Abstract:
In a system and method for peripheral impedance plethysmography, an electrode for application to the patient&#39;s limb includes two outer current electrodes and two inner voltage electrodes. A distance between the two inner electrodes is automatically input into an analyzing device, either as a pre-stored value or as determined automatically from the electrode. Peripheral blood flow is calculated in accordance with that distance.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to an electrode, system and method for peripheral impedance plethysmography and more particularly to such an electrode, system and method in which the distance between the inner voltage electrodes is detected in one of several manners and need not be input manually. 
       DESCRIPTION OF RELATED ART 
       [0002]    The measurement of peripheral blood flow is important in medicine, since there are many specific diseases in which peripheral blood flow is impaired, e.g., diabetes and atherosclerosis. Also, the peripheral blood flow is altered as the total cardiac output is increased or decreased. Cardiac output is particularly important in patients who are under anesthesia, are in the post-operative state, or are critically ill or unstable. As blood flow from the heart falls, the peripheral blood flow is dramatically reduced to preserve flow to the brain and vital organs. 
         [0003]    Blood flow to an extremity can be measured painstakingly and invasively by dissecting out the main blood vessels to the limb (e.g., brachial artery in an arm) and encircling it with an electromagnetic flow probe. That is clearly not a technique suitable for clinical use. It is therefore desired to measure peripheral blood flow non-invasively. 
         [0004]    Peripheral impedance (or conductance) plethysmography is a technique for non-invasively measuring peripheral blood flow by measuring peripheral pulse volume, which is the small change in the volume of a limb segment occurring within the cardiac cycle. The technique works by obtaining a raw pulse volume analog signal and applying a selective signal averaging algorithm to the raw pulse volume signal. The technique is described in U.S. Pat. No. 4,548,211 to Marks. 
         [0005]    In the technique as currently practiced, the raw pulse volume analog signal is obtained by measuring the electrical impedance (or conductance) of a limb segment with an electrode such as that of  FIG. 1 . The electrode  102  is made of a flexible material  104 , so that it can be wrapped around the limb. The flexible material is configured to define a connecting portion or vertical member  106 , which is insulated from direct electrical contact with the patient, and two extending members  108  for being wrapped around or otherwise applied to the extremity. Each of the two extending members  108  contains an outer current electrode  110  paired with an inner voltage electrode  112 . An electrical connector  114  allows the outer current electrodes  110  and the inner voltage electrodes  112  to be connected to a source of current and a voltage measuring device, respectively. 
         [0006]    An alternating current on the order of 1 ma amplitude and 40 kHz frequency is applied to the two outer current electrodes, while the inner voltage electrodes are used to measure the voltage resulting from the applied current. The ratio of the amplitude of the voltage waveform to the amplitude of the current waveform is the limb impedance, Z. Measurements of Z over time provide the baseline impedance Z 0  of the limb segment and the pulsatile change ΔZ of the impedance. Once the resistivity ρ of the blood and the distance L between the two inner voltage electrodes are known, the change in volume ΔV can be calculated as: 
         [0000]      Δ V=ρL   2   ΔZ/Z   0 . 
         [0000]    The resistivity ρ is either calculated or approximated from the patient&#39;s hematocrit. The distance L must be measured with a measuring device, such as a ruler, and then the value of L must be manually input into the device which performs the calculations. That step is cumbersome and time-consuming. It is particularly a problem in one of the most important applications of peripheral impedance plethysmography, i.e., the management of trauma victims in whom hemorrhage has produced peripheral vasoconstriction. In that setting, it is desirable to apply the electrode to the patient and to obtain measurements in as few steps as possible. 
         [0007]    The problem of measuring the spacing between electrodes in peripheral impedance plethysmography has been considered in U.S. Pat. No. 3,957,037 to Fletcher et al. That patent teaches a pair of readout ring electrodes for impedance plethysmography. The ring electrodes are held at a fixed distance from each other by a pair of rods. The rods have measurement indicia on them to permit visual inspection of the axial distance between them. However, Fletcher et al. do not teach or suggest automatic determination and input of that axial distance and thus do not offer a complete solution. Other previous patents, such as U.S. Pat. No. 4,166,455 to Findl et al, teach devices in which the electrodes are relatively movable; however, those previous devices suffer from the same deficiency previously noted for the Fletcher et al. device. 
       SUMMARY OF THE INVENTION 
       [0008]    It will be readily apparent from the above that a need exists in the art to obtain the measurements automatically and in as few steps as possible. It is therefore an object of the invention to eliminate the need to measure L and to input the value manually. 
         [0009]    It is another object of the invention to input L automatically, either by measuring it automatically or by using a pre-stored value. 
         [0010]    To achieve the foregoing and other objects, the present invention is directed to an electrode, system and method for peripheral impedance plethysmography in which L is automatically determined, so that it is available for calculations without having to be input manually into the device. In various embodiments, L can have a single predetermined value, be determined from the electrode, or be calculated directly from the signals received from the electrode. 
         [0011]    For limb impedance measurements, circumferential electrodes are preferable. Spot electrodes can be used to make impedance measurements, albeit less accurately. Of course, the shape of the electrode can be varied for any contemplated use. 
         [0012]    Perhaps the simplest embodiment is a quadripolar, circumferential, electrode system, in which the electrodes are positioned at a known, fixed distance from one another. In this electrode configuration, the inner and outer electrodes are paired by attaching them to a common insulating base. The distance between the circumferential pairs is fixed. Such an electrode can be applied to a limb by attaching the vertical component to the limb, preferably the anterior aspect of the calf (or shin) with an adhesive exposed by a peel-off strip and then sequentially applying the proximal and distal circumferential electrodes around the limb, also with adhesive exposed by a peel-off strip. The distance between the inner electrodes is known and can therefore be preprogrammed as a default value into the device. 
         [0013]    A modification of the first embodiment is to provide a number of different sizes of electrodes in which L varies, thus providing sizes for individuals with different sized limbs and, in addition, to provide a means to communicate the size of the electrode back to the device. This can be done, for example, by having the length L coded into the electrical connector that connects the electrode to the plethysmograph. Alternatively, this can be accomplished by having an additional pair of wires connecting the device and the electrode. A resistor of a particular value is incorporated into the electrode and attached to the additional leads. The device then reads the value of the resistor and uses a look-up table to determine which size electrode is being used. 
         [0014]    A second, more complex, but more versatile embodiment allows the distal and proximal pairs of electrodes to be spaced at varying distances from one another, but with a means built into the electrode to measure L and to convey the measured value of L back to the device. The electrodes can, for example, be mounted on a rod and slidably positioned closer or farther from one another. A distance transducer, mechanically coupled to the electrodes, such as a rheostat, conveys the value of L back to the device so that it may be appropriately included in the calculations. 
         [0015]    A third embodiment incorporates features from both the first and second embodiments. The vertical member is folded to reduce the distance between the inner electrodes to a minimum value. By unfolding the vertical member, the distance between the inner electrodes is increased to a greater value, thus providing more than one electrode spacing to accommodate different size limbs. A means is provided to indicate to the device, which of the lengths is active. One means to accomplish this is to provide an electrical connection that has continuity only when the vertical member is folded and is broken when the vertical member is unfolded and expanded to increase L. It is a straightforward technique to communicate this information to the device with an additional connection indicating the electrical continuity across the vertical member. Many other means can be employed to accomplish this task (e.g., capacitive coupling present only in the folded state). 
         [0016]    A fourth embodiment uses an electronic means to actually calculate L. This allows the paired connectors to be positioned at arbitrary, varying distances from one another. For example, assume that the distance between the outer and inner electrodes in a given pair is d and the distance between the inner electrodes is L. If a current with amplitude I is applied to the outer electrodes, there will be voltage with amplitude V i  detected between the inner electrodes and a voltage with amplitude V o  between the outer electrodes. The ratio V o /V i  will be proportional to the distance between the outer electrodes divided by the distance between the inner electrodes or (L+2d)/L. Therefore, L may be calculated as L=2dV i /(V o −V i ). 
         [0017]    If it is desired to avoid using the exciting, outer current electrodes also as voltage electrodes, a modification of the fourth embodiment may be used. In this modification, a third electrode is added to each pair so that there is an outer, exciting, current electrode, a middle, measuring, voltage electrode and an inner “reference” electrode comprising each of the two electrode sets (proximal and distal). As before, the distance between the middle, measuring electrodes, L, is arbitrary. For this explanation, assume that the distance between the middle, measuring voltage electrode and the inner, reference electrode is d. If a current with amplitude I is applied to the outer electrodes, there will be voltage with amplitude V m  detected between the middle electrodes and a voltage with amplitude V r  between the inner, reference electrodes. The ratio V m /V r  is proportional to the distance between the middle, measuring electrodes divided by the distance between the inner, reference electrodes or L/(L−2d). Therefore, L may be calculated as L=2dV m /(V m −V r ). 
         [0018]    Similarly, this method can be employed using a single middle electrode positioned between the paired electrodes. If the distance between this electrode and one of the inner voltage electrodes is d, the voltage between the middle electrode and this inner voltage electrodes is V 1  and the voltage between the middle electrode and the other inner voltage electrode is V 2 , then d/V 1  equals L/(V 1 +V 2 ) or L=d(V 1 +V 2 )/V 1 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Various preferred embodiments of the invention will be set forth in detail with reference to the drawings, in which: 
           [0020]      FIG. 1  shows an electrode for peripheral impedance plethysmography according to the prior art; 
           [0021]      FIG. 2  shows a system for peripheral impedance plethysmography according to a first preferred embodiment of the present invention; 
           [0022]      FIG. 3  shows a system for peripheral impedance plethysmography according to a first variation of the first preferred embodiment of the invention; 
           [0023]      FIG. 4  shows electrical connectors used in the system of  FIG. 3 ; 
           [0024]      FIG. 5  shows a system for peripheral impedance plethysmography according to a second variation of the first preferred embodiment of the invention; 
           [0025]      FIG. 6  shows a system for peripheral impedance plethysmography according to a second preferred embodiment of the invention; 
           [0026]      FIG. 7  shows a system for peripheral impedance plethysmography according to a third preferred embodiment of the invention; 
           [0027]      FIG. 8  shows a portion of an electrode usable with the system of  FIG. 7 ; 
           [0028]      FIG. 9  shows a portion of another electrode usable with the system of  FIG. 7 ; 
           [0029]      FIG. 10  shows a system for peripheral impedance plethysmography according to a fourth preferred embodiment of the invention; 
           [0030]      FIG. 11  shows an electrode for use with a first variation of the fourth preferred embodiment of the invention; 
           [0031]      FIG. 12  shows an electrode for use with a second variation of the fourth preferred embodiment of the invention; and 
           [0032]      FIG. 13  shows an electrode for use with a modification of any of the preferred embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    Various preferred embodiments of the invention will now be set forth in detail with reference to the drawings, in which like reference numerals refer to like elements throughout. In each of the preferred embodiments and the variations thereof, the electrode can be structured like the known electrode of  FIG. 1 , except for the modifications to be disclosed below. 
         [0034]      FIG. 2  shows a block diagram of a first preferred embodiment of the present invention. The system  200  uses an electrode  102  that is essentially similar to the electrode  102  of  FIG. 1 ; in other words, the electrode of the prior art can be used without modification if desired. However, the processing device  220  is modified from those of the prior art in a manner to be explained below. 
         [0035]    The electrode  102  and the processing device  220  are connected by way of electrical connectors  114 ,  222 . In the processing device  220 , a current source  224  applies alternating current to the two outer current electrodes to induce a voltage in the two inner voltage electrodes. In the processing device  220 , a voltage measuring device  226  measures the induced voltage and supplies the measured value to a microprocessor  228  or other suitable processing element. The microprocessor  228  receives the value of L, the distance between the two inner voltage electrodes, from a memory  230 , which can be a ROM, an EEPROM, or other suitable non-volatile memory. The microprocessor  228  uses that value of L to calculate ΔV and outputs the calculated value to any suitable output  232 . 
         [0036]    The first preferred embodiment, as described above, works with electrodes  102  having a single inter-electrode spacing. However, in practice, it is desirable to use electrodes of multiple inter-electrode spacings. Therefore, two variations of the first preferred embodiment having that capability will be described with reference to  FIGS. 3-5 . 
         [0037]    In the first variation of the first preferred embodiment, the system  300  of  FIG. 3  does not use a memory such as that of the system  200  of  FIG. 2 . Instead, the electrode  302  can be any one of multiple electrodes having different values of L. Thus, the person using the system has flexibility in terms of choosing an electrode to accommodate the patient and the extremity in question. In the system  300 , as shown in  FIG. 4 , the electrode  302  can be structured essentially like the electrode of the prior art, except that the electrical connector  314  of the electrode  302  has information on the distance L encoded into it. As one example, the encoding is mechanical, in the form of protrusions  402  which actuate mechanical switches  404  in the electrical connector  322  of the processing device  320 , although any other suitable form of encoding can be used instead. The processing device  320  includes an element  334  that reads the encoding (e.g., by receiving signals from the mechanical switches) and outputs the reading of the encoding to the microprocessor  228 , which thus knows L. The element  334  can include a look-up table or other suitable device for determining L from the switch signals. 
         [0038]    Similarly, in the second variation of the first preferred embodiment, in the system  500  of  FIG. 5 , the electrode  502  can be structured essentially like the electrode of the prior art except that the electrode  502  includes a resistor  536  whose resistance is chosen to represent L, as well as additional leads  538  for electrical connection of the resistor  536  through the electrical connectors  514 ,  522  to the processing device  520 . In the processing device  520 , an ohmmeter  540  determines the resistance and outputs the value to a look-up table  542  or other suitable device, which determines L and passes the value of L to the microprocessor  228 . 
         [0039]    The first preferred embodiment and its two variations presuppose that any given electrode has a single value of L. However, it is possible to construct electrodes with variable values of L and to construct processing devices that determine the variable value of L, thus providing greater flexibility of use. Preferred embodiments implementing such a feature will now be described. 
         [0040]    In the second preferred embodiment, the electrode includes a component for determining L and outputting its value to the processing device. For example, as shown in  FIG. 6 , the system  600  includes an electrode  602  that is modified from the electrode of the prior art such that the two sets  608  of current and voltage electrodes are connected through a rod  644 , somewhat similarly to what is disclosed in the aforesaid Fletcher et al patent. However, a distance transducer  646 , such as a rheostat, is mechanically coupled to the two sets  608  of electrodes to measure L and to output that value to the processing device  620 , which includes a component  648 , such as an analog-to-digital converter, to receive that value and to transmit it to the microprocessor. 
         [0041]    The third preferred embodiment is similar, but does not require a rod or the complexities of a distance transducer. Instead, in the system  700  of  FIG. 7 , the electrode  702  is modified from the electrode of the prior art to have an electrical characteristic that changes as the connecting portion  706  is unfolded or unrolled. The processing device  720  includes a unit  750  for detecting the electrical characteristic to determine the extent to which the connecting portion  706  has been unfolded or unrolled and thus to determine L. 
         [0042]    Various electrode designs which permit determination of the degree of unfolding or unrolling for use in the third preferred embodiment will now be described. The electrode designs can be based on the electrode of the prior art, except for the modifications to be set forth below. 
         [0043]      FIG. 8  shows a portion of the connecting portion or vertical member  806  of one such electrode. In addition to the leads  810 ,  812  for the current and voltage electrodes, the vertical member  806  includes two exposed conductive pads  852 , each with its own lead  854 , one on either side of a folding line A. The exposed pads  852  are located on the opposite surface of the electrode from the surface that contacts the patient, so that the pads themselves do not contact the patient. When the vertical member  806  is folded along the fold line A, the pads are in direct electrical contact, whereas when the vertical member is unfolded, the contact is broken. Thus, the processing device can determine whether or not the vertical member has been unfolded by determining whether the direct electrical contact is intact or broken. 
         [0044]      FIG. 9  shows a portion of an essentially similar member  906 , except that the pads  852  are not exposed at all. In that case, the contact to be detected is capacitive rather than direct. Of course, if the vertical member of  FIG. 8  or  FIG. 9  has multiple folding lines, multiple pairs of such pads can be provided. 
         [0045]    In a fourth preferred embodiment, L can be calculated directly. For example, as shown in the system  1000  of  FIG. 10 , assume that the distance between the outer and inner electrodes  1010 ,  1012  in a given pair in an electrode  1002  is d and the distance between the inner electrodes is L. If a current with amplitude I is applied to the outer electrodes, a voltage with amplitude V i  will be detected between the inner electrodes and a voltage with amplitude V o  will be detected between the outer electrodes. The ratio V o /V i  will be proportional to the distance between the outer electrodes divided by the distance between the inner electrodes or (L+2d)/L. Therefore, L may be calculated as L=2dV i /(V o −V i ). In the system  1000 , the processing device  1020  includes a component  1056  for determining V o  and V i  and supplying those values to the microprocessor. In the system of  FIG. 10 , the electrode of the prior art could be used without modification. 
         [0046]    If it is desired to avoid using the exciting, outer, current electrodes also as voltage electrodes, a first variation of the fourth preferred embodiment may be used. In this modification, as shown in  FIG. 11 , the electrode  1102  is modified from the electrode of the prior art to add a third electrode  1158  to each pair so that there are an outer, exciting, current electrode  1110 , a middle, measuring, voltage electrode  1112  and an inner reference electrode  1158 . As before, the distance between the middle, measuring electrodes  1112 , L is arbitrary. For this explanation, assume that the distance between the middle, measuring voltage electrode and the inner, reference electrode is d. If a current with amplitude I is applied to the outer electrodes, there will be voltage with amplitude V m  detected between the middle electrodes and a voltage with amplitude V r  between the inner, reference electrodes. The ratio V m /V r  is proportional to the distance between the middle, measuring electrodes divided by the distance between the inner, reference electrodes or L/(L−2d). Therefore, L may be calculated as L=2dV m /(V m −V r ). 
         [0047]    Similarly, as shown in  FIG. 12 , a second variation of the fourth preferred embodiment uses an electrode  1202  modified from the electrode of the prior art to have a single middle electrode  1260  positioned between the paired electrodes  1210 ,  1212 . If the distance between this electrode  1260  and one of the inner voltage electrodes  1212  is d, the voltage between the middle electrode and one inner voltage electrode is V 1  and the voltage between the middle electrode and the other inner voltage electrode is V 2 , then d/V 1  equals L/(V 1 +V 2 ) or L=d(V 1 +V 2 )/V 1 . 
         [0048]      FIG. 13  shows a modification which can be used in the context of any of the preferred embodiments. In the modification of  FIG. 13 , not only is the distance L input, but also, a circumference of the extremity at a location to which one or both of the extending members are applied is determined and input. As shown in  FIG. 13 , one or both of the extending member  1308  include scales  1368  for measuring the circumference. The scales  1368  may be manually read, in which case the operator inputs the values into the processing device. Alternatively, they may be automatically read in a manner like that explained above with reference to  FIGS. 8 and 9 . Either way, the processing device performs the calculations in accordance with the measured circumference, e.g., by using the measured circumference and the distance L to calculate a cylindrical or frustro-conical volume. 
         [0049]    While four preferred embodiments and variations thereon have been described above in detail, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. Therefore, the invention should be construed as limited only by the appended claims.