WO2019053276A1

WO2019053276A1 - Device for determining diastolic blood pressure of a subject

Info

Publication number: WO2019053276A1
Application number: PCT/EP2018/075107
Authority: WO
Inventors: Jozef Hubertus GELISSEN; Francesco SARTOR; Koen Theo Johan De Groot; Mustafa Ghassan RADHA
Original assignee: Koninklijke Philips N.V.
Priority date: 2017-09-18
Filing date: 2018-09-17
Publication date: 2019-03-21

Abstract

The invention relates to a device (1) for determining diastolic blood pressure in an unobtrusive and painless way such that it can be determined, for instance, during sleep. A PPG signal of the skin of a subject is provided by, for instance, a PPG sensor (4), which has been measured at a measurement location, while a force applied to the skin at a force application location has been increased, wherein the measurement location and the force application location are the same or the force application location is proximal relative to the measurement location. A force value of the increasing force is determined by a force value determination unit (6), at which an amplitude of an oscillating component of the PPG signal starts to diminish in reaction to the increasing force, wherein this force value is used by a blood pressure determination unit (8) for determining the diastolic arterial blood pressure.

Description

DEVICE FOR DETERMINING DIASTOLIC BLOOD PRESSURE OF A SUBJECT

FIELD OF THE INVENTION

The invention relates to a device, method and computer program for determining diastolic blood pressure of a subject. BACKGROUND OF THE INVENTION

WO 2017/152098 Al discloses a device for detecting blood pressure. The device comprises a display unit and a sensing unit coupled to the outside of a housing of the device, wherein the sensing unit includes a photoplethysmography (PPG) sensor configured to measure blood volume oscillations and a pressure sensor configured to measure an applied pressure value. The device is adapted to display on the display unit a target pressure graph to guide a user to steadily increase an applied pressure with a finger, wherein the applied pressure is measured over a predetermined period of time using the pressure sensor. Blood volume oscillations are measured by using the PPG sensor and an oscillogram is generated based on the blood volume oscillations and the applied pressure, wherein a blood pressure value is calculated based on the oscillogram.

US 2017/251935 Al discloses a biometric monitoring device for estimating a user's blood pressure. The device comprises a PPG sensor comprising a light emitter and a light detector configured to generate PPG sensor data representing blood volume pulses of variable amplitude. The device further comprises a force or pressure sensor located and oriented with respect to the PPG sensor to generate variable force or pressure data when a user presses the PPG sensor against a location of the user's body where the PPG sensor generates the PPG sensor data. A processor is configured to obtain the PPG sensor data from the PPG sensor, to obtain the variable force or pressure data from the force or pressure sensor, and to determine, from the PPG sensor data and the variable force or pressure data, a variation of PPG blood volume pulse amplitude with a variation in pressure applied to the location where the PPG sensor is pressed against the user's body. The processor is further configured to determine an estimate of the user's blood pressure from the variation of the PPG blood volume pulse amplitude with variation in applied pressure. US 2005/0228296 Al discloses a device for monitoring a patient's blood pressure. The device comprises an optical module comprising an optical source component and an optical sensor that generates a first set of information and a flexible, thin-film pressure sensor that generates a second set of information. The device further comprises a processing module that is configured to receive the first and second sets of information and that comprises a processor that processes this information to calculate a blood pressure value.

US 2003/0036685 Al discloses a manually operated user input device for sensing a physiological characteristic of a user and for providing input of data unrelated to the physiological characteristic, wherein the device comprises a housing having a surface in intermittent contact with a portion of the user's finger and a PPG sensor disposed on the surface for sensing the physiological characteristic of the user. The input device further comprises manually operated means for inputting data to the user input device, wherein the data are unrelated to the physiological characteristic.

The article "Blood pressure measurement on the cheek" by S. Schneider et al., Current Directions in Biomedical Engineering, volume 2, pages 237 to 240 (2016) discloses a device for measuring blood pressure of a patient based on blood flow in the cheek assessed by using a combination of an infrared light source and a detector while pressure is applied to the cheek.

Blood pressure cuffs are often used for determining the diastolic blood pressure of a subject. However, measuring the diastolic blood pressure by using a blood pressure cuff, which is firmly wrapped around an upper arm of the subject, is perceived as being painful. Moreover, it is nearly impossible to obtain indicative 24 hour blood pressure results, without disturbing the subject's sleep, because the blood pressure measurements require cuff inflations.

SUMMARY OF THE INVENTION

It is regarded as being an object of the present invention to provide a device, method and computer program which allow for an improved determination of diastolic blood pressure of a subject.

In a first aspect of the present invention a device for determining diastolic blood pressure of a subject is presented, wherein the device comprises:

a PPG signal providing unit for providing a PPG signal of the skin of the subject, which has been measured at a measurement location, while a force applied to the skin at a force application location has been increased, wherein the measurement location and the force application location are the same or the force application location is proximal relative to the measurement location,

an oscillating component determination unit for determining an oscillating component of the PPG signal by high-pass filtering the PPG signal,

- a force value determination unit for determining a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force,

an assignments providing unit for providing assignments between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to the increasing force, and diastolic arterial blood pressures, and

a blood pressure determination unit for determining the diastolic arterial blood pressure based on the determined force value and the provided assignments.

The measurement of a PPG signal is unobtrusive and not painful. Already this leads to an improved determination of the diastolic blood pressure. Moreover, only very little force needs to be applied for determining the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force such that, if the force is automatically applied, the force can be applied without disturbing a night rest of the subject. This further improves the determination of the diastolic blood pressure.

The subject is a human being or an animal. Moreover, the PPG signal providing unit can be or comprise a storing unit in which the PPG signal is stored and from which the PPG signal can be retrieved for providing the same. The PPG signal providing unit can also be or comprise a receiving unit for receiving the PPG signal from a PPG sensor measuring the PPG signal and for providing the received PPG signal. The PPG signal providing unit can also be or comprise the PPG sensor itself. The PPG signal providing unit can also be adapted to provide the corresponding force values defining at which time which force has been applied at the force application location while the PPG signal was measured. For instance, the PPG signal providing unit can comprise a storing unit in which the force values are stored and from which the force values can be retrieved for providing the same. The PPG signal providing unit can also comprise a receiving unit for receiving the force values from, for instance, a force sensor or a force providing unit like a force applicator, and for providing the force values. The PPG signal providing unit can also be itself adapted to measure the force values. The assignments providing unit can be a storing unit in which the assignments are stored and from which the assignments can be retrieved for providing the same.

Moreover, the assignments providing unit can be a receiving unit for receiving the assignments from another device in which the assignments may have been stored and for providing the received assignments. The force values of these assignments correspond to force values, which have been or would be measured at a force application location being the same or proximal to a measurement location at which the PPG signal has been measured or would be measured. Preferentially, the assignments have been determined in a calibration or training procedure, wherein for one or several subjects force values of the increasing force, at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force, have been determined, while the corresponding diastolic arterial blood pressures were known from, for instance, standard blood pressure cuff measurements, and wherein the determined force values and the corresponding known diastolic arterial blood pressures are used for determining the assignments. A fitting procedure might be used for determining further assignments. The assignments can also be determined by using a simulation. The assignments may be provided, for instance, as a function, a table, et cetera.

In an embodiment the PPG signal providing unit is adapted to provide as the PPG signal of the skin of the subject a PPG signal, which has been measured at the measurement location a) also before the increasing force is applied to the skin and/or b) while the increasing force corresponds to an arterial blood pressure being below the diastolic arterial blood pressure and/or c) also while a constant force, which corresponds to an arterial blood pressure below the diastolic arterial blood pressure, is applied to the skin at the force application location, wherein the amplitude of the oscillating component of the PPG signal varies over time due to breathing, wherein the device further comprises a breathing indicator providing unit for providing as a breathing indicator being indicative of the breathing behavior of the subject a breathing threshold being indicative of a minimum or maximum of the amplitude a) also before the increasing force is applied to the skin and/or b) while the increasing force corresponds to an arterial blood pressure being below the diastolic arterial blood pressure and/or c) also while a constant force, which corresponds to an arterial blood pressure below the diastolic arterial blood pressure, is applied to the skin at the force application location, wherein the force value determination unit is adapted to determine as the force value a force value of the increasing force at which the minimum amplitude or maximum amplitude, respectively, of the oscillating component of the PPG signal starts to diminish below the breathing threshold in reaction to the increasing force. It should be noted that the expression "A and/or B " includes a) A without B, b) B without A, and c) A and B.

Thus, for determining the breathing indicator, i.e., for instance, the minimum of the amplitude varying due to breathing, a part of the PPG signal is used which is not influenced by the increasing force. This part of the PPG signal varies with the respiration cycle such that, if the PPG signal covers several respiration cycles, for example, several local minimum amplitudes can be identified, wherein these minimum amplitudes can be combined, for instance, averaged, for determining the breathing threshold being indicative of the minimum of the amplitude. For ensuring that the part of the PPG signal, which is used for determining the breathing indicator, is really not influenced by the increasing force, this part corresponds to no applied force, or to a constant force or an increasing force being well below a force value to which a diastolic arterial blood pressure of a subject can correspond. For instance, this force is smaller than to a force value corresponding to 40 mniHg. This allows assessing the influence of respiration on the amplitude of the PPG signal and using the result of this assessment, i.e., for instance, the determined minimum amplitude, for determining the force value of the increasing force, which indicates the diastolic arterial blood pressure, without necessarily requiring eliminating or at least reducing respiratory influences on the PPG signal itself.

The device preferentially further comprises a force providing unit for providing the increasing force to the skin while measuring the PPG signal of the skin. The first providing unit can be a force applicator for automatically applying the increasing force to the skin while measuring the PPG signal of the skin. However, in another embodiment the pressure may be applied manually by a user, for instance, by pressing on the device, particularly on a PPG sensor, arranged on the skin. The device preferentially further comprises a force sensor for determining the increasing force applied to the skin. The force providing unit can also comprise an output unit for indicating to the user that manually more pressure or less pressure should be applied, wherein the force providing unit can be adapted to indicate whether more pressure or less pressure should be manually applied depending on the currently applied force as determined by the force sensor and a given pressure which should be currently applied, for instance, in order to generate a desired pressure profile or force profile, wherein it should be noted that pressure and force profiles correspond to each other in the present context.

In an embodiment the device further comprises a heart rate providing unit for providing a heart rate value being indicative of the heart rate of the subject, wherein the force providing unit is adapted to provide the increasing force depending on the provided heart rate value. The heart rate providing unit can be adapted to, for instance, determine the heart rate from the PPG signal. The heart rate providing unit can also be a receiving unit for receiving the heart rate from a heart rate sensor and for providing the received heart rate. The heart rate providing unit can also be the heart rate sensor itself. The force providing unit can be adapted to provide the increasing force with a slope which depends on the provided heart rate value. For instance, the force providing unit can be adapted to provide the increasing force with a slope depending on the provided heart rate value such that a predetermined resolution of determining the diastolic arterial blood pressure or a better resolution is achieved. The resolution defines how accurately the diastolic arterial blood pressure can be determined. If it is assumed that, since the diastolic arterial blood pressure is determined based on the determination of the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force, the resolution is inherently limited by the heart rate, the resolution can be defined by the quotient of the slope and the heart rate, i.e. if the slope is increased, the resolution increases, and, if the heartrate increases, the resolution decreases, wherein here a decreased resolution is a better resolution, because it defines the distance between different diastolic arterial blood pressure values which can be distinguished from each other, i.e. a smaller resolution indicates a smaller corresponding distance and hence a better resolution. For instance, if the heart rate is 60 beats per minute, i.e. one beat per second, and the slope of the increasing force corresponds to 10 mmHg/s, the resolution can be determined as being 10 mniHg, i.e. two different diastolic arterial blood pressures having a distance from each other being 10 mniHg or larger can be distinguished from each other. If in this example the slope is 5 mmHg/s, the resolution is 5 mniHg. Preferentially, the force providing unit is adapted to provide the increasing force depending on the provided heart rate value such that the resolution is 5 mniHg or smaller, further preferred 4 mmHg or smaller and even further preferred 3 mniHg or smaller for the determination of the diastolic arterial blood pressure. In an embodiment the force providing unit is adapted to average provided heart rate values over a period of time, which might be predetermined, in order to determine an average heart rate value, wherein the force providing unit is further adapted to provide the increasing force depending on the averaged heart rate value, especially such that a desired resolution of determining the diastolic arterial blood pressure or a better resolution is achieved.

In an embodiment the force providing unit is adapted to provide the increasing force with a non-constant slope over time. In particular, the force providing unit is adapted to provide the increasing force with a first slope and then with a second slope, wherein the first slope is larger than the second slope. Preferentially, the first slope and the second slope are each constant. Moreover, preferentially the larger first slope is used until a force value has been reached which is surely below a force value corresponding to a possible diastolic arterial blood pressure of a subject. For instance, the first slope is applied up to a force value corresponding to a blood pressure of 40 mmHg or corresponding to a smaller blood pressure like 20 mmHg. The smaller second slope is preferentially applied in a region of force values in which a detection of the diastolic arterial blood pressure of a subject can be expected. By using the relatively large first slope the overall measurement time can be reduced, wherein the relatively small second slope ensures that the diastolic arterial blood pressure can be determined with a sufficient accuracy. In an embodiment the larger first slope and the smaller second slope are directly adjacent to each other. The second slope can depend on the heart rate value, wherein the second slope can be provided such that a desired resolution of determining the diastolic arterial blood pressure can be achieved or such that an even better resolution can be achieved. For instance, the second slope can be chosen depending on the heart rate value such that the resolution is 5 mmHg or smaller. In an embodiment, the second slope is chosen depending on the heart rate value such that the resolution is 2 mmHg or smaller.

In an embodiment the force providing unit is adapted to provide the increasing force with a third slope, after it has been provided with the second slope, wherein the third slope is larger than the second slope. The third slope is preferentially constant. The larger third slope can be used for also detecting the systolic arterial blood pressure, wherein this detection of the systolic arterial blood pressure can also be carried out with a larger slope such that the overall blood pressure determination time for determining a diastolic arterial blood pressure and a systolic arterial blood pressure can be reduced, without diminishing the accuracy of determining these blood pressures.

The force providing unit can be adapted to provide the increasing force with the third slope, after the diastolic arterial blood pressure has been determined. The third slope can be directly adjacent to the second slope. Also the third slope can be provided depending on the heart rate value. In particular, the third slope can be provided depending on the heart rate value such that a desired resolution of determining the systolic arterial blood pressure is used. For instance, the third slope can be provided depending on the heart rate value such that the resolution is equal to or smaller than a predetermined resolution of, for instance, 5 mmHg. In an embodiment the force providing unit is adapted to provide a constant force after having used the first slope and before using the second slope. This constant force can hence be applied, before the diastolic arterial blood pressure is determined, wherein the PPG signal, which is measured while the constant force is applied, can be used for determining the breathing threshold as described above.

In an embodiment the force value determination unit is adapted to only consider changes of the amplitude of the oscillating component, which are larger than a predefined threshold, for determining the force value at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. The changes are preferentially changes with respect to consecutive amplitudes. The changes can also be changes of the minimum amplitude or the maximum amplitude. Thus, changes of the minimum amplitude or of the maximum amplitude of the oscillating component might be considered only, if these changes are larger than a predefined threshold. If changes are not considered, which are not larger than the predefined threshold, it can be prevented that fluctuations in the amplitude of the oscillating component of the PPG signal, which are not related to the application of the increasing force, lead to a wrong determination of the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. This can lead to a further improved determination of the diastolic blood pressure.

In an embodiment the force value determination unit is adapted to determine the force value such that it is the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to monotonically diminish in reaction to the increasing force. Moreover, in an embodiment the oscillating component determination unit is adapted to high-pass filter the PPG signal such that the oscillating component is the pulsatile component of the PPG signal. The PPG signal comprises a pulsatile component, which might also be regarded as being a pulsatile physiological waveform, attributed to cardiac synchronous changes in the blood volume with each heart beat, and which is superimposed on a slowly varying baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. For more details regarding the different components of the PPG signal reference is made to the article " Photoplethysmography and its application in clinical physiological measurement" by John Allen, Physiological Measurement, IOP Publishing, 28, R1-R39 (2007), which is herewith incorporated by reference. In an embodiment the oscillating component determination unit is adapted to determine the oscillating component such that it is or corresponds to the pulsatile component attributed to the cardiac synchronous changes in the blood volume. In order to achieve this, the oscillating component determination unit is preferentially adapted to allow components of the PPG signal, which have a frequency being equal to or larger than 0.3, further preferred equal to or larger than 0.4 and even further preferred equal to larger than 0.5 to pass the high-pass filtering. However, in another embodiment all smaller frequencies might pass the high-pass filtering such that at least a non-oscillating component, i.e. a DC component, is removed from the signal which is used for further processing.

The PPG signal can also be high-pass filtered in another way. In an

embodiment the oscillating component determination unit is adapted to determine the oscillating component of the PPG signal by high-pass filtering the PPG signal such that respiration induced low frequency baseline oscillations are removed, wherein the respiratory induced amplitude and frequency changes are preserved. This can be achieved by applying a high-pass filtering technique that removes for each individual cardiac cycle, i.e. for each individual beat cycle, estimated baseline changes in the PPG signal, i.e. in the PPG waveform, due to respiration. Thus, the oscillating component determination unit can be adapted to determine for each cardiac cycle a baseline and to remove the baseline for each individual cardiac cycle. In particular, the PPG signal, i.e. the PPG waveform, can be segmented into cardiac cycles which could also be regarded as being interbeat intervals (IBIs), wherein for this segmentation a known beat detector might be used. The start of a cardiac cycle might be defined by a local minimum of the PPG signal. Next, for each segment of the PPG signal representing a full cardiac cycle, a straight line can be fitted through the points marking the start and the end of the cardiac cycle, wherein the fitted line can be regarded as representing a new baseline for this segment and can be subtracted from the original segment. Thus, after subtraction the baseline corrected segment has a value of zero at the start point and the endpoint of the cardiac cycle. This can be carried out for all segments in this embodiment, in order to obtain the high-pass filtered PPG signal and thereby determine the oscillating component.

In an embodiment the PPG signal providing unit is adapted to provide a further PPG signal of the skin of the subject, which has been measured at a further measurement location, while the force applied to the skin at the force application location is increased, wherein the further measurement location is proximal relative to the force application location, wherein the oscillating component determination unit is adapted to high- pass filter the further PPG signal for determining an oscillating component of the further PPG signal, wherein the force value determination unit is adapted to determine the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force by determining when a relation between the amplitude of the oscillating component of the PPG signal and the amplitude of the oscillating component of the further PPG signal starts to change. Thus, the PPG signal providing unit can comprise, besides a first PPG sensor for measuring the PPG signal, a further PPG sensor for measuring a further PPG signal of the skin of the subject at a further measurement location, while the force applied to the skin at the force application location is increased, wherein the further measurement location is proximal relative to the force application location. The increase of the force preferentially starts with a zero force. Thus, when the force increase starts, both PPG signals are substantially not influenced by the force application and changes in the amplitude of the oscillating components of both PPG signals, if present at all, are generally only caused by breathing. However, it is also possible that the changes in the amplitude are caused by other effects like a body's temperature regulation, movements, especially posture changes, et cetera. These possibly present changes should be similar for both signals, i.e. the relation between the amplitudes of the oscillating components of the two PPG signals should be substantially constant. If the force is further increased, at a certain force value the amplitude of the oscillating component of the PPG signal, which has been measured at the measurement location, which coincides with the force application location or which is distal to the force application location, starts to diminish. However, the amplitude of the oscillating component of the other PPG signal is not influenced or less influenced by the application of the force, because the other PPG signal has been measured at the further measurement location being proximal relative to the force application location. This leads to a change in a relation between the amplitudes of the oscillating components of the two PPG signals which can be determined, in order to determine the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal, which has been measured at the force application location or distal to the force application location, starts to diminish in reaction to the increasing force. This allows for an accurate determination of the force value and hence for an accurate determination of the diastolic arterial blood pressure, even if the PPG signal is strongly influenced by breathing.

In an embodiment the force value determination unit is adapted to correct the amplitude of the oscillating component of the PPG signal for breathing influences and to determine the force value at which the corrected amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. In particular, the device further comprises a breathing indicator providing unit for providing a breathing indicator being indicative of the breathing behavior of the subject, wherein the force value

determination unit is adapted to determine the breathing influences based on the provided breathing indicator and to use these determined breathing influences for correcting the amplitude of the oscillating component of the PPG signal which has been measured while the force applied to the skin was increased. For instance, the PPG signal providing unit can be adapted to also provide a PPG signal which has been measured before the increasing force is applied to the skin, wherein the breathing indictor providing unit can be adapted to determine the breathing indicator based on the provided PPG signal. It is also possible that the breathing indicator providing unit comprises a breathing sensor for measuring a breathing indicator. A breathing indicator is, for instance, a variable over time which changes with the breathing, i.e., for instance, an amplitude changing in accordance with the breathing over time. The breathing indicator providing unit can comprise any sensor being able to provide a signal from which information about breathing can be determined. For instance, the breathing indicator providing unit can comprise a heart rate sensor, wherein the breathing indicator providing unit can be adapted to determine breathing from the inter-beat-interval provided by the heart rate sensor. The breathing indicator can also comprise an accelerometer, wherein the breathing indicator providing unit can be adapted to determine the breathing indicator based on an accelerometer signal provided by the accelerometer. The force value

determination unit can be adapted to model the breathing influences based on the provided breathing indicator and to use the modeled breathing influences for correcting the amplitude of the oscillating component of the PPG signal which has been measured while the force applied to the skin was increased. By considering the breathing for determining the diastolic arterial blood pressure the determination of the diastolic arterial blood pressure can be further improved.

The PPG signal providing unit preferentially comprises a PPG sensor being adapted to use green light for measuring the PPG signal. It has been found that, if green light is used for measuring the PPG signal, the determination of the diastolic arterial blood pressure can be even further improved. Moreover, the PPG signal providing unit

preferentially comprises a reflective PPG sensor or a transmissive PPG sensor for measuring the PPG signal.

In another aspect of the present invention a method for determining the diastolic arterial blood pressure of a subject is presented, wherein the method comprises:

providing a PPG signal of the skin of the subject, which has been measured at a measurement location by using a PPG signal providing unit, while a force applied to the skin at a force application location is increased, wherein the measurement location and the force application location are the same or the force application location is proximal relative to the measurement location,

determining an oscillating component of the PPG signal by high-pass filtering the PPG signal by using an oscillating component determination unit,

determining a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force by using a force value determination unit,

providing assignments between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to the increasing force, and diastolic arterial blood pressures by using an assignments providing unit, and

determining the diastolic arterial blood pressure based on the determined force value and the provided assignments by using a blood pressure determination unit.

In a further aspect of the present invention a computer program for determining the diastolic arterial blood pressure of a subject is presented, wherein the computer program comprises program code means for causing a device for determining the diastolic arterial blood pressure of the subject as defined in claim 1 to carry out the steps of the method for determining the diastolic arterial blood pressure of the subject as defined in claim 14, when the computer program is run on a computer controlling the device.

It shall be understood that the device of claim 1, the method of claim 14 and the computer program of claim 15 have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the present invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

Fig. 1 shows schematically and exemplarily an embodiment of a device for determining diastolic blood pressure of a subject,

Fig. 2 illustrates schematically and exemplarily a preferred position of the device while measuring a PPG signal, Fig. 3 illustrates schematically and exemplarily a start of a diminishing of an amplitude of an oscillating component of a PPG signal in reaction to an increasing force,

Fig. 4 illustrates exemplarily assignments between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to an increasing force, and diastolic arterial blood pressures,

Fig. 5 shows a flowchart exemplarily illustrating an embodiment of a method for determining the diastolic arterial blood pressure of a subject,

Fig. 6 shows exemplarily assignments between force values, at which an amplitude of an oscillating component of a PPG signal becomes zero in reaction to an increasing force, and systolic arterial blood pressures,

Fig. 7 illustrates schematically and exemplarily a further embodiment of a device for determining the diastolic arterial blood pressure of a subject,

Fig. 8 exemplarily illustrates how a user could apply pressure to a device for determining the diastolic arterial blood pressure of a subject,

Fig. 9 schematically and exemplarily illustrates a further embodiment of the device for determining the diastolic arterial blood pressure of a subject, which comprises two PPG sensors,

Fig. 10 exemplarily illustrates a first slope and a second slope of the force applied to the skin of the subject,

Fig. 1 1 exemplarily illustrates a first slope, a second slope and a third slope of the force applied to the skin of the subject,

Fig. 12 exemplarily illustrates also the application of a constant force to the skin of the subject, and

Fig. 13 illustrates a determination of a breathing threshold based on an average of minimum amplitudes.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically and exemplarily an embodiment of a device for determining diastolic blood pressure of a user being a person. The device 1 comprises a PPG sensor 4 for measuring a PPG signal of the skin of the user at a measurement location, while a force applied to the skin at a force application location is increased, wherein in this embodiment the measurement location and the force application location are the same.

In this embodiment the PPG sensor 4 is adapted to use green light for generating the PPG signal. The PPG sensor 4 can be regarded as being a blood pulse volumetric sensor, wherein the underlying concept is pulse oximetry which tracks the average skin tone which varies with the blood volume and blood oxygenation. The PPG sensor 4 comprises a light emitting diode (LED) 20 which shines light on the skin. The PPG sensor 4 further comprises a photodiode 21 for measuring the amount of light reflected by the skin, wherein during each cardiac cycle the amount of reflected light varies as more or less blood flows through the arterioles. The variation in skin tone over time contains information on the blood volume and the photodiode 21 converts the reflected light into an electrical signal, i.e. into the PPG signal.

The device 1 is a wrist worn device including a flexible strap or wrist band 2 having two ends, wherein securement means are present for circumferentially fasten the strap around the wrist. In this embodiment the PPG sensor 4 is integrated in the wrist band 2. The device 1 further comprises a force applicator for applying the increasing force to the skin while measuring the PPG signal of the skin. In this embodiment the force applicator, which might also be regarded as being a compression means, comprises a distensible airbag 11 in a cavity 12 and an air pump and valve combination 10 for inflating and deflating the airbag 1 1. The force applicator can be regarded as being a force providing unit.

The PPG sensor 4 is arranged within the device 1 such that it is in direct contact with the skin of the wrist of the user, when the device 1 is worn by the user.

Moreover, the force applicator is arranged within the device 1 such that it applies the force onto the PPG sensor 4 and thereby on the measurement location on the skin where the PPG signal is measured. The force applicator hence pushes the underlying PPG sensor onto the skin by exerting a force on the underlying PPG sensor 4. In other embodiments other means for applying the force can be used. For instance, a force applicator can be used, which is adapted to shrink the inner circumference of the wrist band 2, in order to apply an increasing force to the skin of the user's wrist.

The device 1 further comprises a force sensor 13 for measuring the increasing force applied to the skin. The force sensor 13 is arranged in between the force applicator 10, 1 1 and the PPG sensor 4. The force sensor 13 can include, for instance, a force transducer, a strain gauge or a piezoelectric sensor. However, the force sensor 13 can also comprise other means for measuring the increasing force applied to the skin.

The device 1 further comprises an oscillating component determination unit 19 for determining an oscillating component of the PPG signal by high-pass filtering the PPG signal. In this embodiment the oscillating component determination unit 19 is adapted to use a cut-off frequency of 0.5 Hz for the high-pass filtering. In particular, the oscillating component determination unit 19 can be adapted to high-pass filter the PPG signal such that the oscillating component is the pulsatile component of the PPG signal.

The device 1 further comprises a force value determination unit 6 for determining a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. For instance, the force value determination unit 6 can be adapted to only consider changes of the amplitude of the oscillating component, which are larger than a predefined threshold, for determining the force value at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. The changes are preferentially changes with respect to consecutive amplitudes of the oscillating component. If changes are not considered, which are not larger than the predefined threshold, it can be prevented that fluctuations in the amplitude of the oscillating component of the PPG signal, which are not related to the application of the increasing force, lead to a wrong determination of the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. The predefined threshold can be determined by, for instance, calibration measurements during which the increasing force is not applied and changes of the amplitude of the oscillating component are determined, wherein these changes are than not caused by the increasing force and can define the threshold.

Moreover, the force value determination unit 19 can be adapted to determine the force value such that it is the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to monotonically diminish in reaction to the increasing force.

Furthermore, before determining the force value, the amplitude of the oscillating component of the PPG signal can be corrected for breathing influences. For this reason the device 1 comprises a breathing indicator providing unit 15 for providing a breathing indicator being indicative of the breathing behavior of the subject, wherein the force value determination unit 6 is adapted to determine the breathing influences based on the provided breathing indicator and to correct the oscillating component of the PPG signal for these determined breathing influences. For instance, the force value determination unit 6 can be adapted to model the breathing influences based on the provided breathing indicator and to use the modeled breathing influences for correcting the amplitude of the oscillating component of the PPG signal which has been measured while the force applied to the skin was increased. The breathing indicator is preferentially a value or a signal which changes over time in accordance with the user's breathing. The breathing indicator providing unit 15 can be, for instance, an accelerometer which provides an accelerometer signal in accordance with the breathing. The breathing indicator is preferentially measured at or preferentially corresponds to a time before the increasing force is applied to the skin, in order to obtain a breathing indicator which is less likely corrupted by the increasing applied force. The breathing indicator providing unit 15 can also comprise another sensor like a sensor for measuring the heart rate and/or the inter-beat-interval, wherein the breathing indicator providing unit 15 can be adapted to derive the breathing indicator from this kind of sensor. For deriving the breathing indicator from a measurement of the heart rate and/or the inter- beat-interval known techniques can be used like the techniques disclosed in the article

"Developing an algorithm for pulse oximetry derived respiratory rate (RRoxi) : a healthy volunteer study", Paul S. Addison et al., volume 26, pages 45 to 51, Journal of Clinical Monitoring and Computing (2012), which is herewith incorporated by reference.

The breathing indicator providing unit 15 can also be a receiving unit for receiving the breathing indicator from another means measuring breathing information like a breathing belt. In fact, the breathing indicator providing unit 15 can comprise or receive breathing information from any means which allows measuring the breathing behavior of the subject, especially the breathing rate.

Besides breathing, the oscillating component of the PPG signal can be influenced by other factors like posture (orthostatic pressure), arm position, obstruction of the arterial path if, for instance, the user lies on his/her arm, and vasomotion. In order to control the posture, i.e. the orthostatic pressure, the arm position and a possible obstruction of the arterial path, the measurement of the PPG signal and also of the breathing indicator is preferentially standardized. Preferentially, the measurement of the PPG signal and also of the breathing indicator is carried out, while the user 30 is sitting or standing and the wrist is placed at heart height as schematically and exemplarily illustrated in Fig. 2.

The breathing indicator providing unit 15 can also comprise a further PPG sensor for measuring a further PPG signal, wherein this further PPG signal can be used for determining the breathing behavior, i.e. for determining the breathing indicator being indicative of the breathing behavior. This further PPG signal used for determining the breathing behavior can be measured before applying the force to the skin or, if the force application does not influence the further PPG signal, while the force is applied to the skin. It is also possible that the PPG sensor 4 measures a PPG signal before the increasing force is applied to the skin, wherein the force value determination unit 6 can be adapted to determine the breathing influences based on this PPG signal and to use these determined breathing influences for correcting the amplitude of the oscillating component of the PPG signal which is then measured while the force applied to the skin is increased. In this case any further sensor, i.e. a breathing sensor, for measuring a signal from which the breathing behavior can be derived is preferentially not present. The breathing indicator providing unit 15 can in this case derive and provide the breathing indicator being indicative of the breathing behavior based on the PPG signal measured by the PPG sensor 4 before applying the increasing force. In particular, since the breathing rate is normally about 12 breaths per minute, before applying the increasing force to the skin and measuring the actual PPG signal which will be used for determining the force value of the increasing force at which the amplitude of the oscillating component of this PPG signal starts to diminish, a PPG signal can be measured over a time period of two full cycles or more, i.e., for instance, over 10 seconds or more, in order to provide enough information for assessing how the oscillating component of the PPG signal behaves due to breathing, wherein this information can be used to model the influence of the applied force on top of that.

For instance, the breathing rate can be determined by using the high-pass filtered PPG signal, i.e. the oscillating component, or the non-filtered PPG signal before applying the increasing force and/or when the application of the increasing force just started. For determining the breathing rate it is possible to, for instance, use the technique disclosed in the above mentioned article by John Allison, which is based on detecting a DC variation, an AC variation and an R-R variation, or to use other known techniques. For the modeling of the respiratory behavior of the high-pass filtered PPG signal the determined breathing rate can be extrapolated or it can be assumed that the R-R variation continues.

In order to determine the diastolic blood pressure of the user 30, the user puts on the wrist worn device 1 on either wrist. Preferentially, in order to output a blood pressure reading, the user sits comfortably and keeps the device 1 at heart level as illustrated in Fig. 2. The device 1 may comprise a blood pressure reading button, wherein, if the blood pressure reading button is pressed, the measurement procedure starts. During the measurement procedure the built-in force applicator pushes the PPG sensor 4 onto the skin by inflating the distensible airbag 1 1 enclosed in the cavity 12. The applied force stimulus preferentially linearly increases from 0 N to a predefined maximum value. Simultaneously with pushing the PPG sensor 4 onto the skin, the PPG sensor 4 registers the blood volume pulse in the skin arterioles, i.e. the PPG sensor 4 measures a PPG signal. The output of the PPG sensor 4 and the registered force applied onto the PPG sensor 4 and hence onto the skin as measured by the force sensor 13 are provided to the force value determination unit 6 which, as described above, determines a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force. This is schematically and exemplarily illustrated in Fig. 3.

In Fig. 3 it can be seen that the amplitude of the oscillating component of the

PPG signal 40 is firstly constant, although the applied force F is increased, wherein at a force value indicated by the line 41 the amplitude of the oscillating component of the PPG signal 40 starts to diminish. At this force value 41 the external pressure applied on the arterioles exceeds the diastolic pressure. It should be noted that Fig. 3 is of course only an illustration used for explaining how the force value can be determined, at which the amplitude starts to diminish, i.e. Fig. 3 does not show a real measured signal which might still be influenced by respiration.

The device 1 further comprises an assignments providing unit 7 for providing assignments between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to an increasing force, and diastolic arterial blood pressures. The assignments can be generated in a training or calibration procedure, wherein the device 1 is used for determining force values of the increasing force for different subjects, at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force, wherein the diastolic arterial blood pressure is known from measurements with a blood pressure cuff. Such measurements are illustrated in Fig. 4 showing the force values FVD of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force versus the diastolic arterial blood pressure CD which has been measured by using a blood pressure cuff. In Fig. 4 the points 61 are measurement point and the line 60 is an exponential fit through the points. In this example the assignments between force values FVD and diastolic arterial blood pressures CD are provided by the line 60.

The device 1 further comprises a blood pressure determination unit 8 for determining the diastolic arterial blood pressure based on the force value determined by the force value determination unit 6 and the assignments 60 provided by the assignments providing unit 7. The determined diastolic arterial blood pressure can be shown on a display 5. The display 5, the oscillating component determination unit 19, the force value

determination unit 6, the assignments providing unit 7, the blood pressure determination unit 8, the breathing indicator providing unit 15 and a controller 9 for controlling the different components of the device 1 are integrated into a casing 3 arranged on the wrist band 2. After the PPG signal has been measured while the applied force has been increased, the airbag 1 1 is deflated.

In the following an embodiment of a method for determining the diastolic arterial blood pressure of a subject will exemplarily be described with reference to a flowchart shown in Fig. 5.

In step 101 a PPG signal of the skin of the subject is measured at a

measurement location by using the PPG sensor 4, while the force applicator applies an increasing force at a force application location, wherein the measurement location and the force application location are the same. In another embodiment the force application location can be proximal relative to the measurement location. Moreover, also in step 101 the force sensor 13 measures the actual increasing force applied to the skin.

In step 102 the oscillating component determination unit 19 determines an oscillating component of the PPG signal by high-pass filtering the PPG signal, and in step 103 a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force is determined by using the force value determination unit. In step 104 assignments 60 between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to the increasing force, and diastolic arterial blood pressures are provided by using the assignments providing unit 7. In step 105 the diastolic arterial blood pressure is determined based on the determined force value and the provided assignments by using the blood pressure

determination unit 8. In step 106 the determined diastolic arterial blood pressure is shown on the display 5.

The device 1 and the method allow for a determination of diastolic arterial blood pressure from locally induced pressure on the skin. The PPG sensor 4 is preferentially a single wavelength reflective PPG sensor. However, in another embodiment the PPG sensor can also be a multispectral PPG sensor. Moreover, it can be a transmissive PPG sensor. The blood flow Q , when passing through a vessel that is too small to allow all blood to flow freely, is directly related to blood pressure P through the resistance of the blood vessel R , expressed in the following physical relationship:

(1) This means that at a certain location within a blood vessel, the blood flow through is determined by the pressure difference before and after the location. However, the resistance in the location serves as a mediator: when AP is kept constant but R is increased, Q will decrease proportional to R . The resistance of the vessel can be expressed in terms of the vessel properties as follows: 1 η

R = (2)

nr wherein L is the vessel length, η is the blood viscosity and r is the vessel radius. Thus:

8Z?7

Assuming blood viscosity to be constant, the vessel radius has a polynomial relationship with blood flow.

When r is decreased with a known amount and the resulting decrease in Q is measured, AP can be computed after applying a multiplier c that takes into account the remaining factors:

£> = APr⁴ - ^— = cAPr⁴ . (4)

8Z?7

It is known that A , the differential between pressure before and after the vessel, is not a constant but continuously modulated by the pulsating heart, which propagates a blood pulse with high pressure (i.e. systolic pressure) through the arterial system. However, even without the direct blood pulse (i.e. during diastole) there is a continuous pressure referred to as diastolic pressure that keeps blood circulating.

The PPG measurement is determined by the sum of absorptions from venous blood at very low pressure and arterial blood which increases at systole and decreases during diastole. Next to that the signal is also affected by non-blood tissue which should not be affected by pressure changes. Upon applying pressure on the skin, the following will happen. Firstly, venous blood volume will start to decrease which is expressed in the PPG signal as an increase in the baseline of the signal (overall higher reflectivity). After that, the arterial resistance will be high enough to affect the lowest level of diastolic pressure. At this point the pulsatile component of the PPG will start to shrink. This property is preferentially used by the device 1 for determining the diastolic arterial blood pressure. As the pressure on the skin is further increased, eventually the vessel resistance will be high enough to stop systolic pressure. At this point the pulse will be completely diminished and the PPG will become a non-pulsatile signal. This property is preferentially used by the device 1 for determining the systolic arterial blood pressure as will be explained in the following.

Preferentially, the force value determination unit is also adapted to determine a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal becomes zero, in order to determine the systolic pressure. Correspondingly, the assignments providing unit is preferentially also adapted to provide assignments between force values, at which the amplitude of the oscillating component of the PPG signal becomes zero, and systolic arterial blood pressures. Moreover, the blood pressure determination unit is preferentially adapted to determine the systolic arterial blood pressure based on the determined force value, at which the amplitude of the oscillating component of the PPG signal becomes zero, and the provided assignments between force values, at which the amplitude of the oscillating component of the PPG signal becomes zero, and systolic arterial blood pressures. Also the systolic arterial blood pressure can be shown on the display.

Also the assignments between the force values FVS, at which the amplitude of the oscillating component of the PPG signal becomes zero, and the systolic arterial blood pressures CS can be determined in a calibration or training procedure, wherein these force values FVS are determined while the systolic arterial blood pressures CS are known from, for instance, blood cuff measurements. Fig. 6 schematically and exemplarily illustrates such measurements 70 and a fitted line 71, wherein the assignments providing unit can be adapted to provide the fitted line 71 as the assignments to be used for determining the systolic arterial blood pressure.

Fig. 7 shows schematically and exemplarily another embodiment of a device for determining diastolic blood pressure of a subject. The device 210 of this embodiment is similar to the device 1 described above with reference to Fig. 1, except for the force applicator, i.e. in this embodiment the increasing force is manually applied by the user by pressing on the display 5 of the device 201 as schematically and exemplarily illustrated in Fig. 8. Thus, in use the user may keep the device 201 at heart level as illustrated in Fig. 2 and then the user may apply an increasing force on the casing 3 with the display 5, while the PPG sensor 4 measures the PPG signal. While measuring the PPG signal the actually applied force is measured by using the force sensor 13, wherein this force measurement is used together with the measured PPG signal for determining the force value of the increasing force, at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force and wherein this force value is used by the blood pressure determination unit 8 together with the assignments provided by the assignments providing units 7 for determining the diastolic arterial blood pressure which is finally shown on the display 5. Also in this embodiment the device 201 can in addition be adapted to determine the systolic arterial blood pressure as described above based on a determination of a force value at which the amplitude of the oscillating component of the PPG signal becomes zero.

The controller 9 of the device 201 can be adapted to guide the user when exerting pressure on the casing 3 with the display 5. For instance, the controller 9 can comprise a force protocol defining a sequence of increasing force values to be applied by the user on the casing 3 with the display 5. The controller 9 can be adapted to indicate to the user that the pressure should be increased or decreased based on a) the force protocol and b) the actual force and hence the actual pressure as measured by the force transducer 13. The indication to increase or decrease the pressure can be provided via the display 5 or via another output unit. For instance, it might be provided acoustically.

In a further embodiment of the device for determining diastolic blood pressure of a user a further PPG sensor might be provided as schematically and exemplarily illustrated in Fig. 9.

The device 310 shown in Fig. 9 can be similar to the devices 1 and 201 shown in Figs. 1 and 7, except for an additional, second PPG sensor 14 besides the PPG sensor 4 which in the following is regarded as being a first PPG sensor. The second PPG sensor 14 measures a further, second PPG signal of the skin of the user at a further measurement location, while the force applied to the skin at the force application location is increased, wherein the further measurement location is proximal relative to the force application location. In this embodiment the oscillating component determination unit 19 is adapted to determine the oscillating component of the second PPG signal by high-pass filtering and the force value determination unit 6 is adapted to determine the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force by determining when a relation between the amplitude of the oscillating component of the PPG signal measured by the first PPG sensor, i.e. of the first PPG signal, and the amplitude of the oscillating component of the second PPG signal measured by the second PPG sensor starts to change. Thus, on the first PPG sensor 4 a force is applied that increases, preferentially linearly increases, from 0 N to a predefined maximum. No force is exerted on the second PPG sensor 14. The core function of the second PPG sensor 14 is to assess the influence of breathing on the oscillating component of the PPG signal measured by the first PPG sensor 4. Differences between the amplitudes of the oscillating components of the PPG signals measured by using the first PPG sensor 4 and the second PPG sensor 14 during corresponding cardiac cycles are continuously monitored. In case no force is applied, it can be assumed that fluctuations of the amplitude of the oscillating component of the first PPG signal measured by using the first PPG sensor 4 are proportional to fluctuations of the amplitude of the oscillating component of the second PPG signal measured by using the second PPG sensor 14. If the relation between the amplitude of the oscillating component of the first PPG signal and the amplitude of the oscillating component of the second PPG signal is constant within a series of consecutive cardiac cycles, it is started to apply the force on the first PPG sensor 4. At the beginning the applied force yields an external pressure on the probed arteriole wall which is below diastolic pressure in the arterioles. If the applied pressure is further increased, the ratio between the amplitudes of the oscillating components of the PPG signals measured by the first and second PPG sensors 4, 14 starts to deviate, wherein at this force value, at which this deviation starts, the applied force yields a pressure on the probed arteriole wall that exceeds the internal diastolic pressure. This force value that initiates a reduction in the amplitude of the oscillating component of the first PPG signal measured by using the first PPG sensor 4, as the external pressure applied on the probed arteriole wall exceeds the diastolic pressure, is translated to a pressure value representing diastolic arterial blood pressure by using the assignments provided by the assignments providing unit 7. The mapping from applied force to diastolic arterial blood pressure is learnt during an offline training or calibration stage as explained above.

The force providing unit can be adapted to provide the increasing force at the force application location with a constant slope. However, it is also possible that the force providing unit is adapted to provide the increasing force depending on a heart rate value being indicative of the heart rate of the subject, wherein the heart rate value may be provided by a heart rate providing unit being adapted to determine the heart rate based on the PPG signal or in another known way. For instance, the heart rate providing unit can be a heart sensor. If the heart rate providing unit is adapted to determine the heart rate based on the PPG signal, this function can be carried out by, for instance, the force providing unit, especially a controller of the force providing unit, another unit of the device described above or by a further dedicated heart rate providing unit of the device.

The force providing unit can also be adapted to provide the increasing force with a non-constant slope over time in accordance with, for instance, Fig. 10. Thus, the force providing unit can be adapted to provide the increasing force with a first slope 410 and then with a second slope 420, wherein the first slope is larger than the second slope. In this example up to a force value, which corresponds to a blood pressure of 40 mniHg, the first slope 410 is applied and then the second slope 420 is applied. It is preferentially ensured that the determination of the force value of the increasing force, at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force, is carried out while the second slope 420 is applied. The second slope can depend on the provided heart rate frequency. Thus, in this example the applied force quickly increases to a value corresponding to 40 mniHg (first slope 410) and then continues with a smaller slope 420, which preferentially depends on the current heart rate frequency of the subject, until, for instance, a diastolic and a systolic arterial pressure are detected or a set maximum force value has been reached like 20 N or a force value which corresponds to 200 mniHg. The force value, at which it is switched from the first slope to the second slope, can be a fixed force value or it can be a force value which depends on the respective subject. For instance, if from previous measurements it is known that the diastolic arterial blood pressure cannot be smaller than a blood pressure value corresponding to a certain force value, it might be switched from the first slope 410 to the second slope 420 at this certain force value.

The second slope can be provided such that a desired resolution of determining the diastolic arterial blood pressure is achieved. In an embodiment the second slope preferentially increases with increasing heart rate.

In a further embodiment the force providing unit is adapted to provide the increasing force, after it has been provided with the second slope, with a third slope, wherein the third slope is larger than the second slope, as illustrated in Fig. 1 1. Preferentially, the second slope is smaller than the first slope. Also the third slope can depend on the heat rate value such a desired resolution for determining the systolic arterial blood pressure can be achieved.

In Fig. 1 1 the force quickly increases to a force value corresponding to 40 mniHg with a first slope 510 or to a force value close to a force value corresponding to an expected diastolic arterial blood pressure as known from previous measurements like to a force value corresponding to 20 mniHg below a previously measured diastolic arterial blood pressure. Then, the force application continues with a smaller second slope 520 at least until the diastolic arterial blood pressure has been detected. The speed of applying the force, i.e. the slope, is then increased again, wherein the corresponding third slope 530 is still smaller than the first slope 510 and wherein it is continued with this third slope until either the systolic arterial blood pressure has been detected or a set maximum force value has been reached. The smaller second slope 520 is chosen, because the detection of the diastolic arterial blood pressure requires more "delicacy" than the detection of the systolic arterial blood pressure. Also in this embodiment the second slope and optionally also the third slope can depend, as mentioned above, on the heart rate, wherein the respective slope might increase with the heart rate.

If the application of the force is carried out relatively fast, the measurement of the blood pressures can also be carried out relatively fast, but the resolution of this measurement is relatively high, i.e. only blood pressure values having a relative large distance to each other can be distinguished, and hence its accuracy is relatively low. On the other side, if the application of the force is relatively slow, the resolution of the measurement of the blood pressures is relatively low, i.e. blood pressure values having a relative small distance to each other can be distinguished, and hence the accuracy can be relatively high, whereas the overall measurement time would be relatively low. By using the different slopes, a compromise can be found between overall measurement time and measurement accuracy.

As explained above, the force providing unit can be adapted to provide the increasing force depending on the provided heart rate value. Thus, the rapidity of the force and hence of the pressure applied to the skin can be adapted to the heart rate value of the subject, wherein the rapidity might be slower, i.e. the respective slope might be smaller, for lower heart rate values. Moreover, as also explained above, the rapidity of the force and hence the pressure increase over time might match the specificity of the vital signs, i.e. of the diastolic arterial blood pressure and the systolic arterial blood pressure, to be detected, wherein the corresponding force profile, i.e. the applied force over time, can be a non- constant profile which might be predetermined. Preferentially, the force increases more rapidly during one or several periods of time where the blood pressure measurements are predeterminedly assessed as being less sensitive and less rapidly where the pressure measurements are predeterminedly assessed as being more sensitive. A predetermined first expected diastolic arterial blood pressure might be used to determine a first force limit in the respective profile until which a respective first slope with a high increase of the force with relatively low force values is applied. This can be followed by a second period of a lower increase by using a second slope with relatively high force values as illustrated in Fig. 10. A predetermined first expected systolic arterial blood pressure might be used to determine a further limit in the profile between the second period, i.e. the second slope, with a relatively low increase of the force and a third period, i.e. a third slope, in which the increase of the force is higher as illustrated, for instance, in Fig. 1 1.

In a further embodiment a constant zero slope, i.e. a constant force, is used after the first slope and the second slope as schematically and exemplarily illustrated in Fig. 12. In Fig. 12 firstly the relatively large first slope 610 is used, whereafter a constant force 620 is applied, wherein during the application of the constant force 620 a breathing indicator can be determined. The first slope 610 and the constant force 620 are applied with force values being well below a force value corresponding to the diastolic arterial blood pressure to be determined. The adjacent second slope 630 is smaller than the first slope 610 and used within a range in which the diastolic arterial blood pressure to be measured is expected. After the diastolic arterial blood pressure has been determined, a third slope 640 is used in which the systolic arterial blood pressure is determined and which is larger than the second slope 630.

The force values, at which it is switched from, for instance, the first slope to the second slope and/or from the second slope to the third slope, can depend on the activity level of the person, particularly on the immediate previous activity level which might still influence the blood pressure like the activity level in the last 15 minutes. Thus, the activity level of the subject can be determined by using known algorithms based on, for instance, the PPG signal, wherein assignments can be provided between activity levels and force values at which it is switched from one of the slopes to another of the slopes. This can be helpful, because the expected diastolic arterial blood pressure and the expected systolic arterial blood pressure depend on the activity level of the of the subject, which was present before carrying out the blood pressure measurement. By using the activity level it can be better ensured that, for instance, the second slope is really used while determining the diastolic arterial blood pressure and the third slope used while determining the systolic arterial blood pressure.

In an embodiment the PPG signal providing unit is adapted to provide as the PPG signal of the skin of the subject a PPG signal which has been measured at the measurement location a) also before the increasing force is applied to the skin and/or b) while the increasing force corresponds to an arterial blood pressure being below the diastolic arterial blood pressure and/or c) also while a constant force, which corresponds to an arterial blood pressure below the diastolic arterial blood pressure, is applied, wherein the amplitude of the oscillating component of the PPG signal varies over time due to breathing. Thus, the PPG signal also comprises a part which is not influenced by the increasing force such that the changes in the amplitude within this part of the PPG signal can be attributed to respiratory influences. In this embodiment, the oscillating component determination unit is adapted to high-pass filter the PPG signal such that respiration induced low frequency baseline oscillations are removed, wherein the respiratory induced amplitude and frequency changes are preserved. This is achieved by applying a high-pass filtering technique that removes for each individual beat cycle, i.e. cardiac cycle, the estimated baseline changes in the PPG signal due to respiration. In particular, the PPG signal is segmented into cardiac cycles by applying a known beat detector, wherein the start of a cardiac cycle can be defined by a local minimum of the PPG signal. Next, for each PPG signal segment representing a full cardiac cycle a straight line is fitted through the points marking the start point and the endpoint of the respective cardiac cycle, wherein the fitted line represents the new baseline for the PPG signal segment and is subtracted from the original PPG signal segment. This can be carried out for each PPG signal segment such that, after this high-pass filtering has been carried out, each PPG segment has a value of zero at the start point and at the endpoint of the respective cardiac cycle. This approach leads to a determination of the oscillating component as illustrated in Fig. 13, wherein respiration induced low frequency baseline oscillations are removed and wherein, as mentioned above, respiratory induced amplitude and frequency changes are preserved.

The high-pass filtering for removing the respiration induced low frequency baseline oscillations, wherein the respiratory induced amplitude and frequency changes are preserved, in order to thereby determine a baseline corrected oscillating component of the PPG signal, can also be carried out in another way. For instance, in another embodiment the segments might not represent each a full cardiac cycle, but they may represent less than a full cardiac cycle or more than a full cardiac cycle. Thus, the PPG signal can also be segmented in another way. For instance, a segment can be defined by a given number of minima and/or maxima covered by a respective segment.

Also in this embodiment the device comprises a breathing indicator providing unit, wherein in this embodiment the breathing indicator providing unit is adapted to provide as a breathing indicator being indicative of the breathing behavior of the subject a breathing threshold being indicative of a minimum or maximum of the amplitude of the oscillating component of the PPG signal, which is not influenced by the increasing force applied to the skin. The force value determination unit can then be adapted to determine as a force value a force value of the increasing force at which the minimum amplitude or maximum amplitude, respectively, of the oscillating component of the PPG signal starts to diminish below the breathing threshold in reaction to the increasing force. The determination of the breathing threshold will in the following be described with reference to Fig. 13.

Fig. 13 schematically and exemplarily shows how the amplitude of the oscillating component of the PPG signal changes with time due to breathing, wherein the brackets r indicate respiration cycles and the ellipses 730 indicate minimum amplitudes. In this embodiment these minimum amplitudes 730 are averaged over the several respiration cycles r for generating a breathing threshold 720. If, while the force applied to the skin is increased, the minimum amplitude decreases below the breathing threshold 720 by more than a predetermined value, it is detected that the amplitude starts to diminish in reaction to the increasing force, wherein the corresponding force value is then used together with the provided assignments for determining the diastolic arterial blood pressure. In this example not all changes of the amplitude of the oscillating component of the PPG signal are considered, but only changes being larger than a predefined threshold, i.e. the diastolic arterial blood pressure is only determined based on a minimum amplitude which is significantly below the breathing threshold 720.

Although in above described embodiments the PPG sensor uses green light for measuring the PPG signal, in other embodiments the PPG sensor can also be adapted to use another wavelength like red or infrared light.

Although in above described embodiments the different components of the device for determining diastolic blood pressure of a user are distributed in a certain way in the wrist band and in the casing attached to the wrist band, these components can also be distributed in another way among these components. Moreover, it is also possible that a casing or a wrist band is not present, wherein in the latter case the casing is held in place on the wrist by another attaching means. It is also possible that only a wrist band is present, wherein all components are integrated in the wrist band. Furthermore, the device can also comprise separate units which communicate with each other via a wired or wireless data connection. For instance, the components needed for carrying out the measurements can be included in a first unit being adapted to be attached to the user for carrying out the measurements, especially the measurement of the PPG signal, and the measured data can be provided to a further unit of the device, which is separate from the unit attached to the subject, wherein this further separate unit includes the components for data processing and finally determining the diastolic arterial blood pressure. For instance, this further separate unit of the device can comprise the oscillating component determination unit, the force value determination unit, the assignments providing unit and/or the blood pressure determination unit. The further separate unit can be a correspondingly adapted smartphone or another correspondingly adapted computer, particularly another handheld computer.

Although in above described embodiments the device is adapted to measure the PPG signal at a wrist, the device can also be adapted to measure the PPG signal at another part of a subject like a finger.

The oscillating component can be adapted to further process, in addition to the high-pass filtering, the PPG signal for determining the oscillating component.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Procedures like determining an oscillating component of a PPG signal, determining a force value, providing assignments, determining a diastolic arterial blood pressure, determining a systolic arterial blood pressure, et cetera performed by one or several units or devices can be performed by any other number of units or devices. For instance, the determination of the oscillating component of the PPG signal, the determination of the force value, the provision of the assignments and the determination of the diastolic arterial blood pressure can be performed by a single unit or by any other number of different units. These procedures and/or the control of the device for determining diastolic blood pressure of a subject depending on the method for determining diastolic blood pressure of a subject can be implemented as program code means of a computer program and/or as dedicated hardware.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope. The invention relates to a device for determining diastolic blood pressure in an unobtrusive and painless way such that it can be determined, for instance, during sleep. A PPG signal of the skin of a subject is provided by, for instance, a PPG sensor, which has been measured at a measurement location, while a force applied to the skin at a force application location has been increased, wherein the measurement location and the force application location are the same or the force application location is proximal relative to the

measurement location. A force value of the increasing force is determined by a force value determination unit, at which an amplitude of an oscillating component of the PPG signal starts to diminish in reaction to the increasing force, wherein this force value is used by a blood pressure determination unit for determining the diastolic arterial blood pressure.

Claims

CLAIMS:

1. A device for determining diastolic blood pressure of a subject, the device (1) comprising:

a photoplethysmography (PPG) signal providing unit (4) for providing a PPG signal of the skin of the subject, which has been measured at a measurement location, while a force applied to the skin at a force application location has been increased, wherein the measurement location and the force application location are the same or the force application location is proximal relative to the measurement location,

an oscillating component determination unit (19) for determining an oscillating component of the PPG signal by high-pass filtering the PPG signal,

a force value determination unit (6) for determining a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force and which indicates the diastolic arterial blood pressure,

an assignments providing unit (7) for providing assignments between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to an increasing force, and diastolic arterial blood pressures, and

a blood pressure determination unit (8) for determining the diastolic arterial blood pressure based on the determined force value and the provided assignments.

2. The device as defined in claim 1, wherein the PPG signal providing unit (4) is adapted to provide as the PPG signal of the skin of the subject a PPG signal, which has been measured at the measurement location a) also before the increasing force is applied to the skin and/or b) while the increasing force corresponds to an arterial blood pressure being below the diastolic arterial blood pressure and/or c) also while a constant force, which corresponds to an arterial blood pressure below the diastolic arterial blood pressure, is applied to the skin at the force application location, wherein the amplitude of the oscillating component of the PPG signal varies over time due to breathing, wherein the device (1) further comprises a breathing indicator providing unit (15) for providing as a breathing indicator being indicative of the breathing behavior of the subject a breathing threshold being indicative of a minimum or maximum of the amplitude a) also before the increasing force is applied to the skin and/or b) while the increasing force corresponds to an arterial blood pressure being below the diastolic arterial blood pressure and/or c) also while a constant force, which corresponds to an arterial blood pressure below the diastolic arterial blood pressure, is applied to the skin at the force application location, wherein the force value determination unit (6) is adapted to determine as the force value a force value of the increasing force at which the minimum amplitude or maximum amplitude, respectively, of the oscillating component of the PPG signal starts to diminish below the breathing threshold in reaction to the increasing force.

3. The device as defined in claim 1, wherein the device (1) further comprises a force providing unit (10, 1 1) for providing the increasing force to the skin while measuring the PPG signal of the skin.

4. The device as defined in claim 3, wherein the device (1) further comprises a heart rate providing unit (15) for providing a heart rate value being indicative of the heart rate of the subject, wherein the force providing unit (10, 1 1) is adapted to provide the increasing force depending on the provided heart rate value.

5. The device as defined in claim 3, wherein the force providing unit (10, 1 1) is adapted to provide the increasing force with a non-constant slope over time.

6. The device as defined in claim 5, wherein the force providing unit (10, 1 1) is adapted to provide the increasing force with a first slope and then with a second slope, wherein the first slope is larger than the second slope.

7. The device as defined in claim 6, wherein the force providing unit (10, 1 1) is adapted to provide the increasing force with a third slope, after it has been provided with the second slope, wherein the third slope is larger than the second slope.

8. The device as defined in claim 7, wherein the force providing unit (10, 1 1) is adapted to provide the increasing force with the third slope, after the diastolic arterial blood pressure has been determined.

9. The device as defined in claim 7, wherein the force providing unit (10, 1 1) is adapted to provide a constant force after having used the first slope and before using the second slope.

10. The device as defined in claim 1, wherein the force value determination unit

(6) is adapted to only consider changes of the amplitude of the oscillating component, which are larger than a predefined threshold, for determining the force value at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force.

1 1. The device as defined in claim 1, wherein the PPG signal providing unit is adapted to provide a further PPG signal of the skin of the subject, which has been measured at a further measurement location, while the force applied to the skin at the force application location is increased, wherein the further measurement location is proximal relative to the force application location, wherein the oscillating component determination unit is adapted to high-pass filter the further PPG signal for determining an oscillating component of the further PPG signal, wherein the force value determination unit (6) is adapted to determine the force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force by determining when a relation between the oscillating component of the PPG signal and the oscillating component of the further PPG signal starts to change.

12. The device as defined in claim 1, wherein the force value determination unit (6) is adapted to correct the amplitude of the oscillating component of the PPG signal for breathing influences and to determine the force value at which the corrected amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing.

13. The device as defined in claim 12, wherein the device (1) further comprises a breathing indicator providing unit (15) for providing a breathing indicator being indicative of the breathing behavior of the subject, wherein the force value determination unit (6) is adapted to determine the breathing influences based on the provided breathing indicator and to use these determined breathing influences for correcting the amplitude of the oscillating component of the PPG signal which has been measured while the force applied to the skin was increased.

14. A method for determining the diastolic arterial blood pressure of a subject, the method comprising:

providing a PPG signal of the skin of the subject, which has been measured at a measurement location by using a PPG signal providing unit (4), while a force applied to the skin at a force application location is increased, wherein the measurement location and the force application location are the same or the force application location is proximal relative to the measurement location,

determining an oscillating component of the PPG signal by high-pass filtering the PPG signal by using an oscillating component determination unit (19),

- determining a force value of the increasing force at which the amplitude of the oscillating component of the PPG signal starts to diminish in reaction to the increasing force by using a force value determination unit (6) and which indicates the diastolic arterial blood pressure,

providing assignments between force values, at which an amplitude of an oscillating component of a PPG signal starts to diminish in reaction to an increasing force, and diastolic arterial blood pressures by using an assignments providing unit (7), and

determining the diastolic arterial blood pressure based on the determined force value and the provided assignments by using a blood pressure determination unit (8).

15. A computer program for determining the diastolic arterial blood pressure of a subject, the computer program comprising program code means for causing a device for determining the diastolic arterial blood pressure of the subject as defined in claim 1 to carry out the steps of the method for determining the diastolic arterial blood pressure of the subject as defined in claim 14, when the computer program is run on a computer controlling the device.