Patent Description:
During a cardiac cycle of an individual, i.e. the period between two heart beats, blood is pumped through the vascular system of the individual in a pulsating fashion. In other words, during a cardiac cycle, the volume of blood in for example a finger, a nostril, an ear, a forehead, the inside of a mouth, a toe, a wrist, an ankle, etc., increases and decreases cyclically. The blood comprised in arteries is called arterial blood. The blood comprised in veins is called venous blood.

A common method for measuring this fluctuation in blood volume is optical plethysmography, or photoplethysmography, during which an optical plethysmogram, or PhotoPlethysmoGram, or PPG, is used to detect changes in blood volume in the microvascular bed of the tissue. A PPG is often obtained by shining light from one or more light sources, such as for example LEDs, onto an investigated volume, and detecting collected light corresponding to the light being reflected or transmitted in the investigated volume on a sensor, wherein the sensor may for example comprise or correspond to a photodetector, such as for example a photodiode. The light sources and the sensor can for example be arranged on opposite sides of a finger of an individual, hereby allowing the measurement of a transmission mode PPG, or at the same side of a finger of the individual, hereby allowing the measurement of a reflectance mode PPG. With each cardiac cycle the heart pumps arterial blood to the investigated volume. The physical events corresponding to a change in arterial blood volume in the investigated volume for example in a cardiac cycle can be captured by optical plethysmography. For example, <CIT> describes an apparatus and method for the non-invasive evaluation, detection, and monitoring of a physiological state or medical condition by assessing peripheral circulation. For example, the scientific publication by <NPL>" describes that the variability of the PPG parameters shows promise for the assessment of the function of the autonomic nervous system.

In addition to cyclic fluctuations in arterial blood volume, the arterial blood volume in the tissue is also influenced by the diameters of small arteries, or arterioles, in the investigated volume. These arterioles have muscular walls which can contract to reduce the diameter arterioles. In other words, when these arterioles contract and reduce in diameter, the volume of blood comprised in the corresponding arterioles evidently reduces. Optical plethysmography is a measurement technique which can be used to monitor changes in the volume of blood comprised in arterioles when these arterioles contract in diameter. Monitoring changes in the arterial blood volume with optical plethysmography therefore empirically provides information on relative changes in muscle tension or 'tone' of the smooth muscle tissue of the arterioles, also referred to as Peripheral Arterial Tone or PAT.

Because arterial blood flow to the investigated volume can be modulated by multiple other physiological phenomena, optical plethysmography can also be used to monitor breathing, hypovolemia, other circulatory conditions, and for example can be used for diagnosing sleep disorders. Sleep disorder diagnosis is a medical field wherein a patient's sleep is monitored during a certain time, e.g. one or more nights. Based on the monitoring, different sleep-related events may be identified such as for example apneic events, snoring, or limb movements.

The reuptake of breathing at the end of a sleep apnea usually coincides with an adrenaline release. Adrenaline is released in the bloodstream and binds to adrenergic receptors in the arterioles in the investigated volume, which triggers an increase in muscle tension of the arterioles resulting in a reduction of the arteriole diameter and a reduction of arterial blood volume in the investigated volume. Monitoring Peripheral arterial tone through for example optical plethysmography can therefore provide valuable information on the occurrence of sleep-related events such as for example sleep apnea. Several scientific publications widely report that a signal acquired by optical plethysmography provides information on changes in arterial tone through changes in the amplitude of the pulses in the signal. For example, the scientific publication entitled "<NPL>, reports that the Pulse PlethysmoGraphic Amplitude, or PPGA, of the photoplethysmograph is due to pulsatile changes in tissue volume, mainly arterial blood and that during sympathetic activation, or vasoconstriction, PPGA decreases.

The article "<NPL>, describes PAT analysis for sleep apnea.

However, monitoring changes in the amplitude of pulses in the signal acquired by optical plethysmography does not accurately reflect changes in arterial tone. In other words, considering the amplitude of pulses in the signal and monitoring changes in this amplitude does not allow to accurately determine changes in sympathetic tone.

It is thus an object of embodiments of the present invention to propose a computer-implemented method and an apparatus which do not show the inherent shortcomings of the prior art. More specifically, it is an object of embodiments of the present invention to propose a method and an apparatus to accurately and robustly determine changes in peripheral arterial tone by optically measuring changes in the arterial blood volume in the investigated volume.

There is a need for accurately and robustly determining changes in the monitored arterial blood volume on the optical measurement of peripheral arterial tone.

This object is achieved, according to a first example aspect of the present disclosure, by a computer-implemented method according to claim <NUM>.

The computer-implemented method according to the present disclosure allows determining peripheral arterial tone in an accurate and robust manner. By determining a logarithm or a function approximation thereof of a function of the light intensities, the determination of changes in arterial blood volume in the investigated volume between two points in time with the computer-implemented method according to the present disclosure is more accurate than by monitoring changes in amplitude of pulses in the optical plethysmography signal. Therefore, with the computer-implemented method according to the present disclosure, it becomes possible to assess PAT of an individual more accurately and more robustly than with existing prior art solutions. In other words, the resulting evaluation of changes in arterial blood volume in the investigated volume through the determination of a logarithm or a function approximation thereof of a function of the light intensities therefore provides a more accurate and robust assessment of peripheral arterial tone of the individual.

In the context of the present disclosure, an investigated volume of an individual is for example a volume defined in an investigated tissue of the individual which is monitored by optical plethysmography and in which light emitted by optical plethysmography propagates and is collected on a sensor for optical plethysmography. In other words, an investigated volume of an individual is for example a volume defined in an investigated tissue of the individual for which an optical plethysmography signal is acquired. For example, the investigated volume is a peripheral tissue volume of the individual. For example, the investigated volume is a volume defined in finger, a tip of a finger, a distal end of a digit of the individual, a nostril, an ear, a forehead, the inside of a mouth, a toe, a tip of a toe, a wrist, an ankle of the individual. In the context of the present disclosure, the investigated volume of an individual comprises the skin of the individual comprised in the investigated volume and further comprises the blood volume present in the investigated volume. In the context of the present disclosure, peripheral arterial tone is understood as arterial tone changes in investigated arterial beds in the investigated volume of an individual. In other words, determining pulsatile volume changes in the vascular beds of the investigated volume of the individual allows determining or assessing information indicative for muscle tension or 'tone' of the smooth muscle tissue of the arterioles in the investigated volume and therefore allows determining or assessing peripheral arterial tone which is modulated by the sympathetic nervous system. Determining peripheral arterial tone is non-invasive and can for example be used to detect heart diseases, erectile dysfunction, sleep apnea, obstructive sleep apnea, cardiovascular conditions, etc..

In the context of the present disclosure, an optical plethysmography signal is a signal measured by optical plethysmography. For example, the optical plethysmography signal is an optical plethysmogram. For example, the optical plethysmography signal is a PPG. The optical plethysmography signal is for example measured at the tip of a finger of the individual by an optical plethysmography setup comprising at least one light source and a sensor. In the context of the present disclosure, a light intensity corresponds to the intensity of light collected on the sensor of the optical plethysmography setup, wherein the light collected on the sensor corresponds to light generated by one or more light sources being transmitted through or reflected in the investigated volume of the individual.

In the context of the present disclosure, the oxygen saturation estimate or SpO<NUM> or hemoglobin composition corresponds to a fraction of oxygenated hemoglobin related to a total amount of hemoglobin in the arterial blood volume in the investigated volume. For example, the oxygen saturation estimate or SpO<NUM> or hemoglobin composition corresponds to a ratio of the concentration of oxygenated hemoglobin on the sum of the concentrations of oxygenated and deoxygenated hemoglobin in the arterial blood volume being monitored in the investigated volume. Alternatively, the oxygen saturation estimate or SpO<NUM> or hemoglobin composition corresponds to a ratio of the volume fraction of oxygenated hemoglobin on the sum of the volume fractions of oxygenated and deoxygenated hemoglobin in the arterial blood volume being monitored in the investigated volume.

In the context of the present disclosure, deoxygenated hemoglobin is defined as the form of hemoglobin without the bound oxygen, and without any other bound molecule such as for example carbon monoxide, carbon dioxide, or iron. In the context of the present disclosure, oxygenated hemoglobin is defined as the form of hemoglobin with the bound oxygen. In the context of the present disclosure, light emitted by the light sources of an optical plethysmography setup comprises photons which reach the sensor through a probabilistic path of one or multiple scattering events. This optical path is not straight and is often assumed to follow a curved spatial probability distribution. The investigated volume along this curved optical path forms the volume which is sampled or investigated by optical plethysmography. The computer-implemented method according to the present disclosure evaluates one or more changes in arterial blood volume in the investigated volume between the two or more points in time and hereby assessing PAT of the individual. In the context of the present disclosure, a change in arterial blood volume in the investigated volume between two points in time corresponds to a relative change between the volume of arterial blood present in the investigated volume at a first point in time and the volume of arterial blood present in the investigated volume at a second point in time different from the first point in time. The blood comprised in arteries is called arterial blood. The blood comprised in veins is called venous blood.

In the context of the present disclosure, a chromophore is a molecular unit which absorbs or scatters light in the investigated volume. For example, in the context of the present disclosure, examples of chromophores are melanin molecules, oxygenated hemoglobin, deoxygenated hemoglobin, etc. In the investigated volume, the decay in light intensity of the incident light emitted by a light source of an optical plethysmography setup follows the Beer-Lambert law which can be formulated as in equation (<NUM>): <MAT> wherein:.

the Beer-Lambert law as formulated in equation (<NUM>) can be evaluated at the first point in time and at the second point in time. When taking a ratio of both expressions, equation (<NUM>) is obtained: <MAT>.

Then, equation (<NUM>) is obtained by taking the natural logarithm of both sides of equation (<NUM>), as follows: <MAT>.

If a logarithm with another base b than Euler's number e is used, equation (<NUM>) then becomes: <MAT>.

It can be seen that the above equation (<NUM>') is equal to equation (<NUM>) up to a constant <MAT>.

Equation (<NUM>) can be simplified to equation (<NUM>) when when the difference Vi,s - Vi,d is being renamed ΔVi, thereby obtaining: <MAT>.

From equation (<NUM>), it can be seen that the logarithm of the fraction of light intensities at the first point in time and at the second point in time is linearly related to the difference in either volume fraction or concentration of chromophore i between the first point in time and the second point in time.

Some chromophores remain attached to the epidermis of the individual between the two points in time along the optical plethysmography signal. For example, melanin molecules remain fixed to the investigated volume between the two points in time along the optical plethysmography signal. The difference in either volume fraction or concentration of such chromophores, such as for example melanin molecules, between the two points in time is therefore null. The contribution to the right-hand side of equation (<NUM>) of such chromophores is therefore null.

The main chromophores of which either the volume fraction or the concentration fluctuates between the two points in time along the optical plethysmography signal are the oxygenated and deoxygenated hemoglobin in the arterial blood volume. The two predominant forms of hemoglobin in the context of the present disclosure, i.e. oxygenated hemoglobin and deoxygenated hemoglobin, demonstrate absorption and scattering coefficients which are significantly different for most wavelengths of light.

The effect of all other chromophores, wherein all other chromophores are not oxygenated hemoglobin nor deoxygenated hemoglobin, of which either the volume fraction or the concentration fluctuates between the two points in time along the optical plethysmography signal can be written as the product of their combined extinction coefficient, εother, and one minus the sum of either the volume fractions or the concentrations of oxygenated hemoglobin and deoxygenated hemoglobin, wherein the sum of either all volume fractions or all concentrations equals <NUM>.

Taking the above considerations into account, equation (<NUM>) can then be rewritten as follows into equation (<NUM>): <MAT>.

It is known that the oxygen saturation estimate can be defined according to equation (<NUM>): <MAT> wherein:.

Additionally, Vblood is defined as the total volume fraction or the total concentration of oxygenated and deoxygenated hemoglobin comprised within the arterial blood in the investigated volume and is defined as follows in equation (<NUM>): <MAT>.

It is assumed that under normal circumstances, the sum of the volume fractions or of the concentrations of oxygenated and deoxygenated hemoglobin within the arterial blood volume or the sum of the concentrations of oxygenated and deoxygenated hemoglobin within the arterial blood volume remains approximately constant throughout the measurement of the optical plethysmography signal. Indeed, only the ratio of oxygenated hemoglobin and deoxygenated hemoglobin, i.e. only the oxygen saturation estimate, might significantly change during the monitoring of the individual by optical plethysmography, for example during sleep apnea.

From equations (<NUM>) and (<NUM>), the following can be obtained: <MAT>.

By substituting expression (<NUM>) into equation (<NUM>), the following can be obtained: <MAT>.

Two predetermined calibration coefficients Q<NUM> and Q<NUM> , which are two constants, can be defined as follows: <MAT>.

After rewriting equation (<NUM>) in view of the two predetermined calibration coefficients Q<NUM> and Q<NUM>, equation (<NUM>) can be obtained, wherein the left-hand side of equation (<NUM>) corresponds to an evaluation function, and wherein Q<NUM>SpO<NUM> + Q<NUM> corresponds to a compensation function which is a function of the oxygen saturation estimate: <MAT>.

Equation (<NUM>) highlights a term on the left-hand side, referred to as the evaluation function, which shows a linear relationship with:.

Under the assumption of a constant volume fraction or concentration of the sum of oxygenated and deoxygenated hemoglobin in the arterial blood, ΔVblood is a linear proxy for arterial blood volume fluctuations within the investigated volume, and hence corresponds to a measurement of peripheral arterial tone.

For a constant value of the oxygen saturation estimate, a change in arterial blood volume in the investigated volume between two points in time is thus evaluated when determining the logarithm of the ratio of the light intensities collected on the sensor when measuring with optical plethysmography at said two points in time.

As only a linear relationship between a measurable parameter and ΔVblood is relevant to investigate relative changes in ΔVblood rather than the exact ΔVblood value, determination of ΔVblood up to a constant factor is sufficient, as is performed by determining the evaluation function.

If the SpO<NUM> is not constant, the evaluation function will also change with changes in SpO<NUM>. However, certain compensations methods can be used to compensate for the effect of changes in SpOz.

According to example embodiments, the function of the light intensities corresponds to a ratio of the light intensities.

According to equation (<NUM>), the evaluation function corresponds to the natural logarithm of a function of the light intensities. Alternatively, starting from equation (<NUM>), any other evaluation function defined as a function of the light intensities could be used, for example a linear approximation of the logarithm of a function of the light intensities, or for example a Taylor series approximation of a function of the light intensities, or for example a linear approximation of other base logarithms of a function of the light intensities. Alternatively, the evaluation function corresponds approximately to the ratio of the pulsatile waveform or AC component of the optical plethysmography signal and of the slowly varying baseline or DC component of the optical plethysmography signal, resulting in equation (<NUM>): <MAT>.

According to example embodiments, the function of the light intensities include a first measure corresponding to the light intensity measured by the sensor of the optical plethysmography setup at a first point in time and a second measure corresponding to the light intensity measured by the sensor of the optical plethysmography setup at a second point in time.

According to example embodiments, the function of the light intensities that corresponds to the ratio of the light intensities (<NUM>; <NUM>) is a ratio of a first measure corresponding to the light intensity measured by the sensor of the optical plethysmography setup at a first point in time and a second measure corresponding to the light intensity measured by the sensor of the optical plethysmography setup at a second point in time.

According to example embodiments, the first point in time corresponds to the diastole in a first cardiac cycle and the second point in time corresponds to the systole in a second cardiac cycle different from the first cardiac cycle.

According to example embodiments, the first point in time corresponds to the systole in a first cardiac cycle and the second point in time corresponds to the diastole in a second cardiac cycle different from the first cardiac cycle.

According to example embodiments, the first point in time corresponds to the diastole in a cardiac cycle and the second point in time corresponds to the systole in the same cardiac cycle.

According to example embodiments, the first point in time corresponds to the systole in a cardiac cycle and the second point in time corresponds to the diastole in the same cardiac cycle.

According to example embodiments, the evaluation function corresponds to a logarithm of a ratio of the light intensities; and the evaluation function depends on one or more of the following:.

According to example embodiments, at least one of the points in time corresponds to the diastole in a cardiac cycle of the individual and/or wherein at least one of the points in time corresponds to the systole in a cardiac cycle of the individual.

During systole, the volume of arterial blood in the investigated volume of the individual is maximum, resulting in the largest absorption and scattering of light of any point in time within a cardiac cycle, i.e. the period between two heart beats, since hemoglobin is one of the main absorbers and scatters of photons in the investigated volume, hence resulting in the lowest measured light intensity on the sensor of the optical plethysmography setup. Conversely, during diastole, the volume of arterial blood in the investigated volume of the individual is minimum, resulting in the lowest absorption and scattering of light of any point in time within a cardiac cycle and hence highest measured light intensity on the sensor of the optical plethysmography setup. At least one first point in time corresponds for example to the diastole in a first cardiac cycle and/or at least one second point in time corresponds for example to the systole in a second cardiac cycle different from the first cardiac cycle. Alternatively, at least one first point in time corresponds for example to the systole in a first cardiac cycle and/or at least one second point in time corresponds for example to the diastole in a second cardiac cycle different from the first cardiac cycle. Alternatively, at least one first point in time corresponds for example to the systole or to the diastole in a cardiac cycle and at least one second point in time corresponds to any point in time within the same cardiac cycle or within a different cardiac cycle.

According to example embodiments, the method further comprises the steps of:.

Optical plethysmography technology uses a simple and noninvasive setup probe or biosensor. The optical plethysmography biosensor non-invasively measures pulsatile arterial volume changes in the investigated volume, and thereby assesses PAT, by collecting the optical plethysmography signal. The light source is for example a LED or any other suitable light source which can be miniaturized to fit in the optical plethysmography biosensor. The wavelength is for example comprised in the red spectrum. Alternatively, the wavelength is comprised in the infra-red spectrum. A physical distance between the light sources and the sensor is for example a few millimeters, such as for example less than <NUM>.

According to example embodiments, the method further comprises the step of:
detecting the occurrence at the individual under monitoring of the sleep-related event by determining a drop in value of the logarithm of the function of the light intensities below a predetermined threshold value.

According to a second example aspect, an apparatus according to claim <NUM> is disclosed.

The apparatus according to the present disclosure allows determining peripheral arterial tone in an accurate and robust manner. By determining a logarithm or a function approximation therefore of a function of the light intensities, the determination of changes in arterial blood volume in the investigated volume between two points in time with the apparatus according to the present disclosure is more accurate than by monitoring changes in amplitude of pulses in the optical plethysmography signal. Therefore, with the apparatus according to the present disclosure, it becomes possible to assess PAT of an individual more accurately and more robustly than with existing prior art solutions. In other words, the resulting evaluation of changes in arterial blood volume in the investigated volume through the determination of a logarithm or a function approximation thereof of a function of the light intensities therefore provides a more accurate and robust assessment of peripheral arterial tone of the individual.

According to example embodiments, a system is provided, wherein the system comprises an apparatus according to a second example aspect of the invention, and further comprises:.

The sensor collects propagated light by optical plethysmography, wherein the propagated light corresponds to the light being transmitted or reflected when propagating in the investigated volume of the individual, such as for example a distal end of a digit of the individual, at the two or more points in time. The system further optionally comprises a wireless transmitter comprising a wireless communication interface, wherein the wireless transmitter is configured to transmit the determined peripheral arterial tone wirelessly for further processing by the apparatus. The wireless communication interface is preferably a low power communication interface, e.g. a Bluetooth Low Energy, BLE, wireless interface.

According to a third example aspect, a computer program product comprising computer-executable instructions for causing a system to perform the method according to a first example aspect is provided.

According to a fourth example aspect, a computer readable storage medium is provided, wherein the computer readable storage medium comprises computer-executable instructions for performing the method according to a first example aspect when the program is run on a computer.

According to a fifth example aspect, a use of a logarithm or a function approximation thereof for assessing peripheral arterial tone, PAT, of an individual monitored by optical plethysmography, is provided, wherein assessing peripheral arterial tone comprises:.

<FIG> illustrates an example embodiment of an apparatus <NUM> according to the present disclosure. The apparatus <NUM> comprises at least one memory <NUM>, at least one processor, wherein the memory <NUM> includes computer program code configured to, with the at least one processor, cause the apparatus <NUM> to perform the following:.

The apparatus <NUM> obtains the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from an external device. According to an alternative embodiment, the apparatus <NUM> obtains the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from the memory <NUM>. According to a further alternative embodiment, the apparatus <NUM> the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from the memory <NUM> and/or from an external device. Optionally, at least one point in time <NUM> corresponds to the diastole of a cardiac cycle of the individual and/or at least one point in time <NUM> corresponds to the systole of a cardiac cycle of the individual. The apparatus <NUM> is configured to determine an evaluation function <NUM> which corresponds to a logarithm <NUM> or a function approximation thereof <NUM> of a function of the light intensities <NUM>; <NUM>. The evaluation function <NUM> depends on one or more of the following: an optical path length, a function of the oxygen saturation estimate, a change in arterial blood volume in the investigated volume.

<FIG> illustrates an example embodiment of a system <NUM> according to the present disclosure. Components having identical reference numbers to numbers on <FIG> perform the same function. The system <NUM> of <FIG> comprises an apparatus <NUM> according to the present disclosure. Optionally, the system <NUM> further comprises a light source <NUM> and a sensor <NUM>. A light source <NUM> is configured to emit light <NUM>. The apparatus <NUM> is configured to:.

The apparatus <NUM> obtains the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from an external device from an external device <NUM> comprising at least one light source <NUM> and/or the sensor <NUM>. For example, the external device <NUM> determines the arterial blood volume pulse in the investigated volume <NUM> of the individual <NUM>. The external device <NUM> comprises a battery for powering the different electrical components <NUM>;<NUM>. The light source <NUM> is configured to emit light, i.e. to transmit the light <NUM> into the investigated volume <NUM> of the individual attached to the external device <NUM>, for example into the finger <NUM> of the individual <NUM> as illustrated, more particularly to a distal end <NUM> of a finger of the individual. The external device <NUM> further comprises control circuitry for controlling the light source <NUM>, i.e., for enabling or disabling the light source <NUM> and for receiving the measurements for example of the light intensities from the sensor <NUM>. Control circuitry may further comprise a memory component for temporarily storing the obtained measurements. Control circuitry is further coupled to a wireless interfacing circuitry <NUM> and configured to forward the measurements to the wireless interfacing circuitry <NUM>. Wireless interface <NUM> may support a short range and/or low power wireless communication protocol for efficient transmission of the measurements to a receiving part of the system. Wireless interface <NUM> may for example operate according to the Bluetooth Low Energy, BLE, protocol as defined by the Bluetooth Special Interest Group or according to a Near Field Communication, NFC, protocol. Operation by such protocols together with forwarding of the raw optical plethysmography signal <NUM> allows miniaturization of the external device <NUM> such that it fits on a finger or a nostril and allows operation during multiple nights According to an alternative embodiment, the apparatus <NUM> obtains the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from the memory <NUM>. According to a further alternative embodiment, the apparatus <NUM> the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from the memory <NUM> and/or from an external device. Optionally, at least one point in time <NUM> corresponds to the diastole of a cardiac cycle of the individual and/or at least one point in time <NUM> corresponds to the systole of a cardiac cycle of the individual. The apparatus <NUM> is configured to determine an evaluation function <NUM> which is a function of the light intensities <NUM>;<NUM>. The external device <NUM> collects, by optical plethysmography on the sensor <NUM>, propagated light <NUM> corresponding to light <NUM> emitted by the light source <NUM>, wherein the light <NUM> is transmitted or reflected by the investigated volume <NUM> when propagating in the distal end of the digit of the individual <NUM> at the two or more points in time <NUM>;<NUM> along the optical plethysmography signal <NUM>. In other words, the external device <NUM> collects, by optical plethysmography on the sensor <NUM>, a light intensity <NUM> corresponding to propagated light <NUM> corresponding to light <NUM> emitted by the light source <NUM>, wherein the light <NUM> is transmitted or reflected by the investigated volume <NUM> when propagating in the distal end of the digit of the individual <NUM> and is collected on the sensor <NUM> at a first point in time <NUM>; and the external device <NUM> collects, by optical plethysmography on the sensor <NUM>, a light intensity <NUM> corresponding to propagated light <NUM> corresponding to light <NUM> emitted by the same light source <NUM>, wherein the light <NUM> is transmitted or reflected by the investigated volume <NUM> when propagating in the distal end of the digit of the individual <NUM> and is collected on the sensor <NUM> at a second point in time <NUM>. The apparatus then determines an evaluation function <NUM>. The evaluation function <NUM> for example corresponds to a logarithm of a function of the light intensities <NUM>;<NUM>. The evaluation function <NUM> for example corresponds to a logarithm of a ratio of the light intensities <NUM>;<NUM>. The evaluation function <NUM> depends on one or more of the following: an optical path length, a function of the oxygen saturation estimate, a change in arterial blood volume in the investigated volume.

<FIG> illustrates an example embodiment of a system <NUM> according to the present disclosure. Components having identical reference numbers to numbers on <FIG> or <FIG> perform the same function. The system <NUM> of <FIG> comprises an apparatus <NUM> according to the present disclosure. Optionally, the system <NUM> further comprises at least one light source <NUM> and a sensor <NUM>, comprised in the apparatus <NUM>. A light source <NUM> is configured to emit light <NUM>. The apparatus <NUM> is configured to:.

The apparatus <NUM> obtains the optical plethysmography signal <NUM> and/or the light intensities <NUM>;<NUM> from the light source <NUM> and/or the sensor <NUM>. For example, the apparatus <NUM> determines the arterial blood volume pulse in the investigated volume <NUM> of the individual <NUM>. The apparatus <NUM> comprises a battery for powering the different electrical components <NUM>;<NUM>;<NUM>. The light source <NUM> is configured to emit light, i.e. to transmit the light <NUM> into the investigated volume <NUM> of the individual attached to the apparatus <NUM>, for example into the finger <NUM> of the individual <NUM> as illustrated, more particularly to a distal end <NUM> of a finger of the individual as illustrated. The apparatus <NUM> further comprises control circuitry for controlling the light source <NUM>, i.e., for enabling or disabling the light source <NUM> and for receiving the measured arterial blood volume pulse values from the sensor <NUM>. Control circuitry may further comprise a memory component for temporarily storing the obtained measurements. Control circuitry is further coupled to a wireless interfacing circuitry <NUM> and configured to forward the measurements to the wireless interfacing circuitry <NUM>. Wireless interface <NUM> may support a short range and/or low power wireless communication protocol for efficient transmission of the measurements to a receiving part of the system. Wireless interface <NUM> may for example operate according to the Bluetooth Low Energy, BLE, protocol as defined by the Bluetooth Special Interest Group or according to a Near Field Communication, NFC, protocol. Operation by such protocols together with forwarding of the raw optical plethysmography signal <NUM> allows miniaturization of the apparatus <NUM> such that it fits on a finger or a nostril and allows operation during multiple nights. According to an alternative embodiment, the apparatus <NUM> obtains one or more of the optical plethysmography signal <NUM>, the light intensities <NUM>;<NUM> from the memory <NUM>. According to a further alternative embodiment, the apparatus <NUM> obtains one or more of the optical plethysmography signal <NUM>, the light intensities <NUM>;<NUM> from the light source <NUM> and/or the sensor <NUM> and/or from the memory <NUM>. Optionally, at least one point in time <NUM> corresponds to the diastole of a cardiac cycle of the individual and/or at least one point in time <NUM> corresponds to the systole of a cardiac cycle of the individual. The apparatus <NUM> is configured to determine an evaluation function <NUM> which is a function of the light intensities <NUM>;<NUM>. The apparatus <NUM> collects, by optical plethysmography on the sensor <NUM>, propagated light <NUM> corresponding to light <NUM> emitted by the light source <NUM>, wherein the light <NUM> is transmitted or reflected by the investigated volume <NUM> when propagating in the distal end of the digit of the individual at the two or more points in time <NUM>;<NUM> along the optical plethysmography signal <NUM>. In other words, the apparatus <NUM> collects, by optical plethysmography on the sensor <NUM>, a light intensity <NUM> corresponding to propagated light <NUM> corresponding to light <NUM> emitted by the light source <NUM>, wherein the light <NUM> is transmitted or reflected by the investigated volume <NUM> when propagating in the distal end of the digit of the individual <NUM> and is collected on the sensor <NUM> at a first point in time <NUM>; and the apparatus <NUM> collects, by optical plethysmography on the sensor <NUM>, a light intensity <NUM> corresponding to propagated light <NUM> corresponding to light <NUM> emitted by the same light source <NUM>, wherein the light <NUM> is transmitted or reflected by the investigated volume <NUM> when propagating in the distal end of the digit of the individual <NUM> and is collected on the sensor <NUM> at a second point in time <NUM>. The apparatus then determines an evaluation function <NUM>. The evaluation function <NUM> for example corresponds to a logarithm of a function of the light intensities <NUM>;<NUM>. The evaluation function <NUM> for example corresponds to a logarithm of a ratio of the light intensities <NUM>;<NUM>. The evaluation function <NUM> depends on one or more of the following: an optical path length, a function of the oxygen saturation estimate, a change in arterial blood volume in the investigated volume.

<FIG> illustrates an example comparison between PPG-amplitude-based peripheral arterial tone <NUM> both in function of time <NUM> of an individual, wherein the PPG-amplitude-based peripheral arterial tone <NUM> is assessed by considering changes in amplitude of pulses in an optical plethysmography signal as described in the prior art, and evaluation-function-based peripheral arterial tone <NUM> in function of time <NUM> of the same individual, wherein the evaluation-function-based peripheral arterial tone <NUM> is determined by the computer-implemented method according to the present disclosure, or by the apparatus according to the present disclosure, i.e. wherein the evaluation-function-based peripheral arterial tone <NUM> is assessed by determining a logarithm of a ratio of light intensities measured from an optical plethysmography signal. For clarity reasons, the evaluation-function-based peripheral arterial tone <NUM> is plotted by calculating <MAT>. According to an alternative embodiment, the evaluation-function-based peripheral arterial tone <NUM> is plotted by calculating <MAT>. As can be seen on <FIG>, the PPG-amplitude-based peripheral arterial tone <NUM> and the evaluation-function-based peripheral arterial tone <NUM> evolve in a similar manner as a function of time <NUM> at a pre-vasoconstriction-event baseline value. But during the time periods <NUM> and <NUM>, i.e. during occurrence of an event such as for example a sleep-related event, it can be seen on <FIG> that the PPG-amplitude-based peripheral arterial tone <NUM> and the evaluation-function-based peripheral arterial tone <NUM> evolve in a similar manner but do not overlap anymore in the corresponding time periods <NUM> and <NUM>. Indeed, in period <NUM>, the PPG-amplitude-based peripheral arterial tone <NUM> drops to a level <NUM> which value is higher than the one of the level <NUM> to which the evaluation-function-based peripheral arterial tone <NUM> drops. Similarly, in period <NUM>, the PPG-amplitude-based peripheral arterial tone <NUM> drops to a level <NUM> which value is higher than the one of the level <NUM> to which the evaluation-function-based peripheral arterial tone <NUM> drops. It can be seen on <FIG> that assessing peripheral arterial tone <NUM> by determining a logarithm of a function of light intensities acquired by optical plethysmography allows to detect the event occurring more accurately. Indeed, a drop in peripheral arterial tone <NUM> is indicative for a vasoconstriction of the arteries and the arterioles in the investigated volume under monitoring. This vasoconstriction can be related to the occurrence of an event at the individual under monitoring such as for example a sleep-related event, such as for example sleep apnea. As it can be seen on <FIG>, the drop in the PPG-amplitude-based peripheral arterial tone <NUM> between the pre-vasoconstriction-event baseline value and the lowest point of the PPG-amplitude-based peripheral arterial tone <NUM> is smaller than the drop in the evaluation-function-based peripheral arterial tone <NUM> between the pre-vasoconstriction-event baseline value and the lowest point of the evaluation-function-based peripheral arterial tone <NUM>. For example, a predetermined threshold value <NUM> for peripheral arterial tone <NUM> can be used to detect whether an event is occurring at the individual under monitoring such as for example a sleep-related event, such as for example sleep apnea: when the peripheral arterial tone <NUM> is above this predetermined threshold value <NUM>, no event is detected, but when the peripheral arterial tone <NUM> is below this predetermined threshold value <NUM>, an event is detected. The drop in the PPG-amplitude-based peripheral arterial tone <NUM> between the pre-vasoconstriction-event baseline value and the lowest point of the PPG-amplitude-based peripheral arterial tone <NUM> is such that the PPG-amplitude-based peripheral arterial tone <NUM> stays above the predetermined threshold value <NUM>, which results in the absence of detection of an event occurring at the individual under monitoring such as for example a sleep-related event, such as for example sleep apnea. On the other hand, the drop in the evaluation-function-based peripheral arterial tone <NUM> between the pre-vasoconstriction-event baseline value and the lowest point of the evaluation-function-based peripheral arterial tone <NUM> is such that the evaluation-function-based peripheral arterial tone <NUM> drops below the predetermined threshold value <NUM>, which results in the detection of an event occurring at the individual under monitoring such as for example a sleep-related event, such as for example sleep apnea. It can therefore be seen that assessing peripheral arterial tone <NUM> by determining a logarithm of a function of light intensities acquired by optical plethysmography allows to more accurately and more robustly detect the occurrence of an event occurring at the individual under monitoring such as for example a sleep-related event, such as for example sleep apnea.

<FIG> illustrates an example embodiment of a computer-implemented method for assessing peripheral arterial tone, PAT, of an individual monitored by optical plethysmography, wherein said method comprises the steps of:.

<FIG> shows a suitable computing system <NUM> enabling to implement embodiments of the system. Computing system <NUM> may in general be formed as a suitable general-purpose computer and comprise a bus <NUM>, a processor <NUM>, a local memory <NUM>, one or more optional input interfaces <NUM>, one or more optional output interfaces <NUM>, a communication interface <NUM>, a storage element interface <NUM>, and one or more storage elements <NUM>. Bus <NUM> may comprise one or more conductors that permit communication among the components of the computing system <NUM>. Processor <NUM> may include any type of conventional processor or microprocessor that interprets and executes programming instructions. Local memory <NUM> may include a random-access memory (RAM) or another type of dynamic storage device that stores information and instructions for execution by processor <NUM> and/or a read only memory (ROM) or another type of static storage device that stores static information and instructions for use by processor <NUM>. Input interface <NUM> may comprise one or more conventional mechanisms that permit an operator or user to input information to the computing device <NUM>, such as a keyboard <NUM>, a mouse <NUM>, a pen, voice recognition and/or biometric mechanisms, a camera, etc. Output interface <NUM> may comprise one or more conventional mechanisms that output information to the operator or user, such as a display <NUM>, etc. Communication interface <NUM> may comprise any transceiver-like mechanism such as for example one or more Ethernet interfaces that enables computing system <NUM> to communicate with other devices and/or systems, for example with other computing devices <NUM>, <NUM>, <NUM>. The communication interface <NUM> of computing system <NUM> may be connected to such another computing system by means of a local area network (LAN) or a wide area network (WAN) such as for example the internet. Storage element interface <NUM> may comprise a storage interface such as for example a Serial Advanced Technology Attachment (SATA) interface or a Small Computer System Interface (SCSI) for connecting bus <NUM> to one or more storage elements <NUM>, such as one or more local disks, for example SATA disk drives, and control the reading and writing of data to and/or from these storage elements <NUM>. Although the storage element(s) <NUM> above is/are described as a local disk, in general any other suitable computer-readable media such as a removable magnetic disk, optical storage media such as a CD or DVD, -ROM disk, solid state drives, flash memory cards,. could be used. Computing system <NUM> could thus correspond to the apparatus <NUM> in the embodiment illustrated by <FIG> or <FIG> or <FIG>.

Claim 1:
A computer-implemented method for assessing peripheral arterial tone (<NUM>), PAT, of an individual (<NUM>) monitored by optical plethysmography, wherein said method comprises:
- obtaining:
∘ an optical plethysmography signal (<NUM>) measured at an investigated volume (<NUM>) of said individual (<NUM>) by a sensor of an optical plethysmography setup; and
∘ light intensities (<NUM>;<NUM>) acquired by optical plethysmography at two or more points in time (<NUM>;<NUM>) along said optical plethysmography signal (<NUM>); and
- determining changes in arterial blood volume (<NUM>) in said investigated volume (<NUM>) between said two or more points in time (<NUM>;<NUM>) by determining an evaluation function that determines a logarithm (<NUM>), or a function approximation thereof (<NUM>), of a ratio of said light intensities (<NUM>;<NUM>), thereby assessing PAT (<NUM>) of said individual (<NUM>); said two or more points in time including at least one first point in time corresponding to a systole or to a diastole in a cardiac cycle and at least one second point in time corresponding to any point in time within the same cardiac cycle or within a different cardiac cycle; and
- determining a drop in said PAT (<NUM>) indicative for a vasoconstriction in said investigated volume (<NUM>), by determining a drop in value of the evaluation function that determines the logarithm (<NUM>), or the function approximation thereof, of the ratio of said light intensities, below a predetermined threshold value.