Abstract:
A system and device for mitigating interference in patient physiological monitoring is provided, particularly in surgical environments. One or more sets of electrodes are placed on a patient&#39;s body and connected to corresponding terminals of an input extender. The terminals of the input extender are connected to a set of signal wires encased by a ferrous shielded cable. The ferrous shielded cable connects to a signal processing unit, which includes a differential amplifier and an active drive topology to drive the shield with a common mode signal. The signal processing unit connects to physiological monitoring equipment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application no. 60/301,918, filed on Jun. 29, 2001, the disclosure of which is hereby expressly incorporated by reference, and priority from the filing date of which is hereby claimed under 35 U.S.C. § 119(e). 

   FIELD OF THE INVENTION 
   In general, the present invention relates to patient monitoring systems, and in particular, to a system and device for reducing electrical interference associated with patient monitoring systems. 
   BACKGROUND OF THE INVENTION 
   In a typical neurological monitoring environment, one or more sets of electrodes are attached to a patient&#39;s body. In turn, the electrodes are electrically connected to one or more pieces of monitoring equipment through signal wires. Depending on the placement of the electrodes, the monitoring equipment can be utilized to monitor various physiological signals to conduct patient tests, such as an electrocardiogram (“ECG”), an electroencephalogram (“EEG”), an electromyogram (“EMG”), and the like. Often, the physiological signals obtained from a patient may be of a low amplitude. For example, a typical physiological signal may have an amplitude as small as 0.2 μV. Accordingly, the low amplitude signals are highly susceptible to interference from environmental electrical/magnetic sources to the extent that the ability for the monitoring system to function properly is affected. 
   One skilled in the relevant art will appreciate that neurophysiological monitoring in surgical environments can present significant amounts of environmental interference. Generally described, a typical surgical environment can generate an electrically hostile environment due to the wide variety of electrical devices present in the surgical environment and their relative proximity to the patient and electrode wires. Additionally, hospital sterility regulations often require that some monitoring devices be outside of the direct surgical environment, thereby requiring a long set of electrode wires. Accordingly, the long length of the wires can increase the susceptibility of the low amplitude signals to interference generated by the various electrical devices along the path of the long length electrode wires. Additionally, long length wires can also be more susceptible to environmental electromagnetic interference. Accordingly, most remote monitoring systems attempt to mitigate interference caused by electrical and magnetic sources to better improve the accuracy of the system. 
   One attempt to mitigate the amount of interference relates to the shielding of some portion of the electrode wires with a braided copper shield. Although copper shielding may reduce some portion of electrical interference, a copper braided shield is generally ineffective against magnetic interference. More specifically, in a surgical environment, copper shielding is generally ineffective in reducing magnetic interference caused by cathode ray tube (“CRT”) displays, drills, cutters, microscope lights, blood warmers, and anesthesia machines. 
   Another attempt to mitigate the amount of environmental interference relates to the use of a differential amplifier in the monitoring system. One skilled in the relevant art will appreciate that a differential amplifier will reject a common interfering signal received from a set of electrode inputs according to a factor known as a common mode rejection ratio (“CMRR”). For example, a high CMRR can result in smaller amplitude interference, which is especially effective for reducing low frequency electrostatic interference. However, because a CMRR is finite, the effectiveness of a differential amplifier may be reduced for lower amplitude signals, such as physiological signals, especially for environments, such as a surgical environment, that experience higher amounts of interference. Additionally, a differential amplifier&#39;s CMRR will generally be reduced with an increase in frequency. Thus, in surgical environments, the use of differential amplifiers alone is generally ineffective in mitigating the effects of electrical interference upon the physiological signals. 
   Another attempt to mitigate environmental interference relates to utilizing a differential amplifier in combination with physically twisting the signal wires together to cancel out the effects of low frequency magnetic interference. One skilled in the relevant art will appreciate that in a set of twisted wires (e.g., electrodes), the induced current in one twist tends to cancel out the same induced current generated in an adjacent twist. Generally described, the effectiveness of the twisting of two wires can be limited to reducing interference in environments in which an essentially uniform magnetic field strength is present. For example, in a surgical environment, magnetic interference may be caused by a near-field source that generates a non-uniform field from twist to twist. Additionally, if the spatial geometry of the twists is not uniform throughout the entire cable, the effectiveness is further reduced. Referring again to a surgical environment, the spatial geometry of the wires is often at issue because of the often great lengths of wire required. Thus, twisting signal wires is not sufficient to adequately mitigate interference associated with physiological monitoring. 
   Yet another attempt to mitigate environmental interference relates to the use of a ferrous metal hose to shield the wires. Although a shield including a ferrous hose can reduce low frequency magnetic interference, the application of a ferrous hose shielded cable can become impractical in several environments. Referring again to a surgical environment, ferrous metal hose shielded cables are generally not suited to be handled and easily manipulated, as they are bulky, semi-rigid, and weighty. Moreover, a ferrous hose shielded cable is not well suited for standard mass production cable manufacturing techniques by requiring the addition of insulation to the outside of the hose. Accordingly, this increases the overall cost of monitoring systems. 
   Thus, there is a need for a system and device capable of mitigating environmental interference in high interference environments. 
   SUMMARY OF THE PRESENT INVENTION 
   A system and device for mitigating interference in patient physiological monitoring is provided. One or more sets of electrodes are placed on a patient&#39;s body and connected to corresponding terminals of a terminal block of an input extender. The terminals of the terminal block are connected to a cable consisting of a set of signal wires encased by a ferrous braided shield. The shielded cable connects to a signal processing unit, which includes a differential amplifier and an active drive topology to drive the shield with a common mode signal. The signal processing unit connects to physiological monitoring equipment. 
   In accordance with an aspect of the present invention, a system for mitigating signal interference associated with a patient monitoring system is provided. The system includes a plurality of patient electrodes operable to obtain one or more patient physiological signals and a cable assembly operable for transmitting patient physiological signals received from the plurality of patient electrodes. The cable assembly includes electrically conductive signal wires corresponding to each of the plurality of patient electrodes. The cable assembly also includes a cavity formed by an outer ferrous shield. The system further includes a signal processing unit connected to the electrically conductive signal wires that includes an active drive topology operable to drive the outer ferrous shield with a common mode signal. 
   In accordance with a further aspect of the present invention, a means for mitigating signal interference associated with a patient monitoring system is provided. The means for mitigating signal interference further includes a means for obtaining a plurality of patient physiological signals, a shielding means for transmitting patient physiological signals and a processing means for actively removing signal interference. 
   In accordance with a further aspect of the present invention, a system for mitigating signal interference associated with a patient monitoring system is provided. The system includes an input extender that includes a terminal block having a plurality of terminals operable to electrically couple with a plurality of patient electrodes. The terminal block further includes a connector operable to electrically couple with a cable assembly having a plurality of conductive signal wires corresponding to the plurality of patient electrodes encased within a ferrous braided shield. 
   In yet still further aspects of the invention, the system for mitigating signal interference associated with a patient monitoring system includes a method for generating the common mode signal. The method includes obtaining patient physiological signals, generating a common mode signal, driving the outer ferrous shield of the input extender with the common mode signal, and processing the patient physiological signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a diagram illustrative of a representative embodiment of a system and device for reducing signal interference in patient monitoring systems in accordance with the present invention; 
       FIG. 2  is a perspective view of an input extender having a ferrous shielded cable, a connector operable to connect to a signal processing unit on a first end, and an input terminal block on a second end in accordance with the present invention; 
       FIG. 3  is cross-sectional view of the ferrous shielded cable of  FIG. 2  taken substantially through SECTION  3 — 3 , showing a set of signal wires housed within a PVC-jacketed braided ferrous shield formed in accordance with the present invention; 
       FIG. 4  is a perspective view of a ferrous shielded cable electrically coupling a terminal block of an input extender to a signal processing unit, illustrating the correlation between one or more input terminals of the terminal block and one or more connectors of the signal processing unit in accordance with the present invention; 
       FIG. 5  is a schematic illustrative of an active drive topology utilized to drive a ferrous shielded cable in accordance with the present invention; and 
       FIG. 6  is a perspective view of an alternate embodiment of an input extender consisting of a connection box and a ferrous shielded cable connected to a signal processing unit suitably arranged for use in electroencephalogram physiological testing in accordance with an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is directed to a system and device for mitigating environmental electrical interference. More specifically, in an embodiment of the present invention, a system and device for mitigating environmental interference associated with neurophysiological monitoring in a surgical environment are provided. One skilled in the art will appreciate that the disclosed embodiments are illustrative in nature and should not be construed as limiting. 
     FIG. 1  is a diagram illustrative of a representative embodiment of a neurophysiological patient monitoring system  10  formed in accordance with the present invention. As illustrated in  FIG. 1 , one or more pairs of electrodes  12  may be connected to a patient  14 . The location and placement of the electrodes  12  will coincide with the particular type of monitoring desired, such as neurological monitoring. For example, the electrodes  12  may be connected to the patient  14  to monitor brain wave activity, electric muscle activity, electric heart activity, and the like. Each of the electrodes  12  is connected to an electrode input terminal  18  (see  FIG. 2 ) of a terminal block  17  of one or more input extenders  16 , which will be explained and described in greater detail below. 
   Each electrode input terminal of the terminal blocks  17  of the input extenders  16  corresponds to a conductive signal wire contained within a ferrous shielded cable  20  operatively connected to the terminal blocks  17 . The length and dimension of the ferrous shielded cable  20  may vary according to the placement of the terminal block  17  relative to the patient  14  and the function of the electrodes  12  corresponding to the input extender  16 . 
   One or more ferrous shielded cables  20  are then connected to one or more signal processing units  22  that may include a differential amplifier and various components for creating an active drive topology. The output from the signal processing unit  22  is then connected to one or more monitoring devices  24 , such as display screens, computer terminals, and the like. In accordance with an actual embodiment of the present invention, the terminal blocks  17  are positioned in close proximity to the location of the patient electrodes  12  point of connection with the patient  14  so as to reduce the length of the patient electrodes  12 . This arrangement, in combination with the ferrous shielded cable  20  and a signal processing unit  22  including a differential amplifier and an active drive topology, promotes the mitigation of environmental interference in monitoring devices, as will be discussed in further detail below. 
     FIG. 2  is a perspective view of an input extender  16  formed in accordance with the present invention. As illustrated in  FIG. 2 , the input extender  16  includes a ferrous shielded cable  20  which includes a connector  26  adaptable to releasably couple to a signal processing unit. One skilled in the relevant art will appreciate that the connector  26  may be constructed of any one of a variety of materials, such as molded plastics, and that the connector  26  may also include one or more attributes to facilitate connection/retention with the signal processing unit  22  (see FIG.  1 ), such as threads, twist locks, and the like. 
   Also connected to the ferrous shielded cable  20  is a terminal block  17 . As illustrated in  FIG. 2 , in an illustrative embodiment of the present invention, the terminal block  17  is generally of a rectangular shape having a substantially flat top and bottom surface. The terminal block  17  also includes one or more side surfaces presenting an array of electrode inputs terminals  18 . In an actual embodiment of the present invention, each electrode input terminal  18  in the array of inputs corresponds to a conductive signal wire  28 , as best seen in  FIG. 3 , running through the ferrous shielded cable  20 . 
   Still referring to  FIG. 2 , in an actual embodiment of the present invention, the bottom surface of the terminal block  17  allows the input extender  16  to rest in a stable manner on a substantially flat surface, such as an operating room table or patient bed. Additionally, the terminal block  17  may also include one or more gripping devices, such as a rubber coating, suction cups, textured surfaces, adhesives, Velcro, etc., to mitigate the amount of movement experienced by the input extender  16 . 
   Although the terminal block  17  is illustrated as having a single array of electrode input terminals  18 , one skilled in the relevant art will appreciate that the terminal block  17  may have multiple input electrode terminals  18  on various surfaces of the terminal block  17 . For example, the terminal block  17  may have electrode inputs terminals  18  on a top surface, any side surface, an angled surface, and/or a bottom surface. Additionally, as will be illustrated below, the dimensions, including shape and number of inputs, of an input extender  16  may be modified to suit a particular type of neurological monitoring, or other type of monitoring. 
   The terminal block  17  of the input extender  16  further includes an isolated grounding electrode input terminal  19 . Referring now to  FIG. 1 , an isolated grounding electrode  19  is affixed to the patient  14 , electrically coupling the isolated grounding system of the patient monitoring system  10  directly to the patient  14  as is well know in the relevant art. 
     FIG. 3  is a cross-sectional view of the ferrous shielded cable  20  of.  FIG. 2  taken substantially through Section  3 — 3  of FIG.  3 . As illustrated in  FIG. 3 , the ferrous shielded cable  20  includes a number of conductive signal wires  28  housed in a central cavity  30  of the cable  20 . In an actual embodiment of the present invention, each conductive signal wire  28  is a  28  gauge tinned copper wire whose inner conductive core is formed from 40 44-gauge strands. The conductive signal wire  28  is surrounded with an outer 0.010-inch thick PVC jacket  32 . As explained above, in an illustrative embodiment of the present invention, the number of wires  28  in the ferrous shielded cable  20  corresponds to the number of electrode input terminals in the terminal block. However, in an alternative embodiment of the present invention, the number of conductive signal wires  28  in the ferrous shielded cable  20  may not match with the number of the electrode input terminals in the terminal block. 
   With continued reference to  FIG. 3 , the conductive signal wires  28  are surrounded by a ferrous metal braided shield  34 . In an actual embodiment of the present invention, the ferrous metal braided shield  34  is formed from 40-gauge nickel/iron ferrous alloy strands. However, one skilled in the relevant art will appreciate that a variety of metals may be utilized to provide the ferrous metal braid  34 . In turn, the ferrous metal braid  34  is surrounded by a protective coating, such as a PVC jacket. By utilizing a ferrous metal braid  34 , the present invention mitigates low frequency environmental interference, while maintaining an effective flexibility and weight in the cable  20 . 
     FIG. 4  is a perspective view of a ferrous shielded cable  20 , electrically coupling a terminal block  17  to a signal processing unit  22 , illustrating the correlation between one or more input terminals  18  of the terminal block  17 , with one or more connectors of the signal processing unit  22 . In an actual embodiment of the present invention, the signal processing unit  22  includes a number of connectors  36  for accepting the connectors  26  of the cables  20  of a multiple number of input extenders  16 . Additionally, the signal processing unit  22  also includes one or more rows of connectors  38 , such as on a top surface  23 , that correspond to the electrode input terminals  18  of one or more input extenders  16  as indicated by the phantom lines in FIG.  4 . The signal processing unit  22  may also have one or more outputs  40  that transmit signals obtained from the input extender(s)  16  to physiological monitors. 
   The signal processing unit  22  and the terminal block  17  of the input extender  16  may be imprinted with indicia operable to identify one or more electrical connections. In the actual embodiment depicted, these indicia include both letters and numbers. The indicia are located adjacent to each electrode input terminal  18  on the terminal block  17  and adjacent to each input connector  38  and each ferrous shielded cable connection  36  on the signal processing unit  22 . Similar indicia are placed adjacent to corresponding components that are in electrical continuity with one another. 
   For example, referring to the top of the signal processing unit  22  illustrated in  FIG. 4 , the input connectors  38  are arranged in a matrix having a first row  52  marked by the letter “A,” a second row  54  marked by the letter “B,” a third row  56  marked by the letter “C,” and a fourth row  58  of connectors  38  marked by the letter “D.” Corresponding letters are located adjacent to the cable connectors  36  to indicate to the user that a cable  20  coupled to a connector  36  marked with a letter “A,” for example, is in electrically continuity with the row  52  of input connectors  38  marked with the letter “A.” On the input extender  16 , each electrode input terminal  18 , except the isolated grounding electrode  19 , is marked with a number from one to eight. The connectors  38  in each row  52 ,  54 ,  56 , and  58  are also marked with a number from one to eight. The numerical indicia next to each electrode input terminal  18  indicate to which connector  38  in the row “A”  52  the electrode input terminal  18  is in correspondence. Marking the terminal blocks  17  and signal processing units  22  as described allows a user to quickly and visually identify which electrodes  12  (see  FIG. 1 ) are in electrical continuity with which connectors  38 . Monitoring devices can then be electrically coupled to specifically selected individual electrodes through coupling a wire (not shown) with the connectors  38  on the signal processing unit in an efficient and accurate manner. 
   In an actual embodiment of the present invention, the signal processing unit  22  includes a differential amplifier and an active drive topology that work in conjunction with one another. One skilled in the relevant art will appreciate that the combination of a shielded cable  20  with a differential amplifier may diminish the common mode rejection of the differential amplifier. More specifically, referring to  FIG. 3 , a capacitance formed between the conductive signal wires  28  and the ferrous metal shield  34  creates a voltage divider between the capacitance and the impedance of the patient at the point of connection of the electrode. Because the impedance of a patient is not uniform, some or all of the electrical signals on the conductive signal wires  28  would experience a different voltage division. Thus, any environmental interference signals common to all the electrical signals on the conductive signal wires  28  would be modified by the voltage division, thereby reducing the effectiveness of the differential amplifier. 
   Referring now to  FIG. 5 , in an illustrative embodiment of the present invention, the signal processing unit  22  utilizes an active drive topology to drive a signal to the ferrous metal shield  34 . As illustrated in the schematic of  FIG. 5 , a signal from each of the conductive signal wires  28  is fed to a buffer amplifier  44  to isolate the signals for transmission to an averaging component  46  located in a first section  60  of the signal processing unit  22 . The use and operation of buffer amplifiers  44  are known to those skilled in the relevant art and will not be explained in greater detail. Examples of buffer amplifier  44  configurations are described in Adel S. Sedra and Kenneth C. Smith,  Microelectronic Circuits , 3 rd  ed., 1991, the disclosure of which is incorporated by reference herein. 
   The averaging component  46  sums and normalizes the inputs to generate an average input signal. The output from the averaging component  46  is fed to a band pass filter  48  to filter out any additional DC and high frequency signals. The output of the filter  48  is then connected to another buffer amplifier  50 , whose output is connected to the ferrous metal shield  34 . Accordingly, by driving the ferrous metal shield with the averaged signal, the present invention reduces the capacitance between the ferrous shield  34  and the conductive wires  28 . As stated before, this capacitance reduces the common mode rejection of the signal processing unit  22 . 
   In an illustrative embodiment of the present invention, the signal wires  28  are also connected to a switching matrix  47  and a plurality of differential amplifiers  42  located in a second section  62  (depicted in dotted lines) of the signal processing unit  22 . The use and operation of differential amplifiers  42  are known to those skilled in the relevant art and will not be explained in greater detail. Examples of differential amplifier  42  configurations are described in Adel S. Sedra and Kenneth C. Smith,  Microelectronic Circuits,  3 rd  ed., 1991, the disclosure of which is incorporated by reference herein. 
   A switching matrix  47  is a well known component which allows the signals received from the signal wires  28  to be selectively directed to a pair of input terminals  70  and  72  associated with each differential amplifier  42  for processing. For example, the switching matrix  47  may selectively direct a signal carried along a first signal wire  28  to a first input terminal  70  and a signal carried along a second signal wire  28  to a second input terminal  72  of one of the differential amplifiers  42 . Configured as such, the differential amplifier  42  would amplify the differential voltage present between the first and second signal wires  28 , each signal wire in electrical communication with a separate patient electrode. 
   The switching matrix  47  is adaptable to selectively route any signal to any differential amplifier  42  input terminal in any manner desired by the user. For instance, the switching matrix  47  may reconfigure the routing of the signals so that the same differential amplifier  42  discussed above will receive the signal carried along the first signal wire  28  upon its first input terminal  70  and the signal carried along a third signal wire  28  upon its second input terminal  72 . Thus, the differential amplifier  42  would now amplify the differential voltage present between the first and third signal wires  28 . As should be apparent to one skilled in the art, the switching matrix is a highly adaptable device that may be configured in a wide range of configurations well beyond the illustrative examples described above. 
   The differential amplifiers  42  of the second section  62  of the signal processing unit  22  help to further mitigate signal interference. More specifically, the differential amplifiers  42  produce an output only in response to a potential difference sensed between the first input terminal  70  and the second input terminal  72  of the differential amplifier  42 . By producing the output from only the difference present between the input terminals  70  and  72 , signal interference from common-mode interference voltages are therefore suppressed as will be appreciated by one skilled in the art. In the illustrated embodiment, the first input terminal  70  and the second input terminal  72  receive the signals conveyed upon the conductive signal wires  28  as routed by the switching matrix  47 . 
   Although the second section  62  of the signal processing unit  22  depicted in  FIG. 5  is shown as housed within the signal processing unit  22 , it will be appreciated by one skilled in the art that the differential amplifiers  42  and switching matrix  47  of the second section  62  may be placed in other locations in the neurophysiological patient monitoring system remote of the signal processing unit  22 . 
   Still referring to  FIG. 5 , the signals transmitted along the conductive signal wires  28  housed within the ferrous shielded cable  20  may also be further processed within the signal processing unit  22 . The signals may be processed by means (not shown) well know in the art, such as by filtering, converting, and amplifying. The signal processing unit processes the signals to mitigate interference effects, modify the format of the signals into a form receivable by the monitoring device if required, and to aid in the transmission of the signals within the cable  21 , as is well known in the art. 
   Referring to  FIG. 1 , it is apparent to one skilled in the relevant art that the input extender  16  and the signal processing unit  22  may be modified in a variety of manners to suit the type of monitoring/testing being performed. For example, the number of electrodes  12 , the number and location of the electrode input terminals  18 , and/or the type of or quantity of connectors or terminals may vary to conform to the requirements of the test conducted or monitoring device utilized. However, the ferrous shielded cable  20 , differential amplifier, and active drive topology may remain the same to mitigate environmental interference. For example,  FIG. 6  shows a perspective view of a ferrous shielded cable  20  electrically coupling an alternate embodiment of an input extender  116  to an alternate embodiment of a signal processing unit  122 . 
   In  FIG. 6 , the actual embodiment shown is suitably designed for accommodating electroencephalogram testing, having a sufficient number of electrode input terminals  118  on the top surface of the terminal block  117  for this purpose as is well known in the art. The signal processing unit  122  is also suitably designed for accommodating electroencephalogram testing, having a sufficient number of input connectors  138  on the top surface of the signal processing unit  122  to allow a monitoring device to selectively and individually receive a signal from each patient electrode coupled to the terminal block  117 . 
   While illustrative embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.