Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-10-05
2003-09-02
Vo, Hieu T. (Department: 3747)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S005000, C607S009000, C607S017000, C607S028000
Reexamination Certificate
active
06615082
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to an implantable medical device, e.g., a cardiac stimulation device, and is particularly directed to an automatic capture/threshold pacing method for use in such a device.
BACKGROUND OF THE INVENTION
Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation or a pathologic rapid organized rhythm and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functions of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation pulses when they are needed and inhibit the delivery of cardiac stimulation pulses at other times. This inhibition accomplishes two primary functions. Firstly, when the heart is intrinsically stimulated, its hemodynamics are generally improved. Secondly, inhibiting the delivery of a cardiac stimulation pulse reduces the battery current drain on that cycle and extends the life of the battery which powers and is located within the implantable cardiac stimulation device. Extending the battery life will therefore delay the need to explant and replace the cardiac stimulation device due to an expended battery. Generally, the circuitry used in implantable cardiac stimulation devices have been significantly improved since their introduction such that the major limitation of the battery life is primarily the number and amplitude of the pulses being delivered to a patient's heart. Accordingly, it is preferable to minimize the number of pulses delivered by using this inhibition function and to minimize the amplitude of the pulses where this is clinically appropriate.
It is well known that the amplitude of a pulse that will reliably stimulate a patient's heart, i.e., its threshold value, will change over time after implantation and will vary with the patient's activity level and other physiological factors. To accommodate for these changes, pacemakers may be programmed manually by a medical practitioner to deliver a pulse at an amplitude well above an observed threshold value. To avoid wasting battery energy, the capability was developed to automatically adjust the pulse amplitude to accommodate for these long and short term physiological changes. In an existing device, the Affinity® DR, Model 5330 L/R Dual-Chamber Pulse Generator, manufactured by the assignee of the present invention, an AutoCapture™ pacing system is provided. The User's Manual, ©1998 St. Jude Medical, which describes this capability is incorporated herein by reference. In this system, the threshold amplitude level is automatically determined for a predetermined duration level in a threshold search routine and capture is maintained by a capture verification routine. Once the threshold search routine has determined a pulse amplitude that will reliably stimulate, i.e., capture, the patient's heart, the capture verification routine monitors signals from the patient's heart to identify pulses that do not stimulate the patient's heart (indicating a loss-of-capture). Should a loss-of-capture (LOC) occur, the capture verification routine will generate a large amplitude (e.g., 4.5 volt) backup pulse shortly after (typically within 80-100 milliseconds) the original (primary) stimulation pulse. This capture verification occurs on a pulse-by-pulse basis and thus, the patient's heart will not miss a beat. However, while capture verification ensures the patient's safety, the delivery of two stimulation pulses (with the second stimulation pulse typically being much larger in amplitude) is potentially wasteful of a limited resource, the battery capacity. To avoid this condition, the existing device, monitors for two consecutive loss-of-capture events and only increases the amplitude of the primary stimulation pulse should two consecutive loss-of-capture (LOC) events occur, i.e., according to a loss-of-capture criteria. This procedure is repeated, if necessary, until two consecutive pulses are captured, at which time a threshold search routine will occur. The threshold search routine decreases the primary pulse amplitude until capture is lost on two consecutive pulses and then, in a similar manner to that previously described, increases the pulse amplitude until two consecutive captures are detected. This is defined as the capture threshold. The primary pulse amplitude is then increased by a safety margin value, e.g., 0.3 volts, to ensure a primary pulse whose amplitude will exceed the threshold value and thus reliably capture the patient's heart without the need for frequent backup pulses. In a copending, commonly-assigned U.S. patent application Ser. No. 09,483,908 to Paul A. Levine, entitled “An Implantable Cardiac Stimulation Device Having Autocapture/Autothreshold Capability”, improved loss-of-capture criteria are disclosed which are based upon X out of the last Y beats, where Y is greater than 2 and X is less than Y. The Levine application is incorporated herein by reference in its entirety.
Whether a stimulation pulse successfully captures muscle, e.g., cardiac, tissue and thus causes the muscle to contract is related to an amplitude component, i.e., voltage or current, and a duration component of the stimulation pulse. This relationship was described in 1909 by Lapicque as a strength-duration curve (see an exemplary curve
10
in
FIG. 1
) which is expressed by the equation:
I=I
R
*(1
+d
c
/d
)
where I
R
represents the current at the rheobase, i.e., the lowest current pulse (independent of duration) that can stimulate the body tissue and d
c
represents the chronaxie time duration, i.e., a duration at which stimulation requires twice the rheobase current value.
This relationship is readily apparent by setting d equal to d
c
which results in I=2*I
R
.
This equation can be adjusted to display voltage by multiplying each side by the lead impedance, resulting in:
V=V
R
*(1
+d
c
/d
)
The energy used for each pulse is a function of the amplitude level (i.e., voltage or current) and the duration of the delivered pulse as shown in the equation:
E
=(
V
2
*d
)/
R
where V is the amplitude of the voltage pulse, d is its duration and R is the lead impedance.
It has been observed and can be shown that the minimum energy point on the strength-duration curve is at a chronaxie point
12
(as shown in
FIG. 1
which shows a prior art implementation of a stimulation energy curve), i.e., where the amplitude component is twice the rheobase
10
and the duration component is the chronaxie duration. Known automatic capture/threshold algorithms adjust the threshold amplitude, e.g., voltage, at a fixed duration, preferably the chronaxie duration. It appears that these algorithms are based on the assumption that changes in the strength-duration curve solely effect the rheobase, i.e., if the chronaxie is essentially fixed, the strength-duration curve will solely shift vertically during the life of the patient (see curve
14
relative to curve
10
). Since the known existing automatic capture/threshold algorithms only alter the amplitude component (see stimulation energy curve
16
), the belief that the chronaxie is “fixed” for a given patient is inherent in these algorithms. If in fact the chronaxie is fixed, an amplitude shift alone will result in the minimum energy dissipation since the stimulation point would shift from the chronaxie point
12
of strength-duration curve
10
to the chronaxie point
18
of the subs
Hoang Johnny H.
Pacesetter Inc.
Vo Hieu T.
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