Abstract:
Mechanical activity of a heart is sensed by a cardiac lead that carries a triboelectric sensor. The triboelectric sensor produces a triboelectric signal in response to cardiac contractions. A lead fabricated according to the invention can be used for a variety of purposes, including without limitation, pacing capture verification, electromechanical conductivity status of the myocardium (including detecting relatively reduced myocardial activity indicative of ischemia, myocyte necrosis, arterial stenosis and the like). The sensor allows detection of mechanical activity without signal blanking traditionally utilized to stimulate and sense cardiac activity. Traditional circuitry can be employed to stimulate/sense while a triboelectric sensor unit(s) detect evoked and/or intrinsic mechanical cardiac activity. A reduction from a prior amplitude signal can be used to set patient (or clinician) alert signals, set a logical flag regarding possible lead dislodgement, compare prior and current signals, store same in memory, and/or provide via telemetry for display.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to cardiac therapy systems. In particular, the present invention relates to a system including a triboelectric sensor for sensing signals related to cardiac contractions.  
         [0002]     Implantable medical devices (IMDs) such as cardiac pacemakers, cardioverter defibrillators, and neurostimulators deliver electrical signals to a portion of the body and/or sense electrical signals from the body. A pacemaker includes a pulse generator and one or more leads for delivering generated stimulation pulses to the heart and for sensing cardiac signals and delivering sensed signals from the heart back to the pacemaker. Electrodes on the lead are electrically coupled to an inner lead conductor, which carries the stimulating current or sensed cardiac signals between the electrodes and the implanted device.  
         [0003]     The inner lead conductor of the cardiac lead defines a channel within the cardiac lead. This channel enables control of lead implantation with a stylet.  
         [0004]     After proper positioning of the lead tip, the stylet is pulled out of the stylet channel. As a result, the implanted cardiac lead has an empty stylet channel.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention senses mechanical activity of a heart. A cardiac lead carries a triboelectric sensor that produces a triboelectric signal in response to cardiac contractions. A lead fabricated according to the invention can be used for a variety of purposes, including without limitation, pacing capture verification, electromechanical conductivity status of the myocardium (including detecting relatively reduced myocardial activity indicative of ischemia, myocyte necrosis, arterial stenosis and the like). Such a lead allows detection of mechanical activity without signal blanking oftentimes used with electrodes traditionally utilized to stimulation and sense cardiac activity. Thus, in one form of the invention traditional circuitry and components are employed to stimulate and sense while one or more triboelectric sensor units are employed to detect evoked and intrinsic mechanical cardiac activity. In another form of the invention a reduction from a prior amplitude signal is utilized to provide a patient (or clinician) alert signal, a logical flag is set regarding possible lead dislodgement, a compare prior and current signals, stored same to memory, and/or provide the results via telemetry for remote display and processing and the like. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic view of an implantable medical device including atrial and ventricular leads.  
         [0007]      FIG. 2  is a cross-section view of proximal and distal ends of a lead stylet for insertion into a cardiac lead.  
         [0008]      FIG. 3A  is a cross-sectional view of a proximal end of a cardiac lead having the proximal end of the lead stylet shown in  FIG. 2  inserted therein.  
         [0009]      FIG. 3B  is a cross-sectional view of a portion of the cardiac lead and lead stylet at a location subject to mechanical stresses due to cardiac contractions.  
         [0010]      FIG. 4  is a schematic view of an equivalent circuit of a charge differential amplifier connected to an inner conductor of the cardiac lead and a lead stylet conductor.  
         [0011]      FIG. 5  is a graph showing the output signal of the charge differential amplifier shown in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  is a schematic view of implantable medical device (IMD)  10  including atrial lead  12  and ventricular lead  14  implanted in heart  16 . IMD  10  may be a pacemaker, defibrillator, cardioverter, pacemaker/cardioverter/defibrillator (PCD), heart function monitor having pacing capabilities, or other implantable device that includes the capability of providing therapy to heart  16 . IMD  10  includes connector module or header  18  and housing  20 . Atrial lead  12  and ventricular lead  14  extend from connector module  18  into the right atrium RA and right ventricle RV, respectively, of heart  16 . Proximal ends of atrial lead  12  and ventricular lead  14  are connected at header  18  to sensing, signal processing, and therapy delivery circuitry (not shown) within housing  20 .  
         [0013]     Atrial lead  12  and ventricular lead  14  enter right atrium RA through superior vena cava  24 . Atrial lead  12  is a J-shaped bipolar lead including tip electrode  30  and ring electrode  32  at its distal end, while ventricular lead  14  is an elongated bipolar lead including tip electrode  34  and ring electrode  36  at its distal end. While bipolar leads  12  and  14  are disclosed, unipolar leads can alternatively be implanted in the same anatomic relation to the heart chambers.  
         [0014]     When heart  16  contracts, atrial lead  12  and ventricular lead  14  are deflected. The atrial contraction causes bending or deformation of atrial lead  12  along bending portion  40 , while the ventricular contraction causes bending or deformation of ventricular lead  14  along bending portion  42 . The magnitude of the deflection along bending portions  40  and  42  depends on the radial stiffness of atrial lead  12  and ventricular lead  14 , respectively, and on the muscle contraction forces of heart  16 . In addition, the magnitude of the deflection depends on the initial bending forces caused by the specific implantation position. For instance, atrial lead  12  implanted on the anterior atrial wall (as shown in  FIG. 1 ) has a larger J-shape radius than a lead implanted in the atrial appendage. Atrial lead  12  and ventricular lead  14  are strongly mechanically coupled to the heart muscle, especially in the chronic phase of cardiac pacing when fibrotic tissue anchors the lead tips to the endocardium.  
         [0015]     The various types of cardiac rhythms have differing hemodynamics (i.e., different magnitudes and frequency spectra of contraction movements). For example, ventricular tachycardia impedes cardiac contractions significantly, causing a decrease in the contraction magnitude. The different cardiac rhythms cause forces that result in different mechanical tension in atrial lead  12  and ventricular lead  14 .  
         [0016]     The stylet channel formed by the inner lead conductor of the cardiac lead enables control of lead implantation with a steel wire (i.e., stylet). After positioning of the lead tip, the stylet is pulled out of the stylet channel. As a result, the implanted cardiac lead has an empty stylet channel. The stylet channel may thus be used for permanent insertion of a stylet having sensing capabilities. The present invention is directed to sensing a triboelectric signal produced by surface contact (friction) effects between an inner conductor of the lead and an insulator on a lead stylet.  
         [0017]      FIG. 2  is a cross-section view of proximal and distal ends of lead stylet  50  for insertion into atrial lead  12  or ventricular lead  14 . At the distal end, lead stylet  50  includes stylet conductor  54 , insulating sheath  56 , and stopper  58 . At the proximal end, lead stylet  50  includes stylet conductor  54 , insulating sheath  56  (which terminates at the proximal end of stylet  50  with insulating connector seal  60 ), and connector pin  62 . Insulating sheath  56 , which is made of a flexible polymeric material, has a diameter such that its exterior surface is coupled to the inner lead conductor that forms the stylet channel in atrial lead  12  or ventricular lead  14  (see  FIG. 3 ). The tip of lead stylet  50  is closed by stopper  58 , which may be made of silicone.  
         [0018]     When lead stylet  50  is inserted within atrial lead  12  or ventricular lead  14  through the stylet channel formed by the inner lead conductor, the outer surface of insulating sheath  56  mechanically couples with the inner surface of the inner lead conductor. The length of lead stylet  50  is such that a portion of lead stylet  50  is positioned within bending portion of the lead (e.g., bending portion  40  of atrial lead  12  or bending portion  42  of ventricular lead  14 ). It should be noted that the configuration of lead stylet  50  shown in  FIG. 2  is merely illustrative, and any lead stylet including an insulating material surrounding a conductive portion may be employed in accordance with the present invention.  
         [0019]      FIG. 3A  is a cross-sectional view of a proximal end of a cardiac lead (e.g., atrial lead  12 , ventricular lead  14 ) including stylet channel  70  defined by inner lead conductor  72  having lead stylet  50  inserted therein.  FIG. 3B  is a cross-sectional view of the cardiac lead shown in  FIG. 3A  along a portion of the cardiac lead subject to mechanical stresses due to cardiac contractions (e.g., bending portion  40  on atrial lead  12 , bending portion  42  on ventricular lead  14 ). Stylet  50  is inserted within stylet channel  70 , which is defined by inner lead conductor  72  of the cardiac lead. Stylet conductor  54  is terminated at the proximal end of lead stylet  50  with connector pin  62 . Connector pin  62  is isolated from inner lead conductor  72  by insulation seal  60 . Connector pin  62  and the proximal end of inner lead conductor  72  provide an interface for electrically connecting stylet conductor  54  and inner lead conductor  72  with circuitry within IMD  10 .  
         [0020]     When lead stylet  50  is subjected to mechanical stresses due to cardiac contractions, an electrical charge differential is generated between insulating sheath  56  and both stylet conductor  54  and lead conductor  72  due to the triboelectric effect. The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after coming into contact with a different material, e.g., by frictional contact. This may occur when lead stylet  50  is subjected to mechanical stresses, such as shock or bending forces. The polarity and strength of the charges generated on the materials differ according to the materials, surface roughness, temperature, strain, and other properties. After coming into contact, a chemical bond is formed between some parts of the contacting surfaces. When the chemical bond is formed between the materials, charges move from one material to the other to equalize their electrochemical potential. This creates a net charge imbalance between the materials. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some have a tendency to give them away.  
         [0021]     The triboelectric series is a list of materials, provided in order from materials that have a greater tendency to attain a positive charge after separation, to those that have a greater tendency to attain a negative charge after separation. Thus, a material towards the negative end of the triboelectric series, when touched to a material closer to the positive end of the series, will attain a more negative charge, and vice versa. The further away two materials are from each other on the series, the greater the charge transferred.  
         [0022]     Insulating sheath  56  of lead stylet  50  is made of a material that is far on the triboelectric series from the conductive materials that comprise stylet conductor  54  and inner lead conductor  72 . In one embodiment, insulating sheath  56  is made of a material toward the negative end of the triboelectric series (e.g., silicone, polyurethane), while stylet conductor  54  and inner lead conductor  72  are made of a conductive material closer to the positive end of the triboelectric series (e.g., platinum, aluminum, steel). Thus, when cardiac contractions bend the portion of lead stylet  50  that is disposed in the bending portion of the cardiac lead (e.g., bending portion  40  of atrial lead  12 , bending portion  42  of ventricular lead  14 ), a negative electrical charge is accumulated in insulating sheath  56  due to the triboelectric effect. The amount of charge that is accumulated in insulating sheath  56  is proportional to the bending angle of the cardiac lead and lead stylet  50  disposed therein. The charge in insulating sheath  56  modulates at a frequency related to the rate of cardiac contractions. Thus, the triboelectric signal occurs at a low frequency (e.g., less than 50 Hz), which is around the frequency of cardiac mechanical activity.  
         [0023]     The electrical charges that accumulate on insulating sheath  56  can be measured with a charge or voltage amplifier connected to stylet conductor  54  and inner lead conductor  72 . The charge or voltage amplifier may be implemented in IMD  10 . Connector pin  76  and the portion of inner lead conductor  72  at the proximal end of lead stylet  50  (which functions as a connector pin for inner lead conductor  72 ) provide interfaces for electrically connecting stylet conductor  54  and inner lead conductor  72 , respectively, to connector module  18  of IMD  10 . The output of the charge amplifier is connected to signal processing circuitry in IMD  10 , which produces a signal related to cardiac contractions from the measured charge variation.  
         [0024]      FIG. 4  is a schematic view of an equivalent circuit for charge differential amplifier  100  with inner lead conductor  72  and stylet conductor  54  connected to inputs  102  and  104 , respectively, of charge differential amplifier  100 . The distributed capacitance of insulating sheath  56  is represented by capacitors C I , the distributed resistance of inner lead conductor  72  is represented by resistors R LC , and the distributed resistance of stylet conductor  54  is represented by resistors R SC . It should be noted that while charge differential amplifier  100  is shown and described with regard to  FIG. 4 , any device capable of measuring signals from lead stylet  50  due to the triboelectric effect may alternatively be connected to stylet conductor  54  and inner lead conductor  72  (e.g., a voltage differential amplifier).  
         [0025]     Charge differential amplifier  100  includes operational amplifiers A 1 , A 2 , and A 3 , feedback resistors R f , feedback capacitors C f , reference resistor R R , and resistors R 1 ,-R 4 . Lead conductor  72  is connected to the inverting input of operational amplifier A 1 , and stylet conductor  54  is connected to the inverting input of operational amplifier A 2 . Feedback resistors R f  and feedback capacitors C f  are connected between the inverting input and the output of amplifiers A 1  and A 2 . Reference resistor R R  connects the non-inverting inputs of operational amplifiers A 1  and A 2  to ground. The outputs of operational amplifiers A 1  and A 2  are connected via resistors R 1  and R 2  to the inverting and non-inverting inputs, respectively, of operational amplifier A 3 . Resistor R 3  is connected between the non-inverting input of operational amplifier A 3  and ground, and resistor R 4  is connected between the inverting input and the output of operational amplifier A 3 .  
         [0026]     Operational amplifier A 1 , feedback resistor R f , feedback capacitor C f , and reference resistor R R  function as a low-pass filter for the signal from lead conductor  72 . Operational amplifier A 2 , feedback resistor R f , feedback capacitor C f , and reference resistor R R  function as a low-pass filter for the signal from stylet conductor  54 . The time constant for each of the low-pass filters is R f C f . As described above, the triboelectric signal occurs at a low frequency (i.e., &lt;50 Hz). Thus, the time constant is selected such that only the signal related to the triboelectric effect is provided at the outputs of operational amplifiers A 1  and A 2 .  
         [0027]     Operational amplifier A 3  and resistors R 1 -R 4  function as a differential amplifier for the signals provided from operational amplifiers A 1  and A 2 . The differential amplifier provides an output signal V SENSE  as a function of the difference between the non-inverting input signal and the inverting input signal to operational amplifier A 3 . This difference is amplified by a factor determined by the ratio of resistor R 4  to resistor R 1 . The V SENSE  signal provided at the output of operational amplifier A 3  is related to the varying charge that accumulates in insulating sheath  56  as lead stylet  50  is subjected to mechanical stresses due to cardiac contractions.  
         [0028]      FIG. 5  is a graph showing signal at output V SENSE  of charge differential amplifier  100  for lead stylet  50  disposed in a cardiac lead. In the tested device, insulation sheath  56  was made of silicone, and stylet conductor  54  and lead conductor  72  was made of a conductive material such as platinum, silver, or steel. Lead stylet  50  was positioned in the cardiac lead so as to enable pendulum movements with an excursion amplitude (i.e., the distance moved by the distal end of the cardiac lead during a cardiac cycle) of about 10 cm. Charge differential amplifier  100  included feedback capacitors C f  having a capacitance of 100 pF and feedback resistors R f  having a resistance of 1 GΩ. The graph shows that the output signal V SENSE  was a sinusoidal signal having a peak-to-peak amplitude of about 0.7 V. This translates to a charge variation in insulation sheath  56  of around 70 pC.  
         [0029]     The output signal V SENSE  may be used by IMD  10  to detect and monitor cardiac contractions in heart  16 . This information may be used by IMD  10  to determine frequency, amplitude, and velocity characteristics of the contractions. In addition, the V SENSE  signal may be provided to signal processing circuitry in IMD  10  to derive data related to other cardiac parameters, such as contraction parameters for arrhythmia detection, or to produce information for heart failure monitoring. The processed information may be used by IMD  10  to control the therapy delivered by cardiac leads  12  and  14 .  
         [0030]     In summary, the present invention senses mechanical activity of a heart. A cardiac lead carries a triboelectric sensor that produces a triboelectric signal in response to cardiac contractions. In one embodiment, triboelectric sensor includes an inner lead conductor that defines a stylet channel and a stylet that includes a stylet conductor within an insulating sheath. At least a portion of the stylet is disposed in the stylet channel at a sensing location. A charge amplifier is connected to the inner lead conductor and the stylet conductor for producing an output signal based upon a triboelectric charge on the insulating sheath caused by movement of the lead due to the cardiac contractions. The output signal of the charge amplifier may be processed to produce information related to cardiac activity that may be used in cardiac diagnosis and therapy.  
         [0031]     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.