Cardiac pacemaker circuit with fast stored charge reduction

A cardiac pacemaker circuit for use with a stimulating electrode embedded in a patient's atrium or ventricle and a reference electrode positioned outside the heart or on the same lead within the heart. Stimulus pulses are applied by the circuit across the two electrodes, these stimulus pulses replacing evoked pulses. On termination of each stimulus pulse the amount of charge stored between the two electrodes is monitored and pulses of short duration are applied to the stimulating electrode progressively to reduce this charge over a very short time period to a level at which the intrinsic pulses of the evoked heart response can be monitored. The evoked heart response can then be used for diagnosis while the pacemaker circuit is in operation. If the stored charge was not reduced quickly then the charge would mask the intrinsic pulse and the pacemaker would have to be switched off before the intrinsic pulses could be monitored for diagnosis.

This invention relates to a cardiac pacemaker circuit. 
BACKGROUND OF THE INVENTION 
Intrinsic cardiac pulses may be measured using an electrode placed in or in 
the vicinity of a heart and an example of such pulses for a healthy 
patient is shown in FIG. 1A. For each heartbeat, the pulses comprise a 
wave labelled P which causes the atria to contract, and a complex wave 
form labelled QRST associated with the contraction of the ventricles. A 
pacemaker circuit comprises a pulse generator connected to a stimulating 
electrode embedded in the patient's atrium or ventricle and a reference 
electrode positioned outside the heart in a unipolar configuration, or on 
the same lead within the heart in a bipolar system. The generator applies 
stimulus pulses across these two electrodes which replaces the evoked 
pulses. During each stimulus pulse, electrical charge is stored in the 
interface between the stimulus electrode and the myocardium, and the 
charge then decays upon termination of the stimulus pulse. An example of 
the waveform appearing at the stimulus electrode is shown in FIG. 1B in 
which each stimulus pulse is denoted by St. As may be seen, the charge 
which is stored and its subsequent decay masks the evoked heart response. 
However, it is useful to detect these evoked pulses as they may be used 
both for diagnosis and for modifying the operation of the pacemaker 
circuit. 
SUMMARY OF THE INVENTION 
The subject invention provides a new pacemaker circuit in which the masking 
of the evoked heart pulses by the charge stored between the stimulus 
electrode and the myocardium is reduced or removed. 
According to this invention there is provided a cardiac pacemaker circuit 
including a terminal for connection to a stimulatings electrode, a 
terminal for connection to a reference electrode, means for applying a 
stimulus pulse across said stimulating terminal and said reference 
terminal in response to a timing signal, means for generating said timing 
signal, and means connected to said stimulating electrode and said 
reference electrode which are operable, in use, upon termination of the 
stimulus pulse to remove charge stored between the stimulating electrode 
and the reference electrode. 
By removing the charge which is stored between the stimulating electrode 
and the tissue, the masking which is caused by such charge is removed. 
Said stimulus pulse applying means may comprise a reference voltage source, 
a resistor and capacitor connected in series between the voltage reference 
source and the stimulating terminal, and a switch responsive to the timing 
signal connected between the junction of the resistor and capacitor and 
the reference terminal. 
The charge removing means may be connected to the junction of the resistor 
and capacitor. 
In a preferred embodiment of this invention, said charge removing means is 
arranged to sample the voltage difference between the stimulating terminal 
and the reference terminal upon termination of a stimulus pulse and to 
apply an electrical pulse across the electrodes in accordance with the 
result of the sampling operation. The charge removing means may be 
arranged to form a plurality of sampling operations upon termination of a 
stimulus pulse, each sampling operation being followed by the application 
of an electrical pulse. 
In an alternative arrangement of this invention, the charge removing means 
is arranged to measure the quantity of charge supplied to a stimulus pulse 
and to remove substantially an identical quantity of charge upon 
termination of the stimulus pulse. 
The charge removing means may be arranged to clamp the potential difference 
between the stimulating electrode and the reference electrode for a preset 
period after removing said substantially identical quantity of charge. 
The pacemaker circuit may include a resistor connected in series with one 
of the stimulating electrode ad the reference electrode, the charge 
supplied during the stimulus pulse and the charge subsequently removed 
being measured by integrating the voltage across said resistor with 
respect to time.

DETAILED DESCRIPTION 
Referring now to FIG. 2, there is shown a cardiac pacemaker circuit which 
has a stimulating terminal 10 connected to a stimulating electrode 11 and 
a reference terminal 12 connected to a reference electrode 13. The 
reference terminal 12 is connected to the earth line. The stimulating 
terminal 10 is connected through a capacitor C1 to a terminal 14. The 
terminal 14 is connected through a resistor R1 to a reference voltage 
source 15 and also through the collector emitter path of the transistor T1 
to the earth line. The base of transistor T1 receives a series of pulses 
from a timing circuit 16. The components described so far are conventional 
in pacemaker circuits. In operation, the capacitor C1 is charged by the 
reference voltage source 15 and, each time a pulse is supplied to the base 
of transistor T1, a stimulus pulse is supplied across the electrodes 11 
and 13. Each stimulus pulse is variable both in amplitude and duration. 
The reference voltage source 15 and terminal 14 are connected to the two 
inputs of an amplifier 17 which subtracts the voltage appearing at 
terminal 14 from the reference voltage V.sub.ref, amplifies the result, 
inverts it and adds to it an offset equal in magnitude to V.sub.ref. The 
output of amplifier 17 is connected through an analog switch S1 to a 
sample and hold circuit 19. The output of circuit 19 is connected to an 
amplifier 20 and analog switch S2 to terminal 14. The switches S1 and S2 
are controlled by the timing circuit 16. In operation, immediately upon 
termination of each stimulus pulse, switch S1 is closed and switch S2 is 
opened for a preset period and the output of amplifier 17 is sampled and 
held in circuit 19. Then, switch S1 is opened and switch S2 is closed for 
a preset period and a voltage pulse is applied to terminal 14 having a 
magnitude and polarity which is determined by the output of sample and 
hold circuit 19. This pulse causes some of the charge which has been 
stored between electrodes 11 and 13 during the stimulus pulse to be 
removed. The cycle is then repeated a predetermined number of times 
thereby returning the voltage at terminal 14 to a value substantially 
equal to V.sub.ref. Because there is no appreciable change in the voltage 
across capacitor C1, the series of operations returns the potential at the 
stimulating electrode substantially to that of the earth line. The 
intrinsic pulses may then be observed at terminal 14. 
Thus, the intrinsic pulses are not masked by the charge which has been 
stored between electrodes 11 and 13. 
The detailed circuit diagram of the pacemaker circuit shown in FIG. 2 will 
now be described with reference to FIGS. 3 to 5. 
Referring firstly to FIG. 4, the timing circuit 16 includes an oscillator 
30. The oscillator comprises a type 3140 operational amplifier 31, the 
output of which is connected to its inverting-input through a diode D1 and 
a resistor R2. The output is also connected to the inverting input through 
a diode D2 and a preset resistor R3. The inverting input is connected 
through a capacitor C2 to the junction of a Zener diode ZD1 and a resistor 
R4 which are connected in series between the positive supply rail and the 
earth line to provide a bias voltage. This junction is also connected 
through a resistor R5 and a resistor R6 to the output of amplifier 31 and 
the junction of these two resistors is connected to the non-inverting 
input. The non-inverting input is also connected to the earth rail through 
a switch S3 which may be closed to prevent oscillation. The oscillator 30 
provides the basic timing signal of the pacemaker circuit and the resistor 
R3 may be adjusted to control the frequency of the stimulus pulses. The 
output of amplifier 31 is connected to the A input of a type 4528 
monostable 32 which produces a signal TOUT at its Q output. The Q output 
of this monostable is connected to the input of a further type 4528 
monostable 33 at the Q output of which is produced a signal PULS. As shown 
in FIG. 3, thesignal PULS is connected to the base of transistor T1 
through a resistor R7. The monostable 33 is set to provide pulses having a 
variable duration. 
The Q output of monostable 32 is connected to one input of an OR gate 40, 
the output of which is connected to the enable inputs C.sub.D of a pair of 
monostables 41 and 42. The monostable 41 is set to produce pulses having a 
duration of 0.4 ms and monostable 42 is set to produce pulses having a 
duration of 25 microseconds. The Q output of monostable 41 is connected to 
one input of an OR gate 43, the output of which is connected to the B 
input of monostable 42. The Q output of monostable 42 is connected to the 
B input of monostable 41. The Q ouput of monostable 41 is also connected 
to the other input of OR gate 40 and the Q output of monostable 33 is 
connected to the other input of OR gate 43. The Q output of monostable 41 
produces a signal HOLD and is connected to the control terminal of switch 
S2. The Q output of monostable 41 produces a signal SMPL which is 
connected to the control terminal of switch S1. A waveform diagram is 
shown in FIG. 5 for the signal appearing at the output of oscillator 30 
and for the signals TOUT, PULS, SMPL, and HOLD. As may be observed in FIG. 
5, the signal TOUT controls the number of cycles of operation of switches 
S1 and S2. It may also be observed in FIG. 5 that the switch S1 is always 
closed immediately upon termination of each stimulation pulse. 
Referring more particularly to FIG. 3, the voltage reference source 15 
comprises resistor R10, a potentiometer R11 and resistor R12 connected in 
series between the positive supply rail V.sub.DD and the earth line. The 
tapping on potentiometer R11 provides the reference voltage V.sub.ref. 
This junction is connected to the non-inverting input of a type 3140 
operational amplifier 50. 
The amplifier 17 comprises three type 3140 operational amplifiers 51, 52 
and 53 which are connected as an instrumentation amplifier. The 
non-inverting input of amplifier 51 is connected to terminal 14 and the 
non-inverting input of amplifier 52 is connected to the tapping on 
potentiometer R11. The output of amplifier 51 is connected through a 
resistor R13, a preset resistor R14 and 1 resistor R15 to the output of 
amplifier 52. The junction of resistors R13 and R14 is connected to the 
inverting input of amplifier 51 and the junction of resistors R14 and R15 
is connected to the inverting input of amplifier 52. The output of 
amplifier 51 is connected through a resistor R16 to the inverting input of 
amplifier 53. The output of amplifier 52 is connected through a resistor 
R17, a potentiometer R18, and a resistor R19 to the output of amplifier 50 
and the tapping on potentiometer R18 is connected to the non-inverting 
input of amplifier 53. The output of amplifier 53 is connected to its 
inverting input through resistor R20. 
The output of amplifier 53 represents the output of amplifier 17 and is 
connected through a resistor R21 and switch S1 to a capacitor C4 which 
represents the sample and hold circuit 19. The capacitor C4 is a low 
leakage polyester type capacitor. It is to be noted that the time constant 
of resistor R21 in capacitor C4 is 2.5 microseconds which is one tenth of 
the duration of the pulses of the signal SMPL. 
The amplifier 20 is formed from a type 3140 operational amplifier 60 and a 
pair of transistors T3 and T4 connected between the positive supply line 
V.sub.DD and the negative supply line V.sub.ss. The emitters of 
transistors T3 and T4 are commonly connected to the inverting input of 
amplifier 60 to provide a voltage follower arrangement. The non-inverting 
input of amplifier 60 is connected to the junction of switch S1 and 
capacitor C4 and the emitters of transistors T3 and T4 are connected 
through the switch S2 to terminal 14. 
In order to minimise external interference, the lines between the 
non-inverting input of amplifier 51 and terminal 14 and the non-inverting 
input of amplifier 52 and the tapping on potentiometer R11 are screened 
and the entire circuit is enclosed in a die cast box which is earthed. It 
is also to be noted that the timing circuit 16 is arranged so that there 
are no logic level transitions during the sampling periods as such 
transitions could cause internal interference. 
Referring now to FIG. 6, there is shown the circuit diagram of another 
pacemaker circuit. As will be explained in more detail, in this pacemaker 
circuit the charge which is stored between the stimulating electrode and 
the myocardium during each stimulus pulse is measured by using an 
integrator and a measuring resistor and a substantially equal amount of 
charge is then removed upon termination of the stimulus pulse. 
The circuit shown in FIG. 6 has a stimulating electrode 111 and a reference 
electrode 113. The electrode 113 is connected to the earth line via a 
terminal 112. The stimulating electrode 111 is connected via a terminal 
110 and a capacitor C11 to a terminal 114. Terminal 114 is connected 
through a measuring resistor R30 to a terminal 130. The terminal 130 is 
connected to the earth line through the collector/emitter path of a 
transistor T11. The pacemaker circuit includes a timing circuit 116 which 
provides a pulsed signal PULS similar to the signal PULS produced by the 
circuit shown in FIG. 4. The signal PULS is supplied through a resistor 
R31 to the base of transistor T11. The pacemaker circuit further includes 
a potentiometer R32 connected between the positive supply line V.sub.DD 
and the earth line. A tapping on potentiometer R32 provides a reference 
voltage V.sub.ref and this reference signal is supplied to the 
non-inverting input of a type 3140 operational amplifier 131. The output 
of amplifier 131 is connected through a resistor R33 to terminal 130. The 
output of amplifier 131 is also connected through an analog switch S10 to 
its non-inverting input. Its non-inverting input is also connected through 
an analog switch S11 to terminal 130 and an analog switch S9 is connected 
between the output of amplifier 131 and terminal 130. With the exception 
of switches S9, S10 and S11 and the measuring resistor R30, the components 
described so far are conventional in a pacemaker circuit. In operation, 
with the switch S10 closed and switches S9 and S11 open, amplifier 131 
charges the capacitor C11 to the voltage V.sub.ref. Each time a pulse is 
supplied to the base of transistor T11, the stimulus pulse is applied 
between electrodes 111 and 113. 
Terminal 130 is connected through an analog switch S12 and a resistor R40 
to the inverting input of a type 3140 operational amplifier 140. The 
terminal 114 is connected through an analog switch S13, a resistor R41, a 
potentiometer R42, and a resistor R43 to the junction of a Zener diode ZD2 
and a resistor R44 connected in series between the positive supply line 
and the earth line. The resistor R44 and Zener diode ZD2 operate to 
provide a bias voltage V.sub.b. A tapping on potentiometer R42 is 
connected to the non-inverting input of amplifier 140 and its output is 
connected to its inverting input through a 1 megohm feed back resistor 
R45. Thus, operational amplifier 140 together with its associated 
components amplifiers the voltage appearing across the measuring resistor 
R30. 
The output of operational amplifier 140 is connected through a resistor R46 
to the inverting input of a type 3140 operational amplifier 150. The 
non-inverting input of amplifier 150 receives the bias voltage V.sub.b. 
The output of amplifier 150 is connected through an integrating capacitor 
C13 to its inverting input. The output of amplifier 150 is also connected 
to its non-inverting input through a pair of analog switches S14 and S15, 
the junction of switches S14 and S15 being connected through a resistor to 
the earth line. Thus, amplifier 150 can integrate the current flowing 
through resistor R30 and so operate as an integrator. When the switches 
S14 and S15 are closed the integrator is reset. 
The output of amplifier 150 is connected to the positive input of a type 
311 comparator 151. The junction of resistor R44 and Zener diode ZD2 is 
connected through a resistor R50 and the resistor R51 to the earth line. 
Resistor R50 is bridged by a pair of analog switches S16 and S17 and the 
junction of these switches is connected to the negative input of 
comparator 151. A resistor R52 and a further analog switch S18 are 
connected between the positive supply line V.sub.DD and the earth line. 
The junction of resistor R52 and switch S18 is connected to the control 
terminal of switch S16. The control terminals of switches S17 and S18 are 
commonly connected together to the output of amplifier 151. Also, the 
signal PULS is connected through resistor R31 to the base of a transistor 
T12, the emitter of which is connected to the earth line and the collector 
of which is connected to the enable input (pin 6) of amplifier 151. 
The output of comparator 151 is connected through a resistor R53 and the 
resistor R54 to the positive supply rail V.sub.DD and the junction of 
these two resistors is connected to the base of a transistor T13. The 
emitter of transistor T13 is connected through a resistor R54 and a 
potiometer R55 to the positive supply line V.sub.DD and the collector of 
this transistor is connected to terminal 130. Thus, when transistor T13 is 
on, this transistor and the associated resistors operate as a current 
source. 
The output of amplifier 151 is connected to the A input of a type 4528 
monostable 160. The Q output of this monostable is connected to the 
control terminal of switch S11 and the Q output is connected to the 
control terminal of switch S10. The Q output of monostable 160 is also 
connected to one input of a NAND gate 161. NAND gate 161 is associated 
with another NAND gate 162, one input of which receives the signal PULS. 
The other input of NAND gates 161 and 162 are connected to the positive 
supply line V.sub.DD. The output of NAND gate 161 is connected to one 
input of a NAND gate 163 and the output of NAND gate 162 is connected to 
one input of a NAND gate 164. The NAND gates 163 and 164 are connected as 
a R-S flip flop. The output of NAND gate 163 is connected to the control 
terminals of switches S13 and S14 and the output of NAND gate 164 is 
connected to the control terminals of switches S12 and S13. 
In operation, immediately before the leading edge of each pulse of the 
signal PULS, switches S12 and S13 are open, switches S14 and S15 are 
closed. Consequently, the output of amplifier 150 is clamped to the bias 
voltage V.sub.b. At the leading edge of the pulse of the signal PULS, 
transistors T11 and T12 are turned on, thereby supplying a stimulus pulse 
across electrodes 111 and 113 and disabling comparator 151. The leading 
edge of the pulse also causes the output of NAND gate 164 to go high, 
thereby closing swithes S12 and S13 and opening switches S14 and S15. 
Consequently, during the stimulus pulse, the current flowing through 
resistor R30 is integrated by amplifier 150 and the deviation of the 
output of this amplifier between the beginning and the end of the pulse 
represents the charge stored between electrodes 111 and 113. Upon 
termination of the stimulus pulse, transistors T11 and T12 turn off and 
comparator 151 is enabled. As the output of amplifier 150 is now below the 
bias voltage V.sub.b, the output of comparator 151 goes low thereby 
turning on transistor T13 which acts as a current source for removing the 
charge which has been stored. During charge removal, the current flowing 
through resistor R30 is again integrated by comparator 150. When the 
output of comparator 150 reaches the bias voltage V.sub.b, the output of 
comparator 151 goes high thereby turning off transistor T13, triggering 
monostable 160, closing switches S17 and S18 and opening switch S16. The 
operation of switches S16 to S18 provides hysteresis for the negative 
input of comparator 151. As a result of triggering monostable 160, the 
output of NAND gate 163 goes high and so switches S14 and S15 are closed 
thereby removing the charge on capacitor C13 and switches S12 and S13 are 
opened. Whilst the Q output of monostable 160 is high, switches S9 and S11 
are closed and switch S10 is open. This has the effect of clamping 
terminal 130 to the reference voltage V.sub.ref thereby substantially 
removing any remaining charge stored between electrodes 111 and 113. When 
the Q output of monostable 160 goes low, switches S9 and S11 open and 
switch S10 closes and the reference voltage V.sub.ref is then applied to 
terminal 130 via a resistor R33. 
Therefore, upon termination of each stimulation pulse, the charge stored 
between electrodes 111 and 113 is substantially removed by transistor T13. 
In practice, it has been found that the charge is removed at this stage to 
an accuracy of approximately 0.1%. Because the interface between the 
stimulus electrode 111 and the myocardium has complex non-linear 
properties, it is unlikely that this interface will be returned precisely 
to its initial state and it is for this reason that terminal 130 is then 
clamped for a preset period to the reference voltage V.sub.ref. When the 
terminal 130 is supplied with a reference voltage via resistor R33, the 
intrinsic pulses in the heart may be sensed at terminal 130. 
Thus, in the two embodiments of the invention which have been described, 
the charge which is stored between the stimulus electrode and the 
myocardium during the stimulus pulse is subsequently removed thereby 
making it possible to detect the evoked heart pulses. The evoked pulses 
may therefore be used for diagnosis whilst the pacemaker circuit is 
operating. For example, damaged tissue in a heart may cause a secondary 
wave to follow the due PRST waves and this wave may be detected using the 
present invention. Also, the pacemaker circuit of the present invention 
may be modified so that it can detect a specific component of the evoked 
pulse and use this to modify the operation of the pacemaker circuit. For 
example, the detection of such a pulse may be used to modify the frequency 
of intensity of the stimulus pulses or to suppress or trigger them as may 
be appropriate.