Battery monitoring means for an implantable living tissue stimulator

A battery monitoring means for an implantable living tissue stimulator system in which various battery voltages are telemetered to an external receiving means, these voltages being related to the internal impedance of the implanted battery. More specifically, a battery loading circuit is provided which incorporates a switch means for loading the battery in accordance with a predetermined sequence. In a specific embodiment, first and second resistors are sequentially connected across the battery. The battery output voltage is telemetered to an external receiving means during this sequential connection. By knowing the values of the two resistors, the internal impedance of the battery can be calculated, this impedance being related to the remaining life of the implanted battery.

FIELD OF THE INVENTION 
The invention relates to battery monitoring systems for implantable tissue 
stimulators such as heart pacemakers. 
BACKGROUND OF THE INVENTION 
Implantable tissue stimulators such as implantable pacemakers 
conventionally use a battery as a power means for generating tissue 
stimulation pulses and operating any electronics and telemetry means 
contained therein. Although typical pacemaker batteries have a relatively 
long life, they do eventually require replacement. Such replacement 
requires removal of the pacemaker and its subsequent reimplantation, 
thereby causing the patient to incur an additional although minimal risk. 
Consequently, there has long been a need to determine the life remaining 
in a pacemaker battery so that it can be replaced at an optimum time, 
early replacement subjecting the patient to unnecessary risk and late 
replacement subjecting him to a possibility that his pacemaker will fail. 
Techniques for determining remaining battery life by loading the battery 
with predetermined loads, measuring the output voltages of the battery as 
the load is varied, calculating the internal impedance of the battery from 
the output voltage measurements, and predicting battery life remaining 
from the calculated internal impedance are well understood. However, these 
techniques have not heretofore been utilized in implantable pacemakers 
because of the complexity involved and the relatively long life of 
batteries utilized therein. The present invention provides a simple and 
reliable means for monitoring the life remaining in a pacemaker battery, 
thereby solving a long standing problem of determining when the battery 
should be replaced. 
SUMMARY OF THE INVENTION 
The invention provides a battery monitoring means for an implantable living 
tissue stimulator having a telemetry means for transmitting and receiving 
signals related to the operation of the tissue stimulator. The battery 
monitoring means comprises means for providing the battery output voltage 
to the telemetry means, a load means for altering current through the 
battery, and a means for connecting the load means to the battery. The 
connecting means is cycled in accordance with a predetermined sequence 
initiated by a telemetry operator. In a specific embodiment, the load 
means comprises first and second resistors each of which can be 
sequentially connected across the battery output terminals by a switch 
means. The switch means could comprise any type of electronic switch, one 
example being a plurality of FET switches. Changes in battery output 
voltage as a result of sequentially connecting the first and second load 
resistors across the battery output terminals provide a means for 
calculating the internal impedance of the battery. Comparing this internal 
impedance with predetermined impedance versus battery life remaining 
characteristics of the particular implanted battery type provides a means 
for determining when the battery should be replaced.

DETAILED DESCRIPTION 
As required, a detailed illustrative embodiment of the invention is 
disclosed herein. This embodiment exemplifies the invention and is 
currently considered to be the best embodiment for such purposes. However, 
it is to be recognized that other means for altering current through the 
battery could be utilized. Accordingly, the specific embodiment disclosed 
is representative in providing a basis for the claims which define the 
scope of the present invention. 
A living tissue stimulator system incorporating a battery monitoring means 
provided by the invention is shown in FIG. 1. The human tissue stimulator 
system comprises an implantable tissue stimulator 10 and a receiving and 
programming means 12. The implantable tissue stimulator 10 could be a 
device generally known as a heart pacemaker. A telemetry means is included 
which comprises an impedance reflecting circuit 14 having an impedance 
related to an output voltage from a voltage control oscillator (VCO) 16 
whose frequency is determined by an input signal to be telemetered. A 
signal selection circuit 18 receives input voltages from both a memory 
means 20 which provides digital inputs, and a pulse generator 22 which 
provides analog inputs. The signal selection circuit 18 includes means for 
selecting one of its input voltages to be telemetered, the selection being 
made in accordance with control signals from the memory means 20. The 
selected signal frequency modulates the VCO 16. The frequency modulated 
VCO 16 output signal then alters the impedance of the impedance reflecting 
circuit 14 which is magnetically coupled as schematically represented at 
24 to a programmer head 26 which in turn is coupled to an oscillator 28. 
The output of the oscillator 28 is determined by the combined impedance of 
the programmer head 26 and the impedance of the impedance reflecting 
circuit 14 which is coupled to the programmer head 26. Thus, the 
oscillator 28 output is an FM modulated signal if the coupled impedance is 
reactive, and an AM modulated signal if the coupled impedance is 
resistive. In both cases the modulation on the oscillator 28 output is 
related to the output of the VCO 16 which is FM modulated by the signal to 
be telemetered. 
The implantable tissue stimulator 10 also includes a telemetry receiver 34 
for receiving signals from the receiving and programming means 12 and 
output circuitry 36 which supplies stimulating pulses to the heart 38. The 
output of the oscillator 28 is provided to a demodulator 40, the output of 
which corresponds to the output of the implantable tissue stimulator VCO 
16. This output is then provided to an FM detector 42 which in turn 
provides an output signal to a programmer and display means 44 which is 
proportional to the signal to be telemetered provided by the signal 
selection circuit 18. In addition, the programmer and display means 44 
provides control signals to be telemetered to the implantable tissue 
stimulator 10 to a modulator 46 which modulates the oscillator 28. The 
output of the oscillator 28 is magnetically coupled through the programmer 
head 26 to the impedance reflecting circuit 14 whose output is provided to 
the telemetry receiver 34. In addition, the implantable stimulator 10 is 
powered by a battery 50 which could be of several types, two of which are 
a lithium iodine battery and a lithium bromine battery. The output voltage 
from the battery 50 is provided to the signal selection circuit 18 on an 
output line 51. A battery loading circuit 52 is connected to the output 
voltage line 51 and provides a means for selectively applying an 
additional load across the battery as will be explained below. Although 
the above description relates to a telemetry system utilizing an impedance 
reflecting circuit 14, the invention is in no way limited to this specific 
type of telemetry system and any type of telemetry system incorporated in 
an implantable tissue stimulator could be utilized. 
Referring to FIG. 2, the battery loading circuit 52 is connected in 
parallel across the battery output voltage line 51 and ground, thereby not 
interrupting power to other portions of the tissue stimulator. The battery 
loading circuit 52 comprises a load switch means 56, a first loading 
resistor 58, and a second loading resistor 60. The signal selection 
circuit 18 has a signal selection switch means 62 which provides a means 
for connecting the battery output voltage line 51 to the VCO 16. The load 
switch means could be any type of electronic switch as FET switches. 
In operation, the battery loading circuit 52 is normally configured so that 
the switch means 56 is in a first position 64 thereby unaffecting the 
output voltage of the battery 50. By appropriate signals, the telemetry 
operator through the programmer and display means 44 can cause the memory 
means 20 to initiate a predetermined battery impedance measuring sequence. 
Such a sequence comprises configuring the signal selection switch means 62 
to provide the voltage appearing on a battery telemetry terminal 66 to the 
VCO 16. The memory means then causes the load switch means 56 to step to a 
second position 68 for a predetermined time period, thereby connecting the 
first loading resistor 58 between the battery output voltage line 51 and 
ground. This connection results in the battery output voltage dropping by 
an amount related to the internal impedance of the battery 50 as is well 
understood by those familiar with batteries. After a predetermined time, 
the memory means then causes the load switch means 56 to step to a third 
position 70, thereby connecting the second loading resistor 60 between the 
battery output voltage line 51 and ground. If the first loading resistor 
58 and second loading resistor 60 have different values, then the voltage 
on the battery output voltage line 51 will change, thereby providing a 
second indication of the internal impedance of the battery 50. Although 
two loading resistors 58 and 60 have been illustrated, any number of 
loading resistors could be utilized. Although any value of load resistor 
could be chosen, it is preferable to choose one with a relatively high 
value so as to minimize current drain from the battery. Utilizing a 
lithium iodine battery, it has been found that values of 60,000 ohms for 
the first loading resistor 58 and 15,000 ohms for the second loading 
resistor 60 provide a satisfactory means for determining the internal 
impedance of the battery. 
A typical way in which the battery loading circuit 52 is utilized can be 
seen by referring to FIG. 3. However, it should be remembered that FIG. 3 
is only a qualitative representation and the impedances shown do not refer 
to any particular battery type. Referring now to FIG. 3, a battery may 
have an internal impedance of 500 ohms as shown at 74. During the first 
80% of battery life, the increase in internal impedance is substantially 
linear as shown at 76. However, when only 20 percent of the original 
battery life is remaining, the internal impedance increases significantly 
and becomes very high when the battery is exhausted as shown at 78. Thus, 
calculations of battery internal impedance from the telemetered voltages 
generated as above described provides a means for determining the life 
remaining in the implanted battery 50. The battery should be replaced as 
soon as the internal impedance begins to rise rapidly as can be seen at 
80. 
It should now be apparent from the above description that a battery 
monitoring means has been provided in which voltages related to the 
internal impedance of the battery are telemetered to an external receiving 
means, these voltages then providing a means for determining the internal 
impedance of the battery.