Device For Starting Or Stimulating Heart Contractions

Abstract

A Pacemaker of the demand type having an output control circuit arranged to control the produced pace impulses in such a manner that the amplitude of each impulse is slightly less than that of the preceding impulse. When the amplitude has decreased below the threshold value so that a heart impulse fails to occur within a predetermined period, a reset circuit will cause another pace impulse to be produced having a given initial amplitude.

Description

A Pacemaker is an electronic device which after surgical insertion of electrodes in a patient is capable of emitting pulses of sufficient magnitude to start or stimulate heart contractions. The Pacemaker is used as a means of treatment in case of abnormal conditions where the function of the natural centers of stimulus of the heart is suspended spasmodically or for a prolonged period and where no effective medical treatment is possible.

The main characteristics of such diseases are disturbance in the propagation of electric pulses from the atriums of the ventricles of the heart, the object of which is to start coordinated contractions of the ventricles. The pulse propagation is characterized physiologically by a successive and coordinated depolarization of certain cell membranes, the so-called "HIS" bundles. Normally the pulse issues from a small cell cluster known as the sinus node and located in the upper portion of the right atrium at the entrance of vena cava superior. From the sinus node the depolarization propagates through the atriums.

The depolarization means a change in the electrical potential difference between the inner and outer muscle walls, a phenomenon which may be compared electrically to an increasing double layer charge. The total effect of this change of potential in all atrium cells is shown in the electrocardiographic recordings on the surface of the patient's body as a small peak known as the P peak of a duration of about 0.1 second and a value of about 0.2 mV.

The peak thus corresponds to the contraction of the atriums. From the atriums the depolarization is normally transmitted to the ventricles through a special cell cluster, the atrio-ventricular node, located in the partition wall immediately beside the valve between the right atrium and ventricle. The pulse transmission is delayed 0.07 second in the atrio-ventricular node and is then conducted through two branches in the cardiac partition wall to the base of the right and left ventricles. From here the depolarization of the ventricles is initiated and is recorded on the electrocardiogram in the form of the socalled QRS complex. The R and S peaks are the dominant characteristics of the complex and are of an order of about 1.0- 2.5 mV and of a duration of about 0.08-0.12 seconds. The QRS complex thus corresponds to the contraction of the ventricles.

During the subsequent relaxation repolarization occurs which will probably be reflected in the electrocardiogram by the T PEAK.

The proper pumping action of the heart is associated with the ventricle contractions. It is therefore of decisive significance that the contractions are started by the release of a QRS complex at appropriate time intervals, for instance about 60-80 times per minute. The release of the QRS complex, as explained above, is effected by transmission of pulses from the atriums. Disturbances in the transmission in the form of complete or partial blocking are known as atrio-ventricular blockage. In patients with complete blockage the ventricles may automatically continue to contract in that the muscle cells contract spontaneously, but at a frequency which is substantially lower than that produced by the sinus node. If the natural frequency of the ventricles drops, the patient will not be able to maintain a normal blood circulation. In cases of acute blockage it frequently occurs that the contractions of the ventricles do not start immediately. Such a condition is known as an ADAM-STOKES attack and is characterized by temporary unconsciousness. Complete failure of the natural rhythm of the ventricles is known as heart stoppage or asystoli and will cause certain death unless immediate action is taken. This may be done either by external heart massage or by artificial release of a QRS complex. In the latter case an electric pulse of short duration (about 1.8 msec. and 10 mA) is transmitted through the ventricles, and this is known as pacing the heart to contraction.

Heart failures of the type just described occur in spasms and without warning and, because there is no safe prophylactic medical treatment, research has led to the development of a pulse generator which can be applied externally or by surgical insertion to assist the heart permanently or temporarily. Patients who receive an internal pulse generator are said to be undergoing permanent pacemaker treatment.

In Pacemaker treatment the electrodes can be applied in two different ways, viz. epicardially or endocardially. The epicardial method requires that the thorax be opened and the electrodes inserted into the heart muscle tissue. The Pacemaker to which the other end of the electrodes are connected is placed in a pocket just under the skin of the thorax or the abdomen.

Because of the great risk of complications arising from such an operation, the epicardial method has been almost completely abandoned in favor of the endocardial method which is carried out as follows. One of the electrodes known as the differential (normally the negative pole) is introduced through a neck vein and carried via the right atrium through the tricuspid valve arriving finally at the base of the right ventricle, where it becomes enmeshed in the papillary muscle. The electric pulse is a capacitive discharge between the electrodes. The value of the pulse necessary for triggering the QRS complex depends upon the type and placing of the electrodes. This value is known as the heart's actual pace threshold value and the "pace security" can be defined as the ratio between the actual pace impulse and the actual pace threshold value. At the commencement of treatment this value is normally between 3 and 5.

There are three known categories of pacemakers, divided according to their functioning:

1. Fixed-rate pacemakers

2. Demand pacemakers

3. Atrial-triggered pacemakers.

The fixed-rate Pacemaker emits continuous pulses of a preset frequency. Thus the patient's heart is constantly paced and is committed to a fixed rhythm. This is unsuitable for patients who need only occasional pacing. Moreover, it appears that a considerable number of patients (about 25 percent), after 6-12 months of Pacemaker treatment, regain their normal sinus rhythm. This causes fluctuating pacing, which presents a serious hazard to life due to ventricular fibrillation. The principle of the fixed-rate Pacemaker is that of an astable multivibrator, e.g. the Hook Circuit.

The demand Pacemaker emits pulses only when the natural rhythm of the heart falls below a preset frequency, i.e. the Pacemaker controls currently that the heart itself triggers QRS complexes of adequate frequency and only when the heart frequency has dropped below a certain value does it set in. In this manner the Pacemaker will not compete with the natural rhythm of the heart and pacing sets in only when required, e.g. during an ADAM-STOKES attack. The demand Pacemaker is formed in principle as a resettable astable multivibrator which is reset whenever a pulse trigger picks up an R peak. If no R peak is detected the multivibrator will work currently and emit pacing pulses of constant frequency. After each pacing pulse emitted the pulse trigger will be blocked for about 400 msec. In a construction of this type it is difficult to obtain a pulse trigger which is sufficiently immune to noise and which can be relied upon to pick up only the R peaks.

The atrial-triggered Pacemaker generates an impulse in synchronization with the atrium contractions. It is used only for patients suffering from total atrio-ventricular blockage and having a strong atrial sinus rhythm. If the sinus rhythm fails, the Pacemaker is reduced to a fixed-rate Pacemaker. This pacing enables adaptation of the pumping effect to requirement, but two intra-cardial electrodes are needed, one in the right atrium and one in the right ventricle. The insertion of these electrodes is difficult and atrial-triggered pacing therefore rarely applied.

One of the most serious drawbacks of the known pacing technique is that it is not possible to ascertain the effectiveness of the pacing by a periodical examination without surgical operation; it can only be ascertained that the Pacemaker's ability to trigger pulses is intact. Nothing can be learned about the trigger effectiveness since it depends also on the threshold value of the heart, which cannot be ascertained without surgical operation. It has been found that the threshold value tends to increase during continued triggering and simultaneously the trigger ability of the Pacemaker will decline as a result of battery discharge. Unreliable pacing will therefore occur earlier than foreseen where the Pacemaker's trigger ability is associated with the threshold value ascertained at the implantation.

This invention relates to a pacemaker comprising a pulse generator for producing heart stimulating pacing pulses of substantially fixed frequency and a detector adapted to detect the heart pulses that trigger muscular contractions of the heart and to activate the pulse generator to produce a pace impulse on the absence of a heart pulse for a predetermined period, and it is the aim of the invention to provide a Pacemaker of this type which enables current determination of the actual threshold value of the heart and at the same time ensures effective pacing security.

This aim has been accomplished by providing the Pacemaker with means for gradual reduction of the amplitude of selected pacing pulses and means for producing a new pacing pulse of predetermined initial amplitude after the absence of a heart pulse for a certain period after the provision of a reduced pace impulse, the smallest pacing pulse emitted by this Pacemaker being a measure of the threshold value of the heart.

Where it is considered expedient the Pacemaker may be adapted to generate pacing pulses of full amplitude, i.e. the initial amplitude, in between the selected pacing pulses, but the simplest construction is obtained by adapting the amplitude reducing means to reduce the amplitude of each pacing pulse by a fixed or relative value with respect to the preceding pacing pulse.

The QRS complex normally occurs from 30 to 40 msec. after the triggering pacing pulse on account of physiological delay, and the Pacemaker's inbuilt time limit may suitably be about 100 msec. As long as the pacing pulse triggers the QRS complex within this period of time the following pacing pulse will be reduced. But if a QRS complex is not detected within the 100 m.sec. period a pacing pulse of full amplitude will be generated at the end thereof, for instance of a value five times that of the actual threshold value.

By providing the Pacemaker with an output circuit comprising a condenser connected in series with an electronic switch and a charging circuit for charging the condenser to a controllable potential, it will be possible in a simple manner to generate relatively high energy pacing pulses by means of weak outer power sources and at the same time to control the pulse amplitude. This control may be accomplished by providing a dosage circuit comprising a dosage condenser the voltage of which controls the potential to which the output condenser is charged and which is connected to a charging circuit and a discharging circuit.

In an embodiment of the invention the pulse generator comprises a resetable astable multivibrator which after each completed cycle activates a monostable multivibrator to produce a trigger pulse the leading edge of which activates a second monostable multivibrator to control the electronic switch of the output circuit and the trailing edge of which in the absence of a detected heart pulse activates a restoration circuit which adjusts the dosage circuit to full pacing pulse amplitude, and here the said inbuilt time limit is equal to the length of the trigger pulse.

Simple means for controlling the voltage over the dosage condenser which determines the magnitude of the pacing pulse are provided by inserting in the charging circuit of the dosage circuit a circuit breaker connected in series with the dosage condenser and controlled by the trigger pulses and connecting the discharging circuit of the dosage circuit in parallel with the dosage condenser and inserting therein an electronic circuit breaker which is controlled by the restoration circuit.

All the specified members of the said Pacemaker may be combined into a single implantable device, but the pacemaker may also be divided into an implantation device which is able to work independently as a conventional demand Pacemaker with fixed pulse amplitude and a device kept at the place where the patient undergoes treatment and connected to the implanted device by non-galvanic, for instance magnetic or inductive means to provide a gradual reduction of selected pacing pulses until the threshold value is reached and then restore full impulse amplitude.

The invention will be explained in greater detail here with reference to the drawing, in which

FIG. 1 shows typical electrocardiographic signals,

FIG. 2 shows graphs illustrating the functioning of the Pacemaker according to the invention,

FIG. 3 is a block diagram of an embodiment of the Pacemaker according to the invention,

FIG. 4 is a diagram showing certain circuit details of the Pacemaker illustrated in FIG. 3, and

FIG. 5 is a corresponding diagram of a modified embodiment divided into two devices, only one of which is intended for implantation.

FIG. 1 shows the peaks or pulse elements P,Q,R,S and T which are characteristic of electro-cardiographic registered signals. The time duration is given along the abscissa and the amplitude along the ordinate. The scales of these graph components are also shown. Both the absolute and relative values of the different peaks as well as their time durations and positionings may vary considerably from patient to patient and it is the existence of these differences that makes electro-cardiography such an important diagnostic tool.

The Pacemaker illustrated in FIG. 3 has two input leads 1 for receiving heart pulses and two output leads 2 for emitting pacing pulses. The two pairs of leads may, if desired, be connected to one another as shown by the dotted line and thus be connected to a common set of electrodes to be inserted in the patient. The input leads 1 are connected, through a blocking circuit B that serves to block the receiving member when pacing pulses are emitted, to an amplified F that amplifies the received signals and transmits them to a detector D which is adapted to detect the QRS complexes and on each such detection to generate an output pulse that triggers a 250 msec. monostable multivibrator MV1. The most characteristic feature of the QRS complex being the inclination of the line RS the detector may expediently be adapted in conventional manner to sense this inclination. By allowing the multivibrator MV1 the relatively long pulse period of 250 msec. the risk that after being activated by a QRS complex it will be activated also by the subsequent T peak has been obviated. It will be seen that the already described portion of the Pacemaker will cause the generation of a square pulse of a length of 250 msec. on an output lead 3 from the multivibrator MV1 whenever a QRS complex is received. This impulse will be referred to in the following as the starting pulse.

The starting pulses are transmitted to the reset entrance 4 of a resetable saw-tooth oscillator with an adjustable frequency of about 50-90 pulses per minute, but which is forced to restart its working cycle whenever it receives a reset or starting pulse. The oscillator 0 will thus be allowed to run through a whole working cycle only when the pulse rhythm of the heart becomes slower than the natural rhythm of the oscillator.

After each completed cycle the oscillator 0 activates a monostable multivibrator MV2 which at its exit 5 produces a square pulse of a duration of 100 msec. This pulse is called a trigger pulse because it is responsible for initiating a pacing pulse by activating with its leading edge another monostable multivibrator MV3 which has a pulse time of 1.8 msec. and via lead 6 activates an output circuit U to emit an output pacing pulse to the output leads 2. The square pulses from the multivibrator MV3 are here called control pulses because they control the emission of pacing pulses. The control pulses are also conducted over a lead 7 to the blocking circuit B whereby the said blocking of the receiving member during the emission of pacing pulses is established.

The trigger pulses from the multivibrator MV2 are also conducted over a lead 8 to a dosage circuit C which is adapted on receipt of a trigger pulse to activate the output circuit U over the lead 9 so that the amplitude of the next pacing pulse is reduced by a predetermined absolute or relative value.

Moreover the trigger pulses are transmitted over a lead 10 to a restoration circuit R which is adapted to be activated by the trailing edge of the said pulses and by the starting pulses transmitted through a lead 11 to generate a restoration pulse on an output lead 12 when and only when the trailing edge of the trigger pulse occurs in the absence of a starting pulse, that is when no heart pulse has been received within 100 msec. from the generation of a pacing pulse. The restoration pulse is transmitted to the dosage circuit C, which immediately adjusts the output circuit U so that the next pacing pulse emitted will be of full amplitude, that is a predetermined initial value. A slightly delayed version of the restoration pulse is transmitted through a lead 13 to the multivibrator MV3 and acts as an extra trigger pulse causing the generation of a new pacing pulse of full amplitude 100 msec. after the emission of an ineffective trigger pulse.

In the embodiment of the Pacemaker illustrated here the trigger pulses are fed back over a lead 14 to the input of the saw tooth oscillator 0, which is thereby prevented from starting a new working cycle before the end of the trigger pulse.

FIG. 4 shows practically expedient embodiments of the restoration circuit R, the dosage circuit C and the output circuit U. These circuits are provided with electronic circuit breakers which are illustrated here as transistors T1-T4 but could also be other types of static electrical change-over switches.

The output circuit U contains a condenser C4 inserted in series with the output leads 2 and the transistor T4 which is made by conducting 1.8 msec. control pulses from the multivibrator MV3 via lead 6 and thus causes discharge of condenser C4 through the electrode circuit 2. The pacing pulse size is thus equivalent to the voltage over the condenser C4 at the moment the transistor T4 is switched on. The condenser voltage is controlled by the dosage circuit C over a voltage follower field-effect transistor T5 which determines the potential to which C4 is recharged after each pacing pulse. This potential depends on the voltage over condenser C3 in the dosage circuit, which in turn is controlled by the two transistors T2 and T3.

Every 100 msec. trigger pulse from the multivibrator MV2 makes transistor T3 conductive and effect changing of condenser C3 so that the voltage in the point P connected with the lead 9 drops, resulting in a reduction of the potential to which the output circuit condenser C4 is recharged. In this manner every pacing pulse is reduced by a certain value relatively to the preceding pacing pulse.

A precondition for the new reduced value of the next pacing pulse is the detection of a heart pulse before the trailing edge of a 100 msec. trigger pulse appears. When this happens the 250 msec. multivibrator MV1 will be activated and transmit the necessary basic current for maintaining the transistor T2 of the restoration circuit R in a saturated condition. If no heart pulse is detected, the trailing edge of the 100 msec. trigger pulse will, via the condenser C1, block the transistor T1 for a comparatively short interval of time determined by the magnitude of the condenser C1 and of the resistors R1, R2 and R3. At the moment the transistor T1 is blocked the transistor T2 of the dosage circuit C is made conductive and short circuits condenser C3 which is thus discharged. Thus the voltage at the point P rises and pulls up transistor T5's source electrode voltage so that the output circuit condenser C4 is recharged to the starting value. When the transistor T1 of the restoration circuit is again reset the condenser C4 has been recharged and a pacing pulse can be produced. This is effected by the 1.8 msec. multivibrator MV3 being activated by the condenser C2 when the transistor T1 is short-circuited.

FIG. 2A shows some of the pacing pulses produced by the Pacemaker described above when the heart's threshold value is e.g. 1.2 mA. The starting pulse is set to five times 1.2 mA., i.e. 6 mA., and for every pacing pulse the amplitude is reduced by 5 percent of 1.2 mA., i.e. by 0.06 mA., so that the threshold value is determined with an accuracy of 5 percent. With the said pulse reduction the threshold value is reached after (6.0-1.2)/0.06 = 80 pulses. The threshold value can thus be registered at any time as the lowest pulse amplitude, and the pacing security can be calculated as the ratio of this amplitude to the starting amplitude.

FIG. 2B shows besides the pacing pulses, also the heart pulses triggered by the pacing pulses. FIG. 2C shows on a greater time scale a smaller section of the pacing pulses and the pertaining 100 msec. triggering pulses.

The embodiment of the Pacemaker according to the invention illustrated in FIG. 5 is divided into two separate sections, of which only the section located to the right of the line I--I is intended for implantation while the section located to the left of this line is to be kept at the place where the patient is treated, for instance at the hospital where the patient is subjected to regular control and, if necessary, treatment. This Pacemaker comprises similar circuits R, C, U and MV3 as indicated in FIG. 4, and in FIG. 5 the designations are the same as used in FIG. 4 for identical parts.

In the Pacemaker shown in FIG. 5 the monostable multivibrator MV3 is activated to emit 1.8 msec. control pulses by a circuit DPM, which may be the detector and pulse generator of a conventional demand Pacemaker. In the dosage circuit C the condenser C3 is connected in series with a resistor R4 in lieu of the pulse-controlled field-effect transistor T3, and in the restoration circuit R the base of the transistor T1 is connected over the resistor R3 to the positive pole of the battery through a normally open reed contact RE1 which can be activated from without by a relay coil RE when the latter is magnetized by a device A which together with the relay coil forms the permanent equipment and which comprises the same chain of circuits F, D and MV1 as shown in FIG. 3 and explained in the foregoing.

When the contact RE 1 is open, as shown in the drawing, the transistor T1 will be blocked and the transistor T2 in the dosage circuit C will be connected. Consequently the dosage condenser C3 will be short-circuited and the point P will continually be at maximum potential so that all the pacing pulses emitted by the output circuit U will have full amplitude. Under these conditions the Pacemaker will therefore work as an ordinary demand Pacemaker.

When the implanted section of the Pacemaker is connected at the place of treatment to the permanent equipment by placing the relay coil RE opposite the reed contact RE1 and connecting the contact by magnetization of the coil by means of the device A, the transistor T1 is made conductive while the transistor T2 is blocked. The charging of the dosage condenser C3 is now commenced through the resistor R4 whereby the potential in the point P is gradually reduced with the result that the pacing pulses emitted by the output circuit U will get a correspondingly decaying amplitude. When the amplitude reaches a value that is insufficient to effect heart contraction it is registered in conventional manner by the device A, which may for instance include a cardiograph and which is further adapted on failing activation within a certain period of time of the multivibrator corresponding to MV1 in FIG. 3 to cause demagnetization of the relay coil RE over such a period, for instance 20 msec., that the dosage condenser C3 will be discharged through the transistor T2, which has been made conductive by the connection of the reed contact RE 1, and the output condenser C4 can be charged to full potential. When the transistor T1 after the said period is made conductive again by connection of the contact RE1 an activating pulse will be transmitted, as in the embodiment first described, to the control pulse multivibrator MV3 over the condenser C2 and the lead 13, whereby a pacing pulse of full amplitude will be generated. After this restoration the pacing pulse amplitude will again decay in step with the charging of the dosage condenser C3.

The connection of the implanted section of the Pacemaker to the permanent equipment may also be provided in other ways than by means of a reed relay as shown in FIG. 5, for instance by means of a tuned high frequency transformer whose secondary circuit in the implanted section is connected through a rectifier to an RC combination whose voltage controls the transistor T1.

Other structural details of the Pacemaker according to the invention may likewise be adapted and arranged in other ways than illustrated in the drawing and described in the foregoing. For instance may other types of time metering circuits than monostable multivibrators be employed, and the functions performed by the circuits shown in FIGS. 4 and 5 may be performed by other types of logical circuits.

Claims

What I claim is:

1. A Pacemaker comprising a pair of electrodes adapted to be inserted in or near the heart of a patient, a pulse generator for producing heart stimulating pacing pulses of substantially fixed frequency, which pulses are applied to said electrodes, and a detector connected to the electrodes and to the pulse generator and adapted to detect the heart pulses that trigger muscular contractions of the heart and to control the pulse generator to produce a pacing pulse on the absence of a heart pulse for a predetermined period, characterized in that means are provided in operative relationship for gradual reduction of the amplitude of selected pacing pulses and means are operatively connected for producing predetermined initial amplitude, after the absence of a heart pulse for a certain period after the provision of a reduced pacing pulse.

2. A Pacemaker according to claim 1, characterized in that the amplitude reducing means reduces the amplitude of each pacing pulse by a fixed or relative value with respect to the preceding pacing pulse.

3. A Pacemaker according to claim 1, further comprising an output circuit connected to said electrodes and containing a condenser, an electronic switch connected in series with said condenser, and a charging circuit operatively connected for charging the condenser to a controllable potential.

4. A Pacemaker according to claim 3, characterized in that the amplitude reducing means include a dosage circuit containing a dosage condenser operatively connected so that the voltage of the dosage condenser controls the potential to which the output condenser is charged, and a charging circuit and a discharging circuit connected in circuit relationship with the dosage condenser.

5. A Pacemaker according to claim 4 and in which the pulse generator comprises a resetable astable multivibrator, characterized in that it comprises a first monostable multivibrator which is activated by said astable multivibrator after each completed cycle thereof to produce a trigger pulse, a second monostable multivibrator in operative circuit relationship with said first monostable multivibrator to be activated by the leading edge of said trigger pulse and to control the electronic switch of the output circuit, and wherein said amplitude producing means include a restoration circuit in operative circuit relationship with said first monostable multivibrator to be activated by the trailing edge of said trigger pulse in the absence of a detected heart pulse to adjust the dosage circuit to full pace pulse amplitude.

6. A Pacemaker according to claim 5, characterized in that the charging circuit of the dosage circuit comprises an electronic switch connected in series with the dosage condenser and further connected to said second monostable multivibrator so as to be controlled by the trigger pulses.

7. A Pacemaker according to claim 5, characterized in that the discharging circuit of the dosage circuit is connected in parallel with the dosage condenser and includes an electronic switch operatively connected to the restoration circuit as to be controlled thereby.