Abstract:
Apparatus and methods for non-invasive measurement of a subject&#39;s venous pressure, and particularly the subject&#39;s central venous pressure (CVP). The apparatus comprises a probe with a load cell. The method makes use of a non-invasive medical device or technique, such as an ultrasound system, to visualize the internal jugular (IJ) vein. Once the IJ has been located, the operator pushes on the surface of the neck with the probe until the external pressure is sufficient to collapse the IJ. The load cell within the probe determines the amount of force applied, and the applied force is converted into venous pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Patent Application No. 60/787,065, filed on Mar. 29, 2006, the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates generally to apparatus and methods for measuring venous pressure and, more particularly, to apparatus and methods for non-invasively measuring central venous pressure.  
         [0004]     2. Description of Related Art  
         [0005]     The central venous pressure (CVP) is an important physiological parameter, the correct measurement of which is a clinically relevant diagnostic tool for heart failure patients, amongst others. For example, increased venous pressure is indicative of low cardiac output and higher blood volume in the venous compartment.  
         [0006]     A challenge for physicians is to obtain a quick and accurate measure of a patient&#39;s CVP in a manner that poses minimum discomfort. Current methods of measuring CVP accurately are rather invasive. Typically, a catheter is threaded along a major vein until it is within the vicinity of the right atrial compartment. Pressure readings are then collected directly from inside the vein. U.S. Pat. Nos. 6,592,565 and 6,819,951 describe methods and apparatus for collecting CVP data in this manner.  
         [0007]     However, threading a central line in this fashion carries certain risks. For example, inserting the needle into the vein itself can result in internal bleeding if an artery is accidentally punctured in the process. The risk of infection is also present whenever the skin is punctured. Furthermore, the procedure is also time-consuming and difficult to perform without hospitalization or in primary care settings.  
         [0008]     To avoid performing this procedure unnecessarily, doctors typically first check the CVP non-invasively by treating the superior vena cava as a manometer to the right atrium. The pressure at the right atrium correlates to the height of the column of blood in the vein, which can be estimated by visually identifying small disturbances in the jugular vein. These disturbances are a reflection of the pumping of the atrium and are observed at the topmost part of the column of blood. The actual pressure relative to the heart can be roughly measured by lifting the neck slowly from a supine position until the fluctuations become visible on the surface of the neck. The hydrostatic pressure that corresponds to the height difference between the neck and the heart is a measure of CVP. The procedure is outlined extensively in Lipton, B. “Estimation of central venous pressure by ultrasound of internal jugular vein” American. Journal of Emergency Medicine. 2000 July; 18(4):432-4, the contents of which are incorporated by reference herein in their entirety. This method is prone to error because spotting the exact height where the fluctuations appear is very difficult, especially in patients where layers of fat obscure the jugular vein. The process of physically lifting the patient upward incrementally is also taxing and time-consuming, and not well-suited for emergency conditions.  
         [0009]     Other methods have been described to aid physicians in visualizing the exact location of these fluctuations. For example, using ultrasound to visualize the internal jugular vein has been described in the Lipton article cited above. However, in this situation ultrasound only serves to supplant the less accurate visual identification of the fluctuations; lifting the patient to an appropriate height is still required to make an accurate measurement.  
         [0010]     Another measurement procedure has been outlined in Baumann U, Marquis C, Stoupis C, Willenberg T A, Takala J, Jakob S M. “Estimation of central venous pressure by ultrasound”.  Resuscitation.  64(2005), 193-199, the contents of which are incorporated by reference herein in their entirety. In this study, conducted in Switzerland, the operator uses an ultrasound probe modified with a quartz pressure transducer within a mixture of water and glycerin that is translucent to ultrasound waves. The device records the external pressure needed to collapse the IJ and correlates this value to the CVP. This device requires modification of the ultrasound probe, which makes it unattractive for many clinical care providers, since they would not be able to use their existing ultrasound equipment.  
       SUMMARY OF THE INVENTION  
       [0011]     One aspect of the invention relates to a venous pressure measurement apparatus. The apparatus comprises a probe, a load cell, and a central unit. The probe has a contacting member constructed and arranged to be pressed against skin proximate to a vein. The load cell is mounted on the probe and coupled to the contacting member such that it is arranged to measure an amount of force exerted by or on the contacting member. The central unit is coupled to the load cell and is adapted to read and record the amount of force measured by the load cell and to convert that measurement to a venous pressure.  
         [0012]     Another aspect of the invention relates to a venous pressure measurement system. The system includes a medical device capable of detecting vein closure and a venous pressure measurement apparatus. The apparatus comprises a probe, a load cell, and a central unit. The probe has a contacting member constructed and arranged to be pressed against skin proximate to a vein. The load cell is mounted on the probe and coupled to the contacting member such that it is arranged to measure an amount of force exerted by or on the contacting member. The central unit is coupled to the load cell and is adapted to read and record the amount of force measured by the load cell and to convert that measurement to a venous pressure.  
         [0013]     A further aspect of the invention relates to a method of measuring venous pressure. The method comprises visualizing a vein using a non-invasive medical imaging device, applying pressure to the skin over the vein using a probe capable of sensing the applied pressure until the medical device indicates that the vein has begun to collapse, and recording an initial collapse pressure when the vein has begun to collapse. The method also comprises apply pressure to the skin over the vein using the probe until the medical device indicates that the vein has substantially completely closed, recording a final close pressure when the vein has substantially completely closed, and calculating the venous pressure using the difference between the final collapse pressure and the initial collapse pressure.  
         [0014]     Other aspects, features, and advantages of the invention will be set forth in the description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention will be described with respect to the following drawing figures, in which like numerals represent like features throughout the figures, and in which:  
         [0016]      FIG. 1  is a side elevational view of a probe capable of measuring force, according to one embodiment of the invention;  
         [0017]      FIG. 2  is a cross-sectional view of the probe of  FIG. 1 , taken through Line  2 - 2  of  FIG. 1 ;  
         [0018]      FIG. 3  is a top plan view of a central unit adapted to read and record the amount of force measured by the probe of  FIG. 1 ;  
         [0019]      FIG. 4  is a schematic view of a system adapted to measure central venous pressure according to another embodiment of the invention; and  
         [0020]      FIG. 5  is a schematic flow diagram of a method for measuring central venous pressure using the system of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 1  is a side elevational view of a probe for non-invasively measuring venous pressures, generally indicated at  10 .  FIG. 2  is a cross-sectional view of the probe  10 , taken through Line  2 - 2  of  FIG. 1 . The probe  10  has a body  12  and a contacting member  14 . The body  12  of the probe  10  in the illustrated embodiment has the form of an elongate, generally cylindrical member with an overall length of approximately 15 cm and a diameter of approximately 2 cm, although in other embodiments, it may have substantially any shape and dimensions, so long as the body  12 , or at least a portion thereof, can be held comfortably in a user&#39;s hand. In some embodiments, the body  12  may be made of a plastic; in other embodiments, metal may be a suitable material.  
         [0022]     The contacting member  14  is constructed and arranged to be pressed against a patient&#39;s skin proximate to (e.g., over) a vein to measure the pressure within that vein. In some embodiments, the vein may be the internal jugular (IJ) vein and the pressure measured may be the patient&#39;s central venous pressure (CVP).  
         [0023]     While the inventors do not wish to be bound by any particular theory of operation, certain aspects of this description may assume that the IJ vein and other veins can be modeled according to Laplace&#39;s Law, which is set forth in Equation (1):  
             T   =       P   ×   R     M             (   1   )             
 
 where T is wall tension, P is transmural pressure, R is vessel radius, and M is wall thickness. According to Laplace&#39;s Law, when transmural pressure (the difference between internal and external pressure) is zero, the tension falls to zero and the vessel collapses. Therefore, if the contacting member  14  is pressed against the skin proximate to a vein until the vein collapses, and the pressure exerted to make the vein collapse is measured, the internal pressure of the vein can be determined. In the case of the IJ vein, that determined pressure is the CVP. Specific procedures for determining venous pressures and, in particular, the CVP, will be described below in more detail. 
 
         [0024]     The contacting member  14  of the illustrated embodiment is most advantageously curved, such that when it is pressed against skin, the pressure that it exerts on the skin is concentrated at a single point. (In most applications, that point would be the point along the vein at which pressure is to be measured.) In the embodiment of  FIGS. 1-2 , the contacting member  14  has an overall semi-hemispherical curvature, although other types of curvature and radii of curvature may be used in other embodiments. The contact area may be on the order of approximately two square centimeters in some embodiments. However, any contact area may be used so long as a proper balance is struck - if the contact area is too small, the contacting member  14  may move past or beyond the vein instead of compressing it; if the contact area is too large, the contacting member  14  may compress soft tissue and other structures as well.  
         [0025]     The contacting member  14  may be made of a durable rubber or it may be made of a hard plastic or metal, depending on the embodiment. Generally speaking, it may be advantageous if the contacting member  14  is made of a material that will not deform significantly under the applied loads.  
         [0026]     As is shown particularly in the cross-sectional view of  FIG. 2 , the contacting member  14  is not attached directly to the body  12 . Instead, mounted between the contacting member  14  and the body  12 , in a recess  16  in the contacting member  14  and a corresponding recess  18  in the top end of the body  12 , is a load cell  20 . Positioned as shown in  FIG. 2 , the load cell  20  can perceive all of the forces exerted by or on the contacting member  14 . Moreover, so as to ensure accurate measurement, the contacting member  14  does not make direct contact with the body  12  of the probe  10 . In some embodiments, however, the contacting member  14  may make contact with other structures, so long as load is not transferred to those structures. For example, the contacting member  14  need not directly contact or cover the load cell  20 . Instead, any number of members or elements may be interposed between the load cell  20  and the contacting member  14 , so long as those elements are essentially rigid and transmit the full load perceived by the contacting member  14  to the load cell  20  without absorbing or dissipating it.  
         [0027]     The precise manner in which the load cell  20  is mounted may vary from embodiment to embodiment. The mounting may be by adhesive, mechanical fastener engagement, or interference fit, depending on the embodiment and the type and capabilities of the load cell  20 . In some embodiments, adhesive tape on a flat load cell  20  has been found to be sufficient securement. Adhesives that are flexible when set may also be suitable. In other embodiments, for example, the load cell  20  could include a threaded post on each side, and the contacting member  14  and body  12  could include threaded openings adapted to engage the threads of those threaded posts on the load cell  20 . Generally speaking, the contacting member  14  and load cell  20  need not be user-removable or replaceable, although it is advantageous if the contacting member  14  and load cell  20  can be removed and replaced in order to service them.  
         [0028]     In one embodiment, the load cell may be, for example, an Omegadyne LCKD-5 five pound subminiature compression load cell (Omegadyne, Inc., Sunbury, Ohio, United States). Load cells of other ranges and sensitivities may be used, so long as they have adequate sensitivity in the range of loads expected for the particular application. Additionally, it should be understood that the term “load cell” is to be construed broadly to include any type of device or element capable of perceiving force and converting that perception into a recordable data point, without regard to the underlying technology by which it does so. For example, piezoelectric load cells, load cells based on change in electrical resistance with deformation, and mechanical spring-deflection load cells may all be used in various embodiments of the invention.  
         [0029]     The leads  22  from the load cell  20  pass through a hole  24  in the body  14  bored proximate to the load cell  20 , transit the length of the body  14 , and exit at the back end of the body  14  in a main data cable  26 . Along the interior of the body  14 , the leads  22  may be secured to the interior sidewall of the body  14  to reduce the risk of strain and breakage. Additionally, the leads  22  may be covered by a wire guide or another protective structure. In some embodiments, rather than being secured along the interior, the leads  22  may be secured on the exterior of the body  14  and covered by an appropriate protective cover or guide. Ultimately, the manner in which the leads  22  are held within or outside the body is not critical so long as they are not unduly strained and are protected from breakage and other adverse conditions. For that reason, the main data cable  26  may be provided with additional molded strain relief or any other features that may be desirable to protect the leads  22 .  
         [0030]     On the exterior lateral surface of the body  14 , a switching element  28  is provided. In the illustrated embodiment, the switching element  28  is a button, although in other embodiments, the switching element  28  may be a switch or any other sort of element. The leads  29  from the switching element also enter the body  14 , traverse its length, and exit in the main data cable  26 . As will be explained in greater detail below, when the switching element  28  is actuated, the amount of force measured by the load cell  20  is recorded.  
         [0031]     While in operation, the probe  10  may be covered by a disposable cover, such as a disposable latex cover, so as to prevent contamination and avoid transmitting infection from one patient to the next.  
         [0032]     In some embodiments, the components used to read and display the load values generated by the load cell  20  and to generate venous pressure values may all be internal to and/or a part of the probe  10 , such that the apparatus as a whole comprises only a handheld probe. Ultimately, a unitary probe with all electronics integrated might have a form similar to that of an electronic thermometer, with a display and controls along its exterior sidewall.  
         [0033]     However, in the illustrated embodiment, an external central unit is coupled to the probe  10  through the main data cable  26  in order to read and display the load and pressure values.  FIG. 3  is a top plan view of an exemplary external central unit, generally indicated at  30 . On its exterior, the central unit  30  has controls  32 , including a reset button  33  and a power switch  35 , and an external display  34 . The type of external display  34  may vary from embodiment to embodiment. For example, the display  34  may be an LED display or an LCD display. Additionally, although configured as a stand-alone unit in the illustrated embodiment, the central unit  30  may be configured to interface and communicate with other medical devices and monitoring tools in other embodiments.  
         [0034]      FIG. 4  is a schematic illustration of a system for measuring venous pressure using the probe  10  and central unit  30 .  FIG. 4  also illustrates the internal components of the central unit  30 . As shown, signals from the load cell  20  pass through signal conditioning elements, including an amplifier  36  and a filter  38  which are connected to a processor  40 . In other embodiments, other types of signal conditioning elements may be included and interposed between the load cell  20  and the processor  40 . Moreover, depending on the type of processor  40 , an analog-to-digital converter (ADC) may be used to convert analog voltage signals from the load cell into digital data that can be processed by the processor  40 . However, the processor  40  may include an internal ADC.  
         [0035]     The processor  40  may be a microcontroller, an ASIC, or any other element capable of performing the described functions. In some embodiments, the central unit  30  may be implemented as a software program on a general purpose computer, in which case the processor  40  may be the CPU of the general purpose computer.  
         [0036]     As one example, the amplifier  36  may be an INA128P instrumentation amplifier with a gain of 1,000. The filter  38  may be a low-pass filter based on a LM741 operational amplifier with a cut-off frequency of 0.5 Hz, such that only direct current (DC) signals from the load cell  20  are permitted to pass. The processor may be an 8-bit PIC16F877 microcontroller mounted on an internal circuit board. As shown in  FIG. 4 , a clock  42 , in this case, a 10 MHz crystal oscillator, is coupled to the processor  40 , although some processors  40  may include internal clocks, and thus, the clock  42  may be omitted in some embodiments.  
         [0037]     The processor  40  may have sufficient onboard storage memory, for example, flash memory, to permit the storage of one or more load readings and/or final pressure readings. However, as shown in  FIG. 4 , external storage  44  may be provided. The storage  44  may comprise any combination of random access memory (RAM) read-only memory (ROM), programmable read-only memory, and flash memory. Additionally, the storage  44  may include devices that read and write magnetic or optical media, such as hard disk drives, floppy disk drives, CD-ROM drives, CD-R drives, and DVD/DVD-R drives.  
         [0038]     The central unit  30  also includes a power supply  46 . The power supply  46  may comprise a number of components to allow it to draw power from a number of different sources, including a transformer and AC-to-DC converter to draw power from standard household and industrial power grids, a battery, a rechargeable battery, such as a lithium ion battery, or any combination of those components.  
         [0039]     Additionally, as was noted briefly above, the central unit  30  may include one or more input/output ports and their associated hardware in order to communicate with other medical devices, offload venous pressure readings, or otherwise cooperate with other devices. Examples of suitable input/output ports include Universal Serial Bus (USB) ports, IEEE 1394 Firewire ports, RS232-C serial ports, parallel ports, and infrared communication ports. In addition to “wired” input/output ports, some embodiments of the invention may also be equipped for wireless communication, such as by the 802.11a/b/g and Bluetooth wireless networking standards, or by wireless standards and hardware specific to medical devices.  
         [0040]     In order to measure a venous pressure value, the probe  10  and its central unit are used in combination with a technology that allows the user to determine when the vein in question has begun to collapse and when it has substantially completely closed. A number of different technologies, particularly medical imaging technologies, may be used. For example, ultrasound systems are suitable, as are Doppler imaging systems. However, other technologies may be used, such as auscultation for characteristic noises indicating vein closure and other auditory sensing techniques.  
         [0041]     Certain aspects of the following description may assume the use of ultrasound, which is presently one of the most commonly available types of medical imaging technologies suitable for the purpose. However, the particular type of technology or technique used to determine when the vein has begun to collapse and when it has substantially completely collapsed is not critical to the invention, so long as the technology or technique is appropriately calibrated and verified to function correctly.  
         [0042]     As shown in  FIG. 4 , in order to measure venous pressure, the probe  10  is placed in contact with the skin  100  over the vein  102  in which venous pressure is to be measured. If the venous pressure to be measured is a CVP, then the vein  102  would generally be the IJ vein.  
         [0043]     Placed proximate to the probe  10  is the probe  104  of an imaging device  106 . The placement of the probe  104  of the imaging device  106  relative to that of the probe  10  may vary from embodiment to embodiment and from one application or patient to another. In some embodiments, the probe  104  of the imaging device  106  may be placed closer to the patient&#39;s heart than the probe  10 , because it may be easier to visualize the collapsing vein from that vantage point. However, in other embodiments, the probe  10  may be placed closer to the heart than the probe  104  of the imaging device  106 . Other factors may also come into play to determine the placement of the probes  10 ,  104  relative to one another. For example, it may be necessary or desirable to choose the placement of one relative to another so that there is a suitable muscular backing onto which to push the vein so that it will collapse under the exerted force. Ultimately, it is generally advantageous if the two probes  10 ,  104  are placed close to one another, for example, within about one inch of one another.  
         [0044]     Using the display  108  of the imaging device  106 , the user is able to visualize the changes in the vein  102  as pressure is exerted by the probe  10 . When the vein  102  begins to collapse, the user actuates the switching element  28  on the probe  10  to store that force value; when the display  108  of the imaging device  106  indicates that the vein  102  has substantially completely closed, the user actuates the switching element  28  again to store that final force value and calculate the venous pressure. Generally speaking, the difference between the initial and final force values is taken to be the venous pressure, although, as will be described below, that value may be transformed or modified to account for calibration or other issues. In the view of  FIG. 4 , the vein  102  is slightly compressed where the probe  10  contacts it and has thus begun to collapse.  
         [0045]     More specifically, this process is described in the flow diagram of  FIG. 5 , which illustrates a method  200  for determining a venous pressure. Method  200  begins at task  202  and continues with task  204 . In task  204 , the system is initialized. Initialization may include a number of steps. For example, an initial reading may be taken from the load cell  20  and that reading may be used to zero the load cell  20 . Additionally, if a calibration curve for the load cell or other calibration data is available, that data may be retrieved during the initialization. In some embodiments, task  204  may also involve initializing components internal to the central unit  30  or the processor  40 , such as the analog-to-digital converter,  
         [0046]     Method  200  continues with task  206 , in which the user places the probe  10  over the vein  102 , as illustrated in  FIG. 4 . Once the user has placed the probe  10 , the central unit  30  essentially executes a loop until the switching element  28  is actuated. Specifically, in task  208 , a load data point is gathered from the load cell  20 . Method  200  then continues with task  210 , in which it is determined whether the switching element  28  has been actuated. If the switching element  28  has been actuated (task  210 :YES), indicating initial vein collapse, then method  200  continues with task  212  and the data point gathered in task  208  is stored as the force value at initial vein collapse. If the switching element  28  has not been actuated, then method  200  returns to task  208  and another data point is gathered.  
         [0047]     Once the initial force value, indicating the beginning of vein collapse, is stored in task  212 , another data point is gathered in task  214 . After a data point is gathered in task  214 , method  200  continues with task  216 , another decision task in which it is determined whether the switching element  28  has been actuated to indicate that the vein has substantially completely closed. If the switching element has been actuated (task  216 :YES), method  218  continues with task  218  and data point gathered in task  216  is stored as the final pressure at vein closure. If the switching element has not been actuated (task  216 :NO) method  200  returns to task  214 .  
         [0048]     After task  218 , method  200  continues with task  220 , in which the venous pressure is calculated. In some embodiments, the venous pressure may be calculated as the simple difference between the force applied to cause final vein closure and the force applied to cause initial vein collapse. For purposes of description, the venous pressure established by taking the simple difference between the final and initial applied forces will be referred to as the simple difference pressure.  
         [0049]     As those of skill in the art will realize, it may be necessary to transform the simple difference pressure using linear or nonlinear functions to account for a number of conditions or factors so as to arrive at a precise, accurate final venous pressure reading. For example, it may be advantageous to calibrate the load cell  20  by measuring its response to a series of known weights or pressures and then transforming the simple difference pressure using the calibration data. A number of techniques for calibrating load cells are known in the art and any may be used.  
         [0050]     Additionally, as those of skill in the art will realize, venous pressures measured with this technique may vary with the characteristics of the individual patients, including the patient&#39;s age, gender, and other characteristics. Therefore, it may be useful to calibrate the measurement technique itself to establish calibration data. For example, a probe  10 , and the technique for using it, could be calibrated by performing method  200  on a patient, simultaneously performing a typical venous catheterization to measure venous pressure internally, and comparing the data obtained by the two results.  
         [0051]     If the simple difference pressure is to be transformed, then task  204 , in which the system is initialized, could also comprise retrieving the appropriate transformation factors or functions, or, in some embodiments, allowing the user to select which of a plurality of transformation factors should be used. This could be done, for example, by allowing the user to specify the age, gender, and other characteristics of the patient.  
         [0052]     Although not shown in  FIG. 5 , once it is gathered, a pressure measurement may optionally be stored in the storage  44  so that it can be reviewed at a later point.  
         [0053]     In pseudocode, not specific to any particular computer or machine programming language, the tasks of method  200  may be rendered as:  
                                                   main( )            // start-up waiting period - flash 7-Seg Display while resetting            for (i = 0; i &lt; 2; i++)             display(ZERO)             delay             display(OFF)             delay            end for            // setup A/D converter            setup_adc( );            // calculate initial tension on load cell            init_calib_value = read_adc( );            // set initial tension to part of initial skin compliance            init_skin_value = init_calib_value;            // program loops continuously until reset            while (TRUE)             if not final value then              // keep reading data              ADC_result = read_adc( ) − init_skin_value             end if             // convert ADC value (0-255) to CVP value (0-20)             CVP = convert(ADC_result)             if valid CVP then              display(CVP)             else              display(ZERO)             end if             if button pushed then              if button pushed for first time then               // zero out pressure reading to account for initial tension               init_skin_value = init_skin_value + ADC_result               set indicator LED              else if button pushed for second time then               // record final value and stop further reading               set indicator LED              end if             end if            end while           end main( )                      
 
         [0054]     The above pseudocode assumes that the display  34  is a seven-segment LED display. Additionally, the abbreviation “ADC” refers to the analog-to-digital converter of the processor  40 .  
         [0055]     Although the invention has been described with respect to certain exemplary embodiments, the examples are intended to be illuminating, rather than limiting. Modifications and changes may be made within the scope of the invention, which is determined by the claims.