Apparatus for determining blood pressure
An apparatus for determining blood pressure has a control unit and a device for providing pulse wave data representative of a heartbeat of a human subject. The subject has a body height, an age, and a gender. The control unit is configured for receiving the pulse wave data, selecting a portion of the pulse wave data indicative of one or more heart periods, and, for each respective heart period of the one or more heart periods, determining a systolic component of the respective heart period, approximating the systolic component with a first Gaussian function and a second Gaussian function, and determining a time difference between the first and second Gaussian functions. The blood pressure value of the subject is determined based on the time difference, the body height, and the age.
1. An apparatus for determining blood pressure, comprising:
a memory to store program code;
a processor coupled to the memory and configured to execute the program code having instructions which, when executed by the processor, cause the processor to perform operations, the operations comprising:
receiving pulse wave data, wherein the pulse wave data defines a heartbeat of a human subject, and wherein the subject is associated with a body height value, an age value, and a gender type;
selecting a portion of the pulse wave data indicative of one or more heart periods;
determining a time difference (WWT) for at least one respective heart period of the one or more heart periods, wherein determining the WWT for the at least one respective heart period comprises:
determining a systolic component of the respective heart period, wherein determining the systolic component comprises:
determining a respective global maximum of the respective heart period;
determining a second order derivative of the respective heart period;
determining a maximum value of the second order derivative located at least at a predetermined time difference from the global maximum; and
defining the systolic component as a portion of the heart period between the start of the heart period and the maximum value;
approximating the systolic component with a first Gaussian function and a second Gaussian function, wherein the first and second Gaussian functions have respective first and second standard deviations (σ 1 , σ 2 ), the first and second standard deviations (σ 1 , σ 2 ) being equal to each other; and
determining the WWT as the time difference between the first and second Gaussian functions; and
determining a blood pressure value (BP) of the subject based on the WWT, the body height value, and the age value, wherein determining the BP comprises:
determining a preliminary stiffness index (SI p ) based on the body height value and the WWT;
determining an adjusted stiffness index (SI a ) based on the preliminary stiffness index (SI p ) and the age value, wherein the SI a is determined by adjusting the SI p according to an adjustment factor including the age value of the subject; and
determining the BP based on the adjusted stiffness index (SI a ) and a regression model, wherein the regression model comprises a regression function
f (SI a ,g )=BP sys ,
where SI a is the adjusted stiffness index (SI a ), g is the gender type of the subject, and BP sys is the blood pressure;
wherein determining the BP comprises determining the blood pressure value based on the regression function; and
wherein the regression function comprises a linear function of the type
f ( x )= ax+b,
where a ranges from 1 to 20 mmHg/(m/s) and b ranges from 0 to 80 mmHg; and
a display comprising a user interface, the display providing the determined BP of the subject for display at the user interface, the display commutatively coupled with the processor.
2. The apparatus according to claim 1 , wherein the portion of the pulse wave data is indicative of a plurality of successive heart periods, and wherein determining the time difference (WWT) further comprises:
determining the time difference (WWT) for the plurality of successive heart periods as an average value based on the respective time differences determined for the heart periods of the plurality of heart periods.
3. The apparatus according to claim 1 , wherein the first and second Gaussian functions have a respective maximum amplitude, the maximum amplitude of the first Gaussian function being greater than or equal to the maximum amplitude of the second Gaussian function.
4. The apparatus according to claim 1 , wherein approximating the systolic component comprises:
fitting the first and second Gaussian functions to the systolic component using
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with a, b, c, d, and f being determined using non-linear optimization or curve-fitting.
5. The apparatus according to claim 1 , wherein determining the adjusted stiffness index (SI a ) is based on an adjustment function
f (SI p )=SI a ,
where SI p is the preliminary stiffness index and SI a is the adjusted stiffness index (SI a ).
6. The apparatus according to claim 5 , wherein the adjustment function is a linear function of the type
f ( x )= cx+d,
where c and d are adjustment factors determined based on a plurality of value pairs comprising the age value and an associated stiffness index value.
7. The apparatus according to claim 1 , wherein determining the preliminary stiffness index (SI p ) is based on a function
SI
p
=
h
WWT
,
where h is the body height value, WWT is the time difference, and SI p is the preliminary stiffness index (SI p ).
8. The apparatus according to claim 1 , further comprising:
a light source configured to transmit light into an extremity of the subject; and
an optical sensor configured to receive light reflected from blood flow through the extremity.
9. The apparatus according to claim 1 , wherein the portion of the pulse wave data is indicative of 1 to 50 heart periods.
10. The apparatus according to claim 1 , further comprising an optical sensor and a light source, the optical sensor being configured to detect a signal emitted by the light source and reflected by part of the body of the subject.
11. The apparatus according to claim 1 , further comprising:
a light source configured to transmit light into an extremity of the subject; and
an optical sensor configured for receiving light reflected from blood flow through the extremity; and further comprising instructions, which when executed by the processor, cause the processor to perform operations including receiving the pulse wave data by activating the light source and receiving the pulse wave data based on a signal provided by the optical sensor.
12. The apparatus according to claim 1 , wherein a ranges from 5 to 15 mmHg/(m/s) and b ranges from 20 to 60 mmHg.
13. The apparatus according to claim 2 , wherein the average value is the median value of the determined respective time differences.
14. The apparatus according to claim 6 , wherein:
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=
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-
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range
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with μ=0.109*age+3.699 and range(age)=0.1663*age+4.3858−μ, age being the age of the subject, and
d= 0.
15. The apparatus according to claim 1 , wherein the predetermined time difference from the global maximum is 350 ms or less.
16. The apparatus according to claim 8 , wherein the instructions, which when executed by the processor, further cause the processor to perform operations including receiving the pulse wave data by activating the light source and receiving the pulse wave data based on a signal provided by the optical sensor.
17. The apparatus according to claim 8 , wherein the optical sensor comprises a video sensor, and wherein receiving the pulse wave data further comprises:
receiving a video stream indicative of the reflected light based on the signal; and
selecting a region of interest from the video stream containing a plurality of pixels.
18. The apparatus according to claim 17 , wherein the region of interest has a size of 50×50 pixels.
19. The apparatus according to claim 17 , further comprising instructions which when executed by the processor cause the processor to perform operations, the operations comprising:
selecting a plurality of frames from the video stream, each frame of the plurality of frames having a respective time stamp; and
for each respective frame:
determining, within the region of interest, a first sample value indicative of the sum of the values of a green subcomponent of each pixel of the plurality of pixels;
associating each first sample value with the respective time stamp;
generating a first pulse wave from the first sample value; and
determining a second pulse wave by re-sampling the first pulse wave based on the respective time stamps.
20. The apparatus according to claim 19 , wherein determining the second pulse wave further comprises filtering the second pulse wave using a bandpass filter.
21. The apparatus according to claim 20 , wherein the bandpass filter removes all frequencies not falling within a range from 0.6 Hz to 2.5 Hz.
22. The apparatus according to claim 9 , wherein the portion of the pulse wave data is indicative of a plurality of successive heart periods.
23. The apparatus according to claim 10 , wherein the part of the body of the subject comprises a pulsatile blood flow of the subject.