U.S. patent number 3,661,158 [Application Number 04/884,825] was granted by the patent office on 1972-05-09 for atrio-ventricular demand pacer with atrial stimuli discrimination.
This patent grant is currently assigned to American Optical Corporation. Invention is credited to Barouh V. Berkovits.
United States Patent |
3,661,158 |
Berkovits |
May 9, 1972 |
ATRIO-VENTRICULAR DEMAND PACER WITH ATRIAL STIMULI
DISCRIMINATION
Abstract
An atrial and ventricular demand pacer. The demand pacer of the
present invention provides atrial stimulation and also protects
against ventricular asystole. In patients with atrial bradycardia
but normal AV conduction, only the atria are stimulated. When the
condition is complicated with AV block, both the atria and the
ventricles are pacer controlled. The pacer does not compete with
spontaneous ventricular contractions. An improved design eliminates
the need for a refractory period following each atrial stimulus so
that even a ventricular depolarization which coincides with an
atrial stimulus can be detected.
Inventors: |
Berkovits; Barouh V. (Newton
Highlands, MA) |
Assignee: |
American Optical Corporation
(Southbridge, MA)
|
Family
ID: |
25385487 |
Appl.
No.: |
04/884,825 |
Filed: |
December 15, 1969 |
Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N
1/368 (20130101) |
Current International
Class: |
A61N
1/368 (20060101); A61n 001/36 () |
Field of
Search: |
;128/419P,421,422,423 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dodinot et al., "Annals of the New York Academy of Sciences," Vol.
167, Art. 2, pp. 1,038-1,054, Oct. 30, 1969.
|
Primary Examiner: Kamm; William E.
Claims
What is claimed is:
1. An atrial and ventricular pacer comprising a first pair of
terminals for connection to a patient's heart for atrial
stimulation, a second pair of terminals for connection to said
patient's heart for ventricular stimulation, means coupled to said
second pair of terminals for detecting an electrical signal thereon
resulting from the beating of said patient's heart, means for
generating electrical stimuli on said fist and second pairs of
terminals in accordance with the time at which the beating of said
patient's heart is detected, and means for preventing the operation
of said detecting means responsive to the appearance on said second
pair of terminals of an electrical signal resulting from the
generation of an electrical stimulus on said first pair of
terminals.
2. An atrial and ventricular pacer in accordance with claim 1
wherein stimuli are generated on said first pair of terminals at
intervals which are shorter than the normal interval between
successive R waves in the electrocardiographic waveform of said
patient and are longer than the normal interval between an R wave
and the next P wave in the electrocardiographic waveform of said
patient, and stimuli are generated on said second pair of terminals
at intervals which are longer than the normal interval between
successive R waves in the electrocardiographic waveform of said
patient.
3. An atrial and ventricular pacer in accordance with claim 2
wherein said electrical stimuli generating means includes a pulse
generating circuit and transformer means for coupling said pulse
generating circuit to said first pair of terminals.
4. An atrial and ventricular pacer in accordance with claim 3
wherein the electrical stimuli generated on said first pair of
terminals are much narrower than the width of a normal QRS waveform
and said preventing means includes a low-pass filter for
attenuating the relatively high-frequency signal components
contained in the electrical signal appearing on said second pair of
terminals resulting from the generation of an electrical stimulus
on said first pair of terminals.
5. An atrial and ventricular pacer in accordance with claim 4
further including means responsive to the operation of said
detecting means at a rate greater than a predetermined rate for
controlling the generation of stimuli on said second pair of
terminals at a rate independent of the detection of the beating of
said patient's heart, said controlling means including a capacitor,
first resistance means connected in series with said capacitor,
means responsive to the operation of said detecting means for
applying a current pulse to said first resistance means for
charging said capacitor, second resistance means connected between
said first resistance means and said capacitor for discharging said
capacitor between applications thereto of successive current
pulses, said electrical stimuli generating means operating in
accordance with an instantaneous rise in potential greater than a
predetermined value across said capacitor with the application of
one of said current pulses, and means for reducing the discharge
time constant of said capacitor.
6. An atrial and ventricular pacer in accordance with claim 5
wherein said discharge time constant reducing means includes a
diode connected across said first resistance means poled in the
direction of the flow of discharge current from said capacitor.
7. An atrial and ventricular pacer in accordance with claim 1
wherein said electrical stimuli generating means includes a pulse
generating circuit and transformer means for coupling said pulse
generating circuit to said first pair of terminals.
8. An atrial and ventricular pacer in accordance with claim 7
wherein the electrical stimuli generated on said first pair of
terminals are much narrower than the width of a normal QRS waveform
and said preventing means includes a low-pass filter for
attenuating the relatively high-frequency signal components
contained in the electrical signal appearing on said second pair of
terminals resulting from the generation of an electrical stimulus
on said first pair of terminals.
9. An atrial and ventricular pacer in accordance with claim 8
further including means responsive to the operation of said
detecting means at a rate greater than a predetermined rate for
controlling the generation of stimuli on said second pair of
terminals at a rate independent of the detection of the beating of
said patient's heart, said controlling means including a capacitor,
first resistance means connected in series with said capacitor,
means responsive to the operation of said detecting means for
applying a current pulse to said first resistance means for
charging said capacitor, second resistance means connected between
said first resistance means and said capacitor for discharging said
capacitor between applications thereto of successive current
pulses, said electrical stimuli generating means operating in
accordance with an instantaneous rise in potential greater than a
predetermined value across said capacitor with the application of
one of said current pulses, and means for reducing the discharge
time constant of said capacitor.
10. An atrial and ventricular pacer in accordance with claim 9
wherein said discharge time constant reducing means includes a
diode connected across said first resistance means poled in the
direction of the flow of discharge current from said capacitor.
11. An atrial and ventricular pacer in accordance with claim 1
wherein the electrical stimuli generated on said first pair of
terminals are much narrower than the width of a normal QRS waveform
and said preventing means includes a low-pass filter for
attenuating the relatively high-frequency signal components
contained in the electrical signal appearing on said second pair of
terminals resulting from the generation of an electrical stimulus
on said first pair of terminals.
12. An atrial and ventricular pacer in accordance with claim 11
further including means responsive to the operation of said
detecting means at a rate greater than a predetermined rate for
controlling the generation of stimuli on said second pair of
terminals at a rate independent of the detection of the beating of
said patient's heart, said controlling means including a capacitor,
first resistance means connected in series with said capacitor,
means responsive to the operation of said detecting means for
applying a current pulse to said first resistance means for
charging said capacitor, second resistance means connected between
said first resistance means and said capacitor for discharging said
capacitor between applications thereto of successive current
pulses, said electrical stimuli generating means operating in
accordance with an instantaneous rise in potential greater than a
predetermined value across said capacitor with the application of
one of said current pulses, and means for reducing the discharge
time constant of said capacitor.
13. An atrial and ventricular pacer in accordance with claim 12
wherein said discharge time constant reducing means includes a
diode connected across said first resistance means poled in the
direction of the flow of discharge current from said capacitor.
14. An atrial and ventricular pacer comprising terminal means for
connection to a patient's heart for atrial stimulation, terminal
means for connection to said patient's heart for ventricular
stimulation, a first timing circuit connected to said atrial
terminal means for generating an electrical impulse on said atrial
terminal means, a second timing circuit connected to said
ventricular terminal means for generating an electrical impulse on
said first and said ventricular terminal means, means continuously
operative for detecting a signal on said ventricular terminal means
corresponding to a ventricular contraction of said patient's heart,
means responsive to the operation of said detecting means for
resetting said second timing circuit, and means for preventing the
operation of said resetting means responsive to the operation of
said detecting means when a signal appears on said ventricular
terminal means as a result of the generation of an electrical
impulse on said atrial terminal means.
15. An atrial and ventricular pacer in accordance with claim 14
wherein the electrical impulse generated on said atrial terminal
means is much narrower than the width of a normal QRS waveform and
said preventing means includes a low-pass filter for attenuating
the relatively high-frequency signal components contained in the
electrical signal appearing on said ventricular terminal means
resulting from the generation of an electrical impulse on said
atrial terminal means.
16. An atrial and ventricular pacer in accordance with claim 15
further including means responsive to the operation of said
detecting means at a rate greater than a predetermined rate for
controlling the generation of electrical impulses on said
ventricular terminal means at a rate independent of the detection
of signals on said ventricular terminal means corresponding to
ventricular contractions of said patient's heart, said controlling
means including a capacitor, first resistance means connected in
series with said capacitor, means responsive to the operation of
said detecting means for applying a current pulse to said first
resistance means for charging said capacitor, second resistance
means connected between said first resistance means and said
capacitor for discharging said capacitor between applications
thereto of successive current pulses, said second timing means
operating in accordance with an instantaneous rise in potential
greater than a predetermined value across said capacitor with the
application of one of said current pulses, and means for reducing
the discharge time constant of said capacitor.
17. An atrial and ventricular pacer in accordance with claim 16
wherein said discharge time constant reducing means includes a
diode connected across said first resistance means poled in the
direction of the flow of discharge current from said capacitor.
18. An atrial and ventricular pacer in accordance with claim 14
further including means responsive to the operation of said
detecting means at a rate greater than a predetermined rate for
controlling the generation of electrical impulses on said
ventricular terminal means at a rate independent of the detection
of signals on said ventricular terminal means corresponding to
ventricular contractions of said patient's heart, said controlling
means including a capacitor, first resistance means connected in
series with said capacitor, means responsive to the operation of
said detecting means for charging said capacitor, second resistance
means connected between said first resistance means and said
capacitor for discharging said capacitor between applications
thereto of successive current pulses, said second timing means
operating in accordance with an instantaneous rise in potential
greater than a predetermined value across said capacitor with the
application of one of said current pulses, and means for reducing
the discharge time constant of said capacitor.
19. An atrial and ventricular pacer in accordance with claim 18
wherein said discharge time constant reducing means includes a
diode connected across said first resistance means poled in the
direction of the flow of discharge current from said capacitor.
20. An atrial and ventricular pacer comprising terminal means for
connection to a patient's heart for atrial stimulation, terminal
means for connection to said patient's heart for ventricular
stimulation, a first timing circuit connected to said atrial
terminal means for generating an electrical impulse on said atrial
terminal means, a second timing circuit connected to said
ventricular terminal means for generating an electrical impulse on
said ventricular terminal means, means for detecting a signal
corresponding to a ventricular contraction of said patient's heart,
means responsive to the operation of said detecting means for
resetting said first and second timing circuits, and switch means
for preventing the resetting of only said second timing circuit
responsive to the operation of said detecting means.
21. An atrial and ventricular pacer in accordance with claim 20
wherein said atrial terminal means includes first and second
electrodes and said ventricular terminal means includes third and
fourth electrodes.
22. An atrial and ventricular pacer in accordance with claim 21
wherein said detecting means includes means for providing
continuous operation of said detecting means.
23. An atrial and ventricular pacer in accordance with claim 20
wherein said detecting means includes means for providing
continuous operation of said detecting means.
24. A pacer comprising terminal means for connection to a patient's
heart for atrium and ventricle stimulation, means for detecting
electrical signals on said terminal means resulting from beating of
said ventricle, and means controlled by operation of said detecting
means for generating electrical stimulii on said terminal means for
stimulating said atrium and said ventricle in a predetermined
timing sequence.
Description
This invention relates to pacers, and more particularly to demand
pacers for use with patients exhibiting symptomatic atrial
bradycardia and unpredictable AV block.
The electrical activity of a normal heart begins with a nerve
impulse generated by a bundle of fibers located in the sinoatrial
node. The impulse spreads across the two atria while they contract
and speed the flow of blood into the ventricles underneath them.
The atrial activity of the heart corresponds to the P wave in an
electrocardiogram trace. The electrical impulse continues to spread
across the atrioventricular (AV) node, which in turn stimulates the
left and right ventricles. Typically, an interval of approximately
120-160 milliseconds elapses between atrial and ventricular
stimulation. The ventricular activity corresponds to the QRS
portion of the electrocardiogram, and typically has a duration of
80 milliseconds. Toward the end of each heartbeat, the ventricular
muscles repolarize, and this portion of the electrical activity of
the heart corresponds to the T wave in the electrocardiogram.
Of the two types of contractions, the ventricular is far more
important than the atrial. The atrial contractions cause the
ventricular contractions to be more efficient; the ventricular
contractions are more effective if the ventricles are first filled
with blood. While a patient can survive without proper atrial
action, he cannot survive without ventricular contractions. With an
AV block, that is, an AV node which is open-circulated, life cannot
be sustained (unless the ventricles somehow beat on their own
without AV stimulation, and even in such a case the heartbeat rate
is generally far too slow). With proper ventricular contractions, a
patient can live even with atrial fibrillation. For this reason,
early pacers were generally used to protect against ventricular
asystole. These pacers stimulated the ventricles continuously at a
fixed rate to control their contractions.
Following the use of this type of pacer for many years, the demand
pacer was introduced. In a demand pacer, electrical
heart-stimulating impulses are provided only in the absence of
natural heartbeats. If only a single natural heartbeat is absent,
only a single electrical impulse is generated. If more than one
natural heartbeat is missing, the pacer fills in beats at the pacer
rate as long as the natural beats fail to occur. No matter how many
electrical stimuli are generated, they occur at essentially the
same time spacing from each other and from previous natural
heartbeats -- as would be the case if they were all natural
heartbeats. The result is an overall "integrated" operation, i.e.,
a mutually exclusive cooperation of natural heartbeats and
stimulating impulses. The demand pacer of this type is disclosed in
my U.S. Pat. No. 3,345,990 issued on Oct. 10, 1967.
Generally, a demand pacer is primed to generate an impulse at a
predetermined time after the last natural heartbeat. If another
natural heartbeat occurs during the timing interval of the pacer,
an impulse is not generated and the timing period starts all over
again. On the other hand, if a natural heartbeat does not take
place by the end of the timing period a stimulating impulse is
generated. For the proper operation of a demand pacer, the pacer
circuitry must determine if a natural heartbeat has occurred. The
largest magnitude electrical signal generated by the heart activity
is the QRS complex corresponding to ventricular contraction. To
determine whether a natural heartbeat has occurred, an electrode is
generally coupled to a ventricle. Since in most cases ventricular
stimulation is required, the same electrode can be used for both
stimulating the ventricles and detecting a natural heartbeat, as
disclosed in my aforesaid patent.
In the presence of noise, erroneous operation of a demand pacer of
this type can take place. The noise may result in the generation of
an electrical signal on the ventricular electrode, and the pacer
circuitry may treat this noise as indicative of a natural heartbeat
and inhibit the generation of a stimulating impulse even if one is
required. In my co-pending application Ser. No. 727,129 filed on
Apr. 11, 1968, now U.S. Pat. No. 3,595,242 an improved demand pacer
is disclosed. In this improved demand pacer, in the presence of
noise the pacer timing period is not interrupted. Continuous
stimulating impulses are generated, even if they are not required.
It is better to provide an impulse even if it is not required than
it is not to provide an impulse if it is required.
There are many patients with symptomatic atrial bradycardia even
though they have normal AV conduction. In such a patient, the slow
atrial rate causes the ventricular rate to slow down. Ventricular
pacer stimulation has been used in the past to treat this disorder.
For such patients, however, it would be better to provide atrial
stimulation to thus control both the atrial and ventricular rates,
with the additional benefit of the natural atrioventricular
sequence. But such atrial stimulation would leave the patient
unprotected from unpredictable AV block. Thus, provision should
also be made for ventricular stimulation if it becomes
necessary.
Both types of pacing could be accomplished with the use of two
individual pacers. But even if they are combined in a single
package many problems must be overcome, especially if a demand-type
operation is desired. One of the most obvious problems concerns the
timing sequence of the two types of pacing. A demand pacer for
atrial as well as ventricular stimulation is disclosed in my
co-pending application Ser. No. 810,519 filed on Mar. 26, 1969, now
U.S. Pat. No. 3,595,242. The first function of the pacer is to
generate an atrial stimulating impulse. After a predetermined time
interval, the pacer functions to generate a ventricular stimulating
impulse. Three electrodes are provided -- a neutral electrode, an
electrode for atrial stimulation and an electrode for ventricular
stimulation. The ventricular electrode also serves to detect the
occurrence of a ventricular contraction.
The pacer exhibits two timing or escape intervals. The ventricular
escape interval is 160-250 milliseconds longer than the atrial
escape interval. The ventricular escape interval is greater than
the normal interval between two heartbeats (as in a typical demand
pacemaker). The atrial escape interval is greater than the normal
interval between ventricular and atrial beats (R to P), but less
than the normal inter-beat interval (R to R). Both timing periods
begin with the generation of the last heartbeat (natural or
stimulated). If another ventricular contraction does not occur
within the atrial timing period, that is, in the absence of a
premature ventricular contraction, the atrial stimulating impulse
is generated. The atria contract and fill the ventricles with
blood. In the event the ventricles contract (i.e., there is no AV
block), the detected ECG signal on the ventricular electrode resets
both timing circuits and the ventricular impulse is not generated.
In the event the ventricular contraction does not occur, a
ventricular impulse is generated at the end of the ventricular
timing interval.
The atrial stimulating pulse is large in magnitude, in the order of
5 volts, and causes an electrical signal to appear on the
ventricular electrode. If this signal is treated as one which
results from a ventricular contraction, the ventricular timing
circuit will be reset and a ventricular stimulating pulse will not
be generated even if a ventricular contraction does not occur
following the atrial contraction. For this reason it is necessary
to prevent the pacer from misinterpreting a signal on the
ventricular electrode at the time of an atrial electrical stimulus
as representing a ventricular contraction. In the pacer disclosed
in my co-pending application Ser. No. 810,519, now U.S. Pat. No.
3,595,242 the pacer heartbeat detecting circuit is inhibited from
operating for a short refractory period following the generation of
each atrial stimulating pulse. Typically, the atrial stimulating
pulse has a duration of 2 milliseconds, and the heartbeat detecting
circuitry is inhibited from operating for 8 milliseconds starting
with the leading portion of the atrial stimulating pulse. In this
manner, the ventricular timing circuit cannot be reset when the
atrial stimulating pulse is generated. The ventricular timing
sequence continues in its ordinary course, and if the ventricular
contraction does not occur on schedule a ventricular stimulating
pulse is generated.
However, there are times when a ventricular contraction coincides
with an atrial contraction, or follows it within 8 milliseconds. In
the demand pacer disclosed in my co-pending application, such a
ventricular contraction cannot be detected because the ventricular
contraction detecting circuitry is inhibited from operating during
and immediately following the generation of each atrial stimulus.
If a ventricular contraction does occur during the refractory
period, the ventricular timing circuit does not reset and a
ventricular stimulating pulse is generated at the end of the timing
period even though a ventricular contraction has already
occurred.
It is a general object of my invention to provide a demand pacer in
which an electrical signal on the ventricular electrode resulting
from the generation of an atrial stimulus is not interpreted as a
ventricular contraction so that a pacer atrial refractory interval
is not required and a ventricular contraction which does occur
during or immediately following the atrial stimulus can be
detected.
Briefly, in accordance with the principles of my invention, the
design of the pacer is predicated on an analysis of the signal
frequencies contained in various waveforms, including an atrial
stimulating pulse and the QRS waveform (the electrical signal on
the ventricular electrode corresponding to a ventricular
contraction). The atrial and ventricular stimulating circuits are
isolated from each other by the use of a different pair of
electrodes for each function, as contrasted with the use of three
electrodes (one of which is shared) for the two functions. This
isolation cuts down the magnitude of the electrical signal on the
ventricular electrode resulting from the generation of an atrial
stimulus. However, the signal is not sufficiently small in
magnitude to prevent the pacer detecting circuitry from treating it
as representing a ventricular contraction. Additional circuits are
provided for discriminating between the two events based upon the
different frequency contents of the two respective signals which
appear on the ventricular electrode. In this manner, there is no
need to inhibit the operation of the detecting circuitry following
the generation of an atrial stimulus, and if a ventricular
contraction does occur during or immediately following atrial
stimulation the event can be detected.
It is a feature of my invention to provide separate and isolated
atrial and ventricular electrodes in a demand pacer, together with
a ventricular contraction detecting circuit which can discriminate
against signals corresponding to an atrial stimulus.
Further objects, features and advantages of my invention will
become apparent upon consideration of the following detailed
description in conjunction with the drawing, in which:
FIG. 1 depicts the illustrative demand pacer disclosed in my
copending application Ser. No. 810,519, now U.S. Pat. No.
3,595,242;
FIG. 2 depicts a typical electrocardiogram;
FIG. 3 is a timing diagram which will be helpful in understanding
the operation of the pacer of FIG. 1; and
FIG. 4 depicts the illustrative embodiment of the present
invention.
In the pacer of FIG. 1, electrode E2 is a neutral electrode
(conductor 9 is "grounded"), electrode E1 is the ventricular
stimulating electrode and electrode E3 is the atrial stimulating
electrode. The ventricular timing and stimulating circuit is
contained in the drawing between switch S and electrodes E1 and E2.
Capacitor 65 is initially charged by current flowing from batteries
3, 5 and 7 through resistor 59, terminals E1 and E2, and the
patient's heart in a time much shorter than the interval between
successive heartbeats. The magnitude of resistor 59 is low enough
to permit rapid charging of capacitor 65 but high enough to prevent
significant attenuation of the signal detected across terminal E1
and E2, these terminals being connected to the implanted
electrodes. When transistor T9 is triggered to conduction, the
capacitor discharges through it, current flowing from the capacitor
through the collector-emitter circuit of the transistor, terminal
E2, a cable to one electrode, the heart itself, the other
electrode, and another cable back to terminal E1. The discharge of
capacitor 65 through the electrodes constitutes the impulse to
stimulate the ventricles if necessary. As soon as transistor T9
turns off, capacitor 65 charges once again in preparation for the
next cycle. The capacitor serves simply as a source of current when
an impulse is necessary. Capacitor 65 is not involved with the
various timing sequences used to control the selective generation
of impulses.
The capacitor always charges to the peak battery voltage. Because
it discharges through an essentially short-circuited transistor
switch, the magnitudes of the impulses do not vary as the battery
impedance increases with aging. Nor is there any waste of energy
between manufacture and implantation -- although transistor T9 is
gated on during each cycle, as long as the electrodes are
open-circuited capacitor 65 cannot discharge.
The capacitor charges, as well as discharges, through the heart so
that the net DC current through the electrodes from the pacemaker
is zero. Otherwise, electrolytic processes in the heart cells could
dissolve the electrodes.
Transistors T7 and T8, connected as shown, are the equivalent of a
conventional silicon controlled rectifier. Both are normally
non-conducting. When the emitter electrode of transistor T7 goes
sufficiently positive, the transistors conduct and current flows
through the emitter circuit of transistor T8. Current continues to
flow until the potential at the emitter of transistor T7 drops
below a predetermined value.
Transistor T9 is a simple current amplifier which is normally
non-conducting. When transistor T8 conducts. however, the emitter
current flowing through resistors 61 and 63 causes the potential at
the base of transistor T9 to increase. At such a time transistor T9
is biased to conduction and capacitor 65 can discharge through it
as described above.
The apparatus can be used in a free-running mode, that is, an
impulse can be generated at a 72-pulse-per-minute rate, for
example, independent of the occurrence of natural heartbeats. In
such a case, switch S is closed and the base of transistor T6 is
connected to the negative terminal of battery 3. Transistor T6
therefore remains in a cut-off condition. Pulses transmitted
through capacitor 53 (to be described below) are shorted through
the switch away from the transistor. Initially, capacitor 57 is
discharged and transistors T7 and T8 are non-conducting. Current
flows from batteries 3, 5 and 7 through resistors 35 and 37,
capacitor 57, and resistors 61 and 63. The current through
resistors 61 and 63 is insufficient to turn on transistor T9. As
the capacitor charges, the junction of the capacitor and resistor
37 increases in potential. Thus the emitter of transistor T7
increases in potential. Eventually the potential is sufficient to
trigger the relaxation oscillator consisting of transistors T7 and
T8. Capacitor 57 discharges through resistor 37 and these two
transistors. At the same time current flows from batteries 3 and 5
through the collector-emitter circuit of transistor T8, and
resistors 61 and 63. Transistor T9 conducts and capacitor 65
discharges through it to provide an impulse to the ventricles. As
soon as capacitor 57 has discharged sufficiently and the potential
of the emitter of transistor T7 has dropped to a low enough value,
all of transistors T7, T8 and T9 turn off and the impulse is
terminated. Capacitor 65 immediately recharges, and capacitor 57
starts charging once again in preparation for the next impulse.
The charging period of capacitor 57, that is, the interval between
impulses, is determined by the magnitude of the capacitor, and the
magnitudes of resistors 35, 37, 61 and 63. Resistors 37, 61 and 63
are very small in comparison to the magnitude of resistor 35.
Consequently, it is the magnitude of resistor 35 which determines
the inter-pulse interval. As the magnitude of resistor 35 is
adjusted the rate of the impulses varies.
Similarly, it may be desirable to adjust the width of each impulse
delivered to the heart. Capacitor 57 discharges through resistor 37
and transistors T7 and T8. The width of the impulse delivered by
capacitor 65 is determined by the discharge time of capacitor 57,
that is, the time period during which transistors T7 and T8 conduct
and thereby turn on transistor T9. By varying the magnitude of
resistor 37 the width of each impulse can be adjusted. In the case
of an implantable pacer, the magnitudes of resistors 35 and 37
would be adjusted prior to implanting the apparatus in the
patient.
When switch S is opened, i.e., in the case of a pacer required to
operate in the demand mode, the same type of free-running operation
would take place were there no input to the base of transistor T6
through capacitor 53. Transistor T6 would remain non-conducting and
would not affect the charging of capacitor 57. However, with switch
S open, pulses transmitted through capacitor 53 are not shorted
through the switch away from the base-emitter circuit of transistor
T6. With the switch open, each pulse transmitted through capacitor
53 to the base of transistor T6 causes the transistor to conduct.
Capacitor 57 discharges through the collector-emitter circuit of
the transistor. In such a case, the timing cycle is interrupted and
the junction of capacitor 57 and resistor 37 does not increase in
potential to the point where transistors T7 and T8 are triggered to
conduction. When the apparatus is functioning as a "demand" pacer,
each ventricular contraction causes a pulse to be transmitted
through capacitor 53 to turn on transistor T6. Transistor T6
conducts to discharge capacitor 57 just prior to the time when
capacitor 57 would trigger, and discharge through, transistors T7
and T8 to control the generation of an impulse. After capacitor 57
has discharged through transistor T6, the transistor turns off. The
capacitor then starts charging once again. The new cycle begins
immediately after the occurrence of the last ventricular
contraction so that the next impulse, if needed, will be generated
immediately after the next natural heartbeat should have been
detected were the heart functioning properly.
A similar circuit is provided for generating an atrial stimulating
pulse. The various elements to the right of electrodes E1 and E2
are designated by the same numerals as the equivalent elements in
the atrial timing circuit with the addition of prime symbols.
Except for component magnitudes, the atrial timing circuit is the
same as the ventricular timing circuit.
Electrode E3 is implanted in the patient's heart to stimulate his
atria. Capacitor 57' charges through potentiometers 35' and 37'.
After a predetermined interval, when the capacitor voltage has
reached the level required to control conduction of transistors T7'
and T8', the two transistors conduct and forward bias the
base-emitter junction of transistor T9'. The charge on capacitor
65' flows through transistor T9' and electrodes E2 and E3. The
width of each pulse is determined by the setting of potentiometer
37' which determines the time required for capacitor 57' to
discharge through transistors T7' and T8'. The inter-pulse interval
is determined by the setting of potentiometer 35' which determines
the time required for capacitor 57' to charge to the level which
causes transistors T7' and T8' to conduct.
Any pulse delivered through capacitor 53 as a result of the
detection of an R wave causes transistor T6' to conduct along with
transistor T6. At the same time that capacitor 57 discharges
through transistor T6, capacitor 56' discharges through transistor
T6'. In such an event, the timing period of the circuit of FIG. 3
is not concluded and an atrial stimulating pulse is not generated.
Instead, the timing begins once again.
FIG. 3 depicts a timing sequence which will be helpful in
understanding the pacer operation. Two R waves are shown and
represent two successive beats (ventricular contractions) of the
patient's heart. Typically, the time interval between them is less
than 760 milliseconds. The P wave associated with the second R wave
is shown occurring before it.
Potentiometer 35' has a value such that capacitor 57' charges to
the level required for the conduction of transistors T7' and T8'
after 600 milliseconds have elapsed since the last capacitor
discharge. The atrial stimulating pulse E3 is thus shown occurring
600 milliseconds after the first R wave. It should be noted that
the atria are stimulated following the P wave during a normal
heartbeat. Actually, if a P wave has been generated it is an
indication that the atria have contracted and an atrial stimulating
impulse on electrode E3 is not required. However, if such an
impulse is generated following the atrial contraction, that is,
during the refractory interval of the atria, it has no effect on
the beating action of the patient's heart. (The generation of an
atrial stimulating impulse prior to the natural atrial contraction
can induce an atrial premature beat which is not desirable.)
Potentiometer 35 in FIG. 1 has a value such that the timing
interval for the ventricular stimulation is 800 milliseconds. Thus,
the pulse designated E1 in FIG. 5 is shown occurring 800
milliseconds after the first R wave, which is slightly after the
second R wave should it be present. If the second R wave is
detected on electrode E1, both timing circuits are reset and an
impulse is not generated on electrode E1. This is the desired
demand-type operation. If a natural heartbeat does not occur within
800 milliseconds after the last heartbeat, an impulse is generated
on electrode E1 to stimulate the ventricular contraction. It should
be noted that if the heartbeats naturally, there will be no
ventricular stimulation by the pacer. However, there will be atrial
stimulation because the 600 millisecond atrial timing interval is
less than the natural inter-pulse interval. But in the event a
natural atrial contraction does not take place, the atrial
stimulation is required in order that the heart function more
efficiently. The ventricular stimulation, of course, is provided to
correct any AV block. A normal ventricular contraction can occur
approximately 120-160 milliseconds after the atrial stimulation.
The ventricular timing period is 200 milliseconds longer than the
atrial timing period; sufficient time is allowed for a natural
ventricular contraction before a ventricular stimulating impulse is
generated. In general, the ventricular timing period should exceed
the atrial timing period by 160-250 milliseconds.
It should also be noted that the operation of the atrial timing
circuit is keyed to the detection of a ventricular contraction on
electrode E1. It is highly desirable to key the atrial timing
circuit to the beating of the patient's heart -- were a
free-running generator provided to stimulate the atria, the timing
of the beating of the patient's heart might be seriously affected.
While the natural timing might change, the circuitry timing would
be invariant. For this reason, capacitor 57' is discharged
following any beating of the patient's heart. Theoretically, it
might be possible to detect an atrial contraction, that is, to
detect the P wave, and to discharge capacitor 57' before its timing
period is completed so that an atrial stimulating impulse would not
be generated if it is not required. However, it is exceedingly
difficult to detect the P wave due to its small magnitude as
compared to the R wave. For this reason, in the pacer of FIG. 1 it
is the detection of the R wave which also serves to reset the
atrial timing period. Of course, this results in the continuous
generation of impulses at electrode E3 if the heart is beating
normally (even though impulses at electrode E1 are not generated)
because each R wave is detected after the impulse at electrode E3
has been generated. However, the generation of an atrial
stimulating impulse during the refractory interval of the atria has
been found not to interfere with the normal beating of a patient's
heart. (The same is not true of the generation of a ventricular
stimulating impulse following a ventricular contraction, and this
is the reason for the use of the demand-type pacer in the first
place.)
It is the function of the circuitry to the left of switch S to
detect a natural heartbeat (ventricular contraction), to the
exclusion of other undesired signals, and in response thereto to
apply a positive pulse to the base of transistor T6 for the purpose
of interrupting the charging cycles of capacitors 57 and 57'.
The natural beating action of the heart produces electrical signals
which are characteristic of successive steps in the occurrence of
each heartbeat. A heart beating in normal or sinus rhythm produces
electrical signals conventionally identified as P, Q, R, S and T
waves as shown in FIG. 2.
It is generally recognized by those skilled in the art that it is
preferable to distinguish the QRS complex in an electrocardiogram
from the P and T waves for the purpose of detecting a natural
heartbeat. Actually, with respect to implantable pacers it is the
cellular electrogram in the vicinity of the electrodes which is
important, not the skin electrocardiogram, since the pacer responds
to the electrical signals generated by the cells in the vicinity of
the electrodes. The cellular electrogram is generally considerably
different from the skin electrocardiogram. The latter is the
integral of all the cellular electrograms generated by a beating
action of the heart. Because the various cells generate their
signals at different times during each heartbeat, the integral
(electrocardiogram) is in many respects dissimilar from an
individual cellular electrogram. However, just as the
electrocardiogram exhibits a sharply rising R pulse so does the
cellular electrogram. It is the sharply rising pulse of the
electrogram which is the best indication of a natural heartbeat.
Although references below are made to the QRS complex of an
electrocardiogram, it must be borne in mind that with respect to
the electrodes implanted in the patient's heart it is the sharply
rising pulse of the cellular electrogram which is of importance. It
has become the practice in the art to focus on the QRS complex of
the electrocardiogram, rather than the individual cellular
electrogram, primarily because the R wave in the electrocardiogram
does for the most part correspond to the sharply rising pulse of
the cellular electrogram.
Using the techniques of frequency analysis, it can be shown that
the R peak comprises frequency components primarily in the 20-30 Hz
region. The P and T waves comprise for the most part lower
frequency components. To avoid triggering of transistor T6 by P and
T waves, various filters are provided in the circuit to filter out
frequencies below 20 Hz. Of course, it is advantageous to provide
additional filters to filter out frequencies above 30 Hz, and
particularly 60-Hz frequency signals. Such filters are incorporated
in the pacer depicted in FIG. 1, although it has been found that
such filters are not totally effective in preventing the triggering
of transistor T6 by 60-Hz stray signals. For this reason, while
various filters are associated with amplifying stages T1 and T2, a
rate discrimination circuit (including transistors T3, T4 and T5,
resistors 45 and 47, and capacitors 49 and 53) is provided to
prevent triggering of transistor T6 by 60-Hz stray signals. This
rate discrimination circuit will be described below after the
frequency discrimination circuit is first considered.
Transistor T1 is normally conducting, the emitter terminal of the
transistor being connected through resistor 19 and conductor 9 to
the negative terminal of battery 3, and the base of the transistor
being connected through resistor 15 and conductor 13 to the
positive terminal of the battery. The electrical signals picked up
by the electrodes implanted in the patient's heart are coupled
across capacitor 17 and resistor 15 in the base circuit of
transistor T1. Signals of either polarity are amplified by
transistor T1. The transistor is biased for class A operation
because the polarity of the detected signal may be of either type
depending on the manner in which the electrodes are implanted.
It should be noted that Zener diodes 67 and 67' respectively bridge
electrodes E1 and E2, and electrodes E3 and E2. It is possible that
very high voltages can appear across the electrodes. For example,
if defibrillation equipment is used, a very high voltage may be
applied to the patient's heart. To avoid damage to the pacer
circuitry, the large voltage signals are short-circuited through
the Zener diodes. Each diode conducts in the forward direction (for
voltages above a few tenths of a volt) as well as for voltages in
the reverse direction which are above the breakdown potential of 10
volts.
Capacitor 17 and resistor 15 emphasize the step function in the
cellular electrogram. These two elements comprise a differentiator
which emphasizes the frequency components above approximately 20
Hz. For such signals, the voltage drop across resistor 15 is
appreciable and the input to transistor T1 is relatively large. For
lower frequency signals, however, the voltage drop across capacitor
17 is much greater, and a smaller input signal is applied across
the base-emitter junction of transistor T1.
Resistor 19 and capacitor 21 in the emitter circuit of transistor
T1 serve a similar function. The impedance of the parallel circuit
increases as the frequency decreases. The emitter impedance
provides negative feedback for the transistor, and the overall gain
of the transistor decreases as the frequency decreases.
The amplified signal at the collector of transistor T1 is applied
across the base-emitter junction of transistor T2, this transistor
also being biased for class A operation. Transistor T2 further
amplifies the detected signals. Capacitor 25 and resistor 27 in the
emitter circuit of transistor T2 serve the same function as
resistor 19 and capacitor 21 in the emitter circuit of transistor
T1. This third differentiator further limits the low-frequency
response of the detecting circuit to discriminate against the P and
T waves and any other frequencies well below 20 Hz.
Resistor 29 and capacitor 23 serve as an integrator to reduce
high-frequency noise components well above 30 Hz. The higher the
frequency, the lower the impedance of capacitor 23, the smaller the
overall collector impedance of transistor T1, and the lower the
gain of the stage. Resistor 31 and capacitor 43 in the collector
circuit of transistor T2 serve the same function. Actually, these
four elements serve to attenuate frequencies well above 60 Hz and
have little effect on 60 Hz signals. In the illustrative embodiment
of the invention the rate discrimination stage distinguishes 60-Hz
stray signals from desired signals.
AC signals at the collector of transistor T2 are coupled through
capacitor 41 to the base of transistor T3 and the base of
transistor T4. The overall gain characteristic of stages T1 and T2,
from terminals E1 and E2 to the collector of transistor T2 and
conductor 9, is such that signals in the 20-30 Hz region are
amplified to the greatest extent. The gain curve falls off very
rapidly below 20 Hz such that the frequency components
characteristic of the P and T waves are not amplified sufficiently
for turning on transistors T3 and T4. For frequency components
above 30 Hz, the gain for 60-Hz signals is only slightly less than
the maximum gain. However, for signals considerably higher, e.g.,
above 150 Hz, the gain is low enough to prevent false operation of
transistors T3 and T4.
If transistors T3 and T4 require a signal of approximately 1 volt
to conduct, and the maximum gain of stages T1 and T2 is above 50,
it is apparent that 20-mv signals in the 20-30 Hz region at the
electrodes can trigger transistors T3 and T4 to conduction. The
20-30 Hz components in the electrical signal generated by the
beating of the heart in the vicinity of the electrodes is typically
above 20 mv. The frequency components characteristic of the P and T
waves are not only one to two times smaller in magnitude than those
characteristic of the R wave, but since the gain of stages T1 and
T2 in the region around 5 Hz is only a fraction of the maximum
gain, these signals do not trigger transistors T3 and T4 to
conduction.
The rate discriminator stage includes three transistors T3, T4 and
T5 which collectively comprise a bi-phase switch having two
functions. First, the switch serves to provide unipolar current
pulses to charge capacitor 49. However, the switch is not a true
rectifier because of its second function. This function is to
provide unipolar pulses of constant magnitude independent of the
amplitude of input signals above a threshold value. Any signal
through capacitor 41, either positive or negative, which exceeds a
threshold value (typically, 1 volt) results in a unipolar current
pulse of predetermined magnitude being fed through resistor 45 to
charge capacitor 49.
The emitter of transistor T4 is connected to the positive terminal
of battery 5, while the base of the transistor is connected through
bias resistor 39 to the same potential. Transistor T4 is normally
non-conducting. However, when a negative signal is transmitted
through capacitor 41 the transistor turns on and current flows from
battery 5 through the emitter-collector circuit of the transistor,
resistor 45, and the parallel combination of resistor 47 and
capacitor 49. The capacitor thus charges toward a maximum voltage
determined by batteries 3 and 5, the drop across transistor T4, and
resistors 45 and 47. If the emitter-collector circuit of the
transistor is considered to have negligible impedance, the charging
current is determined solely by the magnitude of the batteries, and
the magnitudes of elements 45, 47 and 49. The magnitude of the
negative input signal is of no moment. As long as it is above the
threshold value necessary for controlling the conduction of
transistor T4, a current pulse of predetermined magnitude will be
delivered to charge capacitor 49.
A positive signal transmitted through capacitor 41, on the other
hand, has no effect on transistor T4. However, it does cause
transistor T3 to conduct, current flowing from battery 7 through
resistor 33 and the collector-emitter circuit of transistor T3. It
is necessary that the positive signal transmitted through capacitor
41 also result in a unipolar pulse of the same polarity to charge
capacitor 49. The collector output of transistor T3 cannot be used
for this purpose because it drops in potential when transistor T3
conducts. For this reason, phase inverter T5 is provided. While the
emitter of this transistor is connected to the negative terminal of
battery 7, the base of the transistor is connected to the junction
of resistors 51 and 69. Normally the transistor is non-conducting.
However, when transistor T3 conducts and the collector voltage
drops, so does the base potential of transistor T5. At this time
transistor T5 conducts, current flowing from the positive terminal
of battery 5 through the emitter-collector circuit of transistor T5
to resistor 45. It is thus seen that any changing signal
transmitted through capacitor 41 above a threshold value causes a
unipolar pulse to be delivered to the charging circuit.
Consider for the moment unipolar pulses delivered by either
transistor T4 or transistor T5, or both of them, occurring at a
very slow rate. Each current pulse causes capacitor 49 to charge,
current flowing through resistor 45 and the capacitor. (Some of the
current flows through resistor 47 but capacitor 49 keeps charging
and the voltage across it keeps increasing). When the pulse
terminates, capacitor 49 starts discharging through resistor 47.
Assuming that each charging pulse is sufficient to fully charge
capacitor 49, the potential across the capacitor will equal the sum
of the magnitudes of batteries 3 and 5, multiplied by the voltage
divider ratio of resistors 47 and 45 (less any drop across
transistor T4). (The exception of narrow RF input pulses will be
described below.) When each unipolar pulse terminates, capacitor 49
starts discharging through resistor 47. If the capacitor fully
discharges by the time the next charging pulse is delivered, the
capacitor will then recharge to the maximum voltage, after which it
will fully discharge once again. The potential across capacitor 49
is AC-coupled through capacitor 53 to the base of transistor T6 and
the base of transistor T6'. Each charging pulse increases the
potential across capacitor 49 from zero to the maximum voltage. The
positive step is sufficient to cause transistors T6 and T6' to
conduct, thereby discharging capacitors 57 and 57' and inhibiting
the next impulses which would otherwise have been generated.
Consider now charging pulses which occur at a faster rate, e.g., at
a rate of 72 per minute which is expected as a result of natural
heartbeats. Each charging pulse charges capacitor 49 to the maximum
voltage. The capacitor then starts to discharge through resistor 47
but before the discharge is complete another charging pulse occurs.
The capacitor immediately charges to the maximum voltage and then
starts to discharge once again. The capacitor never fully
discharges, but the minimum voltage across it (that at the end of
the discharge cycle when the next charging pulse is received) is
low enough such that the increase in the capacitor voltage with the
occurrence of each charging pulse is still sufficient to trigger
transistors T6 and T6'. Consequently, each charging pulse which
results from a natural heartbeat resets both timing circuits.
Consider now the effect of 60-Hz signals on the circuit. If a stray
60-Hz signal is applied to the base of transistor T3 and the base
of transistor T4, each of these transistors conducts during each
cycle, transistor T3 for some time during the positive half-cycle
and transistor T4 for some time during the negative half-cycle.
Consequently, charging pulses are delivered to capacitor 49 at the
rate of 120 per second. This is a rate considerably greater than 72
per minute. Each pulse fully charges capacitor 49 and the next
pulse is delivered before the capacitor has had an opportunity to
discharge to any meaningful extent. Consequently, although each
pulse fully charges the capacitor, the increase in the capacitor
voltage is negligible because the capacitor voltage never decreases
much below the maximum potential. Consequently, steps of negligible
magnitude are transmitted through capacitor 53 to the base of
transistor T6 and the base of transistor T6'. Each transistor
requires a signal of approximately 0.5 volts for conduction, and
the step functions delivered through capacitor 53 are well below
this value as the result of unipolar pulses occurring at a rate of
120 per second.
Activations of transistors T3 or T4 at a rate above 40 per second
(an inter-activation period of 25 milliseconds) are sufficient to
prevent appreciable discharge of capacitor 49 and the triggering of
transistor T6. It will be seen that should any 60-Hz signals, or
signals of any higher frequency, be present in the circuit, step
functions of insufficient magnitude to trigger transistors T6 and
T6' are transmitted through capacitor 53. Transistors T6 and T6'
remain non-conducting and the pacer operates in its free-running
mode. Even if there are natural heartbeats at this time, they have
no effect. Each natural heartbeat causes a charging pulse to be
delivered to capacitor 49, but it has no effect since the capacitor
is at all times charged to almost its peak value. Only in the
absence of undesirable high frequencies does the capacitor have an
opportunity to discharge prior to the delivery of a current pulse
resulting from a natural heartbeat. It is only at this time that
each natural heartbeat results in the conduction of transistors T6
and T6' and the resetting of the timing circuits. In effect,
resistors 45 and 47, and capacitor 49, can be thought of as a high
inertia switch. This switch cannot respond to beats above a rate of
40 per second. Any repetitive signal above 40 per second is
ineffective to de-activate the impulse generating circuits.
Of course, during the time that stray 60-Hz signals, or other
undesirable signals, are present, the pacer operates in its
free-running mode along with the natural beating of the patient's
heart. This may be disadvantageous, but it is far better than
allowing the pacer to cease functioning at all -- a disastrous
condition if at the particular time the patient's heart has stopped
functioning.
While very high-frequency signals have the same effect on capacitor
49 as 60-Hz signals, there is one type of signal which is not
prevented from falsely operating transistors T6 and T6' by the lack
of discharge of capacitor 49. Specifically, single pulses of very
narrow width can cause either of transistors T3 or T4 to conduct
and a charging pulse to be delivered to capacitor 49. If capacitor
49 is discharged at this time (as it would be before the end of
each cycle) the positive step across capacitor 49 can falsely
trigger transistors T6 and T6'. To preclude this possibility,
resistor 45 is provided. Although each charging pulse causes a
rapid rise in potential across capacitor 49, the rise is not a
perfect step function because resistor 45 increases the charging
time constant. With a very narrow pulse, by the time capacitor 49
has begun to charge appreciably, the pulse terminates.
Consequently, capacitor 49 does not charge sufficiently to trigger
transistors T6 and T6'.
Transistors T1 and T2 serve a different function than transistors
T3, T4 and T5. The first two transistors, together with the various
differentiators and integrators connected to them, serve as a
frequency discriminator. Although higher frequencies are somewhat
attenuated, it is the attenuation of the lower frequencies (below
20 Hz) which is of the utmost importance. By attenuating these
signal frequencies and distinguishing between the different waves
in the myocardial signal, it is possible to prevent triggering of
transistors T6 and T6' by P and T waves. Although the frequency
discrimination circuit attenuates signals below 20 Hz, this should
not be confused with beats at a 72-per-minute rate. It is the
emphasis on signal frequencies in the 20-30 Hz region which ensures
that beats at a 72-per-minute rate appear at the base of transistor
T3 and the base of transistor T4 as a result of the R waves to the
exclusion of other signals. As far as signals transmitted through
capacitor 41 are concerned, it is more convenient to analyze the
operation of the pacer in terms of activation rates. The frequency
components in any particular signal are not determinative once the
signal has been transmitted through capacitor 41. From that point
on, the important consideration is the number of activations of
either transistor T3 or T4 during any given period of time. Since
for any signal the bi-phase switch delivers a current pulse of
predetermined magnitude to the capacitor charging circuit, it is
the rate discrimination circuit which prevents cancellation of the
pacer stimuli by competitive sine wave interference or any other
interference from signals occuring at a rate greater than a minimum
value. In the pacer of FIG. 1, intereference is prevented for all
signals occurring at a rate greater than 40 per second.
It is possible in some cases that the atrial stimulating pulse will
produce an electrical signal on electrode E1 which after
amplification will cause the two timing circuits to be reset. For
this reason, FET switch 92 is inserted in conductor 11 between
electrode E1 and capacitor 17 in the base circuit of transistor T1.
The switch is normally conducting due to its connection through
resistor 94 to ground conductor 9. The negative pulse generated at
electrode E3 is transmitted over conductor 84 and through diode 95
to capacitor 93. The capacitor charges and turns off the FET
switch. When the atrial stimulating pulse terminates (after a
typical duration of 2 milliseconds), capacitor 93 discharges
through resistor 94. The time constant of the capacitor-resistor
combination is such that the FET switch remains off for
approximately an additional 6 milliseconds to prevent erroneous
detection of a ventricular contraction for a few additional
milliseconds until after all transients have died down. In this
manner, the heartbeat detection circuit is disabled during each
atrial stimulation and for a short interval thereafter. (Capacitor
91 is provided to short high-frequency transients, arising from the
FET switching, to conductor 9.)
It is possible, however, for a true ventricular contraction to
occur within the 8-millisecond refractory period controlled by FET
switch 92. In the pacer of FIG. 1, such a ventricular contraction
is not detected because the signal on electrode E1 is not
transmitted through the switch and capacitor 17 to the base of
transistor T1. It is desirable that the ventricular contraction be
detected in order to reset the ventricular timing circuit. The
atrial timing circuit re-starts anyway inasmuch as capacitor 57'
discharges in order to generate the atrial pulse in the first
place. But capacitor 57 continues to charge if transistor T6 is not
turned on with the occurrence of the ventricular contraction.
The pacer of FIG. 4 is similar to that of FIG. 1 except that the
inhibiting circuit including FET switch 92 is omitted. The
ventricular timing circuit in the pacer of FIG. 4 can be reset if a
ventricular contraction occurs during or immediately after the
generation of an atrial stimulating pulse. At the same time, in the
absence of such a ventricular contraction, the atrial stimulating
pulse does not cause transistor T6 to turn on thus resetting the
ventricular timing circuit.
The circuit of FIG. 4 is different from the circuit of FIG. 1 in a
number of respects. First, instead of providing a single atrial
stimulating electrode E3, which shares ground electrode E2 with
ventricular stimulating electrode E1, two separate atrial
stimulating electrodes E3 and E4 are provided. Thus the collector
circuit of transistor T9' in FIG. 4 is different from that of FIG.
1. Second, instead of providing FET switch 92 as in FIG. 1, the
circuit of FIG. 4 includes filter circuits for attenuating the
signal on electrode E1 which results from the generation of an
atrial stimulating pulse. Third, instead of providing a direct
connection between the upper terminals of resistor 47 and capacitor
49 as in FIG. 1, a parallel connection of resistor 38 and diode 42
is provided in the circuit of FIG. 4. Fourth, the arrangement of
switch S is slightly different in the circuit of FIG. 4, and two
separate capacitors 53 and 54 are provided instead of the single
capacitor 53 of FIG. 1.
In the pacer of FIG. 1, the atrial stimulating pulse on electrode
E3 can cause a significant signal on ventricular-contraction
detecting electrode E1. This is due to the fact that ground
electrode E2 is shared by the two other electrodes. Each of
electrodes E1 and E3 is connected through the heart to electrode
E2. Electrode E2 has some impedance and it is apparent that when an
atrial stimulating pulse is generated and current flows between
electrodes E3 and E2, the potential on electrode E2 rises due to
the voltage drop across it. The increased potential is in turn
extended through the heart to electrode E1.
In order to eliminate the atrial refractory period of the pacer, as
a first step it is desirable to attenuate the signal on electrode
E1 which results from the generation of an atrial stimulating
pulse. This is achieved in the pacer of FIG. 4 by providing a
separate pair of electrodes E3 and E4 for the atrial stimulating
circuit. When an atrial pulse is generated, current flows between
electrodes E3 and E4. Because electrode E2 is not shared by
electrodes E1 and E3, a significant rise in potential is not
extended to electrode E1.
Capacitor 65' is no longer used to store charge preparatory to the
generation of each atrial stimulating pulse. Instead, the
conduction of transistor T9' causes a pulse to be transmitted
through transformer 44 to electrodes E3 and E4. When the transistor
turns on, current flows from batteries 3, 5 and 7 through the
primary winding of transformer 44 and the collector-emitter circuit
of transistor T9' to ground conductor 9. The current pulse through
the primary winding of transformer 44 causes a voltage pulse to
appear across the secondary winding of the transformer. This pulse
is transmitted through capacitor 48. Electrode E3 goes positive
with respect to electrode E4 and current flows through the atrium
to which the electrodes are connected.
Capacitor 48 is provided to prevent the flow of direct current in
the event the electrodes become polarized. Zener diode 52 is
provided in place of Zener diode 67' in FIG. 1. The diode is poled
such that it is reverse biased when the current stimulating pulse
is generated. However, in the event of an excessive signal
appearing across the electrodes resulting from an external source,
the diode conducts in the forward direction if the signal exceeds a
few tenths of a volt and it conducts in the reverse direction if
the signal exceeds 10 volts. In either case, the magnitude of the
pulse extended in the reverse direction through transformer 44 to
the collector circuit of transistor T9' can cause no damage.
Diode 46 is provided to allow the current through the primary
winding of the transformer to dissipate when transistor T9' turns
off at the end of each pulse. The current continues to flow in the
same direction through the primary winding but it now flows through
diode 46 instead of transistor T9'. The diode is reverse biased
when transistor T9' conducts so that during the generation of the
current pulse it has no effect on the circuit operation. The use of
the diode in this manner allows the rapid disappearance of the
magnetic field produced by the current flow through the primary
winding at the end of the atrial stimulating pulse.
Because the current path for the atrial stimulating pulse is
through the heart tissue between electrodes E3 and E4, and does not
include electrodes E1 and E2 or the heart tissue in which they are
implanted, the potential on electrode E2, and the potential on
electrode E1 reflected through the heart tissue between electrodes
E1 and E2, is much less than the potential which is developed on
the same electrode in the pacer of FIG. 1. As a practical matter,
however, the potential on electrode E1 may still be sufficient,
after amplification by transistors T1 and T2, to cause the
ventricular timing circuit to be reset. For this reason the
pacemaker of FIG. 4 includes additional circuitry for preventing
resetting of the ventricular timing circuit.
The atrial stimulating pulse has a typical duration of 2
milliseconds. Due to its short width, the pulse is characterized
primarily by relatively high-frequency components. In the pacer of
FIG. 1, electrode E1 is connected through FET switch 92 directly to
capacitor 17. If the FET switch is omitted, it is apparent that
electrode E1 is coupled directly to the base of transistor T1
through the capacitor. In the pacer of FIG. 4, electrode E1 is
coupled through resistor 32 to capacitor 17. One end of the
resistor is connected through capacitor 91 to ground conductor 9.
Capacitor 91 in the pacer of FIG. 1 serves to short high-frequency
switching transients of FET switch 92 to ground. In the pacer of
FIG. 4, resistor 32 and capacitor 91 comprise a low-pass filter
which reduces the spike transmitted through capacitor 17 to the
base of transistor T1 when an atrial stimulating pulse is
generated. Typically, the cut-off frequency of the low-pass filter
comprising resistor 32 and capacitor 91 is 15 Hz. The filter has
little effect on the QRS signal which is detected at electrode E1
because the QRS waveform is characterized primarily by frequencies
above 15 Hz. But the low-pass filter does attenuate the relatively
high-frequency signal on conductor 11 which appears when the atrial
stimulating pulse is generated.
In the pacer of FIG. 1, the base bias potential for transistor T1
is derived by the connection of conductor 13 from the junction of
batteries 3 and 5 to resistor 15. Although ideally the batteries
have no source impedance, as a practical matter the batteries do
exhibit some impedance. When the atrial stimulating pulse is
generated, the current which flows through the batteries does cause
a change in the potential exhibited across each battery. Thus even
though the use of an isolated pair of electrodes in the atrial
stimulating circuit and the incorporation of an additional low-pass
filter do cut down the magnitude of the spike transmitted through
capacitor 17 to the base of transistor T1 when an atrial
stimulating pulse is generated, the change in the voltage across
battery 3 when the atrial stimulating current flows can cause the
base potential of transistor T1 to change. This change in base
potential produces the same effect as the transmission of a spike
through capacitor 17 -- the amplified signal can cause the
ventricular timing circuit to be reset. To minimize changes in the
base potential of transistor T1 as a result of the battery source
impedance, resistor 36 and capacitor 34 are provided.
The resistor and capacitor form another low-pass filter. The atrial
stimulating current pulse which flows through battery 3 causes a
2-millisecond voltage spike to appear across the battery terminals.
The pulse is characterized by relatively high frequencies and they
are attenuated by the low-pass filter so that the resulting spike
transmitted through resistor 15 to the base of transistor T1 is
minimal. The cut-off frequency of the low-pass filter comprising
resistor 36 and capacitor 34 can be much higher than the 15-Hz
cut-off frequency of the low-pass filter comprising resistor 32 and
capacitor 91. The latter filter should not attenuate the
frequencies which characterize the QRS waveform and consequently
the cut-off frequency is only 15 Hz. But since there is no need to
transmit the QRS signal through resistor 15 to the base of
transistor T1, resistor 36 and capacitor 34 can attenuate even the
higher frequencies characteristic of the QRS waveform. For this
reason the cut-off frequency of the low-pass filter inserted in
conductor 13 can be orders of magnitude higher than the cut-off
frequency of the low-pass filter inserted in conductor 11.
Even with these modifications to the circuit of FIG. 1, however, it
is possible for the signal on electrode E1 which results from the
generation of an atrial stimulating pulse to be great enough such
that after amplification one of transistors T4 and T5 conducts. In
such a case, the rise in potential across capacitor 49 would cause
transistor T6 to turn on and the ventricular timing circuit to be
reset. For this reason, yet another modification is made in the
pacer of FIG. 1 to reduce the possibility of false triggering of
transistor T6.
The short-duration (2-millisecond) atrial stimulating pulse, even
if it causes one of transistors T4 and T5 to conduct, causes the
transistor to turn on only for 2 milliseconds. This short-duration
pulse is similar to an RF spike for which the circuit including
resistor 45, resistor 47 and capacitor 49 is designed to attenuate.
Resistor 45 is included in the circuit so that capacitor 49 charges
slowly with respect to the width of an RF spike. Only a relatively
wide spike, such as that produced with the detection of a QRS
waveform signal on electrode E1, results in the charging of
capacitor 49 to a sufficient level for triggering transistor T6. To
further reduce the possibility of a spike resulting from the
generation of an atrial stimulating pulse charging capacitor 49 to
a level sufficient to trigger transistor T6, resistor 45 can be
increased in magnitude. With a sufficiently large resistor,
capacitor 49 can be prevented from charging sufficiently from a
2-millisecond spike to trigger the transistor. However, if resistor
45 is increased in magnitude even a spike resulting from the
detection of a QRS waveform signal may not charge capacitor 49
sufficiently to trigger transistor T6. This is due to the fact that
resistor 45 and resistor 47 form a voltage divider, and the larger
resistor 45 the smaller the source potential for charging capacitor
49. Even a spike resulting from the detection of a QRS waveform may
not charge capacitor 49 sufficiently to trigger transistor T6 if
resistor 45 is increased greatly in magnitude.
In the circuit of FIG. 4, instead of connecting capacitor 49
directly to the junction of resistors 45 and 47, the capacitor is
connected to the resistor junction through the parallel circuit
including resistor 38 and diode 42. The current which flows from
the collector of one of transistors T4 and T5 flows through
resistor 45 and resistor 38 in the capacitor charging path. The
incorporation of resistor 38 in the circuit is equivalent to
increasing the magnitude of resistor 45 insofar as delaying the
charging of capacitor 49 is concerned. However, by connecting
resistor 38 to the junction of resistors 45 and 47, rather than
increasing the magnitude of resistor 45, the maximum charging of
capacitor 49 is not affected to as great an extent. Consequently,
while resistor 38 delays the charging of capacitor 49 so that the
relatively narrow spike resulting from the generation of an atrial
stimulating pulse does not charge capacitor 49 sufficiently to
trigger transistor T6, the relatively wide spike resulting from the
detection of a ventricular contraction does cause capacitor 49 to
charge sufficiently to trigger the transistor.
However, the use of resistor 38 affects the operation of the
circuit in yet another manner. It will be recalled that the spike
resulting from each QRS waveform charges capacitor 49 in FIG. 1,
the capacitor thereafter discharging through resistor 47. The
capacitor must discharge sufficiently during the inter-beat
interval in order that the spike resulting from each QRS waveform
be capable of increasing the voltage across the capacitor. If the
capacitor does not discharge sufficiently, the rise in potential
across the capacitor with the conduction of transistor T4 or T5
will not be sufficient for turning on transistor T6. In fact, the
rate discrimination circuit operates in such a manner that pulses
at too rapid a rate do not permit the capacitor to discharge
sufficiently -- each pulse charges the capacitor only slightly and
transistor T6 does not turn on. With resistor 38 included in the
circuit of FIG. 4, it is apparent that the discharge path now
includes this resistor as well as resistor 47. The increased time
constant of the discharge circuit may not permit capacitor 49 to
discharge sufficiently between beats; transistor T6 may not conduct
when each QRS waveform is detected.
For this reason, diode 42 is included in the circuit. The diode
conducts current only when capacitor 49 is discharging and current
flows in the forward direction through the diode. In such a case,
resistor 38 is effectively short-circuited by the diode whose
impedance is negligable. Consequently the discharge circuit in the
pacer of FIG. 4 is basically the same as that in the pacer of FIG.
1 since resistor 38 is effectively removed from the circuit. The
resistor is effective only during the charging cycle of capacitor
49 since when current flows from one of transistors T4 and T5, it
flows through resistor 38 from left to right and diode 42 is
reverse biased. The incorporation of diode 42 in the circuit allows
resistor 38 to be used to increase the charging time constant of
the rate discrimination circuit, without affecting the discharge
time constant. The circuit can be adjusted, for example, such that
one of transistors T4 and T5 must conduct for at least 10
milliseconds (at intervals corresponding to the normal heartbeat
rate) in order for the voltage rise across capacitor 49 to be
sufficient to turn on transistor T6. The atrial stimulating pulse
which is only 2 milliseconds in width, even if it causes one of
transistors T4 and T5 to conduct, will now allow capacitor 49 to
charge sufficiently to turn on transistor T6.
Referring to the pacer of FIG. 1, it will be recalled that at the
end of the atrial timing period when transistors T7' and T8'
conduct, the voltage drop across resistors 61' and 63' increases.
This increase in potential is fed back through resistor 55' to
conductor 82. Of course, it is necessary that the increased
potential on conductor 82, connected to the base of transistor T6,
not be sufficient to turn on transistor T6 which would in turn
reset the ventricular timing circuit. In the circuit of FIG. 1,
transistor T6 does not turn on because resistor 55', and capacitors
53 and 49, form an integrating circuit. The potential at the
junction of capacitor 53 and resistor 55' (the base of transistor
T6) cannot rise instantaneously when the potential at the junction
of resistors 55' and 61' rises with the generation of the atrial
stimulating pulse. The atrial stimulating pulse is sufficiently
short in duration such that the voltage at the base of transistor
T6 does not rise sufficiently to turn the transistor on by the time
the atrial stimulating pulse is terminated.
However, I have found that it is possible to provide better
isolation between the two timing circuits and this is achieved in
the circuit of FIG. 4 with the use of an additional capacitor 54.
Conductor 82 is coupled through this additional capacitor to the
junction of capacitor 53 and capacitor 49. The rise in potential
across capacitor 49 with the conduction of one of transistors T4
and T5 now results in the transmission of a pulse through capacitor
53 (when switch S is closed) to the base of transistor T6 and the
transmission of a pulse through capacitor 54 to the base of
transistor T6'. EAch transistor conducts as required when a QRS
waveform is detected. However, when the atrial stimulating pulse is
generated, the pulse reflected back along conductor 82 is now
transmitted through capacitor 54 to the junction of capacitors 49
and 53, rather than being coupled directly to the base of
transistor T6 as in the circuit of FIG. 1. Capacitor 49 is greater
in magnitude than capacitor 53 so that the pulse transmitted
through capacitor 53 to the base of transistor T6 is even further
reduced in magnitude to prevent conduction of transistor T6. The
use of an additional capacitor 54 in this manner further isolates
the two timing circuits from each other.
It should be noted that switch S in the pacer of FIG. 4 is
connected in series with the base of transistor T6 rather than in
parallel with it as in the circuit of FIG. 1. In the circuit of
FIG. 1, when switch S is closed, the base of transistor T6 and the
base of transistor T6' are shorted to ground and both pulse
generating circuits operate in the continuous mode. When switch S
is open, both pulse generating circuits function in the demand
mode. In the circuit of FIG. 4, a slightly different arrangement is
used -- switch S is placed in series with the base of transistor
T6. When the switch is open, the ventricular timing circuit
operates in the continuous mode since no pulses are transmitted
through capacitor 53 to trigger transistor T6 and discharge
capacitor 57. When the switch is closed, on the other hand, the
ventricular timing circuit operates in the demand mode. As for the
atrial timing circuit, it operates in the demand mode at all times.
Even with switch S open and the ventricular stimulating circuit
operating in the continuous mode, each ventricular contraction is
detected and causes a pulse to be extended through capacitor 54 to
reset transistor T6'. The atrial timing period thus starts over
again with each ventricular contraction. This arrangement is most
satisfactory because it allows the two timing circuits to be
synchronized to each other even when the ventricular timing circuit
operates in the continuous mode.
Although the invention has been described with reference to a
particular embodiment, it is to be understood that this embodiment
is merely illustrative of the application of the principles of the
invention. Numerous modifications may be made therein and other
arrangements may be devised without departing from the spirit and
scope of the invention.
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