Method of deriving systolic blood pressure and/or diastolic blood pressure of a subject

Abstract

A method (200) of deriving systolic blood pressure and/or diastolic blood pressure of a subject is disclosed. The method comprises: (i) receiving (202) data related to at least one cardiac cycle of a bio-signal from the subject; (ii) calculating (208) a rise time and a fall time of the at least one cardiac cycle based on the received data; (iii) calculating (208) a parameter derived from a function of the rise time and fail time; and (iv) determining (210) the systolic blood pressure and/or diastolic blood pressure of the subject based on the calculated parameter. A related apparatus is also disclosed.

Claims

1. A method of deriving systolic blood pressure and/or diastolic blood pressure of a subject, the method comprising: (i) measuring, by a single cuffless portable optical measurement device, one or more bio-signals including at least one cardiac cycle; (ii) calculating, by one or more processors based on the measured one or more bio-signals, a rise time and a fall time of the at least one cardiac cycle; (iii) calculating, by the one or more processors, a parameter according to the equation:
f=(T.sub.r).sup.X×(T.sub.f).sup.Y, where ƒ is the parameter, T.sub.r is the rise time, T.sub.f is the fall time, and x and y are predetermined constants selected from a range of between −3 to 3, and exclusive of 0; (iv) determining, by the one or more processors based on the calculated parameter, at least one of the systolic blood pressure or the diastolic blood pressure of the subject; and (v) outputting, by a display, the determined at least one of the systolic blood pressure or the diastolic blood pressure.

2. The method of claim 1, wherein the rise time is calculated between a start of the at least one cardiac cycle to a systolic peak of the at least one cardiac cycle.

3. The method of claim 2, wherein the fall time is calculated from the systolic peak of the at least one cardiac cycle to an end of the at least one cardiac cycle.

4. The method of claim 1, wherein the rise time is calculated between 10% and 90% of a systolic peak of the at least one cardiac cycle.

5. The method of claim 1, wherein the one or more bio-signals further includes an arterial photoplethysmography (PPG) signal.

6. The method of claim 5, further comprises processing the PPG signal to digitally filter noise signals in the PPG signal.

7. The method of claim 6, wherein digitally filtering the noise signals in the PPG signal includes using a band pass filter configured to only permit signals having a frequency of between 0.5 Hz to 8.0 Hz to pass through the band pass filter.

8. The method of claim 1, wherein the one or more bio-signals relates to a plurality of cardiac cycles, and the method further includes: calculating the rise time and the fall time of each of the plurality of cardiac cycles; calculating, based on a function of the rise time and the fall time of each of the plurality of cardiac cycles, parameters; and calculating, based on the respectively calculated parameters as the parameter in step (iii), an average parameter.

9. The method of claim 8, wherein the plurality of cardiac cycles include being arranged consecutively.

10. The method of claim 1, wherein the one or more bio- signals includes time intervals of the at least one cardiac cycle relating to a systolic peak, start time and end time of the at least one cardiac cycle.

11. The method of claim 1, further comprises determining a systolic peak, a start time and an end time by a first electronic device and transmitting, by the one or more processors, the determined systolic peak, start time and end time to a second electronic device for calculating the rise time and fall time.

12. The method of claim 1, wherein determining the systolic blood pressure includes determining the systolic blood pressure according to the equation:
SBP=c.sub.1׃+c.sub.2+c.sub.3, where SBP is the systolic blood pressure; ƒ is the parameter; and c.sub.1, c.sub.2, and c.sub.3 are predetermined constants.

13. The method of claim 1, wherein determining the diastolic blood pressure includes determining the diastolic blood pressure according to the equation:
DBP=√{square root over (2×eQM.sup.2−SBP.sup.2)}+c.sub.6, where DBP is the diastolic blood pressure; eQM is an estimated quadratic mean; SBP is the systolic blood pressure; and c.sub.6 is a predetermined constant.

14. The method of claim 13, wherein the estimated quadratic mean is determined according to the equation:
eQM=c.sub.4×SBP+c.sub.5, where eQM is an estimated quadratic mean; SBP is the systolic blood pressure; and c.sub.4, and c.sub.5 are predetermined constants.

15. A non-transitory computer readable medium storing instructions for deriving systolic blood pressure and/or diastolic blood pressure of a subject, which when executed by one or more processors of a single cuffless portable optical measurement device configured to capture one or more bio-signals cause the one or more processors to: (i) measure the one or more bio-signals including at least one cardiac cycle; (ii) calculate, based on the measured one or more bio-signals, a rise time and a fall time of the at least one cardiac cycle; (iii) calculate a parameter according to the equation:
f=(T.sub.r).sup.X×(T.sub.f).sup.y, where ƒ is the parameter, T.sub.r is the rise time, T.sub.f is the fall time, and x and y are predetermined constants selected from a range of between −3 to 3, and exclusive of 0; (iv) determine, based on the calculated parameter, at least one of the systolic blood pressure or diastolic blood pressure; and (v) output to a display the determined at least one of the systolic blood pressure or the diastolic blood pressure.

16. A single cuffless optical measurement device for deriving systolic blood pressure and/or diastolic blood pressure of a subject, the single cuffless optical measurement device comprising: (i) a sensor configured to capture one or more bio-signals including at least one cardiac cycle; (ii) a display; and (iii) a processor configured to: (a) measure the one or more bio-signals; (b) calculate, based on the measured bio-signals, a rise time and a fall time of the at least one cardiac cycle; (c) calculate a parameter according to the equation:
f=(T.sub.r).sup.X×(T.sub.f).sup.Y, where ƒ is the parameter, T.sub.r is the rise time, T.sub.f is the fall time, and x and y are predetermined constants selected from a range of between −3 to 3, and exclusive of 0; (d) determine, based on the calculated parameter, at least one of the systolic blood pressure or the diastolic blood pressure; and (e) output, to the display, the determined at least one of the systolic blood pressure or the diastolic blood pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are disclosed hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic block diagram of an apparatus for measuring the blood pressure of a subject, based on a first embodiment of the invention;

(3) FIG. 2 is a flow diagram of a method, performed by the apparatus of FIG. 1, for deriving systolic blood pressure and/or diastolic blood pressure of the subject;

(4) FIG. 3 is a diagram of a PPG signal having at least a cardiac cycle;

(5) FIG. 4 is a diagram showing a rise time and a fall time of a cardiac cycle included in a PPG signal;

(6) FIG. 5 is a diagram depicting the correlation between systolic blood pressure values versus measured average of percentage of rise time over fall time;

(7) FIG. 6 is a diagram depicting the correlation between eQM values versus measured systolic blood pressure values;

(8) FIG. 7 is a flow diagram of step 210 of the method of FIG. 2 for calculating the systolic blood pressure and/or diastolic blood pressure of the subject;

(9) FIG. 8 is a table comparing measurement results obtained using the apparatus of FIG. 1 versus results obtained using a conventional blood pressure monitoring device;

(10) FIG. 9 is a diagram of an arterial waveform, in which effect of age on the diastolic peak and dicrotic notch of the arterial waveform is shown, according to the prior art; and

(11) FIG. 10 is a diagram depicting the correlation between systolic blood pressure values and a function of the rise time and fall time.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(12) FIG. 1 is a schematic block diagram of an apparatus 100 for measuring blood pressure of a (human or animal) subject 102 according to a first embodiment. In particular, measuring blood pressure herein refers to deriving systolic blood pressure and/or diastolic blood pressure of the subject 102. The apparatus 100 includes an optical measurement device 110 comprising a signal sensing module 112 for obtaining a bio-signal from the said subject 102. The optical measurement device 110 further includes a data processing module 114 (e.g. a processor) which is arranged to receive and process the bio-signal from the signal sensing module 112, and a wireless/wired transmission module 116 for transmitting data processed from the bio-signal. It is to be appreciated that the optical measurement device 110 is preferably arranged to be conveniently portable, for example in a palm-sized form factor.

(13) The transmission module 116 of the optical measurement device 110 is arranged to communicate wirelessly/non-wirelessly with a telecommunications device 120 (e.g. a mobile phone or other portable electronic devices). The telecommunications device 120, which is part of the apparatus 100, includes a receiving module 122 for receiving signals from the optical measurement device 110, a calculation module 124 (e.g. a processor) and a display unit 126 for displaying a result or information to a user of the apparatus 100. The receiving module 122 is configured to receive the signals from the optical measurement device 110 wired/wirelessly, depending on the corresponding setup of the transmission module 116 of the optical measurement device 110.

(14) FIG. 2 shows a method 200 for deriving systolic blood pressure and/or diastolic blood pressure of the subject 102, whereby said method 200 is performed by the apparatus 100 of FIG. 1. A brief overview of steps 202-210 of the method 200 is first described, with detailed description of each step to follow further below. At step 202, signal acquisition is performed to obtain a bio-signal from the subject 102, and at next step 204, the acquired bio-signal is filtered to provide a processed signal. In this embodiment, the bio-signal is an arterial PPG waveform signal 300 comprising at least one cardiac cycle 302, for example as depicted in FIG. 3. But in other variant embodiments, it is however to be appreciated that a plurality of cardiac cycles 302-306 (as depicted in FIG. 3) may also be utilised to provide better accuracy of results, and the plurality of cardiac cycles 302-306 are arranged consecutively together. At following step 206, the processed signal (from step 204) is transmitted to the calculation module 124 of the telecommunications device 120, i.e. transmission is made from the wired/wireless transmission module 116 (of the optical measurement device 110) to the receiving module 122 (of the telecommunications device 120). Then at step 208, the received processed signal is data processed by the calculation module 124 of the telecommunications device 120 to obtain a rise time and a fall time of the least one cardiac cycle 302. Finally, at step 210, the systolic blood pressure and/or diastolic blood pressure of the subject 102 are calculated based on a parameter derived from a function of the rise time and fall time, i.e. see equation (1). It is to be appreciated that steps 206, 208 may be performed by the apparatus 100 sequentially or in parallel, depending on a specific desired implementation.

(15) The above mentioned steps 202-210 of the method in FIG. 2 are respectively described in greater detail set out below.

(16) 1. Step 202 of the Method

(17) At step 202, signal acquisition is performed by the optical measurement device 110 using the signal sensing module 112 to obtain a bio-signal from the subject 102 and as mentioned, the bio-signal is the arterial PPG waveform signal 300 (hereafter “PPG signal” for brevity) shown in FIG. 3. It is to be appreciated that the PPG signal 300 includes time intervals relating to a systolic peak, a start time and an end time of the at least one cardiac cycle 302. The PPG signal 300 may be acquired from any peripheral sites of a subject's body such as wrist and/or finger. It has been empirically determined that an accuracy suitable for calculating the subject's blood pressure may for example, but without being limiting, be achieved by configuring the signal sensing module 112 to employ a measurement window of at least 30 seconds to obtain the subject's bio-signal.

(18) 2. Steps 204 and 206 of the Method

(19) The acquired PPG signal 300 is then passed to the data processing module 114 for processing at step 204. The data processing module 114 may include a predefined digital band pass filter (not shown), or a low pass filter for digitally filtering any noise and signal artifacts present in the acquired PPG signal 300 to beneficially provide a filtered PPG signal (not shown). In this case, reference to the filtered PPG signal will instead be made to FIG. 4 as the filtered PPG signal 400, which includes a (filtered) cardiac cycle 402 of the at least one cardiac cycle 302.

(20) Following from step 206, the filtered PPG signal 400 is transmitted to the calculation module 124 of the telecommunications device 120 to be further processed. It is to be appreciated that the filtered PPG signal 400 received by the telecommunications device 120 may be further processed with band pass filtering. In this respect, a digital band pass filter may be provided at the calculation module 124, and configured to only permit signals in the frequency range from 0.5 Hz to 8.0 Hz to pass through.

(21) 3. Step 208 of the Method

(22) Detecting Peak and Valley

(23) At step 208, (with reference to FIG. 4) the calculation module 124 is configured to detect a first valley position 404, a start point 404 (which coincides with the first valley position 404), a peak position, a systolic peak 406 and a second valley position 408, an end point 408 (which coincides with the second valley position 408) of the cardiac cycle 402. To elaborate, the time intervals representing the start point 404, end point 408 and the systolic peak 406 of the cardiac cycle 402 are obtained by the calculation module 124 to determine data relating to at least one cardiac cycle of the filtered PPG signal 400. It should be appreciated that the filtered PPG signal 400 also includes a dicrotic notch 410 and a diastolic peak 412, but this embodiment however does not make use of the dicrotic notch 410/diastolic peak 412 to determine the blood pressure (but is not to be construed as limiting in any way).

(24) Calculating Rise and Fall Time

(25) The calculation module 124 is configured to calculate a rise time based on the start point 404 and the systolic peak 406 of the cardiac cycle 402. Mathematically, referring to FIG. 4, the rise time for the cardiac cycle 402 is thus the time interval (denoted by reference numeral 414) measured between the start point 404 and the systolic peak 406. More preferably, it is to be appreciated that the rise time is calculated between 10% and 90% of a systolic peak of the at least one cardiac cycle. Further, the calculation module 124 also obtains a fall time of cardiac cycle 402. Mathematically, the fall time is the time interval (denoted by reference numeral 416) measured between the systolic peak 406 and the end point 408.

(26) Calculating a Parameter Derived from a Function of Rise Time and Fall Time

(27) Next a parameter to be derived from a function of the rise time and fall time, as obtained previously, is calculated according to equation (1):
f=(T.sub.r).sup.X×(T.sub.f).sup.Y  (1)
in which ƒ is the parameter, T.sub.r is the rise time, T.sub.f is the fall time, and x and y are predetermined constants respectively selected from a range of between −3 to 3 (i.e. −3≤x, y≤3, where x and y are not equal to zero). The value of x is preferably in range of 0 to 3 (i.e. 0<x≤3), more preferably in range of 1 to 3 (i.e. 1≤x≤3), and more preferably in range of 1 to 2 (i.e. 1≤x≤2). The value of y is preferably in range of −3 to 0 (i.e. −3≤y<0), more preferably in range of −3 to −1 (i.e. −3≤y≤−1), and more preferably in range of −2 to −1 (i.e. −1≤y≤−2). It is to be appreciated that in this embodiment, the respective values of x and y are 1 and −1. So this means equation (1) takes the simplified form set out in equation (2) below:

(28) $\begin{array}{cc}f=\frac{T_r}{T_f}& \left(2\right)\end{array}$

(29) With reference to equation (2), in the event that the filtered PPG signal 400 includes a plurality of (filtered) cardiac cycles, an average ratio corresponding to dividing the sum of all ratios (of rise time over fall time) over the total number of cardiac cycles.

(30) Moreover, additional mathematical operations may be performed to improve accuracy of the eventually calculated blood pressure values. For example, the calculation module 124 may also be configured to further perform a further outlier filtering process to identify and remove outlier percentage of rise time over fall time that are affected by any noise and signal artifacts. Statistical operations such as calculating the mean, moving average, standard deviation or combinations thereof, may be used by the calculation module 124 to define outlier values. In one exemplary but non-limiting example, value of percentage of rise time over fall time that are not determined to be within 50% of data at the centre, or one standard deviation (sigma) derived from all values of percentage of rise time over fall time in the measurement window are classified as outliers and will be removed from subsequent calculations. Accordingly, an average percentage of rise time over fall time is calculated based on all the non-outlier values based on equation (3):
Average % of rise time/fall time=Sum of all (non-outlier % rise time/fall time)/No. of (non-outlier % rise time/fall time)  (3)

(31) For good order, it is to be noted that equation (3) is used only in the event that the filtered PPG signal 400 includes a plurality of cardiac cycles, and is also optional under such circumstances.

(32) 4. Step 210 of the Method

(33) At step 210, the calculation module 124 (of the telecommunications device 120) calculates the systolic blood pressure and/or diastolic blood pressure of the subject 102 using the parameter derived from equation (1), and more specifically in this embodiment, equation (2). In particular, performance of the step 210 is detailed in a method 700 of FIG. 7. At step 702, the calculation module 124 calculates a systolic blood pressure (SBP) of the subject 102 according to equation (4):
SBP=c.sub.1×f+c.sub.2+c.sub.3  (4)
in which SBP is the systolic blood pressure; ƒ is the parameter; and c.sub.1, c.sub.2, and c.sub.3 are predetermined constants. It is to be appreciated that the constants, c.sub.1, and c.sub.2, may be determined based on correlation of actual clinical data as shown in FIG. 5, which is a diagram 500 depicting the correlation between systolic blood pressure values versus measured average of percentage of rise time over fall time.

(34) Once the constants, c.sub.1, and c.sub.2 are determined, equation (4) is input into the calculation module 124 for future calculations of arterial pressure, without beneficially need for further user-specific calibration procedures (relating to SBP measurements for the same user for which c.sub.1, and c.sub.2 were determined).

(35) Then at next step 704, the calculation module 114 is configured to calculate an estimated quadratic mean (eQM) based on equation (5), in which FIG. 6 depicts a diagram 600 about the correlation between eQM values versus measured systolic blood pressure values:
eQM=c.sub.4×SBP+c.sub.5  (5)
in which eQM is an estimated quadratic mean; SBP is the systolic blood pressure; and c.sub.4, and c.sub.5 are predetermined constants. In this instance, the SBP value used in equation (5) is based on the same calculated in equation (4).

(36) In last step 706, the calculation module 124 is arranged to compute a diastolic blood pressure (DBP) of the subject 102, using the SBP and eQM values respectively obtained from equations (4) and (5) above, according to equation (6):
DBP=√{square root over (2×eQM.sup.2−SBP.sup.2)}+c.sub.6  (6)
in which DBP is the diastolic blood pressure; eQM is an estimated quadratic mean; SBP is the systolic blood pressure; and c.sub.6 is a predetermined constant.

(37) FIG. 8 is a table 800 comparing measurement results obtained using the apparatus 100 of FIG. 1 versus results obtained using a reference conventional blood pressure monitoring device arranged with an arm cuff, such as Omron HEM-7203.

(38) The remaining configurations will be described hereinafter. For the sake of brevity, description of like elements, functionalities and operations that are common between the different configurations are not repeated; reference will instead be made to similar parts of the relevant configuration(s).

(39) In a second embodiment, instead of comprising two separate devices, i.e. the optical measurement device 110 and telecommunication device 120, or the apparatus 100 itself may be implemented as a single equivalent electronic device, in which the optical measurement device 110 and telecommunication device 120 are (hardware) integrated and configured to perform all the same functions described in the first embodiment. Further, all the steps 202-210 of the method 200 (in FIG. 2) may be implemented as a computer program product downloadable over the internet for storing on a memory of the said single electronic device. So, if there are improvements to the method 200 in future, the electronic device may also be updated (as and when required) with those improvements by way of the downloadable computer program product.

(40) In a third embodiment, all the steps 202-210 of the method 200 are performed by one electronic device which may be the telecommunications device 120 or the optical measurement device 110 (which may also be equipped with a display unit). In other words, it is envisaged that the various modules—signal sensing, data processing and calculation modules 112, 114, 124 may form parts of the same electronic device, possibly as part of the optical measurement device 110 or the telecommunications device 120. The single electronic device may also be realised as a wearable sensing device to be worn on the subject's body.

(41) In a fourth embodiment, step 210 of the method 200 may be performed by the data processing module 114 of the optical measurement device 110, instead of the calculation module 124 of the telecommunications device 120, if it is determined that the data processing module 114 (for example) has a higher processing power than the calculation module 124, but is however not to be construed as a sole limiting criterion for the data processing module 114 to execute step 210. Also, performance of step 210 may also dynamically be allocated to the data processing module 114 or calculation module 124, based on a desired arrangement configured by a user of the apparatus 100.

(42) In summary, the proposed method 200 of FIG. 2 (and related apparatus 100) advantageously enables an accurate blood pressure value (i.e. the systolic blood pressure and/or diastolic blood pressure) to be measured (from a single measurement site of the subject 102) using a single optical sensor device, without needing an inflatable cuff, and further without requiring identification of the dicrotic notch and the diastolic peak of an arterial waveform for analysis. Specifically, the proposed method 200 broadly includes the following steps of receiving data related to at least one cardiac cycle of a bio-signal from the subject 102; calculating a rise time and a fall time of the at least one cardiac cycle based on the received data; calculating a parameter derived from a function of the rise time and fall time; and determining the systolic blood pressure and/or diastolic blood pressure of the subject 102 based on the calculated parameter. So, by detecting blood volume changes in the arteries of the subject 102 through PPG techniques, and utilizing various important graphical features of the PPG signal waveform, blood pressure information, i.e. the systolic and diastolic blood pressures, may be calculated from the above disclosed equations (4)-(6).

(43) 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. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention.

(44) For example, in step 208 of the method 200, if the filtered PPG signal 400 includes a plurality of filtered cardiac cycles, then a corresponding first valley position, a start point 404, a peak position, a systolic peak 406 and a second valley position of each filtered cardiac cycle are to be detected. Further, the SBP value used in equations (5) and (6) need not always be based on the SBP value computed in equation (4). Indeed, the SBP value used in equations (5) and (6) may instead be obtained using suitable conventional methods/apparatuses, independent of the value derived from equation (4). Then, the respective SBP values obtained using the conventional methods/apparatuses, and equation (4) are compared to determine which SBP value actually provides a more accurate result, and consequently, the more accurate SBP value is substituted into equations (5) and (6) for calculating values of the eQM and DBP. Then alternatively, with reference to step 208, the systolic peak 406, start time (i.e. based on the start point 404) and end time (i.e. based on the end point 408) may instead be determined by the data processing module 114 (instead of the calculation module 124), and subsequently, the determined systolic peak 406, start time and end time are transmitted to the calculation module 124 for calculating the rise time and fall time.

(45) It is also to be appreciated that for the condition −3≤x, y≤3, where x and y are not equal to zero, the predetermined constants, c.sub.1 to c.sub.6 (listed in afore disclosed equations (4)-(6)), have the following respective ranges: −4.11≤c.sub.1≤22.18, 38≤c.sub.2+c.sub.3≤220, 0.7082≤c.sub.4≤0.8170, −5.22≤c.sub.5≤21.71, and −27≤c.sub.6≤27.

(46) For good order, FIG. 10 is a diagram 1000 showing the correlation between systolic blood pressure values and a function of the rise time and fall time for the following three sets of conditions: (i). x=2, y=−2 (i.e. see line denoted by reference numeral 1002), (ii). x=1, y=−1 (i.e. see line denoted by reference numeral 1004), and (iii). x=1, y=−2 (i.e. see line denoted by reference numeral 1006).