U.S. patent number RE30,387 [Application Number 05/901,962] was granted by the patent office on 1980-08-26 for automatic cardioverting circuit.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Thomas E. Davis, Rollin H. Denniston, III.
United States Patent |
RE30,387 |
Denniston, III , et
al. |
August 26, 1980 |
Automatic cardioverting circuit
Abstract
Pulse generating apparatus which provides electrical
heart-stimulating pulses only in the absence of normal heart
activity. If the patient's heart has developed a life threatening
arrhythmic condition the inventive apparatus automatically applies
an electrical shock to the heart having sufficient magnitude to
restore normal heart activity. The inventive apparatus features a
redundant heartbeat sensing system which monitors two dynamic
characteristics of heart function, for example, heart contraction
and EKG. An electrical heart stimulating pulse is delivered to the
patient's heart following the elapse of a specified period of time
since the sensing of a dynamic characteristic indicative of a
normal functional heart. Sensing control is automatically regained
following successful heart stimulation, thereby inhibiting the
application of further electrical pulses. In the event that the
patient's heart fails to resume normal heartbeat action, the
inventive apparatus will continue delivering intermittant shocks--a
lower energy pulse is applied first followed by higher energy
pulses. .Iadd.
Inventors: |
Denniston, III; Rollin H.
(Golden, CO), Davis; Thomas E. (Duluth, MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
26929195 |
Appl.
No.: |
05/901,962 |
Filed: |
May 1, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
235756 |
Mar 17, 1972 |
03805795 |
Apr 23, 1974 |
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Current U.S.
Class: |
607/6 |
Current CPC
Class: |
A61N
1/36585 (20130101); A61N 1/3956 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61N 1/39 (20060101); A61N
001/32 () |
Field of
Search: |
;128/2.6A,2.6F,2.6R,2.6E,2R,2S,21E,21R,404,419D,419R,419P |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Schuder et al., "Transactions of the American Society for
Artificial Internal Organs", vol. XVI, 1970, pp. 207-212. .
Stratbucker et al., "Rocky Mountain Engineering Society", 1965, pp.
57-61..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Forest; Carl A. Breimayer; Joseph
F. Schwartz; Lew
Claims
We claim: .[.1. Heart contraction sensing and stimulation circuit
comprising:
a. first detecting means responsive to a change in a monitored
electrical parameter produced by the normal beating action of the
heart for providing a first identifiable electrical signal
corresponding with each heart contraction;
b. second detecting means responsive to monitored heart electrical
activity corresponding to heart contractions for providing a second
identifiable signal corresponding with each heart contraction;
c. gating means electrically connected to receive electrical
signals from said first and second detecting means, said gating
means for providing an electrical output signal in response to
either of said identifiable electrical signals produced by a single
heart contraction;
d. electrical energy storage means capable of storing sufficient
energy to cardiovert a malfunctioning heart;
e. electrical energy source means;
f. control means connected in controlling relation to said energy
storage means and said energy source means and operatively
connected to said gating means and responsive thereto, said control
means for controlling the transmission of electrical energy from
said source means to said storage means only in the absence of
electrical signals from both the first and second detecting means
for a predetermined period of time;
g. output means adapted for connection to the heart;
h. regulating means connected in controlling relation to said
energy storage means, said regulating means for permitting energy
to be transmitted from said storage means to said output means when
the energy stored by said storage means becomes greater than a
predetermined
level..]. 2. Heart contraction sensing and stimulation apparatus
comprising:
a. first heart monitoring means being in the form of an elastomer
body means having conductive particles imbedded therein and
exhibiting a change in electrical impedance upon flexing, the body
means including means adapted to be positioned adjacent heart
muscle so that each heart contraction causes the body means to flex
thereby changing its impedance;
b. first detecting means responsive to a change in impedance of
said elastomer body means for providing a first identifiable
electrical signal corresponding with each heart contraction;
c. second heart monitoring means in the form of conductive
electrode means being adapted for insertion within the human
vascular system and positioned adjacent the heart for monitoring
heart electrical activity and transmitting electrical energy to the
heart,
d. second detecting means responsive to the monitored heart
electrical activity corresponding to heart contractions for
providing a second identifiable signal corresponding with each
heart contraction;
e. gating means electrically connected to receive electrical
signals from said first and second detecting means, said gating
means for providing an electrical output signal in response to
either of said identifiable electrical signals produced by a single
heart contraction;
f. electrical energy storage means capable of storing sufficient
energy to cardiovert a malfunctioning heart;
g. electrical energy source means;
h. control means electrically connected in controlling relation to
said energy storage means and said energy source means and
operatively connected to said gating means and responsive thereto,
said control means for controlling the transmission of electrical
energy from said source means to said storage means only in the
absence of both said first and second identifiable signals for a
predetermined period of time;
i. regulating means electrically connected to said electrode means
and further connected in controlling relation to said energy
storage means, said regulating means for permitting transmission of
energy from said storage means to said electrode means when the
energy stored by said storage means is greater than a predetermined
level; and
j. disabling means electrically connected to said first monitoring
means for sensing a break in the electrical circuitry of said first
monitoring means, said disabling means for preventing electrical
energy from being
delivered to said electrodes when such a break occurs. 3. The
apparatus of claim 2 further comprising flexible enclosure means
substantially inert in living body fluids and tissue, the enclosure
means being adapted for insertion within the vascular system of a
living animal, the enclosure means further for enclosing, at least
some of, the apparatus elements,
thereby sealing them from living body fluids and tissues. 4. The
apparatus of claim 2 wherein the second monitoring means includes
two conductive electrode means, one of the electrode means being
adapted to be positioned within the heart and the other electrode
means being adapted to be positioned outside the heart to monitor
heart electrical activity produced
by the natural beating action of the heart. 5. The apparatus of
claim 2 wherein the second detecting means includes discrimination
means, said discrimination means for responding to the heart
electrical activity produced by the natural beating action of the
heart while discriminating against the heart electrical signals
produced by an abnormally functioning
heart and those artificially induced by a heart pacing device. 6.
The apparatus of claim 2 wherein said gating means includes a
conforming means, said conforming means for transforming the pulses
having varying amplitudes and widths into pulses having
substantially the same pulse
amplitude and pulse width. 7. The apparatus of claim 2 wherein said
control means has a transmitting and a non-transmitting state and
includes a timing means for maintaining said control means in the
non-transmitting state for a predetermined time interval following
each signal received
from said gating means. 8. The apparatus of claim 7 wherein the
timing means includes capacitive means for switching said control
means from the non-conductive state to the conductive state when
said capacitive means becomes charged above a predetermined level,
said capacitive means being operatively connected to said gating
means so that a signal from said gating means causes the capacitive
means to discharge rendering said
control means non-conductive. 9. The apparatus of claim 2 further
comprising a counting means operatively connected to said energy
storage means for disabling the transmission of electrical pulses
from said energy storage means to said electrode means after a
predetermined number of electrical pulses have been transmitted
without an intervening heart contraction having been detected by
said first or second detecting means.
0. The apparatus of claim 2 wherein the regulating means includes
multiple level control means, said level control means being
constructed and arranged such that energy will begin being
transmitted from said energy storage means to said electrodes when
the energy stored by said storage means is above a certain first
predetermined level and will cease being transmitted when the
stored energy becomes less than a second lesser predetermined
level, thereby causing the energy stored by said storage means to
be transmitted to said electrodes in a truncated capacitive
discharge electrical pulse waveform. 11. The apparatus of claim 2
wherein the regulating means includes means for setting the time
interval between the generation of the latest pulse from said gate
means and the generation of the first electrical pulse transmitted
to said electrode means so that said time interval is substantially
greater than the time interval between each of the succeeding
electrical pulses transmitted to said electrodes, during the time
interval in which no pulses are provided by said gate
means. 12. The apparatus of claim 2 wherein the regulating means
includes means for increasing the energy content of all but the
first electrical pulse of the series of pulses transmitted to said
electrodes during the time interval in which there is an absence of
normal heart contractions.
. In an automatic cardioverting system comprising intravascular
electrical lead means having electrodes adapted for transmitting
electrical pulses to the heart of a living animal, sensing means
including electrical by conductive means within said lead means and
flexible therewith for sensing the action of a heart contration
upon said lead means, means electrically connected with and
responsive to said sensing means for continuously monitoring heart
activity and producing an identifiable electrical signal
corresponding to each sensed heart contraction, and stimulation
means responsive to the identifiable signal and operatively
connected to said intravascular lead for providing cardioverting
shocks to said electrodes, the improvement comprising:
disabling means electrically connected to said sensing means for
sensing a break in said electrically conductive means and its
electrical connection with said monitoring means, said disabling
means for preventing cardioverting shocks from being transmitted
from said stimulating means to said electrode upon sensing a break
in said electrically conductive means or its electrical circuitry.
Description
This is a divisional reissue application based on U.S. Pat. No.
3,805,795 issued Apr. 23, 1974 on an application Ser. No. 235,756
filed Mar. 17, 1972; a related divisional reissue application is
the application Ser. No. 901,963 filed May 1, 1978. .Iaddend.
BACKGROUND OF THE INVENTION
During the past several decades, coronary heart disease has come to
occupy the first position among the causes of death in the
developed areas of the world. In the United States, for example,
this disease is responsible for over one-half million deaths
yearly. And of this number, more than half occur suddenly, outside
the hospital, and therefore before the patient is able to obtain
the necessary medical assistance. Although the precise cause of
sudden death in coronary heart disease has not yet been entirely
clarified, the available evidence permits the medical field to
ascribe death in the majority of these cases to grave disturbances
in cardiac electrical activity culminating in ventricular
fibrillation.
Another frustrating but related problem is the present inability to
deal efficiently with lethal and nonlethal arrhythmias outside of a
hospital setting. Within the hospital environment, however, recent
experience has clearly demonstrated that ventricular fibrillation
and its frequent precursor, ventricular tachycardia, are reversible
phenomena when prompt cardioversion of the heart is instituted.
Under such circumstances, cardiac function can frequently be
restored to normal without the patient suffering from residual
disability. Unfortunately, however, the present state of the art
makes cardioversion very dependent upon a highly specialized
medical environment, thus limiting such treatment to fully
equipped, modern hospitals.
There is no question that a great need exists for a defibrillator
which would be carried by those who are prone to having one of the
many life-threatening arrhythmias generally discussed above. Thus,
in some patients having coronary heart disease, a fatal outcome
from ventricular tachycarida or ventricular fibrillation could be
avoided, even in the absence of immediate medical assistance. The
first step, of course, is the detection of those prone to suffering
from cardiac malfunctions leading to ventricular tachycardia or
ventricular fibrillation.
While it is not possible to predict with unerring exactness which
patients suffering from coronary heart disease will die from
ventricular fibrillation or ventricular tachycardia, several high
risk groups of patients can be recognized. For example, patients
who have experienced myocardial infarction, even though they may be
surviving in good health, run a substantial risk of dying suddenly,
a risk several times greater than that associated with the general
population. Further, if patients with myocardial infarction have a
history of serious ventricular arrhythmias and/or of cardiac
arrest, or if evidence of persistent myocardial irritability is
present, it may be logically assumed that the risk of sudden death
is increased substantially. A patient like those described above
would greatly benefit from an automatic defibrillator.
Also, such an automatic defibrillator would be an asset to those
patients who have suffered myocardial infarction in the coronary
care unit and remain hospitalized in the coronary care unit or some
other area of the hospital. Under such circumstances, the
defibrillator could be used temporarily for the remainder of the
expected hospital stay; or the automatic defibrillator could be
permanently implanted for use both in the hospital and after
discharge. And another recognizable class of patients particularly
in need of an automatic defibrillator is the class composed of
those who have not shown prior histories of myocardial infarction
but who show severe symptoms of coronary heart disease, such as
ventricular arrhythmias resistant to medical treatment or angina
pectoris.
From the brief discussion above, there should be little doubt that
the possible applications for an automatic defibrillator are
numerous. Such an automatic defibrillator has been developed by
Medtronic, Inc. and is described in U.S. Pat. application Ser. No.
124,326, filed Mar. 15, 1971, now abandoned by Mieczyslaw Mirowski,
et al. and entitled "CARDIOVERTER HAVING SINGLE INTRAVASCULAR
CATHETER ELECTRODE SYSTEM."
The automatic standby defibrillator described in the
above-identified patent application employs a pressure sensing
element attached to a body implantable electrical lead such that it
can be positioned within the right ventricle of the heart. Since
the pressure in the heart drops severely when the heart goes into
the fibrillation state, ventricular fibrillation can be easily
detected by monitoring heart pressure. However, several
difficulties with measuring heart pressure are encountered. One
disadvantage with using pressure as an indicator of the
fibrillation state is that the small pressure sensing elements
which are suitable for use with body implantable electrical leads
are quite expensive. A second disadvantage with using these
pressure sensing elements is that they must either be located
alongside, on the outer surface, or at the tip of the body
implantable electrical lead to obtain accurate pressure readings.
Locating the detection means alongside the catheter tends to make
the catheter bulky and inflexible. Fibrotic tissue tends to
build-up around the sensing element when it is positioned within
the heart. This build-up tends to dampen the pressure transducer
mechanism, thus giving inaccurate pressure readings. Inaccurate
pressure readings may also result when the element is located at
the tip and wedged into the apex of the right heart ventricle as
the transducer will then, of course, tend to be dampened by the
surrounding heart muscle.
The apparatus of this invention uses a single intravascular
electrode of the type described in U.S. Pat. application Ser. No.
202,238, filed Nov. 26, 1971, by Rollin H. Denniston, III, entitled
"MUSCLE CONTRACTION DETECTION APPARATUS," to perform three
functions; namely, (1) detecting heart contractions; (2) detecting
heart electrical activity in the form of R waves; and (3) applying
electrical impulses to the heart for cardioverting it.
Thus the apparatus of this invention overcomes many difficulties
existent in the prior art devices while providing a compact and
practical automatic cardioverting system.
SUMMARY OF THE INVENTION
The present invention relates to a cardioverter, an electronic
system which, after detecting one of the above-noted lethal or
non-lethal arrhythmias, automatically cardioverts the heart of the
user. "Cardioverting" or "cardioversion" as used herein is intended
to mean a method of correcting a number of arrhythmic heart
conditions including atrial tachycardia, atrial fibrillation,
junctional rhythms, ventricular tachycardia, ventricular flutter,
and ventricular fibrillation, and any other non-pacing related
arrhythmic condition which may be corrected by applying electrical
shocks to the heart. Obviously then, "defibrillation" is included
in the term "cardioversion" as a method of applying electrical
shocks to the heart to defibrillate a fibrillating atrium or a
fibrillating ventricle. The system of the present invention may be
installed in patients particularly prone to develop ventricular
tachycardia and/or ventricular fibrillation, or other types of
tachyarrhythmias which may be corrected by cardioverting, either on
a temporary or a permanent basis. And, because of extremely small
and compact size, the system including both electrodes may be
totally and completely implanted under the skin of the patient, or
alternatively, may be carried externally, save for the sensing
probe carrying the two electrodes.
More particularly, the present invention relates to an automatic
cardioverting circuit for monitoring cardiac contraction and
sensing when the heart has developed an arrhythmic heart condition,
and which then automatically applies a cardioverting shock to the
heart of sufficient magnitude to restore effective heart rhythm.
The device is adapted to continue delivering intermittent shocks to
the heart in the event that the heart fails to return to its normal
behavioral pattern, and has the ability of automatically regaining
sensing control over a functional heart, thereby insuring that
further shocks are inhibited after successful cardioversion has
taken place.
The automatic cardioverting circuit comprises two basic subsystems;
a sensing system, which continuously monitors heart activity; and a
stimulation system which upon receiving a signal from the sensing
system applies a cardioverting shock to the heart myocardium
through an intravascular electrical lead.
The sensing system of the present invention monitors two dynamic
characteristics of the heart and provides an electrical signal
corresponding to each heart contraction. The absence of both these
characteristics for a predetermined period of time is required
before the stimulation system will be activated to transmit a
cardioverting shock to the heart. One of the characteristics
monitored is the EKG. The EKG is obtained from the electrodes
located on the intravascular lead. The second characteristic
monitored is muscle contraction. The muscle contraction signal is
obtained from a contraction sensing device positioned in the
intravascular lead and consisting of a conductive elastomer body
having carbon particles imbedded therein. The contraction signal is
generated whenever the contraction sensing device is flexed by a
heart contraction.
The EKG and the heart contraction signals are fed to a gating
device. The gating device will allow a cardioverting shock to be
delivered to the heart only if both signals are absent for a
predetermined period of time. Thus a heart contraction detected by
either the EKG monitoring system or the heart contraction
monitoring system is sufficient to prevent a cardioverting pulse
from being delivered to the heart. Consequently, each of the
monitoring devices provides a back-up signal for the other.
The stimulation portion of the present invention applies energy to
the heart in the form of electrical pulses delivered through the
electrodes located on the intravascular lead. The application of
these electrical pulses to the patient's heart is delayed for a
preset period of time (on the order of 15-20 seconds) following the
sensing of abnormal heart activity. If normal cardiac action
resumes during this period, the application of the cardioverting
pulses is automatically inhibited. This delay gives the heart the
opportunity to convert spontaneously to normal cardiac rhythm if it
is able to do so, and also insures that the cardioverting pulses
are applied only when they are needed.
The present invention comprising the sensing system and the
stimulation system provides an automatic cardioverting device
capable of cardioverting a malfunctioning heart at relatively low
energy levels. This device senses when the heart is malfunctioning
and then automatically delivers a cardioverting shock to the heart.
The device lies dormant during normal heart activity and applies a
shock to the heart only when the heart functions become abnormal.
This device is extremely compact and features an electrode system,
in the form of an intravascular lead, which is totally and
completely implantable in the body of a patient. This single
intravascular lead is used for sensing the difference between a
normally functioning heart and one which is functioning abnormally,
and also for transmitting cardioverting shocks to the heart through
the electrodes positioned on the same lead. The intravascular lead
is also capable of being used for sensing heart conditions
requiring heart pacing and for transmitting pacing pulses to the
heart.
The invention features a redundant heart contraction sensing
system. Two dynamic heart characteristics of the heart
function--EKG and heart contraction--are monitored by the
invention. A cardioverting shock is transmitted to the heart only
following the elapse of a specified period of time since the
sensing of a dynamic characteristic indicative of a normally
functioning heart. This aids in assuring that cardioverting shocks
will be delivered to the heart only when they are needed, and thus
largely eliminates the concern over possible heart damage being
caused by the delivery of cardioverting shocks to a properly
functioning heart.
A disabling feature of the invention further guards against
unnecessary cardioverting shocks being applied to the heart. A
fracture in the contraction sensor will be automatically detected
by a disabling means which will then disable the portion of the
circuit which generates and transmits cardioverting shocks to the
heart. Therefore, a fracture in the contraction sensor will not
result in an unnecessary cardioverting shock being applied to the
heart. Further, using an endocardial implantable electrical lead,
the invention provides a reliable way of cardioverting a
malfunctioning heart without causing serious damage to the heart by
the application of high energy densities directly to the heart
endocardium. The truncated capacitive discharge waveform used in
the circuit of the invention to apply energy to heart helps
minimize the peak and total energy required to cardiovert the
heart. Another way the invention minimizes the energy densities
applied to the heart is by applying a lower energy pulse first, and
then, if that pulse does not restore normal heart functioning,
applying cardioverting pulses having higher energy content. The
invention also features a counting means which automatically
disables the cardioverting circuit after a predetermined number of
pulses have been delivered to the heart; thereby preventing
cardioverting shocks from being applied when they have little
chance of restoring the heart to normal functioning, and could
damage the heart.
The invention additionally provides a means for delaying
application of the first cardioverting pulse for a period of time,
for example, 15 to 20 seconds, following the sensing of abnormal
heart functioning. This delay gives the heart the opportunity to
convert spontaneously to a normal cardiac rhythm, and also insures
that cardioverting pulses are not applied if the heart condition is
not critical. There is, of course, no need for the long built-in
delay period with the succeeding cardioverting shocks as there is
no longer any doubt but that the heart condition is now critical.
Accordingly, the succeeding shocks are separated by significantly
shorter time intervals.
Other features and advantages of the present invention will be set
forth in, or become apparent from, the following description and
claims and illustrated in the accompanying drawings, which disclose
by way of example and not by way limitation, the principle of the
invention and the structural implementation of the inventive
concept.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating basic components of the
apparatus provided by .[.this.]. .Iadd.the companion divisional
reissue application to the present .Iaddend.invention;
FIG. 2 is a graph indicating the shape of electrical waves produced
by the heart during normal heartbeat action;
FIG. 3 shows electrical circuitry of the heartbeat sensing means
embodied in the apparatus of this invention;
FIG. 4 is a schematic diagram illustrating the power supply and the
low battery indicator incorporated in the apparatus of this
invention;
FIG. 5 is a schematic diagram illustrating the control means
incorporated in the apparatus of this invention;
FIG. 6 is a schematic diagram illustrating the system disabling
means incorporated in the apparatus of this invention;
FIG. 7 is a schematic diagram illustrating the regulating means
incorporated in the apparatus of this invention;
FIG. 8 is a voltage v. time diagram illustrating the voltage on the
energy storage means embodied in the inventive apparatus and the
states of the reed switch and the SCR embodied in the inventive
apparatus, and how these voltages and component states effect the
wave form and voltage magnitude of the cardioverting pulses applied
to the patient's heart by the apparatus of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring generally to FIG. 1, .Iadd.there is shown .Iaddend.the
cardioverting apparatus .[.of this.]. .Iadd.according to the
companion reissue application to the present invention, i.e.
divisional reissue application Ser. No. 901,963 filed May 1, 1978
by Rollin H. Denniston III, Thomas E. Davis and Mieczyslaw Mirowski
and based on U.S. Pat. No. 3,805,795 issued April 23, 1974. The
present invention may be more clearly understood by considering the
companion invention shown in FIG. 1 which .Iaddend.invention
includes: sensing means shown in block 10 adapted to sense each
contraction of the patient's heart; stimulation means shown in
block 12 adapted to automatically provide electrical impulses which
can be used to cardiovert the patient's heart; an intravascular
electrical lead represented in block 16 adapted to detect heart R
waves and contractions and to apply electrical impulses to the
patient's heart; disabling means shown in block 14 adapted to
disable stimulation means 12 whenever electrical lead 16 is
rendered inoperative; and a power supply for the system .Iadd.not
.Iaddend.shown in FIG. .[.4.]. .Iadd.1.Iaddend.. Sensing means 10
monitors heart activity and provides an electrical signal to
stimulation means 12 which corresponds with each heart contraction.
When no electrical signal is received from sensing means 10 for a
predetermined period of time, stimulation means 12 is automatically
activated to transmit a cardioverting electrical impulse to the
heart through lead 16.
It will be understood that the normal beating of the human heart
produces electrical signals or waves which are representative of
the various stages in the occurrence of each heartbeat. Thus a
heart beating in sinus rhythm produces electrical waves
conventionally identified as P, Q, R, S and T waves, as shown in
FIG. 2. The R wave, for example, is representative of a heart's
ventricular contraction and can be detected by the electrodes of a
conventional electrical intravascular lead of the type commonly
used in heart pacing.
An intravascular electrical lead is diagrammatically shown as block
16 in FIG. 1. This block represents an intravascular electrical
lead which is adapted to detect the EKG and contractions of the
heart and to apply cardioverting electrical pulses to the patient's
heart. In a preferred embodiment of this lead the EKG is detected
using electrically conductive electrodes; .Iadd.according to the
present invention .Iaddend.the heart contractions are detected by
an elastomer body which changes impedance whenever it is flexed, as
for example, by heart contraction; and the cardioverting electrical
impulses are applied to the heart via the same electrodes as used
to detect the EKG. However, it will be understood that the
above-described embodiment is only one of the many different
intravascular lead embodiments which can be advantageously used
with the apparatus of this invention.
Sensing means 10 comprising EKG sensor 20, contraction sensor 22,
"or" gate 24, and wave conformer 26 is shown in FIG. 3. Sensing
means 10, using intravascular electrical lead 16, is adapted to
sense R waves and heart contractions and to provide an electrical
signal corresponding with each sensed normal heartbeat.
.[.With.]. .Iadd.Turning now to the description of the present
invention, with .Iaddend.reference to FIG. 3, contraction sensor 22
comprises fixed resistor 114, capacitor 112, and operational
amplifier 110. One side of fixed resistor 114 is connected to the 4
volt power supply. The other side of resistor 114 is connected to
the junction between electrical line 11 and to capacitor 112 for
convenience denoted as junction 113. Capacitor 112 is used to AC
couple junction 113 to the input side of contraction amplifier 110.
The output of amplifier 110 is transmitted on line 21.
Contraction sensor 22 is connected to electrical lead 16 by
electrical line 11 and to "or" gate 24 by electrical line 21 and is
adapted to provide a usable electrical signal corresponding with
each heart contraction.
In a preferred embodiment, electrical line 11 is connected to a
conductive elastomer body within electrical lead 16 which changes
impedance when flexed by a heart contraction. This change in
impedance is easily detectable as it will cause a change in the
current flowing from the 4 volt power source through resistor 114,
junction 113, electrical line 11, and the elastomer body.
The resulting change in voltage at junction 113 is AC coupled by
capacitor 112 to operational amplifier 110 where an electrical
output signal in the form of an electrical pulse, is generated in
response to the voltage and transmitted to the "or" gate 24 on
electrical line 21.
EKG sensor 20 is adapted to amplify each R wave signal detected by
the electrical lead 16 corresponding to a normal heartbeat. More
specifically, EKG sensor 20 amplifies R waves produced by a human
heartbeat, discriminating against the electrical heart waves
produced by a heart in fibrillation or otherwise abnormally
functioning, as well as the pacer pulses applied using lead 16.
EKG sensor 20 is electrically connected to electrical lead 16 via
electrical line 13 and line 17 from stimulation means 12 and to
"or" gate 24 by electrical line 23. EKG sensor 20 comprises R wave
amplifier 120, compensated monostable multivibrators 122 and 130,
capacitor 124, resistor 126, and "and" gate 140. The input side of
R wave amplifier 120 and multivibrator 130 are connected to
electrical line 13 at junction 119. The output of multivibrators
130 is directly connected to the input side of "and" gate 140,
whereas the output side of amplifier 120 is connected to the input
side of "and" gate 140 through the series combination of
multivibrator 122 and capacitor 124. One side of resistor 126 is
connected to the junction between capacitor 124 and "and" gate 140
and the other side is connected to the system ground. The output of
"and" gate 140 is transmitted on electrical line 23.
R wave amplifier 120 is an amplifier which operates in the same
manner as those commonly used in demand pacer. It is adapted to
select and amplify the R waves produced by heartbeats while
discriminating against electrical heart waves produced by an
abnormally functioning heart. The selection of the R waves is
commonly performed by amplitude and frequency filtering.
Monostable multivibrators 122 and 130 are of a conventional design.
In a preferred embodiment, the multivibrators used are conventional
RCA Monostable Oscillators using COS/MOS Digital Integrated
Circuits. The exact multivibrator circuitry used in this preferred
embodiment is described and shown schematically in FIG. 9 of RCA
Application Note ICAN-6267.
The preferred embodiment multivibrators have two states, a high
state and a low state. They are normally in the high state, and are
switched into the low state, for a predetermined period of time
(T.sub.a), upon receipt of an electrical pulse of sufficient
magnitude. The period of time (T.sub.a) the multivibrator is in the
low state and the threshold voltage (V.sub.1) of the pulse required
to trigger the multivibrator into the low state can, of course, be
varied by varying the component values of the circuitry associated
with the multivibrators. Accordingly, the output from the
multivibrators, upon receipt of a pulse of sufficient magnitude
(V.sub.1), will be an electrical pulse of a predetermined pulse
width (T.sub.a).
Capacitor 124 and resistor 126 are electrically connected to
multivibrator 122 in such a way that they differentiate the output
from multivibrator 122. Thus if multivibrator 122 remains in the
high state, there will be no input from multivibrator 122 through
the differentiating circuit of capacitor 124 and resistor 126 to
"and" gate 140. However, this differentiating circuit will provide
"and" gate 140 with a negative spike pulse followed by a positive
spike pulse at a time (T.sub.a) later when a negative electrical
pulse is generated by multivibrator 122.
Gate 140 is a conventional "and" gate. It is adapted so that it
will transmit an electrical pulse, if at the time it receives the
positive pulse from the differentiating circuit of capacitor 124
and resistor 126, the electrical signal received from multivibrator
130 is in a high state. If at the time the positive spike pulse is
received from the differentiating circuit the electrical signal
received from multivibrator 130 is in the low state, then gate 140
will not transmit an electrical pulse.
Selecting multivibrators 122 and 130 having the proper threshold
voltages (V.sub.1) and the proper pulse widths (T.sub.a) will allow
EKG sensor 20 to differentiate between pacer pulses and R waves. In
a preferred embodiment, multivibrator 122 will have a threshold
V.sub.1 of 10 m-v and a pulse width of T.sub.a of 1 m-sec and
multivibrator 130 will have a threshold V.sub.1 of 0.5 v and a
pulse width of 5 m-sec. R waves from a human heart beating in
normal sinus rhythm commonly have a magnitude in the 20 m-volt
range when sensed through the intracardiac lead system; whereas
pacer pulses are commonly in the 1.0 to 2.0 volt range.
Accordingly, a normally produced R wave will be insufficient to
trigger multivibrator 130 into its low state but the R wave
amplified by amplifier 120 will be sufficient to trigger
multivibrator 122 into its low state and thus will cause the
differentiating circuitry to supply a positive spike pulse to gate
140 when multivibrator 122 returns to its high state 1 m-sec.
later. This positive spike pulse will cause gate 140 to transmit an
electrical pulse on line 23 as multivibrator 130 will be in the
high state. Conversely, pacer pulses will trigger both
multivibrator 122 and multivibrator 130. Since the output of
monostable multivibrator 122 is effectively delayed 1 m-sec. by
differentiation elements 124 and 126, monostable multivibrator 130
will be in the low state when gate 140 receives the positive pulse
from multivibrator 122. Thus gate 140 will not transmit a pulse on
line 23.
Gate 24 comprises transistors 150 and 160. The base 151 of
transistor 150 is electrically connected to contraction sensor 22
via electrical line 21; the emitter 155 is connected directly to
the system ground; and the collector 153 is electrically connected
to wave conformer 26 via electrical line 25. The base 161 of
transistor 160 is electrically connected to EKG sensor 20 via
electrical lead 23; the emitter 165 is connected directly to the
system ground; and the collector 163 is electrically connected to
the electrical line 25 and the collector 153 of transistor 150.
Gate 24 functions as a conventional "or" gate. Transistors 150 and
160 are normally in the non-conductive state; however, if an
electrical pulse from contraction sensor 22 is received at the base
151 of transistor 150, it will render transistor 150 conductive,
thus providing a low resistance electrical path from electrical
line 25 to ground. Likewise, an electrical pulse from EKG sensor 20
will render transistor 150 conductive; thus providing a low
resistance electrical path from electrical line 25 to ground.
Consequently, whenever an electrical pulse is received from
contraction sensor 22 or EKG sensor 20 or from both, gate 24 will
provide a low resistance path from electrical line 25 to
ground.
Wave conformer 26 is electrically connected to gate 24 via
electrical line 25 and to stimulation means 12 via electrical line
15, and is adapted to conform the electrical signals received from
gate 24 into pulses having substantially the same pulse width and
amplitude. The conformed electrical pulses received from wave
conformer 26 have a predetermined pulse amplitude and width which
is sufficient to effect the functioning of stimulation means
12.
Wave conformer 26 comprises a programable unijunction transistor
180 electrically connected in a monostable multivibrator
arrangement. Programable unijunction transistor 180 has a gate
input 178, an anode input 179, and a cathode 177. This type of
transistor is commonly referred to as a PUT in the engineering
literature. It is rendered conductive, thereby providing a low
impedance from both the PUT anode and gate to its cathode, when the
anode voltage exceeds the gate voltage by a specified amount, for
example, 0.7 volts.
PUT 180 is connected with its gate 178 electrically connected to
junction 181, its anode 179 electrically connected to junction 183,
and its cathode 177 electrically connected to electrical line 15.
Resistor 194 is electrically connected between electrical line 15
and the system ground. Junction 181--the junction between resistors
182 and 184--is electrically connected to gate means 24 via
electrical line 25. Resistors 182 and 184 are connected in series
between the 4 volt power supply and ground, thereby forming a
voltage divider which establishes the voltage at junction 181 at a
predetermined value. Junction 183 is the junction between resistor
186 and diode 188. Resistor 186, diode 188 and the parallel
combination of resistors 192 and capacitor 190 are connected
between the 4 volt power supply and the system ground. The
component values of resistor 186 and 192 and capacitor 190 are
chosen such that the voltage at junction 183 is kept at a
predetermined value which normally forces PUT 180 into a
non-conducting state.
Wave conformer 26 is adapted to provide a pulse having a
predetermined amplitude and width in response to each electrical
signal received from gate 24. Whenever an electrical pulse is
received by gate 24 from contraction sensor 22 or EKG sensor 20 or
from both, gate 24 becomes active, providing a low resistance path
from electrical line 25 to ground. Junction 181 is electrically
connected to electrical line 25 and thus becomes connected to
ground via a low resistance path whenever gate 24 is active.
Consequently, whenever gate 24 is active, the voltage at junction
181 is decreased and falls to a voltage such that PUT 180 is
rendered conductive.
Transistor 180 will remain conductive for a predetermined period of
time. This time period is determined by the discharge time of
capacitor 190.
Once PUT 180 becomes conductive, capacitor 190 is prevented from
discharging through it by diode 188. Consequently, capacitor 190
must discharge through resistor 192. The capacitor 190 and resistor
192 component values are selected such that capacitor 190
discharges at a predetermined rate, thus keeping the voltage at
junction 183 sufficiently high to keep PUT 180 conductive for a
predetermined period of time. Accordingly, this time period
establishes the pulse width of the pulse generated by wave
conformer 26. The amplitude of the generated pulse is established
by the voltage at which junction 183 is maintained while PUT 180 is
conductive. The generated pulse having a predetermined amplitude
and width is transmitted to stimulation means 12 on output line
15.
Referring to FIG. 4, the power supply for the cardioverting system
is shown schematically. With reference to FIG. 1, the system power
supply (not shown) must provide the energy needed to charge storage
capacitor 34 as well as providing the energy needed to drive and
bias the circuitry of sensing means 10, and the associated
circuitry of stimulation means 12. This requires that it have a
substantially constant output, as the sensing and stimulation
circuitry do not function well when they are driven and biased by a
supply that fluctuates significantly; that it be able to supply the
relatively large amount of energy required to charge storage
capacitor 34; and that it be as compact as possible.
The above stringent requirements are met by the power supply
embodiment shown in FIG. 4 which comprises a 6 volt battery 210
which drives a 4 volt supply 220. The 6 volt battery 210, via a DC
to DC converter 32 (FIG. 1), supplies the energy needed to charge
capacitor 34 (FIG. 1), whereas the 4 volt supply 220 supplies a
constant driving and biasing voltage for the circuitry of sensing
means 10 and the associated circuitry of stimulation means 12. This
particular embodiment prevents fluctuations in the output of the 6
volt battery 210, caused by the drain put on it when the storage
capacitor 34 is charged, from affecting the 4 l volt source 220
output, provided the output of the 6 volt battery 210 remains above
4 volts.
A low battery indicator 230 is also shown in FIG. 4. The indicator
230 is set so that it is activated whenever the power source 210
output falls below a predetermined voltage level, for example, 4.0
volts, and is used to drive a light emitting diode which indicates
that the power source output is below this predetermined level.
Controller 30 is shown schematically in FIG. 5. It comprises
programable unijunction transistor 340, transistor 310, diode 328,
capacitor 330 and resistors 326, 352, 354 and 346. Programable
unijunction transistor 340 has a gate input 341, an anode input
343, and a cathode 345. This type of transistor is commonly
referred to as a PUT in the engineering literature. It is rendered
conductive, thereby providing a low impedance from both the PUT
anode and gate to its cathode when the anode voltage exceeds the
gate voltage by a specified amount, for example, 0.7 volts.
Resistors 352 and 354 are electrically connected in series between
the 4 volt power source and the system ground. Resistor 326, diode
328, and capacitor 330 are likewise electrically connected in
series between the 4 volt source and the system ground. The
junction between resistors 352 and 354, designated junction 335, is
electrically connected to gate 341 of PUT 340--the junction between
resistor 326 and diode 328 designated junction 333, is electrically
connected to anode 343 of PUT 340. The cathode 345 of PUT 340 is
electrically connected to electrical line 31 and also to the system
ground through resistor 346. Transistor 310 has its collector 313
electrically connected to the junction between diode 328 and
capacitor 330, designated junction 320; its emitter 315
electrically connected to the system ground; and its base 311
electrically connected to sensing means 10 via electrical line
15.
The junction 335 voltage is maintained constant as it is the
junction between resistors 352 and 354 which form a voltage
divider; whereas the junction 333 voltage varies depending upon the
charge on capacitor 330. Capacitor 330 is charged by the 4 volt
energy source through the series connection of resistor 326 and
diode 328. In a preferred embodiment the component values of
capacitor 330, diode 328 and resistor 326 are chosen so that it
takes approximately 5 seconds to charge capacitor 330 to the
predetermined level where it will render PUT 340 conductive. Diode
328 prevents capacitor 330 from discharging through PUT 340. Since
transistor 310 is connected across capacitor 330 to the system
ground, whenever transistor 310 is rendered conductive capacitor
330 rapidly discharges to ground. However, transistor 310 is in the
non-conductive state unless it receives an electrical signal from
sensing means 10 on electrical line 15. This signal comes from wave
conformer 26 and is of sufficient pulse amplitude and width to keep
transistor 310 on long enough for capacitor 330 to totally
discharge.
Consequently, whenever an electrical signal is received from
sensing means 10, capacitor 330 will discharge, rendering PUT 340
non-conductive. PUT 340 will remain non-conductive for at least 5
seconds; longer if another pulse is received from sensing means 10
during that five second interval. However, when PUT 340 becomes
conductive, it will remain conductive until a pulse is received
from sensing means 10. This is the case since capacitor 330 cannot
discharge except through transistor 310, and thus will remain at
substantially the same voltage until transistor 310 is rendered
conductive by a pulse from sensing means 10.
System disable 14, shown schematically in FIG. 6, continuously
monitors the contraction sensing circuitry of intravascular lead
16. It is adapted and connected so that if an open circuit should
occur in the contraction sensor, whether due to a break in lead 16
or in the sensor itself, a visual alarm is activated and the
controller 30 is clamped so that it is non-conducting.
System disable 14 comprises conventional Darlington amplifier 210,
visual alarm 220 in the form of a light emitting diode, and
resistors 230 and 240. Visual alarm 220 is electrically connected
between the six volt power supply and input 212 of amplifier 210.
Amplifier 210 is electrically connected to the contraction sensor
of intravascular lead 16 at input 214 via electrical line 11. The
two outputs 216 and 218 from amplifier 210 are electrically
connected to the system ground through resistors 230 and 240
respectively. Output 216 is additionally electrically connected to
controller 30 via electrical line 15.
When an open circuit occurs in the contraction sensing circuit,
system disable 14 will prevent stimulation means 12 from applying a
cardioverting pulse to the patient's heart. Specifically, when the
contraction sensing circuit is broken, the increase in voltage at
input 214 will render amplifier 210 conductive. Amplifier 210 will
remain conductive until the break in the contraction sensing
circuit is repaired. When amplifier 210 conducts, visual alarm 220
will be activated and an electrical signal will be transmitted to
controller 30 on electrical line 15. This electrical signal clamps
controller 30 in an off state thereby preventing controller 30 from
activating DC--DC converter 32 and thus prehibiting a cardioverting
pulse being applied to the patient's heart.
Regulator 36 is shown schematically in FIG. 7. It controls the
application of stimulating pulses to the heart. That is, it allows
stimulating pulses to be transmitted from capacitor 34 to the heart
only when they have an energy content which is sufficient so that
they are likely to be able to stimulate heart activity, but not so
great so that it is likely to cause permanent heart damage. The
energy content of the applied pulses is determined by regulator 36.
Specifically, regulator 36 is adapted to apply a relatively low
energy cardioverting pulse first, and then if that does not restore
normal heart function to apply a higher energy cardioverting
pulse.
Referring generally to FIG. 7, regulator 36 will allow energy from
capacitor 34 to be applied to the heart when silicon controlled
rectifier (SCR) 410 is in the conductive state and reed relay 420
is in the position designated position B. If SCR 410 is in the
non-conductive state or reed switch 422 is in the position
designated position A, then energy cannot be applied from capacitor
34 to the heart. SCR 410 and reed switch 422 (when it is in
position B) are electrically connected in series between electrical
line 35 and electrical line 17--electrical line 17 is electrically
connected to intravascular lead 16 so that it is capable of
transmitting a cardioverting pulse to the heart. Reed switch 420,
when it is in position A, electrically connects EKG sensor 20 to
lead 16. Specifically, reed relay 420 electrically connects EKG
sensor 20 via electrical line 13 to lead 16 via electrical line 17.
Accordingly, EKG sensor 20 is electrically disconnected from lead
16, whenever reed switch 422 is capable of transmitting energy from
capacitor 34 to lead 16 (position B). Conversely, EKG sensor 20 is
electrically connected to lead 16 whenever reed switch 422 is
incapable of transmitting energy from capacitor 34 to lead 16
(position A). This isolates EKG sensor 20 from the cardioverting
pulse being applied to the heart.
Reed relay 420 is of conventional design. It comprises a coil 430
which is adapted to mechanically move switch 422 from one terminal
to another. Coil 430 is electrically connected at one end to 6 volt
power supply and at the other to a conventional Darlington
amplifier 435. When amplifier 435 is active, current flows from the
6 volt source through coil 430. This causes reed switch 422 to
mechanically move from terminal A to terminal B.
SCR 410 is connected with its input 411 electrically connected to
electrical line 35; its output 413 electrically connected to
terminal B of reed switch 422; and its base 415 electrically
connected to a Photo-Darlington relay 440. In a preferred
embodiment relay 440 is a Monsanto MCA2 solid state relay. This
relay is particularly adapted to isolate SCR 410 from the other
circuit components of regulator 36. It is capable of rendering SCR
410 conductive whenever it becomes active. SCR 410 will remain
conductive as long as the current path through it is not
interrupted. Specifically, SCR 410 will remain conductive, once it
is rendered conductive, as long as reed switch 422 is at position B
or until the potential on line 35 is essentially zero.
The two voltage level detectors shown generally at 460 and 480
control SCR 410 and reed relay 420. More particularly, voltage
level detector 460 must be active to render SCR 410 conductive and
voltage level detector 480 must be active to move reed switch 422
to position B. Accordingly, since SCR 410 must be conductive and
switch 422 must be at position B for energy to be transferred to
the heart in the form of cardioverting pulses, detectors 460 and
480 effectively control the application of the cardioverting
pulses.
The elemental unit denoted level detector 480 comprises programable
unijunction transistor 490, zener diode 484 and resistors 482, 486,
488 and 492. Programable unijunction transistor (PUT) 490 has a
gate input 491, an anode input 493, and a cathode 495. PUT 490,
like PUT 320 of controller 30 and PUT 180 of wave conformer 26, is
rendered conductive thereby providing a low impedance from both the
PUT anode and gate to its cathode when the anode voltage exceeds
the gate voltage by a specified amount, for example, 0.7 volts.
Resistor 482 and zener diode 484 are electrically connected in
series between electrical line 35 and the system ground. Resistors
486 and 488 are likewise electrically connected in series between
electrical line 35 and the system ground. The junction between
resistor 482 and diode 484, designated junction 483, is
electrically connected to the gate 491 of transistor 490--the
junction between resistor 486 and 488, designated junction 487, is
electrically connected to the anode 493 of transistor 490. The
cathode 495 of transistor 490 is electrically connected to
Darlington amplifier 435 and also to the system ground through
resistor 492.
Level control 480 is voltage sensitive. It will be rendered active
when the voltage on electrical line 35 is above some predetermined
level, for example, 550 volts and will remain active as long as the
voltage of line 35 remains above 550 volts. Junction 483 is held at
some predetermined voltage, for example, 6.0 volts by zener diode
484 over a broad range of electrical line 35 voltages, for example,
6.0 to 1,500 volts. Resistors 486 and 488 form a voltage divider,
thus determining the voltage at junction 487--the junction 487
voltage bears the same relation to the electrical line 35 voltage
as the resistive value of resistors 486 and 488. By a proper
selection of the resistive values of resistors 486 and 488 the
voltage on electrical line 35 which is required to establish a
voltage at junction 487 sufficient to render transistor 490
conductive can be easily set at 550 volts.
Level detector 480 is electrically connected in controlling
relation to reed relay 420. More particularly, the cathode 495 of
transistor 490 is electrically connected to the coil 430 of reed
relay 420 through Darlington amplifier 435. The electrical signal
produced at cathode 495 of transistor 490 when transistor 490
becomes conductive is transmitted to, and sufficient for activating
Darlington amplifier 435. This will cause current to flow through
coil 430 of reed relay 420, thus switching reed switch 422 from
terminal A to terminal B.
The element denoted level detector 460, is elementally and
functionally quite similar to that of level detector 480. Level
detector 460 comprises programable unijunction transistor (PUT)
470, zener diode 464, transistor 478, and resistors 462, 466, 468,
472 and 474. PUT 470 is similar to the PUT 490 of level detector
480. Resistor 462 and zener diode 464 are electrically connected in
series between electrical line 35 and the system ground. Resistors
466 and 468, and the parallel combination of resistors 474 and
transistor 478 are likewise connected in series between electrical
line 35 and the system ground. Transistor 478 is connected with its
collector 481 connected to the junction between resistors 468 and
474 and its emitter 477 connected directly to the system ground.
The junction between resistor 462 and zener diode 464 for
convenience denoted junction 463 is electrically connected to the
gate 471 of PUT 470--the junction between resistors 466 and 468 for
convenience denoted junction 467 is electrically connected to the
anode 473 of PUT 470. The cathode 475 is electrically connected to
Darlington amplifier 445 and to the system ground through resistor
472.
Level control 460 is voltage sensitive. It will be rendered active
when the voltage on electrical line 35 is above some predetermined
level, for example 900 volts. The junction 463 voltage is held at
some predetermined value, for example, 6.0 volts by zener diode
464. A voltage of a predetermined amount, for example, 0.7 volts
above the junction 463 voltage is required to render PUT 470
conductive. The junction 467 voltage bears the same relation to the
electrical line 35 voltage as the resistive value of the series
combination of resistor 468 and the parallel combination of
resistor 474 and transistor 478 bears to the resistive value of the
series combination of resistors 466, 468, and the parallel
combination of resistor 474 and transistor 478. The resistive value
of the parallel combination of resistor 474 and transistor 478
will, of course, be greater when transistor 478 is in the
non-conductive state than when it is in the conductive state. Thus,
the resistance of the series combination of resistor 468 and the
parallel combination of resistor 474 and transistor 478 will be
larger in relation to the resistance of the series combination of
resistors 466 and 468 and the parallel combination of resistor 474
and transistor 478 when transistor 478 is non-conducting, than when
it is conducting. Accordingly, for any given electrical line 35
voltage, the voltage at junction 467 will be greater when
transistor 478 is non-conductive than when transistor 478 is
conductive. The resistive values of transistors 466, 468 and 474
may be selected, so that the electrical line 35 voltage required to
render PUT 470 conductive is 700 volts when transistors 478 is
nonconductive, and 900 volts when transistor 478 is conductive. The
normal state of transistor 478 is the nonconductive state.
Level control 460 is electrically connected in controlling relation
to SCR 410. Specifically, cathode 475 of PUT 470 of level control
460 is connected through Darlington amplifier 445 and
Photo-Darlington amplifier 440 to the gate 415 of SCR 410. The
electrical signal produced at cathode 475 of PUT 470 when PUT 470
is rendered conductive is amplified and processed by Darlington
amplifiers 445 and 440. The amplified and processed signal is
transmitted to SCR gate 415 and is capable of causing SCR 410 to
conduct. Accordingly, SCR 410 conducts when level detector 460 is
active and level detector 460 is active when the voltage on
electrical line 35 reaches 700 volts if transistor 478 is
non-conductive, but does not become active until the electrical
line 35 voltage reaches 900 volts if transistor 478 is conductive.
Since reed switch 422 will normally be in position B when the
voltage on electrical line 35 is above 550 volts, whenever SCR 410
conducts, a cardioverting pulse will be applied to the patient's
heart.
Still with reference to FIG. 7, the element denoted 590 is a
conventional binary counter. In a preferred embodiment, an
RCA-CD4004E COS/MOS seven-stage Binary Counter is used.
Functionally speaking, it will provide a different output signal
for each electrical signal it receives. In a preferred embodiment
an input 591 to counter 590 is electrically connected to the
cathode 475 of PUT 470. Thus, each time PUT 470 is rendered
conductive, the electrical signal produced at cathode 475 of PUT
470 is transmitted to input 591 of counter 590. Output terminal 595
is electrically connected to junction 335 of controller 30 via
electrical line 19. When four electrical signals have been received
at input terminal 591 of counter 590, an electrical signal will be
provided at terminal 595 of counter 590. This electrical signal
keeps junction 335 at a voltage sufficiently high so that
controller 30 is disabled. Controller 30 cannot now activate DC--DC
converter 32. Terminal 596 of counter 590 is connected to a reset
circuit 500. When a positive electrical pulse is received at
terminal 596, counter 590 will be reset to the "zero" state (the
state in which there is no output signal from counter 590).
In the preferred embodiment the counter 590 is electrically
connected via output terminals 592, 593 and 594 to the base 479 of
transistor 478 in such a manner that the signal from counter 590
corresponding to each and every electrical input signal at input
terminal 591 will render transistor 478 conductive.
As discussed above, the conduction or non-conduction of transistor
478 determines whether the cardioverting pulse applied to the heart
is of a 700 or a 900 voltage magnitude. Transistor 478 is
nonconductive when counter 590 is in the "zero" state and
conductive when counter 590 is in any other state. Accordingly, the
first cardioverting pulse applied to the patient's heart will have
a 700 volt magnitude, and if that does not restore normal heart
activity each succeeding pulse will have a 900 volt magnitude.
Reset circuit 500 comprises inverters 510, 530 and 550, diode 512,
resistors 514 and 526, capacitor 516, transistor 520 and "nand"
gate 540. The inverters and the "nand" gate are of conventional
design. The input of inverter 510 is electrically connected to the
cathode 475 of PUT 470 and the output is electrically connected to
one side of diode 512. The other side of diode 512 is electrically
connected to input terminal 542 of "nand" gate 540. It is also
electrically connected through resistor 514 to the 4 volt power
supply and through capacitor 516 to the system ground. Transistor
520 is connected having its base 521 electrically connected to
electrical line 15, its emitter 525 electrically connected to the
system ground, and its collector 523 electrically connected through
resistor 526 to the 4 volt power source. The emitter 523 of
transistor 520 and one side of resistor 526 are electrically
connected to the input side of inverter 520. The output side of
inverter 530 is connected to input terminal 544 of "nand" gate 540.
The output terminal 546 of "nand" gate 540 is electrically
connected through inverter 550 to input terminal 596 of counter
590.
Reset circuit 500 will reset counter 590 to the "zero" state
whenever an electrical pulse corresponding to a normal heartbeat is
received from sensing circuit 10 on electrical line 15. Circuit 500
is capable of differentiating between the heart activity associated
with a normal heartbeat and the activity induced by a cardioverting
pulse being applied to the heart. Circuit 500 is non-responsive to
the induced heart activity, but responsive to the normal heart
activity and capable of resetting counter 590 to the "zero" state
in response thereto.
Each electrical pulse corresponding to a normal heartbeat produced
by sensing circuit 10 is transmitted to the base 521 of transistor
520. This pulse causes transistor 520 to conduct which allows
current to flow through resistor 526 and transistor 520 to the
system ground as long as transistor 520 is conductive, and thus
lowers the voltage at the input 531 of inverter 530 for this time
period. This negative pulse is inverted into a positive pulse by
inverter 530 and transmitted to input 544 of "nand" gate 540.
"Nand" gate 540 will invert this pulse and transmit the inverted
pulse to inverter 550, provided the voltage at input 542 is not
decreased. The voltage at input 542 is decreased only when a pulse
is received from an active level detector 460 which occurs only
when a cardioverting pulse is applied to the patient's heart.
Specifically, when level detector 460 is active a positive pulse of
short duration is transmitted to inverter 510 where it is inverted
and transmitted on through diode 512. This negative pulse being
transmitted through didode 512 allows capacitor 516 to discharge
thus decreasing the voltage input at terminal 542 of "and" gate 540
for a period of time equal to the time required to recharge
capacitor 516 from the 4 volt power source through resistor 514.
This period of time typically is of a long enough duration so that
is keeps the voltage at terminal 542 depressed during the time in
which the heart activity induced by the cardioverting pulse is
exhibited.
Inverter 550 inverts the negative pulse received from gate 540 into
a positive pulse which is transmitted to the input 596 of counter
590. This pulse is sufficient to reset counter 590 to the "zero"
state. In this manner, reset circuit 500 differentiates between a
normal heart activity and the activity induced by a cardioverting
pulse and resets counter 590 to the "zero" state when the heart
activity is normal.
The apparatus of this invention comprises a sensing means 10 for
monitoring heart activity and a stimulation means 12 for applying a
shock to the patient's heart of sufficient magnitude to restore
normal heart activity. The sensing means 10 controls the
stimulation means 12 allowing the stimulation means 12 to apply a
cardioverting shock to the heart only after normal heart activity
has ceased. Upon monitoring life threatening arrhythmias, the
apparatus of this invention automatically cardioverts the patient's
heart.
As shown in FIG. 1, sensing means 10 incudes EKG sensor 20,
contraction sensor 22, "or" gate 24, and wave conformer 26. The EKG
sensor 20 amplifies the R wave signal detected by electrical lead
16 corresponding to normal sinus rhythm of the human heart and
filters out all other heart electrical activity. The contraction
sensor 22 is responsive to the heart contractions detected by
electrical lead 16 and is adapted to provide an electrical signal
corresponding to each heart contraction. Gate 24 is constructed so
that it provides an electrical output signal whenever it receives
an electrical signal from either the contraction sensor 22 or the
EKG sensor 20, or both. Consequently, if either the contraction
sensor 22, relying on detected heart contraction, or the EKG sensor
20, relying on detected R waves or both provide an electrical
signal to gate 24 corresponding to a normal heartbeat, gate 24 will
provide an electrical signal in the form of a pulse corresponding
to the heartbeat. Wave conformer 26 is adapted to transform these
electrical pulses received from gate 24 which have varying
amplitudes and widths into pulses having substantially the same
pulse width and amplitude. Accordingly, sensing means 10 is
responsive to each normal sinus heartbeat--detected by
intravascular electrical lead 16 in the form of an R wave or as a
heart contraction--and is adapted to provide an electrical pulse
having a predetermined pulse amplitude and pulse width
corresponding with each detected heartbeat.
Stimulation means 12 is adapted to apply electrical pulses to the
heart via intravascular lead 16 for cardioverting a malfunctioning
heart. These cardioverting pulses are not applied immediately upon
the sensing of abnormal heart functioning, but their application is
delayed for a period of time. This delay gives the heart the
opportunity to convert to normal heart functioning, if it is able
to do so. Stimulation means 12 applies a cardioverting pulse having
a low energy content first, and then, if that pulse does not
restore normal heart functioning, cardioverting pulses having
higher energy content will be applied until the heart resumes
normal functioning or the cardioverter is automatically
disabled.
As shown in FIG. 1, stimulation means 12 includes controller 30,
DC--DC converter 32, capacitor 34, regulator 36, and alert system
40. Stimulation means 12 is electrically connected to sensing means
10 by an electrical connection between wave conformer 26 of sensing
means 10 and controller 30 of stimulation means 12 and to
intravascular lead 16 via electrical line 17.
Controller 30 functions much like a timing device. Specifically, it
provides an electrical signal if a predetermined period of time,
for example, 5 seconds has elasped without an electrical signal
being received from wave conformer 26. Controller 30 continues to
supply an electrical signal until it receives an electrical signal
from wave conformer 26 corresponding to normal heart functioning.
This electrical signal activates alert system 40 comprising both a
visual and an audio alarm and activates DC--DC converter 32.
Converter 32 is electrically connected to capacitor 34 and capable
of charging capacitor 34 to a 1,000 volt level.
Converter 32 is a DC--DC converter of conventional design which is
capable of increasing the power supply 33 voltage from 6 volts to
1,000 volts. Voltages in the 700-1,000 volt range are necessary to
charge capacitor storage means 34 to a sufficient level so that it
is capable of providing cardioverting pulses of the necessary
magnitude. It takes a predetermined period of time, for example
10-15 seconds, to charge capacitor means 34 to the necessary level.
However, any normal heartbeat during this interval will deactivate
controller 30 which will then disable converter 32 and thus stop
the charging cycle of energy storage means 34.
Accordingly, capacitor 34 is charged to the level required for
cardioverting within 15-20 seconds (five second delay in controller
30 plus the 10-15 seconds needed to charge capacitor 34) following
the last sensed normal heartbeat.
Regulator 36--electrically connected between capacitor 34 and
intravascular lead 16--controls the application of energy from
capacitor 34 to the patient's heart. It determines the energy
content of the applied pulses, allowing only pulses to be applied
when they have an energy content which is likely to be sufficient
to stimulate heart activity.
The functional operation of stimulation means 12 can be best
described with reference to the voltage diagram of FIG. 8--a
chronological description of the operation is possible using this
diagram in explaining the differences between the first pulse
generated and suceeding pulses. All times and waveforms are merely
illustrative--the actual times and waveforms depend upon the
particular components and component values used. FIG. 8 shows the
voltage waveforms representing the voltage on capacitor 34, the
state of reed switch 422, the state of SCR 410, and the voltage
applied to the patient's heart. Specifically, waveform (a) shows
the voltage on capacitor 34 as represented by the electrical line
35 voltage; waveform (b) shows the times when a voltage is applied
across coil 430 of reed relay 420 as represented by the voltage at
cathode 495 of control PUT 490 in regulator 36; waveform (c) shows
the times when a voltage signal is applied to gate 415 of SCR 410
as represented by the voltage at cathode 475 of PUT 470 in
regulator 36; and waveform (d) shows the voltage waveform of the
pulse applied to the patient's heart as represented by the voltage
at electrical line 17.
The active operation of stimulation means 12 begins when a pulse
representing normal heart activity has not been received from
sensing means 10 for 5 seconds. When this occurs controller 30
becomes active supplying an electrical signal to converter 32.
Converter 32 becomes active and charges capacitor 34. It takes
converter 32 approximately 9 seconds to charge capacitor 34 to the
550 volt level. When capacitor 34 becomes charged to the 550 volt
level (line 601 in FIG. 8) PUT 490 of level control 480 becomes
active thus switching reed relay 420 to position B. It takes
converter 32 an additional three seconds to charge capacitor 34 to
the 700 volt level. When this occurs (line 602 in FIG. 8) PUT 470
of level control 460 becomes active thus rendering SCR 410
conductive. With reed switch 422 in position B and SCR 410
conductive, capacitor 34 has a discharge path through the patient's
heart. Capacitor 34 begins to discharge immediately upon SCR 410
being rendered conductive (line 602, FIG. 8) and discharges through
the patient's heart until reed switch 422 is switched to position A
(line 603, FIG. 8). Reed switch 422 is switched to position A when
the capacitor 34 voltage is reduced to the 500 volt level.
Accordingly, 17 seconds (5 sec.+9 sec.+3 sec.) following the last
sensed normal heart activity a cardioverting pulse is applied to
the patient's heart. This cardioverting pulse is in a truncated
capacitive discharge waveform having a peak magnitude of 700 volts
and being truncated at the 500 volt level.
If this first cardioverting pulse stimulates normal heart activity,
sensing means 10 senses the resumed normal heart activity and
disables stimulation means 12--but if this first pulse did not
stimulate normal heart activity, a second cardioverting pulse is
needed and is supplied by stimulation means 12. Assuming that
normal heart activity has not been restored, controller 30 will
become active again 5 seconds following the first cardioverting
pulse. This occurs since the contraction sensing portion of sensing
means 10 is responsive to the heart contraction caused by the
cardioverting pulse. The pulse it generates which corresponds with
the cardioverting pulse deactivates controller 30. It takes 5
seconds for controller 30 to be activated again. Capacitor 34 is
still charged to nearly 500 volts 5 seconds after the first
cardioverting pulse (line 604, FIG. 8). Thus it will take only 1
second to charge capacitor to the 550 volt level necessary to
switch reed switch 420 to position B (line 605, FIG. 8). However,
900 volts are now required to render SCR 410 conductive instead of
the 700 volts required for the pulse. This is due to transistor 478
in SCR level control 460 being rendered conductive as a result of
the first applied pulse. It takes approximately 5 seconds to charge
capacitor 34 to the 900 volt level required to render SCR 410
conductive (line 606, FIG. 8). Once the 900 volt level is reached
SCR 410 becomes conductive and capacitor 34 discharges until reed
switch 422 is switched to position A. Accordingly, 11 seconds (5
sec.+1 sec.+5 sec.) after the first cardioverting pulse a second
cardioverting pulse also having a truncated capacitive discharge
waveform is applied to the heart.
The second applied pulse has a greater energy content than the
first pulse. The energy content is greater as the peak voltage of
the second pulse (900 volts) is greater than the peak voltage of
the first pulse (700 volts) and both pulses truncate at 500
volts.
If the second cardioverting pulse stimulates normal heart activity,
sensing means 10 will sense this and disable stimulation means 12.
If not, a third cardioverting pulse which is similar to the second
pulse will be applied in a manner similar to that of the second
pulse. If the third pulse still does not restore normal heat
functioning, stimulation means 12 will be disabled
automatically.
Although the invention has been described with reference to a
particular embodiment, it will be understood that this embodiment
is merely illustrative of the applications of the principles of
this invention. It will be further understood that numerous
modifications in the inventive embodiment may be made and other
arrangements may be devised without departing from the spirit and
scope of this invention.
By suitable modifications in the inventive circuitry many
modifications in the functional operation of the invention can be
achieved. For example, a P-wave amplifier could be used instead of
the described R-wave amplifier in EKG sensor 20. A gating means
which is nonresponsive to electrical signals having a repetitive
rate greater than a predetermined amount could be used in place of
or in addition to "or" gate 64 to discriminate against certain
types of tachyarrhythmias and thus allow a cardioverting pulse to
be applied when the heart is functioning in this manner. A dynamic
heart characteristic such as heart pressure could be monitored
instead of either EKG or heart contractions. Additionally, each
cardioverting pulse could have an increased energy content; the
time between cardioverting pulses could be decreased for additional
applied cardioverting pulses; or the cardioverting pulse could be
applied using an intravascular lead which is distinct from the
intravascular lead which is used to sense heart activity.
Many substitutions may, of course, be made in the circuit elements
used in the inventive circuit without materially affecting the
operation of the invention. For example, two independent power
sources could be used instead of having a 6 volt battery drive or 4
volt constant voltage source; various arrangements of SCR's and/or
transformers as well as solid state switching devices could be used
to control the transmission of the cardioverting pulse to the
patient's heart instead of the particular arrangement of an SCR and
a reed relay actually used; and devices of various types could be
used to perform the level detecting function performed by level
detectors 460 and 480. Many more examples are possible--the above
listing is anything but exhaustive.
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