Device and method for the physiological frequency control of a heart pacemaker equipped with a stimulating electrode

Physiological frequency control of a heart pacemaker having a stimulating electrode is accomplished by providing an oxygen measuring electrode and placing it in the body tissue, loading the oxygen measuring electrode with stimulating pulses in parallel with the stimulating electrode, measuring the potential of the oxygen measuring electrode relative to another electrode between stimulating pulses, and controlling the frequency of the pacemaker as a function of the measured potential.

BACKGROUND OF THE INVENTION 
This invention relates to a device and a method for the physiological 
frequency control of a heart pacemaker equipped with a stimulating 
electrode by means of a control unit as well as to a method for operating 
such a device. 
The most common applications of pacemaker therapy are the permanent and the 
temporary electrostimulation of the heart. On occassion, a combination of 
both methods may be necessary in cases which cannot be judged 
unequivocally. Permanent electrostimulation of the heart is used if 
Adams-Strokes attacks occur or in the event of total AV block 
(AV=atrioventricular node). In cases of bradycardic heart arrhythmia, 
temporary electrostimulation of the heart must be provided so that the 
physical capacity of the patient is improved. 
For the permanent electrostimulation of the heart, heart pacemakers are 
known which contain a fixed frequency generator which delivers, for 
instance, 70 current pulses per minute at a constant rate. These heart 
pacemakers are of simple design and have a long service life even with 
standard chemical batteries. Such heart pacemakers can be used 
particularly for older people for whom a heart time-volume on the basis of 
70 beats per minute is sufficient for their still tolerable extent of 
physical stress. In addition, the rhythm of the patient's heart itself is 
suppressed at least approximately. If spontaneous actions by the patient 
occur from one or several automation centers, the parasystolic behavior 
can lead not only to an irregular beat sequence with bunched occurrence of 
disturbed pacemaker pulses; it also can trigger, in particular, 
tachycardic states all the way to chamber fibrillation if the artificial 
stimuli fall into the valnerable phase, i.e., the T-wave of the intrinsic 
preceding action. 
In addition, different types of so-called demand pacemakers are known. In 
demand sets, the pulse of the heart pacemaker is inhibited via an 
electrode located in the ventricle by the potential of the R-spike of the 
intrinsic actions as long as its frequency is above, for instance, 70 
beats per minute. If it drops below this value, the device is switched on 
automatically and takes over the stimulation. In the "stand-by-pacer," the 
R-spike of the intrinsic rhythm acts, via the electrode, as a trigger 
pulse, to which the pacemaker is subordinated in the frequency range, for 
instance, of between 70 and 150 beats per minute with matched signal 
lapse. If intrinsic pulses are missing or if the R-spike spacings are 
smaller than between 300 and 400 msec, artificial stimulation is applied. 
If, however, the latter exceeds an upper predetermined pulse per minute 
value of, for instance, 150, the heart pacemaker cuts the frequency in 
half, i.e., it takes over the electrical stimulation of the heart with a 
correspondingly reduced pulse delivery. With these two types of demand 
pacemakers, the parasystolic state is avoided and an orderly side by side 
arrangement of the internal rhythm and artifical stimulation is obtained. 
Furthermore, an electrochemical device for determining the oxygen content 
of a liquid is known. The measuring cell of this device consists of a 
tubular body in which a cathode and an anode are arranged in an 
electrolyte. The one end face of the measuring cell is provided with a 
diaphragm which is fastened by a sealing ring and a cap provided with an 
opening. This diaphragm separates the liquid to be examined from the 
electrode arrangement. The measurement principle consists of the 
electrochemical reduction of oxygen (O.sub.2) where an oxygen diffusion 
limiting current is brought about at the electrode through the diaphragm. 
Thereby a measuring signal proportional to the concentration if obtained 
(U.S. Pat. No. 2,913,386). With a measuring cell of such a design, the 
oxygen concentration in the blood or tissue can be measured in vivo, 
however, only for a short time, for instance, for several days since the 
measuring cell becomes surrounded by developing connective tissue layers, 
and the measuring signal is thereby falsified. 
It is, thus, an object of the present invention to describe a heart 
pacemaker which makes possible a mode of operation which is adapted to the 
physiology, is simple and trouble-free. 
SUMMARY OF THE INVENTION 
According to the present invention this problem is solved by a pacemaker to 
which an oxygen measuring electrode is connected. A control unit senses 
the potential between the charged oxygen electrode and either the 
stimulating electrode or a reference electrode and utilizes the measuring 
oxygen level to set a desired heart rate with appropriate outputs then 
provided to the pacemaker so that the stimulating electrode is stimulated 
at the desired rate. 
In a first embodiment of the device according to the present invention, the 
O.sub.2 measuring electrode is always loaded by a stimulating pulse in 
parallel with the stimulating electrode. In each instance, prior to the 
next loading of the O.sub.2 measuring electrode (and of the stimulating 
electrode) the potential of the O.sub.2 measuring electrode is measured 
referred to a reference electrode. The measured potential corresponds to 
the oxygen concentration of the blood or the heart muscle tissue. An 
electronic processing circuit assigns to each potential of the O.sub.2 
measuring electrode, an oxygen concentration level and regulates the 
frequency, i.e., the number of beats per minute of the heart, as an 
inverse function of oxygen concentration. Thus, a heart pacemaker with an 
implantable oxygen sensor which makes possible a mode of operation adapted 
to the physiology is obtained. 
The invention also includes a method for physiological frequency control of 
heart pacemaker device having a stimulating electrode, a heart pacemaker 
and an oxygen level measuring electrode connected in parallel and coupled 
to the heart pacemaker. The method comprises the steps of placing the 
oxygen level measuring electrode in the blood or the body tissue, loading 
the oxygen level measuring electrode in the stimulating electrode with 
stimulating voltage pulses at a variable frequency, measuring the 
potential of the oxygen level measuring electrode relative to the 
stimulating electrode between stimulating voltage pulses and controlling 
the stimulating voltage pulse frequency of the pacemaker as an inverse 
function of the measured potential. 
In a second embodiment of the device according to the present invention, 
the O.sub.2 measuring electrode is likewise loaded in parallel with the 
stimulating electrode by a stimulating pulse. In this case, however, the 
potential difference between the O.sub.2 measuring electrode and the 
stimulating electrode prior to the next loading of the O.sub.2 measuring 
electrode is sensed. The O.sub.2 measuring electrode preferably is 
comprised of smooth vitreous carbon and the stimulating electrode 
preferably of activated vitreous carbon. Such a stimulating electrode has 
a large double-layer capacity, which results in low polarization. If the 
oxygen concentrations in the blood or the tissue change quickly, the 
stimulating electrode of activated vitreous carbon maintains its 
potential, but the O.sub.2 measuring electrode of smooth vitreous carbon 
changes its potential as a function of the oxygen concentration. By 
forming the difference of the potentials, possible influences which may 
become active for both electrodes, of substances of the body, the 
concentrations of which change more slowly than those of the oxygen, are 
eliminated. Thus, oxygen concentrations in the blood or the tissue which 
change quickly can be measured and the frequency of the heart pacemaker 
can be controlled accordingly. 
In the device according to the present invention, the heart pacemaker and 
the control unit advantageously form a common structural unit. In 
addition, the lines of the O.sub.2 measuring electrode and of the 
reference electrode can be arranged together with the line of the 
stimulating electrode in an electrode cable. In this manner, a 
physiologically controlled heart pacemaker is obtained, the design of 
which is not appreciably larger than known heart pacemaker designs. 
Furthermore, the operative intervention does not become more complicated 
by this design.

DETAILED DESCRIPTION 
FIG. 1 illustrates the device of the present invention which is generally 
indicated by the elements within the dotted block 100. Included is a heart 
pacemaker 2 of conventional design and a control unit 4. Within the 
control unit 4 is a processor 101, a sample and hold circuit 103, an 
analog to digital converter 105, and switches indicated as S2 and S3. 
Associated with the pacemaker is a counter electrode 6 and a stimulating 
electrode 10 each at the end of a line 24. These are both connected to the 
patient's body 8, the stimulating electrode being arranged in the heart 
muscle tissue 14. Also provided is an oxygen sensor electrode or measuring 
electrode 12 which is also disposed in body 8 or heart muscle tissue 14. 
All three lines 24 can be formed into a single cable 26. In the embodiment 
of FIG. 1 the stimulating electrode 10 and oxygen measuring electrode 12 
are coupled as inputs to the sample and hold circuit. In the alternative 
embodiment of FIG. 1A, there is also provided a reference electrode 18 at 
the end of a line 24 in cable 26 which is also inserted in the body 
tissue. In that case, it is the reference electrode 18 and the oxygen 
electrode 12 which are provided as inputs to the sample and hold circuit 
103. The stimulating electrode 10 is coupled to an output of the pacemaker 
2 through switch S2. Similarly, the oxygen electrode is coupled to the 
pacemaker through switch S3. 
In operation, as shown in FIG. 3, during a first time period between 
t.sub.0 and t.sub.2, a switch S1 in the pacemaker 2 is closed to allow a 
capacitor C within the pacemaker to charge from a battery B through a 
resistor R1. The upper curve of FIG. 3 illustrates the capacitor current 
which will initially be high and then drop off as the capacitor becomes 
fully charged. After the capacitor is charged, at an appropriate time 
t.sub.2, an output from the processor 101 opens switch S1 and closes the 
switch S2 to provide a stimulating pulse 107 to the stimulating electrode 
10. This is conventional operation in the pacemaker. However, as 
illustrated by FIG. 3, switch S3, which was closed at time t.sub.1, is 
still closed at this time. Thus, the oxygen electrode 12 is also provided 
with the stimulating pulse. 
The oxygen measuring electrode 12 preferably consists of smooth vitreous 
carbon, and the stimulating electrode 10 consists preferably of activated 
vitreous carbon. Although various shapes are possible, preferably both of 
these electrodes have a hemispheric shape. During the stimulating pulse, 
low current also initially flows through the oxygen measuring electrode 
12. The small amount of current is due to the smooth surface and very low 
capacitance of electrode 12. This low current is however sufficient for 
measurement without adversely affecting stimulation. 
In operation, the stimulating electrode 10 and the oxygen measuring 
electrode 12 are thus loaded in parallel by the cathodic stimulating 
pulses of the heart pacemaker 2. After this stimulation, and after the 
switch S2 has been opened and the switch S3 opened, at time t.sub.3, the 
processor 101 directs the sample and hold circuit 103 to take a sample of 
the voltage between the stimulating electrode 10 and the oxygen sensor 12 
or alternatively in the case of FIG. 1A between the sensor electrode 12 
and the reference electrode 18. This is done between stimulating pulses, 
i.e., before the next stimulating pulse loads the oxygen measuring 
electrode 12. The time interval during which the potential of the oxygen 
measuring electrode can be measured is, for example, 0.5 to 1 msec. Within 
this time span the potential of the oxygen measuring electrode 12 is at 
least approximately constant. In accordance with stored data which 
comprises a digitized form of the curves of FIG. 2 to be explained below, 
the microprocessor 101 assigns a pulse rate based on the measured 
potential. 
The flow of the program in the processor 101 is illustrated by FIG. 4. As 
indicated, the potential is sampled by providing an output to sample and 
hold circuit 103 as indicated by block 120. For this sampled potential, as 
indicated by block 122 a pulse rate is calculated using stored data. The 
pulse rate is then used, as indicated in block 124, to calculate opening 
and closing times for the switches. The nature of the data stored and from 
which the pulse rate is calculated is that with increasing oxygen 
concentration in the blood, the number of beats per minute of the 
pacemaker 2 drops. Conversely, with decreasing oxygen concentration in the 
blood, the rate of the pacemaker is increased. Once the opening and 
closing times are calculated, in accordance with block 124, the program 
can then cause the opening and closing of the switches as indicated by 
FIG. 3. During the sampling, S1 was closed to allow charging. Now using 
the calculated pulse time, switch S1 is opened and switch S2 closed to 
provide an output to the stimulating electrode and to the oxygen 
electrode. This is shown by block 126. As shown by block 128, switch S3 is 
then opened and, as indicated by block 130, S1 is closed and S2 opened to 
carry out charging. Thereafter, switch S3 is again closed as indicated by 
block 132. Switch S3 is closed during a portion of the charging in order 
to avoid potential drift of the sensor electrode. The program then loops 
back to block 120. 
In the calibration curves according to FIG. 2, the potential .phi./AgCl of 
the O.sub.2 measuring electrode 1 is plotted versus the oxygen 
concentration. A straight line a with positive slope represents the course 
of the potential of the O.sub.2 measuring electrode 12 in an electrolyte 
which is loaded with cathodic stimulating pulses of, for instance, 5V and 
a pulse length of about 0.5 msec. This electrolyte contains, for instance, 
0.9% sodium chloride (NaCl) and, for instance, 0.1% sodium hydrogen 
carbonate (NaHCO.sub.3) and forms the base electrolyte. A straight line b 
with positive slope likewise represents the course of the potential of the 
O.sub.2 measuring electrode 12. There, however, physiological substances 
such as glucose, urea and amino acid mixtures at their physiologically 
maximal concentration were added to the base electrolyte. The straight 
line a and b always start from the same origin but have different slopes. 
The straight line b, for instance, has a slope of about 50 mV/20% oxygen. 
The straight line b, which does not deviate substantially from the 
straight line a, shows that the oxygen concentration can be measured by 
this measuring method in vivo over an extended period of time in blood or 
tissue even in the presence of accompanying physiological substances.