Abstract:
In a method, device and computer program product for extracting an EMG signal out of a raw signal obtained with a number of electrodes, the electrodes being adapted to interact with a patient to obtain signals from the patient&#39;s diaphragm on respective channels associated with the electrodes, a signal-to-noise-ratio is determined for the raw signal in each channel, and a weighting factor is automatically determined dependent on the signal-to-noise ratio. The respective raw signals from the channels are weighted according to the weighting factors, and are summed in order to generate a sum signal that represents the total EMG signal contained in all of the raw signals.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention concerns a method, device and computer program product for the purpose of filtering an EMG signal out of a raw signal, i.e., for extracting an EMG signal from a raw signal that contains the EMG signal, among other signal components. 
   2. Description of the Prior Art 
   Sensing of EMG-signals in a patient&#39;s diaphragm by placing a catheter with a number of electrodes in the esophagus is a known technique, which is described in, among others, U.S. Pat. No. 5,671,752. The EMG-signals received can be used in connection with mechanical ventilation of patients, which among others is described in U.S. Pat. No. 5,820,560 and WO 98/48877. 
   Sensing of EMG-signals from the diaphragm can even take place outside the body with electrodes placed on the patient, such as described in e.g. U.S. Pat. No. 4,248,240. 
   If a catheter with electrodes is guided down in the esophagus the electrodes lie on both sides of the diaphragm and at different distances therefrom. Each electrode&#39;s position relative to the diaphragm is normally not known and furthermore can vary when the patient breathes or moves in another way. 
   Since the EMG-signal from the diaphragm is relatively weak, in particular compared to interferences from EKG, a continuous desire is to in the best way attain the highest quality possible signal handling of the raw signal which the sensors detect. This is evident even in WO 01/03579. In WO 01/03579 it is assumed that the electrodes&#39; location in relation to the center of the diaphragm is known. Then the electrodes are weighted based on location and symmetry, in which the EKG signal is taken into account in a conventional way. 
   Known methods for compensating for the EKG signal include, among others, using a band pass filter which filters out the frequencies where the EKG signal normally appears. It is also known to measure the EKG signal separately and then remove an equivalent signal from the measured EMG signal. 
   None of these methods takes into account the actual disturbance the EKG signal creates in a particular measuring situation in a particular patient. This disturbance also varies with time. 
   The known methods also fail to consider that the electrodes location relative to the diaphragm often varies during a single measurement. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method and a device and a computer program product for measuring EMG signals that take into consideration that the location of the electrodes varies during a measurement. 
   The above object is achieved in accordance with the present invention by a method, device and a computer program product (i.e., a computer-readable medium encoded with computer program data) wherein a raw signal is obtained from a number of electrodes that are adapted for placement in a patient to capture signals from the patient&#39;s diaphragm, each electrode having an associated signal channel, wherein a signal-to-noise ratio is determined for each signal channel and a weighting factor is generated for each signal channel based on the signal-to-noise ratio, and the channels are summed dependent on the weighting factors. 
   By selectively estimating the signal-to-noise ratio for each signal channel and arranging significance factors depending upon the signal-to-noise ratio, the channels with the highest quality can be used more effectively for determining the EMG-signal. 
   Since EKG-signals constitute a primary disturbance source, the determination can in large part be done in relation to these signals. The signal-to-noise ratio can then be determined as the quotient between an estimated EKG-activity and an estimated EMG-activity. The quotient can be determined so that a high estimated EMG-activity is always awarded, e.g. by applying the equation 
             R   2       R   +   S           
where R represents the EMG-activity and S represents the EKG-activity.
 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a device according to the invention connected to a patient who is undergoing respiratory therapy. 
       FIG. 2  shows a first example of electrode coupling for receiving a raw signal. 
       FIG. 3  shows a second example of electrode couplings for receiving a raw signal. 
       FIG. 4  is a flowchart describing the steps that are included in the signal treatment with the method according to the invention. 
       FIG. 5  is a flowchart showing how an ECG-activity is estimated with the method according to the invention. 
       FIG. 6  is a flowchart showing how an EMG-activity is estimated with the method according to the invention. 
       FIG. 7  and  FIG. 8  are flowcharts showing, respectively, how an ECG activity and an EMG activity are estimated according to the method according to an alternative embodiment. 
       FIG. 9  is a flowchart showing how a weighing of signals on different signal channels can be obtained with the method according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The signal and raw signal are defined to encompass n-1 signals from the n electrodes/sensors, and for the n-1 signals there are n-1 channels. 
     FIG. 1  shows a device  2  for filtering and analysis of EMG-signals according to the invention. The device  2  can in a known way be connected to a patient  4  via a catheter  6  with a number of electrodes  8 A,  8 B,  8 C,  8 D in one end (four electrodes are shown, but the number can be larger or smaller). By placing the catheter  6  in the esophagus of the patient  4 , the electrodes  8 A,  8 B,  8 C,  8 D can be placed in different locations in the diaphragm  10  (the size of which has been exaggerated in  FIG. 1  to indicate the relative placement of the electrodes  8 A,  8 B,  8 C,  8 D). In an analysis unit  12  in the device  2 , filtering and analysis of the raw signal from the catheter  6  is done to extract the highest quality EMG-signal possible. 
   In this connection, the raw signal can be received in many different ways.  FIGS. 2 and 3  show two embodiments. From  FIG. 2  it is evident that the electrodes  8 A,  8 B,  8 C,  8 D are coupled together in pairs via three couplers  14 A,  14 B,  14 C and in that way give rise to a three-channel raw signal (with e.g. nine electrodes, eight channels are received). 
   In  FIG. 3  an example is shown where the respective electrodes  8 A,  8 B,  8 C,  8 D are connected to a reference electrode  16  (which e.g. can be grounded) via four couplers  14 D,  14 E,  14 F,  14 G. This results in a four-channel raw signal (for eight channels in this arrangement, eight electrodes and one reference are required). 
   Naturally an arbitrary number of electrodes can be used and for n electrodes, n-1 signals are allowed, and thereby n-1 channels. 
   More information regarding the catheter, the sensors and the entire process to capture raw signals from the diaphragm via esophagus can be found in e.g. U.S. Pat. No. 5,671,752 and WO 01/03579. As already noted, electrodes connected outside the body can even be used instead, for completely non-invasive reception of EMG-signals. 
   The patient  4  can also be connected in conventional ways to a ventilator system  18 , which in turn can be connected to the device  2  via a suitable connection  20 . The respiratory therapy given via the ventilator system  18  in that way can be influenced by the EMG-signal, which is extracted from the raw signal from the diaphragm  10 . This influence can be done in many different ways, of which some are described in U.S. Pat. No. 5,820,560 and WO 99/43374. 
   The present invention is directed to the device  2  and, to be precise, the analysis unit  12 . The analysis unit  12  filters EMG-signals out of the raw signal from the catheter  6 . The analysis unit can be operated by a computer program encoded on a computer-readable medium, such as a CD-ROM  13 . In that connection a number of signal channels are used, as noted above. 
   To receive the highest quality possible in the EMG-signals, the raw signal contains in addition to EMG even EKG, alternating current noise, noise, movement artifacts and other low frequency disturbances, filtering in the analysis unit  12  is done according to the method described below, which can be performed in an analog or digital manner, or as a combination thereof and can be realized in hardware, software or in a combination thereof. 
   The method is described in connection with the flowcharts in  FIGS. 4 ,  5 ,  6 ,  7 ,  8  and  9 . 
     FIG. 4  shows a flowchart which describes the overall signal handling in the analysis unit  12 . The general handling is as follows. 
   The raw signals from the sensors are input by a (multi-channel) input  22  and first pass a first high pass filter  24 . The purpose of this is to filter away movement artifacts and other low-frequency interferences. The breaking frequency should be lower than 10 Hz. 
   In the next step, the signals are filtered in a non-linear low pass filter  26 . The purpose of this is to smooth the signal when it has a high amplitude, which typically happens with the existence of EKG-signals. The breaking frequency should lie within an interval of about 50-700 Hz. Where in the interval the breaking frequency lies depends on the energy or the amplitude of the signal. Higher amplitude results in lower breaking frequency. The non-linear low pass filter  26  should even have a dynamic component, namely that the breaking frequency changes with a time constant. 
   Subsequently, the signals pass a second high pass filter  28 . The purpose of the second high pass filter  28  is to select the frequency interval where the EMG-signal lies. The breaking frequency is therefore chosen to the lower regions of the bandwidth of the EMG-signal which is about 100 Hz. 
   The block  26  and  28  can be replaced with an adaptive band pass filter which is described in Swedish application 0303061-6 filed Nov. 19, 2003 and corresponding to Ser. No. 10/599,980 filed Mar. 26, 2007 and assigned to the same assignee as the present application, in connection with  FIG. 6  in that application. 
   In the next step, nearby channels are differentiated from each other in a differentiator  30 . The purpose of this is to remove ringing in the filter and is based on the assumption that nearby channels are correlated with reference to common-mode interferences. 
   Subsequently, the energy content of the signal is determined in an RMS-former  32  (Root Mean Square). To reduce any remaining spikes in EKG-signals (foremost related to the QRS-complex in the EKG-signal), the derivative of the signal from the RMS-former  32  is limited in a rate limit block  34 . 
   In the next step, a summation is done in a summing unit  36 . The purpose of the summing unit  36  is to weigh together the channels. This is done by multiplying the signals in the respective channels by a weight factor (see below), summing and normalizing the signals. In this connection the weighting factor can be squared to more selectively promote the channels with good signal-to-noise ratio. In principle, the summing unit  36  can be seen as a channel selector in which the channels that have the highest weighting factor are selected for use while the channels with poorer SNR can be allocated the value 0 in extreme cases. 
   The weighting factor for the respective channels is determined as follows. 
   After the first high pass filter  24 , the signals in the channels are also passed to an EKG-detector  38 . The purpose of the EKG-detector  38  is to establish the presence of the EKG-signals in the channels. If a single channel indicates the presence of an EKG-signal, an estimated EKG-activity is calculated for all of the channels. The determination of the estimated EKG-activity is done in a first calculation block  40 . The output signal from the EKG-detector  38  is supplied to a second calculation block  42 . 
   In the manner described above, the presence of EMG-signals can also be established. This is done in an EMG-detector  44 , which is fed with the signal from the second high pass filter  28 . 
   The output signals from the EMG-detector  44  are led to the second calculation block  42 . 
   In addition to the signals mentioned above, the first calculation block  40  and the second calculation block  42  have a further input signal, namely the signal after the rate limit block  34 . 
   The EKG detector  38  and the EMG detector  44  can be designed in different ways. In one formulation the EKG detector  38  is designed for each channel and thus for each signal which is received from an electrode, to detect if the EKG signal exceeds a limit value which is defined for the EKG signals. To make this comparison the raw signal is filtered in a band pass filter to take away the relevant frequency band for an EKG signal and the output signal from the band pass filter is compared with the set threshold. If the output signal is higher than the threshold the EKG signal is considered present. 
   In an alternative embodiment both the detectors  38 ,  44  are designed so that a first probability function P EKG  is determined for each channel. P EKG  indicates the probability that an EKG signal is present in the signal. A second probability function P EMG =1−P EKG  indicates the probability that an EMG signal is present in the signal. 
   The probability functions are calculated from a frequency analysis for the respective EKG and EMG signals. 
   The function of the first calculation block  40  is evident from  FIG. 5 . The signal from the EKG-detector  38  goes into a first decision block  46 . Here it is established whether an EKG-signal is present in one of the channels (output yes) or not (output no). If there is no EKG-activity, the activity level is set to 0 in block  48  (estimated EKG-activity=0). If an EKG-signal is present in one of the channels, the estimated EKG-activity S is calculated for all the channels, which is done via a low pass filter  50 , which also receives the filtered signal from the rate limit block  34 . In the low pass filter  50 , the D.C. voltage level for each channel is in principle determined, the filtered signal from rate limit block  34 , (a breaking frequency of a few Hz can be accepted), which in that connection represents the estimated EKG-activity S. 
   The function of the second calculation block  42  is explained in  FIG. 6 . The signal from the EKG-detector  38  goes into a second decision block  52 . If an EKG-signal exists (output yes), the estimated EMG-activity is set to 0 in block  54 . If no EKG-signal exists (output no), it is investigated whether there exists some EMG-signal (from the EMG-detector  44  ) in a third decision block  56 . If no EMG-signal exists (output no), the activity is set to 0 in the block  54 . If EMG-signals exist, the estimated EMG-activity R is calculated by passing the signal, the filtered signal from Rate limit block  34 , through a low pass filter  58 . 
   Another embodiment of the blocks  40  and  42  are shown in  FIGS. 7 and 8 , respectively. 
     FIG. 7  shows an alternative embodiment of the first calculation block  40  for estimating the EKG signal. From the rate limit block  34  a low pass filter  50  in the first calculation block  40  receives the filtered raw signal. From the EKG detector  38  it receives as previously the estimation of the presence of EKG signal. In contrast to the above-described embodiment the value which is received from the EKG detector is a probability function P ECG  with a value between 0 and 1. The output signal from the calculation block  40  is an estimation of the effect contents in the EKG signal, which depends on the probability function. 
     FIG. 8  shows an alternative embodiment of the second calculation block  42  for estimating the EMG signal. The output signal from the EKG detector  38 , P ECG  is fed to a second calculation block  42  where the probability 1−P ECG  is fed to a low pass filter  58 . The low pass filter  58  receives the filtered signal from the rate limit block  34 . Before the filtered signal is fed to the low pass filter  58  the noise level in the signal is estimated in an estimating block  59  and cancelled from the signal. The output signal from the calculation block  42  is an estimation of the effect contents in the EMG signal, which depends on the mentioned probability function. 
   As shown, the embodiments shown in  FIGS. 5 and 6  can be seen as a special case of that shown in  FIGS. 7 and 8 , where the probability P ECG  can assume the value 0 or 1. 
   The estimated EMG-activity R and the estimated EKG-activity S are transferred to a SN-block  60  for determining a signal-to-noise ratio for each channel between the estimated EMG-activity R and the estimated EKG-activity S. The signal-to-noise ratio can be determined in different ways. One way to determine a signal-to-noise ratio T is by the quotient 
           T   =     R     R   +   S             
where R is the estimated EMG-activity and S is the estimated EKG-activity.
 
   To promote channels with high estimated EMG-activity (regardless of interferences), it is advantageous to instead use the quotient 
   
     
       
         
           T 
           = 
           
             
               R 
               2 
             
             
               R 
               + 
               S 
             
           
         
       
     
   
   Instead of R 2 , R n  can naturally be used, where n suitably is considerably larger or equal to 1. 
   The signal-to-noise ratio is transferred to a weighting factor block  62 , where a weighing is determined for each channel. The determinations in the weighting factor block  62  are shown in  FIG. 9 . A time block  64  counts time interval t, for example for a few seconds. In the example of  FIG. 9 , t must be longer than 2.5 seconds. During the respective time interval t, the maximum signal T 1  is determined for the signal-to-noise ratio T from the SN-block  60  in a maximizing block  66 . This maximum signal T 1  is then filtered in a low pass filter  68 . The filtered signal T 2  then represents the base for the determination of a weighting factor for each channel in a calculation block  70 . In this example, the weighting factor is set to 
             T   ⁢           ⁢   2       max   ⁢           ⁢   T   ⁢           ⁢   2           
where maxT 2  is the maximum T 2  for all the channels. In other words, the channels are normalized to the strongest signal-to-noise ratio of all the channels, such that the weighting factor for the respective channel receives a value between 0 and 1.
 
   The determined weighting factors are then transferred to the summing unit  36  ( FIG. 4 ), where handling is done as previously described. Finally, the signal can be smoothed in a low pass filter  72  and put out as a ready EMG-signal. 
   Preferably, a computer program is arranged in the analysis unit  12 , which directs the function of the different blocks in the analysis unit according to the above. 
   In the method according to the present invention, the determination of weighting factors for the respective signal channels plays a major role. The numerical values given above, e.g. breaking frequencies and multiplication factors, are only examples and in no way exclude other values. Similarly, certain portions of the signal handling can be obtained in other ways without deviating from the invention. 
   Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.