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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to medical stimulators and more
specifically to implantable cardioverters and defibrillators.
When implanting an implantable defibrillator, it is desirable to test the
device's operability to ensure that it is capable of reliably
defibrillating the heart. In order to accomplish this, it is necessary to
first induce fibrillation in the patient's heart, and then determine
whether the implantable defibrillator is capable of terminating the
induced fibrillation. Typically, a 60 cycle type fibrillator has been used
in inducing fibrillation.
The inventors of the present application have determined that it would be
desirable to incorporate the fibrillation induction function into an
implantable defibrillator, to allow for a more fully automated testing
regimen and to simplify the implantation procedure. However, incorporation
of a 60 cycle defibrillator into an implantable device poses substantial
technical difficulties. In any case, 60 cycle fibrillators frequently fail
to induce fibrillation.
SUMMARY OF THE INVENTION
The present invention is directed toward providing an implantable
fibrillator, preferably as part of an implantable
cardioverter/defibrillator, which can reliably and automatically induce
fibrillation. The inventors have determined that by accurately timing the
delivery of a high voltage pulse, fibrillation can be reliably induced in
most cases, using a single high voltage pulse. Fibrillation is induced
immediately, so that the two second additional period of hemodynamic
compromise which could occur during a 60 cycle fibrillation pulse is
avoided. The inventors have further determined that appropriate timing of
the high voltage pulse can be derived from a measurement of the patient's
effective refractory periods measured directly or derived from a
measurement of the patient's Q-T interval and that sufficient accuracy of
the timing of the fibrillation inducing pulse is facilitated by using a
pacing pulse delivered during overdrive pacing as a timing reference.
Typically, in implantable cardioverters and defibrillators, delivery of
high voltage pulses for purposes of cardioversion or defibrillation is
timed from sensing of natural ventricular contractions. However, the
duration of the "R-wave" signal corresponding to an actual ventricular
contraction is typically at least 50 ms., and the sense amplifiers
typically used in implantable cardioverters and defibrillators are
responsive to both variations in amplitude and frequency. As such,
depending upon the configuration of the particular R-wave in question, the
sense amplifier may detect the occurrence of the corresponding ventricular
contraction at different points respective to the initiation of the
R-wave. The inventors of the present application have determined that by
timing the delivery of the high voltage pulse intended to induce
fibrillation from an immediately preceding pacing pulse, a consistent
timed relationship between the paced R-wave and the fibrillation pulse can
be provided. This in turn allows for extremely accurate placement of the
fibrillation pulse, relative to the refractory period of the patient's
heart.
Further, the device of the present invention determines the refractory
period of the heart based on paced contractions of the heart, paced at the
same rate as the paced contraction immediately preceding delivery of the
fibrillation pulse. This further enhances the accuracy of the timing of
the delivery of the fibrillation pulse relative to the patient's present
effective refractory period, and substantially increases the likelihood
that a single pulse will be sufficient to fibrillate the heart.
Alternative embodiments of the invention forego the actual measurement of
the patient's refractory period and instead employ a fixed delay based on
known typical values for effective refractory periods at the rate of
pacing in effect prior to delivery of the fibrillation pulse. Additional
alternative embodiments may employ measurement of the effective refractory
period of the patient's heart by means of measurement of the patient's Q-T
interval and use the measured Q-T interval to control timing of
fibrillation inducing pulses following spontaneous heartbeats.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a simulated EKG strip illustrating the functioning of a first
embodiment of the present invention.
FIG. 1b is a simulated EKG strip illustrating the functioning of a second
embodiment of the present invention.
FIG. 2 is a functional block diagram of a device in which the present
invention may be embodied.
FIG. 3a is a flow chart illustrating the operation of the present invention
when practiced in an embodiment corresponding to that illustrated in FIG.
1a.
FIG. 3b is a flow chart illustrating the operation of the present invention
when practiced in an embodiment corresponding to that illustrated in FIG.
1b.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1a is a simulated EKG strip, illustrating the function of an
implantable defibrillator incorporating the present invention. The EKG
strip can be broken into two functional sections, with the preliminary
portion of the EKG strip illustrating the method in which the device
determines the patient's effective refractory period, and the later
portion of the strip illustrating the timing and delivery of a high
voltage pulse to induce fibrillation.
In its preferred embodiment, the fibrillation induction function is
initiated by the physician by means of a programmer located exterior to
the body which and provides programming signals to the implanted
defibrillator. It is preferred that the fibrillation induction function be
embodied in the form of a temporarily activatable feature, which is
automatically disabled upon removal of or turn-off of the programmer, as
disclosed in U.S. Pat. No. 4,253,466, issued to Hartlaub, incorporated
herein by reference in its entirety. This is believed to provide a
particularly useful safety function, which is especially desirable in the
context of the fibrillation induction function of the present invention.
After the physician initiates the fibrillation induction function of the
defibrillator, the device begins pacing the heart at a rate in excess of
the patient's resting heart rate, so that it may overdrive the patient's
heart. The rate at which this is accomplished is preferably selected by
the physician, as a temporary parameter, as described in the above-cited
Hartlaub patent. The amplitude of the pacing pulse is preferably twice the
measured pacing threshold. It is contemplated that threshold measurement
will be performed automatically, using the method and apparatus
illustrated in U.S. Pat. No. 4,250,884, issued to Hartlaub et al., and
incorporated herein by reference in its entirety. However, the invention
may also be usefully practiced employing a fixed, high amplitude pacing
pulse, e.g. a 5 volt, 1 ms pulse.
As illustrated, a rate of 150 beats per minute has been chosen so that the
interval T1 separating pacing pulses 10 and 12 is approximately 400 ms.
Overdrive pacing at this rate continues for eight pacing pulses, separated
by T1 intervals. Only the first T1 interval is illustrated, in order to
simplify the diagram. After the eighth such pacing pulse 13, a subsequent
pacing pulse 14 is generated at an interval T2 thereafter. The duration of
T2 is selected to be between a time interval TMIN measured from pacing
pulse 13 during which it is expected that most patients' hearts will be
refractory (e.g. TMIN equals 100 ms.), and a second time interval TMAX
measured from pacing pulse 13, at the expiration of which it is expected
that the patient's heart will not be refractory. TMAX may be selected by
the physician, or may be set equal to a predetermined percentage of the
overdrive pacing rate, e.g. 80 or 90%. As illustrated, TMAX is set at
approximately 320 ms. T2 as illustrated is intermediate TMIN and TMAX,
and, for purposes of the illustration in FIG. 1, is equal to TMIN plus
TMAX divided by 2. As such, using the suggested values for TMIN and TMAX
discussed above, T2 would be equal to 210 ms.
Following delivery of pacing pulse 14, the implanted device determines
whether pacing pulse 14 has been successful in capturing the heart and
causing a corresponding ventricular depolarization. While this function
will be discussed in more detail below, for the purposes of the present
invention it can be accomplished using any of a number of prior art
methods for determining whether a pacing pulse has successfully captured
the heart. For example, either the method disclosed in U.S. Pat. No.
4,305,396, issued to Wittkampf, et al, or the method disclosed in U.S.
Pat. No. 4,766,900 issued to Callaghan, both of which are incorporated
herein by reference in their entireties, may be used. However, any of the
numerous alternative capture detection methods and devices known to the
art may also be employed.
As illustrated in FIG. 1, pacing pulse 14 has been successful in capturing
the heart. The implanted defibrillator again paces the heart at the
overdrive pacing rate of 150 beats per minute, triggering generation of a
eight pacing pulses separated by 400 ms. intervals. Following the eighth
such pacing pulse 16, a subsequent pacing pulse 18 is generated, at an
interval T3, which is intermediate between T2, known to be outside the
refractory period of the heart, and TMIN, believed to be within the
refractory period of the heart. As illustrated, the time interval T3 is
selected to be 155 ms., equal to one-half of the sum of T2 and TMIN. As
illustrated, this pulse is not successful in capturing the heart,
indicating that pacing pulse 18 is within the refractory period of the
heart.
Following generation of pacing pulse 18, the defibrillator returns to the
overdrive pacing mode, and generates eight additional pacing pulses at 400
ms. intervals. In a manner similar to the calculation of the interval T3,
a subsequent interval T4 is calculated separating the eighth such pacing
pulse 20 from pacing pulse 22. In this case, because pacing pulse 18,
which occurred at 155 ms. following the preceding paced contraction was
found to be within the refractory period of the heart, pacing pulse 22 is
generated at a time equal to one-half of the sum of T2 and T3. Pacing
pulse 22 is thus generated 182.5 ms. after pacing pulse 20. As
illustrated, pacing pulse 22 is unsuccessful in capturing the heart,
indicating that it is within the refractory period of the patient's heart.
The defibrillator returns to the overdrive pacing mode, and generates eight
subsequent overdrive pacing pulses at 400 ms. intervals. In a manner
similar to the calculation of the time interval T4, a subsequent pacing
pulses 26 and 30 are generated at time intervals T5 and T6 following
pacing pulses 24 and 28. The duration of time interval T5 is equal to
one-half of the sum of T4 and T2, i.e. approximately 196 ms. The duration
of time interval T6 is equal to one-half of the sum of T4 and T5, i.e.
approximately 190 ms. As illustrated, pacing pulse 26 is successful in
capturing the heart and pulse 30 is not.
Because the time differential between T5, outside the refractory period of
the heart and T6, within the refractory period of the heart is less than a
predetermined value, for example 10 ms., the implantable defibrillator
determines that T5 constitutes a sufficiently accurate measurement of the
refractory interval of the heart. The value of T5 will be used to control
the timing of the generation of a high voltage pulse to induce
fibrillation. Following generation of pacing pulse 30, the defibrillator
returns to the overdrive pacing mode, and generates overdrive pacing
pulses 31 and 32 separated by 400 ms.
After generation of pacing pulse 32, the defibrillator begins timing of
time interval T7, the interval between pacing pulse 32 and the generation
of high voltage pulse 34, intended to induce fibrillation. The duration of
time interval T7 is equal to the duration of time interval T5, plus a
predetermined increment, typically on the order of 40 to 80 ms. The value
of this incremental time interval should be selected by the physician, as
should the amplitude of high voltage pulse 34. Typically, pulse 34 should
have an amplitude of approximately 50 to 200 volts and a pulse width of 2
to 20 ms.
In the event that the high voltage pulse 34 is unsuccessful in inducing
fibrillation, additional fibrillation inducing pulses may be delivered,
employing a different incremental time interval added to the measured
refractory period of the heart or employing a higher output fibrillation
inducing pulse. In conjunction with repetition of the fibrillation
induction function, the patient's refractory period may be remeasured or
the fibrillation induction method represented by pulses 32 and 34 can
immediately be repeated after verification that fibrillation has not been
induced. In conjunction with repetition of the fibrillation induction
function, either the time of delivery of the high voltage pulse or its
amplitude may be scanned by regularly incrementing the interval added to
the measured refractory period of the heart or by regularly incrementing
the amplitude of the high voltage pulse.
As illustrated in FIG. 1, pulse 34 is successful in inducing fibrillation.
At this point, the device exits the temporary fibrillation induction mode
automatically, and returns to its underlying normal functionality,
including operation of its tachycardia and fibrillation detection and
terminating functions. While not discussed specifically in this
application, the tachycardia and fibrillation detection and termination
functions may correspond to any of those disclosed in previous patents
relating to implantable cardioverter/defibrillators. For example, they may
correspond to the detection and termination methodologies disclosed in
U.S. Pat. No. 4,548,209, issued to Wielders, et al, U.S. Pat. No.
4,693,253, issued to Adams, U.S. Pat. No. or U.S. Pat. No. 4,830,006,
issued to Haluska, et al, all of which are incorporated 5 herein by
reference it their entireties. The present invention is believed to
workable in the context of any of the numerous available fibrillation
detection and termination methodology.
As can be seen from FIG. 1a, the delivery of the high voltage pulse 34
occurs when the heart is being paced at the same rate as it was paced
during measurement of the refractory interval. As such, precise
correspondence between the measured effective refractory period and the
timing of the high voltage pulse is provided. As an alternative to the
refractory period measurement method described above, measurement may be
made according to the teaching of U.S. Pat. No. 4,280,502, issued to Baker
et al, incorporated herein by reference in its entirety.
While actual measurement of the patient's refractory period is desirable,
the invention may also be usefully practiced without actual measurement of
the refractory period, allowing for substantial simplification of the
apparatus embodying the invention. In such an embodiment, the duration of
the interval separating the initial fibrillation pulse from the
immediately preceding overdrive pacing pulse may be a fixed, predetermined
interval based on the typical duration of patient's refractory periods
when paced at the overdrive pacing rate. If the first fibrillation
inducing pulse is unsuccessful, this fixed, predetermined interval may be
incremented or the fibrillation inducing pulse level may be incremented as
discussed above, until fibrillation is induced.
FIG. 1b illustrates an alternative embodiment of the present invention, in
which the timing of the high voltage fibrillation inducing pulse is
derived from a measurement of the Q-T interval. In FIG. 1, a series of
pacing pulses 40, 42, 44, 46 and 48 are provided, generated at a rate of
150 beats per minute (400 ms. intervals). Following each pacing pulse, a
measurement is made of the time interval between the pacing pulse and the
peak of the T-wave or some other identifiable feature of the T-wave. In
FIG. 1b these are illustrated as time intervals T2, T3, T4 and T5.
In a device according to the embodiment illustrated in FIG. 1b, time
intervals T2 through T5 are averaged to produce an average Q-T interval. A
predetermined interval of time, is added or subtracted from this average
Q-T interval to provide interval T6, separating the final overdrive pacing
pulse 48 from the high voltage, fibrillation inducing pulse 50. The
duration of the time interval will depend upon the particular feature of
the T-wave identified. Like the embodiment illustrated in FIG. 1a,
accurate timing of the location of fibrillation inducing pulse 50 is
possible because it is timed from the preceding ventricular pacing pulse
48. Again, delivery of the high voltage pulse 50 occurs when the heart is
being paced at the same rate as it was paced during measurement of the Q-T
interval. As such, precise correspondence between the depolarization
induced by the pacing pulse 48 and the delivery of the high voltage pulse
50 is also provided.
While the disclosed embodiment of the invention discussed above measures
intervals between pacing pulses and subsequent T-waves, the invention may
also be usefully practiced by employing other methods of measuring time
intervals between cardiac depolarizations (R-waves), including spontaneous
depolarizations and subsequent T-waves. Further, In such embodiments, the
interval separating the fibrillation inducing pulse from the preceding
depolarization may in some cases be timed from some reference point with
regard to the depolarization other than the pacing pulse.
FIG. 2 is a functional block diagram of an implantable
cardioverter/defibrillator/pacemaker of the type in which the present
invention may be practiced. The disclosed embodiment takes the form of a
microprocessor controlled device. However, it is believed that the
invention might usefully be practiced in other types of devices, including
those employing dedicated digital circuitry, and perhaps even in devices
comprised primarily of analog timing and control circuitry. As such, FIG.
2 should be considered exemplary, rather than limiting with regard to the
scope of applications of the present invention.
The primary elements of the apparatus illustrated in FIG. 2 are a
microprocessor 100, read only memory 102, random access memory 104, a
digital controller 106, input and output amplifiers 110 and 108
respectively, and a telemetry/programming unit 120.
Read only memory 102 stores the basic programming for the device, including
the primary instructions set defining the computations performed to derive
the various timing intervals performed by the device. Random access memory
104 serves to store the values of variable control parameters, such as
programmed pacing rate, programmed cardioversion and defibrillation
intervals, pulse widths, pulse amplitudes, and so forth, which are
programmed into the device by the physician. Random access memory also
stores derived values, such as the intervals separating the overdrive
pacing pulses 12, 16, 20, 24 and 28 (FIG. 1) from the subsequent
refractory interval testing pulses 14, 18, 22 and 26, or from the
subsequently generated high voltage pulse 30. Reading from random access
memory 104 and read only memory 102 is controlled by RD-line 146. Writing
to random access memory 104 is controlled by WR-line 148. In response to a
signal on RD-line 146, the contents of random access memory 104 or read
only memory 102 designated by the then present information on address bus
124 are placed on data bus 122. Similarly, in response to a signal on
WR-line 148, information on data bus 122 is written into random access
memory 104 at the address specified by the information on address bus 124.
Controller 106 performs all of the basic timing and control functions of
the device. Controller 106 includes at least one programmable timing
counter, initiated on ventricular contractions, paced or sensed, and
timing out intervals thereafter. This timing counter is used to define the
timing intervals referred to above, including the overdrive pacing
interval ODINT, the intervals (TEST) separating the refractory interval
testing pulses from immediately preceding overdrive pacing pulses, and the
derived interval separating the delivery of the high voltage pulse from
the immediately preceding overdrive pacing pulse. It is also anticipated
that the controller 106 would also perform the basic timing functions of
the pacing, cardioversion and tachycardia detection and termination
routines performed by the device, as described in the above-cited prior
art patents.
Controller 106 also triggers output pulses from output stage 108 as
discussed below, and it generates interrupts on control bus 132 waking
microprocessor 100 from its sleep state to allow it to perform the
mathematical calculations referred to in conjunction with FIG. 1 above.
Generally, it is anticipated that the controller 106 will generate
interrupts to microprocessor 102 following either delivery of output
pulses by output stage 108 or following detection of natural ventricular
contractions by input stage 110, as discussed below. The time intervals
which the timing counter in controller 106 counts are controlled by data
stored in random access memory 104, applied to controller 106 via data bus
122.
Controller 106 also serves to control the capture detection function
described in conjunction with FIG. 1 above. Initiation of the capture
detection function is controlled by microprocessor 106, by means of
control bus 132. Corresponding flags are generated by controller 106
indicating the success or failure of the pacing pulse to capture, and are
placed on control bus 132 for use of the microprocessor in calculating the
value of the TEST intervals, discussed above.
Controller 106 further serves to define the Q-T interval measurement
function, performed in conjunction with the alternative embodiment
discussed in conjunction with FIG. 1b. The Q-T interval measurement
function can be performed as described in U.S. Pat. No. 4,228,803 issued
to Rickards or in the above cited U.S. Pat. No. 4,644,954 issued to
Wittkampf et al., both of which are incorporated herein by reference in
their entireties. Basically, the controller defines a predetermined short
blanking period, for example on the order of 70 or 80 ms., during and
following the generation of a ventricular pacing pulse. The controller 106
then enables signals from electrodes 138 and 142 or from electrodes 140
and 142 to pass through. On detection of the peak amplitude, a signal from
amplifier 110 is passed through to controller 106, which performs a
measurement of Q-T interval used as described below to define the interval
between an overdrive pacing pulse and the subsequent generation of a high
voltage fibrillation inducing pulse.
Output stage 108 contains a high output pulse generator capable of
generating cardioversion and defibrillation pulses. For purposes of the
present invention, it is important that output stage 108 be also able to
generate a high voltage pulse, of at least 100 volts, for use as a
fibrillation inducing pulse in conjunction with the present invention.
High output pulses, including cardioversion, defibrillation and
fibrillation inducing pulses are applied to the patient's heart via
electrodes 134 and 136, which are typically large surface area electrodes
mounted on the heart, electrodes mounted in the heart, or some combination
thereof. Any prior art implantable defibrillation electrode system made
and used in conjunction with the present invention. Output circuit 108 is
also coupled to electrodes 138 and 140 which are employed to accomplish
ventricular bradycardia pacing. Electrode 138 is typically located on the
distal end of a endocardial lead and is typically placed in the apex of
the right ventricle. Electrode 140 is typically an indifferent electrode
mounted on or adjacent to the housing of the implantable defibrillator.
Output circuit 108 is controlled by control bus 122, which allows the
controller 106 to determine the time, amplitude and pulse width of the
pulse to be delivered and to determine which electrode pair will be
employed to deliver the pulse.
Sensing of heart activity, both for normal sensing of ventricular
contractions and for determining whether pacing pulses have successfully
captured the heart is accomplished by input amplifier 110, coupled to
electrodes 138, 140 and 142. Electrode 1 | | |
