Abstract:
A method of measuring blood pressure and velocity proximally and distally of a stenosis in a vessel carrying blood includes the steps of providing a guide wire having both a pressure sensor and a velocity sensor disposed on a distal region of the guide wire, introducing the guide wire into the vessel, advancing the guide wire to position the pressure sensor and the velocity sensor proximally and distally of the stenosis, and measuring the blood pressure and velocity proximally and distally of the stenosis with the pressure sensor and the velocity sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 11/650,064, filed Jan. 4, 2007, now U.S. Pat. No. 7,967,762, which is a continuation of U.S. application Ser. No. 11/303,249, filed Dec. 15, 2005, which is a continuation of U.S. application Ser. No. 10/247,043, filed Sep. 19, 2002, now U.S. Pat. No. 6,976,965, which is a continuation of U.S. application Ser. No. 09/644,111, filed Aug. 21, 2000, now U.S. Pat. No. 6,767,327, which is a continuation of U.S. application Ser. No. 08/912,879, filed Aug. 15, 1997, now U.S. Pat. No. 6,106,476, which is a continuation-in-part of U.S. application Ser. No. 08/710,062, filed Sep. 9, 1996, now U.S. Pat. No. 5,715,827, which is a continuation of U.S. application Ser. No. 08/300,445, filed Sep. 2, 1994, now abandoned, all of which are hereby expressly incorporated by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     This invention relates to an ultra miniature pressure sensor and guide wire and apparatus using the same and method, which is particularly suitable for making pressure measurements in coronary arteries of human beings. 
     It has been well known that it is desirable to make pressure measurements in vessels and particularly in coronary arteries with the advent of angioplasty. Typically in the past, such pressure measurements have been made by measuring the pressure at a proximal extremity of a lumen provided in a catheter advanced into the coronary artery of interest. However, such an approach has been less efficacious as the diameters of the catheters became smaller with the need to advance the catheter into smaller vessels. This made necessary the use of smaller lumens which gave less accurate pressure measurements and in the smallest catheters necessitated the elimination of such a pressure lumen entirely. In an attempt to overcome these difficulties, ultra miniature pressure sensors have been proposed for use on the distal extremities of catheters. However, it has not been feasible prior to the present invention to provide such ultra miniature pressure sensors which are capable of being incorporated in a guide wire for making pressure measurements in a very small arterial vessels. There is therefore a need for a new and improved ultra miniature pressure sensor and a guide wire and apparatus utilizing the same. 
     In general it is an object of the present invention to provide an ultra miniature pressure sensor and guide wire and apparatus utilizing the same making possible pressure and velocity measurements. 
     Another object of the invention is to provide a sensor which can be utilized on the distal extremity of a guide wire 0.018″ or 0.014″ in diameter. 
     Another object of the invention is to provide a sensor of the above character which is formed of a silicon chip of a small dimension which is reinforced by an additional member to provide reinforcement for the chip. 
     Another object of the invention is to provide a sensor of the above character in which a thin diaphragm is formed in the crystalline silicon chip. 
     Another object of the invention is to provide a sensor of the above character in which the reinforcing member extends for approximately 200 microns beyond the silicon diaphragm. 
     Another object of the invention is to provide a guide wire with the above character in which the number of conducting wires required is kept to a minimum. 
     Another object of the invention is to provide a guide wire and method in which simultaneous pressure and velocity measurements can be made. 
     Another object of the invention is to provide a guide wire of the above character in which the diaphragm area has been maximized. 
     Another object of the invention is to provide a guide wire with the above character in which two pressure sensors are provided on the guide wire which are spaced apart so that pressure measurements can be made on both sides of a stenosis. 
     Another object of the invention is to provide a guide wire of the above character in which the sensors are covered to prevent the formation of blood clots. 
     Another object of the invention is to provide an apparatus of the above character which includes a guide wire with an integral inflatable balloon. 
     Another object of the invention is to provide an apparatus of the above character in which temperature compensation can be provided. 
     Another object of the invention is to provide an apparatus of the above character which can be utilized in a half-bridge configuration. 
     Additional features and objects of the invention will appear from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration showing use of a guide wire incorporating a pressure sensor of the present invention and apparatus utilizing the same in conjunction with a patient undergoing a catheterization procedure for diagnosis or treatment. 
         FIG. 2  is a side elevational view of a guide wire incorporating an ultra miniature pressure sensor of the present invention. 
         FIG. 3  is an enlarged side elevational view of the distal extremity of the guide wire shown in  FIG. 2  and showing the pressure sensor mounted therein. 
         FIG. 4  is a top plan view looking along the line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a bottom plan view looking along the line  5 - 5  of  FIG. 3 . 
         FIG. 6  is an isometric view of the pressure sensor shown in  FIGS. 3 ,  4  and  5  with the lead wires connected thereto. 
         FIG. 7  is a side elevational view of the pressure sensor shown in  FIG. 6 . 
         FIG. 8  is a top plan view of the pressure sensor shown in  FIGS. 6 and 7 . 
         FIG. 9  is a cross-sectional view taken along the line  9 - 9  of  FIG. 8 . 
         FIG. 10  is a cross-sectional view taken along the line  10 - 10  of  FIG. 8 . 
         FIG. 11  is a cross-sectional view taken along the line  11 - 11  of  FIG. 8 . 
         FIG. 12 , is a schematic diagram of the circuitry utilized in the pressure sensor shown in  FIGS. 6-11 . 
         FIG. 13  is a side elevational view of the distal extremity of another guide wire incorporating the pressure sensor with the sensor of the present invention being mounted in the tip housing. 
         FIG. 14  is a side elevational view of the distal extremity of a guide wire having first and second pressure sensors mounted in the distal extremity of the same spaced apart to permit simultaneous measurements of proximal and distal pressures with respect to a stenosis. 
         FIG. 15  is a partial side elevational view of another guide wire incorporating the present invention with an enclosed pressure sensor. 
         FIG. 16  is a side elevational view partially in section of the distal extremity of another guide wire incorporating the present invention in which the pressure sensor is enclosed in a transition housing. 
         FIG. 16A  is a side elevational view in section showing an end-mounted pressure sensor incorporating the present invention. 
         FIG. 17  is a side elevational view in section of a guide wire housing a tip-mounted sensor incorporating the present invention with an integral balloon. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, the guide wire of the present invention having pressure sensing capabilities is comprised of a flexible elongate element having proximal and distal extremities and having a diameter of 0.018″ and less. The pressure sensor is mounted on the distal extremity of a flexible elongate element. It is comprised of a crystal semiconductor material having a recess therein and forming a diaphragm bordered by a rim. A reinforcing member is bonded to the crystal and reinforces the rim of the crystal and has a cavity therein underlying the diaphragm and exposed to the diaphragm. A resistor having opposite ends is carried by the crystal and has a portion thereof overlying a portion of the diaphragm. Leads are connected to opposite ends of the resistor and extend within the flexible elongate member to the proximal extremity of the flexible elongate member. 
     More in particular, the guide wire  21  of the present invention having pressure measuring capabilities as shown in  FIG. 1  is one that is adapted to be used in connection with a patient  22  lying on a table or a bed  23  in a cath lab of a typical hospital in which a catheterization procedure such as for diagnosis or treatment is being performed on the patient. The guide wire  21  is used with apparatus  24  which consists of a cable  26  which connects the guide wire  21  to an interface box  27 . Interface box  27  is connected by another cable  28  to a control console  29  which has incorporated as a part thereof a video screen  31  on which a waveform  32  displaying ECG measurements may be provided as well as two traces  33  and  34  displaying pressure measurements being made by the guide wire  21 . 
     The guide wire  21  is shown more in detail in  FIG. 2  and as shown therein, the guide wire  21  can be constructed utilizing the various constructions as shown in U.S. Pat. Nos. 5,125,137; 5,163,445; 5,174,295; 5,178,159; 5,226,421; and 5,240,437. As disclosed therein, such a guide wire consists of a flexible elongate element  41  having a proximal and distal extremities  42  and  43  and which can be formed of a suitable material such as stainless steel having an outside diameter for example of 0.018″ or less and having a suitable wall thickness as for example, 0.001″ to 0.002″ and conventionally called a “hypotube” having a length of 150-170 centimeters. Where a smaller guide wire is desired, the hypotube  41  can have an exterior diameter of 0.014″ or less. Typically such a guide wire includes a core wire (not shown) of the type disclosed in the above identified patents which extends from the proximal extremity to the distal extremity of the flexible elongate element  41  to provide the desired torsional properties for guide wires (See U.S. Pat. No. 5,163,445, col. 18:40-51) to facilitate steering of the guide wire  21  in the vessel. 
     A coil spring  46  is provided and is formed of a suitable material such as stainless steel. It has an outside diameter of 0.018″ and is formed from a wire having a diameter of 0.003″. The spring  46  is provided with a proximal extremity  47  which is threaded onto the distal extremity  43  of the flexible elongate member  41 . The distal extremity  48  of the coil spring  46  is threaded onto the proximal extremity  49  of an intermediate or transition housing  51  such as disclosed in U.S. Pat. No. 5,174,295, formed of a suitable material such as stainless steel having an outside diameter of 0.018″ and having a suitable wall thickness as for example, 0.001″ to 0.002″. The housing  51  is provided with a distal extremity  52  which has the proximal extremity  53  of a coil spring  54  threaded thereon. The coil spring  54  is formed of a highly radiopaque material such as palladium or a tungsten platinum alloy. The coil spring  46  can have a suitable length as for example 27 centimeters whereas, the coil spring  54  can have a suitable length such as 3 centimeters. The intermediate or transition housing  51  can have a suitable length as for example, one to five millimeters. The use of the two coils  46  and  54  on opposite ends of the housing  61  provides a very flexible floppy tip for the guide wire  21  as described in U.S. Pat. No. 5,174,295. The coil  54  is provided with a distal extremity which is threaded onto an end cap  57  also formed of a suitable material such as stainless steel and having an outside diameter of 0.018″ and a wall thickness of 0.001″ to 0.002″. An ultrasonic transducer  58  is mounted in the end cap in a manner described in U.S. Pat. No. 5,125,137 and has conductors  61  and  62  secured to the front and rear sides of the same which extend interiorly to the proximal extremity of the flexible elongate member  41 . 
     A torquer  66  of the type described in U.S. Pat. No. 5,178,159 is mounted on the proximal extremity  42  of the flexible elongate member  41  for causing a rotation of a guide wire  21  when used in connection with catheterization procedures in a manner well known to those skilled in the art. 
     The proximal extremity  42  is also provided with a plurality of conducting sleeves (not shown) of the type disclosed in U.S. Pat. No. 5,178,159. In the present invention, one or more additional sleeves can be provided to make connection to the conductors hereinafter described. The proximal extremity  42  of the flexible elongate member is removably disposed within a housing  68  of the type described in U.S. Pat. Nos. 5,178,159, 5,348,481 and 5,358,409 that makes electrical contact with the sleeves on the proximal extremity  42  while permitting rotation of the sleeves and the flexible elongate member  41 . The housing  68  carries female receptacles (not shown) which receive the sleeves and which are connected to a cable  71  connected to a connector  72 . The connector  72  is connected to another mating connector  73  carried by the cable  26  and connected into the interface box  27 . 
     The portion of the guide wire  21  therefore described is substantially conventional. In accordance with the present invention it is provided with a pressure measuring capability in the form of a pressure sensor assembly  76  which is mounted within the intermediate or transition housing  51 . The pressure sensor assembly  76  consists of a diaphragm structure  77  supported by a base plate  78 . The diaphragm structure  77  is formed of suitable materials such as “n” type or “p” type  100  oriented silicon with a resistivity of approximately 6-8 ohm-centimeters. The diaphragm structure  77  is a die made from such a wafer. In accordance with the present invention, the die has a suitable length, as for example, 1050 microns and for a 0.014″ guide wire has a width of 250 microns and for a 0.018″ guide wire has a width of between 250 and 350 microns. It can have a suitable thickness, as for example, 50 microns. A rectangular diaphragm  79  is formed in the diaphragm structure  77  of a suitable thickness, as for example, 2.5 microns and having dimensions such as a length of 350 microns. The diaphragm  79  has first and second or top and bottom surfaces  80  and  81 . The diaphragm is formed by utilization of conventional masking and crystal etching techniques which create a die with two parallel sloping endwalls  82  and two parallel sidewalls  83  extending at right angles to the end walls  82  leading down to the top surface  80  of the diaphragm  79  to form a well  84 . As hereinafter explained, the diaphragm  79  is made relatively wide in comparison to the diaphragm structure  77  so that what remains is a relatively narrow rim  86  formed by side portions  87  and  88  and an end portion  89 . As can be seen from  FIGS. 6 ,  7  and  8 , the diaphragm  79  is located at or near one end of the diaphragm structure or die  77 . It has been found that it is desirable to provide a rectangular geometry for the diaphragm  79  rather than a square geometry in order to obtain the highest possible sensitivity for pressure measurements. For example, it has been found that the rectangular diaphragm provides approximately 1.5 times more sensitivity than does a square diaphragm for the same diaphragm thickness and width. 
     In etching the well  84  to form the diaphragm  81 , an impurity can be implanted into the backside of the diaphragm structure  77  before the etching process is commenced so that etching will stop at the desired depth, as for example, within 2 to 3 microns of the bottom surface  81  to provide a diaphragm  79  having a thickness ranging from 2 to 5 microns, and for example, the preferred thickness of 2.5 microns. Because the rim  86  provided on the diaphragm structure  77  surrounding the rectangular diaphragm  79  is relatively thin, the base plate  78  provides support for this rim to provide the necessary strength for the pressure sensor  76 . 
     In order to obtain adequate performance characteristics such as sensitivity in the miniaturized pressure sensor assembly  76  hereinbefore described, it has been found desirable to have as much of the width of diaphragm structure  77  as possible be occupied by the diaphragm  79  and at the same time to minimize the portion of the diaphragm structure  77  occupied by the rim. In order to achieve a diaphragm width ratio of at least 0.45 to 0.9 with respect to the width of the diaphragm  79  to the width of the structure  77  and therefore to obtain the largest diaphragm possible in the diaphragm structure  77 , diaphragm  79  is made relatively large compared to rim  86 . With current manufacturing technology, it has been found feasible to have a width of rim  86  of 40 microns, which provides for a diaphragm  79  of 170 microns in a 250 micron-wide diaphragm structure  77  to provide a diaphragm width ratio of 0.68. In a larger diaphragm structure such as 350 microns wide, the pressure sensor assembly  76  can be made stronger by increasing the rim width to 90 microns. Alternatively, it can be made more sensitive by increasing the diaphragm width up to 270 microns. This results in a diaphragm width ratio for a 350 micron-wide device of between 0.49 and 0.77, depending on what combination of sensitivity and strength is desired. 
     Prior to or after the formation of the rectangular diaphragm  79 , a plurality of V-shaped recesses or grooves  91  are formed in the diaphragm structure  77  on the end opposite the end at which the diaphragm  79  is located and on the side opposite the side in which the well  84  is formed. These V-shaped recesses  91  also can be formed in a conventional manner by the use of a conventional etch. It should be appreciated that if desired, the etching can be stopped so that the recesses formed are short of a complete V. By way of example, if the etching for the V-shaped recess was stopped at a depth of 12 microns, the bottom of the substantially V-shaped recess or trench  91  would be approximately 8 microns wide. 
     After the V-shaped or substantially V-shaped recesses have been formed, a P+ diffusion utilizing a suitable material such as boron can be carried out to create a V-shaped region  92  (in the structure  77 ) which underlies the V-shaped recess  91 . Utilizing suitable masking a common layer  93  of a suitable material such as chromium is sputtered into the V-shaped recess  91  to a suitable thickness as for example, 300 Angstroms followed by a layer  94  of a suitable material such as gold of a suitable thickness as for example 3000 Angstroms. The layers  93  and  94  overlie the bottom surface  81  to form pads  96  thereon. In depositing the gold in the V-shaped recess  91  it is desirable to terminate the gold just short of the leftmost extremity of the V-shaped recess as viewed in  FIG. 8  in order to minimize the likelihood of lead-to-lead shorting during the dicing operation when a wafer is sawed up into individual sensor chips. 
     By way of example, the spacing between V-grooves  91  from center to center can be 75 microns with the V-groove having a width of 25 microns and having a typical depth of 18 microns. The metal pads  96  formed by the chromium and gold layers  93  and  94  can have a suitable width as for example, 50 microns with the overlap on each side being approximately 12.5 microns to provide a spacing of approximately 25 microns between adjacent V-shaped pads  96 . The bottom of the V-shaped groove can have a total length of approximately 250 microns. 
     The regions  92  formed from the P+ diffusion have patterns that extend to the right from the three V-shaped recesses  91  as viewed in  FIG. 8  for a distance so that they underlie the approximate midpoint of the diaphragm  81  on opposite sides to provide generally U-shaped portions or resistors  92   a  which are located on the diaphragm in areas of a maximum stress to provide maximum sensitivity to pressure changes. The resistors  92   a  are provided with opposite ends, one end being connected to one each of the V-grooves and the other end being connected to the center or common V-groove. Contact is made to these P+ diffused regions by the chromium and gold layers  93  and  94  hereinbefore described. 
     The base plate  78  can be formed of a suitable material such as Pyrex supplied by Corning Glassworks and can have the same width as the diaphragm structure  77  but has a length which is less than the length of the diaphragm structure  77  so that the V-shaped grooves  91  are exposed on the underside of the diaphragm structure  77  as shown in  FIG. 6 . It also can have a suitable length such as 850 microns. It is provided with a rectangular recess or cavity  101  having substantially the same size as the diaphragm  79 . It can be etched into the Pyrex by suitable means such as a conventional etching process utilizing hydrochloric acid. After the etching has been completed to form the rectangular recess  101  it is bonded to the lower surface of the diaphragm structure  77  to form a hermetic seal with respect to the same so that the cavity  101  underlies the diaphragm  79  and is exposed to the bottom surface  81  of the diaphragm  79 . The cavity  101  below the diaphragm  79  serves as a reference pressure chamber and can be filled with a suitable fluid. For example, it can be filled with air to half an atmosphere to provide a partial vacuum. Alternatively, the cavity  101  can be filled to one atmosphere or it can be completely evacuated. 
     A trifilar lead structure  106  is connected to the rectangular diaphragm structure  77 . It has insulated copper leads  107  of a suitable diameter as for example 48 AWG soldered into place to the V-shaped recesses  91  so that the leads  107  extend outwardly therefrom and lie in a plane parallel to the plane of the diaphragm structure  77 . The trifilar lead construction  106  provides insulation around each lead and in addition there is provided additional insulation which surrounds the leads and which interconnects the leads into a single unit which can be readily extended through the hypotube forming the flexible elongate member  41 . 
     The pressure sensor assembly  76  is mounted within a cutout  111  provided in the transition housing  51  and secured therein by suitable means such as an epoxy  112  so that the outer surface of the pressure sensor assembly  76  is generally flush with the outer surface of the transition housing  51  (see  FIG. 3 ) and so that the diaphragm  79  is exposed to ambient and the leads  106  extend through the flexible elongate member  41  to the proximal extremity  42  of the same where they are connected to the sleeves (not shown) carried by the proximal extremity  42  disposed within the housing  68 . Also, the conductors  61  and  62  of the velocity sensing transducer  58  are connected to two of such sleeves (not shown) provided on the proximal extremity  42 . 
     A schematic of the wiring for the pressure sensor assembly  76  is shown in  FIG. 12 . The two generally U-shaped portions  92   a  on opposite sides of the diaphragm  79  are represented as resistors and are connected to the three leads  107  in the manner shown. One of the first of the outside leads  107  is “SIGNAL OUT” (+) and the second or other outside lead is “SIGNAL OUT” (−) and the third or middle lead is a common lead as shown. This pattern makes it possible to not cross leads and has the third lead going up the middle or center of the die or the diaphragm structure  77 . It can be seen that the two resistors  92   a  connected as shown form a half bridge one of the resistors responds positively to pressure change and the other resistor responds negatively to a pressure change. Thus, as a pressure is supplied to the diaphragm  79 , one resistor increases in value and the other resistor decreases in value to provide a voltage change. By applying the same current to both resistors at the same time, temperature effects can be measured because temperature change will affect both of the resistors in the same way so that the pressure measurements can be compensated for any changes in temperature which are sensed by the resistors  92   a . The changes in resistivity caused by the temperature changes in the resistors will cancel each other out because of the half bridge configuration used. In connection with  FIG. 12  it can be seen that with the use of three leads it is possible to obtain temperature compensation by utilizing a half-bridge configuration for the pressure sensor. Alternatively, a more precise temperature compensation can be provided by directly measuring the two resistances, and then solving the mathematical equations which relate temperature and pressure to the two sensor resistances. 
     Operation and use of the guide wire  21  in performing a catheterization procedure such as angioplasty may now be briefly described as follows: Let it be assumed that a guiding catheter (not shown) has been introduced into the femoral artery of the patient  22  shown in  FIG. 1  with the distal extremity near the desired location in the heart in which it is desired to perform an angioplasty. The guide wire  21  of the present invention is inserted into the guiding catheter. At the time that its distal extremity is in close proximity to the distal extremity of the guiding catheter, the pressure output signal from the guide wire is compared with that of the guiding catheter assuming that the guide wire is provided with pressure sensing capabilities. If there is a difference between the two pressure measurements, the pressure measurement from the guide wire  21  is equalized with that from the guiding catheter at the control console  29 . The distal extremity of the guide wire  21  is then advanced so that it is proximal of the stenosis to be treated at which time a pressure measurement is made. After this pressure measurement has been recorded, the distal extremity of the guide wire is then advanced through the stenosis and another pressure measurement made to determine whether the stenosis is severe enough to require treatment by angioplasty. Alternatively, the distal extremity of guide wire  21  can be immediately advanced to the distal side of the stenosis rather than making a pressure measurement proximal of the stenosis and thereafter comparing the pressure measurement on the distal extremity being measured by the guide wire  21  with the pressure measurement being provided proximal of the stenosis by the guiding catheter. If it is determined that the stenosis causes a partial occlusion which is severe enough to warrant use of an angioplasty procedure, an angioplasty catheter having a balloon thereon (not shown) can be advanced over the guide wire  21  and advanced into the stenosis to dilate the stenosis. After dilation has occurred, the angioplasty balloon can be withdrawn from the stenosis and pressure measurements can be made proximal and distal of the stenosis to ascertain the effect of the angioplastic treatment. If the pressure measurements indicate that the original dilation by the angioplasty balloon has been inadequate, another balloon catheter as for example, one having a balloon of a greater diameter can then be positioned over the guide wire  21  by utilizing an exchange wire if appropriate. The larger angioplasty catheter can be advanced through the stenosis and inflated to again dilate the stenosis to a larger size after which it can be withdrawn. Thereafter, pressure measurements proximal and distal of the stenosis can again be made to ascertain whether or not the second dilation which has been performed is adequate. The decisions to be made in connection with such procedures can be readily made by use of the control console  29  by observing the traces  33  and  34  on the video monitor  31 . 
     It also should be appreciated that at the same time Doppler velocity measurements can be made by the transducer  58 . That information can be used in connection with the pressure measurements to ascertain the need for performing the angioplasty procedure or for determining the efficacy of the angioplasty procedure performed. Because of the very small diameters of the guide wires as for example, 0.018″ or 0.014″, it is possible to utilize the guide wire  21  of the present invention with very small coronary vessels in the heart. In connection with the leads from the Doppler transducer  58  it should be appreciated that if desired some of the conductors provided for the Doppler ultrasound transducer can be shared with the wires or conductors provided for the pressure sensor assembly  76 . Thus, two of the wires for the pressure sensor can be utilized for the Doppler transducer because the pressure sensor operates at DC or up to a few hundred Hz or KHz whereas the Doppler sensor operates at 10 MHz and above. These frequency ranges can be readily separated by one skilled in the art by using simple filters and the appropriate circuitry. 
     In connection with the present invention it should be appreciated that rather than bonding the leads  107  into the V-grooves or V-shaped recesses  91 , the Pyrex base plate  78  can be formed so it has the same length as the diaphragm structure  77 . V-shaped or U-shaped grooves can be formed in the base plate underlying the V-shaped grooves to in effect form little tunnels which can be utilized for receiving the wires  107  and for them to be soldered therein. Such a construction aids in the placement of wires which are of the very small diameter, as for example, 1 mil. 
     Another embodiment of a guide wire  121  incorporating the present invention is shown in  FIG. 13 . In the guide wire  121 , pressure sensor assembly  76  is mounted in a tip housing  122 . The tip housing  122  can be substituted at the end cap  57  and threaded into the distal extremity  56  of the coil  54 . The tip housing  122  can be formed of a suitable material such as stainless steel having an outside diameter of 0.018″ and a wall thickness of 0.001″ to 0.002″. The sensor assembly  76  can be of the type hereinbefore described and can be mounted in a cutout  123  provided in the tip housing  122  much in the same manner as the sensor assembly  76  was mounted in the cutout  111  in the transition housing  51  such as by use of an epoxy  124 . An hemispherical end cap  126  formed of a radiopaque material such as palladium or tungsten platinum alloy can be mounted on the distal extremity of the tip housing  122 . Alternatively, the end cap  126  can be formed of a non-radiopaque material such as epoxy or silicone rubber. 
     Thus it can be seen with the embodiment of the guide wire  121  shown in  FIG. 13 , the guide wire  121  can be utilized in the same manner as the guide wire  21  hereinbefore described with the exception of it cannot be used for making velocity measurements because that capability has been removed from the guide wire  121 . 
     Another guide wire  131  incorporating the present invention is shown in  FIG. 14  in which two pressure sensors  76  have been provided. The sensors  76  have been spaced apart a suitable distance as for example, 3 centimeters with one of the pressure sensors being mounted in the transition housing  51  and the other pressure sensor being mounted in a tip housing  122  of the type shown in  FIG. 13 . With such an arrangement, it can be seen that the distal extremity of the guide wire  131  can be advanced across a stenosis in a vessel with the pressure sensor  76  mounted in the tip housing being distal of the stenosis to measure distal pressure and the pressure sensor  76  in the transition housing  51  being proximal of the stenosis to measure proximal pressure. Thus, it can be seen that it is possible to measure simultaneously the distal pressure and the proximal pressure with respect to a stenosis in a vessel. This may give more accurate measurements than utilizing the proximal pressure being sensed by the guiding catheter. 
     When using two pressure sensors  76  in the same guide wire as shown in  FIG. 14 , it is possible to utilize the same common wire for both of the transducers, thus making it necessary to provide only five wires rather than six wires for the two pressure sensors. 
     Still another guide wire  141  incorporating the present invention is shown in  FIG. 15  in which a cover  142  is provided for covering the pressure sensor assembly  76  provided in the transition housing  51 . The cover is elongate and extends the length of the cutout  111  and is arcuate in cross-section so that it conforms to the conformation of the transition housing  51 . The cover  142  can be secured in place by a suitable means such as an adhesive. The cover  142  overlying the pressure sensor assembly  76  is provided with a pin hole  143  which immediately overlies the diaphragm  79 . The pin hole  143  can be of a suitable size as for example 2-5 mils in and preferably 3 mils in diameter. The cover  142  serves to prevent the large opening provided by the cutout  111  from collecting blood which could possibly clot. The cover  142  also serves to protect the sensor  76  from damage. It also prevents the sensor  76  from being broken loose during use of the guide wire  141 . It should be appreciated that if desired, the volume beneath the cover  142  can be filled with viscous fluid such as oil which can be utilized for transmitting pressure from the pin hole  143  to the diaphragm  81 . With a small size pin hole  143 , the viscous fluid provided would not have a tendency to bleed out of the transition housing  51 . The viscous fluid would be held in place because of the surface tension of the fluid. Because there is a very short distance between the pin hole  143  and the diaphragm  79 , there would be very little tendency for the viscous fluid to damp any pressure signal transmitted from the blood in which the guide wire  141  is disposed to the diaphragm. 
     Another guide wire  151  incorporating the present invention is shown in  FIG. 16  having a transition housing  152  formed of a suitable material such as stainless steel and having an OD of 0.018″ or less. A pressure sensor assembly  76  of the type hereinbefore described is mounted within the bore  153  of the transition housing  152  and is secured therein by mounting the same in an epoxy  154  while leaving the area immediately above the diaphragm  79  exposed to a pin hole  156  provided in the transition housing  152 . The space overlying the diaphragm  81  exposed to the pin hole  156  can be filled with a viscous fluid  157  such as oil. The viscous fluid  157  can be retained within the desired location by a barrier  158  formed on the proximal side of the pressure sensor  76  having the trifilar lead structure  106  extending therethrough, in sealing engagement therewith. To seal the other end of the bore  153 , an intermediate end cap  161  can be provided which is provided with a barrier  182  extending thereacross to seal the bore  153 . The intermediate end cap  161  can be bonded to the transition housing  152  by a suitable means such as an adhesive (not shown). The coil  54  can be threaded onto the intermediate end cap  161  and can be threaded onto a tip housing  166  that carries a rounded hemispherical tip  167 . With such a construction it can be seen that the pressure sensor assembly  76  is protected within the transition housing  152 . 
     In  FIG. 16A  a guide wire  168  is shown which is very similar to the guide wire  151  with the exception that the housing  152  has been provided on the distal extremity of the coil  46  with the tip  167  directly mounted on the housing  152  for closing the bore  153 . 
     In  FIG. 17  there is shown another embodiment of a guide wire  171  incorporating the present invention which has an integral balloon carried thereby. A guide wire with an integral balloon is described in U.S. Pat. No. 5,226,421. The guide wire  171  consists of a flexible elongate tubular member  173  in a manner formed of a suitable material such as plastic which is provided with a distal extremity  174 . An inflatable balloon  176  is secured to the distal extremity  174  of the flexible elongate member  173  in a manner well known to those skilled in the art. Such a balloon can be formed integral with the distal extremity and can be formed of the same material as the flexible elongate tubular member  173 . Alternatively, it can be formed of a different material or the same material and be formed as a separate part and secured to the distal extremity  174  by suitable means such as adhesive. 
     The balloon  176  is provided with a distal extremity which is closed and which is secured to the proximal extremity of a coil spring  178  formed of a radiopaque material such as a palladium or tungsten platinum alloy threaded onto a tip housing  179 . The tip housing  179  can be formed in a manner similar to the tip housing  122  shown in  FIG. 13  having a pressure sensor  76  mounted therein and carrying an end cap  181 . The trifilar leads  106  connected to the sensor  76  extend through the coil  178  and through the balloon  176  and through the flexible elongate tubular member  172  to the proximal extremity thereof. A core wire  186  formed of a suitable material such as stainless steel is provided in the flexible elongate member  173  and can be provided with a diameter such as disclosed in U.S. Pat. No. 5,226,421. The core wire  186  is provided with a tapered portion  186   a  extending through the balloon which has a distal extremity secured to the housing  179  by a suitable means such as the epoxy utilized for mounting the sensor  76  within the housing. The flexible elongate tubular member  172  is provided with a balloon inflation lumen  187  which can be used for inflating and deflating the balloon  176 . 
     The guide wire  171  with an integral balloon  171  can be utilized in a manner similar to that hereinbefore described for the other guide wires. Rather than deploying a separate catheter with a balloon thereon over the guide wire, the guide wire  171  itself carries the balloon  176  which can be inflated to dilate the stenosis after the proximal and distal pressure measurements have been made by the tip mounted sensor  76 . After the balloon  176  has been deflated, the pressure measurement can be made to ascertain the pressure in the distal extremity after dilation has occurred. If necessary, the balloon  176  can be re-inflated to perform another dilation of the stenosis to obtain improved blood flow through the stenosis. 
     After an appropriate dilation has occurred, the guide wire  171  with integral balloon can be removed in a conventional manner. The angioplasty procedure can then be completed in a conventional manner. 
     From the foregoing, it can be seen that there has been provided an ultra miniature pressure sensor which can be utilized on guide wires having a diameter of 0.018″ and less which can be utilized for making accurate measurements proximal and distal of a stenosis in the coronary vessel. This is made possible because of the small size of the pressure sensor incorporated into the distal extremity of the guide wire. In addition to sensing pressure, flow velocity can also be obtained by the use of a distally mounted velocity transducer provided on the same guide wire as on which the pressure sensor is mounted. Alternatively, additional first and second pressure sensors can be provided on the distal extremity of a guide wire so that pressure measurements can be made simultaneously, proximally and distally of the stenosis. The pressure sensor is constructed in such a manner so that it can be readily incorporated within the confines of a small guide wire as for example, 0.018″ and less. It can be constructed to avoid a large opening in the distal extremity of the guide wire to inhibit or prevent the formation of clots. The pressure sensor also can be protected so that it cannot be readily damaged or broken loose. In addition, where desired, the guide wire can be provided with an integrally mounted balloon on its distal extremity so that the guide wire can be utilized for performing an angioplasty procedure while at the same time facilitating the making of pressure measurements, proximal and distal of the stenosis being treated.