U.S. patent number 3,662,759 [Application Number 04/859,900] was granted by the patent office on 1972-05-16 for cardiac pacer system.
This patent grant is currently assigned to American Hospital Supply Corporation. Invention is credited to Laurence L. Dabolt.
United States Patent |
3,662,759 |
Dabolt |
May 16, 1972 |
CARDIAC PACER SYSTEM
Abstract
An implanted pacer system includes a resettable oscillator for
generating a train of pulses having a period slightly greater than
the period of a natural heartbeat. The pulses generated in the
oscillator are coupled to a pulse shaper network which transmits
pulses to an electrode contacting the heart. If no natural
heartbeat is sensed, the oscillator operates at its natural
frequency and the shaper network transmits heart-stimulating pulses
to the electrodes. If a natural heartbeat is detected, the
oscillator is reset and timing circuitry in the shaper network is
modified so that a non-stimulating pulse is transmitted to the
electrode. The non-stimulating pulses are detectable by a receiver
outside the body to permit an examining physician to ascertain
whether the pacer system is operative when the heart is functioning
normally even though the system is non-competing. Further, the
non-stimulating pulses do not produce an artifact on an EKG machine
monitoring the patient's heartbeat, and negligible energy is
dissipated in the non-stimulating pulses.
Inventors: |
Dabolt; Laurence L. (Newport
Beach, CA) |
Assignee: |
American Hospital Supply
Corporation (Evanston, IL)
|
Family
ID: |
25332002 |
Appl.
No.: |
04/859,900 |
Filed: |
September 22, 1969 |
Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N
1/365 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61n 001/36 () |
Field of
Search: |
;128/419P,422,421 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
I claim:
1. In a cardiac pacer system adapted to be implanted in a body, the
combination comprising electrode means adapted to contact the
heart, excitation means for generating a continuous train of
electrical pulses to energize said electrode means, and generator
means for generating a control signal in timed relation with a
natural heartbeat, said excitation means including circuit means
for generating either a first electrical pulse in said train to
stimulate the heart in the absence of said control signal or a
second electrical pulse in said train incapable of stimulating said
heart in the presence of said control signal.
2. In combination with the system of claim 1, detector means
adapted for use external to said pacer system and responsive to
said second electrical pulses for detecting the occurrence of said
non-stimulating pulses whereby when said pacer system is implanted
said system is non-competing and an examining physician may
ascertain that the pacer system is operative in the presence of a
normally functioning heart.
3. The system of claim 1 wherein said excitation means includes an
oscillator for generating a train of electrical pulses at a
repetition rate slightly less than the normal repetition rate of
the heart, pulse shaping circuit means receiving the output signal
of said oscillator means for generating a heart-stimulating first
pulse for each received pulse from said oscillator means in the
absence of said control signal, and conductive means coupling said
control signal to said pulse shaping circuit means for generating
said non-stimulating second pulse in timed relation with the
occurrence of said natural heartbeat.
4. The system of claim 3 further comprising means for resetting
said oscillator means in the presence of a natural heartbeat
whereby said oscillator means operates in a fixed rate mode to
generate said first pulses in the absence of a natural heartbeat
and generates said second electrical output pulses in synchronism
with a naturally occurring heartbeat.
5. The system of claim 4 wherein said oscillator means has a
refractory period of about 300 milliseconds after it is reset
wherein it is not capable of being triggered into oscillation
within said refractory period, thereby to reduce the sensitivity of
said system to external noise sources having a repetition rate
greater than about 3 cycles per second.
6. The system of claim 1 wherein said generator means includes
sensor means adapted to contact the ventricle of the heart to sense
a QRS complex therein.
7. A non-competing pacer system capable of either a fixed rate or a
demand mode of operation comprising electrode means for contacting
the heart of a patient, first pulse generator means for generating
a train of heart-stimulating signals and coupling the same to said
electrode means at an interval greater than the natural heartbeat
interval only when said natural heartbeat does not occur within
said interval, whereby said system operates in a fixed rate mode in
the absence of a natural heartbeat, and sensing means including
circuit means for generating externally detectable, non-stimulating
pulses in timed relation with natural heartbeats and for coupling
said non-stimulating pulses to said electrode means only when said
natural heartbeat occurs whereby said pacer operates in a demand
mode.
8. In combination with the system of claim 7 wherein said
non-stimulating pulses extend for a duration of about 10
microseconds; and further comprising detection means exterior of
said patient for selectively detecting the occurrence of said
non-stimulating pulses whereby an examining physician may ascertain
the operativeness of said system when said system is operating in
said non-competing mode and the heart of the patient is functioning
normally.
9. The combination of claim 8 wherein said detection means further
includes means for generating an audible signal when said
non-stimulating pulse occurs.
10. The system of claim 8 wherein said first signal generator means
includes a fixed-rate oscillator for generating a train of pulses
at intervals greater than the natural heartbeat interval and
shaping circuitry for coupling a heart-stimulating pulse to said
electrode means from said oscillator means only after said natural
heartbeat interval has lapsed.
11. The system of claim 10 wherein said sensing means comprises
means adapted to sense the occurrence of a natural QRS complex in a
heart for generating a control signal in response thereto, means
for resetting said oscillator to generate a non-stimulating pulse
immediately in timed relation with said control signal, and means
coupling said control signal to said shaping means for modifying
the time constant thereof to generate said non-stimulating pulse
immediately and in timed relation with the occurrence of a natural
heartbeat.
12. The system of claim 7 characterized in that said stimulating
pulses occur for about one millisecond and said non-stimulating
pulses occur for about 10 microseconds.
13. A method of pacing a heart comprising sensing a natural QRS
complex, generating an electrical control signal in response to
said sensed QRS complex, generating a non-stimulating, externally
detectable pulse in response to said control signal, and
stimulating said heart in the absence of said control signal at a
predetermined time after a natural heartbeat should have
occurred.
14. The method of claim 13 further comprising coupling said
non-stimulating pulse to said heart.
15. In combination, an implantable, non-competing demand cardiac
pacer system for generating heart stimulating pulses and
transmitting said stimulating pulses to the heart only in the
absence of a natural heartbeat; and means responsive to the
presence of a natural heartbeat for generating externally
detectable, non-stimulating pulses in mutually exclusive relation
with said stimulating pulses and for transmitting said
non-stimulating pulses to said heart, whereby the detection of said
non-stimulating pulses is indicative that the pacer is operating in
a demand mode and that said system is operational.
Description
BACKGROUND AND SUMMARY
The present invention relates to an artificial system which is
implanted in a body for stimulating the heart in the absence of a
natural heartbeat; such systems are commonly referred to as cardiac
pacers.
For purposes of understanding the present invention, the heart may
be thought of as including two upper chambers (the left and right
auricles), and two lower chambers (the left and right ventricles).
The two auricles taken together form the atrium. Blood is pumped
from the atrium to the ventricles and thence into either the lungs
(from the right ventricle) or the aorta (from the left
ventricle).
The contraction of the atrium is accompanied by a depolarization of
the muscle cells in the atrium which coincides with a small but
detectable electrical pulse referred to as the P wave. The
ventricular contraction which accompanies ventricular pumping
action is accompanied by a depolarization of the muscle cells of
the ventricles, referred to as the QRS complex, or simply the R
wave. In a properly functioning heart, the R wave follows the P
wave by a fixed and constant time period (usually about 0.2 sec. in
man) because blood is pumped from the atrium to the ventricles and
then through the body). For a period of about 0.1 second after the
contraction of the ventricle, the heart remains insensitive to
electrical pulses; and this is sometimes referred to as the
"refractory" period of the heart. After the refractory period,
there is a short period of the order of a few milliseconds during
which an electrical impulse transmitted to the heart and capable of
stimulating it, may cause damage to the heart by sending it into a
state of fibrillation.
The constant period of time between the successive occurrences of a
P wave and an R wave (signaling respectively the contraction of the
atrium and verticles) is provided by a conductive nerve system
extending from the atrium to the ventricles and commonly referred
to as the AV conduction system. In some types of heart disorders
the AV conduction system becomes intermittently discontinuous
whereby ventricular pumping action does not follow contraction of
the atrium by the fixed delay period. Cardiac pacer systems are
employed to insure stimulation of the ventricles in these
cases.
Cardiac pacer systems may be divided into two broad categories,
fixed-rate or competing pacers and non-competing pacers. In a
fixed-rate pacer, a main oscillator establishes a time base for
generating a train of pulses having a constant repetition rate. It
can be seen that a fixed-rate pacer can operate in modes wherein it
"competes" (i.e. generates pulses not timed with the occurrence of
an R wave) with a natural heartbeat. A non-competing pacer system
is one in which the stimulating pulses are always synchronized with
the natural R wave if there is one; and there are two broad
subclasses here. The first is referred to as the synchronous pacer,
and the second is referred to as a demand or standby pacer.
Early cardiac pacer systems of the so-called "synchronous" type
employed two separate electrodes, a sensing electrode contacting
the atrium and an exciting electrode contacting the ventricle. In a
synchronous pacemaker, the sensing electrode detects an atrial P
wave, delays it for a predetermined and fixed time, then uses that
pulse to initiate or generate a stimulating pulse which is
transmitted to the ventricle coincident with the time that a
natural R wave should occur. That is, a stimulating pulse is
transmitted to the exciting electrode in the ventricle in timed
relation with a natural heartbeat. If a natural heartbeat occurs,
the stimulating pulse merely reinforces it; whereas, if the AV
conduction system is not functioning normally, the pulse stimulates
the ventricle.
A demand or standby pacer provides an electrical discharge into the
ventricle only on demand--i.e., a main oscillator circuit is reset
so as not to provide a stimulating pulse if a natural QRS complex
is detected before the end of the fixed period of the oscillator,
which fixed period is set to be slightly longer than the natural
period between heartbeats.
Pacer systems currently available are permanently implanted in the
patient's body so as to avoid the psychological effect of having
any part of the system in the patient's view. The power supply will
normally operate reliably for a period of 18 to 30 months; and a
patient having an implanted cardiac pacer periodically visits a
physician for check-ups on the operability of the system. However,
the predominant use of implanted pacers presents a problem to the
physician in that with competing pacers the stimulating pulses
discharged into the ventricle normally have energy comparable to
the naturally occuring R wave so that the artificial pulse alters
the appearance of a natural QRS complex. Hence, although an
examining physician may determine that the cardiac pacer is
operable by detecting the magnitude of the stimulus, he is not
easily able to determine with certitude whether the AV conduction
system is functioning normally, or whether the heart is being
artificially stimulated.
Conversely, in the case of an implanted non-competing demand or
standby cardiac pacer if the patient's heart is acting normally
during his periodic check-up, an examining physician has no way of
telling whether the dormant pacer is functioning properly because
the occurrence of each natural heartbeat suppresses the generation
of a stimulating pulse by the pacer.
In the inventive system, a main oscillator generates pulses to
establish a time base; and the oscillator circuit energizes a pulse
shaping circuit which transmits output pulses to the ventricle
whether or not a natural heartbeat is sensed. In the absence of a
natural heartbeat, the pulse shaping circuit generates a
stimulating electrical pulse to cause the ventricle to contract.
However, if a natural heartbeat is detected, the oscillator is
reset, and at the same time, timing circuitry in the pulse shaping
network is modified so that the pulse transmitted to the heart is
very narrow--of the order of less than ten microseconds. This
narrow pulse is transmitted in synchronism with the occurrence of
an R wave; but its energy content is such that it is
non-stimulating so that the narrow pulse has no effect on the
heart. However, because the pulse is so narrow, it has a
substantial high frequency content and may be externally detected
by separate receiving means. Thus, during a routine examination, an
examining physician may determine whether the heart is functioning
naturally and, if so, whether the implanted pacer is also fully
operative. The non-stimulating high frequency pulse is not apparent
on the standard EKG record which is used to analyze the electrical
functioning of the heart. The oscillator has a built-in refractory
time which inhibits re-triggering of the pulse shaping circuit by a
pacer-generated pulse and filters out noise frequencies outside the
physiologic range.
Other features and advantages of the present invention will be
apparent to persons skilled in the art from the following detailed
description of a preferred embodiment accompanied by the attached
drawing wherein identical reference numerals will refer to like
components or elements in the various views.
THE DRAWING
FIG. 1 is a functional block diagram of a cardiac pacer system
according to the present invention;
FIGS. 2A and 2B comprise a detailed circuit schematic diagram for
the system of FIG. 1;
FIG. 3 is a diagrammatic illustration of a detection system;
and
FIG. 4 is a diagram illustrating the time relationship between
various idealized voltage wave forms in the inventive system.
DETAILED DESCRIPTION
Referring first to FIG. 1, reference numeral 10 generally denotes
an electrode in contact with the ventricle of the patient's heart.
A lead connects the electrode 10 to a junction 10a within the pacer
housing. Preferably the housing encloses the circuitry and may be
located in the abdomen of the patient and a current return is
provided through the body fluids to the pacer circuitry. A
conductive wire or lead 11 connects the junction 10a to the signal
input of a protective network 12 which clips or limits the
amplitude of the voltages fed to the system. The output of the
protective network 12 then feeds an input amplifier 13 which
amplifies the signals detected at the ventricle electrode. The
input amplifier 13 feeds a pulse rectifier circuit 14. The pulse
rectifier 14 generates output pulses of one polarity regardless of
the polarity of an input pulse; and the output pulses from the
pulse rectifier circuit 14 trigger a monostable multivibrator
circuit 15. The monostable multivibrator circuit 15 has two outputs
designated respectively 16 and 17, the former feeding an oscillator
circuit 18 for resetting it. The output 17 of the monostable
circuit 15 triggers a second monostable (or "one-shot") circuit 19.
The oscillator circuit 18 feeds a first input A to a pulse shaping
circuit 20; and the output of the monostable multivibrator 19 feeds
a second input B to the pulse shaping circuit 20. The output pulse
of the pulse shaping circuit 20 is coupled to an output amplifier
21 which, in turn, is connected to the junction 10a to energize the
ventricle electrode 10.
Referring to FIG. 4, there is shown on line 1 an idealized
illustration of the electrochemical pulses occuring in the heart,
namely, the above described P, Q, R, S and T waves. For simplifying
an understanding of the invention, there are two sets of waveforms
shown in FIG. 4. The left side are those waveforms generated when a
natural heartbeat is detected (the heartbeat waveform being
identified as the solid line 22); this may be referred to as the
demand mode of system operation. The right side shows those
waveforms occuring when no natural heartbeat occurs (but the dashed
line 23 indicates a natural heartbeat to show the time relation);
and this may be called the fixed rate mode.
The period of oscillation of the oscillator circuit 18 is set to be
slightly greater than the time between successive beats of a
normally functioning heart. Typically, the period of the output
train of pulses generated by the oscillator 18 is 0.85 sec. In the
absence of a natural heartbeat, the oscillator 18 acts as a
free-running oscillator to trigger the input A of the pulse shaping
circuit 20 which, in turn, generates an output pulse having a
duration of about 1 millisecond. This output pulse, when amplified
by the output amplifier 21 is sufficient to stimulate the
heart.
If, on the other hand, a natural heartbeat is detected, it is fed
through the protective network 12, the input amplifier 13, and the
pulse rectifier circuit 14 to trigger the monostable multivibrator
15. The monostable multivibrator 15 transmits a first pulse along
the line 16 to reset the oscillator 18. At the same time, the
monostable multivibrator circuit 15 transmits a pulse along the
line 17 to trigger the monostable multivibrator 19. When reset,
oscillator 18 produces an output pulse which is transmitted to
input A of the pulse shaping circuit 20. At the same time,
multivibrator 19, when triggered, produces an output pulse
transmitted to input B. Thus, both inputs A and B of the pulse
shaping circuit 20 are energized when a natural heartbeat occurs;
and the function of the pulse shaping circuit under these
circumstances is to generate a much narrower, non-stimulating
output pulse of the same amplitude. This non-stimulating output
pulse has a duration of the order of 10 microseconds or less; and
it is coupled through the output amplifier directly to the heart
and in timed relation with the occurrence of the QRS complex that
generates it.
The narrow, 10 microsecond pulse contains sufficient high frequency
energy so that it may be easily detected by a detector means held
in proximity to the pacer system. One detector means is shown in
FIG. 3 and includes a pick-up coil generally designated by
reference numeral 25. The terminals of the coil 25 are connected to
an amplifier 26 which amplifies the high frequency detected pulse
and energizes an indicator 27 (preferably to generate an audible
signal) to indicate the presence of such pulse. Although the
illustrated embodiment employs a non-stimulating pulse which is of
much shorter duration than the stimulating pulse and of equal
amplitude, persons skilled in the art will realize that other forms
of non-stimulating pulses may be used although it is preferred that
these pulses have the following characteristics in addition to
being incapable of stimulating the heart: (1) they should be easily
externally detectable; (2) they should not produce an artifact on
an EKG machine monitoring the patient; and they should dissipate
only a negligible amount of electrical energy from the storage
batteries. Further detection means other than the one illustrated
may be used. For example, it has been found that a portable
transistor radio receiver, tuned to a frequency between
broadcasting stations, will produce an audible signal (in the form
of a "blip") for each pulse when its antenna is placed near the
pacer.
Turning now to FIGS. 2A and 2B, the protective network is enclosed
within a dashed line again designated 12; and it includes a
resistor 30 for limiting the input current, and diodes 31 and 32
which are connected anode-to-cathode and in parallel between the
resistor 30 and the negative terminal of a supply battery 33 via a
negative supply bus 33b. It will be appreciated that the battery 33
may include a plurality of individual 1 1/2 volt cells connected in
series to generate the desired bias voltage; and its positive
terminal is connected to a positive supply bus 33a.
The other terminal of the resistor 30 is directly connected to the
previously described line 11 connected to the electrode 10. The
output signal from the protective network 12 is coupled to the base
of a transistor Q1 through a capacitor 34. Unless indicated
differently, all transistors are of the NPN type. The transistor Q1
is biased in the linear amplifying region as a common emitter
amplifier with feedback resistor in the emitter circuit. Since the
biasing and feedback circuitry is conventional, it need not be
further described here. The transistor Q1 comprises the first stage
of the three-stage input amplifier 13. Thus, a second transistor Q2
receives the collector signal of the transistor Q1; and the
transistor Q2 is similarly biased in the active region in a
common-emitter configuration. The input amplifier 13 also includes
a third stage--a common emitter amplifier including, as the active
element, transistor Q3. The output signal (see pulse 34a on line 2
of FIG. 4) from the collector of transistor Q3 is coupled by means
of a first capacitor 35 and a first diode 36 to the base of a PNP
transistor Q4. The diode 36 is arranged to permit the passage of
negative pulses only. A first resistor 37 connects the cathode of
the diode 36 to the positive power bus 33a; and a resistor 38
connects the common junction between the base of transistor Q4 and
the anode of diode 33 to the positive power bus 33a. The emitter of
transistor Q4 is connected directly to the positive power bus 33a;
and its collector terminal is connected to the negative power bus
33b through a series resistor 39.
The output of the collector terminal of transistor Q3 is also
coupled to the base of a transistor Q5 through a capacitor 40 in
series with diode 41 connected to transmit only positive pulses.
The junction between the anode of the diode 41 and capacitor 40 is
connected to the negative power bus 33b by means of a resistor 42;
and the cathode of the diode 41 is connected directly to the common
junction between the base of transistor Q4 and resistor 39.
Transistors Q4 and Q5 comprise active elements of the circuit
previously described as the pulse rectifying circuit identified by
reference numeral 14. In operation of the circuitry thus far
described, an R wave detected by the ventricle electrode 10 is fed
to the input of the protective network 12. Although line 1 of FIG.
4 shows the R wave as positive-going, the present system is
designed to accommodate either polarity. That is, the protective
diodes 31 and 32 provide a means for clipping or limiting input
pulses of both polarities; and the input amplifier comprising the
three stage amplifier including transistors Q1-Q3 will amplify both
positive and negative polarity voltages.
Normally, transistors Q4 and Q5 are biased in the cut off region.
If the output pulse from the collector of transistor Q3 is
negative-going, it will be blocked by diode 41 and transmitted
through the capacitor 35 and the diode 36 to forward-bias the
emitter-base junction of transistor Q4 and drive that transistor
into saturation. Collector current from transistor Q4 will then
flow into the base of transistor Q5 to forward-bias that transistor
and drive it into saturation. If, on the other hand, the output
pulse from the collector of transistor Q3 is positive-going, it
will be blocked by the diode 36, but be transmitted through
capacitor 40 and diode 41 to forward-bias the base-emitter junction
of transistor Q5 directly. Thus, whether the output pulse from the
input amplifier section is positive or negative, the pulse
rectifier 14 will draw collector current at transistor Q5.
Transistors Q6 and Q7 comprise the active elements of monostable
circuit 15. The emitter of PNP transistor Q6 is connected to the
positive supply bus 33a by means of a resistor 43, and its base is
connected to the positive bus by means of resistor 44. The
collector of transistor Q5 is connected by means of a resistor 45
to the cathode of a diode 46, the anode of which is connected by
means of a resistor 47 to the common junction between the base of
transistor Q6 and the resistor 44. The collector of transistor Q7
is connected to the positive supply bus 33a by means of a resistor
48 and to the cathode of diode 46 by means of a coupling capacitor
49. The base of transistor Q7 is connected to the collector of
transistor Q6 through resistor 50; and it is connected to the
negative power bus 33b through a resistor 51. The resistors 50 and
51 thus form a voltage divider network; and that series circuit is
shunted by a resistor 52. A diode 53 is connected in the emitter
circuit of the transistor Q7 with its cathode connected to the
negative supply bus 33b.
As already mentioned, normally the transistor Q5 of the pulse
rectifying circuit 14 is cut off; but either polarity pulse from
the input amplifier 13 will cause it to conduct thus drawing
collector current through the diode 46 and causing the junction
between the voltage divider network of resistors 44 and 47 to go
relatively negative. This, in turn, will draw base current from
transistor Q6 causing it to conduct. When transistor Q6 conducts,
base current will flow in transistor Q7; and it will saturate. The
diode 53 in the emitter circuit of transistor Q7 insures that under
normal circumstances, transistor Q7 will be biased at cut off.
Thus, at the occurrence of an input pulse to the monostable circuit
15, transistors Q6 and Q7 will conduct; and the right terminal of
capacitor 49 will be clamped to a potential slightly above the
potential of the negative terminal of the supply battery 33. As
current flows through resistors 44 and 47 to charge the capacitor
49, the left terminal thereof will become positively charged until
it reaches a point at which transistor Q6 will be biased again at
cut off; and both transistors Q6 and Q7 will thereafter be
non-conductive. The time constant of the output pulses from the
monostable circuit 15 is determined primarily by the values of the
resistor 44 and capacitor 49, the value of resistor 47 being very
much smaller than that of the resistor 44. In practice, the output
pulses of the monostable circuit 15 are 100 milliseconds; and they
are seen in FIG. 4 on the left section of lines 3 and 4, identified
respectively by reference numerals 55a and 55b. The output of the
collector of transistor Q6 is the positive pulse 55a on line 3; and
it is coupled through a capacitor 57 to the line 16 which, as
already disclosed, feeds the input of the oscillator 18, the
detailed circuitry of which is illustrated in FIG. 2B and will be
described in detail after a description of the monostable circuit
19. The output pulse 55b of the collector of transistor Q7 is seen
on line 4 of FIG. 4.
The monostable circuit 19 includes a PNP transistor Q8 and an NPN
transistor Q9. The emitter of transistor Q9 is connected directly
to the negative bus 33b; and the collector of transistor Q9 is
connected to the base of transistor Q8 through a resistor 61. The
collector of transistor Q9 is also connected to a line 63 which
feeds the input B of the pulse shaping circuit 20 of FIG. 1 and to
the positive power bus 33a by means of a resistor 65. The emitter
of transistor Q8 is directly connected to the bus 33a, the
collector is connected to line 33b by means of resistor 66, and the
base is connected to line 33a by means of resistor 67. The
collector of transistor Q8 is also connected through capacitor 68
and resistor 69 to the base of transistor Q9; and a resistor 70
connects line 33b with the junction between the base of transistor
Q9 and the resistor 69. The base of transistor Q8 is also connected
to the collector of transistor Q7 by means of a coupling capacitor
71. It will be remembered that the monostable multivibrator circuit
15, when it is triggered, will produce a positive pulse (reference
numeral 55a, line 3 of FIG. 4) along the line 16 which is fed to
the oscillator 18 as well as a negative pulse (reference numeral
55b, line 4 of FIG. 4) which is coupled along the line 17 and
through capacitor 71 to the base of transistor Q8.
Normally, both transistors Q8 and Q9 are cut off so there is no net
charge across capacitor 68. When a negative pulse is transmitted
along line 17 through capacitor 71, it causes transistor Q8 to
conduct and the resulting positive pulse at the collector of
transistor Q8 is coupled through capacitor 68 to the base of
transistor Q9 to cause it to conduct. When transistor Q8 saturates,
the right terminal of capacitor 68 is clamped to the positive
potential of the power supply 33; and the left terminal thereof
begins to charge to the potential of the negative terminal of the
power supply 33 through resistors 69 and 70. When the left terminal
of capacitor 68 reaches a sufficiently low voltage, transistor Q9
will again cut off; and its collector, which is coupled directly to
the line 63 which feeds the input B of the pulse shaping circuit
20, goes positive, ending the negative output pulse.
This negative pulse is shown on line 5 of the left section of the
timing diagram of FIG. 4; and it is denoted by reference numeral
73. The width of the pulse 73 is normally about 30
milliseconds.
Turning now to FIG. 2B, and in particular to the left-hand side,
the lines 11, 16, 33a, 33b, and 63 are repeated. These lines
represent respectively the sense line from the ventricle electrode,
the positive power bus, the output of transistor Q6 of the
monostable circuit 15, the negative power bus, and the output of
the collector of transistor Q9 of the multivibrator circuit 19.
The oscillator 18 includes first and second transistors Q10 and
Q11, the latter being a PNP transistor. The collector of transistor
Q11 is directly connected to the base of transistor 10 and to the
line 16 through a diode 74 arranged to conduct positive pulses to
the base of transistor Q10. The collector of transistor Q10 is
directly connected to the base of transistor Q11. The emitter of
transistor Q10 is connected to line 33b through a resistor 75 and
to line 33a through a capacitor 76. The emitter of transistor Q11
is directly connected to the bus 33a, and the base of transistor
Q11 is connected to the bus 33a by means of a parallel circuit
comprising a resistor 78 and a capacitor 79. The base of transistor
Q10 is connected to a bias network including a resistor 80 in
series with a resistor 81; both are connected between the power
buses 33a and 33b. The junction of the bias resistors is connected
directly to the base of transistor Q10 and to the movable contact
of a single pole, single throw switch S. The fixed contact of the
switch S is connected to the positive power bus 33a through a
resistor 82. When switch S is open, the oscillator 18 generates a
train of pulses at the rate of 70 per minute. When the switch S is
closed, the oscillator 18 generates pulses at the rate of 90-95 per
minute.
In operation, the bias network including resistors 80 and 81 (and
resistor 82 if switch S is closed) bias the transistor Q10 to a
conducting state by causing its base to be positive relative to its
emitter. When the transistor Q10 conducts, however, its collector
draws current through the base of transistor Q11 which saturates
immediately; and the lower terminal of capacitor 76 is thereupon
directly connected to the positive power bus 33a through the
saturated transistors Q10 and Q11. Thus, the charge on capacitor 76
will dissipate very rapidly through the saturated transistors Q10
and Q11. However, as the lower terminal of capacitor 76 rises to a
potential sufficient to reverse-bias the emitter-base junction of
transistor Q10, both transistors in the oscillator circuit will cut
off; and the capacitor 76 will again begin to charge. The cycle is
repetitive for as the lower terminal of capacitor 76 reaches a
sufficiently low potential (depending upon the base bias
established by the resistors 80 and 81) the transistor Q10 will
again conduct.
The output pulse of the oscillator circuit 18 is taken from the
collector of transistor Q11; and it is diagrammatically illustrated
on line 6 of the timing diagram of FIG. 4 as the positive pulse 85.
The duration of the pulse 85 is 20-30 milliseconds; and as already
mentioned, the period between pulses is approximately 850
milliseconds. It will be observed that when the transistor Q10 is
cut off by depletion of the charge on capacitor 76, its base
voltage is slightly less than the voltage on the positive power bus
33a because transistor Q11 is saturated at that time. However, when
the transistors Q10 and Q11 are cut off, the base bias voltage for
transistor Q10 is very much lower--i.e., it is more nearly that of
the negative power bus 33b. Hence, at the instant at which the
transistor Q10 is turned off, the lower terminal of the capacitor
76 is at a relatively high potential; and it must recover down to a
relatively low potential before the transistor Q10 can be
re-triggered. This action provides a built in refractory time for
the operation of the oscillator 18. That is, for a definite period
of time after the triggering of the oscillator 18, it may not be
re-triggered.
In a preferred design, this refractory period extends for 300
milliseconds, sufficient to extend beyond the critical period
wherein a pacer-produced pulse could stimulate the pacer detection
circuit, and lead to an additional pulse. This refractory period
further renders the system insensitive to external noise sources--a
primary problem with implanted cardiac pacers. Thus, the refractory
period has the effect of rendering the system incapable of response
at repetition rates above the physiological range of interest
(i.e., repetition rates above 180 per minute). Beyond the
refractory period, however, the oscillator 18 may be reset by a
positive pulse transmitted from the monostable circuit 15 along the
line 16 through the diode 74 to cause the transistor Q10 to
conduct.
It will be observed that any such advance triggering automatically
establishes a set of initial conditions in the oscillator 18 so
that it continues to operate in the same mode, but will have a new
triggering time. As already mentioned, if a natural heartbeat is
detected, it will be amplified by the input amplifier and fed
through the pulse rectifier circuit 14 to trigger the one-shot
monostable circuit 15. This will advance the output pulse of the
oscillator circuit 18 as indicated by the solid pulse 85 in line 6
of FIG. 6, the dashed pulse 85a representing the naturally occuring
pulse if a natural heartbeat had not been detected.
If the switch S is closed, the resistor 82 is coupled in parallel
with the resistor 80; and the bias voltage at the base of
transistor Q10 is raised, thereby raising to a relatively more
positive level the voltage to which the capacitor 76 must charge in
order to cause the transistor Q10 to conduct and decreasing the
time between pulses even in the absence of a detected
heartbeat.
The collector of transistor Q11 thus forms the input A to the pulse
shaping circuit 20 which includes three transistors, Q12, Q13, and
Q14. The base of transistor Q12 is connected to the input terminal
A by means of a capacitor 85 and a diode 86 which is arranged to
pass positive pulses. The transistors Q13 and Q14 are PNP
transistors. The junction between the capacitor 85 and anode of
diode 86 is connected to the negative bus 33b by means of a
resistor 87; and the cathode of diode 86 is connected to the
negative bus 33b through a resistor 88. The base of transistor Q12
is connected to the collector of transistor Q13 by means of a
series circuit comprising a resistor 89 and a capacitor 90. The
collector of transistor Q12 is connected to the positive power bus
33a through a resistor 91; and the emitter of transistor Q12 is
connected to the negative power bus 33b by means of a resistor 92.
The output signal of transistor Q12 is taken from its collector and
fed directly to the base of transistor Q13. The emitter of
transistor Q13 is directly connected to the positive power bus 33a;
and the collector of transistor Q13 is directly connected to the
emitter of transistor Q14. The collector of transistor Q14 is
connected to the emitter of transistor Q12 through a resistor 94;
and a resistor 95 is connected between the emitter and collector of
transistor Q14. The base of transistor Q14 is connected to the
input B of the pulse shaping circuit 20 through a resistor 96. It
will be recalled that the input B of the pulse shaping circuit 20
is received from the monostable circuit 19 along the line 63 which
is connected to the collector of transistor Q9. The pulse
transmitted to the input B when a natural heartbeat is detected is
shown on the left side of line 7 of the timing diagram of FIG. 4 as
the negative pulse 97, there being no corresponding pulse when the
system is operating in the fixed rate mode.
The pulse shaper circuit 20 is a complementary single-shot
multivibrator circuit in which normally all of the transistors
Q12-Q14 are biased in the cut off region. In the fixed rate mode of
operation when there is no detected heartbeat (see the right side
of the timing diagram of FIG. 4) so that the oscillator 18 is not
advanced, a positive pulse (denoted 85b on line 6) is transmitted
from the oscillator 18 to the input A, but no pulse (right side of
line 7) is transmitted from the monostable multivibrator 19 to the
input B of the pulse shaping circuit. Since all of the transistors
in the pulse shaping circuit 20 are normally off, there is no net
charge on the capacitor 90. When the positive pulse received at
input A is transmitted through the diode 86, it causes transistor
Q12 to conduct; and this will, in turn, cause transistor Q13 to
conduct and saturate. Thus, current will flow through the capacitor
90 in a regenerative action to saturate the transistor Q12 because
the right terminal of the capacitor 90 is directly connected to the
positive bus 33a through the saturated transistor Q13. The RC
network including the input capacitor 85 and resistor 87
differentiates the positive input pulse so that it supplies only a
triggering action; and when the current flowing through the
capacitor 90 into the base of transistor Q12 subsides to a point at
which it is incapable of sustaining conduction of transistor Q12,
transistor Q12 will become nonconducting and shut off the
transistor Q13. Under these conditions, the output pulse
transmitted to the output amplifier 21 has a duration of 1
millisecond. It will be observed that in this mode of operation,
the transistor Q14 remains nonconducting so that the resistor 95
(which is very much larger than the resistence 94) in effect
isolates the collector of transistor Q13 from the emitter of
transistor Q12. The width of the output pulse is determined
primarily by the time constant established by the values of the
resistor 89 at the capacitor 90. Under these circumstances, the
output pulse generated by the output amplifier 21 is the negative
pulse 98 in the right side of line 8 of FIG. 4.
The output amplifier includes transistors Q15 and Q16 as the active
elements. Transistor Q15 is connected in a conventional common
emitter arrangement. Transistor Q16, normally in a non-conducting
state, allows capacitor 100 to charge thru resistor 99, and
discharges capacitor 100 thru the heart during the 1 millisecond
on-time of a stimulating pulse, or the 10 microsecond on-time of a
non-stimulating pulse. The line 11 is connected to the junction
between the inductor 101 and the heart electrode 10.
It will be appreciated that the output pulse caused by the pulse 98
of the pulse shaping circuit 20 when the pacer is operating in the
fixed rate mode is also detected along line 11 and fed to the input
amplifier 13 through the protective network 12, resulting in pulse
34b on the right side of line 2 of FIG. 4. Pulses 55c and 55d on
lines 3 and 4 will also result from the monostable circuit 15.
However, these pulses do not affect the remainder of the operation
because there is a slight delay through the three-stage input
amplifier and the pulse rectifier network, and this delay is
sufficient so that any such pulse received at the oscillator 18 is
received while the oscillator is in its refractory period and
insensitive to any received input pulses. Hence, there is no effect
on the pulse shaping circuit 20.
Turning now to the case in which a natural heartbeat is detected by
the input amplifier along the line 11 (the left side of FIG. 4),
signal inputs are received at both the input A and the input B of
the pulse shaping circuit 20. The negative pulse received from the
collector of transistor Q9 is fed along the line 63; and it causes
the transistor Q14 to conduct thereby shunting the resistor 95 and
connecting the collector of transistor Q13 to the emitter of
transistor Q12 via resistor 94 which, it will be remembered, has a
relatively low resistance. Simultaneously, the pulse received at
the input A causes the transistor Q12 to conduct; and this, in
turn, causes the transistor Q13 to conduct and again feed
regenerative current through the capacitor 90 into the base of
transistor Q12. However, since transistor Q14 is saturated, the
voltage at the emitter of transistor Q12 is approximately the same
(less the drops through resistor 94 and across the
emitter-collector junction of transistor Q14) as the voltage at the
right terminal of the capacitor 90. Thus, almost immediately as
soon as the capacitor 90 begins to charge, the emitter-base
junction of transistor Q12 is reverse-biased thereby cutting it off
and terminating the pulse at the collector of transistor Q13 in a
very short time--nominally 10 microseconds. This output pulse is
shown as the narrow pulse 105 on the left side of line 8 of FIG.
4.
It will be realized that in FIG. 4, the waveforms shown are
idealized for the purposes of explanation, exaggeration being made
in amplitude and in time in certain instances to illustrate the
operation of the inventive system.
This narrow pulse 105 is also amplified by transistors Q15 and Q16,
and coupled to the heart through the capacitor 100 and inductor
101. However, since there is so little energy involved it is
non-stimulating and, therefore, non-competing. At the same time, it
provides an examining physician with an indication that the entire
implanted cardiac pacer system is in an operative condition since
it is capable of being picked up by the coil 25 of FIG. 3,
amplified, and detected.
As already mentioned, it is convenient to have the detection means
generate an audible "blip" when the narrow, non-stimulating pulse
occurs. This means audible detection means may be used in
conjunction with a standard EKG machine which typically has a
single recording channel. When a patient is examined who has an
implanted pacer but whose AV conduction system is functioning
properly, the standard EKG recording is made. It will show no pacer
artifact; thus it is immediately obvious to the physician that the
patient is pacing himself, i.e., does not have heart block. The
physician then wants to know whether the pacer would be functional
in the event of a later episode of heart block. He places a
transistor radio on the pacemaker generator bulge and listens for
the "blip." By watching the recorder at the same time, he can see
that the non-stimulating pulse he hears is synchronized with the
QRS complex; and he therefore can conclude that the pacemaker
generator is functioning and the lead system is intact and sensing
the natural QRS complex.
Having thus described in detail a preferred embodiment of the
present invention, persons skilled in the art will be able to
substitute elements for those disclosed to perform the same or
similar functions, or to otherwise modify the illustrated
embodiment while continuing to practice the inventive principle;
and it is, therefore, intended that all such modifications and
substitutions be covered as they are embraced within the spirit and
scope of the invention. As used in the claims, the word "pulse" is
intended to describe any electrical signal other than a non-varying
signal (such as zero voltage or any other dc level) or a continuous
wave signal such as a continuous sine wave.
* * * * *