METHOD AND APPARATUS FOR IDENTIFYING HEARTBEAT

Abstract: The invention relates to a method and an apparatus for identifying a heartbeat. In the method, an arterial pressure signal is measured in the form of multichannel measurement, and the required signal-processing operations, such as filtration, are carried out if necessary. The method also comprises carrying out signal detection and decision-making concerning the acceptance as a heartbeat signal. According to the invention, the signal detection includes multichannel channel-specific detection for the purpose of identifying signal components of different channels, and the obtained channel-specific detected channel signals are used as input data during the decision-making stage.

Description: METHODANDAPPARATUS FORIDENTIFYING HEARTBEAT The invention relates to a method for identifying a heartbeat, the method comprising measuring an arterial pressure signal in the form of multichannel measurement, and carrying out the required signal-processing operations, such as filtration, if necessary, and performing signal detection and decision-making concerning the identification of a signal as a heartbeat signal. The invention also relates to an apparatus for identifying a heartbeat, the apparatus comprising a multichannel sensor, optionally signal- processing means for processing a signal obtained from the multichannel sensor, detection means for detecting a signal, and decision-making means for identifying a signal as a heartbeat signal. Prior art conventional arrangements for heartbeat measurement performed on an arterial pressure pulse are based on the use of one sensing element, in other words the measurement is carried out via one channel. The problem is, however, that heartbeat measurement, based on the use of one sensing element, from an arterial pressure pulse is only possible at rest and in a situation with no interference. A movement of a limb, a tendon or a muscle causes great interference on the signal measured by the sensor, which may result in unsuccessful measurement. The problem is aggravated by the near similarity of the frequency bands and waveforms of the interference and the actual signal in an individual measurement channel, which makes it more difficult to distinguish a signal and interference from one another. Arrangements utilizing multichannel measurement are also known. Multichannel measurement of a pressure pulse provides better capacity than a one-channel method. In a multichannel pressure pulse signal, the interference is mainly common-mode or similar in different channels, which enables elimination of interference caused by movements. A known method is disclosed in US 5,243,992. The method disclosed in this reference utilizes a multichannel tonometer sensor. The signal processing utilizes known methods for decreasing interference signals, such as band-pass filtration and subtraction of an average of signals of all channels from each channel signal. In US 5,243,992 the heartbeat detection is based on the selection of the best signal channel. The pulse in the selected channel is accepted as a heartbeat pulse by means of a correlation function. A few preceding pulse amplitudes are used as the selection criteria for the channel. Such selection of one channel leads to uncertain operation during interference situations and when the sensor moves with respect to the artery. Due to the high interference level, the method based on the selection of one channel may lead to the selection of a channel other than the one with the best heartbeat signal. Also, when the heartbeat of a moving person is measured, the movement of the sensor at the wrist with respect to the artery may transfer the heartbeat signal to a different channel, and therefore the signal may be lost for a while until a channel providing a good signal is located again. Also, the use of mathematical operations, such as a correlation function, requires a great deal of calculation capacity, in which case implementation of a signal processor is required, which in turn leads to a large size and great consumption of energy. Other known multichannel measurement methods are disclosed in EP 404,594 and SE 425,290. The purpose of the present invention is to provide a new type of method and system avoiding the problems of the prior arrangements. In order to achieve this, the method according to the invention is characterized in that in the method the signal detection includes multichannel channel-specific detection for the purpose of identifying signal components of different channels, and that the obtained channel-specific detected channel signals are used as input data in the decision-making stage. In order to achieve this, the apparatus according to the invention is characterized in that the detection means are arranged to carry out multichannel channel-specific detection on several different channels, and that the decision-making means are connected to perform identification of a heartbeat signal from among said several channel signals detected specifically for each channel. The invention provides several advantages. The arrangement according to the invention provides better reliability and capacity with the use of more than one signal channel for the detection of heartbeat or pulse. Transfer of the sensor with respect to the artery does not necessarily remove the signal from all the channels, as might happen in a one-channel method. The algorithm according to the method of the invention can be implemented in a simple manner, for example with a general-purpose processor component or a digital or preferably analog ASIC, which provides a wrist-watch-like small and light apparatus operated with a battery. The preferred embodiments of the invention and other more detailed embodiments accentuate the advantages of the invention. The invention will be described in greater detail below by means of examples with reference to the accompanying drawings, in which Figure 1 shows an apparatus according to the invention, Figure 2 shows one signal-processing channel of Figure 1 , Figure 3 shows a multichannel pressure pulse signal, Figure 4 shows a multichannel pressure pulse signal after the wrist has been rotated, Figure 5 shows a heartbeat pulse component of a pressure signal, and detection thresholds used for detecting it, Figure 6 shows a pulse rate meter wrist strap placed on the wrist and comprising an apparatus according to the invention integrated therein. A block diagram of the apparatus/method according to the invention is shown in Figure 1. The invention relates to an apparatus for identifying a heartbeat. The apparatus comprises a multichannel sensor 2 and optionally signal-processing means 3, most preferably filtration means 3, for processing a signal obtained from the multichannel sensor 2. In Figure 2, the filtration is divided into two blocks 3a and 3b. The apparatus for identifying a heartbeat also comprises detection means 4 for carrying out signal detection, and decision-making means 5 for identifying a signal as a heartbeat signal. Figures 1 and 6 show that the multichannel sensor 2 comprises, for example, eight sensing elements 21 to 28. Correspondingly, Figures 3 and 4 illustrating pressure pulse signals S1 to S8 show channels C1 to C8 corresponding to the sensing elements 21 to 28. The multichannel sensor 2 may be any sensor, for example a PVDF sensor based on the piezoelectric effect or, for example, an electromechanical film (EMF) sensor based on an electret film. Figure 6 shows a pulse rate meter wrist strap 30 wrapped around a person's wrist 29 with an associated apparatus 1 according to the invention for identifying a heartbeat. The block 30 that represents a pulse rate meter or some other so-called host apparatus is also shown in Figure 1. The pulse rate meter 30 or some other host apparatus comprises calculation means 31 and a display 32 and possibly also other components. The connection between the apparatus 1 and the apparatus 30 may be wired or wireless. If the connection is wired, the apparatus 1 and the apparatus 30 are most preferably integrated into one and the same apparatus complex. Reference numeral 33 denotes a wired connection. In the situation shown in Figure 6, the connection is wireless, i.e. between the apparatus 1 and the apparatus 30 there is a telemetric remote readable connection 34 via remote readable means 35 and 36, such as remote readable coils 35 and 36. The remote reading technique which is based on magnetic inductive coupling is based on oscillating circuits producing magnetic oscillation. In this respect, reference is made to previous patent applications and patents of the Applicant. The apparatus 1 is characterized in that the detection means 4 are arranged to carry out multichannel channel-specific detection for several different channels C1 to C8 and that the decision-making means 5 are connected to identify a heartbeat signal from among the numerous channel signals detected specifically for each channel. In the method, a pressure signal in an artery 40 is measured in the form of multichannel measurement, and if necessary, the required signal-processing measures, such as filtration, are carried out, as well as signal detection and decision-making concerning the identification of a signal as a heartbeat signal. The method is characterized in that the signal detection includes multichannel channel-specific detection for the purpose of identifying signal components of different channels, and that the obtained channel-specific detected channel signals are used as input data in the decision-making stage in the block 5. In the preferred embodiment of the invention, the multichannel channel-specific detection includes the identification of the heartbeat pulse component of the pressure signal as part of the identification of the signal components of the different channels. Figure 6 shows that the multichannel sensor 2 is positioned such that at least one sensing element of the sensor 2, for example elements 23 to 26, is placed over the artery 40, and that at least one other sensing element of the sensor 2, for example elements 21 to 22 and 27 to 28, is not situated over the artery 40. In addition to a measurement carried out from a point with a heartbeat signal, the method also comprises measurement from a point with no heartbeat signal. Correspondingly, in addition to the formation of at least one channel signal having a heartbeat pulse in the channel-specific detection, at least one channel signal with no heartbeat pulse is also formed. The measurement of a channel containing no heartbeat, e.g. channel C8, is utilized in the signal filtration, since by means of the signal S8 of the channel C8 having no heartbeat it is possible to determine the average value of the common-mode interference signal. Signals obtained from the multichannel pressure sensor are thus filtered both specifically for each channel and with respect to the other channels. The procedure is such that the signal of each channel C1 to C8 is emphasized and the interference is reduced by subjecting the signal S1 to S8 of each channel C1 to C8 to band-pass filtration and by subtracting the value that describes the common-mode interference signal of the signals on the channels adjacent to the aforementioned channels. The order in which the filtrations are carried out may be different from the one disclosed above. In a preferred embodiment of the invention, a pressure pulse is detected from the signal of each channel by detecting positive peaks H and negative peaks L of the filtered signal within a time window W, which is preferably predetermined. In the preferred embodiment, the detection of the channel signal utilizes a fixed or changing time window W. Detection of a peak value is simple and advantageous. In order to simplify the arrangement, it is noted with reference to Figures 2 and 5 that the detection utilizes detection thresholds THR1 and THR2, and that these detection thresholds are either fixed and common to all channels, adaptive and common to all channels or adaptive specifically for each channel. Figure 5 may represent for example signal S3 as regards its heartbeat pulse component SC3. Detection threshold THR1 shows the detection threshold of the positive peak value H, and detection threshold THR2 shows the detection threshold of the negative peak value L. With reference to Figure 2, it is stated that this time delay between the peak and the minimum, i.e. the time window W, is most preferably about 100 to 200 milliseconds. To further improve the efficiency and adaptivity of the invention, in the preferred embodiment the apparatus 1 comprises weighting means 6 for weighting the meaning of one or several channels differently from the other channels. The apparatus 1 most preferably comprises a rule block 7 that controls the weighting means 6. The rule block 7 is also connected to control the decision-making block 5. In the preferred embodiment, the channel signals C1 to C8 are weighted for the purpose of decision-making depending on how the heartbeat pulse, e.g. SC3, has occurred previously in the aforementioned channels, e.g. in channel C3. The weighting can be carried out either before the detection or most preferably after the detection, as shown in the figures. In the preferred embodiment of the invention, the weighting utilizes weighting coefficients p1 to p8 that can be updated, as shown in Figure 1. The weighting coefficients are then updated on the basis of empirical rules stored in the block 7. In the preferred embodiment of the invention, an effective empirical rule determines that if a similar signal has been detected in adjacent channels in previous measurements, during the decision-making these channels are weighted more than the other channels. Figure 3 shows that channels C3 to C6 comprise similar signals in adjacent channels, and therefore the weighting coefficients p3 to p6 are updated so that in later measurements, either during the same measurement period or in a separate subsequent measurement, the channels C3 to C6 will be weighted more. In the preferred embodiment of the invention, if a similar signal has been detected in non-adjacent channels in previous measurements, during the decision-making these channels are weighted less than the other channels. In such a case, the weighting coefficients are updated so that in subsequent measurements either during the same measurement period or in a separate measurement these non-adjacent channels will be weighted less. The weighting is therefore most preferably adaptive and emphasizes the channels from which a pressure pulse signal has been detected previously. The decision-making concerning the identification as a heartbeat signal is based on the number of channels C1 to C8 in which the desired signal has occurred on average. The weighting coefficients p1 to p8 are continuously updated on the basis of the empirical rules, so that as the sensors 21 to 28 move with respect to the artery 40, the algorithm is capable of rapidly locating the channels where the desired signal occurs. The movement of the sensing elements 21 to 28 of the sensor 2 with respect to the artery 40 and the resulting transfer of the heartbeat signals to different channels is illustrated by a comparison of Figures 3 and 4. In Figure 3, the heartbeat signals can be detected on channels C3 to C6. In Figure 4, as a result of a rotation of the wrist 29 or some other movement or some other interfering factor, the heartbeat signals have moved to other channels and they now constitute signals S5 to S8 on channels C5 to C8. In the preferred embodiment of the invention, the decision concerning the identification as a heartbeat signal is carried out in the decision-making stage on the basis of the cross-correlation of signals in two or more adjacent channels. In a preferred embodiment, the decision is carried out in the decision-making stage on the basis of common-mode signals occurring simultaneously in two or more adjacent channels. According to the Applicant, the comparison of two or more adjacent channels is an effective manner to reach a decision. Figure 3 shows that adjacent channels C3 to C6 contain common-mode signals S3 to S6 that occur simultaneously. When a heartbeat signal cannot be identified on the basis of monitoring two or more adjacent channels, in the next step of the invention the decision in the decision-making stage is carried out on the basis of a signal that occurs regularly in one channel. The apparatus may be implemented by means of analogue or digital technology, microprocessor technology, or a combination thereof. As regards the components, they may consist of separate components, integrated circuits or a combination thereof. Even though the invention is described above with reference to examples according to the accompanying drawings, it is clear that the invention is not restricted thereto, but it can be modified in several ways within the scope of the inventive idea disclosed in the appended claims.

Claims: CLAIMS 1. A method for identifying a heartbeat, the method comprising - measuring an arterial pressure signal in the form of multichannel measurement, and - carrying out the required signal-processing operations, such as filtration, if necessary, and performing signal detection and decision-making concerning the identification of a signal as a heartbeat signal, c h a ra c te r ized in that in the method the signal detection includes multichannel channel- specific detection for the purpose of identifying signal components of different channels, and that the obtained channel-specific detected channel signals are used as input data in the decision-making stage. 2. A method according to claim 1, characterized in that for the purpose of decision-making the channel signals are weighted depending on how the heartbeat pulse has occurred previously in the aforementioned channels. 3. A method according to claim 2, characterized in that the weighting utilizes weighting coefficients that can be updated. 4. A method according to claim 3, characterized in that the weighting coefficients are updated on the basis of empirical rules. 5. A method according to claim 4, characterized in that if a similar signal has been detected in adjacent channels, during decision-making these channels are weighted more than other channels. 6. A method according to claim 4, characterized in that if a similar signal has been detected in non-adjacent channels, during decision- making these channels are weighted less than other channels. 7. A method according to claim 2, characterized in that the weighting is carried out after the channel-specific detection before the decision-making. 8. A method according to claim 1, characterized in that in addition to a measurement carried out at a point with a heartbeat signal, the method also comprises a measurement carried out at a point with no heartbeat signal. 9. A method according to claim 1 or 8, characterized in that in addition to the formation of at least one channel signal having a heartbeat pulse in channel-specific detection, the method also comprises forming at least one channel signal with no heartbeat pulse. 10. A method according to claim ^characterized in that a pressure pulse is detected from the signal of each channel by detecting the positive and negative peaks of the filtered signal within a time window, which is preferably predetermined. 11. A method according to claim 10, characterized in that the detection of a channel signal utilizes a fixed or changing time window. 12. A method according to claim 10, characterized in that the detection utilizes detection thresholds, and that these detection thresholds are either fixed and common to different channels, adaptive and common to different channels or adaptive specifically for each channel. 13. A method according to claim 1, characterized in that signals obtained from a multichannel pressure sensor are filtered both specifically for each channel and with respect to the other channels. 14. A method according to claim 1 or 13, characterized in that a signal of each channel is emphasized and the interference is reduced by subjecting signals of all channels to band-pass filtration and by subtracting the value that describes the common-mode interference signal of the signals on the channels adjacent to the aforementioned channels. 15. A method according to claim 1, characterized in that a decision is made in the decision-making stage on the basis of the cross- correlation of signals occurring in two or more adjacent channels. 16. A method according to claim 1, characterized in that a decision is made in the decision-making stage on the basis of the common- mode signals occurring simultaneously in two or more adjacent channels. 17. A method according to claim 1, characterized in that a decision is made in the decision-making stage on the basis of a signal occurring regularly in one channel, and that this procedure is most preferably used when a heartbeat signal cannot be identified on the basis of monitoring two or more adjacent channels. 18. A method according to claim 1, characterized in that the measurement signals are obtained from the artery at the wrist by using a wrist strap containing a sensor. 19. A method according to claim 1, characterized in that in multichannel channel-specific detection, the heartbeat pulse component of the pressure signal is identified as part of the identification of the signal components of the different channels. 20. An apparatus for identifying a heartbeat, the apparatus comprising a multichannel sensor (2, 21-28), optionally signal-processing means (3, 3a, 3b) for processing a signal obtained from the multichannel sensor, detection means (4) for detecting a signal, and decision-making means (5) for identifying a signal as a heartbeat signal, characterized in that the detection means (4) are arranged to carry out multichannel channel-specific detection on several different channels (C1-C8), and that the decision-making means (5) are connected to perform identification of a heartbeat signal from among said several channel signals detected specifically for each channel. 21. An apparatus according to claim 20, characterized in that the apparatus comprises weighting means (6) for weighting the meaning of one or more channels differently from the other channels. 22. An apparatus according to claim 21, characterized in that the apparatus comprises a rule block (7) that controls the weighting means (6). 23. An apparatus according to claim 20, characterized in that the multichannel sensor (2) is positioned such that at least one of its sensing elements (23-26) is placed over the artery (40) and that at least one other sensing element (21-22, 27-28) of the sensor is not situated over the artery.

Inventors: MIETTINEN JARI, NISSILAE SEPPO, RUHA ANTTI, SORVOJA HANNU, MIETTINEN, JARI, NISSILAE, SEPPO, RUHA, ANTTI, SORVOJA, HANNU
Applicants: MIETTINEN JARI, NISSILAE SEPPO, POLAR ELECTRO OY, RUHA ANTTI, SORVOJA HANNU, MIETTINEN, JARI, NISSILAE, SEPPO, RUHA, ANTTI, SORVOJA, HANNU
Application year: 1997
Main classification: A61B5
IPC codes: A61B5