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We claim:
1. An implantable rate responsive pacer arranged to be implanted in a
patient and comprising: pacer control means for producing control pulses
for controlling cardiac activity at a rate which is variable between
selected upper and lower limits as a function of the metabolic demand of
the patient; means connected to said pacer control means for producing
heart stimulation pulses in response to the control pulses; and
measurement circuit means responsive to respiratory activity of the
patient for producing a signal representative of the change in volume of
air in the patient's lungs, corresponding to pulmonary minute ventilation,
that is the quantity of air inhaled by the patient in unit time, said
measurement circuit means being connected to said pacer control means for
controlling the rate of the control pulses on the basis of the signal.
2. The pacer of claim 1 wherein said measurement circuit means comprise
means for detecting variations with respect to time in the geometry of at
least part of the chest of the patient due to pulmonary activity.
3. The pacer of claim 2 wherein said means for detecting variations
comprise two electrodes implantable for monitoring such variations in
chest geometry, and monitoring means connected to said electrodes for
monitoring the electrical impedance present between said electrodes, and
wherein said measurement circuit means further comprise signal processing
means connected to said monitoring means for producing the signal
dependent on minute ventilation, and wherein said electrodes are arranged
to be placed in a position which guarantees that the activity monitored is
substantially unaffected by movements other than respiration.
4. The pacer of claim 3 further comprising an implantable conductive case
constituting the other one of said electrodes, and wherein said one of
said electrodes is arranged to be implanted at a selected distance from
said case.
5. The pacer of claim 3 further comprising an implantable conductive case
having: two electrically conductive parts each constituting a respective
one of said electrodes; and means electrically insulating said two
conductive parts from one another.
6. The pacer of claim 3 wherein said monitoring means comprise means for
monitoring the impedance at regularly spaced sampling times.
7. The pacer of claim 6 wherein the sampling times occur at a rate of
approximately 10 Hz.
8. The pacer of claim 6 wherein: said monitoring means comprise an
impedance/voltage converter for producing signal pulses V(Z.sub.AB) which
correspond in amplitude to the impedance present between said electrodes
at successive sampling times; and said signal processing means comprise a
calculating circuit producing a first signal V'(.DELTA.A.sub.AB)
representing the absolute values of the amplitude variations between
successive signal pulses, and low-pass filter means having a time constant
of a few tens of seconds and connected to said calculating circuit for
producing a second signal Vm(.DELTA.Z.sub.AB) which is based on the first
signal and which represents the mean values or the average of the absolute
values of the amplitude variations between successive signal pulses, said
second signal constituting said signal dependent on pulmonary minute
ventilation.
9. The pacer of claim 8 further comprising a programmable correlation
connected to receive the second signal from said filter means and for
associating two selected values of said second signal with two respective
control pulse rate values, whereby the rate at which cardiac activity is
controlled depends on the value of said second signal.
10. A control circuit for an active device implantable in a patient, the
active device being operable at a controllable rate, and said control
circuit producing an output signal which controls the operating rate of
the active device, said control circuit comprising: means for detecting
variations with respect to time in the geometry of at least part of the
chest of the patient due to pulmonary ventilation and for producing a
first signal representative of such variations; and circuit means
connected to said means for detecting variations, and responsive to the
first signal, for producing a second signal which is representative of the
change in volume of air in the patient's lungs, corresponding to the
patient's minute ventilation and which constitutes the output signal from
said control circuit.
11. The control circuit of claim 10 wherein said means for detecting
variations comprise two electrodes implantable for monitoring such
variations in chest geometry, and monitoring means connected to said
electrodes for monitoring the electrical impedance present between said
electrodes, and wherein said circuit means comprise signal processing
means connected to said monitoring means for producing the signal
dependent on minute ventilation, and wherein said electrodes are arranged
to be placed in a position which guarantees that the activity monitored is
substantially unaffected by movements other than respiration.
12. The control circuit of claim 11 further comprising an implantable
conductive case constituting the other one of said electrodes, and wherein
said one of said electrodes is arranged to be implanted at a selected
distance from said case.
13. The control circuit of claim 11 further comprising an implantable
conductive case having: two electrically conductive parts each
constituting a respective one of said electrodes; and means electrically
insulating said two conductive parts from one another.
14. The control circuit of claim 11 wherein said monitoring means comprise
means for monitoring the impedance at regularly spaced sampling times.
15. The control circuit of claim 14 wherein the sampling times occur at a
rate of approximately 10 Hz.
16. The control circuit of claim 14 wherein: said monitoring means comprise
an impedance/voltage converter for producing signal pulses V(Z.sub.AB)
which corresponds in amplitude to the impedance present between said
electrodes at successive sampling times; and said signal processing means
comprise a calculating circuit producing a first signal
V'(.DELTA.Z.sub.AB) representing the absolute values of the amplitude
variations between successive signal pulses, and low-pass filter means
having a time constant of a few tens of seconds and connected to said
calculating circuit for producing a second signal Vm(.DELTA.Z.sub.AB)
which is based on the first signal and which represents the mean value or
the average of the absolute values of the amplitude variations between
successive signal pulses, said second signal constituting said signal
dependent on pulmonary minute ventilation.
17. The control circuit of claim 16 further comprising a programmable
correlation connected to receive the second signal from said filter means
and for associating two selected values of said second signal with two
respective control pulse rate values, whereby the rate at which the active
device operates is controlled in dependence on the value of said second
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Claims |
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Description |
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BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to implantable cardiac pacers and more particularly
to a rate responsive pacer which automatically alters the stimulation rate
of the heart in response to the metabolic demand of the patient. At the
present stage, a wide range of cardiac pacers is used to replace, by an
implantable device, the mechanisms producing and driving the electrical
signals associated with the heart function in the human body. These pacers
are identified by internationally agreed abbreviations. Hereafter we will
use the word "Pacemaker" (registered trademark) in place of implantable
cardiac electrostimulator. The pacers wherein the stimulation of the
ventricle is driven and synchronized with the naturally occurring activity
of the atrium are generally considered physiologic as the artificial
stimulation rate is not preset but is induced by the atrial natural
activity and is, therefore, always proportional to the latter one. As a
consequence this kind of pacemaker can be used only in those cases wherein
the atrial activity is physiologic, that is, only 30/60% of the present
pacemaker patients. Hence, in the present conditions a high percentage of
patients cannot use an electrostimulation based on a physiologically
variable rate according to the above description. This is the reason why
many experts have long tried to realize an implantable pacemaker able to
detect an alternative parameter other than the atrial activity, parameter
which can change in response to the physiologic demand of the patient and
can be used as a reference variable in order to automatically and
physiologically regulate the heart electrostimulation rate. As yet the
pacemakers theorized and designed to solve said problem are those
hereafter summarized wherein the electrostimulation rate is subject by an
algorithm to changes in the following parameters: atrial activity--blood
pH--body temperature--oxygen saturation in venous blood--QT interval
obtained from the endocavitary E.C.G.--respiratory rate--mechanical
activity--cardiac output--electromyogram of the diaphragm. An overall
discussion on the characteristics of these proposals can be found in:
Clin. Prog. Pacing and Electrophysiol. Vol. 1 n. 1--1983 "Rate Responsive
Pacing" by Anthony F. Rickards M.D. and Robert M. Donaldson M.D.--From the
National Heart Hospital--London W 1 England -
and in:
"A Physiologically Controlled Cardiac Pacemaker"--Krasner--Voukydis and
Nardella--J.A.A.M.I., Vol. 1 n. 3--1966; 14-20--
and more specifically in:
"A Pacemaker which automatically increases its rate with physical activity"
by Kenneth Anderson, Dennis Brumwell, Steve Huntley--From Medtronic
Inc.--Minneapolis--Minn.--U.S.A.
and:
"Variation of Cardiac Pacemaker rate relative to
respiration"--IEEE/Engineering in Medicine and Biology Society First
Annual Conference--p. 50, 1979--by Getzel W., Orlowski J., Berner B.,
Cunnigham B., Esser M., Jacob M., Jenter D. Other pacemakers of the kind
are described in the European patent application No. EP-A-0 080 348 as
well as in U.S. Pat. Nos. 3,593,718
--4,228,803--4,313,442--4,202,339--4,140,132--4,009,721--and in the
European patent application No. EP-A-0 089 014 and corresponding U.S.
application Ser. No. 06/474,241 filed by the same applicants of the
present application.
All the literature highlights the existing need and desire to realize a
pacemaker able to adjust the stimulation rate to the metabolic demand by
means of a suitable physiologic variable sensor circuit. The present
invention consists of a rate responsive pacer the inhibition and/or
stimulation rate of the ventricle and/or of the atrium of which is
automatically driven in the range between minimum and maximum programmable
values by a measuring and processing circuit which detects the variations
in time of the geometry of a part of the chest of the patient as a
consequence of pulmonary ventilation. The circuit also produces a signal
which depends on said physiologic variable and which automatically changes
the pacemaker stimulation rate between said minimum and maximum values.
DESCRIPTION OF THE DRAWINGS
The features of such a pacemaker and its resulting advantages are apparent
in the following description of a preferred embodiment, shown by way of
non-limiting example, in the annexed sheets of drawings, in which:
FIG. 1 is a diagram showing the normal relation in man between pulmonary
minute ventilation and oxygen consumption;
FIG. 2, 2a and 3 show different possible arrangements of the pacer and of
its electrodes and leads in the patient's body;
FIG. 4 shows two correlation slopes identifying minute ventilation (1/min)
and the average value of the absolute electrical impedance variation speed
in a part of the chest with said slopes being relevant to two different
patients examined at different physical activity levels;
FIG. 5 shows the block diagram of a single chamber demand pacemaker which
incorporates the invention;
FIGS. 6, 7, 8, 9 and 10 show the input signals of the different blocks of
the diagram of FIG. 5;
FIG. 11 shows an example of wiring diagram of the second block of the
circuit of FIG. 5;
FIG. 12 shows signals processed by the circuit of FIG. 11.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Pulmonary minute ventilation (VE), as shown in FIG. 1 of the annexed
drawings, strictly depends on VO.sub.2 oxygen consumption in man and is
controlled by chemical-humoral and nervous stimuli, similarly to the heart
activity. It is important to note that the ventilation which is defined as
liters of air inhaled during the time unit is quite different from the
respiratory rate which is defined as the number of respirations in the
time unit. While the respiratory rate measurement is simply confined to
the detection of the presence or absence of the breathing act, apart from
its duration and amplitude, the minute ventilation requires continuous
direct or indirect measurement of the airflow in and out of the lungs.
Moreover, the respiratory rate, as we will see in the following examples,
is not always an indicator of physical activity, whereas the proportional
correlation between minute ventilation and oxygen consumption up to the
anaerobiosis threshold is well proven and is accepted in all scientific
literature. In fact, in absence of physical activity, for example in
sleep, the respiratory rate may increase even if minute ventilation and
oxygen uptake is reduced due to the minimal metabolic demand of the human
body. On the other hand, the athletes, in order to increase the minute
ventilation in response to the increased oxygen demand, are trained to
increase the tidal volume more than the respiratory rate. As the minute
ventilation increase occurs simultaneously to the exercise, similarly to
cardiac activity, we suggest using minute ventilation as a reference
parameter to control the heart artificial stimulation rate. In pulmonary
ventilation, the variation in the inhaled and exhaled gas volume, in the
preset time unit, corresponds to an equivalent variation in the chest
cavity volume and, therefore, to its geometric variation.
In order to detect the minute ventilation it is sufficient to detect the
electrical impedance time variation of a part of the chest by means of two
electrodes A and B subcutaneously positioned, for example with the
arrangement show in FIG. 2 wherein an electrode could correspond to the
heart stimulation lead or with the arrangement of FIG. 3 wherein an
electrode (B) corresponds to the pacemaker conductive case (P) or with the
arrangement of FIG. 2a wherein both electrodes A and B are placed on the
pacemaker case and are separated by an electrically insulating part (C) of
the same case. It is understood that electrode A can be placed in any
suitable and non critical position vis-a-vis electrode B keeping in mind
that such position has to allow the detection of geometric variations of a
part of the chest barely affected by the movements of the upper limbs of
the patient. In view of these premises we will now describe the block
diagram of a single chamber demand pacemaker. FIG. 5 shows block 1
consisting of a strobed Impedance/Voltage converter using sampling
frequencies of approx. 10 Hz and including means which allow to send very
narrow pulses of proper intensity to the electrodes A and B. The duration
and repetition period ratio of these pulses is very high, for example
1/1000. In this manner, the means designed for said impedance measurement
consume a minimum quantity of energy and can therefore be supplied by the
same electric battery supplying the implanted pacemaker without being
remarkably detrimental to the life-time of the battery. A possible circuit
with these features has been described in detail in No. EP-A-0 089 014 of
the same applicants, as already mentioned. The strobed pulses between
electrodes A and B are outlined in the diagram of FIG. 6.
Block 2 consists of means to measure the absolute variations of the signal
corresponding to the impedance between one pulse and the previous one. The
function of block 2 is remarkable in that it allows to turn signal V
(Z.sub.AB) coming from block 1 (FIG. 7) into a signal V' (
.DELTA.Z.sub.AB) proportional to the chest volume variation speed, that is
a signal proportional to the instantaneous respiratory flow, in absolute
value, taking the flow itself as a variation in time of the chest volume.
The signal, shown in FIG. 8, will have peaks corresponding to the phases
of the respiratory cycle in which the expiratory and inspiratory speeds
reach their maximum values and will have a zero value when any respiratory
dynamics is absent.
Another peculiar function of block 2 consists in off-setting the slow
variations of the impedance between electrodes A and B due for instance to
the histologic changes in the tissues surrounding the same electrodes or
due to the change in the relative position of the same electrodes or to
the slow variation of the bodily mass of the patient or to the posture
variation of the patient or, finally, to the variation of the lung
residual functional capacity which shows a positive increase under strain
conditions.
Block 3 is made up by a low-pass filter with a time constant of a few tens
of seconds, e.g. approx. 30 seconds. The task of this block is to
determine the mean value or the average of the absolute values of the
impedance variations with a time constant such as to minimize the ripple
in the output signal and to be sufficiently fast to physiologically adjust
the heart stimulation rate. Signal Vm (.DELTA.Z.sub.AB) corresponding to
the mean value of the variation of the input signal, as shown in FIG. 9,
is available at block 3 output. Data has been collected which shows that
signal Vm (.DELTA.Z.sub.AB) is proportionally dependent on the minute
ventilation (FIG. 4) which is the physiologic variable driving the
pacemaker stimulation/inhibition rate according to the present invention.
We referred to the absolute values of the variations of the impedance but,
likewise, reference could be made either only to the positive increases or
only to the negative increases, having to avoid calculating the whole
signal with the relevant polarities as the mean value would always be
zero, in fact during the respiration it is agreed that the gas volume
inhaled on average is well balanced with that exhaled. FIG. 4 shows the
two slopes R1 and R2 which outline the correlation between minute
ventilation VE (1/min) and the output signal Vm (.DELTA.Z.sub.AB) from
block 3 with the two slopes being relevant to two different patients
examined at different activity levels. According to what has been
expounded and what is shown in FIG. 4, it can be stated that output signal
Vm (.DELTA.Z.sub.AB) from block 3 is proportional to minute ventilation
with a different proportional coefficient characterizing each patient.
This coefficient can be easily ascertained if the minute ventilation of
the patient during physical activity is evaluated by traditional means
together with block 3 output.
Block 4 is a programmable correlator that via a telemetry link associates
two values Vml (.DELTA.Z.sub.AB) and Vm2 (.DELTA.Z.sub.AB) of the output
signal from block 3, which are "a priori" programmable or obtained in two
different physical activity situations of the patient, with two
stimulation/inhibition rates (f1 and f2) of the pacemaker circuit. The
rates define a possible operational mode of the pacemaker on the basis of
which f1 and f2 may, not necessarily but possibly, coincide with the
minimum and maximum working rates of the pacer. The stimulation/inhibition
rate (f) indication sent to block 5 is proportional to the signals Vm
(.DELTA.Z.sub.AB) dynamically performed by block 4.
Block 5 represents a typical pacemaker circuit well known to persons
skilled in the art, realized in such a way as to guarantee the
stimulation/inhibition rate (f) sent by the previous block 4. With
reference to FIGS. 11 and 12, there is described now, as non-limiting
example, a possible circuitry realization of block 2. The signal V
(Z.sub.AB) coming from block 1 is made up by pulses the amplitude of which
is proportional to the impedance cyclically detected between electrodes A
and B. SH1 and SH2 indicate two sample and holds whereas AMP-1 indicates a
differential amplifier. If SH1 stores the amplitude of the n.sup.th of D
pulses, SH2 stores the amplitude of N.sup.th -1 pulse. A CK time signal
synchronous with the pulses of block 1 first sends the output signal from
SH1 to SH2 and then, after a delay due to DEL component, is stores in SH1
the new value of n.sup.th +1 pulse amplitude. At this point AMP-1 performs
the difference between n.sup.th amplitude stored in SH2, after the
transfer performed by CK, and n.sup.th +1 amplitude stored in SH1 and so
on with the following pulses. D1 signal coming out of AMP-1 is shown in
FIG. 12.
It is understood that blocks 3 and 4 are not described in every
constructional detail as they can be easily realized by any expert in the
field, taking into consideration the role they are designed to play.
Blocks 2-3 and 4 can be realized with analog and/or digital or
microprocessor based circuits.
Although this invention is disclosed within the context of a single chamber
demand pacemaker, the same invention can be applied to other pacing
modalities, including the so called double chamber ones (DVI) or (DDD),
which maintain the atrioventricular sequentiality, or even applied to
other therapeutic or diagnostic, portable or implantable devices or to
artificial organs, for example to an artificial heart, operating in
response to the minute ventilation of the patient. It is therefore to be
underlined that the invention is not confined to the given examples, but
it can be considerably changed or modified without altering the guiding
principle above expounded and described and hereafter claimed.
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