Abstract:
A guidewire is disclosed that is constructed using tubular members that create a hollow lumen that runs from the proximal end of the guidewire to a window towards the distal end of the wire. This internal lumen is filled with a fluid that allows pressure exerted at the window to exert pressure at the proximal end of the guide wire proportional to the pressure exerted at the window. This pressure exerted at the proximal end of the guidewire is measured using a pressure transducer external of the guidewire. The pressure transducer converts the measured pressure into an electrical signal that is proportional to the pressure at the window. The electrical signal is manipulated to correct for errors that are due to the pressure signal traveling though the inner lumen of the guidewire to ensure the electrical signal matches the pressure exerted at the window.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefits of U.S. Provisional Patent Application No. 61/429,026, filed on Dec. 31, 2010, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to the field of guidewires used to diagnose and treat maladies in humans and more specifically to pressure-sensing guidewires used in intravascular procedures. 
         [0004]    2. Description of the Related Art 
         [0005]    It is often desirable to determine the severity of a stenosis or occlusion in the coronary arteries by measuring the pressure distally and proximally of the stenosis or occlusion. Devices today that are used for this purpose include catheter like members with some type of pressure-sensing device incorporated therein. Such devices are often referred to as a pressure-sensing guidewire since they can provide the dual function of guidewire and a pressure measuring device. Many of these devices today are constructed by incorporating a piezo-resistive pressure sensing device towards the distal end of a hollow guidewire body. Three electrical wires are then run the length of the hollow guidewire in order to connect the piezo-resistive pressure sensing device to the proper measurement instrumentation. 
         [0006]    One problem associated with the currently available pressure-sensing guidewires is the cost to manufacturer such a device. These devices can be up to 10 times more expensive to manufacturer than a standard guidewire and up to 20 times more expensive than a standard catheter with invasive blood pressure sensor. The main reason for the high cost is the piezo-resistive device itself and the labor required to run electrical wires through the length of guidewire and terminate them on the proximal end of the wire. Thus adoption of the current pressure-sensing guidewires by medical professionals is inhibited due to the large cost difference between these devices and standard guidewires. 
         [0007]    A second problem associated with the currently available pressure-sensing guidewires is that the accuracy of the piezo-resistive pressure sensor inside the guidewire is not as accurate as a standard invasive blood pressure sensor. Due to limited space inside the guidewire, the piezo-resistive pressure senor must use a half bridge design in order to minimize the number of electrical wires that must be run the length of the guidewire. As a result, the currently available pressure sensing guidewires have reduced zero drift stability and increased susceptibility to thermal variations over standard full bridge invasive blood pressure sensors. 
         [0008]    A third problem associated with the currently available pressure-sensing guidewires is reliability of such devices. These devices have electrical wires running the length of the guidewire that terminate to the piezo-resistive sensor on the distal end and to an electrical connector on the proximal end. These electrical connections must be sealed from the surrounding fluid in the body and any tiny breach will cause errors in the pressure measurement. As a result, there is high rate of failure with such a device because of the number of electrical interconnects and their proximity to fluids. 
         [0009]    A guidewire with a single fluid filled lumen from the distal end to the proximal end, with a pressure transducer attached at the proximal end would solve the three problems mentioned above because it does not require electrical wires to run the length of the guidewire  10  and a full bridge, low cost pressure transducer can be used at the proximal end of the wire. However measuring pressure through a fluid filled tube creates distortions in the pressure waveform that creates measurement error because the pressure exerted on one end of the tube is no longer directly proportional to the pressure measured on the other end of the tube. In order to accurately measure pressure through a fluid filled tube, these distortions created by the fluid filled tube must be corrected for. 
         [0010]    As shown in  FIG. 1 , when pressure is applied to one end  1  of a fluid filled tube  2  having a pressure transducer  3  on the other end  4  of the tube  2  the relationship between pressure p(t) and an electrical output E of the pressure transducer  3  is governed by the following differential equation: 
         [0000]    
       
         
           
             
               
                 E 
                 ¨ 
               
               + 
               
                 
                   c 
                   
                     ρ 
                      
                     
                         
                     
                      
                     LA 
                   
                 
                  
                 
                   E 
                   . 
                 
               
               + 
               
                 
                   k 
                   
                     ρ 
                      
                     
                         
                     
                      
                     LA 
                   
                 
                  
                 E 
               
             
             = 
             
               
                 K 
                 
                   ρ 
                    
                   
                       
                   
                    
                   L 
                 
               
                
               
                 p 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ρ is the density of the fluid  5  in the tube  2 , L is the length of the tube  2 , A is the cross-sectional area of the inner diameter of the tube  2 , K is a constant related the pressure transducer  3 , k is a constant related to elastic forces inside the tube  2  and c is a constant related to the fluid column velocity. An alternative form of the equation above is: 
         [0000]    
       
         
           
             
               
                 E 
                 ¨ 
               
               + 
               
                 2 
                  
                 
                   ζω 
                   n 
                 
               
               + 
               
                 E 
                 . 
               
               + 
               
                 
                   ω 
                   n 
                   2 
                 
                  
                 E 
               
             
             = 
             
               
                 K 
                 
                   ρ 
                    
                   
                       
                   
                    
                   L 
                 
               
                
               
                 p 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ω n  is the natural frequency of the lumen filled tube  2  and pressure transducer  3 , and ζ is the damping ratio of the lumen filled tube  2  and pressure transducer  3 . 
         [0011]    Accordingly, there is a need for a device and system that can function both as a pressure-sensing device and a guidewire but measures pressure by using a fluid filled lumen inside the guidewire and corrects for the errors that are created when pressure is measured though this fluid filled lumen or tube. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is a guidewire that is constructed using tubular members that create a hollow lumen that runs from the proximal end of the guidewire to a window towards the distal end of the wire, where the interior of the tubular member is exposed to the exterior of the tubular member. This internal lumen is filled with a fluid that allows pressure exerted at the window to exert pressure at the proximal end of the guide wire that is proportional to the pressure exerted at the window. This pressure exerted at the proximal end of the guidewire can then be measured using a pressure transducer that is external of the guidewire. The pressure transducer will then convert the pressure at the proximal end of the guidewire to an electrical signal that is proportional to the pressure at the window. The electrical signal is then manipulated to make the pressure measured at the proximal end of the guidewire equal to the pressure exerted at the window, correcting for errors that are due to the pressure signal traveling though the inner lumen of the guidewire. 
         [0013]    There are many objects of the present invention in its various embodiments that may be addressed individually or in combinations and permutations. Each embodiment may address one or several of the following objectives. 
         [0014]    An object of the invention in one or more embodiments to create device that functions both as a pressure-sensing device and a guidewire. 
         [0015]    It is an object of the invention in one or more embodiments to create a device that measures pressure by using a fluid filled lumen inside a guidewire and correct for errors that are created when pressure is measured though this fluid filled lumen. 
         [0016]    These and other objects and advantages of the invention will be clear in view of the following description to the invention including the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be described hereafter in detail with particular reference to the drawings. Throughout this description, like elements, in whatever embodiment described, refer to common elements wherever referred to and referenced by the same reference number. The characteristics, attributes, functions, interrelations ascribed to a particular element in one locations apply to that element when referred to by the same reference number in another location unless specifically stated otherwise. All Figures are drawn for ease of explanation of the basic teachings of the present invention only; the extensions of the Figures with respect to number, position, relationship and dimensions of the parts to form the preferred embodiment will be explained or will be within the skill of the art after the following description has been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength and similar requirements will likewise be within the skill of the art after the following description has been read and understood. 
           [0018]      FIG. 1  is a schematic view of the elements related to determining pressure in a tube. 
           [0019]      FIG. 2  is a top view of an embodiment of the present invention where the guidewire has a lumen that runs from proximal end to a distal window. 
           [0020]      FIG. 3  is a lengthwise cross-sectional view of the guidewire of  FIG. 2 . 
           [0021]      FIG. 4  is a side view of the guidewire in  FIG. 2  attached to a pressure transducer housing device. 
           [0022]      FIG. 5  is a lengthwise cross-sectional view of the arrangement of  FIG. 4 . 
           [0023]      FIG. 6  is a schematic, partial cross-sectional view of an embodiment of the present invention. 
           [0024]      FIG. 7  is a chart showing a typical frequency response of fluid within the lumens of the guidewire and mating device of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The pressure sensing guidewire of the present invention is shown in  FIGS. 2-6  and generally labeled  10 .  FIG. 2  shows the external surface  12  of the guidewire  10  which also has a distal end  14  and a proximal end  16 . Guidewire  10  also has a window  18 , located at or near the distal end  14 , and an inner lumen  20  that extends from the window  18  to the proximal end  16 . The window  18  exposes the external surface  12  of the guidewire  10  to the inner lumen  20  of the guidewire  10 . The guidewire  10  has a continuous external surface  12  from the proximal end  16  to the window  18  such that no part of the inner lumen  20  is exposed to the external surface  12  between the proximal end  16  and the window  18 . The guidewire  10  has a flexible tip coil  22  and flexible working section  24  that are typical features of a guidewire and consequently well known and understood in the art. The overall diameter of the guidewire  10  is typically 0.014 inches, however the invention is not limited to this dimension. In addition, it is common to keep the outer diameter constant throughout the length of the guidewire  10 . But, the invention is not limited to this requirement and there are embodiments of this invention where the outer diameter of the guidewire changes throughout the length of the guidewire. 
         [0026]      FIG. 3  is the cross-section lengthwise view of the guidewire  10  of  FIG. 2 .  FIG. 3  shows the hollow inner lumen  20  that runs from the proximal end  16  of the guidewire  10  to the window  18 . The only points where the inner lumen  20  of the guidewire  10  is exposed to the exterior of the guidewire  10  and surrounding environments is at the proximal end  16  and at the window  18 . The window  18  shown in  FIGS. 2 and 3  is only one representation of where the window  18  could be located on the guidewire  10 . There are many different embodiments of the window  18  location and geometry and this invention is not limited just to the embodiment shown. The inner lumen  20  shown in  FIG. 3  that connects the proximal end  16  of the guidewire  10  to the window  18  is filled with a fluid, gel or any media that can transmit the pressure exerted at the window  18  to the proximal end  16  of the guidewire  10 . A key feature of the fluid, gel or any other media within the inner lumen  20  is that such fluid, gel or any other media is virtually incompressible in the range of physiological pressures so that pressure applied to the fluid, gel or other material at the window  18  is transferred by the fluid, gel or other material to the proximal end  16  of the guidewire  10 . 
         [0027]      FIG. 4  shows the guidewire  10  shown in  FIGS. 2 and 3  mated to a mating device  26  that houses a pressure transducer  28 .  FIG. 5  shows, in lengthwise cross-section, the mating device  26  of  FIG. 4  mated with the guidewire  10 . The mating device  26  has a chamber  30  dimensioned to conformally receive and grip the outer surface  12  of the proximal end  16  of the guidewire  10 . The mating device  26  contains an inner lumen  32  that is also filled with a fluid, gel or any media that is virtually incompressible at physiological pressures and can transmit pressure and that is in fluid communication with the inner lumen  20  at the proximal end  16  of the guidewire  10 . The pressure transducer  28  is in fluid contact with the fluid filled inner lumen  32 . The pressure transducer  28  converts mechanical pressure exerted in the fluid filled inner lumen  32  to an electrical signal that is proportional to the pressure of the fluid in the inner lumen  32 . 
         [0028]    As mentioned above, the mating device  26  attaches to the outer surface  12  of the guidewire  10 . In the preferred embodiment of the invention, this attachment is temporary, meaning that the mating device  26  can be easily removed from or attached to the guidewire  10  by the user. However, the present invention is not limited to this feature and the mating device  26 , in another embodiment of the invention, is permanently attached to the guidewire  10 . 
         [0029]    When the mating device  26  is attached to the guidewire  10  by placing the proximal end  16  of the guidewire  10  in the chamber  30 , the fluid filled inner lumen  32  and the fluid filled inner lumen  20  are fluidly connected so that there is continuous fluid from the pressure transducer  28  to the window  18  on the guidewire  10 . As a result, pressure exerted at the window  18  of the guidewire  10  exerts a pressure on the pressure transducer  28  that is proportional to the pressure exerted at the window  18 . The pressure transducer  28  then converts the pressure measured at pressure transducer  28  to an electrical signal that is proportional to the pressure exerted on the fluid at the window  18 . 
         [0030]      FIG. 6  shows the guidewire  10  of the present invention mated to a mating device  26  that contains a pressure transducer  28  that is in turn connected to a measurement instrument  34  via an electrical connection  36 . This measurement instrument  34  reads the electrical signals generated by the pressure transducer  28  that is proportional to the pressure exerted at the window  18  of the guidewire  10  and converts this electrical signal to a pressure reading corresponding to the pressure exerted at the window  18  of the guidewire  10 . The measurement instrument  34  conveys this pressure information to a user via a display  38  or by other means of communication well understood in the art or to another medical device or instrument. 
         [0031]    In order for the measurement instrument  34  to create a reading that is equal to the pressure exerted at the window  18 , the measurement instrument  34  corrects the electrical signal produced by the pressure transducer  28  for distortion and errors caused by the pressure signal traveling though the fluid filled lumens  20 ,  32  of the guidewire  10  and the mating device  26 . These distortions and errors are due to amplification or damping of certain frequencies of the pressure signal as it travels though the fluid filled lumen. As a result the pressure signal measured at the proximal pressure transducer  28  will have a different shape, peak to peak magnitude, and possibly mean pressure compared to what is exerted at the distal window  18 . There are many different methods of how this correction can be done and this invention is not limited to the methods that are described here. 
         [0032]    One method for correcting for the errors caused to the pressure signal as it travels though the fluid filled lumens  20 ,  32  of the guidewire  10  and mating device  26 , respectively, is to measure the frequency response of the fluid within the lumens  20 ,  32  with the pressure transducer  28 . In order to measure frequency response, a pressure generator is used to create an oscillating pressure signal with a known magnitude and known frequency at the distal window  18 . During this time, the pressure is measured by the proximal pressure transducer  28  and its magnitude is compared to the known magnitude of the pressure signal generated at the distal window  18 . This process is repeated for all frequencies of interest, which is typically 0-30 Hz for a blood pressure signal. Once this data is collected, the frequency response of the lumen based pressure sensing guidewire can be determined and plotted for the frequencies of interest. 
         [0033]    An example of such a frequency response in shown in  FIG. 7 . In the plot, the Y axis is the magnitude of the pressure measured at the proximal pressure transducer  28  divided by the magnitude of the pressure generated at the distal window  18 . The X axis is the frequency of the oscillating pressure signal generated at the distal window  18 . Once the frequency response is known, the pressure measured by pressure transducer  28  is corrected so that it measures more precisely the pressure at the window  18  of the guidewire  10 . 
         [0034]    In order to do this, the pressure measured at the proximal pressure transducer  28  has to be converted into the frequency domain using a version of a Fourier transform or any method that decomposes a signal into its constituent frequencies. Once in the frequency domain, the signal can be scaled based on the measured frequency response such that the magnitude of the measured signal at the proximal pressure transducer  28  equals the magnitude of pressure exerted at the distal window  18 , at a given frequency. Once this is done for all frequencies of interest, the scaled frequency domain signal is converted back into the time domain and the result is a time domain pressure signal that is measured at the proximal transducer  28  but is equal to the pressure exerted at the distal window  18 . 
         [0035]    Another method of corrected the pressure measured by the pressure transducer  28  for distortions and errors is to correct for these distortions and errors by comparing the pressure measured by the pressure transducer  28  attached to the guidewire  10  to another pressure transducer in the body, a so called “Reference Transducer,” that is measuring the same pressure that is exerted at the guidewire  10 &#39;s window  18 . The reference transducer can be placed anywhere in the body&#39;s vasculature (e.g., at or near the end of the guide catheter, typically at the start of the coronary tree) as long as the window  18  of the guidewire  10  is at or near the same location as where the reference transducer is measuring and where there is a pulsatile signal due to the heart beat. 
         [0036]    Since blood pressure in the body is pulsatile, the scale of the pressure measured by the pressure transducer  28  attached to the guidewire  10  can be corrected by determining the scale factor required to make the peak to peak signal measured by the pressure transducer  28  equal to the peak to peak signal measured by the Reference Transducer. Offset for the pressure transducer  28  can be corrected by making the mean pressure measured by the pressure transducer  28  equal to the mean pressure measured by the Reference Transducer. Once the pressure transducer  28  is equalized to the Reference Transducer, the guidewire  10  can be positioned in any artery or vein of the body, including those into which the Reference Transducer cannot fit, and measure the pressure there accurately. 
         [0037]    Another method to correct for these distortions and errors is to compare the frequency domain of the pressure measured by the pressure transducer  28  to the frequency domain of the Reference Transducer that is measuring the same pressure exerted at the guidewire  10 &#39;s window  18 . A Fourier Transform or FFT (Fast Fourier Transform) is performed on the pressure data measured by the pressure transducer  28  and compared to the Fourier Transform or FFT of the pressure data from the Reference Transducer. At each frequency, the data from the pressure transducer  28  is scaled so that the magnitude at the frequency is equal to the magnitude of the pressure data from the Reference Transducer at the same frequency. By determining the correct scale values at each frequency, a transfer function is then created where data from the pressure transducer  28  is adjusted in the frequency domain by the appropriate scale value and then converted back to the time domain so that the resulting pressure data is equal to the time domain pressure signal of the Reference Transducer. Once this transfer function is determined, the guidewire  10  can be positioned in any artery or vein of the body, including where the Reference Transducer cannot fit, and measure the pressure there accurately. 
         [0038]    The present invention has been described in connection with several different embodiments. The present invention also anticipates that more than one embodiment or correction method may be applied or combined into a single device. Further, although the window  18  has been shown as being directed radially from the inner lumen  20 , the window  18  may be directed axially at the distal end  14 . Further, more than one window  18  may be present wherever located. 
         [0039]    The present invention has been described in connection with certain embodiments, combinations, configurations and relative dimensions. It is to be understood, however, that the description given herein has been given for the purpose of explaining and illustrating the invention and are not intended to limit the scope of the invention. In addition, it is clear than an almost infinite number of minor variations to the form and function of the disclosed invention could be made and also still be within the scope of the invention. Consequently, it is not intended that the invention be limited to the specific embodiments and variants of the invention disclosed. It is to be further understood that changes and modifications to the descriptions given herein will occur to those skilled in the art. Therefore, the scope of the invention should be limited only by the scope of the claims.