Abstract:
An implantable heart stimulator has a heart signal detector adapted to detect electrical heart signals and to apply the detected signals to at least two detection channels. Each detection channel includes a filter, with each filter having a passband that differs from the passband of the other filters. Each channel also includes a threshold detector and a peak amplitude determining unit connected to the output of the filter in that channel. A heart event identifying unit is connected to the outputs of each channel and unambiguously identifies a type of signal which produced a detected heart event by applying predetermined identifying criteria to the outputs of the threshold detector and the peak amplitude determining unit from each channel.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an implantable heart stimulator, e.g. a pacemaker or a defibrillator. 
   2. Description of the Prior Art 
   In conventional pacemaker technology often a single band-pass filter is used in the sensing circuit of the pacemaker in order to detect electrical heart signals. When using this known technique the origin of a signal that caused a sensed event is difficult to determine. 
   A ventricular event occurring early in the heart cycle (prior to a normally timed QRS-complex) and arising from a focus in the ventricles is often referred to as a premature ventricular contraction (PVC). 
   If a PVC not is detected due to undersensing it can cause inappropriately timed, asynchronous or competitive stimulation pulses to be delivered. Undersensing is defined as a failure of the pacemaker to sense an electrical signal related to a heart event, e.g. a PVC, due to the sensitivity of the sensing circuit of the pacemaker being set too low. This can often be corrected by programming the pacemaker to a more sensitive setting, i.e. decreasing the value of the sensitivity level. 
   U.S. Pat. No. 4,880,004 discloses an implantable cardiac stimulator for detecting and treating cardiac arrhythmias. The stimulator includes a sense amplifier responsive to sensed cardiac signals for detecting and distinguishing normal and abnormal cardiac activity within the sensed signals. The sense amplifier includes an automatic gain control amplifier, a filter and a comparator having a pair of signal channels for processing the sensed signals according to different frequency bandpass characteristics to establish sensing thresholds, margins and signal gain. One of the signal channels constitutes a feedback loop for determining the signal gain and the sensing margin for the other channel. 
   In U.S. Pat. No. 5,350,402 an atrial defibrillator is disclosed including a first detector for detecting R-waves of the heart and a second detector for detecting T-waves of the heart. The detection criterion is based on a predetermined time interval relationship between the R-wave and the T-wave. According to a software implementation of the T-wave detector, a microprocessor may be implemented for filtering the output of a sense amplifier with a high-pass filter and a low-pass filter. The derivative of the filtered signal is calculated by discrete differentiation of the filtered data and the derivative is re-filtered with a low-pass filter. These values are used in further calculations to determine if a T-wave is detected. 
   In U.S. Pat. No. 5,755,739 an adaptive and morphological system for discriminating P-Waves and R-waves inside the human body is disclosed. A drawback of a system using morphological recognition is that it probably is not fast enough for real time operation and that it is often implemented by a microprocessor that has unacceptably high energy consumption. 
   In U.S. Pat. No. 4,305,396 an improved automatically rate adaptive pacemaker is disclosed. The theory behind this patent is that a correlation has been identified between e.g. the amplitudes of the R-wave and T-wave and the heart rate. This correlation is then used to control a rate-responsive pacemaker. The peak values of the QRS-wave and T-wave, respectively, are detected in detection windows using conventional techniques. The detected values are then applied to a correlation block where a rate-controlling signal is generated. 
   European Application 0 917 887 discloses a cardiac event detecting system for an implantable heart stimulator intended to be connected to the heart of a patient via at least two unipolar electrode leads, or at least one bipolar electrode lead having one electrode pole in the atrium and one electrode pole in the ventricle, for sensing heart signals. This system has at least two signal channels for signals sensed between the two electrode poles and between one of the electrode poles and the stimulator capsule, respectively. 
   European Application 0 646 390 discloses a heart stimulator having an atrial electrode in an atrium of a heart and a ventricular electrode in a ventricle in the heart. In order to sense stimulated events in the heart a detector is connected between the atrial and ventricular electrodes to measure electrical heart signals between them. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to improve the safety in detecting electrical heart signals and to make it possible to determine the origin of detected signals. The heart stimulator according to the invention is in particular useful for a safe detection of premature ventricular contractions (PVCs). 
   Another object of the invention is to arrange an implantable heart stimulator having a detection of electrical heart signals that is fast and low energy consuming. 
   The above objects are achieved in accordance with the principles of the present invention in an implantable heart stimulator having at least one heart signal detector adapted to detect electrical signals originating from either a ventricle or an atrium, at least two detection channels connected to the detector, each channel including a filter with a predetermined filter characteristic, a threshold detector with a predetermined threshold, and peak amplitude determining unit. In each detection channel, the filter therein generates a filtered signal that is supplied to the threshold detector, which emits a detection signal if the filtered signal exceeds the threshold, and the filtered signal is also supplied to the peak amplitude determining unit which generates a signal representing the peak amplitude value of the filtered signal. 
   Each of the detection channels is connected to the same cardiac lead electrode, and each filter has a passband that is different from the passband of the other filters. Each of the channels is continuously active, and the respective signals therefrom are supplied to a heart event identifying unit that unambiguously identifies the type of signal that caused a detected heart event by applying predetermined heart event identifying criteria to the detection channel signals. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an implantable heart stimulator. 
       FIG. 2  is a block diagram of the implantable heart stimulating device according to the invention. 
       FIG. 3  is a block diagram of a detection channel according to the invention. 
       FIG. 4  is a block diagram of a preferred embodiment of a part of a detection channel according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an implantable heart stimulator formed by a heart stimulating device  2  and an electrode lead  4  inserted into the ventricle of a heart  6 . The electrode lead  4  is inserted into the heart  6  and arranged in the ventricle according to procedures well known to persons skilled in the art. The heart stimulator in  FIG. 1  relates to a single chamber heart stimulator, which means that the electrode lead is arranged in one chamber of the heart, in this case the right ventricle. However, it should be noted that the invention is equally applicable in a dual chamber heart stimulator that has two heart electrode leads adapted to stimulate the heart both in the atrium and in the ventricle, as well as in a multi-chamber heart stimulator adapted to stimulate three or four chambers of the heart. 
     FIG. 2  illustrates the implantable heart stimulating device  2  according to the invention, that has a pulse generator  8  for generating heart stimulating pulses to the heart via the electrode lead  4 . The heart stimulating device  2  further has a heart signal detector  10  connected to the electrode lead  4  and adapted to receive electrical heart signals  12  and to generate detected electrical heart signals  14  to three detection channels  16 ,  16 ′,  16 ″. Each channel is adapted to generate a detection signal  18 ,  18 ′,  18 ″ and a peak amplitude value  20 ,  20 ′,  20 ″ to a heart event identifying unit  22  that generates a signal  24  that identifies a detected heart event and applies the signal  24  to a control unit  26 . 
     FIG. 3  illustrates one of the detection channels  16 . The detection channel  16  has a filter  28  that generates a filtered signal  30  that is applied to a threshold detector  32  and to a peak amplitude determining unit  34 . If the filtered signal exceeds a predetermined threshold  36  of the threshold detector  32  the detection signal  18  is generated. The peak amplitude determining unit  34  generates the peak amplitude value  20 . 
   The invention is described in relation to a single chamber heart stimulator, i.e. with one electrode lead placed in the atrium or in the ventricle of the heart. As mentioned above the invention is equally applicable in a dual chamber heart stimulator where, for each electrode lead, a heart signal detection means and at least two detection channels are associated. 
   Each filter  28  has a predetermined filter characteristic, that differs from that of the filter  28  in another each of the other detection channels. 
   If the heart signal detector  10  receives signals detected in the ventricle of the heart, the predetermined respective filter characteristics of the filters  28  in three parallel detection channels are e.g. tuned to be sensitive to R-waves, T-waves and PVCs. 
   The filter  28  sensitive to R-waves is a band-pass filter with a passband in the range 20–50 Hz. 
   The filter  28  sensitive to T-waves is a band-pass filter with a passband in the range 2–10 Hz. 
   And the filter  28  sensitive to PVCs is a band-pass filter with a passband typically in the range 15–40 Hz. 
   If the heart signal detector  10  instead receives signals detected in the atrium of the heart, the predetermined respective filter characteristics of the filters  28  in two parallel detection channels preferably are tuned to be sensitive to P-waves and far-field R-waves. The filter  28  sensitive to P-waves is a band-pass filter with a narrow pass-band around 30 Hz. 
   The filter  28  sensitive to far-field R-waves is a band-pass filter with a pass-band typically in the range 10–35 Hz. 
   It is however possible to arrange further detection channels both for detection in the atrium and in the ventricle, e.g. to be able to detect different kinds of arrhythmia, states of atrial or ventricular fibrillation etc. 
   The filter filters  28  can be implemented using digital or analog filter techniques. 
   If a digital filter technology is used the analog detected heart signal is A/D converted before filtering is performed, and the processing of the filtered signal in the threshold detector  32  and in the peak amplitude determining unit  34  is digital. 
   If an analog filter instead is used the above-mentioned processing might also be performed in an analog threshold detector and in an analog peak amplitude determining means. As an alternative the filtered signal is A/D-converted after the filtration and then applied to the threshold detector  32  and the peak amplitude determining unit  34 . 
   The filter characteristics discussed above could either be set at the time of manufacture of the implantable device or could be set by a physician during implantation of the device or later at a follow-up visit. The filters  28  can be automatically tuned by tuning means in the heart event identifying unit  22 . 
     FIG. 4  illustrates a preferred embodiment of the threshold detector  32  and the peak amplitude determining unit  34 . The filtered signal  30  is a stream of digital bits representing the heart signal. The bit-stream is applied to the threshold detector  32  which is a digital comparator with a threshold  36  that generates the detection signal  18  if the filtered signal exceeds the threshold  36 . The detection signal is applied to the peak amplitude determining unit  34  that, according to this embodiment, is a shift register. When a detection signal is received by the determining unit  34 , the digital bit-stream is clocked into the shift register during a predetermined time, about 10–30 ins. When the predetermined time has elapsed, the content of the shift register is inspected in order to find the maximum value and that value is then generated as the peak amplitude value  20 . 
   According to another preferred embodiment of the invention the heart signal detector  10  receives signals detected in the ventricle of the heart. In  FIG. 2  the detection channel  16  is tuned to be sensitive to R-waves, the detection channel  16 ′ is tuned to be sensitive to T-waves and the detection channel  16 ″ is tuned to be sensitive to PVCs. The detection channel  16  generates detection signal  19  (R det ), indicating a detected R-Wave, and a peak amplitude value  20  (R max ) indicating the peak amplitude of the detected R-wave. According to the same principles T det , T max , PVC det  and PVC max  are generated by the detection channels  16 ′,  16 ″, respectively. 
   The detection signals and the peak amplitude values are received by the heart event identifying unit  22  where a number of heart event identifying criteria are applied. 
   To unequivocally identify an R-wave the following criteria must be fulfilled: 
   Detection signal R det  received, i.e. no T det  or PVC det , and R max /PVC max &gt;1 (also R max /T max &gt; 1  could be checked). 
   The division R max /PVC max  need only be performed if there also was a PVC det . 
   R max −PVC max &gt;0 can be used instead of R max /PVC max &gt; 1 . 
   To unequivocally identify a PVC the following criteria must be fulfilled: Detection signal PVC det  received, i.e. no R det  or T det  and PVC max /R max &gt;1 and PVC max /T max &gt;1 if PVC det  and R det . 
   The division PVC max /R max  need only be performed if there also was an R det . 
   PVC max −R max &gt;* can be used instead of PVC max /R max &gt;1. 
   Typical values for R max  is in the range of 6–12 mV and for PVC max  is in the range of 3–6 mV. T max  has a maximal peak amplitude below 1 mV. 
   According to a second preferred embodiment of the invention the heart signal detector  10  receives signals detected in the atrium of the heart. In  FIG. 2  only two detection channels are used and the detection channel  16  is tuned to be sensitive to P-waves and the detection channel  16 ′ is tuned to be sensitive to far field R-waves. The detection channel  16  generates detection signal  18  (P det ), indicating a detected P-Wave, and a peak amplitude value  20  (P max ) indicating the peak amplitude of the detected P-wave. According to the same principles R (far-field) det  and R(farfield) max  are generated by the detection channel  16 ′. 
   To unequivocally identify a P-wave, the following criteria must be fulfilled: 
   Detection signal P det  received and P max /R(far-field) max &gt;1 if P det  and R(far-field) det.    
   To unequivocally identify a far-field R-wave the following criteria must be fulfilled: 
   Detection signal R(far-field) det . received and R(far-field)  max /P max /&gt;1 if R(far-field) det  and P det . 
   Typical values for P max  when filtered with the P-wave adapted filter  28  is in the range of 3–4 mV and when filtered with the far-field R-wave adapted filter  28  in the range of 2–3 mV. Typical values for R(far-field) max  when filtered with the P-wave adapted filter is in the range of 2–3 mV and when filtered with the far-field R-wave adapted filter  28  in the range of 3–4 mV. 
   It should be noted that the individual variability regarding signal amplitudes may be significant. 
   The heart event identifying unit  22  is implemented either by software in a microprocessor or by a digital network using commonly available programming technique or digital network design. 
   The filters  28  are continuously active which means that each filter  28  in each of the detection channels  16 ,  16 ′ and  16 ″ receives detected electrical heart signals and performs filtering during the whole heart cycle. 
   As soon as a detection signal is received by the heart event identifying unit  22 , the peak amplitude values received during a predetermined time interval, e.g. from * to 30 ms, are used in the above-mentioned identifying criteria to identify the detected heart event. 
   The signal  24  identifying a detected heart event is applied to the control unit  26  where appropriate action is taken in response of the detected heart event. Such action could be a resetting of certain time intervals, a change of mode of operation for the heart stimulator and/or the adjustment of certain parameters, e.g. the sensitivity level. All these actions are well known to a person skilled in the art of heart stimulators and therefore need not be further described in the present application. 
   According to still another embodiment of the invention the heart event identifying unit  22  is provided with means for tuning and adjusting each filter  28  to be more sensitive to the heart event it is intended to detect, e.g. R-waves. That could be done by e.g. changing the band-width or another filter parameter of the filter. 
   In the embodiments of the invention described above the heart signal detection technique is only briefly discussed. It should be noted that any detection technique resulting in a detection of heart signals is applicable in the present invention. The heart signal can be detected by a single bipolar electrode lead by measuring between a tip and a ring electrode surfaces. If instead a unipolar heart electrode is used detection is performed between a tip electrode surface and an electrode surface at the pacemaker housing. Still another possibility is to detect between electrode surfaces at different electrode leads that could be unipolar, bipolar or multipolar. The above-mentioned measurement techniques and expressions are well known to a person skilled in the art of heart stimulators and are therefore not further described. 
   Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.