Abstract:
The ability of an ecg apparatus to detect and display pacing pulses from the surface electrodes on a patient in whom a minute ventilation-based rate adaptive pacemaker is implanted is improved by providing the ecg apparatus with a minute ventilation detection circuit capable of indicating the time of occurrence and repetition rate of bursts of AC carrier signals which the implanted pacemaker generates in deriving a minute ventilation related control signal for the implanted pacemaker. In addition to improving the ability of the ecg system to detect and record paced events, the incorporation of the MV detection into the ecg system accommodates leads-off detection by coordinating the generation of the leads-off drive signal with the MV carrier signal generated by the implanted pacer.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates generally to electrocardiograph apparatus and/or programming equipment for cardiac rhythm management devices (CRMD) employed to monitor and record cardiac depolarization and repolarization signals, and more particularly to the incorporation into such equipment of a minute ventilation detector for providing an indication of when an implanted MV (minute ventilation-based) rate adaptive CRMD is generating a AC carrier signal used in deriving a minute ventilation related control signal for the implanted device. 
     II. Discussion of the Prior Art 
     One type of rate adaptive cardiac pacemaker on the market today uses a patient&#39;s minute ventilation as a rate control parameter. In such rate adaptive pacemakers/defibrillators, transthoracic impedance is measured by applying an AC carrier signal of a predetermined frequency, typically about 30 KHz, from an oscillator between an electrode carried by a pacing lead disposed in the heart and an electrode on the pacemaker can that is usually located in a surgically formed pocket beneath the pectoral muscle in the patient&#39;s chest. This AC carrier signal is modulated by respiratory activity (inspiration and expiration) and a rate control signal is derived by demodulating the carrier. The Hauck et al. U.S. Pat. No. 5,318,597 may be referred to for additional disclosure of the construction and mode of operation of MV based rate adaptive pacemakers. 
     While the frequency of the AC carrier is generally outside of the bandwidth of most physiologic signals, the amplitude of the carrier frequently dominates electrocardiograph input signals and can interfere with the detection of pacing pulses put out by the implanted device. It is also true that electrocardiograph equipment, especially those incorporating leads-off indication, can adversely affect operation of an implanted minute ventilation-based rate adaptive pacemaker, causing erroneously high pacing rates. 
     The need, therefore, exists for a way to better render electrocardiograph and patient programmers used with CRMDs compatible with minute ventilation-based rate adaptive pacemakers. 
     SUMMARY OF THE INVENTION 
     A solution to the foregoing undesired interaction resides in providing an ecg recorder for detecting and displaying cardiac signals picked up on a plurality of skin-contacting surface electrodes disposed on a patient at predetermined body locations in whom a rate-adaptive cardiac rhythm management device is implanted with a MV detector. The cardiac rhythm management device can be of a type that may have minute ventilation as a control parameter for the rate at which the device produces pacing pulses. The minute ventilation based rate adaptive device includes a means for impressing a sub-threshold carrier signal of a given high frequency in timed bursts of a predetermined repetition rate between a first electrode disposed within a patient&#39;s thoracic cavity and a reference electrode. The MV detection circuit in the ecg recorder or CRMD programmer is connected to receive signals picked up by pairs of the plurality of surface electrodes, such signals including cardiac signal components, pacing pulse components, noise components and components due to the attenuating current carrier. The minute ventilation detection circuit indicates the time of occurrence of the components due to the carrier signal. 
     Without limitation, the minute ventilation detection circuit may comprise a matched filter that is coupled to receive the signals picked up by the pairs of surface electrodes along with a template comprising signals of the given high frequency, or, if in the digital domain, approximate filter coefficients. A comparator is coupled to receive the output of the matched filter and a reference signal. The comparator produces an output indicating the time of occurrence of the components due to the carrier signal when the output of the matched filter exceeds the reference signal. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Further features, applications and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, especially when considered in conjunction with the accompanying drawings in which: 
     FIG. 1 is a general block diagram of the system in which the present invention finds use; 
     FIG. 2 is a block diagram of an implantable minute ventilation based rate adaptive pacemaker; 
     FIG. 3 is a block diagram of the minute ventilation detection circuit embodied in the ecg recorder of the present invention; 
     FIG. 4 depicts a state machine for adaptively adjusting the threshold level for the comparator of the minute ventilation detector circuit of FIG. 2; 
     FIG. 5 are exemplary waveforms helpful in explaining operation of the minute ventilation detector in distinguishing pacing pulses from noise and an AC carrier signal; 
     FIG. 6 is a software flow diagram of an improved pace detection algorithm taking advantage of the minute ventilation detector; 
     FIG. 7 illustrates the configuration of a “leads-off indicator” employed in ecg equipment; and 
     FIG. 8 are waveforms showing a time relationship of pacemaker minute ventilation carrier signals to the ecg leads-off signals. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, there is shown an ecg recorder  10  which may be embodied in a programmer/monitor used with implantable cardiac rhythm management devices. It is shown as having a plurality of skin contacting electrodes, including a right arm (RA) electrode  12 , a left arm (LA) electrode  14 , a right leg (RL) electrode  16  and a left leg (LL) electrode  18  on a patient  20 . The patient is also shown as having a minute ventilation rate adaptive cardiac pacemaker  22  implanted and connected by conventional pacing/sensing leads  24  to electrodes disposed within the heart  26 . The ecg recorder  10  is shown as having a telemetry antenna  28  which, when positioned proximate the implanted device  22 , allows two-way communication via a telemetry link. The communications link may also comprise the standard magnetic coil telemetry wand commonly used to exchange information between an external programmer and an implanted medical device. 
     Referring next to FIG. 2, there is illustrated a block diagram of the implantable device  22 . It is seen to include a sense amplifier  30  connected via the lead  24  to electrodes  32  and  34 . Cardiac depolarization signals (R-waves) are amplified and applied to a microprocessor  36 , via a conductor  38 . The microprocessor is programmed to control a pulse generator  40  causing it to emit cardiac stimulating pulses there applied to the heart, via the lead  24 . 
     In order to develop a pacer rate control signal based upon the patient&#39;s minute ventilation, the implantable device  22  includes an oscillator  42  capable of producing timed bursts of relatively high frequency energy which are applied via an electrode  44  placed on or in the heart. This carrier signal is subject to modulation caused by inspiratory and expiratory activity and the resulting modulated carrier signal is picked up by an electrode  46  disposed on the implanted CRMD&#39;s housing and delivered to a sense amplifier  48 . The output from the sense amplifier is demodulated by demodulator circuit  50  such that the modulating envelope is a measure of transthoracic impedance vs. time. This demodulation envelope is signal processed at  52  and converted to a digital format by A/D converter  54  and fed as an input to the microprocessor  36 . Telemetry circuit  56 , which is incorporated into the implantable unit  22 , allows two-way communication with the external monitor/programmer  10 . 
     Those wishing additional information on the design and operation of MV-based rate adaptive pacemakers may refer to the aforereferenced Hauck et al. &#39;597 patent and the Pederson et al. U.S. Pat. No. 5,137,019, both of which are hereby incorporated by reference as if fully set forth herein. 
     Turning then to FIG. 3, there is shown a block diagram of the MV detection circuit incorporated into the ECG recorder/CRMD programmer module  10  of FIG.  1 . It includes a matched filter  58  having first inputs coupled to receive the signals picked up by the body contacting electrodes  12 ,  14 ,  16  and  18  along with a template input  60 . The template may be an analog signal of a predetermined morphology or any filter coefficients for a FIR digital filter. The matched filter  58  may be any one of a number of known devices operative to produce an output on line  62  when input signals to the matched filter correspond to the template  60 . The matched filter output on line  62  is applied as an input to a comparator circuit  64  which operates to produce a high output on line  66  when the input to the comparator exceeds a predetermined reference value  68 . Those skilled in the art will appreciate that the MV detect circuit of FIG. 3 can be implemented in either the analog or the digital domain. 
     In use, the template  60  will be a signal of substantially the same frequency as the AC carrier produced by oscillator  42  of the implanted CRMD  22 . Thus, at the time instances when the MV carrier bursts are picked up by the ECG electrodes, the MV detect circuit of FIG. 3 will produce an output in substantial time coincidence thereof. 
     Most MV-based rate adaptive pacemakers output the 30 KHz carrier signal in timed bursts, for example, every 47.04 ms. This rate information may be used to help distinguish true MV signals from noise. Thus, if the detector circuit of FIG. 3 produces a MV detection at rates other than integer multiples of 47.04 seconds, then it is known that noise is corrupting the detector and producing false MV indications. A frequency detector  69  is coupled to receive the signals from comparator  64  and functions to measure the repetition rate of the MV carrier bursts. 
     A further feature of the present invention is to utilize the rate information to dynamically change the reference threshold  68  of the comparator  64  to optimize noise rejection. For example, if the frequency detector  69  indicates more than one MV detection every 47.04 ms, then the reference threshold is made to increase until the number of detections decreases to the appropriate rate. In this manner, the system dynamically sets the threshold level above the noise floor. FIG. 4 is a state machine configured to adjust the threshold level in the manner just described. 
     Having explained the functioning of the MV detect circuit to identify the time of occurrence of high frequency carrier bursts from the implanted pacemaker device, the utility thereof in improving the ability of the ECG recorder to detect pacing pulses within the surface ECG data will next be explained. As is explained in our pending application Ser. No. 09/516,533, filed Mar. 1, 2000, entitled “System and Method for Detection of Pacing Pulses Within ECG Signals”, which is also incorporated by reference, signals in the 30 KHz band can interfere with the detection of pacing pulses picked up by the body contacting surface electrodes, especially when edge detection is employed to help discriminate pacing pulses from background noise. To distinguish the pacing edges, incoming ECG signals are filtered at frequencies around 1 KHz and about 30 KHz. Because the minute ventilation carrier frequency is close to this range, the MV signals tend to dominate in this band. FIG. 5 illustrates the shape of a pacing pulse and MV signals within ECG data. Because MV signals are often an order of magnitude greater than pacing signatures, the MV signals can inappropriately increase dynamic thresholds of pace detect algorithms if not properly detected as noise. To help discern MV signals as noise during pace detection, the output of the MV detector of FIG. 3 is incorporated into the pacing detector functionality. FIG. 6 is a simplified software flow diagram of an improved pacing detection algorithm which utilizes the output of a pace detection algorithm, such as described in the aforementioned application Ser. No. 09/516,533, along with the output of the MV detector. As is reflected in FIG. 6, once a possible pace is recognized, the system checks the output of the MV detector to verify that no MV detections occurred at the same time as the pace event. If so, a valid pace is declared. On the other hand, if a MV signal is sensed at the same time as the pace, the pace detect algorithm inadvertently detected the MV signal as a pace event. In this situation, the possible pace is declared invalid and any pace detect noise thresholds are not adjusted. In this manner, the additional MV detection criteria improves the rejection of noise from the pace detection algorithm and prevents a seeding of the pace detection thresholds with a grossly large value. 
     In my copending patent application Ser. No. 09/639,037, filed Aug. 15, 2000, I point out that many ECG machines incorporate a “lead-off indicator” to help identify a high impedance ECG electrode patch. By providing such an indicator, a medical professional is able to quickly locate the source of a noisy signal and take appropriate steps to secure the lead patch to the patient&#39;s skin. It is further explained that most conventional leads-off indicators use simple impedance measurements to determine whether an electrode is attached to the patient. Typically, the ECG machine has an AC signal source  70  that is adapted to apply a relatively high frequency drive signal, again in the 30 KHz range, to the patient through electrode  16  affixed to the patient&#39;s right leg as illustrated in FIG.  7 . The ECG then measures this signal through the other input electrodes  12 ,  14  and  18  to determine whether the electrodes are properly attached by comparing the amplitude of the transduced 30 KHz signal to a predetermined reference. When ECG machines with such prior art style leads-off indicators are used with pacemaker patients having a MV-based rate adaptive pacemaker, the leads-off indicator can drive the pacing rate of the pulse generator up to its programmed upper-rate limit. 
     By incorporating the MV detector of the present invention into the ECG recorder, it can be used in conjunction with the leads-off indicator to help prevent this unwanted interaction. Using the output of the frequency detector  69  of the MV detector of FIG. 3 to first recognize the characteristic repetition rate of the MV carrier pulse, the 30 KHz leads-off indicator signal from source  70  is then interlaced in time, operating between the MV carrier bursts produced by the oscillator  42  of the implanted CRMD. The waveforms of FIG. 8 illustrate how the minute ventilation signals from an implanted rate adaptive pacemaker are detected and used to predict the time of occurrence of a succeeding burst allowing synchronization of the generation of the leads-off signal in the ECG recorder apparatus so that the leads-off signal occurs between bursts of the MV carrier signal and thereby prevents interaction with the MV sensor  48  and demodulator  50  by operating in the time intervals between expected MV sensing activity within the implanted device. 
     With continued reference to FIGS. 7 and 8, the incorporation of the minute ventilation detector of FIG. 3 into the CRMD programmer/monitor  10  will allow communication between the external device and the implanted device. By moving the switch  72  from the position shown when used for leads-off indication to its position connecting the digital data source  74  and by configuring the telemetry circuit  56  (FIG. 2) so that its sensing amplifier will detect the data stream sent between the RL reference electrode  16  and the other ECG input electrodes  12 ,  14  and  18 , the MV detector can again establish a time intervals between detected bursts of the MV carrier in which the ECG apparatus transmits the digital data stream. Thus, in the timing diagram of FIG. 8, the digital data stream would be transmitted in place of the 30 KHz leads-off drive signal in the interval between MV carrier bursts once the MV detector and the frequency counter  69  have predicted the time of occurrence of the next succeeding burst. 
     This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.