GB1594902A

GB1594902A - Cardiac pacemakers

Info

Publication number: GB1594902A
Application number: GB1277/78A
Authority: GB
Original assignee: Medtronic BV
Current assignee: Medtronic BV
Priority date: 1977-01-12
Filing date: 1978-01-12
Publication date: 1981-08-05
Also published as: NL7806850A; FR2377190A1; DE2701104A1; NL177080B; JPS5389292A; AU512855B2; JPS5650591B2; FR2377190B1; AU3237078A; DE2701104C2; SE7800351L; CA1098587A; NL177080C

Description

(54) IMPROVEMENTS IN CARDIAC PACEMAKERS (71) We, MEDTRONIC B.V.-KER KRADE, a company organised under the laws of Holland, of Wenckebachstraat 10, Kerkrade-West, Holland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a cardiac pacemaker, in particular to a pacemaker comprising means for detecting and, if required, stimulating ventricular action, and means for detecting and, if required, stimulating atrial action.

In the case of known pacemakers of the above type (e.g. US-Patent 3,595,242) the pacemaker monitors the ventricular endocardial ECG and as a function thereof programs both the atrial and the ventricular stimulation. A predetermined first time interval after the last QRS-complex an atrium stimulation, if required, is triggered, whereas at the end of the second predetermined time interval after the last QRS-complex the next ventricular stimulation is triggered. Accordingly both time periods for atrial and ventricular stimulation are defined by a common reference time, namely the last preceding ventricular action.

Pacemakers of the type outlined above are satisfactory to a limited extent only. In the case of a total AV-block e.g. the atrium and the ventricle are stimulated. In addition the pacemaker does not follow if the atrial rate increases.

Furthermore atrium controlled pacemakers are known in which the atrial depolarization is detected by an electrode positioned within or on the atrium and in which after a predetermined interval of e.g. 150 msec. a stimulating pulse is supplied to the ventricle.

However, such pacemakers stimulate the ventricle only, whereas there is no atrial stimulation. On the other hand pacemakers are known which stimulate the atrium only because in certain types of diseases the atrium only beats slowly and stimulating of the atrium is sufficient to provide for a properly high ventricular rate. Such atrial pacemakers are to be haemadynamically preferred with respect to ventricular pacemakers because the atrial rhythm is maintained in the case of an intact AV-conduction and because the risk of embolism is substantially reduced. However, such an atrial stimulation is not applicable in the case of unreliable AV-conduction.

Finally a so-called synchronized-demand pacemaker is known (The Journal of Thoracic and Cardiovascular Surgery, Volume 61, No. 3) which constitutes a modification of the aforementioned atrium controlled ventricular pacemaker. In the case of this pacemaker ventricular demanding is effected such that ventricular depolarizations occuring within a predetermined time slot cause the suppression of a ventricular stimulation pulse which normally would occur. However, such a pacemaker does not allow to stimulate within the atrium in the case of atrial bradycardia. In view of the fact that this known pacemaker is an external device the output of the stimulating pulse generator may be connected to the atrial electrode rather than to the ventricular electrode.

However, in such a case the atrium only and not the ventricle will be stimulated. Thereby an operation is obtained which is unsuitable for different types of diseases, particularly AV-conduction disturbances. Accordingly the utility of this pacemaker likewise is limited.

The heart activities within the atrium and the ventricle basically may be distinguished as to bradycardia and normal function in accordance with the following system: Atrium and ventricle beat at a sufficient rate (Ist quadrant); the atrium is beating at an insufficient rate and must be stimulated whereas the ventricle properly follows (2nd quadrant); the atrium functions sufficiently, however, the ventricle does not follow (3rd quadrant); both the atrium and ventricle require stimulation (4th quadrant). None of the prior pacemakers may be successfully used in all four bradycardia quadrants.

The present invention provides a cardiac pacemaker including means for detecting ventricular activity, means for detecting atrial activity, means for stimulating ventricular activity and means for stimulating atrial activity, the arrangement being such that said means for stimulating ventricular activity is operative to provide a ventricular stimulation pulse unless spontaneous ventricular activity occurs within a first predetermined time interval after spontaneous or stimulated atrial activity, and the arrangement being such that said means for stimulating atrial activity is operative to provide an atrial stimulation unless spontaneous atrial activity occurs within a second predetermined time interval after spontaneous or stimulated ventricular activity.

A feature of the invention disclosed is to provide a pacemaker which, if required, may stimulate the ventricle and which is able to entrain the ventricle when the atrial rate increases. Accordingly the pacemaker should be able to provide for a correct function within all four aforementioned bradycardia quadrants.

The solution of the invention is based on the concept of completely electronically simulating the normally occurring heart cycle exteriorly of the heart and to force this external control cycle on the heart in the case of physiological disturbances which indicate the need for pacing within the atrium or the ventricle, wherein the heart, to the extent it functions properly, is able to suppress each individual partial function of the pacemaker.

Accordingly the pacemaker may stimulate in the atrium only or in the ventricle only; it may also stimulate both within the atrium and the ventricle. If required an increased atrial rate is conducted to the ventricle. If required, i.e. in the case of all functions of the heart being undisturbed, neither the atrium nor the ventricle are stimulated.

Preferably the means for stimulating ventricular and atrial activity are interconnected so that the end of said first predetermined time interval initiates the said second predetermined time interval and that the end of said second predetermined time interval initiates the said first predetermined time interval. Accordingly a stimulating pulse is delivered to the ventricle if no ventricular contraction follows a spontaneous P-wave or a provoked atrial contraction within the predetermined first time interval. This pulse does not occur in the case of timely spontaneous ventricular activity. An atrial stimulating signal follows at the end of the second predetermined time interval after a ventricular pulse, irrespective of this ventricular pulse occuring spontaneously or being caused by pacemaker depolarization, unless a spontaneous atrial activity has been detected in due time.

Advantageously the means for stimulating ventricular activity and the means for stimulating atrial activity each include a monostable multivibrator which determines said first and said second predetermined time interval, respectively.

Preferably the first and the second predetermined time intervals are between 120 msec.

and 200 msec. and between 600 msec. and 750 msec., respectively.

Preferably unipolar electrodes are provided for detecting and stimulating the atrial and the ventricular activity, respectively, and the pacemaker casing constitutes the common neutral electrode.

According to a further development of the invention the means for stimulating atrial activity include an output capacitor arranged to be charged through first switch means and to be discharged through second switch means. This provides, after the output capacitor having been quickly charged, for a high input impedance of the atrial activity detecting means and accordingly for an improved P-detection, If contrary thereto the circuit means for stimulating ventricular activity are designed in a conventional manner, a safe distinction between atrial and ventricular pulses in the ECG is possible. In this connection a particularly simple circuit arrangement is obtained in that an atrial stimulation triggering signal is arranged to control the second switch means and to control the first switch means via time delay means. Thereby the same signal first provides for the immediate triggering of the second discharging, switch means and then for the correspondingly delayed subsequent actuation of the first charging, switch means.

Preferably the means for detecting ventricular activity have associated therewith disabling means adapted to disable detection of ventricular activity for a predetermined time period when atrial stimulation is triggered.

For the purpose of testing suitably the first and/or the second time interval may be shortened.

In conformity with a further development of the invention the means for detecting atrial activity and/or the means for detecting ventricular activity comprise refractory means for blocking the detection of signals the repetition frequency of which exceeds a predetermined maximum value. When such high-frequency noise signals are received, the circuit means for atrial and/or ventricular detection are disabled; the pacemaker will operate at fixed frequencies.

Two preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings wherein: Fig. 1 illustrates a schematic circuit diagram of a first embodiment, and Fig. 2 illustrates a schematic circuit diagram of a second embodiment.

In the embodiment of Fig. 1 an electrode 10 is provided for supplying stimulating signals to the atrium and for detecting natural atrial activity. In a similar manner an electrode 12 is connected to the ventricle to supply stimulating pulses thereto and for detecting naturally occuring ventricle pulses.

The atrium electrode 10 is connected to a first input 18 of a NOR-gate 19 through an atrium signal amplifier 14, a positive edge retriggering monoflop 15 provided for noise suppression and having a delay period of 100 msec., as well as through a "differentiator consisting of a capacitor 16 and a resistor 17.

The ventricle electrode 12 communicates with a first input 23 of a NOR-gate 24 through a ventricle signal amplifier 20 and an inverter 22. The output of NOR-gate 24 is connected to a first input 29 of a NOR-gate 30 through a positive edge retriggering 100 msec. monoflop 26 and a differentiator consisting of a capacitor 27 and a resistor 28.

Monoflop 26 is provided for noise suppression purposes.

The output of NOR-gate 19 is connected to the input of a negative edge triggering 400 msec. monoflop 32 which defines a refractory period. The output of monoflop 32 is connected to the input of a negative edge triggering monoflop 34 having a delay period of 150 msec. Monoflop 34 defines the PQtime interval. Its output Q is connected through a differentiator formed by a capacitor 35 and a resistor 36, to a first input 37 of a NOR-gate 38. The output of NOR-gate 38 is connected to a second input 39 of NOR-gate 30.

The output of NOR-gate 30 is connected to the input of a negative edge triggering 300 msec. monoflop 42 which defines another refractory period. The output Q of monoflop 42 is connected through a differentiator comprising a capacitor 43 and a resistor 44 to a first input of a NOR-gate 46. The output Q of monoflop 42 is further connected through a lead 47 to a second input 49 of NOR-gate 38. A second input 50 of NOR-gate 46 is connected through a differentiator consisting of a capacitor 52 and a resistor 53 to a second output 54 of monoflop 32. The output of NOR-gate 46 is connected to the input of a negative edge retriggering monoflop 56 which has a delay period of 700 ms and defines the QP-time interval. The output Q of monoflop 56 is connected through a lead 57 to a second input 58 of NOR-gate 19 and to the input of a positive edge triggering monoflop 60. Monoflop 60 has a delay period of 0.5 msec. and serves as a pulse shaper. The output Q of monoflop 60 is connected to the base of a PNP transistor 62 through the series connection of a resistor 63 and a capacitor 64. The base of transistor 62 is further connected through a diode 65 to the positive terminal of the voltage supply. In addition output Q of monoflop 60 is connected to the base of a NPN transistor 68 through a resistor 67. The emitter-collector paths of transistors 62 and 68 are connected in series between positive voltage and ground. The junction 69 of the collectors of transistors 62 and 68 is connected to one side of an output capacitor 70 the other side of which communicates with atrium electrode 10.

The input of a positive edge triggering pulse shaper monoflop 72 having a delay period of 0.5 msec. is connected to the output of NOR-gate 38. The output Q of monoflop 72 is connected via a resistor 73 to the base of an output transistor 74 the collector-emitter path of which is connected in series with a resistor 75 between the positive supply voltage and ground. The collector of transistor 74 is connected to one side of an output capacitor 76, the other side of which communicates with ventricle electrode 12.

The output Q of monoflop 56 is additionally connected to the input of a positive edge triggering 40 msec. monoflop 78. The output Q of monoflop 78 is connected to a second input 79 of NOR-gate 24.

The circuit arrangement of Fig. 1 basically operates as follows: At the input of monoflop 32 information in the form of a pulse step appears when, through input 58 of NOR-gate 19, the information that the atrium has been stimulated is supplied, or when, through input 18 of gate 19, information has been received that a spontaneous atrium activity has taken place. Accordingly a pulse step triggering the 400 msec. period always appears at the input of monoflop 32 when the atrium is depolarized by a stimulating pulse or when there is a spontaneous excitation of the atrium. Simultaneously with setting of monoflop 32 the input of monoflop 34 is triggered from the output Q of monoflop 32 thereby starting the 150 msec. period of monoflop 34. After the 150 msec. period of monoflop 34 has elapsed a pulse is delivered through the differentiator 35, 36, and input 37 of gate 38 to the output of this gate. This input pulse is supplied through the 0.5 msec. pulse shaper monoflop 72 to the conventionaly designed ventricle stimulation output stage 73 to 76 and then to the ventricle through electrode 12.

The pulse appearing at the output of gate 38 is also supplied to the input 39 of gate 30 and is passed to the input of monoflop 42.

The pulse thus starts the refractory period of 300 msec. Furthermore this pulse is supplied to the input 45 of gate 46 and to the input of monoflop 56 which is triggered. The 700 msec. delay period of monoflop 56 is started.

Similar to the conditions at the input of monoflop 32 an information appears at the input of monoflop 42 when a ventricle pulse has been delivered from the output of gate 38 or when an information characteristic of a spontaneous depolarization of the ventricle is received at gate 30. Accordingly it does not make any difference for the triggering of monoflop 42 whether the ventricle is stimulated or has been spontaneously activated.

By the appearence of a pulse step at the input of monoflop 42 the retriggerable monoflop 56 is again started. Therefore 700 msec. must elapse after a ventricle depolarization, irrespective whether it occured spontaneously or was caused by stimulation, before the output Q of monoflop 56 goes high and an atrium pulse is delivered through lead 57.

If the atrial activity spontaneously becomes faster than the rate corresponding to the intervention frequency of the pacemaker, the P-wave upon having been amplified in the atrium signal amplifier 14 again enters monoflop 32 through input 18 of gate 19. The respective pulse is delivered from the output 54 of monoflop 32 to the dynamic input 50 of gate 46. The pulse is transferred from gate 46 to monoflop 56. Accordingly at first no further atrium pulse can be delivered after a spontaneous atrium pulse for a period of at least 700 msec (the delay period of monoflop 56). Because this spontaneous atrium pulse again activates the ventricle through mo nofiop 34 after 150 msec. and the monoflop 56 is retriggered through gates 38 and 30 and monoflop 42, the atrium in fact is not again triggered before a period of 700 msec. + 150 msec. = 850 msec. has elapsed after a spontaneous P-wave.

An atrium pulse should result in a ventricle depolarization after 150 msec., i.e. the delay period of monoflop 34. However, it might be that this ventricle depolarization spontaneously occurs within the desired period. In this case the ventricle depolarizing pulse has no physiological function. Therefore it is desired to suppress this pulse in order to save energy and for other reasons.

For this purpose feedback is provided from the output of monoflop 42 through lead 47 to the input 49 of gate 38. The function of this feedback is as follows: If the 150 msec. pulse of monoflop 34 has nearly reached its end and in case a ventricle pulse therefore would be delivered to the ventricle through the output of gate 38, this is prevented by the ventricle activity having at this time again triggered the 300 msec. delay period at the input of monoflop 42. The potential appearing at the output Q of monoflop 42 now disables gate 38 through lead 47. The pulse step appearing after the 150 msec. delay period of monoflop 34 therefore cannot be transferred to the ventricle through gate 38.

Accordingly the unnecessary ventricle depolarization pulse does not occur if there was a spontaneous ventricle depolarization within the 150 msec. delay period.

In the following the mode of operation of the circuit arrangement of Fig. 1 in the four different bradycardia quadrants is described.

At first it is assumed that the atrium as well as the ventricle beat at a sufficiently high rate and that therefore stimulation is not required (1sot quadrant). The spontaneous P-signal received by the atrium electrode 10 is supplied to the input of monoflop 32 and, through the output 54 of monoflop 32 and the input 50 of gate 46, suppresses the delivery of a P-information through the output Q of monoflop 56, because monoflop 56 is retriggered. The timely occuring ventricle action in turn suppresses the delivery of a pulse to the ventricle by setting monoflop 42 through lead 47 and input 49 of gate 38.

Accordingly the atrium and the ventricle are not stimulated when the atrium activity occurs at a sufficiently high rate and when the venrricle responds in due time.

In the case of atrium bradycardia in which the atrium must be stimulated whereas the conduction to the ventricle is intact (2nd quadrant), the information that an atrium depolarization is to be effected by a pulse, is supplied to the input of monoflop 60 through the output Q of monoflop 56 and lead 57 after termination of the 700 msec. delay period of monoflop 56 because in the meantime monoflop 56 is not retriggered. The 150 msec. delay period of monoflop 34 is started.

Before this 150 msec. period has elapsed a spontaneous ventricle polarization occurs which is delivered to monoflop 42 through ventricle signal amplifier 20, gate 24, monoflop 26 and gate 30. Monoflop 42 is triggered and disables through lead 47 the transfer of the ventricle stimulating pulse from output Q of monoflop 34 to monoflop 72. Thus the ventricle pulse is suppressed in retrograde.

If on the other hand the atrium beats at a sufficient and possibly substantially higher than normal rate up to a frequency of 150 beats min., whereas the conduction to the ventricle is poor (3rd quadrant), the following happens: Spontaneous P-signals are delivered from the atrium electrode 10 to the input of monoflop 32 and start the 400 msec.

refractory period defined by this monoflop.

Simultaneously monoflop 34 is triggered.

After the 150 msec. delay period of monoflop 34 the ventricle depolarization pulse is delivered through gate 38 and monoflop 72 to the ventricle output stage 73 to 76. When using the circuit parameters indicated for the embodiment of Fig. 1 the atrium rate may vary between 70 and 150 beats/min. (the latter corresponding to the 400 msec. refractory period of monoflop 32). The ventricle is entrained at the interval of the delay period of monfiop 34 (150 msec.). There is no atrium stimulation because monoflop 56 is retriggered after each spontaneous atrium pulse so that at first 700 msec. must elapse, and because after depolarization of the ventricle monoflop 56 is retriggered through the out put of gate 30, monoflop 42 and the input 45 of gate 46. Accordingly an atrium pulse could not be delivered before 850 msec.

(corresponding to the sum of the delay periods of monoflops 34 and 56) have elapsed. However, the atrium stimulation does not take place because the atrium has spontaneously functioned already before this time.

Finally, the case is considered that the atrium and the ventricle do not beat and therefore both must be stimulated (4th quadrant). Atrium stimulation now is effected in the manner outlined above. Because the ventricle does not follow within the desired 150 msec. period and because the 300-msec.

delay period of monoflop 42 is not started through ventricle signal amplifier 20, gate 24 and monoflop 26 retrograde suppression of the pulse transfer through gate 38 does not take place. The pulse step occuring at the output of the monoflop 34 after the end of the 150 msec. period appears at the input 37 of gate 38 and is delivered from the output of gate 38 through monoflop 72 to output stage 73 to 76. The ventricle is depolarized through ventricle electrode 12.

In order to provide for a safe detection of the ventricle complex following an atrium pulse, the 40 msec. monoflop 78 is triggered by the atrium stimulating pulse appearing at output Q of monoflop 56. This causes the transfer of the pulses sensed within the ventricle to be disabled through the input 79 of gate 24 during the delay period of monoflop 78. Atrium pulses received by the ventricle signal amplifier are prevented by this time slot from being effective in the ventricular circuit arrangement.

In the ventricular circuit arrangement the ventricle pulse is generated by quickly discharging the output capacitor 76 and slowly recharging this capacitor through resistor 75, whereas the dynamically connected transistor 62 is provided for recharging the atrium output capacitor 70. Recharging is effected through this transistor at a very low resistance and thus is terminated quickly. This ensures that less distortions occur in the ECG, that the atrium pulse may be easily distinguished in the ECG from the ventricle pulse and that upon the capacitor 70 being recharged the atrium signal amplifier has a substantially increased input impedance whereby P-sensing is further improved.

It is evident that output capacitor 76 likewise may be recharged through a transistor corresponding to transistor 62 in a low impedance manner. However, in view of the fact that the ventricle signal amplifier 20 operates in a reliable manner without any specific precautions being required, it normally will be preferable to provide for the described different design of the output stages to safely differentiate in the ECG the atrium pulse from the ventricle pulse to thereby facilitate the postoperative malfunction diagnosis.

In practice it is desirable to likewise check the pacemaker functions in cases in which the patient's natural rhythm is proper and the pacemaker is fully suppressed. This e.g.

may be done by disabling the input amplifiers 14 and 20 through a magnetic switch and by waiting until the stimulating pulses coincide with stimulatible atrium and ventricle phases. Then it can be determined whether the atrium pulse provokes a P-wave and whether the ventricle pulse provokes a QRScomplex. As an alternative the PQ-delay period, which in the described embodiment amounts to 150 msec., may be drastically shortened to e.g. 50 msec. in order to bypass an inact conduction and to electrically stimulate the ventricle if the pacemaker is in proper condition. Simultaneously the delay period of monoflop 56 e.g. is shortened from 700 msec. to 450 msec. so that the sum of the delay periods of monoflops 34 and 56 is 500 msec. corresponding to a frequency of 120 beats/min. In this manner the heart is certainly overridden; the stimulation may be properly checked.

A circuit arrangement which basically corresponds to the circuit arrangement of Fig. 1 may be designed when using integrated circuits utilized for available demand pacemakers. Such an embodiment is illustrated in Fig. 2. It comprises a pair of integrated circuits 84 and 86. Each of circuits 84, 86 includes a pulse generator having a timer adapted to be reset by spontaneous ventricle or atrium signals, respectively (corresponding to monoflop 56) as well as pulse shaping and output units (corresponding to monoflops 60 and 72 as well as to output stages 62 to 70 or 73 to 76, respectively). The refractory members 32 and 42, respectively, the noise suppression members 15 and 26, respectively, as well as the amplifiers 14 and 20, respectively are combined into integrated circuits 88 and 90, respectively.

The integrated circuits 84 and 86 are connected in series a first output 91 tapped before the output unit of circuit 84 being connected through a diode 92 and a capacitor 93 to the input of integrated circuit 86. A first output 95 of integrated circuit 86 which corresponds to output 91 is connected through a lead 96 and a diode 97 to an ONinput 98 of a storage member defined by a pair of mutually coupled NOR-gates 99, 100, the output of this storage member being connected to the input of integrated circuit 84.

Input 98 functionally corresponds to the input of monoflop 32 of Fig. 1. At this junction an information is received whether a pacemaker pulse should stimulate the atrium or whether a spontaneous P-wave has occured. Circuit 84 which is freely running at a frequency of 400 beats/min. corresponding to a pulse interval of 150 msec. is triggered through input 98 of storage member 99, 100, so that 150 msec. after the appearance of the P-information at the input 98 a stimulating pulse is transferred to the ventricle electrode 12 from the second output 103 of circuit 84 through a gate 104 and a conduit 105. The pacemaker thus operates as an atrium controlled ventricle pacemaker.

The circuit 86 which is freely running at a frequency corresponding to 700 msec. in the type of a normal demand pacemaker, is reset through the first output 91 of integrated circuit 84. Accordingly 700 msec. must elapse until an atrium pulse can be delivered to the atrium electrode 10 through the second output 107 of integrated circuit 86 and a conduit 108. Simultaneously an output pulse is fed back from output 95 of circuit 86 to the input 98 of storage member 99, 100 whereby the generator of circuit 84 is restarted and a period of 150 msec. is triggered.

In view of the fact that a repetition rate corresponding to a 150 msec. pulse interval would not make sense, the OFF input 112 of storage member 99, 100 is controlled from output 91 of circuit 84 through a diode 110 so that integrated circuit 84 is allowed to deliver each an individual pulse only. Therefore again an inut signal at input 98 of storage member 99, 100 is required to again trigger circuit 84 and to start the period of 150 msec.

The circuit arrangement of Fig. 2 operates in the four bradycardia quadrants as follows: In the first quadrant-sufficient or fast atrium rate and quick response by spontaneous ventricle activities-the P-wave is received by the atrium electrode 10 and is supplied to the input 98 of storage member 99, 100 through integrated circuit 88, a decoupling diode 114 and a capacitor 115.

The integrated circuit 84 is triggered and tends to deliver after 150 msec. through output 103 a pulse to the ventricle. However, because within this 150 msec. period integrated circuit 90 already has received through ventricle electrode 12 a spontaneous ventricle signal, a disabling signal is fed through a lead 116 and a diode 117 to input 112 of storage member 99, 100 and to one input of gate 104. The latter disables the transfer of the ventricle pulse from output 103 to ventricle electrode 12. Atrium pulses likewise cannot be delivered because the received ventricular response is supplied through integrated circuit 90 and a diode 119 to the input of integrated circuit 86 thereby resetting this circuit for 700 msec. The signal which after 150 msec. leaves output 91 of integrated circuit 84 is used to retrigger through diode 92 the 700 msec. period which already had been triggered through a lead 121 and a diode 122. Therefore an interval of 850 msec. must elapse before a further atrium stimulating signal could be delivered.

However, this delivery does not take place because the P-wave is received at a higher rate. Therefore the pacemaker remains inactive.

In the second quadrant, i.e. in case the atrium rate is too slow and the atrium requires stimulation, whereas conduction is effected in due time, a pulse is generated at the output 107 of integrated circuit 86 700 msec. after a Q-signal. This pulse is supplied to the atrium through conduit 108. The ventricle depolarization preferably may be designed in the manner outlined above in connection with Fig. 1 so that the output capacitor is quickly recharged thereby facilitating the detection of the P-signals and increasing the input impedance of the P-amplifier.

WHAT WE CLAIM IS: 1. A cardiac pacemaker including means for detecting ventricular activity, means for detecting atrial activity, means for stimulating ventricular activity and means for stimulating atrial activity, the arrangement being such that said means for stimulating ventricular activity is operative to provide a ventricular stimulation pulse unless spontaneous ventricular activity occurs within a first predetermined time interval after spontaneous or stimulated atrial activity, and the arrangement being such that said means for stimulating atrial activity is operative to provide an atrial stimulation unless spontaneous atrial activity occurs within a second predetermined time interval after spontaneous or stimulated ventricular activity.

2. A pacemaker according to claim 1, wherein the means for stimulating ventricular and atrial activity are interconnected so that the end of said first predetermined time interval initiates the said second predetermined time interval and that the end of said second predetermined time interval initiates the said first predetermined time interval.

3. A pacemaker according to claim 1 or 2, wherein the means for stimulating ventricular activity and the means for stimulating atrial activity each include a monostable multivibrator which determines said first and said second predetermined time intervals, respectively.

4. A pacemaker according to any of the preceding claims, wherein said first predetermined time interval is between 120 msec. and 200 msec.

5. A pacemaker according to any of the preceding claims, wherein said second predetermined time interval is between 600 and 750 msec.

6. A pacemaker according to any of the preceding claims, wherein unipolar electrodes are provided for detecting and stimulating the atrial and the ventricular activity respectively, and the pacemaker casing constitutes the common neutral electrode.

7. A pacemaker according to any of the preceding claims, wherein the means for stimulating atrial activity include an output capacitor arranged to be charged through first switch means and to be discharged through second switch means.

8. A pacemaker according to claim 7, wherein an atrial stimulation triggering signal is arranged to control the second switch means and to control the first switch means via time delay means.

9. A pacemaker according to any of the preceding claims, wherein the means for detecting ventricular activity have associated therewith disabling means adapted to disable detection of ventricular activity for a predetermined time period when an atrial stimulation is triggered.

10. A pacemaker according to any of the preceding claims, wherein said first and/or second time interval may be shortened for testing purposes.

11. A pacemaker according to any of the preceding claims, wherein the means for detecting atrial activity and/or the means for detecting ventricular activity comprise refractory means for blocking the detection of signals the repetition frequency of which exceeds a predetermined maximum value.

12. Pacemakers substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. preferably may be designed in the manner outlined above in connection with Fig. 1 so that the output capacitor is quickly recharged thereby facilitating the detection of the P-signals and increasing the input impedance of the P-amplifier. WHAT WE CLAIM IS:

1. A cardiac pacemaker including means for detecting ventricular activity, means for detecting atrial activity, means for stimulating ventricular activity and means for stimulating atrial activity, the arrangement being such that said means for stimulating ventricular activity is operative to provide a ventricular stimulation pulse unless spontaneous ventricular activity occurs within a first predetermined time interval after spontaneous or stimulated atrial activity, and the arrangement being such that said means for stimulating atrial activity is operative to provide an atrial stimulation unless spontaneous atrial activity occurs within a second predetermined time interval after spontaneous or stimulated ventricular activity.