 |
Description  |
|
|
FIELD OF THE INVENTION
This invention pertains to an implantable medical device, and more
particularly, to an implantable cardioverter-defibrillator (ICD), which
senses a dangerous cardiac arrhythmia and, in response, provides therapy
to a patient's heart to revert it to a normal sinus rhythm. More
particularly, this invention pertains to an ICD in which a depolarization
pulse is applied after a defibrillation shock, and/or the electrodes used
to deliver the defibrillation shock are shorted together for a brief time
period, to discharge any residual charges, thereby insuring that intrinsic
cardiac signals indicative of fibrillation are not masked.
BACKGROUND OF THE INVENTION
As used herein, the term "arrhythmia" refers to any abnormal heart rhythm
that may be dangerous to the patient and specifically includes
fibrillation, atrial tachycardias, supraventricular tachycardias (SVT),
ventricular tachycardias (VT), ventricular fibrillation and flutter (VF).
As further used-herein, the term "therapy" refers to any means used by the
ICD device to restore normal heart rhythm, such as defibrillation,
cardioversion, and antitachycardia pacing. The term "cardioverter" refers
to a device capable of providing defibrillation therapy, cardioversion
therapy, or both.
Typically, defibrillation therapy consists of the application to cardiac
tissue of one or more electrical shocks of considerable amplitude and
duration. In cases where a first defibrillation shock is not successful, a
second shock having much smaller amplitude applied within about 1-2
seconds after the first shock may suffice to revert the heart to normal
sinus rhythm. It is desirable to apply a subsequent defibrillation shock
as soon as it is discovered that the heart has not reverted despite
earlier attempts.
However, such therapy immediately after the delivery of a defibrillation
shock has not always been possible because it may not be possible to sense
the on-going arrhythmia (including VF) for many seconds after a
defibrillation shock is applied. More particularly, until now such early
therapy additional (e.g., second-shock) could not be applied because the
first defibrillation shock results in a build-up of residual charge on the
electrodes and a local polarization of the tissues which would dissipate
only after about 10 seconds. This may mask any low amplitude VF, and
necessarily lead to a delay in the application of another shock.
SUMMARY OF THE INVENTION
In its broadest sense, the present invention pertains to an ICD wherein a
two-step process is executed after the administration of high energy level
therapy such as a defibrillation shock consisting of one or more high
amplitude pulses. The process consists of first applying a relatively
short duration and low amplitude pulse, preferably having a polarity
opposite to that of the last pulse of the therapy, to depolarize the
electrodes thereby dissipating any residual or parasitic charges therein.
Following this short duration pulse, the electrodes used to apply the
therapy are momentarily shorted together to discharge any remaining
residual charges.
In a particularly advantageous arrangement, a multiphasic shock is
generated by a circuit that includes a capacitor charged to a preselected
voltage of at least 100V and then discharged through a set of electronic
switches arranged in a bridge-like fashion. In this arrangement, the short
duration pulse is generated by applying a charge from the capacitor to the
electrodes using the same switches that control or steer the therapy
shock. After the short duration pulse, additional switches are used to
short the electrodes together.
Advantageously, the electrodes may also be shorted to the conductive case
of the ICD, especially in arrangements where the conductive case acts as
anelectrode as well.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and advantages of the present
invention will be more apparent from the following more particular
description thereof, presented in conjunction with the following drawings
wherein:
FIG. 1 shows a simplified functional block diagram of an implantable
cardioverter-defibrillator (ICD);
FIG. 2 shows a multiphasic defibrillation shock applied to the heart of a
patient to revert the heart to normal sinus rhythm, to depolarize the
defibrillation electrodes, and to dissipate charges within the cardiac
tissues;
FIG. 3 shows a prior art circuit for generating defibrillation shocks and
for sensing an electrogram signal;
FIG. 4 shows a circuit for generating multiphasic shocks and for shorting
the defibrillation electrodes together in accordance with the present
invention; and
FIG. 5 shows a flowchart for the operation of the ICD of FIGS. 1 and 4.
DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best mode presently contemplated for
carrying out the invention. This description is not to be taken in a
limiting sense, but is made merely for the purpose of describing the
general principles of the invention. The scope of the invention should be
determined with reference to the claims.
As indicated above, the present invention may be used with various types of
implantable medical devices, including an implantable
cardioverter-defibrillator (ICD). To better understand the invention, it
will first be helpful to provide a description of the basic functions
performed by the implantable medical device with which the invention is
used, e.g., an ICD device. To that end, reference is first made to FIG. 1,
where there is shown a simplified functional lock diagram of an ICD 20. It
should be noted that in some instances the functions of an ICD may be
combined with the functions of a pacemaker within the same medical device.
A primary function of an ICD device is to detect the occurrence of an
arrhythmia, and to automatically apply an appropriate electrical
defibrillation shock to the heart to terminate the arrhythmia, as
discussed more fully below. Toward this end, the ICD 20 includes a
microprocessor-based control and timing circuit 22 (hereafter a
"control/timing" circuit 22) that controls an output circuit 26. The
output circuit 26 generates output electrical stimulation pulses of
moderate or high energy (cardioversion pulses or defibrillation shocks),
e.g., electrical pulses having energies of from 1 to 5 joules (moderate)
or 6 to 40 joules (high), as controlled by the control/timing circuit 22.
Such moderate or high energy shocks are applied to the patient's heart 28
through a lead 30 coupled to two suitable defibrillator electrodes 38 and
128 implanted in the heart 28. While only one lead and two electrodes are
shown in FIG. 1, it is to be understood that additional defibrillation
leads and electrodes may be used as desired or needed in order to
efficiently and effectively apply to the patient's heart 28, the shock
generated by the output circuit 26.
The ICD 20, disposed in a conductive case 128, includes sense amplifier 42,
coupled to electrodes 32 or 38 and 40 via lead 30. The sense amplifier 42
amplifies the electrical signal indicative of the activity of the heart 28
that appears on the electrodes 32 or 38 and 40. That is, as is known in
the art, an R-wave occurs upon the depolarization, and hence contraction,
of ventricular tissue; and a P-wave occurs upon the depolarization, and
hence contraction, of atrial tissue. Thus, by sensing electrical signals
indicative of R-waves and/or P-waves, amplifying such signals through the
sense amplifier 42, and providing such amplified signals to the
control/timing circuit 22, the control/timing circuit 22 is able to
determine the rate and regularity of the patient's heartbeat. Such data
provides the basis for the control/timing circuit 22 to determine whether
heart 28 is malfunctioning. (As will be understood, if both P-waves and
R-waves are to be sensed, two leads and sense amplifiers must be used.)
The control/timing circuit 22 further has a memory 44 coupled thereto,
wherein the operating parameters used by the control/timing circuit 22 are
stored. Such operating parameters define, for example, a therapy to be
applied to treat the heart 28, including a plurality of shocks, including
the amplitude of each shock to be delivered to the heart 28 within each
defined tier of therapy.
Advantageously, the operating parameters of the ICD 20 may be
non-invasively programmed into the memory 44 through a telemetry circuit
46, in telecommunications contact with an external programmer 48 through a
coupling coil 50. The coupling coil 50 may serve as an antenna for
establishing a radio frequency (rf) communications link 52 with the
external programmer 48; or the coil 50 may serve as a means for
inductively coupling data to and from the telemetry circuit 46 from and to
the external programmer 48, as is known in the art. See, e.g., U.S. Pat.
Nos. 4,809,697 (Causey, III et al.) and U.S. Pat. No. 4,944,299 (Silvian),
incorporated herein by reference. Further, such telemetry circuit 46
advantageously allows status information relating to the operation of the
ICD 20, as contained in the control/timing circuit 22 or memory 44, to be
sent to the external programmer 48 through the established (rf)
communications link 52.
The control/timing circuit 22 includes appropriate processing and logic
circuits for analyzing the output signals of the sense amplifier 42 and
determining if such signals indicate the presence of an arrhythmia.
Typically, the control/timing circuit 22 is based on a microprocessor, or
similar processing circuit, which includes the ability to process or
monitor input signals (data) in a prescribed manner, e.g., as controlled
by program code stored in a designated area or block of the memory 44. The
use, design, and operation of microprocessor-based control circuits
including the control/timing 22 circuit to perform timing and data
analysis functions is known in the art and therefore need not be described
in detail here.
As previously mentioned, a problem associated with the application of
defibrillation therapy is that whenever a shock is applied, the tissue
around the electrodes 38 and 40 becomes polarized. Moreover, the shock
results in a build up of a residual charge, generally of about 1 volt,
across the defibrillator leads that takes several seconds to dissipate.
The problem with the build up of charge on the defibrillator electrodes is
that it masks detection of a possible ventricular fibrillation. More
particularly, ventricular fibrillation is characterized by a signal in the
order of 1 mV which may set in as early as 100 ms after a defibrillation
shock. In the presence of any remaining charge on the electrodes,
fibrillation is difficult to sense. Hence, in prior art ICDs the sensing
of intrinsic cardiac activity was delayed for a period sufficient to
ensure that the remaining charge had dissipated.
This invention resolves the above-described limitation by applying at the
end of a typical defibrillation shock a short duration pulse having a
relatively small amplitude and duration. For example, referring to FIG. 2,
a typical defibrillation shock is shown having two pulses: a positive
pulse 100 and a negative pulse 102. Following the negative pulse 102, an
additional short duration pulse 104 is applied. While the duration of
pulses 100 and 102 is in the order of 6 ms, the pulse 104 is much shorter,
i.e., in the order 1 ms or less. Similarly, while the peak amplitudes of
the pulses 100, 102 are about 750 and 300 volts respectively, the peak
amplitude of short duration pulse 104 is about 100 volts. Since the pulse
104 is positive, it provides for the discharge of any residual charges on
the leads used to apply the defibrillation shock.
Following short duration pulse 104, the defibrillation electrodes are
shorted together to further dissipate any residual charges. We have found
that a positive short duration pulse on one of the cardiac electrodes is
more effective than a negative pulse on the case of the ICD, if the case
is used as one of the electrodes.
FIG. 3 shows a schematic of a typical prior art output circuit 26. In this
figure, the output circuit 26 includes a secondary coil 110 which
cooperates with a primary coil 110A to define a transformer. The primary
coil receives pulses from a power supply, not shown, which results in a
charging voltage in the secondary coil 110. This voltage is applied
through a diode 112 to charge a capacitor 114.
A switching bridge 115 formed of four switches 116, 118, 120, 122 is also
provided in the output circuit 26. These switches 116-122 are electronic
switches whose states (open or closed) are determined by a control circuit
124. One node 126 of bridge 115 is connected to the electrode 38 while the
other node 130 is connected to grounded housing 128 of the ICD 20 by a
wire 129.
The circuit of FIG. 3 can be used to generate a defibrillation shock
between the electrode 38 and the case 128, the stimulation shock being
composed of the two pulses 100 and 102 shown in FIG. 2, by selectively
opening and closing switches 116-122. The cardiac activity before and
after the shock is monitored through an amplifier 136 which senses the
voltage between the electrode 38 and, the electrode 40.
The sequence of opening and closing the switches 116-122 is described in
detail below.
The circuit of FIG. 3 is easily modified to generate the pulses of the
subject invention, as shown in FIG. 4. In FIG. 4, control circuit 124A
operates switches 118-122, thereby generating pulses 100, 102 to be
applied to the electrodes. However, additional control signals are also
generated by control circuit 124A to cause the short duration pulse 104,
shown in FIG. 2, to be applied to the electrode 38, as explained in detail
below. Output circuit 26A of FIG. 4 includes all the components of output
circuit 26 of FIG. 3, and in addition also includes switches 140, 142 and
variable resistors 144, 146. Switch 140, when closed, grounds electrode 38
to case 128 while switch 142, when closed, shorts electrodes 38 and 40
together.
Amplifier 136 measures the intrinsic voltage generated between electrodes
38 and 40 and transmits the value of this intrinsic voltage to the control
circuit 124A as well as to the timing and control circuit 22 (in FIG. 1).
The control circuit 124A senses this value after a defibrillation shock
signal. The control circuit 124A will seek to "drive" the voltage down to
an average value of zero. The polarization is typically negative on the
electrode 38. But the polarization on the housing 128 will, naturally, be
opposite. By adjusting the relative values of input weighting resistors
144 and 146, the control circuit 124A can find a neutral (zero offset
voltage) reference for amplifier 136. Of course, the electrogram signal is
an AC signal superimposed over the decaying polarization signal. Thus, the
feedback speed of control circuit 124A must be limited so that it does not
cancel out the electrogram signal desired at the output of amplifier 136.
In normal operation (i.e., not post-shock), resistor 144 is at an
essentially infinite value so that the sensing reference is purely
electrode 38.
The circuit 26A in FIG. 4 best describes the invention in use with the
so-called "integrated" bipolar sensing. With this scheme, the right
ventricular (RV) coil senses both as shocking electrode (hence, the
connection to node 126) and as the sensing reference electrodes. The dual
purpose role is allowed by the jumper 148 which may be in the ICD, but
most typically in the lead assembly itself.
Another popular lead system has the "true bipolar" sensing scheme. In this
case, there is no jumper 148 and sensing-reference electrode is a distinct
ring (e.g., electrode 32) placed between the RV coil 38 and the tip
electrode 40. The operation of the invention with the true bipolar scheme
would preferably be identical to that described earlier for the dedicated
bipolar lead system. As an alternate embodiment, a third variable resistor
(in addition to 144 and 146) could be used to connect the RV coil
electrode (node 126) to the upper input of amplifier 136. In this way,
control circuit 124A takes advantage of the existence of three choices for
the reference electrode. By suitably balancing the three, it is most
likely to rapidly find a neutral reference.
The operation of the ICD 20 of FIG. 1 and more specifically the output
circuit 26A of FIG. 4 will now be described in conjunction with the flow
chart of FIG. 5.
The heart 28 is monitored at step 200 to determine and classify its
condition. At step 202 the control/timing circuit 22 makes a decision as
to whether the heart 28 is exhibiting an abnormal rhythm. If an abnormal
rhythm is detected, then at step 204 the condition of the heart is
classified as either fibrillation, tachycardia or bradycardia. Procedures
and algorithms for making this determination are described, for example,
in U.S. Pat. No. 5,257,621 (Bardy), incorporated herein by reference.
In the case of a low rate tachycardia, antitachycardia pacing therapy (ATP)
is applied at step 206. In the case of a bradycardia, antibradycardia
therapy is applied at step 208.
If fibrillation is identified at step 204, then a defibrillation therapy is
applied as follows. At step 210, a biphasic defibrillation shock
comprising two pulses 100, 102 as shown in FIG. 2 is applied between
electrodes 38 and 128, by sending a command signal from the control timing
control circuit 22 to the output circuit 26A. In response, the output
circuit 26A defines the four sequential cycles which are identified as
cycle I, cycle II, cycle III, and a shorting cycle S, as illustrated in
FIG. 2.
During cycle I, switches 116 and 122 close causing the capacitor 114 to
discharge and apply positive pulse 100 between electrodes 38 and 128.
At the end of cycle I, the output circuit 26A switches to cycle II by
opening switches 116, 122 and closing switches 118, 120. This causes the
capacitor 114 to continue to discharge at approximately the same rate as
in cycle I but the polarity of the resulting pulse 102 has the opposite,
or negative, polarity with respect to pulse 100 as shown in FIG. 2. The
end of cycle II as determined by output circuit 26A constitutes the end of
the standard biphasic defibrillation shock. Next, at step 212, the short
duration pulse 104 is applied between the electrodes 38 and 128. This
short duration pulse is used to dissipate residual or parasitic charges on
the electrodes, or the polarization of the tissues, around the electrodes.
Importantly, this short duration pulse has a low amplitude so that it does
not stimulate the cardiac tissues. This is accomplished during cycle III
by output circuit 26A which opens switches 120 and 118, and once again
closes switches 116, 122.
At step 214, output circuit 26A initiates the shorting cycle S. Switches
116, 122 are opened, and switches 140 and 142 are closed, causing
electrodes 38, 40 to be connected to each other and case 128, thereby
dissipating any residual charge on the electrodes. At step 216, switches
140, 142 open and the capacitor starts to charge once again.
The monitoring of the heart 28 resumes at step 200. During the monitoring
step, low level cardiac signals, such as the signals associated with
fibrillation, are not masked by charges on the electrodes 38, 40 or
polarization of the tissues around these electrodes. Therefore, any
arrhythmia following a defibrillation shock applied at step 210 can be
detected with far less delay and, accordingly, appropriate therapy (steps
206, 208, 210) can be applied earlier and more effectively than in the
prior art.
While the invention has been described by means of specific embodiments, it
is understood that modifications and variations could be made thereto by
those skilled in the art without departing from the spirit and the scope
of the invention. It is therefore to be understood that within the scope
of the claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *
|
|
 |
|
 |
Description |
|
