respectively, coinciding with the diagnostic window. While a shift of only a portion of the distal or proximal pressure measurement associated with the diagnostic window is utilized in some instances, in other instances all or substantially all of the proximal and distal pressures are aligned before the portions corresponding to a selected diagnostic window are identified. 
         [0074]    Alignment of all or portion(s) of the proximal and distal pressures is accomplished using a hardware approach in some instances. For example, one or more hardware components are positioned within the communication path of the proximal pressure measurement, the distal pressure measurement, and/or both to provide any necessary delays to temporally align the received pressure signals. In some instances, these hardware components are positioned within the interface  170 . In other instances, alignment of all or portion(s) of the proximal and distal pressures is accomplished using a software approach. For example, a cross-correlation function or matching technique is utilized to align the cardiac cycles in some embodiments. In other embodiments, the alignment is based on a particular identifiable feature of the cardiac cycle, such as an ECG R-wave or a pressure peak. Additionally, in some embodiments alignment is performed by a software user where adjustments are made to the delay time of at least one of the proximal and distal pressures until the cardiac cycles are visually aligned to the user. A further technique for aligning the signals is to apply a synchronized timestamp at the point of signal acquisition. Further, in some instances combinations of one or more of hardware, software, user, and/or time-stamping approaches are utilized to align the signals. 
         [0075]    Regardless of the manner of implementation, several approaches are available for the aligning the proximal and distal pressure measurement signals. In some instances, each individual distal pressure measurement cardiac cycle is individually shifted to match the corresponding proximal pressure measurement cardiac cycle. In other instances, an average shift for a particular procedure is calculated at the beginning of the procedure and all subsequent cardiac cycles during the procedure are shifted by that amount. This technique requires little processing power for implementation after the initial shift is determined, but can still provide a relatively accurate alignment of the signals over the course of a procedure because the majority of the signal delay is due to fixed sources that do not change from patient to patient or within the procedure. In yet other instances, a new average shift is calculated each time that the proximal and distal pressure signals are normalized to one another during a procedure. In that regard, one or more times during a procedure the sensing element utilized for monitoring pressure distal of the stenosis is positioned adjacent the sensing element utilized for monitoring pressure proximal of the stenosis such that both sensing elements should have the same pressure reading. If there is a difference between the pressure readings, then the proximal and distal pressure signals are normalized to one another. As a result, the subsequently obtained proximal and distal pressure measurements are more consistent with each other and, therefore, the resulting pressure differential calculations are more accurate. 
         [0076]    With the proximal and distal pressure measurements aligned, the pressure differential for the diagnostic window is calculated. In some instances, the pressure differential is calculated using average values for the proximal and distal pressure measurements across the diagnostic window. The pressure differential calculations of the present disclosure are performed for a single cardiac cycle, in some instances. In other instances, the pressure differential calculations are performed for multiple cardiac cycles. In that regard, accuracy of the pressure differential can be improved by performing the pressure differential calculations over multiple cardiac cycles and averaging the values and/or using an analysis technique to identify one or more of the calculated values that is believed to be most and/or least accurate. 
         [0077]    One advantage of the techniques of the present disclosure for identifying diagnostic windows and evaluating pressure differentials is the concept of “beat matching”. In that regard, the proximal and distal waveforms for the same cardiac cycle are analyzed together with no averaging or individual calculations that span more than a single cardiac cycle. As a result, interruptions in the cardiac cycle (such as ectopic heartbeats) equally affect the proximal and distal recordings. As a result, these interruptions that can be detrimental to current FFR techniques have minor effect on the techniques of the present disclosure. Further, in some embodiments of the present disclosure, the effect of interruptions in the cardiac cycle and/or other irregularities in the data is further minimized and/or mitigated by monitoring the pressure differential calculations to detect these anomalies and automatically exclude the impacted cardiac cycles. 
         [0078]    In one particular embodiment, pressure differential is calculated on two sequential cardiac cycles and the individual pressure differential values are averaged. The pressure differential of a third cycle is then calculated. The average value of the pressure differentials is compared to the average pressure differential using three cycles. If the difference between the averages is below a predetermined threshold value, then the calculated value is considered to be stable and no further calculations are performed. For example, if a threshold value of 0.001 is used and adding an additional cardiac cycle changes the average pressure differential value by less than 0.001, then the calculation is complete. However, if the difference between the averages is above the predetermined threshold value, then the pressure differential for a fourth cycle is calculated and a comparison to the threshold value is performed. This process is repeated iteratively until the difference between the averages of cardiac cycle N and cardiac cycle N+1 is below the predetermined threshold value. As the pressure differential value is typically expressed to two decimal places of precision (such as 0.80), the threshold value for completing the analysis is typically selected to be small enough that adding a subsequent cardiac cycle will not change the pressure differential value. For example, in some instances the threshold value is selected to be between about 0.0001 and about 0.05. 
         [0079]    In some instances, the level of confidence calculation has different thresholds depending on the degree of stenosis and/or an initial calculated pressure differential value. In that regard, pressure differential analysis of a stenosis is typically based around a cutoff value(s) for making decisions as to what type of therapy, if any, to administer. Accordingly, in some instances, it is desirable to be more accurate around these cutoff points. In other words, where the calculated pressure differential values are close to a cut-off, a higher degree of confidence is required. For example, if the cutoff for a treatment decision is at 0.80 and the initial calculated pressure differential measurement is between about 0.75 and about 0.85, then a higher degree of confidence is needed than if the initial calculated pressure differential measurement is 0.40, which is far from the 0.80 cutoff point. Accordingly, in some instances the threshold value is at least partially determined by the initial calculated pressure differential measurement. In some instances, the level of confidence or stability of the calculated pressure differential is visually indicated to user via a software interface. 
         [0080]    Because pressure differential can be calculated based on a single cardiac cycle in accordance with the present disclosure, a real-time or live pressure differential calculation can made while the distal pressure measuring device is moved through the vessel. Accordingly, in some instances the system includes at least two modes: a single-cardiac-cycle mode that facilitates pressure differential calculations while moving the distal pressure measuring device through the vessel and a multi-cardiac-cycle mode that provides a more precise pressure differential calculation at a discrete location. In one embodiment of such a system, the interface  170  is configured to provide the live pressure differential value until the distal pressure measuring device is moved to the desired location and a measurement button is selected and/or some other actuation step is taken to trigger the multi-cardiac-cycle mode calculation. 
         [0081]    Persons skilled in the art will also recognize that the apparatus, systems, and methods described above can be modified in various ways. Accordingly, persons of ordinary skill in the art will appreciate that the embodiments encompassed by the present disclosure are not limited to the particular exemplary embodiments described above. In that regard, although illustrative embodiments have been shown and described, a wide range of modification, change, and substitution is contemplated in the foregoing disclosure. It is understood that such variations may be made to the foregoing without departing from the scope of the present disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the present disclosure.