Source: http://www.google.com/patents/US6445951?dq=5,371,548
Timestamp: 2017-08-23 20:47:29
Document Index: 668936194

Matched Legal Cases: ['art 104', 'art 104', 'art 104', 'art 104', 'art 104', 'art 104', 'art 104']

Patent US6445951 - Implantable cardiac stimulating device incorporating high frequency low ... - Google Patents
An implantable cardiac stimulating device incorporating high voltage leads for the delivery of cardioversion or defibrillation waveforms is provided. The implantable cardiac stimulating device incorporates a lead impedance measurement which measures the impedance of the high voltage lead using a high...http://www.google.com/patents/US6445951?utm_source=gb-gplus-sharePatent US6445951 - Implantable cardiac stimulating device incorporating high frequency low amplitude lead impedance measurement
Publication number US6445951 B1
Application number US 09/415,619
Publication number 09415619, 415619, US 6445951 B1, US 6445951B1, US-B1-6445951, US6445951 B1, US6445951B1
Inventors Gabriel Mouchawar
US 6445951 B1
An implantable cardiac stimulating device incorporating high voltage leads for the delivery of cardioversion or defibrillation waveforms is provided. The implantable cardiac stimulating device incorporates a lead impedance measurement which measures the impedance of the high voltage lead using a high frequency, e.g., 20 KHz or higher, low amplitude, e.g., 500 microamps or lower, signal which results in a lead impedance measurement that has a high degree of correlation to the lead impedance that would occur when the high voltage defibrillation or cardioversion waveform is applied to the heart from the electrode. The implantable cardiac stimulating device is further configured to take corrective action upon the detection of a lead having a high or low impedance that suggests a broken, shorted or damaged lead.
1. An implantable cardiac stimulation device enclosed in a conductive housing and adapted for use with at least one cardiac lead suitable for shocking a patient's heart, the device further adapted to detect lead fractures that would prevent a high voltage shock to a patient's heart, the device comprising:
a current generator coupled to said lead and adapted to supply an impedance measuring current to the lead, said current being of a relatively low amplitude and a relatively high frequency;
a return in electrical communication with the current generator and the lead and forming thereby a current return path between the lead and the current generator; and
an impedance measuring circuit coupled to the lead and return such that upon the supply of the impedance measuring current to the lead the impedance measurement circuit measures the impedance between the lead and the return.
2. The device of claim 1 wherein the current generator generates an impedance measuring current having a frequency in the range from 20 kHz to 50 kHz and an amplitude of no greater than about 500 ua.
applying an impedance measuring current, having a relatively low amplitude and a relatively high frequency, to the lead and return;
measuring the resultant voltage across the lead and return; and
determining the lead impedance based upon the value of the impedance measuring current and the resultant voltage across the lead and return.
15. The method of claim 14 wherein the step of applying an impedance measuring current further comprises the step of applying a current having a frequency in the range from 20 kHz to 50 kHz.
comparing the lead impedance to a preselected maximum value and a preselected minimum value; and
taking corrective action when the lead impedance either exceeds the preselected maximum value or is less than the preselected minimum value.
27. The method of claim 26 comprising the step of providing an annunciator signal when a lead impedance exceeds a predetermined maximum value or is less than a predetermined minimum value.
means for supplying an impedance measuring current, having a relatively low amplitude and a relatively high frequency, to the lead means; and
means for determining the lead impedance based upon the supply of impedance measuring current to the lead means.
29. The device of claim 28 further comprising impedance measuring current return means wherein the supplying means supplies impedance measuring current to the lead means and said return means.
band pass filter means having a center frequency essentially equal to the frequency of the impedance measuring current for filtering the lead impedance signal and for providing thereby a band passed signal; and
demodulator means for demodulating the band passed signal for providing a parameter representative of the lead impedance.
36. The device of claim 35 wherein the cardiac stimulation device further comprises:
pulse generator means for delivering therapeutic electrical stimulation to a patient's heart via the lead means; and
means for adjusting the electrical stimulation to said heart as a function of said parameter.
37. The device of claim 35 further comprising means for disabling a respective lead, when said parameter corresponding to said respective lead, exceeds a preselected maximum value or is less than a preselected minimum value.
The present invention relates to implantable cardiac stimulating devices, including devices having defibrillation or cardioversion leads and, more particularly, concerns an implantable cardiac stimulating device that is adapted to obtain an accurate impedance measurement of the cardioversion or defibrillation lead using a low amplitude, high frequency signal.
Implantable cardiac stimulating devices are devices that are adapted to be implanted within the body of a patient so that therapeutic electrical stimulation can be provided to the patient's heart to regulate heart function. These types of devices include well known pacemakers or implantable cardioverter defibrillators (ICDs) or devices that include the functionality of both a pacemaker and an ICD.
The aforementioned needs are satisfied by the implantable cardiac stimulating device of the present invention which is comprised of a control unit that is adapted to be implanted within the heart of a patient and at least one high voltage lead that is adapted to be positioned adjacent the heart so as to apply high voltage cardioversion or defibrillation shocks to the heart. The control unit also includes an impedance measurement circuit which is adapted to be able to provide a low amplitude, high frequency impedance measurement signal to the lead and then measure the resulting electrical response on a second electrode so that an impedance measurement can be obtained using a low amplitude signal. The resulting impedance measurement has a high correlation to the actual impedance that would occur when a high voltage cardioversion or defibrillation waveform is applied to the lead. In this way, the impedance of the particular lead can be accurately measured without consuming excess power and without being felt by the patient, while still obtaining a measurement that has a high correlation to the impedance that would actually occur when the high voltage cardioversion or defibrillation waveform is applied to the lead.
FIG. 1 is a block diagram of an implantable cardiac stimulating device incorporating an impedance measurement circuit of the present invention;
Reference will now be made to the drawings wherein like numerals refer to like parts throughout. FIG. 1 is a block diagram of an exemplary implantable cardiac stimulating device 100 of the preferred embodiment. The implantable cardiac stimulating device 100 is capable of providing both pacing pulses and higher voltage waveforms, such as cardioversion or defibrillation waveforms.
The voltage between the shocking electrodes 160, 170 is then amplified by an amplifier 202. The resulting voltage has a high frequency due to the use of a 50 KHz measurement signal. Accordingly, band pass filter 203 with a center frequency of the carrier frequency, i.e., 50 KHz, is used to process the signal. The purpose of this band pass filter is to reject erroneous signals that are not modulated at the carrier frequency. Specifically, it rejects any signal offset due to asymmetric ½ cell potentials. A synchronous demodulator 204 is then used to synchronously demodulate the bandpassed signal. The synchronous demodulator 204 is used to discard the carrier, e.g., 50 KHz, and obtain only the modulating signal representing the impedance signal. The synchronous demodulator 204 then provides a signal to a rectifier and filter circuit 206. Preferably, the rectifier and filter circuit 206 includes a low pass filter that will primarily pass only the synchronously demodulated signal that is between approximately 0 to 100 Hz and, more preferably, is between 0 to 20 Hz. The resulting signal contains three components. The first is the D.C. component of the signal. This represents the bulk tissue resistance between the shocking electrodes. The second is the changes to the bulk impedance due to respiration. The third is the change of the bulk resistance due to cardiac activity which can be used as a hemodynamic signal to indicate stroke volume. The use of this technique enables a good measurement while still only injecting a small amount (i.e., 500 microamps or less) of current (and corresponding voltage) into the electrodes. Modulating the signal at the carrier frequency enables the rejection of most noise signals that are not at 50 KHz. Examples of these rejected noise signals include 50 or 60 Hz power line noise and intrinsic cardiac electrograms. It will be appreciated that the impedance reading unit 210 will provide both a varying signal that has a mean or average value and also changes between a maximum and minimum value over the time period of the measurement signal that is supplied by the current source 200. Consequently, the microprocessor 112 will receive both a mean or average impedance value that closely correlates to the actual impedance value that would occur if a high voltage waveform was applied to the heart 104 and a delta impedance value that is indicative of the change of the impedance measurement over the time period that the high frequency impedance measurement signal is applied to the leads 106 from the current source 200.
FIG. 4 is an equivalent circuit which illustrates the interconnection between the current source 200 and the load resistance of the heart 104, designated herein as RL. As illustrated in FIG. 4, the equivalent circuit of the interconnection between the leads 106 and the heart 104 includes a resistance Rf in parallel with a resistance Rw, a V½ cell, and a capacitor Cw. As is understood in the art, the interface between the electrodes, e.g., 160, 170, that are connected in series with the heart 104 (RL), is comprised of a Faraday resistance Rf in parallel with a Warberg resistance Rw and Warberg capacitor Cw and the electrode/electrolyte ½ cell potential, the V½ cell. The equivalent circuit in FIG. 4 demonstrates why the impedance measured using prior art systems that provide low voltage, low frequency waveforms does not provide a realistic approximation of the corresponding impedance of the lead that would occur when a high voltage waveform is applied to the heart.
In particular, there is capacitive coupling between the load resistance RL of the heart 104 and the voltage source that is applied to the heart 104 via the leads 106. Further, the degree of the capacitive coupling Cw varies depending upon the magnitude of the waveform that is applied to the heart 104. In addition, if the electrodes are made of dissimilar metals, then the difference in the ½ cell potential between them will further introduce an error to the impedance measurement. Consequently, the impedance measured using a low voltage, low frequency waveform cannot be readily used to calculate the impedance that would occur when the high voltage waveform is applied. Consequently, the prior art systems that use low voltage waveforms are inherently inaccurate in obtaining an indication as to the magnitude of the impedance that would occur on the leads 106 when a high voltage waveform is applied.
Energy=I 2 R*d=(100 μA*0.707 RMS/peak)2*50Ω*1 second
Energy=2.45 millijoules
Hence, the impedance measurement can be obtained using a limited amount of energy, e.g., less than 5 millijoules. This is particularly advantageous with implantable cardiac stimulating devices since it is generally desirable to limit the dissipation of energy from the battery so as to preserve the active life of the device.
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Cooperative Classification A61N1/3931, A61N2001/083
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