Background Art To reduce the energy consumption of heart stimulators an automatic threshold search function, a so called AUTOCAPTURE function, is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture, cf. e. g. US-A-5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulses. The reason therefor resides in the fact that the evoked response potential is small in amplitude compared to the residual polarization charge. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds after the stimulation. If the polarization is too high, it could be wrongly interpreted by the evoked response detector as a capture, i. e. contraction of the heart. The AUTOCAPTURE algorithm could then by mistake adjust the output amplitude of the stimulation pulse to a value below the actual capture level, which will result in no capture. If the used pacing lead has significant

polarization this could consequently disturb the autocapture function and result in loss of capture.

A unipolar sensed evoked response signal differs from a bipolar sensed evoked response signal both in duration and amplitude, see M. W. Baig et al.,"Comparison of Unipolar and Bipolar Ventricular Paced Evoked Responses", Br. Heart J.

1992,68: 398-402. The duration of the evoked QRS complex is a measure of total ventricular polarization time in the area of the heart subtended by a sensing dipole, and it depends on the extension of the dipole. This means that the unipolar evoked response signal has a longer duration than the bipolar evoked response. Today's evoked response detectors are designed to detect the positive slope of the evoked response signal which occurs within a detection window, typically 15 to about 60 ms after the stimulation pulse, and is not suited for detection of the unipolar evoked response signal.

Thus there is mostly at least one significant slope in the bipolar measured IEGM signal, which makes it possible to discriminate the evoked response signal from slowly varying signals such as polarization signals. Thus in US-A-5,431,693 a method of verifying capture of the heart by a cardiac pacemaker is described. Observing that the non-capture potential is exponential in form and the evoked capture potential, while generally exponential in form, has one or more small amplitude perturbations superimposed on the exponential wave form, these perturbations are enhanced for ease of detection by processing the wave form signal by differentiation to form the second derivative of the evoked response signal for analysis for the evoked response detection.

Unipolar detection of evoked response signal is, however, not possible by this technique. Abrupt slope changes or superimposed small-amplitude perturbations are levelled out if the measurements are made over the longer distance from the electrode to the stimulator casing. Consequently the AUTOCAPTURETM function available today requires a bipolar

ventricular lead for the evoked response detection, whereas stimulation can be accomplished also in unipolar mode.

It has appeared that the true evoked response signal amplitude is fairly constant, independent of the stimulation pulse amplitude, i. e. the evoked response signal amplitude does not vary with the amplitude of the stimulation pulse (provided that the stimulation amplitude is above the stimulation threshold), whereas the electrode polarization is approximately linearly dependent on the stimulation pulse amplitude for a constant pulse duration, see Swedish patent application 9703600-8 and the description below in connection with figure 1. These circumstances make unipolar evoked response detection possible without measurement of the slope of any part of the signal picked up by the lead.

The purpose of the present invention is to provide a technique for setting the threshold value for evoked response detection for an evoked response detector, the function of which is based on these principles.

Disclosure of the Invention This purpose is obtained with an evoked response detector according to the introductory portion of the description having the characterizing features of claim 1.

The detector according to the invention can be used also for unipolar detection of evoked response, which is an important advantage since unipolar leads are less complicated to manufacture and have longer working life than bipolar electrodes. Of course, the detector according to the invention can also be used for unipolar detection of evoked response by using bipolar electrodes in an unipolar mode of operation. The measurement could then be performed between the ring electrode and the casing of the heart stimulator, since severe difficulties in relation to the measurements are avoided if the tip electrode, which is used for stimulation, is not used for the subsequent measurements.

According to an advantageous embodiment of the detector according to the invention an analysing means is arranged to activate an automatic threshold search function of the control means of the heart stimulator provided that the conditions Umeaslow<POLhigh and the |Umeaslow-POLhighl>Y are fulfilled. Thus in this way the use of heart stimulators, like pacemakers with AUTOCAPTURETM function, can be extended also to patients with unipolar leads. For sensing evoked response in the unipolar configuration not only low polarization leads can consequently be used with AUTOCAPTURETM pacemakers but also high polarization leads.

According to yet another advantageous embodiment of the detector according to the invention said pulse generator is controlled to deliver a predetermined number of stimulation pulses of each amplitude Ulstim, U2stim said calculating means being provided to calculate the mean or maximum amplitudes of the measured electrode signals U1, U2 picked by the lead after each pulse of said number of stimulation pulses, said mean or maximum amplitudes being used for setting the evoked response threshold value and for determining activation of said automatic threshold search function of the heart stimulator. In this way small variations in the picked up signals are suppressed.

According to another aspect of the invention a heart stimulator is provided comprising an evoked response detector as described.

Brief Description of the Drawings To explain the invention more in detail an embodiment of the detector according to the invention chosen as example will now be described with reference to the drawings, on which Figure 1 shows measured electrode signals (IEGM) picked up for four different stimulation pulse amplitudes as a function of time from immediately after the delivery of the

stimulation pulse to 70 ms after the pulse, as well as the same signals with the polarization subtracted, Figure 2 is a block diagram of the principal layout of a heart stimulator incorporating the detector according to the invention, and Figure 3 is a block diagram of an embodiment of the evoked response detector according to the invention.

Description of Preferred Embodiments The polarization of a pacemaker electrode can be described as Pol = Ustim x F (dur; RCcutput)(1) a where Pol denotes the polarization signal, Ustim the pacemaker stimulation pulse amplitude, a is a constant, dur designates the duration of the stimulation pulse and RCoutput is a time constant of the pacemaker's output lead system, see Konrad Mund,"Analysis of the Polarization and the Sensing Behaviour of Electrodes for Cardiac Pacemakers", Pacemaker Leads, Elsevier Science Publishers BV, 991, pp. 503-509.

Thus, according to equation (1) above the polarization is a function of the duration dur of the stimulation pulse and the time constant RCoutput. This means that if the duration dur of the stimulation pulse and the time constant RCoutput are constant for different stimulation pulse amplitudes the polarization Pol is only depending on the stimulation pulse amplitude Ustim and this dependency is linear.

Further, studies on animals show that the evoked response signal amplitude ER is roughly constant for different stimulation pulse amplitudes Ustim and independent of the stimulation pulse duration dur, cf. the previously mentioned Swedish patent application No. 9703600-8.

Figure 1 shows the measured electrode signal (IEGM) for different stimulation amplitudes as a function of time. Thus

the electrode signals are recorded from immediately after the delivery of a stimulation pulse, time 0, and until 70 msec after the stimulation. Curve A is obtained for a stimulation pulse amplitude of 0,6 V, curve B is obtained for a stimulation pulse amplitude of 1,5 V, curve C is obtained for a stimulation pulse amplitude of 3.0 V, and curve D for a stimulation pulse amplitude of 4.5 V.

Each curve A-D represents the sum of the evoked response signal and the polarization signal. As the evoked response signal is substantially constant it is apparent from this figure that the polarization signal varies significantly with the used stimulation pulse amplitude, the shown maximum variation amounting to approximately 50 mV.

The family of curves at the top of the figure represents the measured signals with subtracted polarization. As appears the curves are in fact concentrated in a narrow region, thus illustrating the above mentioned fact that the evoked response signal is substantially independent of the stimulation amplitude.

Figure 2 shows a block diagram of the principal layout of a heart stimulator incorporating the detector according to the invention. The stimulator comprises a pulse generator 2 which through a lead 6 is connected to the heart 8 of a patient.

The pulse generator 2 is devised to produce stimulation pulses of varying amplitudes which through the lead 6 are transferred to the heart 8. The evoked response detector 4 comprises an analogue bandpass filter with a cut-off frequency of 1-130 Hz and measuring means 10. The filtered measured signal is supplied to a calculating means 16 where the signal is sampled and digitized. Such a sampling and digitizing procedure is performed before the emission of each stimulation pulse and the mean value of these samples is calculated in the calculating means 16 and represents the DC level. The stimulation pulse followed by the fast discharge pulse then occurs. Timing means 14 are provided for blanking the evoked response detector to 15 msec of the stimulation

pulse, whereupon the picked up IEGM signal is sampled and digitized. The sampling frequency can suitably be 512 Hz and the sampling occurs during an evoked response window of about 40 msec, determined by the timing means 14. This means that about 20 samples are collected for each stimulation. The DC level determined before the delivery of the stimulation pulse is subtracted from each sample and the mean value of all 20 samples is calculated in the calculating means 16. This mean amplitude of the picked up electrode signal is supplied to analyzing means 12. Control means 18 are connected to the analyzing means 12 and the pulse generator 2 for controlling the pulse generator 2 according to the signal received from the analyzing means 12.

To determine the threshold value for evoked response detection ERLimit the following procedure is used.

It is assumed that the stimulation amplitude can be varied from 4.5 V to 0.3 V with an amplitude resolution of 0.3 V.

Further information about the stimulation threshold value is supplied to the analyzing means 12 e. g. by way of a programmer.

Five stimulation pulses of 4.5 V are delivered and the picked up electrode signal after each stimulation pulse is sampled and digitized as described above. The less negative value or the mean value of these five electrode signals Umeas4. 5 is stored.

If the stimulation threshold value is less than a predetermined value X, five stimulation pulses of an amplitude equal to the stimulation threshold value plus 0.3 V are delivered and the corresponding picked up electrode signals are treated as described above and the less negative value or the mean value of these five electrode signals are stored as Umeasthresh+0.3.

The polarization for the stimulation amplitude Ustim = 4.5- (Ustimthresh+0, 3) is given by the equation

P014. 5-(thresh+O 3) Umeas4. 5 Umeasthresh+0.(thresh+O 3) Umeas4. 5 Umeasthresh+0. 3(2) The polarization for a stimulation amplitude of 0.3 V is given by the equation PolO.3= Pol4. 5- (thresh+0.3) K (3) where K = 4.5- (Ustimthresh+0. 3) 0.3 If the stimulation threshold value Ustimtresh> X the procedure will be as follows.

Five stimulation pulses with an amplitude Ustimthresh-0.3v equal to the stimulation threshold value minus 0.3 V are delivered and the picked up electrode signal Umeasthresh-o. 3 is treated as described above and the mean value from the five stimulation pulses is stored as these stimulations will result in loss of capture the stored mean value will represent the polarization Polthresh-0-3 The polarization for a stimulation pulse of 0.3 V can be calculated as Polo. 3 = Polthresh-0.3 K where K = Ustimthresh-0.3 0.3 The polarization for a stimulation pulse of 4.5 V can be calculated by multiplying Polo. 3 by 15, that is P014.5 = 15 x Polo. 3.

The picked up electrode signal Umeaso. 6 for a stimulation amplitude of 0.6 V can be calculated as follows.

UmeasO. 6 = Umeas4.5-13 X POLo 3 This calculation is needed only if the stimulation threshold value is equal to or exceeds 0.6 V. If the stimulation threshold value equals 0.3 V Un, 6 = Umeasthres+0. 3-

For setting the threshold value ERLimit for the measured electrode signal for evoked response detection the difference between Umeaso. 6 and Pol4.5 is studied. If Umeaso. 6 < Pol4.5 and lUmeaso. 6-P014. 5 > Y, where Y denotes a predetermined value, the threshold value is set as <BR> <BR> <BR> <BR> <BR> Umeas0.6-P14. 5<BR> ERLimit-+ POlq, 5 n where n denotes a figure > 1, preferably 1 < n < 10.

Thus, if the measured electrode signal Umeas<Umeasthresh (the electrode signal is negative), capture is indicated and the AUTOCAPTURET function of the control means 18 is activated by the analyzing means 12.

Figure 3 shows in more detail an embodiment of the evoked response detector according to the invention. The heart electrode signal picked up by the lead 6 in figure 2 is then supplied to a highpass filter 20. An amplifier 22 and an A/D- converter 24 are provided for amplifying and A/D-converting respectively the filtered signal. The block 26 comprises a digital signal processor for calculating the polarization Polhigh for a high stimulation amplitude and the picked up electrode signal Umeaslow for the lowest stimulation amplitude and comparing subsequently picked up electrode signals with a threshold value determined from the above mentioned quantities as described above to determine whether an evoked response is detected or not.

Thus in the embodiment shown in figure 3 the algorithm for determining the above mentioned threshold value is implemented in software by use of a microprocessor. Instead of a microprocessor this algorithm can also be implemented in random logic, which means realization by ordinary logic element, that is logic gates.

The detector according to the invention can also be implemented in the pacemaker electronics by use of switch capacitor (SC) technique where different capacitors serve as

memory elements for storing the different electrode potentials and SC-adding, subtracting and multiplying circuits are used for performing the necessary calculations as explained above.

It is obvious to the man skilled in the art that the embodiments described above with reference to figures 2 and 3 should be considered merely as illustrating examples of the realization of the invention and that numerous modifications and variations are possible. Thus e. g. the figures given in the calculations described above are, of course, just examples. 4.5 V is an example of a high stimulation amplitude (preferably the highest available stimulation amplitude) and 0.6 V is an example of the lowest available stimulation amplitude. Obviously these numeral values can be varied.