U.S. patent number 3,631,860 [Application Number 04/869,463] was granted by the patent office on 1972-01-04 for variable rate pacemaker, counter-controlled, variable rate pacer.
This patent grant is currently assigned to American Optical Corporation. Invention is credited to Michael Lopin.
United States Patent |
3,631,860 |
Lopin |
January 4, 1972 |
VARIABLE RATE PACEMAKER, COUNTER-CONTROLLED, VARIABLE RATE
PACER
Abstract
A variable rate pacer having two flip-flops for defining four
possible states. Each state determines the magnitude of a charging
current which in turn determines the rate of the pacer. The
flip-flops, arranged in a counter configuration, are cycled by a
monostable magnetic reed switch which is pulsed by placing an
external magnetic field in the vicinity of the patient's chest. The
magnetic reed switch is used only for cycling the counter to
establish the proper rate of operation; the reed switch is not
required for maintaining the charging current once it is
established. Reliability of operation is improved because, unlike
the prior art, mechanical switches are not required for maintaining
the selected charging current level.
Inventors: |
Lopin; Michael (Cambridge,
MA) |
Assignee: |
American Optical Corporation
(Southbridge, MA)
|
Family
ID: |
25353588 |
Appl.
No.: |
04/869,463 |
Filed: |
October 27, 1969 |
Current U.S.
Class: |
607/30; 327/114;
607/9; 331/113R |
Current CPC
Class: |
A61N
1/365 (20130101); A61N 1/37217 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61N 1/372 (20060101); A61n
001/34 () |
Field of
Search: |
;128/419P ;331/113,179
;307/271 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
What is claimed is:
1. A pacer comprising terminal means for connection to a patient's
heart and pulse-generating means for applying periodic pulses to
said terminal means, said pulse-generating means including
multistate electronic circuit means comprising an electronic
counter for representing at least two states, normally inoperative
pulsing means responsive to the detection of an external signal for
changing the state of said multistate electronic circuit means, and
means responsive to the state of said multistate electronic circuit
means for maintaining a respective rate of operation of said
pulse-generating means when said pulsing means is inoperative.
2. A pacer in accordance with claim 1 wherein said pulse-generating
means comprises a capacitor-charging timing circuit, and a constant
current source for supplying charging current to said timing
circuit.
3. A pacer in accordance with claim 1 wherein said respective
rate-maintaining means comprises a plurality of individual
current-determining sources connected in parallel to and for
feeding said constant current source, and means for controlling the
operation of respective groups of said individual
current-determining sources responsive to respective states being
represented by said multistate electronic circuit means.
4. A pacer in accordance with claim 3 wherein said constant current
source includes means for summing the currents from said individual
current-determining sources and for isolating said timing circuit
from said multistate electronic circuit means to prevent the
spurious switching thereof.
5. A pacer in accordance with claim 4 wherein said pulsing means
includes means comprising a monostable magnetic reed switch for
advancing the state of said counter responsive to the closing of
said switch.
6. A pacer in accordance with claim 5 further including means for
detecting the natural beating of said patient's heart and in
response thereto for inhibiting the generation of the next pulse
which would otherwise be generated by said pulse-generating means,
and means including an additional monostable magnetic reed switch
for selectively preventing the inhibiting of the operation of said
pulse-generating means, said monostable magnetic reed switch and
said additional monostable magnetic reed switch being oriented in
said pacer such that each of said switches can be closed
independent of the other responsive to the application of a
magnetic field having a respective orientation.
7. A pacer in accordance with claim 2 wherein said pulsing means
includes means comprising a monostable magnetic reed switch for
advancing the state of said counter responsive to the closing of
said switch.
8. A pacer in accordance with claim 1 wherein said pulsing means
includes means comprising a monostable magnetic reed switch for
advancing the state of said counter responsive to the closing of
said switch.
9. A pacer in accordance with claim 1 wherein said pulsing means
includes means for making mechanical contacts.
10. A pacer in accordance with claim 9 wherein said mechanical
contacts making means comprises a monostable magnetic reed switch.
Description
This invention relates to pacers, and more particularly to a
variable rate pacer whose rate of operation can be adjusted after
implantation in a patient.
Early pacers generally operated continuously at a fixed rate.
Following the use of this type of pacer for many years, the demand
pacer was introduced. In a demand pacer, electrical
heart-simulating 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. 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
U.S. Pat. No. 3,345,990 issued on Oct. 10, 1967 to Barouh V.
Berkovits.
Generally, a demand pacer is primed to generate an impulse at a
predetermined time after the last natural heartbeat. If another
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 a 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 the aforesaid Berkovits 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 the copending application of Barouh V. Berkovits, Ser.
No. 727,129 filed on Apr. 11, 1968, which has matured into U.S.
Pat. No. 3,528,428, an improved demand pacer is disclosed. In this
improved demand pacer, in the presence of the 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.
In the pacer disclosed in the above-identified copending Berkovits
application, there is a single switch and two potentiometers which
can be manually adjusted to change the pacer operation. Depending
on the setting of the switch, the pacer operates in either a
continuous mode or a demand mode. The first potentiometer controls
the width of each generated pulse. The second potentiometer
controls the interpulse interval. If the switch and the
potentiometers cannot be adjusted after they are implanted in the
patient, it is apparent that the pacer can operate in only one of
the two possible modes, and with only a single pulse width and only
a single interpulse interval.
In some demand pacers marketed in recent years there is provision
for switching between the continuous and demand modes of operation
without requiring access to the pacer after it is implanted. The
switch described above consists of a monostable magnetic reed. When
a magnetic pole of either polarity is placed adjacent to the
patient's chest, the reed is closed and the pacer operates in the
continuous mode. When the magnet is removed, the reed opens and the
pacer functions in the demand mode. One of the most important
reasons for switching to the continuous mode is to allow the
physician to check the pacer operation. If the patient's heart is
beating normally, no pulses are generated by the pacer. Even if the
patient is monitored by an electrocardiographic machine, there is
no way to determine that the pacer is still functional--if the
patient's heart is beating normally, there are no pacer pulses in
the ECG-signal. However, when the magnet controls the closing of
the reed switch, the pacer pulses appear in the ECG-signal and the
physician can determine that the pacer is still functional and will
pulse properly if and when it is required to do so.
With either a demand or continuous pacer, it is often necessary to
adjust the rate of operation after implantation. Of course, it
would be most desirable to accomplish a change in rate without
requiring physical access to the pacer. Magnetic reed switches can
be used for this purpose. An example of the use of a magnetic reed
switch for controlling the rate of a demand pacer is disclosed in
the copending application of Barouh V. Berkovits, Ser. No. 862,695
filed on Oct. 1, 1969.
The magnetic reed switches which have been used in the prior art to
control the rate of a pacer have been of the bistable type. The
magnetic reed is set in one or another of two states depending on
the polarity of the magnetic field brought into the vicinity of the
patient's chest. Each state of the reed controls a different rate.
For example, in the Berkovits pacer disclosed in application Ser.
No. 862,695, a resistor in the pacer-timing circuit is shorted out
when the reed switch is closed, and it is placed in the circuit to
decrease the rate when the reed switch is open. In general, a
problem encountered with the use of bistable magnetic reed
switches, or other types of mechanical switches, in this manner is
one of reliability. It has been found that if a mechanical contact
is required to maintain a selected rate, shock and vibration can
cause inadvertant changes. This is true not only of bistable
magnetic reed switches, but also of many other mechanical contact
switches used for the same purpose. It is apparent that it is
highly undesirable for the pacer rate to accidentally change after
setting by the physician.
It is a general object of my invention to provide a variable rate
pacer (of the demand or continuous type, although the illustrative
embodiment of the invention is disclosed in the context of a demand
pacer) whose rate of operation can be changed, with the selected
rate being independent of the state of a mechanical contact.
It is another object of my invention to provide a variable rate
pacer whose rate of operation is not affected by mechanical shock
or vibration, and whose rate can be adjusted without requiring
physical access to the pacer.
In most pacers, the interpulse interval is determined by an
RC-timing circuit. A capacitor charges from a voltage source
through a resistor, the charging rate being dependent upon the
magnitude of the resistor. In the illustrative embodiment of my
invention, the capacitor is charged directly from a constant
current source, the magnitude of the constant current delivered by
this source is dependent upon the state of a two-bit counter. The
counter can be set in any one of four states, and thus four
different pacer rates are possible. The counter is constructed of
bistable electronic circuits which do not rely upon mechanical
contacts for maintaining them in particular states once they are
set.
Coupled to the counter is a monostable magnetic reed switch. The
switch is normally open. When a magnetic field is brought into the
vicinity of the patient's chest, the switch closes. The closing of
the switch pulses the counter to advance its state. This in turn
results in a change in pacer rate. Once the counter is set in a
particular state, this state is maintained independent of the
condition of the reed switch. When the magnetic field is removed,
the reed switch opens but the counter state and current level
remain the same. Because the pacer rate is not maintained by the
reed switch (the reed switch remains open), the pacer rate is not
affected by mechanical shock or vibration. Of course, it is
possible for other types of devices to be used for pulsing the
electronic circuits to change their states as required. However, it
has been found that a monostable magnetic reed switch is
particularly advantageous for use as the rate-changing control
mechanism.
It is possible to include two monostable magnetic reed switches in
a pacer, one for controlling a change in the pacer rate, and
another for controlling the continuous operation as described in
the two above-identified Berkovits applications. In such a case, if
both reeds close under the influence of a magnetic field, the pacer
would switch to the continuous mode at the same time that its rate
is changed. In most case this is of no concern because as soon as
the proper rate is established the pacer switches back to the
demand mode. However, it is also possible to control the closing of
only one of the two switches at any time by placing them in
perpendicular orientations within the pacer. In such a case a
magnetic field in one direction will close only one of the reeds
and a magnetic field in another direction will close only the
other.
It is a feature of my invention to provide a multistate electronic
circuit which is maintained in any selected state independent of
mechanical contacts for controlling the pacer rate.
It is another feature of my invention to provide a pulsing element
which may be operated without direct access thereto for changing
the state of the electronic circuit.
It is a more particular feature of my invention, in the
illustrative embodiment thereof, to provide a monostable magnetic
reed switch for the pulsing element.
Further objects, features and advantages of my invention will
become apparent upon a consideration of the following detailed
description in conjunction with the drawing, in which:
FIG. 1 is the same as FIG. 1 in the copending application of Barouh
V. Berkovits, Ser. No. 727,129 which has matured into U.S. Pat. No.
3,528,428, and depicts a demand pacer;
FIG. 2 depicts a typical electrocardiogram; and
FIG. 3 depicts the illustrative embodiment of the present
invention.
Referring to the pacer of FIG. 1, electrodes E1 and E2 are
implanted in the patient's heart, electrode E2 being the neutral
electrode and electrode E1 being positioned to stimulate the
ventricles. When switch S is open, current flows between electrodes
E1 and E2 to stimulate the ventricles only when an electrical
stimulus is required.
Capacitor 65 serves to provide a source of current when an impulse
is required. At that time, transistor T9 conducts and the capacitor
discharges through the electrodes. Capacitor 57 charges through
potentiometers 35 and 37 until the voltage across it causes
transistors T7 and T8 to conduct. At that time, capacitor 57
discharges through potentiometer 37 and transistors T7 and T8,
transistor T9 conducts, and an impulse is delivered to the
patient's heart from capacitor 65. The setting of potentiometer 37
controls the time taken for capacitor 57 to discharge, that is, the
width of each impulse. The setting of potentiometer 35 (along with
the setting of potentiometer 37) controls the time required for
capacitor 57 to charge to that level which causes conduction in
transistors T7 and T8, that is, the interpulse interval.
Ordinarily, in the absence of conduction of transistor T6,
capacitor 57 would continuously charge and discharge, and impulses
would be supplied to the patient's heart at fixed intervals
determined by the setting of the potentiometers.
Electrode E1 is coupled over conductor 11 to the base of transistor
T1 through capacitor 17. A representative ECG-trace is shown in
FIG. 2, and transistors T1 and T2 amplify the R-wave signal on
electrode E1 resulting from a ventricular contraction. (Excessive
signals are shorted through Zener diode 67 to prevent damage to
transistor T1.) Following detection of an R wave, a positive pulse
is delivered to the base of transistor T6. Transistor T6 turns on
and capacitor 57 discharges through it. Thus although the capacitor
was previously charging to the level which would have resulted in
the generation of an impulse, it is discharged and a new timing
interval begins. This arrangement insures that an impulse is not
generated if a natural heartbeat has occurred. The timing interval
is such that impulses are generated with an interpulse interval
slightly in excess of the desired natural interbeat interval. Only
if a natural heartbeat is missing is a stimulating impulse
generated.
The remaining transistors in the circuit serve to prevent
conduction of transistor T6 in the presence of noise. In the
presence of noise it would otherwise be possible for transistor T6
to conduct and prevent the generation of an impulse even though one
is required. For this reason, when the pacer detects extraneous
noise, transistor T6 is prevented from operating and impulses are
delivered at a fixed rate. A more complete description of the
operation of the circuit of FIG. 1 is set forth in the
above-identified Berkovits U.S. Pat. No. 3,528,428.
With switch S open, as shown, the pacer operates in the demand mode
as just described. However, with switch S closed, the base of
transistor T6 is held at the potential of conductor 9. In such a
case, pulses transmitted through capacitor 53 do not turn on the
transistor. Capacitor 57 is not discharged through transistor T6
and each time the capacitor voltage rises to the point where
transistors T7 and T8 conduct, a stimulating pulse is generated.
The pacer thus operates in the continuous mode.
The illustrative embodiment of my invention is shown in FIG. 3. The
circuit is identical to that of FIG. 1 except for the elements
shown within heavy outlines--switch S and box 35'. The left half of
the circuit of FIG. 1 is not repeated on FIG. 3, it being
understood that to the left of transistors T3 and T5 the two
circuits are identical. Switch S is shown as a monostable magnetic
reed switch. The switch includes two reeds encased in a glass
enclosure. Such a monostable magnetic reed switch has been used in
pacers for several years. When a magnet is brought into the
vicinity of the monostable magnetic reed switch, the two reeds
engage if the magnet flux is parallel to them. As long as the
magnet is held in position, the base of transistor T6 is shorted
through the reed switch to conductor 9 and the pacer operates in
the continuous mode. As soon as the magnet is removed, the switch
opens and the pacer reverts to the demand mode of operation.
A departure from the circuit of FIG. 1 is the substitution of box
35' for potentiometer 35. In FIG. 1, potentiometer 35 is connected
at one end to the junction of resistor 33 and battery 7, and at the
other end to the junction of potentiometer 37 and the emitter of
transistor T7. Box 35' on FIG. 3 has three input conductors, two of
which are connected between the same two junctions. The third input
to the box couples the base of transistor T14 to the junction of
batteries 5 and 7. In the circuit of FIG. 1, potentiometer 35
serves to control the current (an exponential waveform) which flows
from battery 7 to capacitor 57. In the circuit of FIG. 3, box 35'
serves as a constant current source which delivers current through
potentiometer 37 to capacitor 57. (It is also possible to connect
the collector of transistor T14 directly to capacitor 57 so that
potentiometer 37 is not in the charging current path.)
The constant current is delivered through potentiometer 37 to
capacitor 57 from the collector of transistor T14. The current
flows through the potentiometer and the capacitor, and through
resistors 61 and 63 to ground conductor 9. The charging current is
small so that the voltage developed across resistor 63 is small
enough to prevent conduction of transistor T9. It is only when
capacitor 57 discharges through transistors T7 and T8 that the
current through resistor 63 is large enough to turn on transistor
T9.
There are two requirements for the current source represented by
box 35'. The first is that the current source function over the
entire voltage range of capacitor 57. The maximum voltage across
the capacitor is that which fires transistors T7 and T8. Capacitor
57 is connected at one end through resistors 61 and 63 to ground
conductor 9. The voltage across resistors 61 and 63 during charging
of the capacitor is negligible so one end of the capacitor may be
considered to be grounded during charging. The base of transistor
T7 is held at a potential V.sub.bb determined by batteries 3 and 5.
Transistor T7 conducts when its base-emitter voltage is 0.4 volts.
Consequently, the maximum voltage across capacitor 57 is equal to
V.sub.bb + 0.4 volts. The constant current source must function
over the voltage range from 0 to V.sub.bb + 0.4 volts.
The second requirement of the constant current source is that the
voltage at the emitter of transistor T14 not change when
transistors T7 and T8 conduct. If the voltage changes, it is
possible for the bistable circuits within box 35' to be switched.
To prevent changes in the charging current level, the voltage at
the emitter of transistor T14 should not change even when capacitor
57 discharges through transistors T7 and T8.
The base potential of transistor T14 is held constant by batteries
3 and 5 and is the same as the base potential of transistor T7,
V.sub.bb. Transistor T14 starts to conduct when its emitter voltage
is approximately 0.4 volts greater than its base voltage. Although
an emitter-base forward bias of 0.4 volts is sufficient to cause
the transistor to start conducting, the emitter-base voltage drop
is 0.5 volts when the transistor operates linearly and is
conducting the charging current. Since the base of transistor T14
is held at a constant potential of V.sub.bb volts, the emitter of
transistor T14 is held at a constant potential of V.sub.bb volts,
the emitter of transistor T14 is at a potential 0.5 volts greater
than the base potential essentially independent of the magnitude of
the emitter current. The collector current equals the emitter
current multiplied by the parameter .alpha. of the transistor
(typically, a value of 0.99). Transistor T14 functions as a current
source with the emitter potential being greater than the base
potential by 0.5 volts only if the emitter-collector voltage drop
is greater than approximately 0.1 volts, that is, only if the
collector voltage does not exceed V.sub.bb + 0.4 volts. (Although
transistor T14 functions as a constant current source independent
of the emitter-collector voltage drop, it is necessary that there
be a drop.) Since transistors T7 and T8 turn on to discharge
capacitor 57 when the emitter voltage of transistor T7 rises to
V.sub.bb + 0.4 volts, it is apparent that the collector voltage of
transistor T14 cannot rise higher than that value which would
prevent the transistor from operating as a constant current source.
Thus transistor T14 functions as a constant current source over the
entire voltage range of capacitor 57. Furthermore, the emitter
impedance of transistor T14 is relatively small compared with
resistors 70-73 which, as will be described below, determine the
emitter current of transistor T14. The emitter of transistor T14
thus appears as a voltage source to the counter circuitry in box
35', that is, the transistor conducts any of the four possible
currents without any changes in its emitter potential. Transistor
T14 thus prevents spurious switching of the bistable circuits
within box 35' as a result of the operation of the timing
circuit.
Transistor T14 functions as a current summer; the emitter current
is the sum of the currents flowing into common bus 97 from the
conducting ones of transistors T10-T13, all of the emitter
terminals of these transistors being connected to the emitter of
transistor T14. Transistors T10 and T11 form a first flip-flop and
transistors T12 and T13 form a second flip-flop. The four collector
resistors 70-73 are connected to respective transistors. The
collector of transistor T10 is coupled to the base of transistor
T11 through resistor 78, and the collector of transistor T11 is
coupled to the base of transistor T10 through resistor 79.
Resistors 80 and 81 serve to cross-couple transistors T12 and T13
in a similar manner. Capacitors 74-77 are speed-up capacitors
which, as in known in the art, increase the speed and reliability
of the flip-flop switching.
Assume, that in the flip-flop including transistors T10 and T11,
transistor T10 is conducting and transistor T11 is off. With a
0.1-volt drop across the collector and emitter of transistor T10,
and no current flowing through resistor 78, the base-emitter drop
across transistor T11 is similarly 0.1 volts. Since a 0.4-volt drop
is necessary to cause the transistor to conduct, the transistor
remains off. Current flows from battery 7 through resistors 71 and
79 to the base of transistor T10 to maintain the transistor
conducting.
With transistor T10 conducting and its collector at 0.1 volts
relative to the potential of bus 97, the cathode of diode 86 is
similarly held at this potential. The base-emitter drop of
transistor T10 is 0.5 volts, and consequently the cathode of diode
84 is held at a potential of 0.5 volts relative to the potential of
bus 97. Application of a negative pulse to the anode of either
diode has no effect since the diode does not conduct when the
anode-cathode drop is negative. In order for either diode to
conduct, its anode must be 0.5 volts higher in potential than its
cathode. (Both anodes are held at the potential of bus 97 due to
their connection to the bus through resistor 85.) Since the reverse
bias across diode 84 is 0.5 volts, diode 84 will not conduct until
a positive pulse greater that 1 volt is applied to its anode. Since
the reverse bias across diode 86 is only 0.1 volts, this diode will
conduct when a positive pulse exceeding 0.6 volts is applied to its
anode. Consequently, a positive pulse transmitted through capacitor
83 with a magnitude between 0.6 volts and 1 volt is steered through
diode 86 but not diode 84. A positive pulse transmitted through
diode 86 causes transistor T11 to turn on, which in turn causes
transistor T10 to turn off, thus switching the state of the
flip-flop.
When magnetic reed switch S' closes, the positive potential of
source 7 is applied to capacitor 83. The capacitor and resistor 85
from a differentiating circuit and thus a positive spike is applied
to the anodes of diodes 84 and 86. The component values are such
that the positive pulse has an amplitude greater than 0.6 volts but
less than 1 volt so that the flip-flop switches state as described
above. When the magnetic read switch opens, a negative pulse is
applied through capacitor 83 to the anodes of the two diodes. The
negative pulse, however, has no effect since it further reverse
biases both diodes. Following the closing and opening of the
contacts, capacitor 83 discharges through resistors 85 and 82.
With the flip-flop in the state in which transistor T11 conducts
and transistor T10 is off, the cathode of diode 84 is at 0.1 volts
and the cathode of diode 86 is at 0.5 volts. The next positive
pulse produced by the closing of the reed contacts is steered
through diode 84 to cause transistor T10 to turn on and transistor
T11 to turn off. Successive positive pulses produced by the closing
of switch S' switch the state of the flip-flop.
Elements 88-92 are comparable to elements 82-86 and serve to switch
the state of the flip-flop comprising transistors T12 and T13 when
a positive potential is applied to the junction of resistor 88 and
capacitor 89. The positive potential is derived when transistor T11
turns off and its collector rises in potential. At this time, a
positive step is transmitted through capacitor 87. The capacitor is
provided to isolate the junction of resistor 71 and the collector
of transistor T11 from the second flip-flop in order that the
second flip-flop not affect the bias voltages of the first
flip-flop. The second flip-flop changes state only when transistor
T11 switches from a conducting to a nonconducting state. When the
transistor switches from the nonconducting state to the conducting
state, it is a negative voltage step which is transmitted through
capacitor 87, and the resulting negative pulse transmitted through
capacitor 89 simply further reverse biases diodes 90 and 92 and has
no effect on the second flip-flop. As will be apparent to those
skilled in the art, the two flip-flops function as a 4-state
counter.
The four states can be summarized by the following conduction
conditions of the transistors:
State No. T10 T11 T12 T13
__________________________________________________________________________
1 off on on off 2 on off off on 3 off on off on 4 on off on off
__________________________________________________________________________
Each closure of monostable read switch S' causes the counter state
to advance, with state 1 following state 4. The reed switch is
closed when a magnetic field is produced in the vicinity of the
patient's chest.
The current flowing through the emitter of transistor T14 is the
sum of the currents delivered to bus 97, and its magnitude depends
upon the state of the two flip-flops. Four different currents may
flow through the emitter of transistor T14. For the most part, the
currents are determined by the magnitudes of resistors 70-73. As
long as three of these resistors have different values, four
distinct currents will result. As described above, the emitter
potential of transistor T14 is V.sub.bb + 0.5 volts, where the base
potential of transistor T14 is V.sub.bb volts. The inclusion of
battery 7 in the circuit increases the potential at the upper ends
of each of resistors 70-73 to V.sub.cc volts, where V.sub.cc is
greater than V.sub.bb. Consequently, the potential drop across a
conducting one of transistors T10-T13 in series with its collector
resistor is V.sub.cc - (V.sub.bb + 0.5) volts. Assuming a
collector-emitter drop of 0.1 volts, the drop across one of
resistors 70-73 connected in series with a conducting transistor is
V.sub.cc - V.sub.bb - 0.6 volts. Assume that this drop is 2 volts.
Assume further that resistor 70 has a magnitude of 10.sup.6 ohms,
resistors 71 and 72 both have a magnitude of 2(10.sup.6) ohms and
resistor 73 has a magnitude of 4(10.sup.6) ohms. In such a case,
and neglecting currents which flow through resistors 78-81, 2
microamperes flow through transistor T10 when it is conducting, 1
microampere flows through either of transistors T11 and T12 when it
is conducting, and 0.5 microamperes flow through transistor T13
when it is conducting. Since the current into the emitter of
transistor T14 is the sum of the four transistor currents,
referring to the table above it is seen that when the counter is in
state 1 the total current is 2 microamperes, when the counter is in
state 2 the total current is 2.5 microamperes, when the counter is
in state 3 the total current is 1.5 microamperes, and when the
counter is in state 4 the total current is 3 microamperes.
Actually, the above analysis is only approximate because the effect
of resistors 78-81 has been omitted. For example, when transistor
T10 conducts a current of 2 microamperes flows through resistor 70
and the transistor to bus 97 as described. But current also flows
through resistors 71 and 79, and the base-emitter junction of
transistor T10 to the common bus. However, resistor 79, and the
other three cross-coupling resistors, are very large in magnitude
compared to the four collector resistors, and do not affect
appreciably the current levels described above. In any event, what
is of importance is the four current levels themselves, not the
manner in which they are derived. The values of the collector and
cross-coupling resistors can be selected such that the four
possible emitter currents of transistor T14 are the desired
values.
It should be noted that the prior art monostable reed switch S can
still be used as in the past. The physician can verify that the
pacer is still operational if the electrocardiogram of the patient
shows that the pacer has switched to the continuous mode of
operation when a magnet has been brought into the vicinity of the
patient's chest. The additional switch S' can be arranged so that
the same magnet controls a change in the pacer rate. If the same
magnetic field closes both reed switches, the pacer will function
in the continuous mode at the same time that its rate is changed;
when the magnetic field is removed the pacer switches to the demand
mode and continues to operate at the new rate. However, it is
preferable to operate the two reed switches independently. As shown
in FIG. 3, the orientations of the two monostable magnetic reed
switches are perpendicular to each other. In such a case, a
magnetic field in one direction will cause one of the switches to
close and a magnetic field in a perpendicular direction will cause
the other reed to close. In some cases it may be desirable to
provide for the independent operation of the two switches. Of
course, for a continuous pacer there would be no need for reed
switch S in the first place and the problem of independent reed
operation would not even exist.
Since the flip-flops are stable and self-sustaining, no external
transmitter is required to maintain a selected rate. Nor is the
mechanical reed switch necessary to maintain the selected rate; it
is only used to control the switching from one rate to another.
Since the human body is permeable to magnetic flux, a rate
adjustment may be made simply by holding a magnet against the
patient's chest. However, other techniques can be used to pulse the
counter. For example, the pacer may include an RF detector, with
the counter being cycled each time an RF transmitter is operated in
the vicinity of the patient's chest. Similarly, instead of using
flip-flops, other multistate electronic circuits could be utilized,
e.g., those incorporating tunnel diodes, 4-layer diodes, etc. It
will also be apparent to those skilled in the art that the counter
can be constructed to have any desired number of states. A single
flip-flop could be used for only two states, i.e., if only two
current levels are required. If more than four states are required,
more than two flip-flops could be utilized.
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.
* * * * *