APPARATUS FOR RECOGNISING REJECTION OF TRANSPLANTED HEARTS

Abstract: Apparatus for recognising rejection of transplanted hearts digitalises analogue signals which are obtained when carrying out endocardial or epicardial sensing of the heart and carries out a first differentiation of the digitalised signals. A gradient pattern detector compares the differentiated signals thereby produced with signals of like origin produced when the heart was newly transplanted with respect to amplitude and, optionally, interval, as well as possibly polarity and sequence of amplitudes. Lower amplitudes and slew rates will be indicative of the recipient's body reaction against the implanted heart. The results of the comparison will be transmitted to recording and storage means and/or the patient will be alerted to the rejection which is taking place.

Description: This invention relates to apparatus for recognising rejection of transplanted hearts. The invention relates in particular, but not exclusively, to cardiac implants for detecting and alerting a patient to rejection of a transplanted heart. After a human heart transplantation, it is necessary to monitor the recipient's body reaction against the implanted heart to prevent destruction of the new heart by an immune reaction. Commonly employed methods for detecting rejection are cumbersome and invasive. In one such method according to FR-A-2193320, there is intratissular impedence measurement using electrodes in the zone of interest, by means of an alternating current of appropriate frequency. Changes in impedence indicate rejection. It is known, for example, from US-A-4523595 that the morphology of cardiac electrograms alters during ventricular fibrillation. It is observed that such morphology also changes during a rejection process although this observation has not hitherto found practical use. For practical reasons, the preferred method of monitoring the cardiac electrogram would be by use of an implanted pacemaker which would be able to transmit the electrogram via telemetry to an external device such as a tape recorder or a paper recorder. The main limitation of this method lies in the poor transmission properties of the pacemaker electrocardiogram amplifier and the weakness of the telemetry link to the outside world. More specifically, current pacemaker sensing amplifiers are relatively unsophisticated in the detection of intracardiac signals, relying entirely upon simple amplitude and frequency band pass filtration analysis. For technical reasons, it is difficult to improve on the transmission properties of the electrogram of a pacemaker. Thus, the extraction of information about changes in the electrogram must be done inside the pacemaker before transmission to the external equipment takes place. To do this efficiently in an implantable system is only possible if there exists a simple and powerful algorithm by which relevant information about the signal morphology can be obtained. It is an object of this invention to provide apparatus for monitoring the body reaction against a transplanted heart to allow timely measures to be taken against rejection. According to the present invention, there is provided apparatus for monitoring the body reaction against a heart transplanted thereinto, which is an apparatus which comprises sensing electrodes for attachment to the heart at either an endocardial or an epicardial location, the apparatus storing a reference waveform for comparison with a waveform of like type and identifying from discrepancies therebetween malfunctioning of the heart, the apparatus being characterised by a signal path for receiving electric current from the electrodes and having, in series, an analogue to digital converter, means for carrying out a first differentiation of the digital signals produced, a gradient pattern detector which is adapted to compare differentiated signals thereby produced with differentiated signals of like origin produced when the heart was newly transplanted, with respect to amplitude and, optionally, interval, as well as possibly polarity and sequence of amplitudes, and means for recording and storing the results of a comparison within the apparatus for supply on demand by telemetry to means lying externally of the body for reproducing the results of the comparison in a readable form and/or means for alerting the patient to unfavourable assessment of the results of the comparison as it is produced. The present invention thus encompasses both cardiac implants modified to embody this invention either (A) to be worn by a patient discharged from hospital and/or (B) to be worn by a patient who is in a post-operative phase within a hospital. Apparatus embodying this invention is superior to that previously proposed in that, since the data reduction has been done inside the pacemaker, the requirements on a telemetry link are greatly relieved so that this operation no longer is limited by the quality of the telemetry link. Moreover, insofar as use is by a patient externally of a hospital is concerned, he may be warned by an audible signal when the implanted pacemaker is transmitting signals suggesting rejection of the heart. The patient will then be able to consult his physician who, by obtaining transmission of stored data will be able to obtain the comparison of differentiated signals in readable form and can establish the extent of rejection which is taking place. Infusion of a rejection suppressing drug can then be prescribed for the patient. At a future time it is envisaged that an in board pump for supply to the body of such drug might be activated. Alternatively, if the indications are spurious then this would mean at least that the implant is malfunctioning and should be replaced, repaired or reprogrammed. The concept of gradient pattern detection (GPD) in clinical electrophysiology utilised in the apparatus of this invention is a recent one described, for example, in United States Patent No. 4905708, published on 06/03/90 and issued to Davies entitled "Apparatus for Recognising Cardiac Rhythms" and describing specifically the detection of and response to cardiac arrhythmias. In contrast to other methods of analysis of the morphology of intracardiac electrograms, apparatus of the present invention utilises the principle of gradient pattern detection. This is a simpler algorithm requiring less in the way of computing power and not reducing implant battery life to the same extent as other methods such as fast Fourier transformation and template matching. Although gradient pattern detection (GPD) consumes battery power additional to that consumed when detection of heart implant rejection is carried out using conventional methods, it is not necessary to operate continuously the apparatus embodying this invention. GPD will only need to be effected at predetermined intervals since rejection does not occur instantaneously but over a period of time. Satisfactory operation of apparatus embodying this invention is dependent on the stability of the sensing electrode, as even small movements of the electrode tip would be expected to produce unpredicted differences in electrogram shape. In experiments, recordings obtained using temporary electrodes with conventional "passive" tips yielded stable electrogram morphology within each rhythm despite this movement and the postural and the respiratory variations encountered. The principle behind apparatus embodying this invention will now be described in greater detail with reference to sinus rhythm, that is the normal rhythm of a heart operating stably. Thus, in a first step, electrograms are sensed in the usual manner shortly after implantation of the replacement heart, sensing generally taking place at a signal frequency in the range of 0.016 Hz-1 kHz, with a preferred range being from 0.5 Hz to 500 Hz, the electrograms being obtained during sinus rhythm. The analogue electrograms produced are then converted to digital form preferably using an analogue to digital converted frequency of 1024 Hz. The digital electrograms are then converted to a first differential form where the amplitudes of this processed signal are proportional to the gradients (slopes or slew-rates) of the original analogue signal. The resultant sequence of amplitudes and their temporal spacing is then stored. Subsequently during continued operation of the heart, electrograms are produced and processed in like manner and compared with the processed "normal" electrogram to determine whether changes have taken place which are symptomatic of development of heart rejection. This comparison will be sufficiently sensitive to show the small changes which take place indicating the onset of rejection. The electrograms will become, as a result of rejection, of lower amplitude and slew rates. The apparatus will be specific both to the patient and the location at which the sensors are placed. Put at its simplest, this invention involves comparison of the first differentials of the slopes of the digital electrograms of each beat with what is established for the first differential of the slopes of the digital electrograms before rejection. As will be apparent from Figure 2 of the accompanying drawings, each beat has gradients whose gradient differentials will differ from each other in magnitude, polarity and interval. It may be sufficient for many purposes to rely merely on gradients of and intervals between the processed signals being compared. A particularly sophisticated form of control can be achieved if scanning is carried out only at intervals, with it being necessary for a particular sequence of deflections of particular magnitude to be achieved within a particular time interval and generally in a particular order within that time interval, which parameters are of course established when the heart has been operating stably, that is the programmable window is utilised for comparison purposes. For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made by way of example only to the accompanying drawings wherein:- FIGURE 1 is a schematic plot of amplitude against time for successive beats of a patient showing normal beats; FIGURE 2 shows an approximate first derivative of the signals of Figure 1; FIGURE 3 shows in the upper panel a copy of an experimentally obtained analogue electrogram and in the lower panel a copy of the derivatives of the electrogram; FIGURE 4 is a block diagram of the cardiac implant embodying the invention; and FIGURE 5 is a flow diagram of the steps involved in recognising rejection of a heart using apparatus embodying this invention. As can be seen from Figure 1, each beat is shown to possess two peaks which differ in gradient and amplitude between the respective peaks although there may be a different number of peaks. Insofar as Figure 1 is concerned, there are four slopes to each beat and thus, referring to Figure 2, the first differential of each slope will be correspondingly different as to magnitude, polarity and spacing from the other differentials associated with the beat. This derivative plot, through its simplicity can yield useful indications and requires less power to transmit by telemetry, an advantage in view of the poor transmission properties of a pacemaker electrocardiogram amplifier and the weakness of telemetry links to the outside world. It will enable even small changes in heart performance to be detected. An analogue electrogram obtained in practice is shown in the upper panel of Figure 3 of the drawings, the lower panel showing processed signals. The threshold lines A and B illustrate the mechanism. For recognition of a condition in which rejection is not taking place, line A followed by line B both have to be crossed by the process signal (PS). A failure of either line A or line B to be crossed by PS may denote rejection of the implanted heart. Such sensing coupled with change in separation between successive such intersections will indicate deterioration. In practical determinations to obtain results of the type shown in Figure 3, electrograms have been recorded from 1 cm bipoles in the atrium and either 1 cm bipoles or 0.5 cm bipoles at the right ventricular apex (RVA), a single surface ECG channel being recorded simultaneously. The intracardiac recordings were made either DC to 1 kHz or 0.016 Hz to 1 kHz using a Biodata P400 amplifier onto a Racal store 7 tape recorder. The analogue electrograms recorded were then digitised at a sampling frequency of 1024 Hz. This relatively low sampling frequency was chosen firstly because the electrogram frequency spectrum contains little useful information among higher frequencies and secondly with a view to implantability of a pacemaker embodying the invention and the need to consider battery conservation. The digital electrogram was then converted into a form closely resembling its first time derivative where the amplitudes of the derived signal were proportional to the rates of change of the original electrogram. The derived signal was then analysed by a gradient pattern detection programme of the invention which examined for the sequence of turning point and more particularly for the magnitude of amplitudes and the temporal separation within the derived signal. The values obtained were then compared with those within a reference signal obtained during sinus rhythm shortly after implanting of the heart, recognition of rejection being based upon the difference between the sequences. Such recognition occurs when certain individually adjustable criteria were met by the derived signal and terminated after an adjustable number of gradients within the signal had occurred. The triggering criteria consisted for example of deflections of the processed signal (of either polarity) being less than thresholds for amplitudes and these being sustained for a preset minimum number of sampling points. Satisfaction of these requirements ensured that extraneous "noise" was not misinterpreted. The sequence of gradients within the original electrogram was detected by analysis of the amplitudes of the subsequent deflections of the processed signals. Rejection of the heart was detected when the amplitudes of the deflections differed in magnitude from those seen before rejection particularly if coupled with changing of the intervals between equivalent deflections. Variations of respiration, posture and sinus rate have not been found to alter atrial electrogram morphology in patients in such a way as to affect recognition by GPD despite the fact that there may have been noticable changes in analogue electrogram amplitudes in patients. Moreover, similar types of results have been obtained when producing ventricular electrograms. Finally, Figures 4 and 5 show practical embodiments of the invention and should be viewed in conjunction with each other. Thus an implant located at for example an atrial location may monitor heart rate at pre-programmed intervals using a normal heart beat detector 2 having time base and control 4 whose operation is directed by a microprocessor 5 having a memory 6. At such sampling intervals, gradient pattern detection is switched on at rejection detector 3 which is normally operating in back-up mode. Electrograms produced in detector 3 are subjected to analogue digital conversion and the digitalised signals are subjected to a first differentiation. The amplitude and generally also sequence, interval and polarity of the deflections of the processed signals obtained are examined. With the assistance of memory 6, these first differentials are compared with first derivative values characteristic of the rhythm before rejection. If there is a match, then no device for warning a patient will be actuated. However, if there is a variation indicating possible signs of heart rejection, then the variance will generally be manifested through change in magnitude of the derived function as well as possibly change in sequence, polarity and interval and a signal will be delivered from control 4 to transmitter 7. The purpose of the signal from transmitter 7 is to warn the patient to consult his physician. The transmitter 7 is provided with a storage memory and the physician can interrogate this memory to obtain retransmission of the original signals to means for reproducing the stored signals in a readable form. If the indications are of heart rejection, then the GPD may be switched on externally and further readings obtained. If the physician is satisfied that rejection is taking place, remedial action may be effected. However if it is shown that the readings were spurious, either the GPD will be switched off or reprogrammed, or the GPD may be kept operating on a continuous basis for the purposes of examination. If it should be shown that there is malfunctioning, then surgery will be required to achieve replacement of components of the pacemaker.

Claims: Apparatus for monitoring the body reaction against a heart transplanted thereinto, which is an apparatus which comprises sensing electrodes (1) for attachment to the heart at either an endocardial or an epicardial location, the apparatus storing a reference waveform for comparison with a waveform of like type and identifying from discrepancies therebetween malfunctioning of the heart, the apparatus being characterised by a signal path for receiving electric current from the electrodes (1) and having, in series, an analogue to digital converter, means (5) for carrying out a first differentiation of the digital signals produced, a gradient pattern detector (3) which is adapted to compare differentiated signals thereby produced with differentiated signals of like origin produced when the heart was newly transplanted, with respect to amplitude and, optionally, interval, as well as possibly polarity and sequence of amplitudes, and means (7) for recording and storing the results of a comparison within the apparatus for supply on demand by telemetry to means lying externally of the body for reproducing the results of the comparison in a readable form and/or means (7) for alerting the patient to unfavourable assessment of the results of the comparison as it is produced. Apparatus as claimed in claim 1, which operates as a signal frequency in the range of 0.016 Hz to 1 kHz. Apparatus as claimed in claim 1 or 2, in which the analogue-digital converter operates at a frequency of 1024 Hz. Apparatus as claimed in any of the preceding claims, wherein the means for alerting the patient produces an audible signal. Apparatus as claimed in any preceding claim, which is adapted to provide sensing at periodic intervals. Apparatus as claimed in any preceding claim, which includes programming to require deflections of a processed signal being less than thresholds for amplitudes and these being sustained for a pre-set minimum number of sampling points.

Inventors: DAVIES, DAVID WYN DEPARTMENT OF CARDIOLOGY, LEKHOLM, BENGT ANDERS
Applicants: DAVIES, DAVID WYN, SIEMENS AKTIENGESELLSCHAFT
Application year: 1986
Main classification: A61B5
IPC codes: A61B5