PPG signal processing apparatus and corresponding computer-implemented method

Abstract

1. A photoplethysmography or PPG signal processing device configured to detect atrial flutter, comprising: a processor and a memory including computer program code, the PPG signal processing device comprising: Obtaining (101) at least one PPG signal (200) of a human or animal; Detecting (102) heart beats (201, 202, 203, 204, 205, ...) in at least one PPG signal (200); determining (103) beat intervals (301, 302, 303, 304, ...) between pairs of consecutive heart beats (201-202, 202-203, 203-204, 204-205, ...) in at least one PPG signal (200); detecting (104) beat interval baselines, each of the beat interval baselines (302-304) corresponding to a plurality of consecutive beat intervals (302, 303, 304) that are constant within a predetermined tolerance; determining (105) respective average beat interval values ​​(401, 402, ...) relative to the beat interval baseline (302-304); Detect (110) the relationship between the average beat interval values ​​(401, 402, ...; 501, 502, ...) 4. A photoplethysmography or PPG signal processing device configured to:

Description

[Technical field]

[0001] The present invention relates generally to photoplethysmography (PPG), an optical technique for detecting changes in blood volume that allows various physiological parameters to be monitored. The present invention relates in particular to the processing of one or more PPG signals obtained from a human or animal to facilitate the detection of a wider range of heart rhythm problems. [Background technology]

[0002] Photoplethysmography (PPG) is an optical technique that allows monitoring one or more physiological parameters by detecting changes in blood volume in the peripheral circulation. PPG uses the absorption of light by blood to track these volume changes. When a light source shines on the skin, the reflected light changes depending on the blood flow. A light sensor then converts these changes in light reflection into a digital signal, the so-called PPG signal. PPG signals are typically recorded using a pulse oximeter or a light detector (e.g. a camera integrated into an electronic device such as a human smartphone, a smartwatch, or other smart wearable or non-wearable devices).

[0003] A distinction is made between remote and contact PPG. Remote PPG is less intrusive to the person being monitored, but poses significant challenges in signal detection and processing. As a result, the use of remote PPG remains limited to everyday applications such as leisure or fitness, as its accuracy and reliability are insufficient for medical applications. Contact PPG, where the measuring components are in direct contact with the skin, provides a more reliable and more accurate PPG signal, facilitating medical diagnosis.

[0004] PPG can be used to monitor cardiovascular and hemodynamic parameters such as heart rate, heart rate variability, and blood pressure, as well as other physiological variables such as stress, respiration, and autonomic function, among other applications. Since PPG can be affected by various noise sources, one of the key parts of accurate monitoring with PPG is obtaining a high-quality, artifact-free signal. Methods for accurate and reliable PPG measurements are described, for example, in European Patent Application Publication EP 3 449 820 A1, by applicant Qompium, entitled "Computer-Implemented Method and System for Direct Photoplethysmography (PPG)", and in European Patent Application Publication EP 3 473 173, by applicant Qompium, entitled "Computer-Implemented Method and System for Direct Photoplethysmography (PPG) with Multiple Sensors".

[0005] In medicine, contact PPG is primarily used to detect the risk of atrial fibrillation (AF), the most common heart rhythm disorder. Other heart rhythm disorders are usually detected via an electrocardiogram (ECG), acquired, for example, by a Holter monitoring system.

[0006] It is desirable to use the PPG signal, which can be easily obtained through a wearable device, to detect, for example, tachycardia or other abnormal heart rhythms, such as atrial flutter. Atrial flutter is a type of abnormal heart rhythm caused by abnormal electrical circuits in the upper chambers of the heart, called the atria, which make them beat quickly. In a normally functioning heart, electrical impulses are sent from the sinus node (also called SA node) in the right atrium of the heart. In this way, this node controls the heart rate and the timing of the heart beats. In the case of atrial flutter, an abnormal electrical circuit is formed in the atria. This abnormal electrical circuit governs the heart rhythm and rate, causing the upper chambers to contract abnormally frequently.

[0007] Atrial flutter can cause the lower chambers of the heart (called ventricles) to beat faster, but often not as fast as the atria. In a normally functioning heart, the atrial rate or AR is equal to the ventricular rate or VR. With atrial flutter, the ventricular rate is usually an integer fraction of the atrial rate, and the ratio of AR to VR (called AV conduction rate) is equal to 2:1, 3:1, or 4:1. The AV conduction rate can be constant. In one example where the AV conduction rate remains 3:1 and the AR is 300 bpm (beats per minute), the VR is 100 bpm. The AV conduction rate can also alternate. In one example where the AV conduction ratio alternates between ratios of 2:1, 3:1, and 4:1 and the AR is 300 bpm, the VR alternates between 150 bpm, 100 bpm, and 75 bpm.

[0008] The PPG signal allows the identification of the ventricular rate but not the atrial rate. As a result, the presence of flutter waves is usually not visible on the photoplethysmography or PPG signal plot. Therefore, confirmation by electrocardiogram or ECG is required for reliable detection of atrial flutter.

[0009] The paper "Detecting Atrial Fibrillation and Atrial Flutter in Daily Life Using Photoplethysmography Data" by authors Eerikainen LM et al., published in IEEE J Biomed Health Inform, June 2020, pp. 1610-1618, appears to describe a method to detect atrial flutter using PPG. However, the features extracted from the PPG signal by Eerikainen et al. are not specific to atrial flutter detection. Classical heart rate variability features such as entropy, RMSSD, and pNN70 are used, which are traditional features found in the literature. If these features could reliably detect atrial flutter, we would have seen this in the medical literature. The study on which this article is based included only five patients with atrial flutter, three of which were used to train the classifier and the remaining two were left in the test set. This gives limited insight into performance, as the results may be biased by the limited number of patients. Furthermore, it can be seen that Eerikainen et al. do not perform classification at the patient level, but rather perform classification on individual 30-second strips of the PPG signal.

[0010] According to the current state of the art in PPG, atrial flutter is often listed among a long list of arrhythmias that can be detected, but the existing literature does not explain how the PPG signal needs to be processed to detect atrial flutter and distinguish it from other arrhythmias.

[0011] US Patent Application Publication US2020 / 0100693A1, entitled "Arrhythmia Monitoring Using Photoplethysmography," describes "atrial flutter" in

[0070] with reference to atrial and ventricular rates.

[0076] appears to teach that D14 can use the average RR interval as a parameter for detecting tachycardias such as atrial flutter, but the document does not teach how such RR intervals should be processed to make atrial flutter detectable.

[0012] U.S. Patent Application Publication US2021 / 0007621A1, entitled "Method to Analyse Cardiac Rhythms Using Beat-To-Beat Display Plots," suggests in paragraphs

[0068] ,

[0074] ,

[0105] and FIG. 10 that atrial flutter can be detected from beat-to-beat display plots, but the document does not contain any advice regarding the preferred processing of PPG signals to ensure that atrial flutter can be reliably detected and distinguished from other heart rhythm disorders.

[0013] U.S. Patent Application Publication US2017 / 0032221A1, entitled “Method, Electronic Apparatus and Computer Readable Medium of Constructing Classifier for Disease Detection,” refers to a classifier for atrial flutter at

[0004] and in FIG. 1C, but relies on a feature of the ECG signal, namely the disappearance of the interval between the end of the T wave and the start of the P wave.

[0014] In the U.S. Patent Application Publication US2016 / 0302677A1 entitled “Calibrating for Blood Pressure Using Height Difference,” paragraphs

[0126] ,

[0130] , and FIG. 16B, the following are described: i+1 RR i It has been suggested to detect atrial flutter by looking for changes in heart rate that are multiples of each other, which are detectable through clusters offset from the diagonal of the plot shown in FIG. 16B, which is a plot of atrial flutter vs.

[0015] Paragraphs

[0036] to

[0038] of published U.S. patent application US2014 / 0221845A1 describe obtaining a PPG signal and filtering the PPG signal to detect peaks (heart beats) therein. US2014 / 0221845A1 further describes determining the peak-to-peak interval (RR interval). From the RR interval, the dynamics of the heart rate are established, which is useful for detecting arrhythmias. However, US2014 / 0221845A1 does not describe a technique for detecting atrial flutter. Summary of the Invention

[0016] It is an object of the present invention to provide improved techniques for processing PPG signals in order to extract information therefrom that is useful for detecting atrial flutter or other abnormal heart rhythms that are currently difficult to reliably detect or distinguish based on the PPG signal. [Means for solving the problem]

[0017] According to an embodiment of the present invention, the above object is achieved by a photoplethysmography signal processing device (short: PPG signal processing device) adapted for detecting atrial flutter as defined in claim 1, the PPG signal processing device comprising at least one processor and at least one memory containing a computer program code, the at least one memory and the computer program code together with the at least one processor causing the PPG signal processing device to: Obtaining at least one photoplethysmography signal (abbreviated PPG signal) of a human or animal; Detecting a heart beat in at least one PPG signal; determining a beat-to-beat interval between pairs of successive heart beats in at least one PPG signal; detecting beat interval baselines, each beat interval baseline corresponding to a number of consecutive beat intervals that are constant within a predetermined tolerance; determining a respective mean beat interval value for each of the beat interval baselines; Find the relationship between the average beat interval values, where: determining, for a subset of atrial rates consisting of integer beats per minute values ​​between a lower limit and an upper limit, a respective distance between the average beat interval value of the beat interval baseline and a closest atrial line, the closest atrial line corresponding to an integer portion of the atrial rate; determining, for each atrial rate in the subset of atrial rates, an average distance between the average beat interval value and the closest atrial line for that atrial rate; selecting an atrial rate for which the mean beat interval value has a minimum mean distance to the atrial line; This is achieved by a PPG signal processing device configured to cause atrial flutter to be detected if the minimum average distance is less than a predetermined threshold distance.

[0018] Indeed, the presence and / or nature of the relationship between the central or average beat-to-beat values ​​of the baseline (also called RR baseline) detected based on a single human or animal PPG signal(s) may allow reliable detection of certain arrhythmias, such as atrial alternating flutter. Other cardiac arrhythmias that can be detected in this way include, for example, atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reciprocating tachycardia (AVRT), atrial tachycardia, and multifocal atrial tachycardia. These arrhythmias are part of the category of supraventricular tachycardia, to which atrial flutter also belongs, as explained above.

[0019] According to an embodiment of the present invention, one or more PPG signals are obtained for a single human or animal. In these PPG signals, heartbeats are detected using state-of-the-art algorithms that determine peaks in the PPG signal that correspond to the heartbeats of that human or animal and determine the timing of such peaks. Beat intervals, also called RR intervals or peak-to-peak intervals, are then determined. This means that for each pair of consecutive heartbeats in the PPG signal(s), the time interval between these heartbeats is determined. Instead of looking for heart rate variability to detect heart rate variability indicative of cardiac arrhythmia, the PPG processing device according to the present invention looks for a beat interval baseline, which is a short period during which the beat interval or heart rate remains substantially constant, i.e., constant within a predefined tolerance for the heart rate variability in the beat interval baseline. Thus, all consecutive beat intervals that are equal within a set tolerance together form part of a single beat interval baseline. According to the present invention, multiple beat interval baselines are detected for a single human or animal across one or more PPG signals acquired for that human or animal. These different beat interval baselines usually have different lengths, but each beat interval baseline must include a number of subsequent beat intervals that are substantially equal. It is possible to set a minimum length of the beat interval baseline, which corresponds to the minimum amount of subsequent beat intervals that must be constant to be considered a beat interval baseline. Within each beat interval baseline, the beat intervals are averaged, thus obtaining a mean or median value of the time between successive heartbeats for each detection period during which the heart rate remains substantially constant. The existence of a relationship between these average beat intervals is then searched for and output. For example, the average beat intervals may be equal (within a tolerance range), the average beat-to-beat value may be an integer multiple of a minimum value (within a tolerance range), the average beat-to-beat value may be an integer part of a common multiple value (within a tolerance range), etc.

[0020] The present invention offers the advantage of being able to reliably detect certain arrhythmias (especially arrhythmias that are part of the supraventricular tachycardia category) from PPG signals collected multiple times over time on the same patient, human or animal using a convenient wearable equipped with a photodetector.

[0021] To establish the existence of a relationship between the central beat interval values ​​determined for different beat interval baselines of a single human or animal, these central beat interval values ​​can be compared to the atrial lines. The atrial lines correspond to the integer parts of the common multiple values, which represent the atrial rate or AR value. For a single AR value, for example 180 bpm, the first three atrial lines correspond to 0.67 seconds (or 90 bpm, 1 / 2 the AR), 1.00 seconds (or 60 bpm, 1 / 3 the AR), and 1.33 seconds (or 45 bpm, 1 / 4 the AR). For each central beat interval value, it is determined which atrial line (0.67 seconds, 1.00 seconds, or 1.33 seconds) is closest, and the distance to the closest atrial line is determined. This is then repeated for a subset of atrial rates (for example, 180 bpm, 182 bpm, 184 bpm, ..., 300 bpm). The small distance between the central beat intervals of a single AR and the atrial line indicates that the detected beat interval baselines (time periods during which the intervals between subsequent heart beats or heart rates remain substantially constant) all occur when the human atria are beating at a particular rate.

[0022] To ascertain whether the beat interval baseline detected for a single human or animal occurs when the atrium of that human or animal is beating at a particular rate (AR), the distances measured between each central beat interval and the closest atrial line for that AR are averaged. In this way, a single value is obtained that represents the distance between the baseline beat interval of the human during a short period of time with a constant beat interval and the respective closest atrial line for a single AR across all PPG signals. Such a single value may be determined for each AR value within the subset of AR values ​​considered.

[0023] From the subset of atrial rates whose central beat interval values ​​of the beat interval baselines are compared to the atrial line, an atrial rate is selected that has the smallest average distance of the beat interval baseline to the atrial line. If the beat interval baseline occurs at a particular AR, then the atrial rate so selected is the best candidate for that particular AR.

[0024] The PPG signal processor can be used to detect cardiac arrhythmias that are part of the category of supraventricular tachycardias, such as atrial flutter, atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reciprocating tachycardia (AVRT), atrial tachycardia, and multifocal atrial tachycardia. An embodiment of the invention may include a single classifier for a single arrhythmia, or multiple classifiers to detect multiple arrhythmias. The minimum average distance between the beat interval and the atrial line during the beat interval baseline for a subset of atrial rates may serve as an input parameter for a classifier that determines whether a human or animal is suffering from atrial flutter.

[0025] An easy-to-implement atrial flutter classifier can compare the resulting minimum average distance (the distance between the atrial beat intervals during the atrial beat interval baselines for a subset of atrial rates and the closest atrial line) to a predefined distance threshold. If the minimum average distance falls below this predefined distance threshold, then an atrial rate is found for which the beat intervals during the different beat interval baselines come very close to the atrial line. In other words, there is a very high probability that a beat interval baseline will occur when the atrium is beating at a particular atrial rate. The beat intervals of the different beat interval baselines may vary significantly, but they will all nearly coincide with one of the atrial lines for that particular atrial rate. This is an indication of atrial flutter, and thus the classifier can reliably classify a human or animal as an atrial flutter patient.

[0026] It can be seen that the threshold-based classifier can be used alone, which means that the average distance to the closest atrial line set can be compared to some threshold to classify a human / animal as an atrial flutter patient. If the average distance value is below the threshold, it is atrial flutter. If the average distance value is above the threshold, it is not atrial flutter. The threshold-based classifier can also be used in combination with other methods to extract information from the PPG recording. The current method then serves to calculate a feature which is the average distance to the closest atrial line set. This feature can then be used, together with other features calculated in other ways, to train one or several arrhythmia classifiers (decision trees, support vector machines, k-nearest neighbors, neural networks, etc.). As an example, the following: · Mean distance to nearest atrial line set, Average heart rate, Standard deviation of the RR interval, · Mean amplitude of PPG heartbeat, Standard deviation of PPG heart rate amplitude , and a subset or all of these features can be used to train a classifier. Generally speaking, the present invention contributes to providing a method for computing cardiac rhythm variability features.

[0027] In an embodiment of the PPG signal processing device according to the invention, as defined in claim 2, the average distance corresponds to a weighted squared distance, and the length or duration of the beat interval baseline serves as a respective weight for the squared distance between the beat interval value of the beat interval baseline and the nearest atrial line within the average distance.

[0028] Although the average distance between the central beat interval value of the beat interval baseline and the respective closest atrial line of the atrial rate can be determined as a mean, median, mode, etc., in a preferred embodiment of the present invention, the average distance is determined as a weighted squared distance value, in which each distance between the central beat interval value of the beat interval baseline and the corresponding closest atrial line is squared and weighted with a respective weight value proportional to the length or duration of the beat interval baseline. In this way, distance values ​​determined for longer beat interval baselines get a higher weight or importance in the overall average distance than the weight or importance assigned to distance values ​​determined for shorter beat interval baselines. Weighting the distance to the nearest atrial line using the length of the beat interval baseline improves the reliability of the PPG signal processing device and the disease classification thereon.

[0029] In an embodiment of the PPG signal processing device according to the invention, as defined in claim 3, the atrial lines for one atrial rate of the subset of atrial rates correspond to one half the atrial rate, one third the atrial rate and one quarter the atrial rate.

[0030] Indeed, in the case of atrial alternating flutter, the RR baseline beat intervals may be assumed to lie on lines that are integer fractions of the base atrial rate, with AV conduction ratios equal to 2:1, 3:1, or 4:1. Thus, for each atrial rate in the subset, it is sufficient to consider an atrial line that corresponds to a half, a third, or a quarter of the atrial rate.

[0031] In an embodiment of the PPG signal processing device according to the invention, as defined in claim 4, the subset of atrial rates comprises all integer rates between a lower limit and an upper limit, preferably from 180 beats / min to 400 beats / min.

[0032] Thus, to determine the atrial rate at which the beat interval of the RR baseline is closest to the atrial line, a limited subset of atrial rates may be considered. This limited subset may include, for example, all integer values ​​between the lowest and highest atrial rates. The lowest atrial rate may be selected, for example, equal to 180 bpm. The highest atrial rate may be selected, for example, equal to 400 bpm. In alternative implementations, the lowest and / or highest atrial rates may be selected differently, and instead of respective integer values, a step value may be set that corresponds to a step or distance between two consecutive atrial rates considered.

[0033] In an embodiment of the PPG signal processing device according to the invention, as defined in claim 5, the mean beat interval value of the beat interval baseline is: the average value of the beat interval values ​​belonging to the beat interval baseline; the median of the beat interval values ​​belonging to the beat interval baseline; The most frequent value of the beat interval values ​​that belong to the beat interval baseline, or The median of the beat interval values ​​belonging to the beat interval baseline corresponds to one of the following:

[0034] An embodiment of the present invention determines the average or median value of the beat intervals within each RR baseline as described above. Since the beat intervals are substantially constant within the RR baseline, the beat-to-beat values ​​are assumed to be equal or nearly equal. Nevertheless, it is meaningful to determine the average or median value for subsequent processing (e.g., comparing such average or median value with the atrial line of the atrial rate). The average or median value can be selected to be less sensitive to outliers, such as the median or mode. Alternatively, the average or median value can be used to determine the average or median value, since outliers may not exist as a result of the variation tolerance.

[0035] An embodiment of the PPG signal processing device according to the invention further comprises means for setting a minimum amount forming a beat interval baseline for a number of consecutive beat intervals as defined in claim 6.

[0036] Allowing the user to set the minimum amount of beat-to-beat intervals that constitute the RR baseline provides the user with a parameter for controlling the accuracy and reliability of the device. Setting a longer minimum length for the RR baseline improves accuracy by thus reducing the risk of considering a short period during which the heart rate happens to remain constant as an RR baseline that is indicative of a particular heart rhythm disorder.

[0037] An embodiment of the PPG signal processing device according to the invention further comprises means for setting said predetermined tolerance value as defined in claim 7 .

[0038] In fact, the variation tolerance, when made configurable, constitutes a second parameter that allows the user to control the accuracy and reliability of the device. The smaller the variation tolerance value, the fewer periods with substantially constant beat intervals are considered as RR baselines (due to values ​​outside the variation tolerance, periods may not reach the minimum length required to be considered as RR baselines). Furthermore, within the RR baseline, the values ​​of the beat intervals will generally be close to each other (their difference may be less than or equal to the variation tolerance). A decrease in the variation tolerance reduces the risk of considering short periods in which the heart rate happens to remain constant as RR baselines indicative of certain heart rhythm disorders, and vice versa.

[0039] In addition to a PPG signal processing device, the present invention also provides a corresponding computer-implemented method for processing a photoplethysmography signal (short: PPG signal) for detecting atrial flutter, as defined in claim 8, comprising: obtaining at least one photoplethysmography signal (PPG signal for short) of a human or animal; Detecting a heart beat in at least one PPG signal; determining the beat interval between pairs of consecutive heart beats in at least one PPG signal; detecting beat interval baselines, each beat interval baseline corresponding to a number of consecutive beat intervals that are constant within a predetermined tolerance; determining a respective mean beat interval value for each of the beat interval baselines; Detecting a relationship between average beat interval values, and therefor: determining, for a subset of atrial rates consisting of integer beats per minute values ​​between a lower limit and an upper limit, a respective distance between the average beat interval value of the beat interval baseline and a closest atrial line, the closest atrial line corresponding to an integer portion of the atrial rate; determining, for each atrial rate in the subset of atrial rates, a mean beat interval value and a mean distance between the closest atrial line for that atrial rate; selecting the atrial rate whose mean beat interval value has the smallest mean distance to the atrial line; detecting atrial flutter if the minimum average distance is less than a predetermined threshold distance; The present invention also relates to a computer-implemented method, including:

[0040] According to a further aspect, the invention also relates to a computer program product comprising computer executable instructions for carrying out the method according to the invention, when the program is run on a computer, as defined in claim 9.

[0041] According to yet another aspect, the invention relates to a computer-readable storage medium comprising a computer program product according to the invention as defined in claim 10. [Brief description of the drawings]

[0042] [Figure 1]FIG. 1 illustrates a sequence of steps performed by an exemplary embodiment of a PPG processing device according to the present invention. [Diagram 2] FIG. 2 illustrates the steps of detecting heart beats in a PPG signal 200 as performed by an exemplary embodiment of a PPG processing device according to the present invention. [Diagram 3] FIG. 3 illustrates the steps of determining the beat-to-beat interval and determining the RR baseline based on the PPG signal 200 of FIG. 2 as performed by an exemplary embodiment of a PPG processing device according to the present invention. [Figure 4] FIG. 4 illustrates the steps, as performed by an exemplary embodiment of a PPG processing device according to the present invention, of determining the central beat interval of the RR baseline as detected in FIG. 3 and comparing the RR baseline with the atrial line at an atrial rate of 300 bpm. [Diagram 5] FIG. 5 illustrates the steps, as performed by an exemplary embodiment of a PPG processing device according to the present invention, of selecting the atrial rate having an atrial line with the smallest average distance to the central RR baseline value. [Figure 6] FIG. 6 illustrates a computing system 600 suitable for implementing an exemplary embodiment of a PPG processing apparatus according to this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] 1-5 illustrate one embodiment of a PPG processing device configured to detect an RR baseline and compare such RR baseline to an atrial line corresponding to the atrial rate to detect atrial flutter.

[0044] In a first step, indicated at 101, a PPG processing device acquires one or more PPG signals for a human. Figure 2 shows a 60 second PPG signal 200 acquired for a human in step 101.

[0045] In a second step, indicated at 102, the PPG processing device detects heart beats in the PPG signal. In Fig. 2, points 201, 202, 203, 204, 205 etc. represent heart beats detected in the PPG signal 200. These heart beats are detected by state-of-the-art peak detection algorithms applied to the PPG signal 200 or to portions of the PPG signal 200.

[0046] In a third step, indicated at 103, the PPG processing device determines the length of the beat intervals for pairs of successive heart beats in the PPG signal. These beat intervals can be output or plotted in a tachogram such as the one shown in FIG. 3. In FIG. 3, a point 301 represents a beat interval between a first heart beat 201 and a second heart beat 202 detected in the PPG signal 202. The beat interval corresponds to the time elapsed between the heart beats 201 and 202 and is expressed in seconds. From FIG. 3, it can be seen that the beat interval 301 between the heart beats 201 and 202 corresponds to 0.7 seconds. Similarly, a triangle 302 represents a beat interval between a second heart beat 202 and a third heart beat 203 detected in the PPG signal 202. This second beat interval 302 corresponds to 1.40 seconds. Furthermore, triangle 303 represents the beat interval between the third heart beat 203 and the fourth heart beat 204 detected in the PPG signal 202. This third beat interval 303 corresponds to 1.45 seconds. Triangle 304 in FIG. 3 represents the beat interval between the fourth heart beat 204 and the fifth heart beat 205 detected in the PPG signal 202. This fourth beat interval 304 corresponds to 1.42 seconds. Each subsequent point or triangle in FIG. 3 corresponds to a beat interval between a pair of successive heart beats detected in the PPG signal 200. When plotted on a timeline as is done in FIG. 3, these beat intervals constitute a tachogram.

[0047] In a fourth step, indicated at 104 in FIG. 1, the PPG processing device determines the RR baseline. The RR baseline corresponds to a number of consecutive beat intervals that are equal (within the variability tolerance). In FIG. 3, the detected RR baselines are represented by triangles. Beat intervals that are not part of the RR baseline are represented by dots. For example, beat intervals 302, 303, and 304 constitute the first RR baseline, since they are consecutive beat intervals and their respective values ​​vary by less than 0.05 seconds (which is assumed to be the variability tolerance range in FIG. 3). In addition to the RR baseline formed by beat intervals 302, 303, and 304, FIG. 3 shows five additional RR baselines detected based on the PPG signal 200. Two parameters define the accuracy of the RR baseline detection step: the minimum length of the RR baseline (set to 3 beat intervals in FIG. 3) and the variability allowed in the RR baseline (set to 0.05 seconds in FIG. 3). Increasing the minimum length of the RR baseline and / or decreasing the variation tolerance improves the accuracy and reliability of the PPG processing device because these parameter changes shorten the time interval during which the heart rate by chance remains constant and is considered the RR baseline (reducing false positives).

[0048] In a fifth step, indicated at 105 in FIG. 1, the PPG processing device determines a central beat interval value for each RR baseline detected in step 104. The central or average value can for example correspond to the average value of all beat interval values ​​of the RR baseline. Alternatively, a median, mode, mean or another central value can be considered. In the example of FIG. 3, the average value is determined as the central value for the beat-to-beat values ​​of the RR baseline. Thus, for the RR baseline including the beat intervals 302, 303, 304, this central value corresponds to (1.40+1.45+1.42) / 3=1.42 seconds. Similarly, a central baseline value is determined for each of the detected RR baselines. These central baseline values ​​obtained for one person over one or more PPG signals can be plotted against an index, as is done in FIG. 4 and FIG. 5.

[0049] Depending on the arrhythmia or arrhythmias monitored, the central RR baseline values ​​are then analyzed to see if a particular relationship exists. This is shown by 110 in FIG. 1. In one example where atrial flutter is monitored, the PPG processing device can be configured to detect, for example, whether the central RR baseline values ​​substantially correspond to alternating integer parts (atrial lines) of the common multiple value (atrial rate), where the alternating integer parts represent either a half, a third, or a quarter of the common multiple value. If other arrhythmias are detected by the PPG signal processing device, the existence of other relationships between the central RR baseline values ​​can be investigated.

[0050] To compare the RR baseline with the atrial lines, the distance between the central beat-to-beat value of the detected RR baseline and the respective atrial line corresponding to the atrial rate is determined and the closest distance is kept. This is illustrated by FIG. 4, where 401 represents the central beat-to-beat value of the first RR baseline, 402 represents the central beat-to-beat value of the second RR baseline, etc. In FIG. 4, an atrial rate of 300 bpm is considered and the three atrial lines corresponding to this rate are represented by a dashed line 411 (half the atrial rate corresponds to 150 bpm or a beat interval of 0.40 seconds), a dotted line 412 (one third of the atrial rate corresponds to 100 bpm or a beat interval of 0.60 seconds), and a dash-dotted line 413 (one quarter of the atrial rate corresponds to 75 bpm or a beat interval of 0.80 seconds). For the first RR baseline, the atrial line closest to the central beat interval value 401 is line 412, so the distance between the central beat interval value 401 and atrial line 412 is maintained. For the second RR baseline, the atrial line closest to the central beat interval value 402 is line 413, so the distance between the central beat interval value 402 and atrial line 413 is maintained. Similarly, the distance between each central beat interval value, represented by a point in FIG. 4, and the atrial lines 411, 412, 413 is determined, and the minimum distance (i.e., the distance to the closest atrial line) is maintained. In step 106 of FIG. 1, this process shown in FIG. 4 for an atrial rate of 300 bpm is repeated for a subset of atrial rates (e.g., all integer atrial rate values ​​from 180 bpm to 400 bpm).

[0051] For example, Figure 5 illustrates the above-described distance determination to the atrial line corresponding to an atrial rate of 188 bpm. The points in Figure 5 representing the central beat interval values ​​of the respective RR baselines correspond to the points in Figure 4. In this case, beat interval value 501 corresponds to beat interval value 401, beat interval value 502 corresponds to beat interval value 402, and so on. The three atrial lines corresponding to an atrial rate of 188 bpm are represented by dashed line 511 (one-half the atrial rate corresponds to 94 bpm or a beat interval of 0.64 seconds), dotted line 512 (one-third the atrial rate corresponds to 62.7 bpm or a beat interval of 0.96 seconds), and dash-dot line 513 (one-quarter the atrial rate corresponds to 47 bpm or a beat interval of 1.28 seconds). For the first RR baseline, the atrial line closest to the central beat interval value 501 is line 511, so the distance between the central beat interval value 501 and the atrial line 511 is maintained. For the second RR baseline, the atrial line closest to the central beat interval value 502 is line 512, so the distance between the central beat interval value 502 and the atrial line 512 is maintained. Similarly, the distance between each of the central beat interval values, represented by points in FIG. 5, and the atrial lines 511, 512, 513 is determined, and the minimum distance (i.e., the distance to the closest atrial line) is maintained.

[0052] For each atrial rate from the subset of atrial rates, the average distance from the central beat interval value of the RR baseline to the nearest atrial line is determined according to the distance criterion of step 107 of FIG. 1. The distance criterion can be, for example, the average squared distance over the entire RR baseline. The distance can also be a weighted average squared distance, where the weight corresponds to the length of the RR baseline. In other words, the longer the RR baseline, the higher the weight or importance in the distance criterion. Those skilled in the art will understand that other distance criteria can be considered to determine the average distance from the central beat interval value of the RR baseline to the nearest atrial line for each atrial rate, all of which are encompassed by the present invention.

[0053] In step 108 of Figure 1, the PPG processing device selects the atrial rate for which the smallest average distance between the RR baseline center distance interval value and the closest atrial line has been determined according to steps 106 and 107. In the example shown in Figures 2-5, the average distance for the atrial rate of 188 bmp shown in Figure 5 is the smallest.

[0054] According to step 109 of Figure 1, the PPG processing device compares the minimum average distance with a predefined threshold. If the minimum average distance is below the predefined threshold, the person is classified as an atrial flutter patient.

[0055] In the above embodiment, it is assumed that the flutter heart rhythm includes a pattern with consecutive RR or beat intervals having the same value, forming a so-called RR baseline (usually a short time interval with a very regular heartbeat). A second assumption is that the beat intervals of the RR baseline are all on a line of integer fractions of a common basic atrial rate, and that the AR / VR ratio, or so-called AV conduction ratio, is 2:1, 3:1, or 4:1.

[0056] FIG. 6 illustrates a suitable computing system 600 according to an embodiment of the present invention. The computing system 600 is suitable for implementing an embodiment of a PPG processing device according to the present invention. The computing system 600 is generally formed as a suitable general-purpose computer and includes a bus 610, a processor 602, a local memory 604, one or more optional input interfaces 614, one or more optional output interfaces 616, a communication interface 612, a storage element interface 606, and one or more storage elements 608. The bus 610 may include one or more conductors that enable communication between the components of the computing system 600. The processor 602 may include any type of conventional processor or microprocessor that interprets and executes programming instructions. The local memory 604 may include a random access memory (RAM) or another type of dynamic storage device that stores information and instructions executed by the processor 602, and / or a read-only memory (ROM) or another type of static storage device that stores static information and instructions used by the processor 602. The input interface 614 may include one or more conventional mechanisms that allow an operator or user to input information into the computing device 600, such as, for example, a keyboard 620, a mouse 630, a pen, a voice recognition and / or biometric mechanism, a camera, etc. The output interface 616 may include one or more conventional mechanisms that output information to an operator or user, such as, for example, a display 640. The communication interface 612 may include transceiver-like mechanisms, such as, for example, one or more Ethernet interfaces, that allow the computing system 600 to communicate with other devices and / or systems, such as other computing devices 681, 682, 683. The communication interface 612 of the computing system 600 may be connected to such other computing systems by a local area network (LAN) or a wide area network (WAN), such as, for example, the Internet.The storage element interface 606 may include a storage interface, such as a Serial Advanced Technology Attachment (SATA) interface or a Small Computer System Interface (SCSI), for connecting the bus 610 to one or more storage elements 608, such as one or more local disks (e.g., SATA disk drives), and may control the reading and writing of data to and from these storage elements 608. Although the storage elements 608 above are described as local disks, generally other suitable computer-readable media may be used, such as removable magnetic disks, optical storage media such as CDs or DVDs, -ROM disks, solid-state drives, flash memory cards, etc. It should be noted that the entire method according to the invention may be performed centrally, for example on a server in a management center or in a cloud system, or may be performed partly on a remote electronic device, for example worn by a user, and partly on a central server. Thus, the computing system 600 may correspond to a processing system available centrally or available at an electronic device.

[0057] Although the present invention has been described with reference to specific embodiments, the present invention is not limited to the details of the aforementioned exemplary embodiments, and it will be apparent to those skilled in the art that the present invention can be embodied with various changes and modifications without departing from the scope thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, and the scope of the present invention is indicated by the appended claims, rather than by the foregoing description, and all changes that are within the meaning and range of equivalence of the claims are therefore intended to be included therein. In other words, it is contemplated to cover any modifications, variations, or equivalents that are within the scope of the underlying basic principles and whose essential attributes are claimed in this patent application. Furthermore, readers of this patent application will understand that the words "comprising" or "comprise" do not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality, and that a single element, such as a computer system, a processor, or another integrated unit, can perform the functions of several means recited in the claims. Any signs in the claims shall not be interpreted as limiting the respective claims as applicable. Terms such as "first," "second," "third," "a," "b," "c," and the like, when used in the description or claims, are introduced to distinguish between similar elements or steps and do not necessarily describe a sequential or chronological order. Similarly, terms such as "top," "bottom," "upper," "lower," and the like, are introduced for purposes of description and do not necessarily indicate a relative location. It should be understood that the terms so used are interchangeable under appropriate circumstances and that embodiments of the invention are capable of operating in accordance with the invention in other orders or orientations other than those described or illustrated above.

Claims

1. 1. A photoplethysmography signal processing device (short PPG signal processing device) configured to detect atrial flutter, the PPG signal processing device comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code, together with the at least one processor, causing the PPG signal processing device to: acquiring (101) at least one photoplethysmography signal (200) (short PPG signal) of a human or animal; Detecting (102) heart beats (201, 202, 203, 204, 205, ...) in said at least one PPG signal (200); determining (103) beat intervals (301, 302, 303, 304, ...) between pairs of consecutive heart beats (201-202, 202-203, 203-204, 204-205, ...) in said at least one PPG signal (200); detecting (104) beat interval baselines, each of which corresponds to a number of consecutive beat intervals (302, 303, 304) that are constant within a predetermined tolerance; determining (105) a respective mean beat interval value (401, 402, ...) for each of said beat interval baselines (302-304); A relationship is detected (110) between the average beat interval values ​​(401, 402, ...; 501, 502, ...), whereby: determining (106) for a subset of atrial rates consisting of integer beats per minute values ​​between a lower limit and an upper limit, a respective distance between the average beat interval values ​​(401, 402, ...; 501, 502, ...) of the beat interval baseline and a nearest atrial line (411, 412, 413; 511, 512, 513), the nearest atrial line corresponding to an integer portion of the atrial rate (411, 412, 413; 511, 512, 513); determining (107) for each atrial rate in the subset of atrial rates an average distance between the average beat interval value (401, 402, ...; 501, 502, ...) and the closest atrial line (411, 412, 413; 511, 512, 513) to the atrial rate; selecting (108) the atrial rate whose average beat interval values ​​(401, 402, ...; 501, 502, ...) have a minimum average distance to the atrial line (411, 412, 413; 511, 512, 513); Detecting 109 atrial flutter if the minimum average distance is less than a predetermined threshold distance. The PPG signal processing device is configured to:

2. 2. The PPG signal processing device of claim 1, wherein the average distance corresponds to a weighted squared distance, and a length or duration of a beat interval baseline (302-304) serves as a respective weight for a squared distance between the average beat interval value of the beat interval baseline (302-304) and the closest atrial line within the average distance.

3. 3. The PPG signal processing device according to claim 1, wherein the atrial lines (411, 412, 413; 511, 512, 513) for one atrial rate of the subset of atrial rates correspond to one half of the atrial rate (411; 511), one third of the atrial rate (412; 512), and one quarter of the atrial rate (413; 513).

4. The PPG signal processing apparatus of any one of claims 1 to 3, wherein the subset of atrial rates includes all integer rates between a lower limit and an upper limit.

5. The average beat interval values ​​(401, 402, ...; 501, 502, ...) of the beat interval baseline (302-304) are as follows: an average value of the beat interval values ​​(302, 303, 304) belonging to the beat interval baseline (302-304); the median of the beat interval values ​​(302, 303, 304) belonging to the beat interval baseline (302-304); the mode of the beat interval values ​​(302, 303, 304) belonging to said beat interval baseline (302-304); or The median value of the beat interval values ​​(302, 303, 304) belonging to the beat interval baseline (302-304) The PPG signal processing device according to any one of claims 1 to 4, which corresponds to one of the above.

6. The PPG signal processing apparatus according to any one of claims 1 to 5, further comprising means for setting a minimum amount forming a beat interval baseline for the plurality of consecutive beat intervals.

7. 7. The PPG signal processing device according to claim 1, further comprising means for setting the predetermined tolerance value.

8. 1. A computer-implemented method for processing a photoplethysmography signal (abbreviated PPG signal) for detecting atrial flutter, comprising: Acquiring (101) at least one photoplethysmography signal (200) (short PPG signal) of a human or animal; detecting (102) heart beats (201, 202, 203, 204, 205, ...) in said at least one PPG signal (200); - determining (103) beat intervals (301, 302, 303, 304, ...) between pairs of consecutive heart beats (201-202, 202-203, 203-204, 204-205, ...) in said at least one PPG signal (200); detecting (104) beat interval baselines, each of which corresponds to a number of consecutive beat intervals (302, 303, 304) that are constant within a predetermined tolerance; determining (105) a respective mean beat interval value (401, 402, ...; 501, 502, ...) for each of said beat interval baselines (302-304); A step (110) of detecting a relationship between said average beat interval values ​​(401, 402, ...; 501, 502, ...), for which: determining (106) for a subset of atrial rates consisting of integer beats per minute values ​​between a lower limit and an upper limit, a respective distance between said average beat interval values ​​(401, 402, ...; 501, 502, ...) of said beat interval baseline and a nearest atrial line (411, 412, 413; 511, 512, 513), said nearest atrial line corresponding to an integer portion of said atrial rate; determining (107) for each atrial rate in said subset of atrial rates an average distance between said average beat interval value (401, 402, ...; 501, 502, ...) and the closest atrial line (411, 412, 413; 511, 512, 513) to said atrial rate; selecting (108) the atrial rate whose average beat interval values ​​(401, 402, ...; 501, 502, ...) have a minimum average distance to the atrial line (411, 412, 413; 511, 512, 513); detecting atrial flutter if the minimum average distance is less than a predetermined threshold distance (109); 4. A computer-implemented method comprising:

9. A computer program product comprising computer executable instructions for carrying out the method of claim 8 when the program is run on a computer.

10. 10. A computer readable storage medium comprising the computer program product of claim 9.

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