Technical Field The present invention relates to a medical implant comprising pulse generator means for delivering stimulation pulses to at least one chamber of a pa- tient's heart, an evoked response detector for distinguishing capture from loss of capture from the value of a selected one of a plurality of parameters obtainable from an IEGM signal sensed in an evoked response detection time window fol- lowing delivery of a stimulation pulse, and a setting means for setting a minimum tolerable difference between values of said selected parameter obtained in case of capture and in case of loss of capture respectively. As an alternative, the evoked response detection window can have a fixed length.

Background to the Invention Implantable pacemakers, which automatically detect capture and minimize pacing energy provide many benefits. The use of minimal pacing energy maxi- mizes device longevity and minimizes the size of the device, and most importantly, automatic output regulation protects the patient from loss of capture caused by a rise in the threshold of stimulation.

For automatic capture a cardiac signal sensed in an evoked response detection time window after each stimulation pulse is analysed to determine whether or not the stimulation pulse captured the heart of a patient. The length of the evoked response detection time window is conventionally fixed. If a shorter evoked response detection time window could be used, a stimulation backup pulse could be delivered quicker, however, the shorter evoked response detection time window the greater risk of inaccurate decisions.

The shortest length of an evoked response detection time window that has a tolerable risk of inaccurate decisions depends on the lead type, the lead position, the evoked response of the patient and the parameter used to distinguish capture from loss of capture. Evoked response detection is the heart of the algorithm of automatic capture and thus very important.

There are mainly three different evoked response detection methods to- day, namely methods using the parameters maximum signal amplitude, maximum signal slope of the sensed IEGM signal, or area obtained by integrating the sensed

IEGM signal over the evoked response detection time window. The value of the measured parameter is compared to a pre-set threshold. Values above the threshold indicate capture and values below the threshold indicate loss of capture.

Thus, Boriani et. al. describes in the article titled"Atrial Evoked Response Integral for Automatic Capture Verification in Atrial Pacing", PACE 2003, Vol. 26, Part II, page 1-5, January 2003, the integral of the atrial evoked response signal as a re- source for verification of atrial capture.

The Purpose of the Invention The purpose of the present invention is to provide an improved medical implant which is quick in distinguishing capture from loss of capture and with a tol- erable risk of inaccurate decisions.

Disclosure of the Invention The above-mentioned purpose is obtained by a medical implant compris- ing pulse generator means for delivering stimulation pulses to at least one cham- ber of a patient's heart, an evoked response detector for distinguishing capture from loss of capture from the value of a selected one of a plurality of parameters obtainable from an IEGM signal sensed in an evoked response detection time window following delivery of a stimulation pulse, and a setting means for setting a minimum tolerable difference between values of said selected parameter obtained in case of capture and in case of loss of capture respectively, and having a first calculation means provided to calculate for each of said parameters the length of the evoked response detection time window for which said minimum tolerable dif- ference is obtained, and a first selecting means provided to select that parameter for distinguishing capture and loss of capture for which said minimum tolerable difference is obtained with the shortest evoked response detection time window.

Thus, this medical implant is able to automatically select that parameter for distinguishing capture and loss of capture for which said minimum tolerable difference is obtained with the shortest evoked response detection time window.

The only requirements are that the evoked response detector is able to distinguish capture from loss of capture with at least one of the available parameters if an evoked response detection time window of a maximum length is used, where the maximum length can be very large, e. g. 120 ms, and that a minimum tolerable

difference between values of said selected parameter obtained in case of capture and in case of loss of capture, respectively, is set.

According to an advantageous embodiment of the medical implant ac- cording to the present invention, a third calculation means is provided to calculate a matrix or table of said difference for different lengths of the evoked response detection time window and different ones of said parameters for storage for use in later off-line analysis.

Further, the above-mentioned purpose is also obtained by a medical im- plant according to the invention, and having a second calculation means provided to calculate for each of said parameters the difference between the value of the parameter obtained in case of capture and in case of loss of capture respectively, and a second selecting means provided to select that parameter for distinguishing capture and loss of capture by comparison with said minimum tolerable difference for which a maximum difference is obtained. In this way the risk of inaccurate de- cision is reduced to a minimum.

According to an advantageous embodiment of the medical implant accord- ing to the invention, said setting means is adapted to set said minimum tolerable difference with a safety margin. According to a further advantageous embodiment of the medical implant according to the invention, said minimum tolerable differ- ence is pre-set or programmable.

According to another advantageous embodiment of the medical implant according to the invention, said setting means and said second calculation means are adapted to calculate as said difference the signal-to-noise-ratio SNR from the equation TrilriEapture max (ERMlossofcapture 'capture 'lossofcapture | max (ERMcapture) min (ERMlossof capture) SA2R= kEquation [1] | max (ERMcapulre) min (ERMlossof capture) 'Ecapture G Elossof capture in (ERMcaptre) max (ERMlossof capture) J N Ecapture=- () N i=,

where ERMcapture and ERMloss of capture denote the parameter values obtained in case of capture and loss of capture respectively.

According to yet another advantageous embodiment of the medical im- plant according to the invention, said parameters include maximum signal ampli- tude and maximum signal slope of the sensed IEGM signal, and area obtained by integrating the sensed IEGM signal over the evoked response detection time win- dow.

According to an advantageous embodiment of the medical implant accord- ing to the invention, a differentiating means is provided to calculate the derivative of the sensed IEGM signal for the determination of the maximum slope.

According to a further advantageous embodiment of the medical implant according to the invention, said pulse generator means are controlled to deliver a stimulation back-up pulse at the end of the evoked response detection time win- dow in response to detected loss of capture.

Brief Description of the Drawings The present invention will now, with the purpose of exemplifying, be de- scribed more in detail by way of embodiments and by referring to the appended drawings, wherein : Fig. 1 shows a diagram showing a sensed evoked response signal result- ing from a stimulation pulse, Fig. 2 shows schematically a first preferred embodiment of the medical im- plant according to the present invention, Fig. 3 shows schematically a second preferred embodiment of the medical implant according to the present invention, Fig. 4 shows a flow diagram of a procedure performed by the first pre- ferred embodiment of the medical implant according to the present invention, and Fig. 5 shows a flow diagram of a procedure performed by the second pre- ferred embodiment of the medical implant according to the present invention.

Description of Preferred Embodiments Fig. 1 shows an evoked response resulting from a stimulation pulse. In the diagram an evoked response 1.1 is shown, followed by a T wave 1.2. Further, an evoked response detection time window 1.3 is shown as a rectangle. The signal sensed in this time window 1.3 is analysed to determine whether or not the stimu-

lation pulse has captured the heart. Herein, the length of the window 1.3 is about 39 ms.

Fig. 2 illustrates schematically a first preferred embodiment of the medical implant according to the present invention. The medical implant is connected to a patient's heart 2.1 and comprises pulse generator means 2.2 for delivering stimu- lation pulses to at least one chamber of the patient's heart 2.1. The pulse genera- tor means 2.2 are controlled to deliver a stimulation back-up pulse at the end of the evoked response detection time window in response to detected loss of cap- ture. The medical implant also comprises an evoked response detector 2.3 for dis- tinguishing capture from loss of capture from the value of a selected one of a plu- rality of parameters obtainable from an IEGM signal sensed in an evoked re- sponse detection time window, as shown in Fig. 1, following delivery of a stimula- tion pulse. Said parameters include maximum signal amplitude and maximum sig- nal slope of the sensed IEGM signal, and area obtained by integrating the sensed IEGM signal over the evoked response detection time window. Further, the medi- cal implant comprises a setting means 2.4 for setting a minimum tolerable differ- ence between values of said selected parameter obtained in case of capture and in case of loss of capture respectively. The setting means 2.4 is adapted to set said minimum tolerable difference with a safety margin. The minimum tolerable difference is pre-set or programmable. The setting means 2.4 is adapted to calcu- late as said difference the signal-to-noise-ratio SNR from above-mentioned Equa- tion [1]. A differentiating means 2.5 arranged to calculate the derivative of the sensed IEGM signal for the determination of the maximum slope, an integrating means 2.6 arranged to integrate the sensed IEGM signal over the evoked re- sponse detection window providing above-said area, and a maximum signal am- plitude means 2.7 arranged to provide the maximum signal amplitude are pro- vided. A first calculation means 2.8 is provided arranged to calculate for each of said parameters the length of the evoked response detection time window for which said minimum tolerable difference is obtained, together with a first selecting means 2.9 arranged to select that parameter for distinguishing capture and loss of capture for which said minimum tolerable difference is obtained with the shortest evoked response detection time window. Further, a third calculation means 2.10 is provided to calculate a matrix or table of said difference for different lengths of the

evoked response detection time window and different ones of said parameters for storage for use in later off-line analysis.

Fig. 3 illustrates schematically a second preferred embodiment of the medical implant according to the present invention. As the embodiment of Fig. 2, this embodiment also comprises a pulse generator 3.2, an evoked response de- tector 3.3, a setting means 3.4, a differentiating means 3.5, an integrating means 3.6, and a maximum signal amplitude means 3.7, each with the same function as in the embodiment of Fig. 2. In this embodiment the evoked response detection time window has a fixed length. A second calculation means 3.8 is provided to cal- culate for each of said parameters the difference between the value of the pa- rameter obtained in case of capture and in case of loss of capture respectively, and the second calculation means 3.8 is adapted to calculate as said difference the signal-to-noise-ratio SNR from above-mentioned Equation [1]. Further, a sec- ond selecting means 3.9 is provided to select that parameter for distinguishing capture and loss of capture by comparison with said minimum tolerable difference for which a maximum difference is obtained.

Fig. 4 shows a flow diagram illustrating a procedure performed by the above-mentioned first preferred embodiment of the medical implant according to the present invention, where the procedure includes the following steps: 4.1 Delivering a series of stimulation pulses to at least one chamber of a patient's heart, the amplitude of which ranging from zero to a certain maximum amplitude.

4.2 Recording the electrical activity in an evoked response time window of a certain maximum length after each stimulation pulse. The recording is per- formed by way of a modified VARIO test from the maximum amplitude down to zero without interrupting it.

4.3 Emitting a backup pulse at the end of the evoked response time win- dow after each stimulation pulse.

4.4 Storing the electrical activity in an evoked response time window of a certain maximum length.

4.5 After completion of the recording, calculating the parameters for the evoked response time window of said certain maximum length from the stored values.

4.6 Determining the stimulation threshold for capture.

4. 7 Calculating the signal-to-noise-ratio SNR from the above-mentioned Equation [1] for all parameters and multiple evoked response time window lengths.

4. 8 Selecting that parameter for distinguishing capture from loss of cap- ture for which the SNR is above a pre-set minimum tolerable difference with the shortest evoked response detection time window.

Fig. 5 shows a flow diagram illustrating a procedure performed by the sec- ond preferred embodiment of the medical implant according to the present inven- tion. The length of the evoked response detection window is specified and fixed, and the procedure includes the steps 5.1 to 5.7, which correspond to the steps 4.1 to 4.7 of the procedure of Fig. 4, and step 5.8, which involves selecting that pa- rameter for distinguishing capture and loss of capture by comparison with said minimum tolerable difference for which the greatest SNR is obtained is done.