This invention relates ^" to apparatus for stimulating the activity of the heart, such apparatus ^" being known generall as "Pacemakers" and will be referred to herein by that term.

The normal sequence of electrical activation of the heart begins from the sino-atrial (SA) node in the high right atrium (RA) the electrical impulses traverse the atria to reach the specialised atrio-ventricular (AV) conduction system of the heart through which they pass to reach the ventricles. This sequence results in atrial stimulation causing atrial con traction followed 120 - 200 msec later by ventricular stimulat causing ventricular contraction. • The two main pathologies whic disturb this sequence are atrio-ventricular (AV) block when th impulse fails to reach the ventricles which then beat at their own intrinsically slow and unreliable rate, and SA node diseas wϋich causes a defect in- initial impulse formation and hence a slow and unreliable beating of the whole heart. These two pa ologies, which can co-exist, may cause partial or complete los of consciousness and sudden death. They are ^" treated by the connection of an artificial pulse generator (usually called a ^• "pacemaker") to the heart by means of an electrode (or ' elect- rode catheter ¹ ) sutured to the outside of the heart or passed

to the inside via a vein (transvenous),.

Pacemakers as currently in use may include an el rode which serves to stimulate the atrium or an electrode t stimulate the ^" ventricle of a heart. ^' -Alternatively, it has been proposed to use both

^• atrial and ventricular electrodes sequentially.

Cardiac pacing is, at present, usually accomplis by a ventricular stimulation electrode in order to overcome atrio-ventricular block. Conveniently, the pacemaker is co - ^: - ed through a catheter electrode to one of the ventricles of heart. It has also been proposed, alternatively, to treat node disease in the absence of any atrio-ventricular block, stimulating one of the atria via a catheter electrode. The latter mode has the advantage of retaining both atrial-cont ion and physiological ventricular contraction with increase cardiac performance but it has two main disadvantages;

1, The catheter electrode position in the atri is mechanically less stable when the transvenous route is u .for the catheter than in the ventricular; ' ^" '< .2. The patient with SA node disease may progre to acquire AV conduction dise_a.se with .concommitant loss of ventricular stimulation.

Because of these disadvantages atrial pacing has not been w practised. A disturbance in the position or function of th atrial electrode may prevent the stimulus from activating t • atrium; this is known as atrial exit block. The effect of cardiac pacing is then lost; similarly the onse -of AV cond

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from activating the ventricles with loss of ventricular contraction. In these circumstances there is a need to change from atrial pacing via the atrial electrode to ven¬ tricular pacing via a ventricular electrode. With ' the development of these.complications it would be necessary to change the part of the heart to be stimulated. If the electrodes are implanted surgically such a changeover ^' requires a major operation and even if the catheter carrying the electrode is initially inserted transvenousl , some . disturbance is involved and particularly under circumstances where a changeover might become necessary the disturbance is particularly undesirable.

According to the present invention there is provided apparatus for stimulating the heart comprising a first electrode for stimulating one of the atria of the hear and a second electrode for stimulating one of the ventricles of the heart, means for supplying electrical pulses to one o other of the electrodes and switcjaing means either controll¬ able from externally of the body or automatically within the apparatus, said switching means enabling pulses to be applie selectively to the first and second of the electrodes. If . the apparatus includes provision for sensing a parameter of the operation of the body, then the said switching means, or possibly another switching means, may be used to effect a change over from the sensing of one parameter to another.

Three pacemakers.embodying the invention wi be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

-Figure 1 is a diagram of a dual pacemaker i ^' accordance with the ^' invention;

Figure 2 is a diagram of a triple mode pace in accordance with the invention;

Figure 3 is a diagram of another, embodiment pacemaker; ^" " Figure 4 is a series of electrocardiographi diagrams; and

Figure 5 is a diagram of a modification of embodiment of Figure 3•

The invention is applicable to pacemakers implanted both surgically and by the use of a catheter p in the heart transvenously. Again, the invention can be applied to a variety of different circuits and those ill trated in the accompanying drawings are noc exhaustive o the possible circuits to which the invention is applicab As is conventional the pacemaker may have a polar electrode in the heart'. ith the other ^" electrode on outside of the pacemaker casing. Alternatively, both el rodes may be at the end of a catheter. The invention is applicable to both electrode systems. Re erring now to the drawings and in partic to Figure 1, the pacemaker comprises an energy source 10 example a mercury-zinc or a lithium based cell, connecte

a timing circuit 12 which is arranged to receive messages from a sensing circuit 1 and to deliver an output to an output circuit 16 which supplies electrical pulses select¬ ively either to an atrial electrode 18 or to a ventricular electrode 20 through a switch 22. The sensing circuit receives a feed-back. from the atrial or ventricular electrode 18,22.

Optionally a detector circuit 24 for detecting external programming may be connected to the timing circuit 12 or to the output circuit 16. This external programming enables changes of frequency, amplitude or width of the pulse supplied to the electrode. The energy source, ^" the timing, output, sensing, programming, and detector circuits are all conventional.

The sensing circuit 14 detects intrinsic cardiac electrical activity through the catheter electrode 18,20 which is normally the same as the electrode which carries the pacing pulses. When the intrinsic cardiac activity exceeds a predetermined rate, the amplified signal from the heart either blocks or triggers, as appropriate, the timing circuit

12, thus inhibiting pacemaker activity or stimulating it to fire synchronously, and thus ineffectively, with the heart's own pulses.

The switch device 22 may be electronic, for example a semi-conductor switch, or may be electro-mechanical but should have a low resistance whichever switch is

selected. Such a switch can be actuated by a static or pu sating magnetic field from externally of the patient's bod through a detector circuit 26. Once the switching operati has been- carried out the switch will remain in the same position until subjected to another magnetic field when th is required.

In an alternative unillustrated construction, a reed switch is incorporated, electrical pulses are fed t a detector, a counter and a decoder which will operate the switch 22.

As will be apparent from Figure 1, the output and sensing circuits 14, 16, of the pacemaker are connecte to one of the two electrodes 18, 20. through the switch 22 this enables both sensing and pacing to be effected throug either electrode and further this allows for atrial pacing and sensing or alternatively ventricular pacing and sensin

In the modification illustrated in Figure 2, the basic circuits are the same as in the first embodiment and are given the same reference numerals, but two switche 30, 32 are incorporated, one of which 30 determines the electrode 18, 20 to be connected to the output circuit 16 of the pacemaker and the other 32 of which determines the electrode connected to the sensing circuit 14 of the pace¬ maker. It follows that one electrode can be used for sensing and the other for pacing. The circuit of Figure 2

thus allows for-a third mode,- namely ventricular sensing and atrial pacing which could be used in the event that atrial ^• sensing fails but atrial pacing is retained. The circuit also gives a fourth possible mode, namely atrial sensing and ven- tricular pacing but this is undesirable physiologically and would be avoided by appropriate blocking, circuitry or in¬ corporation of an appropriate cut-out circuit in a programming device.

By the use of either of the hereinbefore describe pacemakers embodying the invention, some of the disadvantages of atrial stimulation are believed to be circumvented. In particular, although pacemakers embodying the-invention as described with reference to Figure 2 are -intended to achieve atrial pacing with its attendant ^' advantages, in the event that atrial sensing fails, the switch can be operated to allow ventricular pacing and further in the event that both atrial sensing and pacing fail, the switch means can be operated to allow both ventricular sensing and pacing.

Reference will now be made to Figures 3 and 4. The output terminal switching may be arranged to occur auto¬ matically by the provision of an additional circuit within the pacemaker so providing an additional safeguard for the patient. This may be in addition to the external programming means described with reference to Figures 1 and 2. An additional sensing circuit 3 is incorporated

in the pacemaker (Fig 3) which receives an input from th timing circuit 12 and from the ventricular electrode- 20 a is capable of delivering an output to -the . switch 22. The switch is activated on receipt of a signal from the timin circuit 12 and is disabled on receipt of a signal from th ventricular electrode 20 at any time in the subsequent 30 msec. If no signal is received from the ventricular electrode 20 within 300 msec after activation it- emits a signal which activates switch 22 changing the output fro the output circuit (16) from the atrial 18 to the ventri electrode 20.

This sequence will be explained further wit reference to Figure 4 which represents the electrocardio in various paced and unpaced modes. "St" represents pac maker stimuli, 'p ¹ represents spontaneous or paced atria activation 'qrs' represents ventricular activation follo normal AV conduction and 'QβS ¹ represents ventricular activation after ventricular pacing.

In A the initial beat is an atrial paced be followed by normal AV conduction and ventricular qrs activation. The second beat is spontaneous and occurs w the escape period of the demand pacemaker the output of which is inhibited according to known methods.

In B both beats follow atrial pacing and normal AV conduction.

In C the initial .atrially paced beat p is followed by delayed AV conduction and ventricular qrs activation occurs more than 300msec after St1 ; sensing circuit 34. therefore switches the output of the pacer to the ventricular electrode and St2 is followed by ventricular activation.

In D there is atrial exit block and St1 is not followed by atrial activation. Although spontaneous atrial activation p is followed by ventricular contraction qrs within 300msec of St1 , after the second stimulus (St2) the activation qrs falls outside the 300msec period, and the output of the pacemaker would be switched to the ventricular electrode for the third beat . . Thus in the event of AV conduction delay or block C or atrial exit block D after atrial pacing the pacer output is switched to the ventricula electrode.

Should AV conduction delay or block occur whils the spontaneous atrial beat p rate is in excess of the deman rate of the pacemaker, which will therefore be in the in- hibited-, demand mode, the sensing circuit 30 will receive no activation from the timing circuit 12 and will not activate switch 22.

This may be prevented by a further arrangement illustrated in Figure 5 in which the sensing circuit 31 receives its actuating signal from the proximal (internal)

side of the switch 22. The sensing circuit 34 would then activate switch 22 if the 'qrs' signal from ventricular electrode 20 fell more than 300 msec beyond either a signa from output circuit 16 or a 'p' wave which traversed the switch 22 from the atrial electrode 18.

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After activation of switch 22 to transmit and deliver signals from ventricular electrode 20 -is and in or to prevent a reversion from ventricular pacing via. ventric electrode 20 to atrial pacing via atrial electrode 18, whi might occur due to triggering of the sensing circuit 3 vi a 'qrs' activation transmitted from ventricular electrode through switch 22, an additional blocking circuit could be inserted between sensing circuit 34 and switch 22, such th the switch 22 could only be activated from sensing circuit 34 once, although external programming by means of detecto circuit 26 in Figures 1 and 2 could still effect further activation of switch 22.

Similar circuitry for automatic switching of > output terminal, to that outlined- above could apply not on to the arrangement in Figure 1 whereby input and output terminals are controlled by.a single switch 22 (Figure 1) also where input and output(sensing -and pacing) are contro by two switches 30 and 32 - Figure 2.

Although these programmable output and input terminal pacemakers are intended primarily to allow the sa

institution of atrial pacing .with the ability to change from atrial to ventricular pacing without the need for surgical intervention in the event of the problems outlined above associated with atrial pacing, these pacemakers may also have additional applications. They would for example allow the connection ^' of the pacemaker by two electrodes to .the same part of the heart such as two ventricular elect¬ rodes, each electrode being conventional, allowing duplication of the-electrode. This would be advantage because at the present time, when more long-lasting energy sources are being developed, it is the electrodes which have become the limiting feature of the pacemaker system; ^' after a period of several years the electrodes are liable to fracture requiring surgical intervention even though the energy source has not -become exhausted. Indeed it is common practice, when the electrodes are sutured to the outside of the heart by means of a major surgical procedure, to attach two electrodes, the second of wnich is normally 'capped' and left lying loose in the event of future need.

By means of a programmable output terminal pacemakers herein described both electrodes could be attached and in the event of failure of one electrode from whatever cause, the second could be used without surgical inter¬ vention.- An alternative use would be for testing experiment-

al electrodes. A variety of experimental and new elect¬ rodes is appearing at the present time and more are likely to be developed in the future. Although they are always te first in the experimental animal, there is always an ethica problem with initial application in the human. The use of programmahie output terminal pacemaker ^' would allow ethicall acceptable use of such an experimental electrode together w a conventional electrode, thus allowing change from experim al to conventional electrode should the former fail from whatever cause.