Abstract:
A subcutaneous implantable cardioverter defibrillator (SubQ ICD) includes a housing carrying electrodes for sensing ECG signals and delivering therapy. A sensor detects local motion in the area of the housing and produces a noise signal related to motion artifact noise contained in ECG signals derived from the electrode array. An adaptive noise cancellation circuit enhances ECG signals based on the local motion noise signal. A therapy delivery circuit delivers cardioversion and defibrillation pulses based upon the enhanced ECG signals.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to implantable medical devices. In particular, the invention relates to a subcutaneous implantable cardioverter defibrillator (SubQ ICD) in which motion artifact noise associated with local motion near the SubQ ICD is sensed and used to enhance sensed subcutaneous ECG signals.  
         [0002]     Implantable cardioverter defibrillators are used to deliver high energy cardioversion or defibrillation shocks to a patient&#39;s heart when atrial or ventricular fibrillation is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met. Defibrillation shocks are typically delivered when fibrillation criteria are met, and the R-wave cannot be discerned from signals sensed by the ICD.  
         [0003]     Currently, ICDs use endocardial or epicardial leads which extend from the ICD housing to the heart. The housing generally is used as an active can electrode for defibrillation, while electrodes positioned in or on the heart at the distal end of the leads are used for sensing and delivering therapy.  
         [0004]     The SubQ ICD differs from the more commonly used ICDs in that the housing is typically smaller and is implanted subcutaneously. The SubQ ICD does not require leads to be placed in the bloodstream. Instead, the SubQ ICD makes use of one or more electrodes on the housing, together with a subcutaneous lead that carries a defibrillation coil electrode and a sensing electrode.  
         [0005]     The lack of endocardial or epicardial electrodes make sensing more challenging with the SubQ ICD. Sensing of atrial activation is limited since the atria represent a small muscle mass, and the atrial signals are not sufficiently detectable thoracically. Muscle movement, respiration, and other physiological signal sources also can affect the ability to sense ECG signals and detect arrhythmias with a SubQ ICD.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     A SubQ ICD includes a local motion sensor for producing a signal related to motion artifact noise contained in ECG signals derived by an electrode array carried on the SubQ ICD housing. An adaptive noise cancellation circuit enhances ECG signals derived from the electrode array based on the signal from the local motion sensor. The enhanced ECG signals are used for arrhythmia detection and delivery of therapy. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  depicts a SubQ ICD implanted in a patient.  
         [0008]      FIGS. 2A and 2B  are front and top views of the SubQ ICD associated electrical lead shown in  FIG. 1 .  
         [0009]      FIG. 3  is a circuit diagram of circuitry of the SubQ ICD.  
         [0010]      FIG. 4  is a block diagram of sensing circuitry of the SubQ ICD, including an adaptive noise cancellation circuit. 
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  shows SubQ ICD  10  implanted in patient P.  
         [0012]     Housing or canister  12  of SubQ ICD  10  is subcutaneously implanted outside the ribcage of patient P, anterior to the cardiac notch, and carries three subcutaneous electrodes  14 A- 14 C and local motion sensor  16 .  
         [0013]     Subcutaneous sensing and cardioversion/defibrillation therapy delivery lead  18  extends from housing  12  and is tunneled subcutaneously laterally and posterially to the patient&#39;s back at a location adjacent to a portion of a latissimus dorsi muscle. Heart H is disposed between the SubQ ICD housing  12  and distal electrode coil  20  of lead  18 . SubQ ICD  10  communicates with external programmer  24  by an RF communication link.  
         [0014]      FIGS. 2A and 2B  are front and top views of SubQ ICD  10 .  
         [0015]     Housing  12  is an ovoid with a substantially kidney-shaped profile. The ovoid shape of housing  12  promotes ease of subcutaneous implant and minimizes patient discomfort during normal body movement and flexing of the thoracic musculature. Housing  12  contains the electronic circuitry of SubQ ICD  10 . Header  26  and connector  28  provide an electrical connection between distal electrode coil  20  and distal sensing electrode  22  on lead  18  and the circuitry with housing  12 .  
         [0016]     Subcutaneous lead  18  includes distal defibrillation coil electrode  20 , distal sensing electrode  22 , insulated flexible lead body  30  and proximal connector pin  32 . Distal sensing electrode  22  is sized appropriately to match the sensing impedance of electrodes  14 A- 14 C.  
         [0017]     Electrodes  14 A- 14 C are welded into place on the flattened periphery of canister  12  and are connected to electronic circuitry inside canister  12 . Electrodes  14 A- 14 C may be constructed of flat plates, or alternatively, spiral electrodes as described in U.S. Pat. No. 6,512,940 entitled “Subcutaneous Spiral Electrode for Sensing Electrical Signals of the Heart” to Brabec, et al. and mounted in a non-conductive surround shroud as described in U.S. Pat. Nos. 6,522,915 entitled “Surround Shroud Connector and Electrode Housings for a Subcutaneous Electrode Array and Leadless ECGs” to Ceballos, et al. and 6,622,046 entitled “Subcutaneous Sensing Feedthrough/Electrode Assembly” to Fraley, et al. Electrodes  14 A- 14 C shown in  FIG. 2  are positioned on housing  12  to form orthogonal signal vectors.  
         [0018]     Local motion sensor  16  is a pressure sensor, optical sensor, impedance sensor or accelerometer positioned to detect motion in the vicinity of electrodes  14 A- 14 C, which are susceptible to motion artifact noise in the ECG signals. As shown in  FIG. 2A , local motion sensor  16  is mounted on the exterior of canister  12 , but is may also be mounted interiorly, so long as it can detect motion in the vicinity of electrodes  14 A- 14 C. Specificity and sensitivity of a signal detection algorithm for electrodes  14 A- 14 C is likely to suffer for a SubQ ICD device due to electrode distance from the heart and the proximity of large muscles in the chest. Local motion sensor  16  provides a way of improving specificity of the detection algorithm. Detection of reliable ECG signals is an essential requirement for proper operation of an implantable device such as an ICD or an external defibrillator. For a device that has no endocardial or epicardial leads, as its electrodes get farther away from the heart, ECG signal strength will degrade. Under these conditions, detection circuitry may be more prone to false detects. Noise due to muscle motion in the vicinity of ECG sensing electrodes may cause spurious electrical signals that could be interpreted as QRS complexes by the detection circuitry and algorithm. This might lead to delivery of unnecessary shocks or a necessary shock being held off, causing adverse outcomes for the patient. However, by using local motion detector  16  in the vicinity of electrodes  14 A- 14 C, a signal representative of the motion that causes motion artifacts in the ECG signals can be acquired. By employing adaptive noise cancellation algorithms, this local motion signal can be used as correlated noise to eliminate motion generated noise present in the ECG channel.  
         [0019]      FIG. 3  is a block diagram of electronic circuitry  100  of SubQ ICD  10 . Circuitry  100 , which is located within housing  12 , includes terminals  102 ,  104 A- 104 C,  106 ,  108  and  110 ; switch matrix  112 ; sense amplifier/noise cancellation circuitry  114 ; pacer/device timing circuit  116 ; pacing pulse generator  118 ; microcomputer  120 ; control  122 ; supplemental sensor  124 ; low-voltage battery  126 ; power supply  128 ; high-voltage battery  130 ; high-voltage charging circuit  132 ; transformer  134 ; high-voltage capacitors  136 ; high-voltage output circuit  138 ; and telemetry circuit  140 .  
         [0020]     Terminal  102  is connected to local motion sensor  16  for receipt of a local motion signal input. Switch matrix  112  provides the local motion signal by sensing amplifier/noise cancellation circuit  114  for use as correlated noise to eliminate motion artifact noise in ECG input signals.  
         [0021]     Electrodes  14 A- 14 C are connected to terminals  104 A- 104 C. Electrodes  14 A- 14 C act as both sensing electrodes to supply ECG input signals through switch matrix  112  to sense amplifier/noise cancellation circuit  114 , and also as pacing electrodes to deliver pacing pulses from pacing pulse generator  118  through switch matrix  112 .  
         [0022]     Terminal  106  is connected to distal sense electrode  22  of subcutaneous lead  18 . The ECG signal sensed by distal sense electrode  22  is routed from terminal  106  through switch matrix  112  to sense amplifier/noise cancellation circuit  114 .  
         [0023]     Terminals  108  and  110  are used to supply a high-voltage cardioversion or defibrillation shock from high-voltage output circuit  138 .  
         [0024]     Terminal  108  is connected to distal coil electrode  20  of subcutaneous lead  18 . Terminal  110  is connected to housing  12 , which acts as a common or can electrode for cardioversion/defibrillation.  
         [0025]     Sense amplifier/noise cancellation circuit  114  and pacer/device timing circuit  116  process the ECG signals from electrodes  14 A- 14 C and  22 , and the local motion signal from local motion sensor  16 . Signal processing is based upon the transthoracic ECG signal from distal sense electrode  22  and a housing-based ECG signal received across an ECG sense vector defined by a selected pair of electrodes  14 A- 14 C, or a virtual vector based upon signals from all three sensors  14 A- 14 C. Both the transthoracic ECG signal and the housing-based ECG signal are amplified and bandpass filtered by preamplifiers, sampled and digitized by analog-to-digital converters, and stored in temporary buffers. In the case of the housing-based ECG signal, adaptive filtering is also performed using the local motion signal from sensor  16  to remove noise caused by local motion artifacts.  
         [0026]     Bradycardia is determined by pacer/device timing circuit  116  based upon R waves sensed by sense amplifier/noise cancellation circuit  114 . An escape interval timer within pacer/device timing circuit  116  or control  122  establishes an escape interval. Pace trigger signals are applied by pacer/device timing circuit  116  to pacing pulse generator  118  when the interval between successive R waves sensed is greater than the escape interval.  
         [0027]     Detection of malignant tachyarrhythmia is determined in control circuit  122  as a function of the intervals between R wave sense event signals from pacer/device timing circuit  116 . This detection also makes use of signals from supplemental sensor(s)  124  as well as additional signal processing based upon the ECG input signals.  
         [0028]     Supplemental sensor(s)  124  may sense tissue color, tissue oxygenation, respiration, patient activity, or other parameters that can contribute to a decision to apply or withhold defibrillation therapy.  
         [0029]     Supplemental sensor(s)  124  can be located within housing  12 , or may be located externally and carried by a lead to switch matrix  112 .  
         [0030]     Microcomputer  120  includes a microprocessor, RAM and ROM storage and associated control and timing circuitry. Detection criteria used for tachycardia detection may be downloaded from external programmer  24  through telemetry interface  140  and stored by microcomputer  120 .  
         [0031]     Low-voltage battery  126  and power supply  128  supply power to circuitry  100 . In addition, power supply  128  charges the pacing output capacitors within pacing pulse generator  118 . Low-voltage battery  126  can comprise one or two LiCF x , LiMnO 2  or Lil 2  cells.  
         [0032]     High-voltage required for cardioversion and defibrillation shocks is provided by high-voltage battery  130 , high-voltage charging circuit  132 , transformer  134 , and high-voltage capacitors  136 . High-voltage battery  130  can comprise one or two conventional LiSVO or LiMnO 2  cells.  
         [0033]     When a malignant tachycardia is detected, high-voltage capacitors  136  are charged to a preprogrammed voltage level by charging circuit  132  based upon control signals from control circuit  122 .  
         [0034]     Feedback signal Vcap from output circuit  138  allows control circuit  122  to determine when high-voltage capacitors  136  are charged. If the tachycardia persists, control signals from control  122  to high-voltage output signal  138  cause high-voltage capacitors  136  to be discharged through the body and heart H between distal coil electrode  20  and the can electrode formed by housing  12 .  
         [0035]     Telemetry interface circuit  140  allows SubQ ICD  10  to be programmed by external programmer  24  through a two-way telemetry link. Uplink telemetry allows device status and other diagnostic/event data to be sent to external programmer  24  and reviewed by the patient&#39;s physician. Downlink telemetry allows external programmer  24 , under physician control, to program device functions and set detection and therapy parameters for a specific patient.  
         [0036]      FIG. 4  is a block diagram showing noise cancellation algorithm used by sense amplifier/noise cancellation circuit  114 .  FIG. 4  illustrates a signal (ECG+Noise), which is received from one or more of electrodes  14 A- 14 C. An additional input is a Noise signal produced by local motion sensor  16 . The Noise signal from sensor  16  is processed by adaptive filter  150  and is subtracted at summing junction  152  from the ECG+Noise signal derived from electrodes  14 A- 14 C. The output of summing junction  152  is an enhanced ECG signal with some or all of the motion artifact noise removed. This enhanced ECG signal is used as a feedback signal to adaptive filter  150  to control the subtraction signal supplied to junction  152 .  
         [0037]     Adaptive filter  150  can use adaptive filtering algorithms based on Least Means Squared (LMS), Recursive Least Squares (RLS) or Kalman filtering methods, or other methods such as multiplication free algorithms that increase computational efficiency and reduce power consumption.  
         [0038]     In order to conserve energy, sense amplifier/noise cancellation circuit  114  may selectively use the noise cancellation feature depending upon the content of the input ECG signals. This can be achieved, for example, by monitoring RMS (Root Mean Square) power of the local motion sensor signal and performing noise cancellation only when the power exceeds a threshold level.  
         [0039]     In another embodiment, the spectrum of the ECG input signals can be analyzed to determine when noise cancellation is appropriate. The ECG signal typically has a narrow band spectrum, which will widen with the presence of noise. Upon detecting spectrum broadening of the ECG signal, the noise cancellation feature is initiated.  
         [0040]     Although a single local motion sensor  16  has been shown and discussed, multiple local motion sensors can be used, with the Noise signal used for cancellation being derived from one or a combination of the motion sensor signals. The motion sensor can be a pressure sensor, an optical sensor, an impedance sensor or an accelerometer.  
         [0041]     For example, an optical sensor used for local motion sensing may include a light emitting diode radiating at an isobestic wavelength for oxygen (such as 810 nm or 569 nm), so that it has no sensitivity to local oxygen change, and a photodetector to collect light scattered by local tissue. Motion will cause changes in tissue optical density, and the amount of light collected by the photodetector will be modulated by motion.  
         [0042]     A local motion sensor using pressure sensing can make use of a piezoresistive, piezoelectric or capacitive sensor located in the housing. Pressure exerted on the surrounding tissue by housing  12  produces a pressure sensor output representing local motion.  
         [0043]     An impedance sensor sharing one or more of ECG electrodes or dedicated electrodes can be used to measure local tissue impedance. Changes in the electrode-electrolyte (tissue) interface due to motion artifacts can be sensed via changes in the magnitude and/or phase of the local impedance signal. Impedance measurement can be performed via narrowband sinusoidal excitation outside of the ECG bandwidth so as not interfere with ECG sensing.  
         [0044]     An accelerometer may also be used to sense motion of housing  12  and electrodes  14 . However, an accelerometer will sense motion globally, and may sometimes detect motion that does not affect the ECG signal. Depending upon the activity of the patient, and other sensor signals that may be used in conjunction with the accelerometer signal, an accelerometer may provide a sufficiently accurate correlation to local motion to permit noise cancellation of the ECG signals.  
         [0045]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.