 |
Description  |
|
|
The present invention relates to a process and devices for aiding the
operation of the heart muscle.
In patients for whom means are provided for aiding the operation of the
heart muscle, one of the important problems resides in the adaptation of
the operation of these means to the various situations resulting from
variations in physiological demands on the circulation or from
physiological states of the heart.
Thus, one of the problems encountered consists in adapting an electrical
stimulation of the heart to the onset of tachycardia.
In some cases, it is highly desirable to prevent the onset of a
tachycardia, for example a ventricular tachycardia the consequences of
which may be dangerous, while, in other cases, it is merely desirable to
avoid a delayed response to a demand of an increase in the heart output or
acceleration of the heart rhythm.
Several methods have already been tried for automatically adapting a
frequency stimulating the heart muscle to the physical effort, this in
particular in the case of a stimulo-dependent heart.
Thus, it has been attempted to utilize the variations in temperature, the
blood pH, the oxygen saturation of the blood, the QT/R-R ratio of the
stimulated complexes, all these factors varying in a more or less regular
manner and with more or less delay as a function of the physical efforts
of a person wearing a pacemaker.
Up to the present time, these various methods have not resulted in truly
satisfactory results.
An object of the present invention is therefore to detect preventively a
physiological demand of an increase in the heart rhythm and/or of the
heart output which cannot be satisfied by the natural means.
Another object of the invention is to detect preventively the onset of a
tachycardia, including a ventricular tachyarrhythmia or tacharrhythmia.
Another object of the invention is to adapt a heart rhythm and/or output to
a modification and in particular to an increase in the physiological
needs.
Another object of the invention is to adapt a stimulation rhythm of a
pacemaker to a modification and in particular to an increase in such
needs.
Another object of the invention is to prevent the onset of a tachycardia
and in particular a ventricular tachycardia.
These objects and others are achieved by a process according to the
invention comprising automatically and periodically sending to the heart
muscle electrical stimulation impulses of sub-threshold power, current or
voltage, i.e. which are insufficient to produce an electro-systole, i.e. a
mechanical contraction of the heart muscle, so long as the heart is in its
normal physiological situation but sufficient to produce such an
electro-systole if the physiological situation of the heart muscle has
changed, and detecting the appearance or the non-appearance of this
electro-systole during a limited period of time consecutive to the impuse.
These impulses will be termed hereinafter "test impulses". In fact, in most
cases, it is sufficient to send such sub-threshold test impulses solely
when the rhythm of the heart appears to be normal. Indeed, in the case
where for example a tachycardia, or a normally physiological increase in
the heart rhythm, has set in, the situation is known and palliative means
may be employed if necessary. On the other hand, it has been found, in
accordance with the invention, that it is possible to detect in advance,
when the operation of the heart appears still normal, a risk of the onset
of tachycardia or a modification in the physiological needs as concerns
the circulation.
According to an improvement of the invention, the value of the power or
current of the sub-threshold electrical test impulse sent out may be
adapted to the value of the impedance between the probe carrying the
stimulation electrode or electrodes and the heart itself. Normally, this
impedance remains constant or varies but slightly, but it is possible to
adapt the value of the sub-threshold test impulse.
Preferably, the value of the sub-threshold test impulse, for example in
current or duration, is 10 to 85% of the value of a stimulation just
sufficient to produce an electro-systolic contraction.
Preferably, the test impulse is always sent out at a given moment of the
heart cycle, this moment being preferably included in the second half of
the heart cycle. This moment, considered as a percentage of the prior
cycle, or measured in duration, may possibly vary along the heart cycle
under the effect of sweep control means or vary as a function of the heart
frequency so as to be closer to the end of the cycle in the case of high
heart frequency.
By way of a modification, a plurality of test impulses may be emitted in
succession during the same heart cycle.
As a modification, the moment of the test impulse may vary periodically.
For example, it may be for a first cycle at 30% of the heart period; for a
second cycle at 50%, etc.; 100% for the eighth cycle and then again at 30%
for the ninth cycle, etc.
In another modification, this variation could intervene only periodically
during a limited number of cycles within a larger number of cycles.
In a first manner of carrying out the invention in which the basic
physiological rhythm corresponding to a situation of rest of the person,
is imposed by natural physiological means, the subthreshold test impulse
is sent to the heart muscle a little before the probable onset of a
spontaneous QRS complex. This may be the case both of a patient in which
the production and conduction of the natural electric heart signals are
preserved (appearance of the P wave followed by a normal QRS wave or
appearance of a normal simple QRS complex) as in the case of a patient
suffering from a conduction block and provided with a DDD type sequential
stimulator responsive to the spontaneous variation of the P wave and then
producing the appearance of a stimulation replacing the defective QRS
complex.
In another manner of carrying out the invention, in which the heart is
stimulated in a constant manner at a rhythm imposed by a pacemaker, the
sub-threshold test impulse is also sent out a little before the sending
out of a stimulation impulse.
In the last-mentioned case, it is easy for a pacemaker to establish the
moment at which the sub-threshold test impulse must be sent out since it
knows the rhythm of its own operation. On the other hand, in the case
where one intervenes in accordance with with the invention, in the
spontaneous rhythm of the heart, it is possible in a simple way to send
the sub-threshold test impulse at the end of a fixed given lapse of time
following on a systole or, in accordance with an improvement, the
spontaneous heart rhythm may be detected, for example by measuring the
duration separating two consecutive spontaneous systoles and, as the case
may be, and there may be varied in accordance with the spontaneous rhythm
the lapse of time between the systole and the sending of the sub-threshold
test impulse, this lapse of time then decreasing when the spontaneous
frequency of the heart increases.
As mentioned before, when the heart exceeds a certain rhythm, test impulses
may cease to be sent out.
The aforementioned two manners of carrying out the invention may moreover
be combined in the case of a heart susceptible to bradycardia and provided
with a pacemaker of synchronous type and beating at certain moments
spontaneously at a normal rhythm and on the other hand at other moments
solely under the effect of a standby stimulation.
In another manner of carrying out the invention, sub-threshold test
impulses may be sent to the heart when it beats at a higher rhythm than is
normal, for example when it is stimulated at a relavely high rhythm by an
anti-tachycardia device stimulating the heart at a high rhythm. In this
case, the sending of sub-threshold test impulses may, for example, be
employed for checking wether the established anti-tachycardia rhythm is or
is not sufficient. In the case where it is sufficient, it can then be
attempted to decrease the stimulation rhythm, progressively or in steps,
while, if it is insufficient, this rhythm may be increased. In the case of
a patient whose heart beats, spontaneously or artificially, at a high
rhythm for taking into account an effort or an emotion, the sending of
sub-threshold test impulses will enable the presence or the absence of a
risk of tachycardia to be checked.
Preferably, the lapse of time between a systole or an electrosystole and
the sending of a sub-threshold test impulse is greater than the duration
of the refractory and vulnerable zone and preferably between 60 and 90% of
the considered heart period.
Preferably, this lapse of time is a fixed proportion, for example between
60 and 90% of the considered heart period, i.e. of the duration between
the last two systoles or electro-systoles.
The sub-threshold test impulse is advantageously sent out by an
intra-cardiac probe, for example a catheter carrying one or more
electrodes.
A sub-threshold test impulse may be sent out after each spontaneous systole
or electro-systole, i.e. a sub-threshold test impulse may be sent out per
heart cycle, or only a single impulse may be sent out for several cycles.
Preferably, at least one sub-threshold test impulse is sent out for a
number of cycles between 1 and 100. For example, in the case of a patient
subject to tachycardias, it is preferable to send out a test impulse for
every cycle or every other cycle or in any case following a small number
of cycles whereas, in respect of patients which do not constitute such
risks, one impulse every ten or even more cycles would be sufficient.
Also, it is possible, in the last analysis, to avoid considerations of
numbers of cycles and to decide to send out a sub-threshold test impulse
after a period of a given duration, for example every two or four seconds
or every ten seconds, it being understood that once this period of 10
seconds has elapsed, the impulse will only be able to be sent out within a
chosen lapse of time which is preferably between 60 and 90% of the period
corresponding to the heart rhythm.
The current of the sub-threshold test impulse is preferably between 10 and
85% of the current which is normally just sufficient to produce a
contraction, i.e. an electro-systole. As mentioned before, this value
depends on the cardiac impedance which may moreover be measured by known
means so as to adapt the stimulation to the possible variations of the
impedance. As to the value, for a given impedance, of the current which is
just sufficient to produce an electro-systole, it may be determined
experimentally by progressive variations of the test impulse, or be fixed
once and for all after examination and experience of the considered
patient.
Preferably, the sub-threshold impulse may have the same voltage as the
stimulation impulses but a reduced duration, for example 10 to 50% of the
duration of a stimulation impulse, but it would also be possible to employ
a different voltage provided there is employed a corresponding duration in
accordance with the chronaxia curve (heart threshold variation in
relationship to applied voltage and impulse length) of the heart muscle.
According to an improvement of the invention, it is considered that a
systole which follows the sending of the test impulse was produced by the
latter if the duration between the test impulse and this systole (QRS) is
less than a value between 20 and 150 milliseconds.
In other words, a response supervising window is opened whose length is
equal to the aforementioned value. In this case, if there occurs within
this window an extra systole, the latter is considered as a response to
the test impulse even if, in fact, the extra systole was not produced by
the test impulse.
According to an improvement of the invention, the detection of such a
systole in response to the sending of a sub-threshold test impulse, i.e.
the detection of an electro-systole stimulated thereby, may produce the
sending of a signal to a monitor in order to enable the nursing personnel
to intervene, for example by the administration of a drug.
According to another improvement of the invention, the detection of a
response may produce the sending of at least one stimulation impulse at a
more rapid rhythm.
In the case of an anti-tachycardia treating device, it is possible, after
having detected such a response, to establish an electric stimulation at a
high frequency for a certain duration.
In a particularly preferred manner, the detection of a response to the test
impulse will produce a moderate increase in the stimulation frequency in a
progressive manner or during a step. Thus, for example, there may be
envisaged a stimulation per minute and stimulation steps of 60, 70, 80, 90
and 100 stimulations per minute. Thus, if in a patient who is stimulated
at the standby rhythm of 50, there is obtained a response to a test
impulse, the stimulation rhythm will automatically and immediately assume
the value of 60 during for example one minute, then redescend to 50 unless
a test impulse during the step of 60 prorduces a response, in which case a
new step at 70 is immediately established etc., the absence of response
during a step causing, at the end of the step, the frequency to descend
preferably to the lower step.
Now if this patient has a spontaneous rhythm of 64 beats per minute (the
standby stimulation being then eliminated), and a response to a test
impulse is detected, there will be immediately and automatically
established a stimulation at the step whose rhythm is just higher than
that of the spontaneous rhythm, in the present instance, the step of 70,
for one minute.
Instead of effecting a treatment by the sending of a sequence of
stimulations at increased frequency, it is possible to act by other means,
for example, by an implanted automatic syringe, such as by beta-blocking
medicinal preparations.
In another manner of carrying out the invention, the detection of a
response may serve to actuate hemodynamic aiding means, for example a
heart assistance auxiliary pump acting on a ventriculoaortic, or
auriculo-aortic branch.
According to a particular improvement of the invention, the current of the
sub-threshold test impulse may be increased when the heart frequency
(spontaneous or artificial) increases. The number of cycles between two
consecutive sub-threshold stimulation impulses may also be varied.
In a preferred manner of carrying out the invention, there may be measured,
during at least a part of the duration of the test impulse, the impedance
of the heart, i.e. the impedance between two electrodes used for the
stimulation, or a parameter having a relationship with this impedance, for
example the electric current passing through an electrode during the test
impulse. The impedance may for example be measured between two
intra-cardiac electrodes in the case of the so-called bipolar stimulation
or between an intracardiac stimulation electrode and a ground electrode in
the case of the so-called monopolar stimulation.
However, the impedance may also be measured during the stimulation
impulses. The impedance measurement may be advantageously employed for
varying the power, for example the voltage of the test impulse and,
possibly of the stimulation impulses, so that the test impulses remain
sub-threshold even in the case of a notable drop in the myocardiac
excitation threshold.
The periodicity of the sending of the electrical impulses serving to
measure the impedance may be situated between the sending of an impulse at
each heart cycle and of an impulse for 100 cycles or several hundred
cycles, or again in the last analysis it is possible to avoid a number of
cycles and decide to send an electrical impulse following on the period of
a given duration, for example every two, or four or every ten seconds, it
being understood that, once this duration has elapsed, it will only be
possible to send the impulse in a chosen period which is preferably
between 60 and 100% of the period corresponding to the heart rhythm which
is or is not spontaneous.
Likewise, in the case where the electrical impulse is sent out at each
cycle of every n heart cycles, this impulse may be sent out in the course
of a period of 60 to 100% of the cycle.
This electrical impulse measuring the impedance may be a subthreshold test
impulse as seen above, but may also be a sub-threshold impulse which is
not used as a test and in this case its power is preferably much lower
than the threshold value, for example 5% of the normal threshold value, so
as to avoid creating a stimulation. By way of a modification, the impulse
could be an electrical stimulation impulse but this solution is not
preferred in a pacemaker of synchronous type since the impedance would not
be measured in the course of the spontaneous beats.
The measurement of the impedance may advantageously be employed for
regulating the power, voltage or duration of the stimulation and/or test
impulses, usually by increasing when the impedance increases and
decreasing when the impedance decreases.
The means for carrying out the invention will be clear from the following
examples which are non-limiting and with reference to the accompanying
drawing in which:
FIG. 1 is a diagram of a pacemaker according to the invention, it being
understood that this diagram is in fact formed principally by a
microprocessor.
FIGS. 2 to 5 show electrocardiograms showing the operation of the pacemaker
.
DESCRIPTION OF A DETAILED FORM OF THE INVENTION
A device according to the invention is incorporated into a pacemaker used
on demand (synchronous pacemaker) which is of the ordinary or DDD type.
The parts of the pacemaker will not be described since they are well
known, except inasmuch as they have a relationship with the invention.
The assembly formed by the device and the pacemaker may be constructed from
a microprocessor and its environment, or from a computer, the structure of
these apparatus having no need to be described since it is well known to
those skilled in the art.
The assembly comprises two electrodes 1, 2 mounted on the same
intra-cardiac catheter and both serving to stimulate and detect.
The ventricular stimulation impulses are set at a potential of 2 volts and
a duration of 0.5 milliseconds. The threshold excitation of the heart was
determined at 1 volt for the same duration of 0.5 milliseconds.
The sub-threshold test impulses always have the same potential of 2 volts
but a duration of 0.1 millisecond so that they have a value which is 20%
of the value of the stimulation impulses and 40% of said threshold.
The standby frequency of the pacemaker is 50 stimulation impulses per
minute. The stimulation increments at increased frequency are 60, 70, 80,
90, 100 stimulations per minute and the duration of an increment is 10
cycles. It is possible to increase the duration of the increment if the
increment is solicited several times in a given interval of time.
The device knows the instantaneous frequency of the systoles and
electro-systoles, i.e. the duration of each heart cycle which has just
finished (interval R--R). On the other hand, the detection is blinded by
an electronic field during a period of 30 milliseconds which starts 2
milliseconds before the beginning of the test impulse. The latter is
therefore never considered as a systole signal.
A test impulse is sent out every 8 heart cycles after an interval of time
of 75% of the length (period) of the preceding cycle. For example, when
the cycle is a second (60 beats per minute), the sending out of the test
impulse starts 0.750 second after the beginning of the cycle (wave K).
A response detection window W is opened for a duration equal to 100
milliseconds. This window opens at the end of the electronic field. For
example, if the cycle is of 1 second, the window is opened 778
milliseconds after the beginning of the cycle and closed at 878
milliseconds after the beginning of the cycle. Any detection during this
window is considered as a positive response to the test impulse.
If such a positive response is detected, the frequency increment
immediately higher than the spontaneous or electro-stimulated rhythm in
process is actuated. At the end of an increment, therefore in the case of
absence of positive response to the test impulses, the immediately lower
frequency increment is actuated. The operation remains of the
"as-required" type during any increment.
Note that if, during an open window, an extra systole which was not
produced by the test impulse appears, this extra systole is however
considered by the device as a positive response to the test impulse and
the immediately higher increment is therefore actuated. The device
therefore operates also as a reducer of tachycardia according to U.S. Pat.
No. 3,857,399 and according to U.S. Pat. No. 4,052,991 (in particular
according to claim 3 in which the predetermined number is equal to 2).
However, two distinct windows could also be provided, one for detecting
the response to the test impulses and the other for detecting the
extra-systoles.
Special cases:
(1) If a positive response is observed during each of the two consecutive
cycles, the new increment produced by the first of the two cycles has no
time to stimulate and the second higher increment is actuated.
(2) If positive responses are observed during the last increment of 100
stimulations/min, this increment remains actuated for a maximum duration
of 300 cycles and then a progressive lowering of frequency increment by
increment is produced. The device may also be associated with
anti-tachycardia means according to said U.S. patents, which would
intervene if the positive responses are observed during the last increment
of 100 stimulations per second.
With reference to FIG. 1, there is shown a diagram of the device in
question. The latter has two endocardiac electrodes, namely a negative
electrode 1 and a positive electrode 2 secured on a catheter (not shown).
The electrode 1 is connected to the usual amplifier 3, followed by the
usual filter 4, then the usual threshold detector 5 which converts the
signals received at the electrode 1 into calibrated impulses provided they
have sufficient characteristics. The threshold detector 5 is connected to
a time interval measuring circuit 6 capable of measuring the time interval
between two successive impulses issuing from the detector 5 and therefore
capable of knowing the instantaneous frequency of these impulses. When it
has just measured a time interval, the circuit 6 sends a corresponding
signal to a circuit 7 recognizing the extra systoles. This circuit sends
to an AND gate 8 a 100 millisecond impulse starting, for a cycle, after a
delay of 75% of the length of the preceding cycle (measured by the circuit
6). If, during this window, an impulse reaches the detector 5, its output
impulse appears on the second input of the gate 8, which sends an impulse
to an increment rhythm generator circuit 9.
The standby stimulation is produced by a standby frequency or generator 10,
set to produce impulses at the rhythm of 50 impulses/min, and sent to an
OR gate 11 whose output pilots a stimulation impulse generator 12 which
sends its stimulation impulses to the electrodes 1 and 2, at the rhythm
controlled by the impulse generator 10. Further, the standby rhythm
generator 10 is responsive, through a path which is not shown but is
usual, to the detection impulses issuing from the detector 5. If no
impulse leaves the detector 5, the generator 10 operates normally. If an
impulse leaves the detector 5, the next impulse produced by the generator
10 is staggered by 1.2 seconds and then re-staggered 1.2 seconds if a new
impulse issues from the detector 5, and so on. If the heart beats
spontaneously at a rhythm higher than 50/min, the generator therefore
remains silent.
A circuit 13 for sequentially controlling test impulses, is also responsive
to the standby rhythm generator 10, to the increment frequency or rhythm
generator 9 and to the detector 5. It is mentioned that the detector 5,
owing to usual means, is not responsive to the electrosignals coming from
the stimulation impulses emitted by the generator 12 when the latter is
piloted by the generator 10 or the generator 9. The circuit 13 for
sequentially controlling the test impulses is therefore responsive to all
the stimulation impulses and to all the spontaneous signal impulses
passing through the threshold of the detector 5. The circuit 13 emits,
every 8 impulses it receives, a test impulse control impulse which is sent
to the OR gate 11 and to a circuit 14 controlling the duration of the test
impulse. Such a control impulse causes the generation of an impulse by the
generator 12 but this impulse only lasts during the period of 0.1
millisecond allowed by the circuit 14, while, if the generator 12 is
actuated by the OR gate while the circuit 14 is silent, it generates a
stimulation impulse having a length of 0.5 millisecond.
The increment rhythm generator 9, each time it is actuated by the AND gate
8, emits 10 increment impulses toward the OR gate 11. The frequency of
these impulses depends on the state of a decoder belonging to the circuit
9 and responsive to the AND gate. The decoder may assume the following
states: 0, 1, 2, 3, 4, 5 respectively corresponding to the rhythms 0, 60,
70, 80, 90, 100. If the decoder is in the state 0, it sends no impulse. If
it is in any state n different from 0, it sends 10 impulses of
corresponding frequency to the OR gate, then returns to the state n-1. If,
in a state n different from 5, it receives an impulse from the AND gate 8,
it immediately changes to the state n+1. If, in state 5, it receives an
impulse from the AND gate 8, it remains in the state 5 for 10 further
cycles and so on during a maximum number of cycles equal to 300. When the
300th cycle is reached, it descends to the state 4 where it remains for
300 cycles, then to the state 3, etc., then it remains in or rises to the
state in which it counted the smallest number of impulses from the AND
gate.
The various circuits 6, 7, 8, 9, 10 are actuated by a high frequency time
base or clock 15. It must also be noted that the detection by the
threshold detector 5 is prevented after each stimulation impulse or each
detection for a duration equal to 40% of the length of the preceding cycle
so as to take into account the refractory period of the heart.
In an improved modification, the impulses issuing from the circuit 13 for
sequentially controlling test impulses are also sent to an impedance
measuring circuit 16 which, on one hand, is connected to the electrode 1
and, on the other hand, measures the strength of the current on the
resistor 17 through which the test impulse passes. The circuit 16 is
moreover responsive to the voltage of the test impulse so that it can
calculate the impedance between the electrodes 1 and 2. If the impedance
varies, the circuit 16 acts on the control of the test impulse duration
circuit 14 so as to vary this duration as a function of the measured
impedance. Preferably, the measurement of the impedance is only effected
during a first fraction of the duration of the test impulse so that it can
subsequently modify the remainder of this duration if the impedance has
varied since the preceding measurement. The circuit 16 operates if, and
only if, it is actuated by the circuit 13.
Also seen in the diagram of FIG. 1 are a number of regulating
potentiometers which may be actuated manually or by remote control:
18 permits the regulation, in percentage of a cycle duration, the moment or
instant of the sending of the test impulse (for example 75%);
19 permits the regulation of the standby rhythm or frequency;
20 permits the regulation of the voltage of the stimulation impulses and
the test impulses;
21 permits the regulation of the duration of the stimulation impulses.
The electronic clamp means and the refractory period locking means have not
been shown.
The operation is also seen in the other Figures.
In FIG. 2, there is seen the detection at the amplifier 3 concerning a
bradycardiac patient whose heart is stimulated by stimulation impulses at
the frequency of 50/min piloted by the standby rhythm generator 10. The
impulses are designated by A.sub.1 . . . A.sub.2 . . . A.sub.7, A.sub.8
for designating successions of 8 cycles. Each eighth cycle, a
sub-threshold test impulse E is emitted followed by the window W. It can
be seen that the third test impulse E is followed by a response signal U
indicating a contraction during the window W so that the AND gate 8 emits
an impulse. The decoder changes from the state 0 to the state 1 and
stimulation impulses B1 to B7 are sent out at a frequency of 60/min. After
B7, i.e. at the eighth cycle, a test impulse E is emitted, followed by no
response. Then arrive the last two impulses B.sub.9 and B.sub.10 of the
increment 60/min. The decoder returns to the state 0 and the standby
stimulation A.sub.1 . . . etc. resumes its action.
It can be seen from FIG. 3 that an impulse E is followed by a response U in
the window W so that the stimulation A'8 has not occurred. The decoder
changes to the state 1. After B7, the test impulse E produces a new
response U in the window W. The decoder changes to state 2 and eliminates
B'.sub.8, B'.sub.9, B'.sub.10. This time the circuit stimulates at the
frequency 70/min. After the seventh stimulation C.sub.7 a new response U
occurs. The decoder changes to the state 3 and the stimulation increment
80/min starts. After the seventh stimulation D.sub.7 the test impulse 6 no
longer produces a response so that the last stimulations of the increment
D.sub.8, D.sub.9, D.sub.10 are allowed, after which the decoder changes to
state 2, etc.
It can be seen from FIG. 4 that, while the window W of a fifth cycle
(devoid of a test impulse) is opened, a ventricular extra systole V is
detected. This extra systole produces an impulse actuating the rhythm
generator 9 whose decoder changes from state 0 to state 1 and produces the
stimulation B.sub.1, B.sub.2 . . . of the 60/min increment.
FIG. 5 shows a spontaneous heart rhythm R, R, R . . . at a frequency of
62/min. For each eighth cycle, a test impulse E is produced. One of the
impulses is followed by a response U in the window W which produces an
impulse issuing from the AND gate. The rhythm generator 9 is therefore
actuated. But in this case, the decoder immediately changes to state 2 and
produces a stimulation C.sub.1, C.sub.2 at the increment 70/min which is
just greater than the spontaneous frequency of 62/min. This may be
obtained by rendering the decoder responsive to a heart frequency
measuring circuit, for example the circuit 6 which puts and maintains the
decoder in the state just lower than the measured spontaneous frequency.
In the present case, this state was state 1 and the impulse of the AND
gate therefore produces the passage to state 2. Thus, the stimulation is
effective at the increment just higher than the preceding spontaneous
activity.
FIG. 5 also shows that a spontaneous systole R occurring after the window W
of the cycle starting with C.sub.2, does not increase the stimulation
rhythm. C'.sub.3 is eliminated and, as the spontaneous rhythm R, R, R is
higher than 70/min, the stimulation at increment 70/min is inhibited. In
other words, at each increment, the device operates as a pacemaker
operating as demanded. However, the state 2 of the decoder remains
operative during the entire duration of this spontaneous activity. If the
frequency of this spontaneous activity decreases, for example becomes
equal to 68/min, the decoder changes to the immediately lower state 1.
In the example described hereinbefore, it is understood that a supervision
window W is established for each cycle including the 7 cycles devoid of
test impulses. However, a window could also be opened only after the test
impulse. A window would also be opened before the test impulse, above all
if it is decided to send out the test impulse rather late in the cycle.
* * * * *
|
|
 |
|
 |
Description | |
