Implantable cardiac stimulator

An implantable cardiac stimulator has an atrial electrode for placement in the atrium of a heart and a ventricular electrode for placement in the ventricle of the heart. In order to sense stimulated events in the heart, a detector is connected to both of the electrodes to measure electrical heart signals between them.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a cardiac stimulator of the type having a 
pulse generator and an electrode system which includes an atrial 
electrode, arranged in an atrium of a heart, and a ventricular electrode, 
arranged in a ventricle of the heart, whereby stimulation pulses, 
generated by the pulse generator are supplied to the atrium via the artial 
electrode or to the ventricle via the ventricular electrode for 
stimulating a heart response. 
2. Description of the Prior Art 
In a dual chamber system of the above-described type, the atrium and 
ventricle can be stimulated in a synchronous sequence which emulates a 
natural heart cycle in a healthy heart. In order to prevent the release of 
needless stimulation pulses to the heart, the atrium and the ventricle can 
be sensed for spontaneous (natural) activity. For example, if the 
ventricle is sensed for spontaneous activity, ventricular stimulation 
pulses are inhibited when a spontaneous ventricular event (ventricular 
systole) is sensed. Such a system is designated DVI. The DVI-system is 
further refined the if the atrium is also sensed for spontaneous activity. 
A sensed spontaneous atrial event (atrial systole) would then inhibit 
emission of an atrial stimulation pulse. This refined system is known 
under the designation DDI. A heart stimulator is designated DDD if it is 
devised for both triggering and inhibiting according to spontaneous 
activity sensed in both the atrium and the ventricle. 
European Application 0 308 536 describes a heart stimulator which can 
stimulate and sense both in the atrium and ventricle. This known cardiac 
stimulator has two electrodes in the atrium (atrial electrodes), two 
electrodes in the ventricle (ventricular electrodes) and an indifferent 
electrode, consisting of the cardiac stimulator's capsule. Stimulation and 
sensing of both the atrium and the ventricle can be achieved either 
between an electrode and the capsule or between the two electrodes in the 
respective chamber, i.e. atrial stimulation and sensing between one atrial 
electrode and the capsule or between two atrial electrodes, and 
ventricular stimulation and sensing between one ventricular electrode and 
the capsule or between two ventricular electrodes. 
The electrodes are implanted in the heart on electrode leads. For this 
known cardiac stimulator, a bipolar electrode lead can be implanted in the 
atrium, and a bipolar electrode lead can be implanted in the ventricle. 
The electrode emitting the stimulation pulses must be in contact with 
heart tissue in order to stimulate that tissue. A sensing electrode, 
however, does not need to be in contact with heart tissue, since blood in 
the heart conducts current better than the tissue itself. It is 
nonetheless preferable to have even the sensing electrode in contact with 
heart tissue, since this will result in more distinct signals. This is 
because an electrode which is not in direct contact with tissue picks up 
signals from a large area of tissue. In addition, interference signals are 
conducted better in blood than in tissue and thus affect the measurement 
signal to a larger degree. 
As an alternative to two bipolar electrode leads, a quadripolar electrode 
lead can be used, provided it is constructed so at least one of the two 
electrodes located in the atrium is in contact with heart tissue for 
stimulating same. The advantage of a quadripolar electrode lead is that it 
facilitates implantation, since only one electrode lead has to be 
introduced into the heart. The disadvantage of a quadripolar electrode 
lead is that it is thicker and stiffer than a bipolar electrode lead. 
In European Application 0 596 319 (published after the filing of the prior 
Swedish application on which the present application is based), a cardiac 
stimulator which has an electrode system for sensing atrial and 
ventricular heart events is described. The electrode system has one atrial 
electrode, one ventricular electrode and an indifferent electrode, 
consisting of the heart stimulator's capsule. Ventricular events are 
sensed in the same way as in the above-described known cardiac stimulator, 
i.e. between the ventricular-electrode and the indifferent electrode. 
Spontaneous atrial activity is sensed between the atrial electrode and the 
ventricular electrode. With this design, the number of conductors in the 
electrode lead can be reduced compared to when a separate sensing 
electrode is used, as in the above-described known heart stimulator. 
In order to operate as effectively as possible in the DDD mode, the cardiac 
stimulator should automatically ascertain whether emitted stimulation 
pulses result in any heart events, i.e. evoked responses. If provided with 
such a function, the heart stimulator will have an ability to 
automatically set the stimulation amplitude at a value which is as close 
to the heart's stimulation threshold value as possible, thereby saving 
energy. This function is known as autocapture. 
One problem associated with the autocapture function is to distinguish 
between signals having different origins in the heart without the need for 
excessively comprehensive electronics or circuit logic. Cardiac 
stimulators are designed so as not to impede or disturb the patient 
receiving the heart stimulator. Therefore, the volume and weight of the 
heart stimulator are restricted as much as possible, and this naturally 
limits the possibility of incorporating comprehensive electronics in the 
stimulator. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a cardiac simulator which 
can simply and safely detect spontaneous and evoked heart responses in a 
manner which avoids complex circuitry and thus allows the pulse generator 
and associated electronics to be contained in a capsule of a size and 
weight which are not disturbing to a patient in whom the cardiac 
stimulator is implanted. 
Such a cardiac stimulator is achieved in accordance with the invention 
wherein a detector is connected both to the atrial electrode and to the 
ventricular electrode for sensing stimulated ventricular heart events. 
In conjunction with the emission of a ventricular stimulation pulse, the 
detector is activated to sense the signal between the ventricular 
electrode and the atrial electrode. If a ventricular heart event is 
stimulated by the ventricular stimulation pulse, the electrical signal in 
the heart tissue (the depolarization signal) will be sensed, and the 
detector will generate a signal which indicates that an evoked response 
has occurred in the heart. Thus only two electrodes are used for sensing 
evoked ventricular heart events. As a result of differences in polarity 
and signal strength between atrial and ventricular signals. and, 
particularly differences in logic structure and function, a detector 
devised according to the known cardiac stimulator in European Application 
0 596 319 is unable to detect any stimulated ventricular events, even 
though it can detect atrial events between an atrial electrode and a 
ventricular electrode. For example, the blanking functions, i.e., the time 
windows in which the detector does not sense any signals, are completely 
different. 
It is advantageous if the cardiac stimulator according to the present 
invention includes a control device connected to the detector for 
activating the detector for a predetermined time window in conjunction 
with the pulse generator's emission of ventricular stimulation pulses, in 
order to sense stimulated ventricular heart events. 
The detector can contain a special electronic circuit for conditioning 
(editing) the measurement signal after a ventricular stimulation pulse, 
this conditioning circuit being activated by a switch controlled by the 
logic circuit. In order to minimize space requirements, activation can be 
devised as a purely logical operation, i.e., a signal from the detector is 
accepted as an evoked response only if received in the predetermined time 
window. This is possible due to an understanding of the heart's 
physiology. A heart signal originates either in the or atrium or the 
ventricle. After an atrial event (spontaneous or stimulated), the atrium 
needs time to recover before a new atrial event (spontaneous or 
stimulated) can occur. The recovery time required (refractory period) 
varies, but it cannot be shorter, in principle, than the repolarization 
time of atrial heart cells. Under normal conditions, the heart beats at a 
rate of 70 to 150 beats a minute. Under normal conditions, atrial 
responses therefore occur at intervals which are never less than about 400 
ms (150 bpm=2.5 Hz, which corresponds to 400 ms). Between these intervals 
the atrial electrode can be used as a reference for the ventricular 
electrode in the sensing signals. The time window is appropriately chosen 
as to minimize the effect in case of abnormal atrial activity. 
In a further version of the cardiac stimulator in accordance with the 
invention, the detector also senses stimulated atrial heart events and in 
the aforementioned control device activates the detector in conjunction 
with every n.sup.th atrial stimulation pulse for sensing stimulated atrial 
heart events, n being a whole number, preferably between 1 and 6. 
In order to ensure patient safety, each ventricular stimulation pulse 
should be checked to determine whether it stimulates a response. For the 
atrium, however, however, it is sufficient to check the stimulation pulse 
at regular intervals to make sure that the atrial stimulation pulses are 
still effective. For the atrium, activation of the detector can be 
performed physically by a switch or by a logic function. 
In conjunction herewith, it is advantageous for the pulse generator to 
generate a biphasic atrial stimulation pulse or some other 
polarization-compensating stimulation pulse when a stimulated atrial event 
is to be sensed. The biphasic, or polarization compensating, stimulation 
pulse ensures that the atrial electrode does not acquire any residual 
polarization after the stimulation pulse is emitted. Detection of the 
stimulated event is thereby facilitated. The ventricular stimulation pulse 
can also be devised in a corresponding manner to facilitate detection of 
stimulated ventricular events. 
In another version of the cardiac stimulator in accordance with the 
invention, the electrode system includes an indifferent electrode, located 
outside the heart, and a further detector is connected to the ventricular 
electrode and to the indifferent electrode for sensing spontaneous 
ventricular events, and the further detector is activated at the same time 
as the other detector is activated in order to sense stimulated atrial 
events. 
If an abnormal ventricular event occurs, i.e., a ventricular extra systole 
(VES), it is advantageous if the heart stimulator is able to distinguish 
between a stimulated atrial event and a spontaneous VES. This is achieved 
by adding the further detector which senses the ventricle at the same time 
as atrial measurement is performed. The indifferent electrode can 
appropriately consist of the capsule (in part or whole). If both detectors 
sense an event, it cannot be determined that a stimulated event has 
occurred in the atrium. Sensing of the atrium should then be repeated. 
In conjunction with sensing of atrial heart events after every n.sup.th 
atrial stimulation pulse, repetition of sensing is advantageously made 
after the next, consecutive atrial stimulation pulse when both detectors 
have sensed an event.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, a cardiac stimulator 2, such as the cardiac stimulator 
of the invention, is connected to a heart 4 by means of an electrode 
system 6. The electrode system 6 has an atrial electrode 8, which is 
arranged in the atrium of the heart 4. Via an atrial electrode connector 
10, electrical signals can be carried between the atrial electrode 8 and 
the heart stimulator 2. Further, a ventricular electrode 12 is arranged in 
the ventricle of the heart 4, and electrical signals can be carried 
between the ventricular electrode 12 and the heart stimulator 2 via a 
ventricular electrode conductor 14. The heart stimulator 2 also has an 
indifferent electrode 16 which can consist of a part or all of the capsule 
of the heart stimulator 2. 
In FIG. 2 the structure of the heart stimulator 2 is schematically shown in 
a block diagram. Both the atrial electrode 8 and the ventricular electrode 
12 are connected to a pulse generator 18, via the atrial electrode 
conductor 10 and the ventricular electrode conductor 14. The pulse 
generator 18 generates and emits atrial stimulation pulses from an atrial 
pulse generator 20 and ventricular stimulation pulses from a ventricular 
pulse generator 22. Both the atrial pulse generator 20 and the ventricular 
pulse generator 22 are controlled by a control device 24. The control 
device 24 controls the shape, amplitude, duration, stimulation interval 
etc. of the stimulation pulses. 
The indifferent electrode 16 is also connected to the pulse generator 18, 
in order to form a common return line for atrial and ventricular 
stimulation pulses delivered to the heart 4 via the atrial electrode 8 and 
the ventricular electrode 12 respectively. A stimulation pulse returns 
through body tissue via the indifferent electrode 16 to the pulse 
generator 18. 
The atrial electrode 8 and ventricular electrode 12 are connected to a 
detector 26 via the atrial electrode conductor 10 and ventricular 
electrode conductor 14. The detector 26 includes a signal conditioning 
unit 27, an atrial comparator 28 and a ventricular comparator 30. Since 
ventricular depolarization signals are much stronger than atrial 
depolarization signals, two different reference signals are used for the 
comparators 28 and 30. Signal conditioning in the signal conditioning unit 
27 can be identical for the signals from both the atrium and the ventricle 
and can include, e.g., signal filtering and amplification. The signal 
conditioning unit 27 can also be devised so that detected signals are 
filtered in different ways, depending on whether they have a spontaneous 
or stimulated origin. Stimulated events can only follow an emitted 
stimulation pulse, so devising the signal conditioning detector 27 to 
achieve this differential filtering is therefore no problem. 
The output signal from the signal conditioning unit 27 is subsequently 
compared in the atrial comparator 28 with an atrial reference potential 
U.sub.A. If the output signal is larger than the reference potential, a 
detection signal is sent from the atrial comparator 28 to the control 
device 24. 
The logic of the control device 24, however, must be activated for 
receiving atrial events if the detection signal from the atrial comparator 
28 is to be accepted as an atrial event. 
In a corresponding manner, the output signal from the signal conditioning 
unit 27 is compared in the ventricular comparator 30 with a ventricular 
reference potential U.sub.V. A detection signal from the ventricular 
comparator 30 is only accepted as a ventricular event if the logic of the 
control device 24 has been activated for receiving ventricular events. 
The control device 24 also controls the detector 26. It determines, e.g., 
when the detectors 28 and 30 are to be activated and the sensitivity with 
which heart signals are to be sensed. 
A further detector 32 is connected to the ventricular electrode 12 and the 
indifferent electrode 16 to sense spontaneous ventricular events in the 
heart. 
In order to detect stimulated ventricular events, the control device 24 
operates as follows: After an approved atrial event, spontaneous or 
evoked, the ventricle is sensed for a spontaneous ventricular event during 
a time period referred to as the A-V interval. If no spontaneous 
ventricular event is sensed during the A-V interval, the control device 24 
orders the emission of a ventricular stimulation pulse. At the same time 
the logic of the control device 24 is activated to sense whether the 
ventricular comparator 30 emits a detection signal in the time window for 
which the logic is activated. If a ventricular event is sensed during the 
time window, evoked response is established. The control device 24 can 
then proceed and activate the logic for sensing a spontaneous atrial 
event. Otherwise, a new ventricular stimulation pulse, containing a higher 
energy than the last, must be delivered to the ventricle. 
Atrial stimulation pulses are emitted when no spontaneous atrial 
stimulation pulse is sensed within an A-A interval from the last atrial 
event (spontaneous or stimulated). For the atrium, every stimulation pulse 
does not have to be checked for evoked response. Checking only, e.g., 
every third atrial stimulation pulse is fully sufficient. In order to 
facilitate detection of the weaker atrial signal, a biphasic stimulation 
pulse is delivered to the atrium. As a result, polarization of the 
electrode 8 does not persist too long. At the same time, the logic of the 
control device 24 is activated in order to identify a detection signal 
from the atrial comparator 28 as an atrial event. For safety, the further 
detector 32 is also activated during the same period of time. The further 
detector 32 ensures that no ventricular extra systoles are spuriously 
sensed as stimulated atrial events. The further detector 32 transmits 
detection signals to the control device 24 and receives control signals 
therefrom in the same way as the detector 26. 
The heart stimulator 2 also contains a telemetry unit 34 which is connected 
to the control device 24 and which can talemetrically receive and transmit 
information to/from an extracorporeal programming unit 36. With the 
programming unit 36, a physician can, e.g., retrieve stored information 
from the control device 24 and even re-program the parameters of the heart 
stimulator 2. 
FIG. 3 shows an alternative design for the detector 26 for detecting both 
atrial and ventricular signals. In a signal conditioning unit 38, input 
signals from the electrode conductors 10 and 14 are conditioned in the 
same way as in FIG. 2. In this instance, however, only one comparator 40 
is used to compare the output signal with the reference potentials U.sub.A 
and U.sub.V. A switch 42, controlled by the control device 24, connects 
the relevant reference potential at any given moment. In other words, the 
atrial reference potential U.sub.A is switched to the comparator 40 when 
atrial events are to be sensed, and the ventricular reference potential 
U.sub.V is switched to the comparator 40 when ventricular events are to be 
sensed. 
FIG. 4 shows yet another version of the detector 26. After signal 
conditioning in a signal conditioning unit 44, the conditioned signal is 
sent to a variable amplifier 46. The gain of the variable amplifier 46 is 
controlled by the control device 24. The output signal from the amplifier 
46 is then compared in a comparator 48 with a uniform reference potential 
U.sub.ref. Depending on whether atrial or ventricular events are to be 
sensed, the control device 24 appropriately changes the gain of the 
variable amplifier 46. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.