U.S. patent application number 12/963239 was filed with the patent office on 2011-03-31 for method and apparatus for detecting fibrillation using cardiac local impedance.
Invention is credited to Andres Belalcazar.
Application Number | 20110077540 12/963239 |
Document ID | / |
Family ID | 39339801 |
Filed Date | 2011-03-31 |
United States Patent
Application |
20110077540 |
Kind Code |
A1 |
Belalcazar; Andres |
March 31, 2011 |
METHOD AND APPARATUS FOR DETECTING FIBRILLATION USING CARDIAC LOCAL
IMPEDANCE
Abstract
A cardiac rhythm management (CRM) system detects tachyarrhythmia
using cardiac local impedance indicative of cardiac local wall
motion. A cardiac local impedance signal indicative of an impedance
of a cardiac region is sensed by using a pair of bipolar electrodes
placed in that cardiac region. Tachyarrhythmia such as VF is
detected by analyzing one or more cardiac local impedance signals
sensed in one or more cardiac regions.
Inventors: |
Belalcazar; Andres; (St.
Paul, MN) |
Family ID: |
39339801 |
Appl. No.: |
12/963239 |
Filed: |
December 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11550923 |
Oct 19, 2006 |
7890163 |
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12963239 |
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Current U.S.
Class: |
600/510 ;
600/518; 607/17 |
Current CPC
Class: |
A61N 1/3956 20130101;
A61N 1/3622 20130101 |
Class at
Publication: |
600/510 ;
600/518; 607/17 |
International
Class: |
A61B 5/0464 20060101
A61B005/0464; A61B 5/0402 20060101 A61B005/0402; A61N 1/365
20060101 A61N001/365 |
Claims
1. A method for detecting tachyarrhythmia, the method comprising:
sensing a cardiac local impedance signal indicative of a cardiac
local wall motion using a first pair of impedance sensing
electrodes at a distal end of a first implantable lead; and
detecting a predetermined-type tachyarrhythmia using the cardiac
local impedance signal.
2. The method of claim 1, further comprising sensing one or more
electrograms, and wherein detecting the predetermined-type
tachyarrhythmia comprises detecting the predetermined-type
tachyarrhythmia using the cardiac local impedance signal and the
one or more electrograms.
3. The method of claim 1, wherein sensing the cardiac local
impedance signal comprises: delivering current pulses through the
first pair of impedance sensing electrodes at a frequency between
approximately 3 and 500 Hz, the current pulses each have an
amplitude between approximately 20 microamperes and 400
microamperes and a pulse width between approximately 10
microseconds and 100 microseconds; sensing a voltage across the
first pair of impedance sensing electrodes; and producing the
cardiac local impedance signal using the sensed voltage.
4. The method of claim 3, wherein producing the cardiac local
impedance signal comprises producing the cardiac local impedance
signal as a ratio of the sensed voltage to the amplitude of the
delivered current pulses.
5. The method of claim 1, wherein detecting the predetermined-type
tachyarrhythmia comprises: producing a ventricular fibrillation
(VF) detection zone specified by one or more threshold amplitudes;
and indicating a VF detection when the cardiac local impedance
signal falls into the VF detection zone.
6. The method of claim 5, wherein detecting the predetermined-type
tachyarrhythmia further comprises adjusting the VF detection zone
based on a trend of the cardiac local impedance signal.
7. The method of claim 1, further comprising producing a cardiac
local impedance derivative signal indicative of a rate of change in
the cardiac local impedance, and wherein detecting the
predetermined-type tachyarrhythmia comprises detecting the
predetermined-type tachyarrhythmia using the cardiac local
impedance derivative signal.
8. The method of claim 7, wherein producing the cardiac local
impedance derivative signal comprises using a high-pass filter
having a cutoff frequency between approximately 0.1 Hz and 1
Hz.
9. The method of claim 7, wherein detecting the predetermined-type
tachyarrhythmia comprises: detecting an impedance event by
comparing the cardiac local impedance derivative signal to an event
threshold, the impedance event representative of a cardiac local
wall motion during a systolic phase of each cardiac cycle; and
adjusting the event threshold based on a trend of the cardiac local
impedance derivative signal.
10. The method of claim 1, wherein sensing the cardiac local
impedance signal using the first pair of impedance sensing
electrodes at the distal end of the first implantable lead
comprises sensing a left ventricular (LV) local impedance signal
(LVZ) indicative of an LV local impedance using a pair of LV
impedance sensing electrodes at a distal end of an implantable LV
lead.
11. The method of claim 10, wherein the LV impedance sensing
electrodes are spaced within approximately 40 millimeters.
12. The method of claim 10, further comprising sensing a right
ventricular (RV) local impedance signal (RVZ) indicative of an RV
local impedance using a pair of RV impedance sensing electrodes at
a distal end of an implantable RV lead.
13. The method of claim 12, wherein the RV impedance sensing
electrodes are spaced within approximately 20 millimeters.
14. The method of claim 12, further comprising: producing an LV
local impedance derivative signal (LV dZ/dT) using the LVZ, the LV
dZ/dT indicative of a rate of change in the LVZ; and producing an
RV local impedance derivative signal (RV dZ/dT) using the RVZ, the
RV dZ/dT indicative of a rate of change in the RVZ, and wherein
detecting the predetermined-type tachyarrhythmia comprises
detecting ventricular fibrillation (VF) using the LV dZ/dT and the
RV dZ/dT.
15. The method of claim 14, wherein detecting VF comprises:
detecting LV impedance events by comparing the LV local impedance
derivative signal (LV dZ/dT) to an LV event threshold, the LV
impedance events each representative of an LV local wall motion
during a systolic phase of a cardiac cycle; and detecting RV
impedance events by comparing the RV local impedance derivative
signal (RV dZ/dT) to an RV event threshold, the RV impedance events
each representative of an RV local wall motion during the systolic
phase of the cardiac cycle.
16. The method of claim 15, wherein detecting VF further comprises
determining whether a pattern of the LV impedance events and the RV
impedance events indicates a degree of dyssynchrony between the LV
and RV local wall motions that exceeds a predetermined threshold
degree.
17. The method of claim 1, wherein detecting the predetermined-type
tachyarrhythmia comprises detecting ventricular fibrillation
(VF).
18. The method of claim 17, further comprising producing a cardiac
local impedance derivative signal indicative of a rate of change in
the cardiac local impedance, and wherein detecting the VF comprises
detecting the VF using the cardiac local impedance derivative
signal.
19. The method of claim 17, further comprising delivering a
defibrillation pulse in response to a detection of the VF.
20. The method of claim 19, wherein sensing the cardiac local
impedance signal comprises sensing the cardiac local impedance
signal using two impedance sensing electrodes spaced at a distance
between approximately 2 millimeters and 40 millimeters.
Description
CLAIM OF PRIORITY
[0001] This application is a divisional of and claims the benefit
of priority under 35 U.S.C. .sctn.120 to U.S. patent application
Ser. No. 11/550,923, filed on Oct. 19, 2006, which is hereby
incorporated by reference herein in its entirety.
TECHNICAL FIELD
[0002] This document relates generally to cardiac rhythm management
(CRM) systems and particularly to an anti-tachyarrhythmia system
that detects fibrillation using cardiac local impedance indicative
of cardiac local wall motion.
BACKGROUND
[0003] Tachyarrhythmias are abnormal heart rhythms characterized by
a rapid heart rate. Tachyarrhythmias generally include
supraventricular tachyarrhythmia (SVT, including atrial
tachyarrhythmia, AT) and ventricular tachyarrhythmia (VT).
Fibrillation is a form of tachyarrhythmia further characterized by
an irregular heart rhythm. In a normal heart, the sinoatrial node,
the heart's predominant natural pacemaker, generates electrical
impulses, called action potentials, that propagate through an
electrical conduction system to the atria and then to the
ventricles of the heart to excite the myocardial tissues. The atria
and ventricles contract in the normal atrio-ventricular sequence
and synchrony to result in efficient blood-pumping functions
indicated by a normal hemodynamic performance. VT occurs when the
electrical impulses propagate along a pathologically formed
self-sustaining conductive loop within the ventricles or when a
natural pacemaker in a ventricle usurps control of the heart rate
from the sinoatrial node. When the atria and the ventricles become
dissociated during VT, the ventricles may contract before they are
properly filed with blood, resulting in diminished blood flow
throughout the body. This condition becomes life-threatening when
the brain is deprived of sufficient oxygen supply. Ventricular
fibrillation (VF), in particular, stops blood flow within seconds
and, if not timely and effectively treated, causes immediate death.
In very few instances a heart recovers from VF without
treatment.
[0004] Cardioversion and defibrillation are used to terminate most
tachyarrhythmias, including AT, VT, and VF. An implantable
cardioverter/defibrillator (ICD) is a CRM device that delivers an
electric shock to terminate a detected tachyarrhythmia episode by
depolarizing the entire myocardium simultaneously and rendering it
refractory. Another type of electrical therapy for tachyarrhythmia
is anti-tachyarrhythmia pacing (ATP). In ATP, the heart is
competitively paced in an effort to interrupt the reentrant loop
causing the tachyarrhythmia. An exemplary ICD includes ATP and
defibrillation capabilities so that ATP is delivered to the heart
when a non-fibrillation VT is detected, while a defibrillation
shock is delivered when VF occurs.
[0005] The efficacy of cardioversion, defibrillation, and ATP in
terminating tachyarrhythmia depends on the type and origin of the
tachyarrhythmia. An unnecessary therapy delivered during a
non-life-threatening tachyarrhythmia episode may cause substantial
pain in the patient and reduces the longevity of the ICD while
providing the patient with little or no benefit. On the other hand,
a necessary therapy withheld during a life-threatening
tachyarrhythmia episode may result in irreversible harm to the
patient, including death. For these and other reasons, there is a
need for accurate tachyarrhythmia detection that ensures patient
safety while reducing unnecessary delivery of anti-tachyarrhythmia
therapy.
SUMMARY
[0006] A CRM system detects tachyarrhythmia using cardiac local
impedance indicative of cardiac local wall motion. A cardiac local
impedance signal indicative of an impedance of a cardiac region is
sensed by using a pair of bipolar electrodes placed in that cardiac
region. Tachyarrhythmia such as VF is detected by analyzing one or
more cardiac local impedance signals sensed in one or more cardiac
regions.
[0007] In one embodiment, a CRM system includes an implantable lead
and an implantable medical device. The implantable lead includes a
proximal end, a distal end, and an elongate lead body coupled
between the proximal end and the distal end. The proximal end is to
be coupled to the implantable medical device. The distal end is to
be placed in the heart and includes a pair of impedance sensing
electrodes for sensing a cardiac local impedance signal. The
implantable medical device includes an impedance sensing circuit
and an impedance-based tachyarrhythmia detector. The impedance
sensing circuit senses the cardiac local impedance signal using the
pair of impedance sensing electrodes. The impedance-based
tachyarrhythmia detector detects a predetermined-type
tachyarrhythmia using the cardiac local impedance signal.
[0008] In one embodiment, a method for detecting tachyarrhythmia is
provided. A cardiac local impedance signal is sensed using a pair
of impedance sensing electrodes at a distal end of an implantable
lead. A predetermined-type tachyarrhythmia is detected using the
cardiac local impedance signal.
[0009] This Summary is an overview of some of the teachings of the
present application and not intended to be an exclusive or
exhaustive treatment of the present subject matter. Further details
about the present subject matter are found in the detailed
description and appended claims. Other aspects of the invention
will be apparent to persons skilled in the art upon reading and
understanding the following detailed description and viewing the
drawings that form a part thereof, each of which are not to be
taken in a limiting sense. The scope of the present invention is
defined by the appended claims and their legal equivalents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The drawings, which are not necessarily drawn to scale,
illustrate generally, by way of example, but not by way of
limitation, various embodiments discussed in the present
document.
[0011] FIG. 1 is an illustration of an embodiment of a CRM system
and portions of the environment in which the CRM system
operates.
[0012] FIG. 2 is an illustration of an embodiment of cardiac local
impedance sensing.
[0013] FIG. 3 is an illustration of examples of signals resulting
from cardiac local impedance sensing.
[0014] FIG. 4 is a block diagram illustrating an embodiment of an
implantable medical device of the CRM system.
[0015] FIG. 5 is a block diagram illustrating an embodiment of an
impedance sensing circuit of the implantable medical device.
[0016] FIG. 6 is a block diagram illustrating another embodiment of
the impedance sensing circuit.
[0017] FIG. 7 is a block diagram illustrating an embodiment of an
arrhythmia detection circuit of the implantable medical device.
[0018] FIG. 8 is a block diagram illustrating an embodiment of an
impedance-based VF detector of the arrhythmia detection
circuit.
[0019] FIG. 9 is a block diagram illustrating another embodiment of
the impedance-based VF detector.
[0020] FIG. 10 is a block diagram illustrating another embodiment
of the impedance-based VF detector.
[0021] FIG. 11 is a flow chart illustrating an embodiment of a
method for detecting tachyarrhythmia using cardiac local
impedance.
[0022] FIG. 12 is a flow chart illustrating an embodiment of a
method for detecting VF using cardiac local impedance.
DETAILED DESCRIPTION
[0023] In the following detailed description, reference is made to
the accompanying drawings which form a part hereof, and in which is
shown by way of illustration specific embodiments in which the
invention may be practiced. These embodiments are described in
sufficient detail to enable those skilled in the art to practice
the invention, and it is to be understood that the embodiments may
be combined, or that other embodiments may be utilized and that
structural, logical and electrical changes may be made without
departing from the scope of the present invention. The following
detailed description provides examples, and the scope of the
present invention is defined by the appended claims and their legal
equivalents.
[0024] In this document, the terms "a" or "an" are used, as is
common in patent documents, to include one or more than one. In
this document, the term "or" is used to refer to a nonexclusive or,
unless otherwise indicated. Furthermore, all publications, patents,
and patent documents referred to in this document are incorporated
by reference herein in their entirety, as though individually
incorporated by reference. In the event of inconsistent usages
between this documents and those documents so incorporated by
reference, the usage in the incorporated reference(s) should be
considered supplementary to that of this document; for
irreconcilable inconsistencies, the usage in this document
controls.
[0025] It should be noted that references to "an", "one", or
"various" embodiments in this document are not necessarily to the
same embodiment, and such references contemplate more than one
embodiment.
[0026] This document discusses a CRM system that detects
tachyarrhythmia episodes using cardiac local impedance indicative
of cardiac local wall motion. A tachyarrhythmia episode is detected
by detecting one or more abnormalities in the mechanical activities
of the heart. A cardiac local impedance signal indicative of a
cardiac local impedance of a cardiac region is sensed by bipolar
electrodes, such as bipolar electrodes on a pacing or
defibrillation lead, placed in that cardiac region. Tachyarrhythmia
such as VF is detected by analyzing one or more cardiac local
impedance signals sensed in one or more cardiac regions, or one or
more cardiac local impedance derivative signals each indicative of
the rate of change in one of the one or more cardiac local
impedances. For example, VF is detected by analyzing a motion
pattern of a cardiac region indicated by the cardiac local
impedance signal sensed from that cardiac region, or by analyzing
the synchrony of local wall motions in two cardiac regions
indicated by the cardiac local impedance signals sensed from those
two cardiac regions.
[0027] In this document, an "impedance signal" or "Z" includes a
signal indicative of impedance. In one embodiment, the impedance
signal is produced as a ratio of a sensed voltage to a current
delivered for impedance sensing. In another embodiment, the
impedance is a sensed voltage signal indicative of impedance, for
example, when the current delivered for impedance sensing is from a
constant-current source. An "impedance derivative signal" or
"dZ/dT" indicates a rate of change in the impedance signal. For
example, a "cardiac local impedance signal (Z)" includes a signal
indicative of a cardiac local (regional) impedance, a "cardiac
local impedance derivative signal (dZ/dT)" in indicates a rate of
change in the cardiac local impedance, a "left ventricular (LV)
local impedance signal (LVZ)" includes a signal indicative of an LV
local (regional) impedance, an "LV local impedance derivative
signal (dZ/dT)" in indicates a rate of change in the LV local
impedance signal, a "right ventricular (RV) local impedance signal
(RVZ)" includes a signal indicative of an RV local (regional)
impedance, an "RV local impedance derivative signal (dZ/dT)" in
indicates a rate of change in the RV local impedance.
[0028] As discussed in this document, the cardiac local impedance
is indicative of cardiac local wall motion, which includes
thickening of the cardiac wall due to systolic contraction and
reorientation of impedance sensing electrodes relative to the
contracting myocardium. The cardiac local impedance is also
affected by displacement of blood in the myocardium due to its
contraction.
[0029] FIG. 1 is an illustration a CRM system 100 and portions of
an environment in which system 100 operates. CRM system 100
includes an implantable medical device 105 that is electrically
coupled to a heart through implantable leads 110, 115, and 125. An
external system 190 communicates with implantable medical device
105 via a telemetry link 185.
[0030] Implantable medical device 105 includes a hermetically
sealed can housing an electronic circuit that senses physiological
signals and delivers therapeutic electrical pulses. The
hermetically sealed can also functions as an electrode for sensing
and/or pulse delivery purposes. In one embodiment, implantable
medical device 105 includes an arrhythmia detection circuit that
detects tachyarrhythmias and determines whether a therapy is to be
delivered from implantable medical device 105. For example, if VF
is detected, implantable medical device 105 delivers a
defibrillation therapy. In one embodiment, implantable medical
device 105 is an ICD with cardiac pacing capabilities. In another
embodiment, in addition to a pacemaker and a
cardioverter/defibrillator, implantable medical device 105 further
includes one or more of other monitoring and/or therapeutic devices
such as a neural stimulator, a drug delivery device, and a
biological therapy device.
[0031] Lead 110 is a right atrial (RA) pacing lead that includes an
elongate lead body having a proximal end 111 and a distal end 113.
Proximal end 111 is coupled to a connector for connecting to
implantable medical device 105. Distal end 113 is configured for
placement in the RA in or near the atrial septum. Lead 110 includes
an RA tip electrode 114A, and an RA ring electrode 114B. RA
electrodes 114A and 114B are incorporated into the lead body at
distal end 113 for placement in or near the atrial septum, and are
each electrically coupled to implantable medical device 105 through
a conductor extending within the lead body. RA tip electrode 114A,
RA ring electrode 114B, and/or the can of implantable medical
device 105 allow for sensing an RA electrogram indicative of RA
depolarizations and delivering RA pacing pulses. In one embodiment,
RA electrodes 114A and 114B function as a pair of RA impedance
sensing electrodes for sensing an RA local impedance signal. The
distance between RA tip electrode 114A and RA ring electrode 114B
is in a range of approximately 2 millimeters to 20 millimeters,
with approximately 5 millimeters being a specific example.
[0032] Lead 115 is a right ventricular (RV) pacing-defibrillation
lead that includes an elongate lead body having a proximal end 117
and a distal end 119. Proximal end 117 is coupled to a connector
for connecting to implantable medical device 105. Distal end 119 is
configured for placement in the RV. Lead 115 includes a proximal
defibrillation electrode 116, a distal defibrillation electrode
118, an RV tip electrode 120A, and an RV ring electrode 120B.
Defibrillation electrode 116 is incorporated into the lead body in
a location suitable for supraventricular placement in the RA and/or
the superior vena cava. Defibrillation electrode 118 is
incorporated into the lead body near distal end 119 for placement
in the RV. RV electrodes 120A and 120B are incorporated into the
lead body at distal end 119. Electrodes 116, 118, 120A, and 120B
are each electrically coupled to implantable medical device 105
through a conductor extending within the lead body. Proximal
defibrillation electrode 116, distal defibrillation electrode 118,
and/or the can of implantable medical device 105 allow for delivery
of cardioversion/defibrillation pulses to the heart. RV tip
electrode 120A, RV ring electrode 120B, and/or the can of
implantable medical device 105 allow for sensing an RV electrogram
indicative of RV depolarizations and delivering RV pacing pulses.
In one embodiment, RV electrodes 120A and 120B function as a pair
of RV impedance sensing electrodes for sensing an RV local
impedance signal. The distance between RV tip electrode 120A and RV
ring electrode 120B is in a range of approximately 2 millimeters to
20 millimeters, with approximately 8 millimeters being a specific
example.
[0033] Lead 125 is a left ventricular (LV) coronary pacing lead
that includes an elongate lead body having a proximal end 121 and a
distal end 123. Proximal end 121 is coupled to a connector for
connecting to implantable medical device 105. Distal end 123 is
configured for placement in the coronary vein. Lead 125 includes an
LV tip electrode 128A and an LV ring electrode 128B. The distal
portion of lead 125 is configured for placement in the coronary
sinus and coronary vein such that LV electrodes 128A and 128B are
placed in the coronary vein. LV electrodes 128A and 128B are
incorporated into the lead body at distal end 123 and each
electrically coupled to implantable medical device 105 through a
conductor extending within the lead body. LV tip electrode 128A, LV
ring electrode 128B, and/or the can of implantable medical device
105 allow for sensing an LV electrogram indicative of LV
depolarizations and delivering LV pacing pulses. In one embodiment,
LV electrodes 128A and 128B function as a pair of LV impedance
sensing electrodes for sensing an LV local impedance signal. The
distance between LV tip electrode 128A and LV ring electrode 128B
is in a range of approximately 2 millimeters to 40 millimeters,
with approximately 11 millimeters being a specific example.
[0034] In various embodiments, one or more pairs of impedance
sensing electrodes are used, with each pair configured to sense a
cardiac local impedance signal. The impedance sensing electrodes of
each pair are spaced to sense an impedance that is indicative of
local wall motion in a cardiac region. Each impedance sensing
electrode may also be used for sensing an electrogram and/or
delivering pacing or defibrillation pulses. The lead configuration
including RA lead 110, RV lead 115, and LV lead 125 is illustrated
in FIG. 1 as an example. Other lead configurations may be used,
depending on monitoring and therapeutic requirements. For example,
additional leads may be used to provide access to additional
cardiac regions, and leads 110, 115, and 125 may each include more
or fewer electrodes along the lead body at, near, and/or distant
from the distal end, depending on specified monitoring and
therapeutic needs.
[0035] External system 190 allows for programming of implantable
medical device 105 and receives signals acquired by implantable
medical device 105. In one embodiment, telemetry link 185 is an
inductive telemetry link. In an alternative embodiment, telemetry
link 185 is a far-field radio-frequency telemetry link. Telemetry
link 185 provides for data transmission from implantable medical
device 105 to external system 190. This may include, for example,
transmitting real-time physiological data acquired by implantable
medical device 105, extracting physiological data acquired by and
stored in implantable medical device 105, extracting therapy
history data stored in implantable medical device 105, and
extracting data indicating an operational status of implantable
medical device 105 (e.g., battery status and lead impedance).
Telemetry link 185 also provides for data transmission from
external system 190 to implantable medical device 105. This may
include, for example, programming implantable medical device 105 to
acquire physiological data, programming implantable medical device
105 to perform at least one self-diagnostic test (such as for a
device operational status), programming implantable medical device
105 to run a signal analysis algorithm (such as an algorithm
implementing the tachyarrhythmia detection method discussed in this
document), and programming implantable medical device 105 to
deliver pacing and/or cardioversion/defibrillation therapies.
[0036] The circuit of CRM system 100 may be implemented using a
combination of hardware and software. In various embodiments, each
element of implantable medical device 105 as illustrated in FIGS.
2-8, including its specific embodiments, may be implemented using
an application-specific circuit constructed to perform one or more
particular functions or a general-purpose circuit programmed to
perform such function(s). Such a general-purpose circuit includes,
but is not limited to, a microprocessor or portions thereof, a
microcontroller or portions thereof, and a programmable logic
circuit or portions thereof. For example, a "comparator" includes,
among other things, an electronic circuit comparator constructed to
perform the only function of comparing two or more signals or a
portion of a general-purpose circuit driven by a code instructing
that portion of the general-purpose circuit to perform the
comparing.
[0037] FIG. 2 is an illustration of an embodiment of cardiac local
impedance sensing. A lead 215 represents portions of lead 115
including RV electrodes 120A and 120B function as a pair of RV
impedance sensing electrodes. A lead 225 represents portions of
lead 125 including LV electrodes 128A and 128B function as a pair
of LV impedance sensing electrodes. RV electrodes 120A and 120B are
used for injecting a current 221 and sensing the resulting voltage
indicative of the RV local impedance. LV electrodes 128A and 128B
are used for injecting a current 229 and sensing the resulting
voltage indicative of the LV local impedance. The cardiac local
impedance is sensed using two closely spaced impedance sensing
electrodes (e.g., within 20 millimeters for the RA or RV, or within
40 millimeters for the LV) placed over or near the myocardium. In
one embodiment, the distance between the two impedance sensing
electrodes is within approximately 20 millimeters. The sensed
cardiac local impedance signal is indicative of local motion and/or
geometrical changes of the myocardial region in the vicinity of the
impedance sensing electrodes.
[0038] In this document, an signal sensed or event detected using
an RV lead such as lead 115 or 215 is referred to as an "RV" signal
or an "RV" event, and an signal sensed or event detected using an
LV lead such as lead 125 or 225 is referred to as an "LV" signal or
an "LV" event. For example, when electrode 120A and 120B are used
to deliver pacing pulse to the RV-LV septum to control LV
activation, the cardiac local impedance sensed using these two
electrodes are still referred to as an RV local impedance
indicative of RV local motion. An "interventricular delay" between
an RV event and an LV event includes a delay between an event
detected using an RV lead such as lead 115 or 215 and an event
detected using an LV lead such as lead 125 or 225.
[0039] FIG. 3 is an illustration of examples of signals resulting
from cardiac local impedance sensing. The illustrated signals
include an LV impedance signal (LVZ), an LV impedance derivative
signal (LV dZ/dT), an RV impedance signal (RVZ), and an RV
impedance derivative signal (RV dZ/dT), sensed during a regular
cardiac rhythm.
[0040] The LVZ represents an impedance signal sensed using LV
electrodes 128A and 128B. The LV dZ/dT represents the rate of
change in the LVZ. The RVZ represents an impedance signal sensed
using RV electrodes 120A and 120B. The RV dZ/dT represents the rate
of change in the RVZ. These signals and their uses are further
discussed below. The LV dZ/dT includes LV impedance events 326. The
RV dZ/dT includes RV impedance events 327. In one embodiment, such
impedance events are each representative of a cardiac local wall
motion during the systolic phase of each cardiac cycle. The LV
impedance events each represent the LV local wall motion during the
systolic phase of each cardiac cycle. The RV impedance events each
represent the RV local wall motion during the systolic phase of
each cardiac cycle. During a normal sinus rhythm or a tachycardia
with a regular rhythm, such illustrated in FIG. 3, the LV and RV
contract in synchrony, and the LV and RV impedance events during
each cardiac cycle occur approximately simultaneously or within a
limited interventricular delay.
[0041] FIG. 4 is a block diagram illustrating an embodiment of an
implantable medical device 405. Implantable medical device 405 is a
specific embodiment of an implantable medical device 105 and
includes a cardiac sensing circuit 430, a pacing circuit 432, a
defibrillation circuit 434, an impedance sensing circuit 436, an
arrhythmia detection circuit 438, an implant controller 440, and an
implant telemetry circuit 442. Cardiac sensing circuit 430 senses
one or more electrograms from the heart through electrodes such as
those selected from RA electrodes 114A and 114B, RV electrodes 120A
and 120B, LV electrodes 128A and 128B, and the can of implantable
medical device 405. Pacing circuit 432 delivers pacing pulses to
the heart through electrodes such as those selected from RA
electrodes 114A and 114B, RV electrodes 120A and 120B, LV
electrodes 128A and 128B, and the can of implantable medical device
405. Defibrillation circuit 434 delivers
cardioversion/defibrillation pulses through electrodes such as
those selected from defibrillation electrodes 116 and 118 and the
can of implantable medical device 405. Impedance sensing circuit
436 produces one or more cardiac local impedance signals each by
sensing a voltage across a pair of impedance sensing electrodes
placed in a cardiac region. In one embodiment, impedance sensing
circuit 436 produces each cardiac local impedance signal as the
ratio of the sensed voltage to a current delivered for the
impedance sensing. In another embodiment, impedance sensing circuit
436 produces each cardiac local impedance signal by isolating the
signal component indicative of the cardiac local impedance from the
sensed voltage, when the current delivered for the impedance
sensing is from a constant-current source. Examples of the pair of
impedance sensing electrodes include the pair of RA impedance
sensing electrodes 114A and 114B, the pair of LV impedance sensing
electrodes 128A and 128B, and the pair of RV impedance sensing
electrodes 120A and 120B. Arrhythmia detection circuit 438 detects
tachyarrhythmias using at least the sensed one or more cardiac
local impedance signals. Implant controller 440 controls the
operation of implantable medical device 405, including delivery of
an anti-tachyarrhythmia therapy in response to the detection of
tachyarrhythmia, such as the delivery of a ventricular
defibrillation therapy in response to a detection of VF. Implant
telemetry circuit 442 receives signals from, and transmits signals
to, external system 190 via telemetry link 185.
[0042] FIG. 5 is a block diagram illustrating an embodiment of an
impedance sensing circuit 536. Impedance sensing circuit 536 is a
specific embodiment of impedance sensing circuit 436 and includes a
current source circuit 546, a voltage sensing circuit 548, an
impedance detector 552, and a differentiator 554.
[0043] Current source circuit 546 includes delivers a current
through a pair of impedance sensing electrodes. In one embodiment,
current source circuit 546 delivers constant current pulses at a
frequency between approximately 3 Hz and 500 Hz, with approximately
20 Hz as a specific example. The constant current pulses each have
an amplitude between approximately 20 microamperes and 400
microamperes, with approximately 80 microamperes as a specific
example, and a pulse width between approximately 10 microseconds
and 100 microseconds, with approximately 40 microseconds as a
specific example. Voltage sensing circuit 548 senses a voltage
across the pair of impedance sensing electrodes and produces a
sensed voltage. Impedance detector 552 produces a cardiac local
impedance signal (Z) using the sensed voltage. In one embodiment,
impedance detector 552 produces the cardiac local impedance signal
(Z) as a ratio of the voltage sensed by voltage sensing circuit 548
to the current delivered from current source circuit 546. In
another embodiment, impedance detector 552 produces the cardiac
local impedance signal (Z) by isolating the signal component
indicative of the cardiac local impedance from the voltage sensed
by voltage sensing circuit 548, when the current delivered from
current source circuit 546 is in the form of constant current
pulses. Differentiator 554 produces a cardiac local impedance
derivative signal (dZ/dT) that indicates the rate of change in the
cardiac local impedance. In one embodiment, differentiator 554
includes a high-pass filter having a cutoff frequency between
approximately 0.1 Hz and 1 Hz, with approximately 0.5 Hz being a
specific example.
[0044] FIG. 6 is a block diagram illustrating an embodiment of an
impedance sensing circuit 636. Impedance sensing circuit 636 is a
specific embodiment of impedance sensing circuit 536 that allows
for sensing of multiple cardiac local impedance signals. In the
illustrated embodiment, impedance sensing circuit 636 includes two
impedance sensing sub-circuits: an LV impedance sensing circuit
636A and an RV impedance sensing circuit 636B sensing. LV impedance
sensing circuit 636A produces an LV local impedance signal
indicative of an LV local wall motion. RV impedance sensing circuit
636B produces an RV local impedance signal indicative of an RV
local wall motion. In other embodiments, impedance sensing circuit
636 includes two or more impedance sensing sub-circuits each
sensing a local impedance signal in a cardiac region.
[0045] LV impedance sensing module 636A includes an LV current
source circuit 646A, an LV voltage sensing circuit 648A, an LV
impedance detector 652A, and an LV differentiator 654A. LV current
source circuit 646A delivers an LV current through a pair of LV
impedance sensing electrodes, such as LV electrodes 128A and 128B.
LV voltage sensing circuit 648A senses an LV voltage across the
pair of LV impedance sensing electrodes. LV impedance detector 652A
produces an LV local impedance signal (LVZ). In one embodiment, LV
impedance detector 652A produces the LV local impedance signal
(LVZ) as a ratio of the LV voltage to the LV current. In another
embodiment, LV impedance detector 652A produces the LV local
impedance signal (LVZ) by isolating the signal component indicative
of the LV local impedance from the LV voltage, when the LV current
is delivered as constant-current pulses. LV differentiator 654A
produces an LV local impedance derivative signal (LV dZ/dT), which
indicates the rate of change in the LV local impedance.
[0046] RV impedance sensing module 636B includes an RV current
source circuit 646B, an RV voltage sensing circuit 648B, an RV
impedance detector 652B, and an RV differentiator 654B. RV current
source circuit 646B delivers an RV current through a pair of RV
impedance sensing electrodes, such as RV electrodes 120A and 120B.
RV voltage sensing circuit 648B senses an RV voltage across the
pair of RV impedance sensing electrodes. RV impedance detector 652B
produces an RV local impedance signal (RVZ). In one embodiment, RV
impedance detector 652B produces the RV local impedance signal
(RVZ) as a ratio of the RV voltage to the RV current. In another
embodiment, RV impedance detector 652B produces the RV local
impedance signal (RVZ) by isolating the signal component indicative
of the RV local impedance from the LV voltage, when the RV current
is delivered as constant-current pulses. RV differentiator 654B
produces an RV local impedance derivative signal (RV dZ/dT), which
indicates the rate of change in the RV local impedance.
[0047] FIG. 7 is a block diagram illustrating an embodiment of an
arrhythmia detection circuit 738, which is a specific embodiment of
arrhythmia detection circuit 438. In the illustrated embodiment,
arrhythmia detection circuit 738 includes an impedance-based
tachyarrhythmia detector 758 and an electrogram-based
tachyarrhythmia detector 760. Impedance-based tachyarrhythmia
detector 758 detects tachyarrhythmia using one or more cardiac
local impedance signals. Electrogram-based tachyarrhythmia detector
760 detects tachyarrhythmia using one or more electrograms. In one
embodiment, arrhythmia detection circuit 738 detects a
predetermined-type tachyarrhythmia using the one or more cardiac
local impedance signals and the one or more electrograms. In one
embodiment, a detection of the predetermined-type tachyarrhythmia
is indicated when impedance-based tachyarrhythmia detector 758 and
electrogram-based tachyarrhythmia detector 760 both indicate a
detection of the tachyarrhythmia. In another embodiment, a
detection of the predetermined-type tachyarrhythmia is indicated
using weighted outputs of impedance-based tachyarrhythmia detector
758 and electrogram-based tachyarrhythmia detector 760. In one
embodiment, impedance-based tachyarrhythmia detector 758 and
electrogram-based tachyarrhythmia detector 760 supplement each
other in tachyarrhythmia detection. For example, electrogram-based
tachyarrhythmia detector 760 detects a fast heart rate, and
impedance-based tachyarrhythmia detector 758 is activated in
response to a detection of the fast heart rate. In one embodiment,
arrhythmia detection circuit 738 includes only impedance-based
tachyarrhythmia detector 758.
[0048] FIG. 8 is a block diagram illustrating an embodiment of an
impedance-based VF detector 858. Impedance-based VF detector 858 is
a specific embodiment of impedance-based tachyarrhythmia detector
758 and includes a comparator 862 and a detection zone generator
864.
[0049] In one embodiment, impedance-based VF detector 858 detects
VF using a cardiac local impedance signal (Z). Detection zone
generator 864 produces a VF detection zone specified by one or more
threshold amplitudes. Comparator 862 has a signal input that
receives the cardiac local impedance signal (Z), one or more
threshold inputs that receives the one or more threshold
amplitudes, and an output that indicates a VF detection when the
amplitude of the cardiac local impedance signal (Z) falls into the
VF detection zone. In one embodiment, detection zone generator 864
adjusts the VF detection zone based on a trend of the cardiac local
impedance signal (Z).
[0050] In one embodiment, impedance-based VF detector 858 detects
VF using a cardiac local impedance derivative signal (dZ/dT).
Detection zone generator 864 produces a VF detection zone specified
by one or more threshold amplitudes. Comparator 862 has a signal
input that receives the cardiac local impedance derivative signal
(dZ/dT), one or more threshold inputs that receives the one or more
threshold amplitudes, and an output that indicates a VF detection
when the amplitude of the cardiac local impedance derivative signal
(dZ/dT) falls into the VF detection zone. In one embodiment,
detection zone generator 864 adjusts the VF detection zone based on
a trend of the cardiac local impedance derivative signal
(dZ/dT).
[0051] FIG. 9 is a block diagram illustrating an embodiment of an
impedance-based VF detector 958. Impedance-based VF detector 958 is
another specific embodiment of impedance-based tachyarrhythmia
detector 758 and includes a motion event detector 966 and an event
threshold generator 968 to detect VF.
[0052] Motion event detector 966 detects an impedance event from a
cardiac local impedance derivative signal (dZ/dT). In one
embodiment, the impedance event is representative of a cardiac
local wall motion during the systolic phase of each cardiac cycle.
Motion event detector 966 indicates a detection of the impedance
event when the cardiac local impedance derivative signal (dZ/dT)
exceeds an event threshold. Event threshold generator 968 adjusts
the event threshold based on a trend of the cardiac local impedance
derivative signal (dZ/dT). Impedance-based VF detector 958 detects
VF using a pattern of the impedance events (i.e., a pattern of
cardiac local wall motion). In one embodiment, impedance-based VF
detector 958 indicates a VF detection when the pattern of the
impedance events becomes irregular while the heart rate falls into
a predetermined VF detection zone.
[0053] FIG. 10 is a block diagram illustrating an embodiment of an
impedance-based VF detector 1058. Impedance-based VF detector 1058
is another specific embodiment of impedance-based tachyarrhythmia
detector 758 and includes a ventricular motion event detector 1066,
an event threshold generator 1068, and an interventricular
synchrony analyzer 1070 to detect VF based on whether the LV and RV
contract in synchrony. During a normal sinus rhythm or a
tachycardia with a regular rhythm, the LV and RV contract in
synchrony, and the LV and RV impedance events during each cardiac
cycle occur approximately simultaneously, such as shown in FIG. 3.
Cardiac disorders such as heart failure may cause a certain degree
of dyssynchrony in the LV and RV local wall motions, but during a
normal or fast but regular rhythm, the LV and RV contractions
generally have a one-to-one relationship and occur within a limited
interventricular delay during each cardiac cycle.
[0054] Ventricular motion event detector 1066 detects an LV
impedance event by comparing the LV local impedance derivative
signal (LV dZ/dT) to an LV event threshold, and detects an RV
impedance event by comparing the RV local impedance derivative
signal (RV dZ/dT) to an RV event threshold. Event threshold
generator 1068 adjusts the LV event threshold based on a trend of
the LV local impedance derivative signal (LV dZ/dT), and adjusts
the RV event threshold based on a trend of the RV local impedance
derivative signal (RV dZ/dT). Impedance-based VF detector 1058
detects VF using a pattern of the LV impedance events and the RV
impedance events. In the illustrated embodiment, interventricular
synchrony analyzer 1070 detects VF by determining whether the
pattern of the LV impedance events and the RV impedance events
indicates a degree of dyssynchrony between the LV and RV local wall
motions that exceeds a predetermined threshold degree. In one
embodiment, interventricular synchrony analyzer 1070 indicates a VF
detection when the degree of dyssynchrony between the LV and RV
local wall motions falls below the predetermined threshold degree
while the heart rate falls into a predetermined VF detection zone.
In one embodiment, the degree of dyssynchrony between the LV and RV
local wall motions is measured by the interventricular delay
between the LV and RV local wall motions. Interventricular
synchrony analyzer 1070 detects an interventricular delay between
the LV impedance event and the RV impedance event during each
cardiac cycle and detects VF by comparing the interventricular
delay to a predetermined threshold delay. Interventricular
synchrony analyzer 1070 indicates a VF detection when the
interventricular delay exceeds the predetermined threshold delay
while the heart rate falls into a predetermined VF detection
zone.
[0055] FIG. 11 is a flow chart illustrating an embodiment of a
method 1100 for detecting a tachyarrhythmia using cardiac local
impedance. In one embodiment, method 1100 is performed by system
100.
[0056] A cardiac local impedance signal is sensed at 1110. The
cardiac local impedance signal is sensed using a pair of impedance
sensing electrodes placed to sense cardiac local wall motion. In
one embodiment, the pair of impedance sensing electrodes includes a
pair of bipolar pacing-sensing electrodes at a distal end of an
implantable pacing or pacing-defibrillation lead. To sense the
cardiac local impedance signal, current pulses are delivered
through the pair of impedance sensing electrodes at a frequency
between approximately 3 Hz and 500 Hz, with approximately 20 Hz as
a specific example. The current pulses each have an amplitude
between approximately 20 microamperes and 400 microamperes, with
approximately 80 microamperes as a specific example, and a pulse
width between approximately 10 microseconds and 100 microseconds,
with approximately 40 microseconds as a specific example. The
voltage across the pair of impedance sensing electrodes is sensed.
In one embodiment, the cardiac local impedance signal (Z) is
produced as a ratio of the sensed voltage to the delivered current.
In another embodiment, the cardiac local impedance signal (Z) is
produced by isolating the signal component indicative of the
cardiac local impedance from the sensed voltage, when the delivered
current is in the form of constant-current pulses.
[0057] In one embodiment, a cardiac local impedance derivative
signal (dZ/dT) is produced, for example, by high-pass filtering the
cardiac local impedance signal (Z) using a cutoff frequency between
approximately 0.1 Hz and 1 Hz, with approximately 0.5 Hz as a
specific example.
[0058] Tachyarrhythmia is detected using the cardiac local
impedance signal at 1120. In one embodiment, tachyarrhythmia is
detected using the cardiac local impedance derivative signal. In
one embodiment, one or more electrograms are also sensed, and
tachyarrhythmia is detected using the cardiac local impedance
signal and the one or more electrograms. In one embodiment, a VF
detection zone specified by one or more threshold amplitudes is
produced, and a VF detection is indicated when the cardiac local
impedance signal (Z) or the cardiac local impedance derivative
signal (dZ/dT) falls into the VF detection zone. In a specific
embodiment, the VF detection zone is adjusted using a trend of the
cardiac local impedance signal (Z) or the cardiac local impedance
derivative signal (dZ/dT). In one embodiment, an impedance event is
detected by comparing the cardiac local impedance derivative signal
(dZ/dT) to an event threshold. The impedance event represents a
cardiac local wall motion during the systolic phase of each cardiac
cycle. The event threshold is adjusted based on a trend of the
cardiac local impedance derivative signal (dZ/dT). VF is detected
using the pattern of the detected impedance events (i.e., pattern
of cardiac local wall motion).
[0059] Delivery of an anti-tachyarrhythmia therapy is controlled at
1130. In one embodiment, if VF is detected at 1120, a
defibrillation pulse is delivered at 1130.
[0060] FIG. 12 is a flow chart illustrating an embodiment of a
method 1200 for detecting VF using LV and RV local impedance
signal. In one embodiment, the method is performed by system
100.
[0061] The LV local impedance signal (LVZ) is sensed at 1210. To
sense the LV local impedance signal (LVZ), an LV current is
delivered through a pair of LV impedance sensing electrodes, and an
LV voltage across the pair of LV impedance sensing electrodes is
sensed. In one embodiment, the LV local impedance signal (LVZ) is
produced as the ratio of the sensed LV voltage to the delivered LV
current. In another embodiment, the LV local impedance signal (LVZ)
is produced by isolating the signal component indicative of the LV
local impedance from the sensed LV voltage, when the delivered LV
current is in the form of constant-current pulses. An LV local
impedance derivative signal (LV dZ/dT) is produced at 1215. LV
impedance events are detected at 1220 by comparing the LV local
impedance derivative signal (LV dZ/dT) to an LV event threshold. In
one embodiment, the LV impedance events are each representative of
an LV local wall motion during the systolic phase of a cardiac
cycle. In one embodiment, the LV event threshold is adjusted based
on a trend of the LV local impedance derivative signal (LV
dZ/dT).
[0062] The RV local impedance signal (RVZ) is sensed at 1230. To
sense the RV local impedance signal (RVZ), an RV current is
delivered through a pair of RV impedance sensing electrodes, and an
RV voltage across the pair of RV impedance sensing electrodes is
sensed. In one embodiment, the RV local impedance signal (RVZ) is
produced as the ratio of the sensed RV voltage to the delivered RV
current. In another embodiment, the RV local impedance signal (RVZ)
is produced by isolating the signal component indicative of the RV
local impedance from the sensed RV voltage, when the delivered RV
current is in the form of constant-current pulses. An RV local
impedance derivative signal (RV dZ/dT) is produced at 1235. RV
impedance events are detected at 1240 by comparing the RV local
impedance derivative signal (RV dZ/dT) to an RV event threshold. In
one embodiment, the RV impedance events are each representative of
an RV local wall motion during the systolic phase of a cardiac
cycle. In one embodiment, the RV event threshold is adjusted based
on a trend of the RV local impedance derivative signal (RV
dZ/dT).
[0063] The pattern of the detected LV and RV impedance events is
analyzed at 1250. A degree of dyssynchrony between the LV and RV
local wall motions is produced based on the pattern. If the degree
of dyssynchrony between the LV and RV local wall motions exceeds a
predetermined threshold degree (i.e., the LV and RV do not contract
in synchrony) at 1255, a VF detection is indicated at 1260. In one
embodiment, an interventricular delay between the LV impedance
event and the RV impedance event during each cardiac cycle is
detected as a measure of the degree of dyssynchrony between the LV
and RV local wall motions. The LV and RV contract in synchrony when
the LV impedance events and the RV impedance events have
approximately a one-to-one relationship and the interventricular
delay is within a predetermined limit.
[0064] It is to be understood that the above detailed description
is intended to be illustrative, and not restrictive. Other
embodiments will be apparent to those of skill in the art upon
reading and understanding the above description. The scope of the
invention should, therefore, be determined with reference to the
appended claims, along with the full scope of equivalents to which
such claims are entitled.
* * * * *