WO1999010044A1

WO1999010044A1 - Dual chamber pacemaker that automatically changes mode, when prolonged vv-interval is detected

Info

Publication number: WO1999010044A1
Application number: PCT/DK1998/000363
Authority: WO
Inventors: Henning Rud Andersen; Peter Thomas Mortensen; Anders Kirstein Pedersen
Original assignee: Henning Rud Andersen; Peter Thomas Mortensen; Anders Kirstein Pedersen
Priority date: 1997-08-22
Filing date: 1998-08-21
Publication date: 1999-03-04
Also published as: AU8851598A

Abstract

The present invention relates to a pacemaker that is capable of switching from one mode of pacemaker function, an atrial pacemaker function such as AAI pacing, to another mode of pacemaker function, a ventricular pacemaker function such as DDD function or VVI function. The invention is characterised in that switching takes place in response to an empirically determined VV interval between heartbeats in the ventricles of the heart of a patient. In this way the atrial pacemaker function is maintained as the primary pacing mode for as long as possible because switching is primarily determined by the VV interval and not the AV delay. In a preferred embodiment the VV interval is set to above 1000 ms, preferably above 2000 ms.

Description

DUAL CHAMBER PACEMAKER THAT AUTOMAΗCALLY CHANGES MODE, WHEN PROLONGED VV-INTERVAL IS DETECTED

The present invention relates to a pacemaker for the sensation of the atria and the ven- tricles of a patient' s heart and for the stimulation of at least one of the ventricles of the patient's heart, said pacemaker comprising at least a first electrode intended for sensing a spontaneous heartbeat in an atrium of the heart, a second electrode intended for sensing a spontaneous heartbeat in a ventricle and an electrode for the stimulation of heartbeats in one of the ventricles as a minimum, means for detecting a time interval, an AV delay, between the heartbeat in the atrium and the heartbeat in the ventricle, means for detecting a time interval between the heartbeats in the ventricles, a spontaneous ventricular activity, and, in addition, means for switching from one type of pacemaker function to another type of pacemaker function.

The natural pacemaker of the heart (the sino-atrial node) is situated at the top of the right atrium. A normal heartbeat is initiated by the sino-atrial node sending out a current impulse spontaneously (being depolarised) after which the current impulse is spread downwards in the surrounding muscle tissue of the heart which results in a pumping beat (contraction) in the atria. When the impulse reaches the bottom of the atria it runs from the atria downwards to the ventricles through a special conduction system (AV node) and runs on through pathways in the ventricles which are activated by this and produce a pumping beat in the ventricles, delayed by approximately 150 ms compared with the atria. This ensures that the atria will pump prior to the ventricles. This synchronisation between the atria and the ventricles is crucial for the normal and optimal pumping function of the heart. Irregularities in the impulse spread and/or slow heart rhythm will be able to affect the pumping function.

Transient or chronic failure in the cardiac impulse formation in the sino-atrial node, "sick sinus syndrome" (SSS) and/or in the impulse conduction in the AV node/conduction system (AV block) are the most frequent reasons for pacemaker therapy. A pacing system consists of a pacemaker (impulse generator) implanted under the skin of the chest, attached to a number of cables (electrodes), typically 1 or 2 cables (electrodes), led through the vein system to the heart where the tip of the electrode is attached to the muscle tissue of the heart. Through the electrode the pacemaker is ca- pable of sensing the spontaneous electrical activity of the heart (1-15 mV amplitude) and in the case of bradycardia of supplying an electrical impulse (such as 2.5 V, 0.5 ms) which initiates a heartbeat.

WO 96/38200 describes a pacemaker for avoiding non-physiological inhibition in- duced by electromagnetic interference. This is achieved by the pacemaker reacting with a mode-switch from a programmed inhibited mode, i.e. XXI, to a trigged mode, i.e. XXT, when the electromagnetic interference exceeds a border value. The border value is set on the basis of differences between the number of sensed impulses or paced beats. Thus the normal pacemakerfunction is not disturbed from electromag- netic interference from the surroundings. However, this pacemaker is not capable of mode-switch in response to abnormalities in heartbeats of the atria or of the ventricles.

US 5.540.727 describes a pacemaker to be used as a DDD pacemaker and being capable of adjusting an AV interval on the basis of sensed physiological parameters. The pacemaker is a conventional DDD pacemaker with the AV synchrony maintained. The pacemaker is provided with a plurality of sensors placed within and outside the heart. The pacemaker works on the basis of establishing an AV delay interval between a sensing of an atrial depolarisation signal and the next subsequent stimulation of ventricular tissue. Thus, the pacemaker does not mode-switch on the basis of a VV inter- val, and the maintenance of the AV synchrony delimits the pacemaker to a specific use.

EP 0 596 540 describes a conventional DDD pacemaker working with an automatic variable AV interval. The purpose is to allow spontaneous conduction from the atrium to the ventricles. The pacemaker maintains AV synchrony and the switch taking place is from DDD-mode to VVT-mode when the frequency in the ventricles is sufficiently high. Thereby it is possible to disconnect the one electrode in the atrium in order to save energy and thereby prolong the lifetime of the pacemaker battery. Thus, the mode-switch from DDD-mode to VVT-mode is established on the basis of high pulse of the patient.

Another known technology is a pacemaker capable of switching from DDD pacing to

VVI pacing if it senses an extra atrial beat, called retrograde P wave, as a result of an extra ventricular beat, called ventricular extrasystole, i.e. an abnormal ventricular beat followed by an abnormal atrial beat compared to the normal heart rhythm. This known pacemaker is also capable of switching back to DDD pacing when a certain number of normal heartbeats have been sensed. Thus, this known pacemaker is capable of switching from one type of pacemaker function to another type of pacemaker function provided that an abnormal event takes place in the atria, in the form of a retrograde P wave caused by an event in the ventricles.

This known type of pacemaker has, however, the disadvantage of being capable of switching from one type of pacemaker function to another type of pacemaker function only on the basis of a sensed atrial event, namely one or more extra atrial beats. The pacemaker makes no qualitative distinction between the extra atrial beat being an isolated event of an otherwise long-lasting, normal heart rhythm and being a manifesta- tion of a long-lasting change of the heart rhythm. Thus, in this known pacemaker, switching from one type of pacemaker function to another type of pacemaker function will occur more frequently than necessary.

For patients with sick sinus syndrome (SSS) the choice is between a single lead atrial pacing system, a single lead ventricular pacing system and a dual chamber pacing system. In the following, an AAI pacing system will be used as an example of a single lead atrial pacing system, a VVI pacing system will be used as an example of a single lead ventricular system and a DDD pacing system will be used as an example of a dual chamber pacing system. However, these examples must not be viewed as a limitation of the possible uses of the invention claimed. An AAI pacing system has one electrode placed in the right atrium and is capable of registering the spontaneous activity of the sino-atrial node/atrium and, when needed, of supplying an impulse which will activate the atria, after which the impulse runs on spontaneously to the ventricles as in the normal heart. Therefore, an AAI pacemaker is suited for the treatment of "sick sinus syndrome" where the problem is unstable impulse formation in the sino-atrial node/atrium and the function of the AV node is normal. In addition, an AAI pacemaker has the advantage of having relatively simple technology and being cost-effective.

A VVI pacing system has one electrode placed in the right ventricle and can detect the spontaneous activity of the ventricle and, when needed, supply an impulse which will activate the ventricles.

A DDD pacing system has two electrodes, one in the right atrium and one in the right ventricle and therefore it is capable of detecting spontaneous activity in and of stimulating both the atria and the ventricles when needed. Thus, a DDD pacemaker is suited for the treatment of AV block where the impulse conduction from the atria to the ventricles is affected. In addition to maintaining the rate of the ventricles, the normal synchrony between the atria and the ventricles is retained. This is ensured in that a given stimulation in the ventricle is started by a preceding, detected or stimulated impulse in the atria. To resemble a physiological synchrony to the pumping beat of the atria and the ventricles, a programmable interval (AV delay) between the stimulation of the atria and the ventricles has been inserted into the pacemaker, corresponding to the normal impulse delay in the AV node/the pathways (typically 150-200 ms). It is, how- ever, a significant disadvantage that the ventricle is stimulated from the tip of the electrode and not through the natural pathways (AV node/pathways). For practical reasons the tip of the electrode has been placed at the bottom of the right ventricle, far from the normal conduction system (AV node/pathways) at the top of the ventricle. As a result, the normal contraction pattern of the ventricles is disturbed, involving the risk of reduced pumping function in spite of retained synchrony between the atria and the ventricles. In general it can therefore be concluded that "sick sinus syndrome" (SSS) with normal AV conduction should be treated with an AAI pacemaker because it ensures normal synchrony between the atria and ventricles and normal activation of the ventricles and, as a result, a normal pumping beat in the ventricles. On the other hand, patients with AV block should be treated with DDD pacemakers. Because SSS has been regarded as part of a more generalised dysfunction of the conduction system, it is still debated whether these patients should be treated with AAI pacemakers because of the risk of developing AV block during follow-up.

Studies have shown that in patients with "sick sinus syndrome" and normal AV conduction at the time of the implantation of an AAI pacemaker, the risk of AV blocks is less than 1% per annum. If an AV block should occur it usually only lasts a few seconds, then the AV conduction returns to normal. Only 1-2% of patients with "sick sinus syndrome" who have a pacemaker implanted need a DDD pacemaker. Thus, more than 95% of all patients with "sick sinus syndrome" could be treated sufficiently with an AAI pacemaker.

For fear of developing significant AV block later on, only 201 out of 560 (36%) patients with "sick sinus syndrome' had an AAI pacemaker implanted in 1995 in Den- mark while "to be on the safe side" the rest received a DDD pacemaker. This tendency is even more pronounced in, for example, the USA where substantially all patients with "sick sinus syndrome" are treated with DDD pacemakers. In the USA it is primarily due to insurance considerations because the physician/hospital is afraid to leave the ventricles unprotected as regards pacing because in the American insurance system it would result in significant compensation claims from the few patients who would develop AV blocks later on.

This desire for maximum security for patients with "sick sinus syndrome" in case they should develop AV blocks later on involves several expenses in addition to those re- lated to the use of a more expensive and technically complicated DDD pacing system.

Studies have shown that, in spite of maintained synchrony between atria and ventricles in DDD pacing, pacemaker stimulation of the ventricles will involve a significant risk of complications, probably because of the abnormal contraction pattern which occurs in the ventricles in this connection. This risk seems to be at a level similar to that previously demonstrated in studies when using pacing systems with only one electrode in the ventricle, so-called "VVI pacing systems" by which only the ventricles are stimu- lated. In VVI pacing systems, there is no synchrony between atria and ventricles, and the ventricles are stimulated frequently.

It has been shown that right ventricular stimulation produces an abnormal contraction pattern which depresses the function and efficiency of the left ventricle. The cardiac pumping efficiency is higher with AAI pacing than with VVI pacing whereas DDD pacing occupies an intermediate position even when the atrial and the ventricular stimulation is properly synchronised. Many studies have also demonstrated that cardiac output is greater during AAI pacing, when the ventricles are activated through the normal ventricular conduction system, than during VVI pacing and during DDD pac- ing. Thus, stimulation in the ventricles (VVI and DDD) always alters the physiological contraction of the ventricular myocardium and may potentially compromise cardiac function.

AAI and DDD pacing more closely resemble the normal physiology of cardiac activa- tion and haemodynamics than VVI pacing does because both AAI and DDD pacing maintain the synchrony of atrial and ventricular contraction. However, VVI pacing is still used in approximately 50% of patients in spite of analyses documenting that DDD pacing is both clinically effective and cost effective compared with VVI pacing. The reluctance to implant more physiological pacemakers may be related to the initial higher device cost and more complex programming and follow-up of DDD pacemakers. Furthermore, the lack of large-scale prospective randomised trials to document which pacing mode is superior may also contribute significantly.

Programming a long AV interval in DDD pacemakers to promote spontaneous AV conduction so that the pacing mode becomes functionally equivalent to AAI mode has been proposed as a method of avoiding ventricular stimulation. On the other hand, an excessively long PR interval can restrict the programming of a sufficiently high upper rate limit and an appropriate atrial refractory period. Furthermore, DDD pacemakers programmed with a long AV interval do not prevent ventricular stimulation. Programming the AV delay to be approximately 20% longer than the spontaneous PR interval results in 80% ventricular stimulation.

Long-term Effects of VVI and AAI/DDD Pacing in Sick Sinus Syndrome

Paroxysmal atrial fibrillation, atrial flutter and supraventricular tachycardia are common findings in SSS patients. In unpaced patients the prevalence of atrial fibrillation rises to 50% if one considers only patients with paroxysmal tachy arrhythmias. Thus atrial fibrillation is a part of the natural history of SSS. The impact of different pacing modalities on the development of chronic atrial fibrillation has been reported in several studies. Data comparing VVI with AAI/DDD pacing reported a significantly higher incidence of atrial fibrillation in the VVI group, see table 1.

Follow-up period was reported for a total study population of 950 patients of which 559 patients (59%) has SSS.

It seems evident that VVI pacing is associated with an increased risk of atrial fibrillation in patients with sick sinus syndrome compared with AAI/DDD pacing. The exact mechanisms which link VVI pacing with the development of atrial fibrillation are unknown. VVI pacing may predispose or AAI/DDD pacing may prevent or delay the natural evolution of SSS to chronic atrial fibrillation. The explanation might be related to atrial overdrive pacing eliminating sinus bradycardia with concomitant homogeni- sation of atrial refractory periods or to preserved AV synchrony or both. However, the observations strongly indicate that the atrium should be sensed and paced in patients with sick sinus syndrome.

There is a definite risk of thromboembolism in patients with SSS, both before and after implantation of a pacemaker. In studies comparing VVI pacing with AAI/DDD pacing, the incidence of thromboembolic events is higher after implantation of a VVI pacemaker than after implantation of an AAI/DDD pacemaker, see table 2.

The explanation for the very high frequency of thromboembolic events in patients treated with a VVI pacemaker is uncertain. It may be related to the abnormal depolari- sation pattern of the ventricular myocardium caused by ventricular stimulation. The change in ventricular contraction has been shown to be followed by decreased coronary artery blood flow, dilatation of the ventricles, increased end diastolic pressure and increased atrial pressure. Such changes may further lead to atrial dilatation and decreased left atrial wall motion and slow the blood flow in perimural areas of the atrium, which may further predispose to development of cardiac thrombus and subsequently increase the risk of thromboembolism.

Table 2. Effect of pacing mode on development of stroke and thromboembolism in trials with more than 50 patients.

* Only one cumulative figure for stroke/thromboembolism (2%) and follow-up period (62 months) reported for the 41 patients with physiologic pacemaker (AAI and DDD). NR: Not Reported

Haemodynamics and Congestive Heart Failure

Normally, cardiac output increases about 200%) - 400%> during maximal exercise; the largest component is provided by an increase in heart rate, whereas increased stroke volume makes only a modest contribution. In a normal heart, AV synchrony at rest contributes about 20% to 30% of the cardiac output. The haemodynamic importance of a properly timed atrial systole has been recognised to be associated with a higher cardiac output at rest with AAI/DDD than with VVI pacing, also in patients with SSS.

Cardiac output is greater during AAI pacing than during VVI pacing. The haemodyna- inic superiority of AAI pacing is partly a result of the effect of the atrial contraction upon ventricular filling. However, the abnormal contraction sequence of the ventricular myocardium induced by ventricular stimulation may obviously also play a negative role for VVI and DDD pacing (see later). However, the lack of large randomised trials designed to prospectively study haemodynanics and heart failure in different pacing modalities (AAI versus VVI versus DDD) makes it difficult to demonstrate that these elements also translate into less congestive heart failure for patients with SSS. Mortality

Early in the 1980s some reports of the natural history of unpaced patients with sick sinus syndrome showed survival to be similar to the normal population. At the same time, VVI patients did not show an improvement in survival compared with unpaced patients. That is, VVI pacing was useful in improving symptoms but was not useful in improving survival. The prognosis for survival is, furthermore, related to the underlying heart disease. Mortality is increased by congestive heart failure and ischemic heart disease. Several studies have compared mortality in AAI/DDD-paced patients with those with VVI pacing, see table 3. There is a tendency for improved survival with AAI/DDD pacing compared with VVI pacing.

Table 3. Effect of mode of pacing on mortality

*Only one follow-up period was reported for the total group. ** Follow-up periods was reported for a total study population of 950 patients of which 559 patients had SSS. Pacing of patients with SSS

Only in patients with chronic atrial fibrillation should VVI pacing be considered as a first choice pacing modality. The crucial question with today's technology is whether patients with SSS and normal AV conduction fare better with AAI than with DDD pacing. Obviously, for SSS patients with impairment of the AV conduction or with concomitant bundle branch block or bifascicular block, DDD pacing should be chosen.

T'he purpose of the present invention is to provide a pacemaker capable of ensuring atrial pacing and in which ventricular single chamber pacing or dual chamber pacing is initiated only when the patient shows a real need for it.

This purpose is achieved with a pacemaker characterised in that the means for switching from one type of pacemaker function to another type of pacemaker function are activated when the means for detecting the time interval between the heartbeats in the ventricles have detected an empirically determined time interval, a programmed VV interval, between the beats in the ventricles.

A preferred embodiment of a pacemaker according to the invention is characterised in that the pacemaker is programmed to work initially as an AAI pacemaker, that the pacemaker is programmed to work as a DDD pacemaker by switching automatically from AAI mode to DDD mode if no spontaneous ventricular activity is sensed and that the pacemaker is programmed to work as an AAI pacemaker again by switching back automatically from DDD mode to AAI mode when spontaneous ventricular activity is sensed again.

In the following, an AAI pacing system will be used as an example of a single lead atrial pacing system, a VVI pacing system will be used as an example of a single lead ventricular system and a DDD pacing system will be used as an example of a dual chamber pacing system. However, these examples must not be viewed as a limitation of the possible uses of the invention claimed. Automatic mode switching from AAI to DDD if the ventricular rate drops significantly and such a DDD pacemaker should be programmed to work as an AAI pacemaker with the ventricular channel continuously sensing the ventricular activity but inactivated from stimulation until significant ventricular bradycardia appears (for example RR interval > 3 s.). Then automatic mode conversion from AAI to DDD should appear and DDD pacing should continue at a normal rate until spontaneous ventricular activity is sensed again through the ventricular channel which triggers reversion to AAI mode at the same rate. This feature allows normal ventricular contraction most of the time and nearly eliminates the inappropriate ventricular stimulation in patients with normal AV conduction.

Automatic mode switching from atrial chamber pacing, such as AAI, to dual chamber pacing, such as DDD, is also suited for the treatment of patients suffering from brady- cardia. In the prior art it is known to perform automatic switching from dual chamber pacing, such as DDD, to ventricular single chamber pacing, such as VVI, which is initiated in patients with atrial tachycardia. With the present invention it is now possible to perform automatic mode switching in patients with bradycardia, primarily paced by atrial chamber pacing, such as AAI, so that they are paced by dual chamber pacing, such as DDD. The automatic mode switching is initiated if the heart rate in the patient drops below an empirically determined value. Thus the pacemaker according to the invention may be used both in patients with transient AV block or atrial fibrillation and in patients with transient bradycardia.

The optimal pacemaker treatment of patients with "sick sinus syndrome" and AV block absent at the time of operation is treatment with an AAI pacemaker which also has the possibility of stimulating the ventricles if, nevertheless, a short-acting AV block should occur later. Such a pacing system may be named AAI/V back up.

This pacing system according to the invention, AAI/V back up has electrodes in both the right atrium and the right ventricle, like a DDD pacemaker. In contrast to the DDD pacemaker, which maintains 1 :1 synchrony between the atria and the ventricles by means of a programmed AV delay, the pacing system according to the invention, AAI/V back up, will allow the ventricles to make a short pause or for a short period of time to beat with another, slower rate than the atria. This will improve the possibilities of the inherent activity of the ventricles to take over earlier and result in more physio- logical pumping beats. Only if the pause/frequency of the ventricles becomes longer/slower than a previously programmed, possibly dynamic, interval of say 3000 ms will the pacing system according to the invention, AAI/V back up, stimulate the ventricles.

In this situation the simpler solution would be that the pacemaker disconnects the atrial lead and works as a ventricular pacing system (VVI) only, stimulating the ventricle during the few seconds when the rate in the ventricle would have been slow. When after a few seconds normal AV conduction is re-established and the rate of the ventricle exceeds the programmed rate of the pacemaker, the pacemaker will auto- matically switch back the atrial lead and the pacemaker will again work as an AAI/V back up pacemaker. As an AAI/V back up pacemaker, the pacemaker maintains the sensing function but the stimulating function in the ventricle is disconnected until another AV block appears and then the automatism is repeated.

An alternative solution may be to omit disconnecting the atrial lead and let the pacemaker work as a DDD pacemaker instead, aiming at synchrony between atria and ventricles until AV conduction returns and the rate of the ventricles exceeds the programmed rate of the pacemaker. The latter is a technically more difficult solution but with the AV synchrony maintained the solution is physiologically more correct and will probably be preferred by the majority.

With the new pacemaker system according to the invention, AAI/V back up, the physiologically most correct pacemaker treatment is achieved with the normal activation of the ventricles being maintained under all normal conditions, and at the same time the ventricles are "protected" as regards pacing without frequent and often unnecessary and potentially harmful stimulation of the ventricles. The short periods of a few seconds in which the ventricles are stimulated in actual practice will not affect the pumping function of the heart to any appreciable extent and will not result in harmful effects similar to those observed in VVI and DDD pacing, in which stimulation in the ventricle is very frequent.

The AAI pacemaker with the option of back up stimulation of the ventricles, AA/V back up, is a new pacemaker principle which combines the advantages of the AAI pacemaker with the security of the DDD pacemaker without including a significant part of the disadvantages of the DDD pacemaker.

In the following the invention will be described in detail with reference to the enclosed drawing in which

fig. 1 shows a pacing sequence of a known DDD pacemaker with a given AV delay. fig. 2 shows a pacing sequence of an AAI/V pacemaker according to the invention with an empirically determined VV interval, fig. 3 shows a pacing sequence of an AAI/V pacemaker according to the invention with an absolute and a relative VV interval, fig. 4 shows a pacing sequence of an AAI/V pacemaker according to the invention, switching from one type of pacemaker function to another type of pacemaker function, fig. 5 shows a pacing sequence of an AAI/V pacemaker according to the invention, programmed with continuous maintenance of the AAI/V back up.

Fig. 1 shows a pacing sequence of a standard DDD pacemaker that works with a maximum, possibly dynamic, AV delay in the way that the ventricle is paced only if the spontaneous AV conduction is longer than the AV delay.

Fig. 2 shows a pacing sequence in AAI/V back up mode. It appears that in the same situation as in Fig. 1 now there is no pacing in heartbeats 3 and 4. This is a result of putting down a programmed VV interval, not an AV delay, each time the ventricle is paced or sensed and this interval is considerably longer than previously known AV delays. In the graph shown the VV interval is 3000 ms; however, it may be chosen arbitrarily depending on the patient in question.

Fig. 3 shows that a pacing sequence in AA/V back up mode may have a VV interval which has been worked out as a programmable, initial, absolute interval and a terminal, relative interval. This aims at AV synchrony during the subsequent pacing of the ventricles.

Fig. 4 shows a pacing sequence in AAI/V back up mode with a VV interval of 3000 ms in the graph shown and with an interval of 1000 ms between each heartbeat, corresponding to a pulse rate of 60. The VV interval determines when the ventricle will be paced at the earliest if no spontaneous ventricular activation has been sensed. When ventricular pacing has taken place in the AAI/V back up mode, the use of various programmable pacemaker functions is intended instead of pacing on the basis of a VV interval. The alternatives are meant to ensure a ventricular rate high enough to protect the patient against symptoms of ventricular bradycardia, for example maintaining a given ventricular frequency in the range of 50 to 70 until normal, spontaneous AV conduction returns.

At each stage, sensing of heartbeats takes place in at least one of the atria (A) and, subsequently, sensing of heartbeats takes place in at least one of the ventricles (V). Detection of heartbeats in the ventricle (V) is stated as "sens" on the abscissa of the figure. After the heartbeat in the ventricles (V) has been detected, a VV interval, which is of 3000 ms as stated in the figure shown, is initiated. This interval constitutes the time interval between the heartbeats in the ventricles that the pacemaker will accept before pacing of the ventricles takes place. In the graph shown, the VV interval of 3000 ms is initiated at stage 1 after the pacemaker has sensed heartbeats in the ventricles (V).

At stages 2, 3 and 4, no heartbeats are sensed in the ventricles because of AV block in the patient. Therefore, between stages 4 and 5, pacing will be performed in the ventricles when the VV interval expires. At the same time, the pacemaker switches from sensing on the basis of the programmed, long VV interval to sensing on the basis of a programmed, short AV delay.

At stages 5 and 6, heartbeats are still not sensed in the ventricles and, therefore, the pacemaker continues to sense and pace on the basis of the AV delay. At stage 7, heartbeats are again sensed in the ventricles and, therefore, after stage 7 (not shown), the pacemaker will switch back to sensing on the basis of the VV interval instead of the AV delay. If, later on, the pacemaker does not again sense any heartbeats in the ventricle within the programmed, long VV interval, stages 1 to 7 will be repeated.

Thus the pacemaker will maintain sensing and pacing in a type of pacemaker function based on the VV interval until heartbeats are no longer sensed in the ventricles within the VV interval. Then the pacemaker will switch from the first type of pacemaker function to a second type of pacemaker function and sense and pace on the basis of, for example, a chosen, programmed AV delay. As soon as heartbeats are sensed in the ventricles again, the pacemaker will switch back from the second type of pacemaker function to the first type of pacemaker function and back to sensing on the basis of the programmed VV interval.

Fig. 5 shows a pacing sequence in which the pacemaker works in AAI/V back up mode. At stages 1 and 2, detection of heartbeats takes place in at least one of the atria (A) and, subsequently, detection of heartbeats takes place in at least one of the ventricles (V). Detection of heartbeats in the ventricle is stated as "sens" on the abscissa of the figure.

After detection of the heartbeat in the ventricles (V) has taken place, a VV interval is initiated which is 3000 ms in the figure shown and constitutes the time interval between the heartbeats in the ventricles that the pacemaker will accept before pacing of the ventricles takes place.

At stages 3, 4 and 5, detection of heartbeats takes place in the atria (A); however, there is no subsequent detection of heartbeats in the ventricles (V). This may indicate that an AV block has occurred between the atria and the ventricles. Because of the relatively long VV interval, the pacemaker initially accepts that there are no heartbeats in the ventricles at stages 3, 4 and 5.

When the empirically determined VV interval, which is 3000 ms in the figure shown, as mentioned, expires and a heartbeat has not yet been sensed in the ventricles, the pacemaker performs pacing of the ventricles between stages 5 and 6. Immediately after this, a new VV interval of 3000 ms is initiated.

At stage 6, a beat is sensed in the atria (A) and a subsequent but. compared with stages

1 and 2, relatively later beat in the ventricles is also sensed. At stages 7 and 8, a beat in the atria is sensed, but no subsequent beat is sensed in the ventricles. However, since the empirically determined VV interval has not yet expired the pacemaker will not yet perform any pacing of the ventricles.

At stage 9, beats in the atria are sensed and subsequent beats in the ventricles are sensed. The beat in the ventricles at stage 9 takes place immediately before the expiry of the VV interval. Thus in this case it is not necessary for the pacemaker to perform pacing of the ventricles. Immediately after the beat in the ventricles has been sensed at stage 9, another VV interval is initiated.

In the event of rapid, irregular rhythm disturbances from the atria (atrial fibrillation), DDD pacing is inappropriate whereas AAI/V back up will work better because of less stimulation and at the same time certainty against too slow a ventricular frequency is achieved. However, in the event of persistent slow ventricular action (such as a pacemaker stimulation frequency of 20 per min. over 10 seconds) there might be a need for automatic adjustment of the stimulation frequency, so-called "hysteresis", which is a well-known technique.

Claims

1. A pacemaker for the sensation of the atria and ventricles of a patient's heart and for stimulation of at least one of the ventricles of the patients heart, said pacemaker com- prising at least a first electrode intended for sensing a spontaneous heartbeat in an atrium of the heart, a second electrode intended for sensing a spontaneous heartbeat in a ventricle, an electrode for stimulation of heartbeats in one of the ventricles as a minimum, means for detecting a time interval, an AV delay, between the heartbeat of the atrium and the heartbeat of the ventricle, means for detecting a time interval be- tween the heartbeats of the ventricles, a spontaneous ventricular activity, and, in addition, means for switching from one type of pacemaker function to another type of pacemaker function, c h a r a c t e r i s e d in that said means for switching from one type of pacemaker function to another type of pacemaker function are activated when the means for detecting the time interval between the heartbeats in the ventricles have detected an empirically determined time interval, a programmed VV interval, between the beats in the ventricles.

2. A pacemaker according to claim 1, characterised in that the means for the stimulation of a heartbeat are not activated until the means for the detection of the time inter- val between the beats in the ventricles, the spontaneous ventricular activity, have detected a time interval, the programmed VV interval, between the beats of the ventricles of more than 1000 ms, preferably more than 2000 ms.

3. A pacemaker according to claim I or claim 2, characterised in that one type of pacemaker function is an atrial chamber function and that the other type of pacemaker function is a ventricular chamber function, primarily a single chamber ventricular function, but preferably a dual chamber function.

4. A pacemaker according to any of the preceding claims, characterised in that switching of the pacemaker function from one type of pacemaker to another type of pacemaker is determined on the basis of the programmed VV interval and that after switching from the one type of pacemaker function to the other type of pacemaker function then the pacemaker function is maintained on the basis of another programmed interval.

5. A pacemaker according to any of the preceding claims, characterised in that switching of the pacemaker function from one type of pacemaker to another type of pacemaker is determined on the basis of the programmed VV interval and that after switching from the one type of pacemaker function to the other type of pacemaker function then the pacemaker function is maintained on the basis of a programmed stimulation frequency.

6. A pacemaker according to any of the preceding claims, characterised in that the time interval, the programmed VV interval, between the beats of the ventricles is determined on the basis of an initial, absolute interval and a subsequent and terminal, relative interval.

7. A pacemaker according to any of the preceding claims, characterised in that the pacemaker is programmed to work initially as an atrial chamber pacemaker, that the pacemaker is programmed to work as a DDD pacemaker by means of switching auto- matically from atrial chamber mode to DDD mode if no ventricular activity is sensed, and that the pacemaker is programmed to work as an atrial chamber pacemaker again by switching automatically from DDD mode back to atrial chamber mode when spontaneous ventricular activity is sensed again.

8. A pacemaker according to any of the preceding claims, characterised in that the pacemaker is programmed to work initially as an AAI pacemaker, that the pacemaker is programmed to work as a ventricular pacemaker by switching automatically from AAI mode to ventricular mode if no spontaneous ventricular activity is sensed, and that the pacemaker is programmed to work as an AAI pacemaker again by switching from ventricular mode back to AAI mode when spontaneous ventricular activity is sensed again.

9. A pacemaker according to any of the preceding claims, characterised in that the pacemaker is programmed to work initially as an AAI pacemaker, that the pacemaker is programmed to work as a DDD pacemaker by switching automatically from AAI mode to DDD mode if no spontaneous ventricular activity is sensed, and that the pacemaker is programmed to work as an AAI pacemaker again by switching automatically from DDD mode back to AAI mode when spontaneous ventricular activity is sensed again.