Note: Descriptions are shown in the official language in which they were submitted.
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Technical Field
This invention relates to implantable body
function control apparatus and particularly, but not
exclusively, to body tissue stimulating devices such as
cardiac pacemakers.
Back~round Art
Pacemakers for generating artificial stimulating
pulses for the heart, and which may be implanted in the
body, are well known. Originally the electrical circuitry
for such pacemakers was of analog design, but in recent
years digital circuitry has been also employed. A digital
approach to pacemakers has led to the evolution of program- ~ :
mable pacemakers - pacemakers having parameters such as
pulse rates which are adjustable (programmable) once the
pacemaker has been implanted. Programmable pacemakers are
described in, for instance, British Specifications
1,385,954 and 1,398,875. Such pacemakers have circuitry
; to detect and decode signals transmitted outside the body
and alter the program accordingly. In British Specifica-
tion 1,385,954, the programming is accomplished by means
of a magnetic field which is sensed by a magnetic reed
switch: the opening and closing of the switch providing
programming pulses to a program store. In British Speci-
fication 1,398,875, the programming is by means of radio
frequency transmission and reception.
It is desirable to miniaturize pacemaker compo-
nents as far as possible, especially where implanted
pacemakers are concerned, and although integrated circuit
techniques can
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help enormously to achieve this aim, the battery size is
still a major problem. The most recently-a~ailable miniature
batteries having sufficient power and life for pacemaker
usage are lithium batteries (both of the "solid" and "-viscous"
types), and such batteries are now employed in pacemakers.
Unfortunately, lithium batteries only generate about 3 volts
and although this voltage is sufficient to provide the supply
voltage for, say, an integrated circuit pacemaker, it is
insufficient, in itself, for the pacemaker output stage which
needs to generate artificial stimulating pulses of at least 5
volts. Under certain circumstances, it is even desirab~e to
generate pacing pulses of greater magnitude (e.g. about 7.5
volts). Although two or more lithium batteries can be employed
for such purposes (and, indeed, have in the past been so
employed), this militates against the desire for maximum
miniaturization.
Disclosure of Invention
We have now designed an implantable body function
control apparatus which employs a single relatively low voltage
source, but which generates, by means of voltage multiplication,
the higher voltages for the pacing pulses issued.
According to the invention, there is provided a body
function control apparatus comprising an oscillator, means
responsive to the oscillator for generating tissue stimulating
pulse control signals, an output stage responsive to signals
from the control signal generating means for providing tissue
stimulating pulses to the body, a volta~e source for said
apparatus, voltage multiplication means responsive to said
oscillator and said voltage source for generating one or more
voltages of magnitude higher than that supplied by said
voltage source and for supplying said higher voltage(s) to
said output stage whereby tissue stimulating pulses of
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voltage magnitude higher than that supplied by said voltage
source are provided.
According to a still further broad aspect of the
present invention there is provided a programmable pacer
system having a remote source of program control signals
and an implantable pacer. The system comprises a voltage
supply, and oscillator means for producing a pulse signal
having a fixed frequency. Counter means is provided for
generating a pacer output control signal upon each occurrence
of a predetermined plurality of oscillator pulses. Memory
means is also provided for receiving from the source and for
storing a program control signal representative of a desired
multiplication factor of the supply voltage for pacer stimu-
lation signals. Output means, responsive to the output con-
trol signal, generates pacer stimulation signals. ~he output
means includes a first capacitor coupled on one side to the
supply, and transistor means, connected in parallel with the
capacitor and enabled by the output control signal, for gene-
rating stimulation signals having an amplitude of the voltage
on the capacitor at the time of occurrence of the control
signal. Capacitor means is also provided and enabled by the
stored program control signal, and charged by a plurality of
successive excursions in one direction of the oscillator
pulse signal, for supplying added charge increments to the
other side of the first capacitor during successive excursions
in the opposite direction of the oscillator pulse signal.
Although this invention is of especial use for
cardiac pacemaker~, it is not restricted to such use and
could be employed for con~rolling other body functions.
Brief Description of the Drawinq
Preferred features of the invention are illustrated
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with reference to the accompanying drawing~ which schematically
illustrates an electrical circuit diagram of a programmable
fixed-rate implanted cardiac pacemaker according to the
invention.
_ st Mode of Carrying Out the Invention
Referring to the drawing, the pacemaker comprises an
oscillator 2 which drives a ripple counter 4~ An output of
the ripple counter (which may actually be the com~ination
of several stages of the counter) supplies an output l`ine 6.
The oscillato~ frequency and the ripple counter output
10~ selected ~ signals on line 6 at an appropriate body
stimulation pulse frequency. Line 6 is connected to an output
amplifier (-within the block formed by the dashed line 14),
from whence amplified stimulating pulses are passed to a
connection 16 which it~elf is connected to the active stimulat~
ing electrode ~not shown) disposed in or on the heart~ Line
6 is also connected to a delay unit 18 which, after an appro-
priate time, resets the counter 4, to enable the next appro-
priately timed pulse to be transmitted.
The output amplifier 14 includes an output transistor
26, across which is connected a capacitor 28. One end of the
latter is connected to the rail VDD of the supply voltage. The
other end of capacitor 28 and the emitter of transistor 26
are connected to a chain of Schottky diodes 30, 32, 34, and
36 (Schottky diodes are chosen for their low voltage drop when
forward-biased~. The cathode of diode 32 is connected to the
rail Vss of the suppIy voltage. For the purposes of illustra-
tion, the supply voltage Vss-VDD is about 3 volts. The output
of oscilIator 2 is supplied to an inverter 38 and via a NAND
gate 40 to a second inverter 42. The inverters feed,
~ respectively, capacitors 44 and 46 which lead into the diode
chain as illustrated. NAND gate 40 is supplied with a further
input, which is from a control line 22 from a pacemaker
program store 20.
The pacemaker program store 20 holds a binary bit of
~35 information which is transmitted on line 22 for voltage
multiplication purposes as to be described. A receiver/decoder
24 is arranged to receive and decode data signals transmitted
from out~ide the patient's body to th~e implanted pacemaker,
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and to employ the decoded signals for changing the
pacemaker program held in program store 20. For the
purpose of illustration, the receiver/decoder 24 and
store 20 have been depicted very simply and as providing -
an output for selecting only the stimulating pulse
amplitude. In practice it would be desirable to make
these features much more sophisticated so that the
program store is employed to provide a varying control
for several different pacemaker parameters (e.g. not
only pulse amplitude, but also pulse rate, pulse width,
hysteresis). The data signals may be transmitted to
the receiver/decoder 24 by any suitable means, but
preferably we employ data signals transmitted by tone
burst modulation (a carrier frequency being pulse width
modulated).
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Many of the pacemaker components described are
constructed as a MOS integrated circuit, and this has
been indicated by the block formed by the dashed line 50.
The integrated circuit is supplied as is customary with
VDD and Vss, but it will be observed that the output
transistor 26 of amplifier 14 is connected across VDD
and Vss in series with the diode chain and with capacitor
28 in parallel.
The pacemaker operates as follows. Each pulse
on output line 6 at the selected rate is passed to output
amplifier 14 where it is amplified and conducted to the
heart; it is also conducted to delay unit 18. After an
appropriate delay corresponding to the pacing pulse
width desired, delay unit 18 resets counter 4 and the
~ 30 count then commences in counter 4 for the next appropriate-
- ly timed pulse to be issued.
; In normal operation VDD is essentially the
circuit ground by reason of being connected to the pace-
maker indifferent electrode and Vss is supplied at about
3 volts. This 3 volt supply is sufficient for the
integrated circuit 50, but insufficient for the output
amplifier 14, which in conventional practice needs to
generate stimulating pulses of at least about 5 volts
for satisfactory pacing. It will be assumed that normally
about 5 volts is desired for each pacing pulse but that,
under certain circumstances, larger pacing pulses (of
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about 7.5 volts) are required. The manner in which the approx-
imately 5 volt pulses are generated ~lill first be described,
and then the manner in which the 7.5 volt pulses are generated.
For 5 volt stimulating pulses, a "0" is held in
program store 20 and is supplied on line 22 to NAND gate 40.
The output of NAND gate 40 will be high and point VT (between
inverter 42 and capacitor 46) will always be low. VT will
therefore not manifest the oscillator square wave output.~
On the other hand, point VD (between inverter 38 and capacitor
44) will manifest the square wave output of oscillator 2.
Point "X", at the junction of diodes 30, 32, 36 will, in the
absence of the oscillator square wave pulse train and ignoring
the voltage drop across diode 32, normally be held at Vss(-3
~ volts). Capacitor 28 charges to ~ volts via diodes 30 and
32. When VD goes to its most positive due to the square wave
pulse train supplied by oscillator 2, a 3 volt drop will exist
across capacitor 44 and the latter will charge via diode 32.
When VD goes negative due to the oscillator pulse train, point
X goes further negative. At this time diode 30 is forward-
biased and it conducts, causing capacitor 28 to acquire an
amount of charge due to the sharing of charge between capacitors
44 and 28. .Taking into account diode voltage drops and
assuming that no current is beîng drawn from the output
amplifier circuit (no pacing pulse being issued), capacitor 28
will charge to about 5 volts over several cycles of the
oscillator pulse train and it will hold its charge until a
pacing pulse is transmitted to the output amplifier 14 from
the integrated circuit 50. With no current being drawn fro~
the output transistor 26, and hence no potential drop across
its collector resistor; the 5 volt potential held by capacitor
28 appears directly across output transistor 26 and hence the
; amplitude of the pacing pulse transmitted to the active elec-
trode at connection 16, when this transistor is switched on,
is at about 5 volts rather than the 3 volts supplied by
VDD~Vss. It will be appreciated that capacitor 28 will charge
to 5 volts gradually over several cycles of the oscillator
frequency, but provided that the latter frequency is much
higher than the stimulating pulse frequency, capacitor 28 will
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always be charged to 5 volts ready for discharge on the next
pacing pulse.
When pacing pulses of about 7 volts magnitude are
desired, a "1" is held in program store 2Q and is supplied on
line 22 to NAND gate 40. The output downstream of inverter
42 (at VT) is the oscillator square wave pulse train but in
anti-phase to the similar pulse train at VD.
Under such circumstances, when VD goes negatiYe, point
~ ~ X goes further negative, as previously explained, and VT will
10~ be positive~charging capacitor 46 (~b~ about -5 volts),
When VD goes positive, VT will go negative and capacitor 46
will share its charge with capacitor 28. The effect is for
capacitor 28 to charge to a higher voltage than it would have
done if charged only from capacitor 44. Taking into account
diode drops, and assuming that no current is drawn from
output transistor 26, capacitor 28 will charge, over several
cycles of the oscillator pulse train, to a potential difference
of about 7.5 volts. When a pacing pulse is transmitted to
the output amplifier 14, this 7.5 volts is employed to proYide
a pacing pulse to connection 16 of corresponding magnitude.
It will thus be appreciated that the presence of a
"0" or "1" on line 22 controls the magnitude of the pacing
pulses.
Although the above embodiment relates to a fixed-rate
cardiac pacemaker, it will be appreciated that this is only
gi~en for the purposes of example, and the invention is
equally-applicable to demand cardiac pacemakers, or to body
function control apparatus employed to provide sti~ulating
pulses of varying amplitude to other parts of the body.
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