Method and apparatus for cuff-less blood pressure measurement in a mobile device
A system and method is presented for cuff-less blood pressure measurement in a mobile device. A key aspect of this disclosure is the discovery of a new location for blood pressure measurement at the fingertip of a subject and that reflectance-mode photoplethysmography can be used to help make this measurement. Through experiments in human subjects, it was discovered that it is indeed possible to measure systemic blood pressure by having a subject press the fingertip against a reflectance-mode photo-plethysmography-force sensor unit under visual guidance and then compute blood pressure from the resulting variable-amplitude blood volume oscillations and applied pressure via an oscillometric algorithm.
1. A system for measuring blood pressure, comprising:
a mobile device including a processor and a display unit; and
an encasing configured to encase the mobile device, wherein the encasing includes:
a sensing unit integrated into the encasing and configured to measure blood pressure at a fingertip of a user, wherein the sensing unit includes a reflectance-mode photo-plethysmography (PPG) sensor configured to measure blood volume oscillations and a force sensor configured to measure pressure applied by the fingertip; and
a visual guide disposed on the encasing and arranged in relation to the sensing unit, where the visual guide comprise indicia to guide the user in placing a base of fingernail and a center of the finger in relation to the sensing unit;
wherein the system includes a non-transitory computer-readable medium that stores instructions that, when executed by the processor, cause the processor to:
measure pressure applied to the sensing unit by a fingertip of a user,
measure blood volume oscillations in the fingertip,
guide the user via the display unit to vary pressure being applied to the sensing unit while the blood volume oscillations and pressure are measured,
generate an oscillogram from the measured pressure and the measured blood volume oscillations, where the oscillogram plots amplitude of blood volume oscillations as a function of the measured pressure,
calculate a blood pressure value from the oscillogram, and
present the blood pressure value on the display unit.
2. The system of claim 1 wherein the PPG sensor includes at least one light emitting diode and an array of photodetectors.
3. The system of claim 2 wherein the maximum amplitude photodetector signal is used as the blood volume oscillations.
4. The system of claim 1 wherein the force sensor is an array of force transducer elements.
5. The system of claim 4 wherein an area of force application is determined based on the force outputs of each element.
6. The system of claim 1 wherein the sensing unit further includes a temperature sensor.
7. The system of claim 1 further includes instructions that, when executed by the processor, cause the processor to guide the user to hold the device at a height aligned with the heart of the user.
8. The system of claim 7 further includes instructions that, when executed by the processor, cause the processor to capture an image of the user with a camera, compare the captured image to a reference image of the user, and instruct the user to hold the mobile device at a height aligned with the heart of the user based on the comparison of the captured image with the reference image, where the reference image was captured when the device was being held at a height aligned with the heart of the user.
9. The system of claim 1 further includes instructions that, when executed by the processor, cause the processor to capture an image of the user with a camera, compare the captured image to a reference image of the user, detect the height at which the device is being held relative to the heart based on the comparison of the captured image with the reference image, where the reference image was captured when the device was being held at a height aligned with the heart of the user, and compensate the calculated blood pressure value in accordance with the detected height.
10. The system of claim 1 wherein the processor generates a blood pressure value by
representing the oscillogram with a mathematical model, wherein the mathematical model is defined in terms of parameters with unknown values, the parameters indicating finger systolic pressure and finger diastolic pressure and specifying a nonlinear blood volume-transmural pressure relationship of a fingertip artery;
estimating the parameters of the mathematical model by fitting the mathematical model to the oscillogram; and
computing brachial systolic pressure and diastolic pressure using the parameter estimates.
11. The system of claim 10 wherein computing brachial systolic pressure and diastolic pressure further comprises use of a transfer function.
12. A system for measuring blood pressure, comprising:
a mobile device including a processor and a display unit; and
an encasing configured to encase the mobile device, wherein the encasing includes:
a sensing unit integrated into the encasing and configured to measure blood pressure at a fingertip of a user, wherein the sensing unit includes a reflectance-mode photo-plethysmography (PPG) sensor configured to measure blood volume oscillations and a force sensor configured to measure pressure applied by the fingertip; and
a visual guide disposed on the encasing and arranged in relation to the sensing unit, where the visual guide comprise indicia to guide the user in placing a surface of the fingertip on an opposite side of the finger from a fingernail on the finger and distal from a distal interphalangeal joint of the finger onto the sensing unit;
wherein the system includes a non-transitory computer-readable medium that stores instructions that, when executed by the processor, cause the processor to:
measure pressure applied to the sensing unit by a fingertip of a the user,
measure blood volume oscillations in the fingertip,
guide the user via the display unit to vary pressure being applied to the sensing unit while the blood volume oscillations and pressure are measured,
generate an oscillogram from the measured pressure and the measured blood volume oscillations, where the oscillogram plots amplitude of blood volume oscillations as a function of the measured pressure,
calculate a blood pressure value at level of brachial artery from the oscillogram, and
present the blood pressure value on the display unit.
13. The system of claim 10 further comprises using a transfer function to calculate the blood pressure value at the level of brachial artery.