Abstract:
An physiological data acquisition apparatus includes three or more leads, at least one AC current source, a switching mechanism structured to selectively couple the current source to selected lead pairs to inject an AC current across the selected lead pairs which produces an AC voltage across the selected lead pair, and a processing device. The processing device is structured to (i) determine an impedance across the current selected lead pair based on the AC voltage, (ii) determine whether the impedance is less than a predetermined threshold, (iii) if the impedance is less than the predetermined impedance threshold cause the current selected lead pair to be used for generating physiological parameter data, and (iv) if the impedance is not less than the threshold cause the switching mechanism to couple the at least one AC current source to a new selected pair of the leads.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to physiological data acquisition systems, such as polysomnography systems, and in particular to a method and apparatus for selecting differential input leads for such a system. 
         [0003]    2. Description of the Related Art 
         [0004]    Polysomnography, also known as a sleep study, is a multi-parametric test that is used for the purpose of diagnosing sleep disorders in people. During polysomnography, physiological data is acquired from a patient while he or she sleeps for subsequent analysis by a trained clinician. In a typical polysomnograph, the monitored parameters include such things as electrical encephalographic activity (via an electroencephalogram (EEG)); eye movements (via an electrooculogram (EOG)), muscle activity (via an electromyogram (EMG), heart rhythm (via an electrocardiogram (ECG)), respiratory effort, nasal and/or oral airflow, blood oxygen saturation (SpO 2 ), body position, exhalation CO 2 , esophageal pH, and breathing sounds (for snoring). These parameters are typically each monitored during sleep by sensors that produce analog signals which are then transmitted to an acquisition device where the data is processed and stored for analysis by a trained clinician. 
         [0005]    For a number of the parameters that are monitored during a polysomnograph, the sensors that collect the data are electrical leads that are attached to the patient&#39;s body. For example, a polysomnograph often includes collection of EMG data relating to leg movements by attaching leads to the legs of the patient and facial muscle movement and tension by attaching leads to the patient&#39;s chin. EEG, EOG, and ECG are also monitored using electrical leads attached to the patient&#39;s body. 
         [0006]    A recurring problem in polysomnography is the detachment of such leads from the patient during the study as a result of, for example, patient movement during sleep. When a lead becomes detached, a signal is lost from a key sleep parameter or diagnostic indicator, which adversely affects the quality of testing. In addition, detached lead(s), if discovered, require intervention by someone during the study to reattach the lead(s) to the patient. Should the clinician choose to try to reattach the lead(s), they will need to enter the patient&#39;s room, turn on the lights, and awaken the patient. This causes a disruption in the patient&#39;s sleep and a disruption in the study, since a minimum number of hours of sleep must be recorded. And, again, the clinician will need to repeat the process of traveling back and forth between the patient room and the central control room to assure the impedance is at an acceptable level upon reapplication of the leads. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, it is an object of the present invention to provided a method and apparatus for selecting differential input leads for a physiological data acquisition system which addresses the problem of lead detachment by ensuring that an optimal pair of leads is used for data acquisition. 
         [0008]    In one embodiment, an apparatus for acquiring physiological data from a patient is provided that includes three or more leads structured to be placed on a body of the patient, each of the leads being adapted to collect a signal relating to a particular physiological parameter from the patient, at least one AC current source, a switching mechanism structured to selectively couple the at least one AC current source to selected pairs of the leads such that at any one time the at least one AC current source will inject an AC current across only a current selected pair of the leads, wherein in response to the injected AC current, an AC voltage will be generated across the current selected pair of the leads, and a processing device. The processing device is programmed/structured to (i) determine an impedance across the current selected pair of the leads based on the AC voltage, (ii) determine whether the impedance is less than a predetermined impedance threshold, (iii) if the impedance is less than the predetermined impedance threshold cause the current selected pair of the leads to be used for generating data relating to the particular physiological parameter; and (iv) if the impedance is not less than the predetermined impedance threshold cause the switching mechanism to couple the at least one AC current source to a new current selected pair of the leads such that the AC current is injected across the new current selected pair of the leads. 
         [0009]    In another embodiment, a method of acquiring physiological data from a patient using three or more leads placed on a body of the patient is provided, wherein each of the leads is adapted to collect a signal relating to a particular physiological parameter from the patient. The method includes injecting an AC current across a first pair of the leads, wherein in response to the injected AC current, a first AC voltage is generated across the first pair of the leads, determining a first impedance across the first pair of the leads based on the first AC voltage, determining that the first impedance is not less than a predetermined impedance threshold, responsive to determining that the first impedance is not less than the predetermined impedance threshold, injecting the AC current across a second pair of the leads, wherein in response to the injected AC current, a second AC voltage will be generated across the second pair of the leads, determining a second impedance across the second pair of the leads based on the second AC voltage, determining that the second impedance is less than the predetermined impedance threshold, and responsive to determining that the second impedance is less than the predetermined impedance threshold, using the second pair of the leads for generating data relating to the particular physiological parameter. 
         [0010]    These and other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of a polysomnography system according to an exemplary embodiment of the present invention; and 
           [0012]      FIG. 2  is a schematic diagram of headbox  20  according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0013]    As used herein, the singular form of “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. As used herein, the statement that two or more parts or components are “coupled” shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs. As used herein, “directly coupled” means that two elements are directly in contact with each other. As used herein, “fixedly coupled” or “fixed” means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other. 
         [0014]    As used herein, the word “unitary” means a component is created as a single piece or unit. That is, a component that includes pieces that are created separately and then coupled together as a unit is not a “unitary” component or body. As employed herein, the statement that two or more parts or components “engage” one another shall mean that the parts exert a force against one another either directly or through one or more intermediate parts or components. As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
         [0015]    Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, upper, lower, front, back, and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. 
         [0016]      FIG. 1  is a schematic diagram of a polysomnography system  2  according to an exemplary embodiment of the present invention. As described in greater detail below, polysomnography system  2  employs hardware and software to add redundancy to the sleep diagnostic testing in situations (e.g., EMG, EEG, etc.) wherein a differential pair of leads is needed to make physiological measurements. More particularly, three or more leads are utilized to acquire parameter signals and polysomnography system  2  continuously monitors signal integrity in differential pairs of the leads to sense degradation in a lead(s) (indicative of a wire that has become detached). If degradation is sensed, polysomnography system  2  will attempt to find and switch to an optimal pair of leads for the measurement in question. 
         [0017]    Referring to  FIG. 1 , polysomnography system  2  includes a plurality of exemplary sensors  4  that are operatively coupled to a patient  6  that is undergoing a sleep study. In the exemplary embodiment, sensors  4  include a pair of EOG leads  8  positioned near the eyes of patient  6  for measuring eye movement, a pressure transducer  10  positioned near the nostrils of patient  6  for measuring nasal and/or oral airflow, three EMG leads  12  ( 12   a,    12   b,    12   c ) positioned near the chin of patient  6  for measuring facial muscle actively and tension, a pair of ECG leads  14  positioned on opposite sides of the chest of patient  6  for measuring heart related parameters, an SpO 2  sensor  16  positioned on the finger (or, alternatively, the ear) of patient  6  for measuring blood oxygen saturation, and three EMG leads  18  ( 18   a,    18   b,    18   c ) positioned near the leg of patient  6  for measuring leg movements. It should be understood that the particular sensors  4  shown in  FIG. 1  are exemplary, and that other sensors in addition to and/or in place of the sensors  4  may also be used in connection with the present invention. 
         [0018]    As seen in  FIG. 1 , polysomnography system  2  further includes a headbox  20  and a base station  22 . Each of the sensors  4  is operatively coupled to headbox  20 . Headbox  20  is an electronic processing device that receives the analog parameter signal from each of the sensors  4 , amplifies and filters each signal and converts each signal to digital form. The digital parameter signals are then output by headbox  20  to base station  22 , which is also an electronic processing device. Base station  22  then packetizes the data and further processes and stores the received digital parameter data. In addition, in the exemplary embodiment, base station  22  includes an Ethernet output that enables base station  22  to be connected to a LAN  24 . LAN  24  carries the digital parameter data to a central station or tech room where a sleep technician and/or other staff can view and analyze the data on a PC  26  using proprietary host software associated with polysomnography system  2 . 
         [0019]      FIG. 2  is a schematic diagram of headbox  20  according to an exemplary embodiment of the present invention. For purposes of illustrating and describing the present invention,  FIG. 2  only shows EMG leads  12   a,    12   b,    12   c  being operatively coupled to (i.e.,. input into) headbox  20 . It will be appreciated, however, that as shown in  FIG. 1 , the other sensors  4  are also operatively coupled to (i.e.,. input into) headbox  20 . 
         [0020]    Headbox  20  includes conductors  30  and  32  to which EMG lead  12   a  is coupled, conductors  34  and  36  to which EMG lead  12   b  is coupled, and conductors  38  and  40  to which EMG lead  12   c  is coupled. Headbox  20  also includes programmable analog switch  42  which can be selectively coupled to any one of conductors  30 ,  34  and  38 , and programmable analog switch  44  which can be selectively coupled to any one of conductors  32 ,  36  and  40 . Programmable analog switches  42  and  44  are controlled by a microprocessor or DSP  46  (or another suitable processing device) provided as part of headbox  20  (as shown by the dotted lines in  FIG. 2 ). In addition, headbox  20  includes an instrumentation amplifier  48  or some other suitable differential amplifier device. As seen in  FIG. 2 , programmable analog switch  42  is also electrically coupled to the non-inverting (+) input of instrumentation amplifier  48  through a conductor  50 , and programmable analog switch  44  is also electrically coupled to the inverting (−) input of instrumentation amplifier  48  through a conductor  52 . Headbox  20  further includes a first AC current source  54  that is coupled to conductor  50  and a second AC current source  56  that is coupled to conductor  52 . First AC current source  54  and second AC current source  56  are structured to output AC current that are  180  degrees out of phase with one another. In the exemplary, non-limiting embodiment, first AC current source  54  and second AC current source  56  is each structured to provide a low level (e.g., 2 nA peak) current set at a particular frequency (e.g., a 100 Hz or a 250 Hz square wave). This current level is well below the safety margins required by IEC60601-1 and is small enough so as to not adversely impact the physiological data carried by leads  12   a,    12   b,  and  12   c.  The output of instrumentation amplifier  48  is provided to an analog-to-digital converter (ADC)  58 . The output of ADC  58  is provided to microprocessor or DSP  46 . 
         [0021]    As described below, headbox  20  is adapted to receive input from the three EMG leads  12   a,    12   b,  and  12   c  and automatically find the first pair of the leads  12   a,    12   b , and  12   c  wherein the impedance between the leads is below a preset impedance threshold. An impedance between the pair of leads in question below the preset impedance threshold indicates that neither of the leads of the pair is detached. That pair of leads may then be used to make the EMG measurement that is needed for the polysomnography study. 
         [0022]    In operation, an initial, default pair of leads  12   a,    12   b,    12   c  is selected by coupling programmable analog switch  42  to a particular one of the leads  12   a,    12   b,    12   c  (through the appropriate one of conductors  30 ,  34  and  38 ) and coupling programmable analog switch  44  to another particular one of the leads  12   a,    12   b,    12   c  (through the appropriate one of conductors  32 ,  36  and  40 ). In the exemplary embodiment shown in  FIG. 2 , that initial, default pair of leads is lead  12   a  and lead  12   b.  Current is then injected across the selected pair of leads  12   a,    12   b  by first AC current source  54  and second AC current source  56 . As stated above, in the exemplary embodiment, the injected AC current is a low level AC current (e.g., a 2 nA peak current set at a 100 Hz or a 250 Hz square wave). In response to the injected current, an AC voltage will be generated across leads  12   a,    12   b  that is proportional to the impedance between the leads  12   a,    12   b.  That voltage difference is input into and differentially measured by instrumentation amplifier  48 . More specifically, as will be appreciated by those of skill in the art, instrumentation amplifier  48  will output an AC voltage that is equal to the difference in the voltage at its two inputs (+ and −) multiplied by a gain factor. Thus, the output of instrumentation amplifier  48  will be an AC voltage that is proportional to the impedance between the leads  12   a,    12   b  because it is equal to the AC voltage across leads  12   a,    12   b  multiplied by the gain factor of instrumentation amplifier  48 . 
         [0023]    Next, the AC voltage output by instrumentation amplifier  48  is passed to ADC  58  where it is converted to digital form. The digital version of the AC voltage output by instrumentation amplifier  48  is then provided to microprocessor or DSP  46 . Inside microprocessor or DSP  46 , the digital AC voltage is first narrow band pass filtered (digitally). The narrow band pass filtering extracts the portion/component of the voltage signal that corresponds to and represents the voltage generated in response to the injected AC current and thus that corresponds to and represents the impedance between the selected leads  12   a,    12   b.  The narrow band pass filtering does not pass the portion/component of the voltage signal that corresponds to physiological parameter measures by leads  12   a,    12   b  (EMG in the exemplary embodiment). The narrow band pass filtered signal is then fully rectified (digitally) inside microprocessor or DSP  46 . The peak voltage of the rectified signal is measured and converted to an impedance value that represents the impedance between the leads  12   a,    12   b  using a standard linear mathematical translation. The translation can be stated as Z=mV+b, where Z is the translated impedance value, V is the measured voltage level, and m and b are the slope and intercept of the linear translation. The values of m and b are a function of the circuit used to create the injected AC current, and are determined in practice by a calibration process which measures the observed voltage level, V, for specific known impedance values Z. 
         [0024]    The resulting impedance value is then compared to the preset impedance threshold. In the exemplary embodiment, the preset impedance threshold is 5000 ohms, although other values may also be appropriate depending on the particulars of the application. If the resulting impedance value is less than the preset impedance threshold, then leads  12   a,    12   b  are deemed to be in satisfactory condition and polysomnography system  2  will use leads  12   a,    12   b  as good leads. This means headbox  20  will extract the EMG signal from the leads  12   a,    12   b,  using a digital notch filtering process, and will pass that digital data on to base station  22  for further processing as discussed elsewhere herein. If, however, the resulting impedance value is not less than the preset impedance threshold, then leads  12   a,    12   b  are deemed to not be a good pair. In response, microprocessor or DSP  46  will select a different pair of the leads  12   a,    12   b,    12   c  (e.g.,  12   a  and  12   c ) by controlling programmable analog switches  42 ,  44  to couple to the selected leads and the verification process just described will be repeated to determine whether that pair of leads is good. This process will continuously cycle through the three possible lead pair combinations ( 12   a  and  12   b,    12   a  and  12   c,    12   b  and  12   c ) until a satisfactory pair is found or until the study is concluded. 
         [0025]    In an alternative exemplary embodiment, when the digital AC voltage signal is received in microprocessor or DSP  46  from ADC  58 , a Discrete Fourier Transform (DFT) or a Fast Fourier Transform (FFT) is performed on the signal. The power level of the DFT or FFT output is then measured at the frequency that corresponds to the frequency of the injected AC current (e.g., 100 Hz or 250 Hz). That power level is then converted to an impedance value that represents the impedance between the leads  12   a,    12   b  using a standard linear mathematical translation similar to the translation used for the main exemplary embodiment. The slope, m, and the intercept, b, of the alternative exemplary embodiment translation function would likewise be determined by a calibration process which measures the observed power level, V, for specific known impedance values Z. The resulting impedance value is then compared to the preset impedance threshold and processing and operation proceeds as described in connection with the main exemplary embodiment. 
         [0026]    It should be appreciated that the present invention as just described in connection with above exemplary embodiments is not limited to using just three leads. Rather, more than three leads examined in pairs as just described may also be used to add further redundancy to polysomnography system  2 . 
         [0027]    Furthermore, while the present invention has been described in connection with EMG leads  12  shown in  FIG. 1 , it should be appreciated that it may also be used with leads that measure other parameters or make measurements at other locations. For example, as noted elsewhere herein, three EMG leads  18  are positioned near the leg of patient  6  for measuring leg movements. Those three leads  18  may be coupled to a circuit configuration within headbox  20  that is identical to that shown in  FIG. 2  so that headbox  20  can find a satisfactory pair of the leads  18  by automatically finding the first pair of the leads  18  wherein the impedance between the leads is below a preset impedance threshold as described above. Also, the present invention may be applied to leads other than EMG leads. For example, an additional one or more EOG leads  8  or ECG leads  14  may be provided (resulting in three or more of such leads) so that the present invention may be employed in connection with EOG and/or ECG measurements. 
         [0028]    Moreover, the present invention is not limited to in connection with polysomnography, but may also be used with other physiological data acquisition systems and applications. For example, and without limitation, the present invention may be employed in dedicated EEG systems and/or studies or dedicated EMG systems and/or studies. 
         [0029]    In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” or “including” does not exclude the presence of elements or steps other than those listed in a claim. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In any device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain elements are recited in mutually different dependent claims does not indicate that these elements cannot be used in combination. 
         [0030]    Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.