METHOD AND APPARATUS FOR ESTIMATING BLOOD PRESSURE

Abstract

A method and apparatus for measuring blood pressure by measuring a photoplethysmographic (PPG) signal of a user with the arm in a raised position and in a lowered position and measuring the difference in timing between them, which represents a change in pulse transit time. The PPG signal is measured in the wrist of the user relative to the PPG signal in the finger of the user. A camera built into a mobile telephone may form a first optical sensor for measuring the PPG signal in the finger and an attached accessory camera, such as an infrared camera, or an optical sensor in a wrist-worn device to obtain the PPG signal in the wrist. Alternatively, a head-worn device may be used as a second optical sensor. Signal averaging based on the timing of the finger-originating PPG signal is used to average the waveforms in the wrist-originating PPG signal for arm-up and arm-down, and the timing difference is measured between the arm-up averaged waveform and arm-down averaged waveform. A calibration process is used to derive a relationship between the change in pulse transit time and the subjects blood pressure allowing a display of an estimate of the blood pressure of the subject on the screen of the mobile telephone.

Claims

1. A method of estimating blood pressure comprising: detecting a first photoplethysmographic signal at a first location on a subject's body using a first optical sensor of a first device held by the subject's hand; detecting a second photoplethysmographic signal at a second location on the subject's body using a second optical sensor of a second device; measuring the relative timings of the detected photoplethysmographic signals and calculating from the relative timings an estimate of the blood pressure.

2. A method according to claim 1 further comprising detecting the first and second photoplethysmographic signals with the subject's arm in a first position and measuring a first relative timing between the detected photoplethysmographic signals, detecting the first and second photoplethysmographic signals with the subject's arm in a second position, different from the first position, and measuring a second relative timing between the detected photoplethysmographic signals, calculating from the first and second relative timings an estimate of the subject's blood pressure.

3. A method according to claim 2 wherein the first position is with the arm raised higher than in the second position.

4. A method according to claim 1 wherein the first location is on the hand of the subject, for example on the finger.

5. A method according to claim 1 wherein the second location is one of: the wrist of the subject, on the head of the subject, on or adjacent to the ear of the subject.

6. A method according to claim 1 wherein at least one of the optical sensors is placed in contact with subject's skin to detect the photoplethysmographic signal.

7. A method according to claim 1 wherein at least one of the optical sensors is an optical reflectance measuring device.

8. A method according to claim 1 wherein at least one of the optical sensors comprises a light source for emitting light towards the subject's skin and a light detector for detecting light emitted from the subject's skin.

9. A method according to claim 1 wherein the first device is a mobile computing device incorporating a camera as said first optical sensor and the second device is one of: an accessory camera as said second optical sensor, a wrist-worn device, a head-worn device, an audio headphone, a headset, spectacles, each comprising an optical sensor as said second optical sensor.

10. A method according to claim 1 further comprising one of the steps of (a) synchronizing clocks of the first and second devices, or (b) acquiring the time difference between clocks of the first and second devices.

11. A method according to claim 1 comprising the steps of averaging together a plurality of photoplethysmographic waveforms of said photoplethysmographic signals to obtain representative average photoplethysmographic waveforms and measuring said relative timing using the representative average photoplethysmographic waveforms.

12. A method according to claim 11 further comprising the step of comparing the signal-to-noise ratios of the first and second photoplethysmographic signals, selecting the photoplethysmographic signal with the highest signal-to-noise ratio and detecting its amplitude maxima or minima; using the timings of the detected maxima or minima as reference timings for the averaging of the plurality of photoplethysmographic waveforms.

13. A method according to claim 1 wherein the first and second devices are connected together for data communication.

14. A method according to claim 1 wherein the first device controls the second device and performs said relative timing measurement.

15. A method according to claim 1 wherein the second device is physically mounted to and electrically connected to the first device.

16. A method of calculating an estimate of blood pressure from a measurement of pulse transit time comprising the steps of: measuring a first pulse transit time in a subject's arm with the subject's arm in a first position; measuring a second pulse transit time in a subject's arm with the subject's arm in a second position, different from the first position, and measuring the difference between the first and second pulse transit times, and calculating from the difference in first and second pulse transit times an estimate of the subject's arterial blood pressure.

17. A method according to claim 16 wherein the first position is with the arm raised higher than in the second position.

18. A method according to claim 16 wherein the pulse transit times are measured between a first location on the hand of the subject, for example on the finger, and a second location elsewhere on the subject.

19. A method according to claim 18 wherein the second location is one of: on the wrist of the subject, on the head of the subject, on or adjacent to the ear of the subject.

20. Apparatus for measuring blood pressure comprising: a first device comprising a first optical sensor for detecting a first photoplethysmographic signal at a first location on a subject's body, the first device being a handheld device; a second device comprising a second optical sensor for detecting a second photoplethysmographic signal at a second location on the subject's body; a programmable data processor configured to control the first and second devices to execute the method of claim 1.

21. Apparatus according to claim 20 wherein the first device is a mobile computing and communications device, the first optical sensor is a camera incorporated into the first device, and the programmable data processor is a data processor of the first device.

22. Apparatus according to claim 21 wherein the mobile computing and communications device is one of a smartphone or a tablet computer.

23. Apparatus according to claim 20 wherein the second device is one of: an accessory camera as said second optical sensor, a wrist-worn device, a head-worn device, an audio headphone, spectacles, each comprising an optical sensor as said second optical sensor.

24. Apparatus according to claim 20 wherein the second device is connected for data communication with said first device.

25. Apparatus according to claim 20 wherein said second device is physically mounted to said first device.

Description

[0018] The invention will be further described by way of non-limitative example with reference to the accompanying drawings in which:

[0019] FIG. 1 illustrates a first embodiment of the invention;

[0020] FIG. 2 illustrates the embodiment of FIG. 1 in use;

[0021] FIG. 3 illustrates the embodiment of FIG. 1 in use;

[0022] FIGS. 4A and B schematically illustrate the embodiment of FIG. 1 in use;

[0023] FIG. 5 is a plot of signals obtained with the embodiment of FIG. 1;

[0024] FIG. 6 is a plot of processed signals from the embodiment of FIG. 1;

[0025] FIG. 7 is an expanded time scale plot of the signals of FIG. 6 (first half—Arm down);

[0026] FIG. 8 is an expanded time scale plot of the signals of FIG. 6 (second half—Arm up);

[0027] FIGS. 9A and B show signal averaged waveforms of the signals from FIGS. 7 and 8;

[0028] FIG. 10 is a flow diagram of an embodiment of the invention;

[0029] FIG. 11 is a calibration plot for a subject for the first embodiment of the invention;

[0030] FIG. 12 illustrates a second embodiment of the invention;

[0031] FIG. 13 schematically shows a third embodiment of the invention;

[0032] FIG. 14 schematically shows a fourth embodiment of the invention;

[0033] FIG. 15 schematically shows a fifth embodiment of the invention; and

[0034] FIG. 16 schematically shows a sixth embodiment of the invention.

[0035] FIG. 1 illustrates an embodiment of the invention in which the first device 1 is a mobile telephone (e.g. smartphone) with an attached accessory camera as the second device 2. In this embodiment the attached accessory camera is a thermal imaging camera including an IR sensor as the second optical sensor 8, though a conventional visible light camera may be used as an alternative 9. The mobile telephone 1 includes a built-in camera 3 as the first optical sensor, which includes a light source 4 (which can be used as the camera flash or for general illumination) and a light sensor 5 which can conventionally operate as a still camera or video camera as is conventional in modern smartphones. It preferably also includes, as is conventional, accelerometers allowing it to detect its own orientation and a touchscreen constituting both a display 6 and user-input device 7.

[0036] In a second embodiment of the invention illustrated in FIG. 12 the thermal camera 2 is not directly attached to the mobile telephone 1, but rather connected through a data communication cable 20 and held to the wrist in the manner of any wrist-worn device. The data cable can be replaced by a wireless communication link such as Bluetooth, wifi, etc.

[0037] As an alternative second device 2 to the attached accessory camera, alternative embodiments of the invention may use an optical sensor fitted to any convenient device which can be held in contact with the subject's skin, such a wristwatch, a conventional pulse oximeter, headphones or earphones, headset, spectacles etc. The second device 2 includes a light source and light detector which together form the second optical sensor 8, so that it can detect the reflectance PPG signal, and is in data communication with the first device 1 so that it can be controlled to perform the PPG measurement and can return the PPG signal to the first device 1 for processing.

[0038] FIGS. 2 and 3 are schematic illustrations of the embodiment of FIG. 1 in use in a blood pressure measurement. As can be seen the mobile telephone 1 is held by the user with the user's finger 10 in contact with the camera 3 and the attached accessory camera 2 facing the user's wrist 12. The second device 2 need not be in direct contact with the subject's skin, but is close-enough that it can detect the PPG signal in the blood vessels of the user's wrist.

[0039] In this embodiment the measurement protocol involves making a first measurement with the arm in a natural lowered position as illustrated schematically in FIG. 4A and a second measurement with the arm vertically raised as schematically illustrated in FIG. 4B. In this embodiment the process is under the control of an executable software application on the mobile telephone which conveniently instructs the user in the protocol, and may also show animation to the user directing them how to take the first and second measurement with the arm positioned downwardly and upwardly. The first device 1 may utilise in-built accelerometers or orientation detectors to check that the user takes the two measurements with the arm in the correct positions, and can thus automatically detect when the two measurements have been completed, enabling it to automatically start processing the measurements to calculate the blood pressure. Alternatively, the user can indicate completion of the protocol by input (clicking an appropriate completion button on a touchscreen of the device 1).

[0040] FIG. 5a illustrates a two minute segment of a PPG signal and its frequency content (FFT) measured in the index finger of a user pressed against the camera 3 as illustrated in FIG. 2. The FFT spectral power plot showing a clear heart rate signal at about 1.26 Hz corresponding approximately to a heart rate of 76 beats per minute. FIG. 5b shows a two minute segment of the PPG signal and its frequency content measured from the wrist of a user using a thermal imaging camera 2 attached to a mobile telephone 1 as illustrated in FIGS. 2 and 3. The spectral power plot of the signal shows that the signal is less clean than that obtained from the finger in contact with the mobile telephone 1, but nevertheless the heart rate signal is clearly present at 1.26 Hz, albeit with other frequency content related to noise and movement. In both of the time plots the left-hand half, approximately the first minute, was performed with the user's arm vertically down, and in this case the signal from the index finger is much stronger than the signal from the wrist. The right-hand half, the second minute, is the signal with the arm in the raised position, and for this period the signal from the wrist is stronger than before.

[0041] FIG. 10 is a flow diagram explaining the measurement protocol and the data processing. In step 100 both optical sensors 3 and 8 are used to record two PPG signals, with the arm of the subject raised and with the arm of the subject lowered. The aim of the method will be to detect the relative timings of the two PPG signals and to calculate from this the blood pressure. To improve the accuracy of the measurement, signal averaging is used to produce a representative averaged PPG waveform.

[0042] Therefore, in step 101 whichever of the two PPG signals is cleaner (i.e. has a higher signal-to-noise ratio) is taken and the maxima (beat peaks) or minima (beat onsets) are detected. FIGS. 6a and b illustrate the signals from FIGS. 5a and b with the beat peaks and onsets in the PPG signal originating from the finger marked with dots. These peaks or beat onsets will be used as reference timing points for the signal averaging.

[0043] In step 102 a window of the two signals in which the arm is pointed down is selected (the window may be from 4-15 seconds long and is selected such that the two signals (finger PPG and wrist PPG) are of good quality and there is not significant hand motion), and in step 103 the second (less clean) PPG signal window is segmented at the reference timing points, e.g. the beat onsets, from the first (cleaner) signal. Then in step 104 each of the segments of the second PPG signal are averaged together resulting in a single representative “arm-down” average waveform for that window. FIGS. 7a and b illustrate examples of magnified 15-second windows of the finger and wrist PPG signals from the “arm down” period of FIG. 6, with the beat onsets and beat peaks illustrated again with dots. The signal of FIG. 7b is segmented at the beat onset timings from FIG. 7a and the segments are averaged together to produce the representative “arm-down” average waveform.

[0044] Steps 102 to 104 are also performed on a period of the PPG signals in which the arm is pointed up, as shown in FIGS. 8a and 8b, and the waveforms in the second PPG signal are averaged by segmenting the second PPG signal of FIG. 8b at the timings of the beat onsets of the first PPG signal illustrated in FIG. 8a to produce a single representative “arm-up” average waveform for the second PPG signal.

[0045] FIGS. 9a and b shows the representative averaged waveforms for each of the two PPG signals in each of the two positions resulting from the signals shown in of FIGS. 7 and 8. It can be seen that there is a relative timing difference between them of about 0.2 seconds resulting from the different effect of gravity on the pulse transit time with the arm up and down.

[0046] In step 105 the relative timing of the arm-down and arm-up PPG signals is taken, for example the timing of their peaks with respect to the t=0 line (the y-axis of FIGS. 9a and 9b), as two pulse transit times PTT.sub.d and PTT.sub.u.

[0047] Then in step 106 the two pulse transit times are used together with a previously obtained relationship between the blood pressure and pulse transit time for this subject to calculate an estimate of the blood pressure of the subject. In step 107 the calculated estimate of blood pressure is displayed to the user on the screen of the mobile telephone 1.

[0048] The relationship between the two pulse transit times PTT.sub.d and PTT.sub.u and blood pressure p is of the form:

[00001]$p=A+B\left(\frac{PT{T}_u^2+PT{T}_d^2}{PT{T}_u^2-PT{T}_d^2}\right)$

[0049] Where A and B are constants for each individual. A and B are obtained by a calibration process comprising measuring the blood pressure of the subject using any of the conventional methods e.g. an inflatable cuff, while also measuring the pulse transit times PTT.sub.d and PTT.sub.u with the arm raised and arm lowered using the device and method of the embodiment above. This is performed over a range of different blood pressures (at least two, but preferably more) which can be induced by having the user perform the measurements at different times of day (blood pressure varies naturally through the day, typically peaking in the morning and evening and dropping in the early afternoon and night), or while having the subject assume different postures, e.g. standing, seated and lying. FIG. 11 illustrates a calibration plot of mean blood pressure as a function of mean pulse transit time (PTT.sub.d+PTT.sub.u)/2 for different arm up/arm down PTT differences.

[0050] Although the invention is not limited to any particular theoretical basis for the relationship between pulse wave velocity and blood pressure as it is based on a measured calibration process as discussed above, the basis for the relationship used above may be derived by starting with the published result for pulse wave velocity derived by S. J. Payne in the paper “Analysis of the effects of gravity and wall thickness in a model of blood flow through axisymmetric vessels”, Med. Biol. Eng. Comput., 2004, 42, 799-806:

[00002]$\begin{array}{cc}{c}^2=\frac{\beta G_0}{2\rho }{\left(\frac{A}{A_0}\right)}^{\beta /2}-\frac{h_0}{R_0}\left(\frac{A_0}{A}\right)\frac{p_{ext}}{\rho }& \left(1\right)\end{array}$

[0051] where the symbols have their standard meaning as defined in Payne (2004). Note that this is the simplified version of Equation 32 in Payne (2004), where it is assumed that the wave speed is much larger than the mean velocity of the flow.

[0052] The pressure-area relationship given by Payne (2004) is then used:

[00003]$\begin{array}{cc}p-p_0=P_{\mathrm{ext}}\left(1+\frac{h_0}{R_0}\frac{A_0}{A}\right)+G_0\left[{\left(\frac{A}{A_0}\right)}^{\beta /2}-1\right]& \left(2\right)\end{array}$

[0053] noting that baseline conditions are denoted by the subscript 0. This is the generalised form of Equation 15 in Payne (2004), where an offset is added in the equation above to account for the fact that we are interested in absolute pressure.

[0054] As a first approximation, taking the external pressure to be negligible; Equation 2 can be substituted into Equation 1 to give:

[00004]$\begin{array}{cc}{c}^2\cong \frac{\beta G_0}{2\rho }\left(p-p_0+G_0\right)& \left(3\right)\end{array}$

[0055] Using this result, the two separate PTT tests are considered, one with arm up (subscript u) and one with arm down (subscript d). The average blood pressure along the arm will be altered due to gravity:

[00005]$\begin{array}{cc}{p}_u=p-\frac{\rho g{z}_u}{2}& \left(4\right)\\ p_d=p+\frac{\rho g{z}_d}{2}& \left(5\right)\end{array}$

[0056] where the changes in height are given by z.sub.u and z.sub.d respectively. Hence three values of wave speed:

[00006]$\begin{array}{cc}{c}_0^2=\frac{\beta G_0}{2\rho }\left(p-p_0+G_0\right)& \left(6\right)\\ c_u^2=\frac{\beta G_0}{2\rho }\left(p-\frac{\rho {\mathrm{gz}}_u}{2}-p_0+G_0\right)& \left(7\right)\\ c_d^2=\frac{\beta G_0}{2\rho }\left(p+\frac{\rho {\mathrm{gz}}_d}{2}-p_0+G_0\right)& \left(8\right)\end{array}$

[0057] Re-arranging gives:

[00007]$\begin{array}{cc}p=p_{\mathrm{off}}+\frac{\rho g\left(z_d+z_u\right)}{2}\left(\frac{c_d^2+c_u^2}{c_d^2-c_u^2}\right)& \left(9\right)\end{array}$

[0058] i.e. based on half the difference in height between the two elevations (chosen as being most robust).

[0059] Note that the relationship in Equation 9 does still require one variable to be calibrated (termed here as offset pressure, p.sub.off) but does remove the need for two variables to be calibrated.

[0060] Equation 9 can also be re-written in more general terms:

[00008]$\begin{array}{cc}p=A+B\left(\frac{c_d^2+c_u^2}{c_d^2-c_u^2}\right)\mathrm{where}\text{:}& \left(10\right)\\ B=\frac{\rho g\left(z_d+z_u\right)}{2}& \left(11\right)\end{array}$

[0061] In terms of pulse transit times, since the lengths are invariant, Equations 9 and 10 become straightforwardly:

[00009]$\begin{array}{cc}p=p_{\mathrm{off}}+\frac{\rho g\left(z_d+z_u\right)}{2}\left(\frac{PT{T}_u^2+PT{T}_d^2}{PT{T}_u^2-PT{T}_d^2}\right)& \left(12\right)\\ p=A+B\left(\frac{PT{T}_u^2+PT{T}_d^2}{PT{T}_u^2-PT{T}_d^2}\right)& \left(13\right)\end{array}$

[0062] noting that the bracketed term is non-dimensional and thus any convenient units can be used for PTT.

[0063] FIGS. 13 to 16 schematically illustrate alternative embodiments of the invention. In FIG. 13 the second optical sensor is in a wrist-worn device 2 and thus is in contact with the skin of the subject. In FIG. 14 the second optical sensor is in an ear piece worn on the head of the user and is thus close to or in contact with skin of the ear. In FIG. 15 the second optical sensor is in spectacles worn on the head of the user and in FIG. 16 the second optical sensor is in a VR headset worn on the head of the user.