WO2010067297A1 - Method and apparatus for the analysis of ballistocardiogram signals - Google Patents

Method and apparatus for the analysis of ballistocardiogram signals Download PDF

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Publication number
WO2010067297A1
WO2010067297A1 PCT/IB2009/055541 IB2009055541W WO2010067297A1 WO 2010067297 A1 WO2010067297 A1 WO 2010067297A1 IB 2009055541 W IB2009055541 W IB 2009055541W WO 2010067297 A1 WO2010067297 A1 WO 2010067297A1
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WIPO (PCT)
Prior art keywords
signal
method
ballistocardiogram
ballistocardiogram signal
step
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Application number
PCT/IB2009/055541
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French (fr)
Inventor
David Friedrich
Xavier L. M. A. Aubert
Andreas Brauers
Hartmut Fuehr
Kurt Stadlthanner
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Koninklijke Philips Electronics N.V.
Philips Intellectual Property & Standards Gmbh
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Priority to EP08171340.6 priority Critical
Priority to EP08171340 priority
Application filed by Koninklijke Philips Electronics N.V., Philips Intellectual Property & Standards Gmbh filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2010067297A1 publication Critical patent/WO2010067297A1/en

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    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00Detecting, measuring or recording for diagnostic purposes; Identification of persons
    • A61B5/103Detecting, measuring or recording devices for testing the shape, pattern, colour, size or movement of the body or parts thereof, for diagnostic purposes
    • A61B5/11Measuring movement of the entire body or parts thereof, e.g. head or hand tremor, mobility of a limb
    • A61B5/1102Ballistocardiography
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00Detecting, measuring or recording for diagnostic purposes; Identification of persons
    • A61B5/02Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
    • A61B5/024Detecting, measuring or recording pulse rate or heart rate
    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61BDIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00Detecting, measuring or recording for diagnostic purposes; Identification of persons
    • A61B5/72Signal processing specially adapted for physiological signals or for diagnostic purposes
    • A61B5/7203Signal processing specially adapted for physiological signals or for diagnostic purposes for noise prevention, reduction or removal
    • A61B5/7207Signal processing specially adapted for physiological signals or for diagnostic purposes for noise prevention, reduction or removal of noise induced by motion artifacts

Abstract

There is provided a method for analyzing a ballistocardiogram signal to detect heart beats, the method comprising filtering the ballistocardiogram signal to extract high frequency components; squaring the filtered ballistocardiogram signal; filtering the squared and filtered ballistocardiogram signal to give a resulting signal; and identifying peaks in the resulting signal, the locations of which correspond to the locations of the high frequency components, each identified peak corresponding to a heartbeat.

Description

METHOD AND APPARATUS FOR THE ANALYSIS OF BALLISTOCARDIOGRAM SIGNALS

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method and apparatus for the analysis of ballistocardiogram signals, and in particular to a method and apparatus that provides for the detection of single heart beat events in ballistocardiogram signals.

BACKGROUND TO THE INVENTION

A ballistocardiograph (BCG) measures the movement of the human body due to the momentum of the blood as it is pumped by the heart.

The BCG has advantages over the electrocardiograph (ECG) in that the measurement of body vital signs is possible without electrodes having to be glued to the body or for special sensors like belts, textiles or the like to be worn. It is particularly useful in obtaining a pulse rate and pulse rate variability data in order to evaluate sleep quality, stress or cardiac performance. Another application that requires a single beat identification is for the detection of arrhythmia, where the periodicity of the pulse is perturbed. These are the applications in which the unobtrusive nature of the BCG monitoring is of prime importance since sensors which are in direct contact with the patient inevitably lead to reduced sleep quality.

In contrast to the evaluation of a standard ECG, in which the R-peak of the ECG trace is identified and an R-R interval can be measured on a beat-to-beat basis, the BCG is usually evaluated over a period of several heart beats either using a spectral method or using methods in the time domain that evaluate the reoccurrence of certain patterns, for example, evaluating the autocorrelation function of the signal. Usually, the signal is filtered prior to the evaluation in order to remove undesired high and low frequency components from the signal. In this approach, segments of the signal have to be considered which last for several seconds such that they cover multiple heart beats. As a result, average heart beats over a period of time are obtained, but no beat-to-beat information is available. This means that the presence of certain arrhythmias, like ectopic beats or missing beats, either perturbs the estimation of the heart rate or remains unnoticed. From this, only the lower frequency portion of the variability of the pulse rate is detectable.

There is therefore a need for a method and system that enables the identification of single heart beats in ballistocardiogram signals, and that is robust in view of the strong variation in the characteristics of the ballistocardiogram signals between patients (and even within the same patient), the measurement situation, the position of the patient's body relative to the sensor and the mechanical environment.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method for analyzing a ballistocardiogram signal to detect heart beats, the method comprising filtering the ballistocardiogram signal to extract high frequency components; squaring the filtered ballistocardiogram signal; filtering the squared and filtered ballistocardiogram signal to give a resulting signal; and identifying peaks in the resulting signal, the locations of which correspond to the locations of the high frequency components, each identified peak corresponding to a heartbeat.

According to a second aspect of the invention, there is provided a computer program product comprising computer program code that, when executed on a computer or processor, is configured to perform the steps of the method described above.

According to a third aspect of the invention, there is provided an apparatus for use with a device for measuring a ballistocardiogram signal of a patient, the apparatus comprising means for receiving a ballistocardiogram signal from the device; and processing means for performing the method described above on the received ballistocardiogram signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the following drawings, in which:

Fig. 1 shows a typical ballistocardiogram signal; Fig. 2 is a flow chart illustrating a method in accordance with the invention;

Fig. 3 is a graph illustrating the signal obtained through the use of the method in Fig. 2;

Fig. 4 is a graph illustrating the results of a refinement procedure; and Fig. 5 is a block diagram of an apparatus in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a typical ballistocardiogram (BCG) that is obtained using a foil- like sensor placed below the thorax of a patient that is lying on a bed or table. The BCG records breathing movements and a periodic pattern of beats related to heart rate.

It can be seen that the BCG has a predominant low frequency component that is related to the breathing movements of the patient, and smaller fluctuations with a higher frequency that are due to the mechanical activity of the heart.

It will be appreciated that the patient must be at rest in order to obtain such a clear BCG signal. Larger movements will lead to predominant movement artefacts in the BCG signal which significantly hamper its analysis. The first step in processing the BCG signal is to divide it into segments in which other movements or perturbations impede an estimation of the heart rate and breathing rate, and segments in which an estimation is possible. This kind of division can be achieved by evaluating the energy level of the signal.

Furthermore, the contributions from breathing movements and the mechanical activity of the heart can be separated by the use of filters. For example, low-pass filtering below 1 Hz yields the breathing component. The heart beat component can be extracted by filtering with a high pass filter (for example a Butterworth filter of order 2 with a cut-off frequency in the range 0.8 to 1.2 Hz). The method described below uses the heart beat component. The aim of the method is to convert the ballistocardiogram signal (for example as shown in Fig. 1) which has a high inter- and intra-patient variation into a much simpler form, such that the individual heart beats can be detected.

A single heart beat interval contains at least two high frequency components. In addition, the signal is corrupted by noise. The method described below strengthens the amplitude of these components by quadratic order, and, since one of the high frequency components has a much higher amplitude than the rest, this amplification makes it possible to discriminate it from noise and other high frequency components. Various methods of peak detection can be used to identify the high frequency component, and a peak-to-peak interval can be calculated. This interval reflects the beat-to- beat interval of the pulse rate.

The method according to the invention will now be described with reference to Fig. 2.

In step 101, signals from a patient are collected by the BCG.

In step 103, a band-pass filter is applied to the ballistocardiogram signal in order to extract the characteristic high frequency components. In a preferred embodiment, a Butterworth filter of order 5 is used for band-pass filtering the BCG signal in the frequency range of 20 to 40 Hz.

In step 105, the band-passed BCG signal is squared.

In step 107, a low-pass filter is applied to the squared band-passed BCG signal to give a new signal whose predominant peaks correspond to the location of the high frequency components. In a preferred embodiment, a Butterworth filter of order 5 with a cut- off frequency L is used for low-pass filtering the BCG signal.

It has been found that the selection of the value of the cut-off frequency L influences the sharpness of the peaks in the resulting signal. Other high frequency components may also appear sharper again, while they are smoothed out when the peaks are less sharp. In a preferred embodiment, a cut-off frequency of 1.5 Hz is used as this has been found to give a good tradeoff between peak sharpness and smoothness. An exemplary filtered signal is shown in Fig. 3. High frequency components in the BCG signal within a frequency range of 20-40Hz correspond to the peaks shown in Fig. 3.

In step 109, a peak detector is used to detect all of the peaks beyond a predetermined threshold, and from this, the location of the high frequency components corresponding to heart beats can be estimated. In one embodiment, the peak detector can search for all local maxima with higher function values than the mean of the signal. It will be appreciated however, that alternative thresholds or methods for the elimination of undesirable local maxima can be used.

The output of the peak detection will provide a reasonable detection of a heartbeat, as the high frequency components typically cover a time period of 80-100 milliseconds. However, robustly fixing the heart beat estimate is not trivial. Furthermore, the smoothing effect inherent in the low-pass filtering step (step 107) decreases the time resolution by a few milliseconds. Thus, if higher accuracy in the detection of a heart beat is required, a refinement procedure can be used (optional step 111) that uses information from the ballistocardiogram signal and the estimates found so far to improve the accuracy of the estimation. In the ballistocardiogram signal, the identified high frequency component is followed by a wave with lower frequency, but very high amplitude, whose peak yields a more exact localization of a heart beat event. Fig. 4 shows the refined estimates that have been found by looking for the maxima within the next 100 milliseconds after the estimates that are due to the maxima in the high frequency signal, and they have been found in a ballistocardiogram signal in which the signal content relating to respiration has been removed.

It will be appreciated, however, that alternative methods for refinement can be used. In particular, these alternative methods can make use of characteristic points in the ballistocardiogram signal, for example the point of highest slope of the wave with lower frequency, but higher amplitude, or methods from periodicity detection that make use of the foreknowledge available from the peaks in the high frequency signal.

After the heart beats have been identified using the method described above, the beat-to-beat intervals can be easily computed.

A beat-to-beat interval length estimation (without the refinement procedure and with the refinement procedure respectively) and a comparison to ECG can be seen in Table 1 below.

Figure imgf000007_0001

Thus, it can be seen that with the refinement procedure, the error in using the BCG analysis against an ECG is within 8 milliseconds. The method described above enables the extraction of beat-to-beat intervals from ballistocardiogram signals. Providing a beat-to-beat estimate using the described method can replace standard ECG devices in various applications such as heart failure management, arrhythmia detection, atrial fibrillation diagnosis and management. The potential of arrhythmia detection will increase the acceptance of the BCG method in professional medical institutions for low acuity monitoring. This is of particular interest since the number of intensive care unit (ICU) beds is limited such that an easy and inexpensive monitoring solution for the general ward is desirable.

The invention can be used in nursing homes, hospitals and for home-care surveillance. In all cases, the general advantage of the BCG over ECG is the unobtrusive monitoring of patients without the necessity to attach electrodes or the like to the patient.

Although the invention has been described in terms of a method or algorithm, it will be appreciated that the invention can be implemented in a BCG system (i.e. a computer apparatus in combination with apparatus for measuring the BCG signals), or as a stand-alone computer system or program. It will be appreciated that the BCG system can provide a ballistocardiogram signal in analog or digital form to the inventive apparatus, and the inventive apparatus can be adapted to receive this signal accordingly. For example, the BCG system can provide the ballistocardiogram signal to the apparatus in analog form, and the apparatus can comprise an anti-alias filter and an analog-to-digital convertor for providing a digital representation of the ballistocardiogram signal to a suitably-programmed digital signal processor in the apparatus. Alternatively, the BCG system can implement an analog-to-digital convertor so the ballistocardiogram signal is provided to the apparatus (and specifically to a digital signal processor in the apparatus) in digital form. The apparatus can receive the ballistocardiogram signal using any appropriate means, such as through a wired or wireless connection to the BCG system. One embodiment of an apparatus for implementing the invention is shown in

Figure 5. A ballistocardiogram signal is provided from a BCG sensor 302 to the apparatus 304. The apparatus 304 receives the BCG signal at an input port 306 and processes the BCG signal as described in the preceding description using a processor 308. Instructions for causing the processor 308 to carry out the method can be stored in a memory 310. A BCG system according to the invention comprises the apparatus 304 and the BCG sensor 302.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CLAIMS:
1. A method for analyzing a ballistocardiogram signal to detect heart beats, the method comprising: filtering the ballistocardiogram signal to extract high frequency components (103); squaring the filtered ballistocardiogram signal (105); filtering the squared and filtered ballistocardiogram signal to give a resulting signal (107); and identifying peaks in the resulting signal (109), the locations of which correspond to the locations of the high frequency components, each identified peak corresponding to a heartbeat.
2. A method as claimed in claim 1, further comprising the step of: using the identified peaks and the ballistocardiogram signal to refine the identification of the heart beats (111).
3. A method as claimed in claim 2, wherein the step of using the identified peaks and the ballistocardiogram signal (111) comprises identifying a heart beat as the wave with lower frequency but high amplitude following the identified peak in the ballistocardiogram signal.
4. A method as claimed in claim 1, further comprising the step of: refining the detection of heart beats using periodicity detection.
5. A method as claimed in claim 4, wherein the periodicity detection uses the peaks identified in the resulting signal.
6. A method as claimed in any preceding claim, further comprising the step of determining a beat-to-beat interval from the identified heart beats.
7. A method as claimed in any preceding claim, further comprising the step of, prior to the step of filtering the ballistocardiogram signal to extract high frequency components (103), prefiltering the ballistocardiogram signal to remove signals related to breathing.
8. A method as claimed in any preceding claim, further comprising the step of, prior to the step of filtering the ballistocardiogram signal to extract high frequency components (103), dividing the ballistocardiogram signal into a plurality of segments based on the presence of movements or other perturbations in the ballistocardiogram signal.
9. A method as claimed in claim 8, wherein the step of dividing comprises estimating the energy level of the ballistocardiogram signal.
10. A method as claimed in any preceding claim, wherein the step of filtering the ballistocardiogram signal (103) comprises applying a band-pass filter to the ballistocardiogram signal.
11. A method as claimed in claim 10, wherein the step of filtering the ballistocardiogram signal (103) comprises applying a Butterworth band-pass filter of order 5 to the ballistocardiogram signal having a pass-band of 20 to 40 Hz.
12. A method as claimed in any preceding claim, wherein the step of filtering the squared and filtered ballistocardiogram signal (107) comprises applying a low-pass filter to the ballistocardiogram signal.
13. A method as claimed in claim 12, wherein the step of filtering the squared and filtered ballistocardiogram signal (107) comprises applying a Butterworth low-pass filter of order 5 to the ballistocardiogram signal having a cut-off frequency of about 1.5 Hz.
14. A method as claimed in any preceding claim, wherein the step of identifying peaks (109) comprises searching for all local maxima in the resulting signal with higher function values than the mean of the resulting signal.
15. A computer program product comprising computer program code that, when executed on a computer or processor, is configured to perform the steps of the method in any of claims 1 to 14.
16. An apparatus (304) for use with a device (302) for measuring a ballistocardiogram signal of a patient, the apparatus comprising: means (306) for receiving a ballistocardiogram signal from the device; and processing means (308) for performing the method defined in any one of claims 1 to 14 on the received ballistocardiogram signal.
17. A ballistocardiograph (BCG) system comprising the apparatus (304) as claimed in claim 16 and a device (302) for measuring a ballistocardiogram signal of a patient.
PCT/IB2009/055541 2008-12-11 2009-12-07 Method and apparatus for the analysis of ballistocardiogram signals WO2010067297A1 (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011013048A1 (en) * 2009-07-31 2011-02-03 Koninklijke Philips Electronics N.V. Method and apparatus for the analysis of a ballistocardiogram signal
CN105447306A (en) * 2015-11-12 2016-03-30 杨松 Ballistocardiogram signal cycle calculating method and apparatus
CN104545863B (en) * 2013-10-10 2017-03-29 上海宽带技术及应用工程研究中心 BCG hearts rate extracting method and system based on Fuzzy Pattern Recognition
EP3492004A4 (en) * 2016-07-27 2020-04-01 Univ Catalunya Politecnica Method and device for detecting mechanical systolic events from a balistocardiogram

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008095318A1 (en) * 2007-02-08 2008-08-14 Heart Force Medical Inc. Monitoring physiological condition and detecting abnormalities

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008095318A1 (en) * 2007-02-08 2008-08-14 Heart Force Medical Inc. Monitoring physiological condition and detecting abnormalities

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
KORTELAINEN J M ET AL: "FFT averaging of multichannel BCG signals from bed mattress sensor to improve estimation of heart beat interval" ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY, 2007. EMBS 2007. 29TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE, IEEE, PISCATAWAY, NJ, USA, 22 August 2007 (2007-08-22), pages 6685-6688, XP031337773 ISBN: 978-1-4244-0787-3 *
None

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011013048A1 (en) * 2009-07-31 2011-02-03 Koninklijke Philips Electronics N.V. Method and apparatus for the analysis of a ballistocardiogram signal
CN104545863B (en) * 2013-10-10 2017-03-29 上海宽带技术及应用工程研究中心 BCG hearts rate extracting method and system based on Fuzzy Pattern Recognition
CN105447306A (en) * 2015-11-12 2016-03-30 杨松 Ballistocardiogram signal cycle calculating method and apparatus
EP3492004A4 (en) * 2016-07-27 2020-04-01 Univ Catalunya Politecnica Method and device for detecting mechanical systolic events from a balistocardiogram

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