Cardiac pacer

Abstract

In a cardiac pacer of the synchronous type and having both atrial and ventricular electrode systems, the atrial electrode senses atrial signals and the ventricular electrode is used to sense the QRS complex and this is used to gate out the QRS complex from the atrial signal and allow the atrial electrode to discriminate against the QRS complex and sense only the P-wave. Consequently the atrial electrode system can be within the heart, and there can be a single catheter with both the atrial and the ventricular electrodes spaced apart on it.

Description

This invention relates to cardiac pacer. For an understanding of the invention it is necessary first to discuss briefly the problems involved in pacing and the various existing proposals that solve these problems to a greater or lesser extent.

The normal human heart has two major pumping chambers, the left and the right ventricles, which by their simultaneous contraction (systole) expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from two thinner walled reservoir chambers, respectively the left and right atrium. The atria also contract in a separate action which precedes the ventricular systole. Although this atrial contraction is not essential to life it does boost the ventricular systole and help the performance of the heart. The contraction of both atria and ventricles results from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation subsequently enters the atrio-ventricular (A-V) node, which delays its passage into the ventricles, to permit the correct temporal relationship between atrial and ventricular contractions.

The spread of excitation in the atria can be recorded externally on the electrocardiograph as a signal known as the P-wave. A larger and more complex signal is caused by the spread of excitation in the ventricles, which can be recorded externally as the QRS complex. The interval from the beginning of the P wave to the beginning of the QRS complex is approximately 150 milliseconds. The repolarisation of the ventricles, i.e., the return to the resting state, is signalled by a small deflection on the cardiograph known as the T-wave. Signals corresponding to the P, QRS- and T-waves can also be recorded from within thecavities of the heart chambers. They are generally referred to as the electrograms.

The whole cycle repeats itself at a frequency ranging in health from about 50 to 190 times per minute depending upon many factors but principally the metabolic requirements of the body (e.g., the rate is fastest during hard exercise). The timing of the heart beat is set by the region high in the right atrium, called the sinoatrial node. In the absence of any external control this gives rise to electrical signals (which cause the spread of excitation referred to above) at the rate of about 105, but the central nervous system acts on the node by way of nerves capable of slowing or accelerating the spontaneous rate of discharge, thus providing a wide range of frequency of heartbeat.

The electrical control of the heart can break down in a variety of ways. One of the most important concerns the propagation of the electrical impulses that activate the heart, usually referred to as heart block. In this conditions the electrical signals may fail to penetrate into the right atrial (sino-atrial block), fail to traverse the region of the A-V node, or fail to activate the ventricles due to disturbances in conduction below the A-V node (two varieties of A-V block). These disorders may cause serious slowing of the heartbeat. The ventricles do have subsidiary (natural) pacers of their own, capable of monitoring the slow rate of electrical activation in the event of heart block but with disease of the conducting system these can also fail. Heart block can then cause temporary or even permanent cessation of the heartbeat.

Artificial pacers are used to restore a satisfactory heartbeat in patients with heart block. An electrode is inserted into the heart, usually by way of a vein, to lie with the tip in the right ventricle. Electrical impulses are generated in to the pacer, which may be outside the patient (external unit) or implanted under the skin and travel by way of the electrode to stimulate the heart. The pacer system can be operated in several different modes which will be discussed briefly.

So-called fixed rate ventricular pacing refers to a system in which the ventricle is stimulated at a preset rate which continues in an uninterrupted fashion. If the heart block is only intermittent, there will be times when the normal electrical signals and the artificial stimuli compete for control of the heart. The heartbeat will therefore be irregular. Moreover an artificial stimulus will fail, from time to time, on a position, on the T-wave, known as the vulnerable period. Under certain adverse conditions such an event can trigger a complete disorganisation of the heart's electrical activity and cause immediate death.

So-called `demand` ventricular pacing is designed to obviate this risk. Naturally occuring heartbeats are detected by the electrode in the ventricle which thus serves both for the sensing and for the delivery of electrical impulses. A spontaneous heartbeat deactivates the pacer for a period determined by its rate setting. The deactivation is always long enough to prevent the pacer impulse coinciding with the T-wave of any previous complex. `Demand` ventricular pacing is in some circumstances safer than fixed rate pacing but important disadvantages remain. First the ventricles beat independently of the atria so that the effect of the atrial boost on the heart is lost. Secondly the ventricular rate depends on the setting of the pacer and cannot accelerate in the normal manner to meet varying metabolic demands.

So-called sequential atrio-ventricular pacing has been used to restore normal A-V relationships. In this technique an atrium and a ventricle are paced by twin stimuli separated by an appropriate interval. However the heart rate is again controlled by the pacer setting and does not vary according to physiological needs. Moreover the pacer rate must be set to exceed the spontaneous rate of the sino-atrial node for otherwise the naturally occuring sino-atrial impulse and the pacer compete for control of the atrium in a manner which precludes satisfactory pacing. TWo electrodes are usually necessary, one to lie with its tip in the right atrium and the other with its tip in the right ventricle. The placing of the two electrodes prolongs the procedure in a patient who may be critically ill.

So-called synchronous pacing approximates most closely to the normal physiological mechanism. The spontaneous atrial electrogram (P-wave) is sensed by an electrode usually in contact with the atrial wall and this is used to trigger the ventricle after an appropriate preset delay. In this way the spontaneous atrial rate (subject to normal nervous control) determines the ventricle rate and normal A-V relationships are maintained. Present techniques have been limited by the need for two electrode systems (atrial and ventricular) and more seriously because an electrograph recorded from within the atrium may show not only P-waves but also QRS complexes, resulting in false triggering. Synchronous pacing has been used satisfactorily with implanted units utilising atrial electrodes sewn onto the atrial surface (which requires exposure of the heart by surgery). It has not previously been developed successfully for pervenous use.

The aim of the present invention is to provide a pacer which will, in normal operation, act in a synchronous mode and maintain sequential A-V operation, yet without the need for extensive heart surgery to implant an external electrode on the atrium.

According to the invention this is achieved in that electrodes are applied both to the atrium and the ventricle and in addition to the atrial electrode acting as a sensing element, with the ventricular electrode acting as a stimulating element (as in known synchronous pacing), the ventricular electrode acts also as a sensing element for QRS complexes and the resulting signals are caused to act on the circuit receiving the signals from the atrial electrode so as to prevent that circuit passing on signals from the atrium which in fact originate from the QRS complex.

In this way the sensing circuit connected to the atrial electrode is able to discriminate against the QRS complex and to identify clearly the atrial P-wave even in the presence of QRS complexes of equal or greater amplitude. The most important consequence of this is that it is no longer essential to apply the atrial electrode intimately to the atrial wall. On the contrary it is possible to achieve synchronous pacing with the atrial electrode simply immersed in the blood in the atrium. This leads to a further important feature of the invention, according to which both the atrial electrode and the ventricular electrode are incorporated in a single catheter. In a preferred arrangement the catheter is designed to be inserted, as in known ventricular pacing, through a vein and the superior vena cava and the right atrium so that its tip lodges in the apex of the right ventricle. The atrial electrode is then formed by a pole or poles exposed at the surface of the catheter at a distance (for example 12cm in a catheter for a normal adult heart) from the tip such as to lie within the atrium.

There may be single active poles in the ventricle and atrium, with a return path formed by an `indifferent` electrode placed elsewhere in or on the body, but preferably each electrode system comprises two respective axially spaced poles on the catheter itself.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a simplified block circuit diagram of the pacer;

FIG. 2 illustrates the location of the catheter in the heart;

FIGS. 3a and 3b show a more detailed circuit diagram of a preferred version;

FIG. 4 shows a possible modification to the circuit of FIG. 3; and

FIG. 5 is an electrocardiogram picture of the waveform of a heartbeat measured by external electrodes, and serving to identify the various parts referred to above.

Referring first to FIGS. 1 and 2, a catheter 1 of flexible inert material forms effectively a double electrode system and contains four wires. It is designed to be inserted into the body through a vein, for example through a vein in the right arm or in the region of the collar-bone, and then passed by known techniques into the heart by way of the superior vena cava 2, passing through the tricuspid valve 3, into the right ventricle 4 until its tip lodges in the apex of the right ventricle. At its tip it has an electrically conducting cap 5 forming one pole, and then spaced about 1 cm from this is a sleeve 6, flush with the material of the catheter, forming a second pole so that the two together form the ventricular electrode. About 12 cm from the tip is a pair of sleeves 7 about 1cm apart, forming the atrial electrode. This is immersed in the blood in the atrium and may or may not be close to the atrial wall. It therefore receives electrical signals present in the region of the atrium and these include not only the P-wave originating in the atrial wall but also the QRS complex from the ventricle. The QRS complex is generally of much greater inherent amplitude than the P-wave, and so even in the interior of the atrium it can have at least a comparable amplitude. That is why hitherto in synchronous pacing it has been considered essential for the atrial electrode to be placed on the outside of the atrial wall by major cardiac surgery.

The signals from the atrial electrode 7 are passed through a variable gain amplifier 8 and used in circuit 9 to produce a corresponding train of pulses of clear and uniform shape and amplitude. Likewise signals are picked up from the ventricular electrode system 5,6; these will only be QRS complexes, either naturally occurring or stimulated, as the P-wave will be barely detectable, if at all, at the apex of the ventricle. These signals are likewise amplified in amplifier 10 and formed in circuit 11 into a train of pulses. These pulses are passed to a circuit 12 in which, while they are present, they block the passage of pulses from the atrial signal amplifier 9. Therefore they eliminate those signals from the atrial electrode 7 that originate in QRS complexes and allow through only those signals originating from the naturally occurring P-wave. The resulting train of pulses is passed through a circuit 13, which sets an upper limit on the pulse repetition rate. If the natural rate rises above this value, indicating very rapid heartbeats or possible interference from external sources, the circuit 13 blocks some of the pulses so as to pass pulses at this fixed maximum rate. The pulses then pass through a circuit 14 which sets a lower limit on the rate. If the rate is below this lower limit or if the atrial pulses are absent altogether the circuit 14 will block the train of pulses and the generator 16, described below, produces a train of pulses at a minimum rate.

The atrio-ventricular delay is introduced in a circuit 15 and will be of the order of 120 milliseconds, but is preferably variable over a range, for example from 18 to 220 milliseconds. This circuit produces a train of pulses which are used to trigger a pulse generator 16 in which the ventricular stimulating pulses are generated. As in known pacer, it is important that the artificially generated pulses should not be applied to the heart in a period, following a natural QRS complex, that might coincide with the subsequent natural T-wave. Therefore, from the circuit 11 that produces pulses when a QRS complex is detected, a further circuit 17 produces pulses of longer duration (about 360 milliseconds) that block the output of the pulse generator 16 in a gate 18 and prevent the transmission of artificial ventricular stimulating pulses for this preset period following any detected signals. Subject to this restriction, the pulses from the generator 16 are fed from an output amplifier 19 to the ventricle through the electrode poles 5 and 6.

Thus the pacer according to the invention, in its normal mode of operation, achieves synchronous pacing, timed from the spontaneous atrial P-wave associated with atrial contraction, yet without requiring an accurately placed external atrial electrode, and in fact requiring only a single catheter, inserted in the manner of known fixed-rate or demand pacer catheters.

FIGS. 3a and 3b show the preferred circuit in sufficient detail to enable the invention to be put into practice. Each of the boxes represents a commercially available integrated circuit and the letters alongside them identify the external connections that enable them to act in the required manner. Mostly they act as monostable multivibrators, i.e., so that an incoming trigger pulse causes the circuit to change its state and it then reverts to its original state automatically after a preset period determined by the values of capacitors and/or resistors (which may be variable) connected externally in accordance with the maker's instructions. This results in a square-wave pulse of length equal to that preset period. Inside each of the boxes in FIGS. 3a and 3b is indicated the length of the pulses it produces. For example it will be seen that the atrial signal, after amplification in amplifier 8, produces pulses in circuit 9 of a duration of 150 milliseconds which in their turn are used to produce very short pulses of only 0.5 milliseconds duration. These pass through a NAND gate (forming the circuit 12 of FIG. 1) to the upper rate limit circuit 13.

It is believed that it will not be necessary to describe the circuit of FIGS. 3a and 3b in great detail as the legends accompanying the boxes identify the characteristics and the components sufficiently for those skilled in the field of modern electronics to put the invention into practice. Where appropriate the reference numerals correspond to those of FIG. 1. The monostable integrated circuits are all of the type 74121 or 74123 marketed by Texas Instruments. In the case of the 74123 unit two of the boxes shown form one such unit.

As stated above the function of circuit 13 is to block the P-wave pulses which have been gated by the circuit 12 and which have a repetition rate above a preset upper limit.

The pulses from gate 12 are negative going pulses (from +5v to 0v hereinafter called negative pulses) and set the bistable formed by gates 21 and 22 to the state where the output of 21 is positive (+5 volts) and the output of 22 is zero (0 volts). Thus, if the monostable 23 is in its state with its output Q positive, the A inputs to monostable 23 receive a negative pulse via gate 24 whenever a negative pulse is delivered from gate 12. As a result monostable 23 is triggered and delivers a positive pulse of 2ms duration at its Q output which passes to the next circuit 14. A complementary negative pulse of 2ms duration appears at the output Q of monostable 23 and this pulse resets bistable 21,22 so that the output of 22 is positive and the output of 21 is zero. The bistable is, thereby, placed in a state where it is ready to receive the next pulse from gate 12.

The trailing edge of the negative pulse from the output Q of monostable 23 also triggers monostable pulses which then produces a negative pulse at its output Q which is applied to the one input of gate 24. If a negative pulse is produced by gate 12 after monostable 25 has reset (i.e., after its output Q has again become positive), it produces a positive pulse from the output Q of monostable 23 in the manner already described above. If, however, a pulse is produced by gate 12 while monostable 25 is triggered (i.e., while its output Q is zero), it is prevented from triggering monostable 23. Instead, it is stored in the bistable 21,22, and, as soon as monostable 25 resets, the pulse then produces a positive pulse at Q from monostable 23. In other words, a pulse from gate 12 cannot trigger monostable 23 until a fixed period, determined by the pulse length of monostable 25, has elapsed following a previous pulse from gate 12. Monostable 25 thus determines the minimum period between output pulses from monostable 23 to circuit 14, that is it determines an upper limit to the repetition rate of pulse which it passes.

Considering now circuit 14, this receives positive pulses from circuit 13 as described above. Each such input pulse is delayed slightly by the C-R network 26 and applied to the input of monostable 27. The trailing edge of the positive input pulse triggers monostable 27 which then produces a positive pulse at its output Q, thereby opening gate 28. The input pulse, if it is the first one to be received, cannot pass through gate 28 as it terminates before monostable 27 opens gate 28. However, if another input pulse is received before monostable 27 resets, it will pass straight through gate 28 to circuit 15 (FIG. 3b) and will also re-trigger monostable 27. Thus, in general, if the input pulses from circuit 13 appear at interval spacings less than the pulse length of monostable 27, the gate 28 is held permanently open by monostable 27 and the pulses will pass straight to circuit 15. However, if the pulses from circuit 13 appear at interval spacings greater than the pulse length of monostable 27 they will all be blocked by gate 28 and none will pass to circuit 15.

Circuit 15 (FIG. 3b) produces the A-V delay. Monostable 29 is triggered by the reading edge of the negative pulses from circuit 14 and after a preset delay resets, triggering the monostable 30 which in turn passes a negative pulse to the circuit 16.

Circuit 16 is a triggered astable. Negative input pulses from circuit 15 pass through gate 31 and trigger the monostable 32 which in turn produces a positive stimulating pulse at output Q to the gate 18. The output Q of monostable 32 delivers a corresponding negative pulse through gate 33 to trigger the monostable 34. Monostable 34 then produces a negative pulse at its output Q, and the trailing edge of this pulse triggers monostable 32. That is, monostable 32 is triggered by either an input pulse from the A-V delay circuit 15 if one arrives before monostable 34 has reset, or by the pulse at Q from monostable 34. In the absence of input pulses the circuit therefore free-runs generating pulses at a fixed minimum rate determined by the pulse length of monostable 34. However, if input pulses are received the circuit follows them in producing output pulses to gate 18.

If a QRS wave is sensed in the ventricle, monostable 11 produces a positive pulse at Q which triggers the monostable 35 and this in turn produces a negative pulse at Q which passes through gate 33 to monostable 34. Monostable 34, which is retriggerable, is thus reset and will not trigger monostable 32 until its full period has elapsed from the advent of the QRS signal. This arrangement provides the demand facility of the pacer. It should be noted that the triggering of monostable 34 by monostable 35 does not produce any output stimulating pulse to gate 18, it merely extends the period for which monostable 34 remains triggered.

The positive output pulses from monostable 11 corresponding to the QRS wave also pass via a gate 36 and trigger a monostable 17 which produces a negative pulse at Q which closes the gate 18 to prevent the passage of the stimulating pulses from circuit 16.

It will be seen that the practical circuit of FIG. 3, includes a light-emitting diode LED1, which is connected via a monostable 37 to the output of the gate 12 and therefore flashes every time a pulse derived from the P-wave is received. It could, if desired, be connected to the atrial pulse circuit ahead of the gate 12, in which case it would flash every time a signal is received in this circuit, i.e., both P-wave and QRS pulses.

Another diode LED2 is connected to the monostable 17 and flashes every time a pulse is received from the ventricular electrode, and a third one LED3, is connected via a monostable 38 to the output of the pulse generator 16 to show the stimulating pulses which the pacer is producing.

The output current amplifier 19 includes at its input an isolating circuit comprising a non-electric link in the form of a light-emitting diode LED4 acting on a phototransistor PT. This provides complete electrical isolation of the input and output. The output amplifier includes a potentiometer P allowing adjustment of the amplitude of the pulse, to a maximum of 20 milliamperes, negative-going.

All of the logic gates used are also formed by commercially available integrated circuits.

As described above, the circuit 13 limits the output pulse rate to a predetermined maximum value. However, when this occurs the output pulses from the pacer are not synchronously related to the input P waves. That is atrio-ventricular synchronism is lost.

The circuit of FIG. 4 is an alternative embodiment which maintains this synchronism even when the input pulse rate is above the upper limit setting. When the switch S is in the right hand position, the circuit operation is as has already been described. When it is in the left hand position, however, the circuit action is modified as follows.

Circuit C is a monostable type 74121 and each input pulse from gate 12 triggers the bistable 21,22 as before and also triggers monostable C. The output Q of the triggered monostable C produces a positive output pulse which opens gate D. Hence, if the input pulse passing through the bistable 21,22, gate 24 and monostable 23 arrives at gate D before monostable C has reset it passes through gate D to circuit 14. However, if it arrives after monostable C has reset it is blocked by gate D. In this latter case monostable 25 is not triggered and no output pulse appears. Since monostable 25 is not triggered it will be in the reset state when the next input pulse arrives from gate 12 and this pulse therefore passes straight through gate 24, monostable 23 and gate D. Hence the action of C is to measure the time interval elapsing between an input pulse from gate 12 and an output pulse to circuit 14. That is, it measures the time for which a pulse is stored in the bistable 21,22. If this time is less than the pulse length of monostable C the pulse passes from gate 12 through gate D to circuit 14. If, however, it is greater than the pulse length of monostable C a pulse is dropped i.e. blocked by gate D. In this situation the next input from gate 12 passes straight to the circuit 14.

To summarize, if input pulses are above the upper rate they are allowed through, at this upper rate, until their delay from the P wave which produces them exceeds a preset value determined by the pulse length of monostable C. When this occurs one output pulse is omitted and the output is resynchronized to the input.

It will be understood that, although the version illustrated employs timing of the various pulse lengths and spacings by the use of monostable multivibrator circuit and capacitors, with variable resistors to alter the values of the timings, it would be within the scope of the invention to achieve the same result employing digital circuits with clock counters for doing the timing.

In a modification of the catheter 1 there could be more than two poles for each electrode system, to allow the same catheter to be used in hearts of different sizes. For example the atrial electrode system could comprise three poles, spaced apart, and one uses the upper pair or the lower pair, whichever is placed best in the atrium in the heart in question.

Claims

We claim:

1. A synchronous cardiac pacer comprising:

atrial electrode means which is adapted so that it can be arranged in relation to an atrium of a heart in order to sense the naturally occurring P-wave of said heart and produce a corresponding sensed P-wave signal;

ventricular electrode means which is adapted so that it can be arranged in relation to a ventricle of said heart in order to sense the naturally occurring QRS complex of said heart and produce a corresponding sensed QRS signal;

gating means which has an output connection to said ventricular electrode means and respective input connections from said atrial electrode means and ventricular electrode means so as to receive said P-wave and QRS signals without disturbing the phase relationship between them, and which responds to said P-wave and QRS signals by blocking any similar signal variations which occur simultaneously so that only a pure P-wave signal without any QRS complex component passes to the output connection;

pulse generating means which is connected in the P-wave channel comprising said atrial electrode means, said gating means, said output connection and said ventricular electrode means to produce a pulse signal corresponding to said P-wave signal, whereby a pure P-wave pulse signal is applied to said ventricular electrode means as a stimulating signal for said heart; and

delay means which is connected in said P-wave channel and introduces an appropriate atrio-ventricular delay in said P-wave signal.

2. A cardiac pacer as claimed in claim 1 in which said gating means comprises a logic gate.

3. A cardiac pacer as claimed in claim 2 in which said P-wave and QRS signals are converted to corresponding pulse signals by respective means connected between said electrode means and said inputs of the logic gate so that pulse signals are received at the latter and pass to said output connection.

4. A cardiac pacer as claimed in claim 1 in which said pulse generating means is connected in said output connection between said gating means and said ventricular electrode means.

5. A cardiac pacer according to claim 1 in which said delay means is connected in said output connection between said gating means and said ventricular electrode means.

6. A cardiac pacer according to claim 5 in which second gating means is connected in said output connection between said delay means and said ventricular electrode means to control the passage of said pure P-wave signal to the latter, said second gating means having a control input, a blocking pulse generating means producing a blocking pulse of fixed length in response to each QRS signal connected to said control input for preventing the passage of said pure P-wave signal through said gating means.

7. A cardiac pacer according to claim 1 in which a single catheter designed for insertion in the heart carries both the atrial electrode means and the ventricular electrode means.

8. A cardiac pacer according to claim 7 in which the catheter is designed for pervenous insertion, the ventricular electrode means being formed on the extreme end of the catheter and the atrial electrode means being formed at a distance spaced from that end.

9. A cardiac pacer according to claim 8 in which the distance between the electrode means is substantially 12cm.

10. A cardiac pacer according to claim 8 in which each electrode means comprises at least two poles, axially spaced apart along the catheter.

11. A cardiac pacer according to claim 10 in which the spacing between the poles of each electrode means is substantially 1cm.

12. A cardiac pacer according to claim 8 in which an end cap on the catheter forms one pole and a second pole on the catheter is spaced axially from said end cap, these two poles comprising ventricular electrode means.

13. A cardiac pacer according to claim 1 in which said pulse generating means includes means for generating said pure P-wave pulse signal at a fixed minimum lower rate in the event of the P-wave signal from the atrial electrode means falling below that rate or vanishing.

14. A cardiac pacer according to claim 1 in which said pulse generating means includes means for generating said pure P-wave pulse signal at a normally fixed upper limiting rate in the event of the P-wave signal from the atrial electrode means rising above that rate.

15. A cardiac pacer according to claim 14 in which said pulse generating means includes means for omitting a pulse of the pure P-wave pulse signal at the upper limiting rate each time this signal falls behind the sensed P-wave signal by an amount exceeding a predetermined value.