EP4009859A1

EP4009859A1 - A pressure cuff for use in a non-invasive blood pressure measurement device

Info

Publication number: EP4009859A1
Application number: EP20757055.7A
Authority: EP
Inventors: Jacobus Jozef Gerardus Maria Settels; Alexander H. Siemons
Original assignee: Edwards Lifesciences Corp
Current assignee: Edwards Lifesciences Corp
Priority date: 2019-08-06
Filing date: 2020-07-27
Publication date: 2022-06-15
Also published as: CN114126483A; WO2021025885A1

Abstract

The present invention relates to a pressure cuff for use in a non-invasive blood pressure measurement device, comprising: a bladder, for applying pressure to a body part; at least one light source, for sending light through the body part under the bladder; at least two light detectors, each for detecting light passed through the body part and each for providing a signal in dependence of the amount of detected light; wherein the at least one light source and at least one of the at least two light detectors are arranged in pairs, in use substantially on opposite sides of the body part; wherein the at least two light detectors and the at least one light source are arranged on the inside of the pressure cuff, and are spaced apart; wherein the at least two light detectors are configured to detect light emitted from a single light source at a time.

Description

A pressure cuff for use in a non-invasive blood pressure measurement device

The present invention relates to a pressure cuff for use in a non-invasive blood pressure measurement device.

It has been known for several years to measure the continuous non-invasive blood pressure waveform wherein a pressure cuff is placed around a body extremity, such as a finger. EP 0 048 060 for instance describes that the pressure of a fluid inside the pressure cuff is controlled on the basis on a signal of a plethysmograph by a pressure valve, in turn controlled by a servo control feedback loop.

The signal of the plethysmograph is representing the volume changes of blood inside the blood vessels of the finger under the cuff. The more blood, the more light from a light source of the plethysmograph is absorbed, which results in a lower signal of the plethysmograph (and vice versa). During every heartbeat, blood is propagated through the blood vessels in the finger. This blood flow also causes a blood volume increase of the vessels, and thus a signal decrease of the plethysmograph.

In the known method, the cuff pressure of the pressure cuff is servo controlled using a variable resistance proportional valve, such that the signal of the plethysmograph, and thus the volume of blood inside the blood vessels under the cuff, is kept constant. The pressure exerted on the blood vessel walls from the inside by the heart pulsations is continuously counteracted by a pressure exerted by the pressure cuff. When these two pressures are dynamically equal, this results in a constant diameter of the blood vessels and an unloading of the vessels. When the vessel wall is unloaded, the counter pressure exerted by the pressure cuff is a direct measure for the actual blood pressure inside the blood vessel, and allows for a continuous non-invasive blood pressure measurement.

It is an objective of the present invention to provide an improved pressure cuff for a non- invasive pressure blood pressure measurement.

The invention thereto provides a pressure cuff for use in a non-invasive blood pressure measurement device, comprising: a bladder, for placement around a body part such as a finger, for applying pressure to the body part; at least one light source, for sending light through the body part under the bladder; at least two light detectors, each for detecting light passed through the body part and each for providing a signal in dependence of the amount of detected light; wherein the at least two light detectors and the at least one light source are arranged on the inside of the pressure cuff, and wherein the at least two light detectors are spaced apart; wherein the at least two light detectors are configured to detect light emitted from a single light source at a time, and are each configured to detect light of the same wavelength. The at least one light source and at least one of the at least two light detectors may be arranged in pairs, in use substantially on opposite sides of the body part. Each pair comprises one light source and one light detector.

The light detectors are for instance spaced apart such that they follow an artery in the body part under the bladder. Typically the body part is a finger, and the detectors are spaced apart in longitudinal direction of the finger, from the proximal end of the finger towards the distal end (or tip) of the finger.

The bladder, which is wrapped around the body part such as the finger, applies pressure to the body part which counteracts the pressure fluctuations of the arteries inside the body part that are caused by the heart. The counteracting pressure is applied in dependence of the amount of light received by the light receiver. The bladder, applying pressure to the body part, in particular the finger, typically has a certain pressure transfer profile within the body part tissues from the outside - the skin - to the inside, which typically means the outside of the wall of the blood vessel within the body part. For instance, such transfer profile has a full (meaning 1 :1) transfer in the middle of the bladder, whereas the transfer on the outer edges of the bladder is significantly less. The bladder is typically used to put the walls of the arteries inside the body part into an unloaded state, in which for instance the pressure exerted on the inside and outside of the arterial wall is substantially the same. This unloaded state requires a certain counter pressure, which may for instance be achieved at certain locations of the bladder only, typically at the centre, or in the middle, of the bladder.

In the middle of the cuff, the arteries of the body part are thus typically brought into an unloaded state, wherein the pressure exerted by the bladder equals intra-arterial pressure, which pressure typically is high and closes the veins of the same body part in the middle of the cuff. On the distal end of the cuff, where the bladder typically does not apply the same pressure, the veins will be left open at least partly, such that in these veins pressure may build up due to entry of blood through the open arteries, but a closed return vein in the middle of the bladder. Secondly, on the proximal end of the cuff the arteries may not be brought completely in the unloaded state, such that they may pulsate in diameter upon beating of the heart. Both effects could disturb an optimal measurement of the arterial volume relative to the counter-pressure applied by the cuff, and thus could disturb an optimal measurement of the blood pressure with the cuff.

The first effect will increase in time when an unloading cuff is applied to the body part, typically a finger. Because the veins are closed in the middle under the cuff, the finger tip distal to the cuff will increase its volume and the arteriolar, capillary and venular beds will gradually fill up and increase their pressure from capillary pressure to mean arterial pressure of the unloaded artery under the cuff.

A third effect is the flow of blood and thus light absorbing red blood cells when the artery is unloaded. When unloaded, the artery is always open - at its unloaded diameter - and with every heart beat blood is flowing in - to the finger tip - during systole and back out - from the finger tip - during diastole because of the positive and negative pressure gradients during these phases.

Additionally, the plethysmograph system may be used to provide input to the servo system maintain an unloaded state of the arteries inside the body part under the cuff. To do so, light is emitted from a light source, which light is influenced - scattered and partly absorbed - by the different tissues - including blood - inside the body part. The amount of light subsequently detected by the light receiver is typically less than the amount of light emitted and is a measure of the volume of blood in the blood vessels - arteries and veins - in the body part under the cuff which absorbs or scatters the emitted light. Such scattering for instance occurs upon tissue interaction, when light for instance encounters cell nuclei and mitochondria inside the body part. Scattering in turn increases the distance the light has to travel, which increases likelihood of encountering other tissues and thus the likelihood of being absorbed.

The pressure cuff relies upon multiple mutually spaced light detectors, and preferably one or multiple spaced light sources. By applying multiple light detectors, and/or sources, for instance spaced longitudinally over the pressure cuff and/or along the artery of the body part, the absorption of light in the body part under the cuff may be determined, by measuring where and how much of the light emitted from one light source is received by the multiple detectors. The light detectors may also be arranged in an array of light detectors, to determine where light emitted from the light sources is received by the multiple detectors. The pressure cuff may further comprise at least two light sources, each for sending light through the body part under the bladder; wherein the two light sources and the two light detectors are arranged in pairs.

With substantially opposite sides of the body part according to the invention is meant that, in a cross-sectional view of the body part each of the elements of the pairs are located on two more or less opposite sides of the body part. Where one of the elements of the pair would for instance be located on the left side, or left half of the cross-sectional view, the other element of the pair would be located on the right side, of right half of the cross-sectional view.

Typical cuff application sites include a phalanx of a human finger, a section of a human upper arm, a human wrist or an animal tail as body part.

In a particular scenario the body part is a finger, and typically is an index or middle finger. The light detector and light source, forming a pair, are located on opposite sides of the finger. For instance, when the index finger is seen from above in stretched position, the light sources are located on the right and the light detectors on the left. In a cross-sectional view of such configuration, seen from the tip of the finger (or distal end of the finger), the light sources are on the left side of the cross-sectional view, and the light detectors are on the right side of the cross-sectional view.

The light sources may emit light of substantially the same wavelength. When each of the light sources of the pressure cuff emits light of the same wavelength the scattering or absorbance of light passing through the body part may be investigated, irrespective of the potential scattering and absorbing differences between light emitted of different wavelengths. Each of the light detectors is further arranged to detect light emitted from the light source(s).

The pressure cuff may for instance comprise at least three light sources and at least three light detectors. More sources and more detectors increases the amount of signals and increases the resolution and thus accuracy of the determination of the arterial volume. Three sources and three detectors furthermore is a practical amount which could still be distributed properly over a pressure cuff to be used around a finger. Each light detector may be arranged to measure the light passed through the body part of each of the light sources. It is expected that a light detector that forms a pair with an emitting light source, or the light detector closest to the light source, receives the majority of the light passing through the body part, and that the other light detectors receive less light from the light source, mainly because the light will need to travel longer to the other light detectors, and is thus subjected to more scattering and absorbance. In a scenario in which all light detectors measure approximately the same amount of light, which light is emitted from a single light source, it is likely that light is not passing through the body part, or finger, in transmission mode but is passing around the body part in reflection mode between the skin and the cuff. In such light piping scenario the signals may be used to determine whether or not the pressure cuff is wrapped around the body part sufficiently tight.

The wavelength of the light sent or emitted by the light sources may lie around the isobestic point of haemoglobin, and in particular is about 800nm. With about 800nm according to the invention is meant wavelengths around 800nm. Around 800nm wavelength the effective attenuation coefficient of haemoglobin molecules, a main constituent of red blood cells, is independent, or at least relatively independent, on the oxygenation thereof. Wavelengths falling within the scope of about 800nm include for instance wavelengths between 750 and 950nm, in particular between 750 and 850nm.

The light sources may be configured for providing a coherent bundle of light, and for instance comprise coherent light emitting diodes and/or lasers. A coherent bundle is different from a normal bundle of light in that the range of wavelengths is smaller, and in particular photons of a single wavelength are emitted. Additionally, when using lasers for instance, emitted light does not diverge, or at least not as much. Less divergence results in less scattering of light in the body part and should thus result in less contamination of received light from the light detectors which do not form a pair with the emitting light source.

The light detectors may be configured to for providing a signal in dependence of the amount of detected light of a certain wavelength, in particular in dependence of a certain range of wavelengths only. This range of wavelengths may for instance coincide with the range of wavelengths, or the single wavelength, as emitted by the at least one light source.

The present invention further relates to a pressure system for use in a non-invasive blood pressure measurement device, comprising: a pressure cuff according to the invention; an actuator, such as a pump, for supplying pressurized fluid to the bladder; a variable flow resistance, located between the actuator and the bladder, to vary the pressure supplied to the bladder by the actuator; and a controller, arranged to provide a controller signal based on the signals of each of the light detectors. If the actuator is made variable, there is no need for variable flow resistance.

The controller of the pressure system may be arranged to provide the controller signal based on the light detected by one pair of light detector and light source of the pressure cuff, and correct this signal for the light detected by each of the other light detectors. The light detected by each of the other detectors may for instance be multiplied with a constant value as weight factor, and subsequently be subtracted from the light detected by the light detector forming a pair with the light source. The correction for instance includes the step of providing a new signal, based on a constant times the pair forming detector minus another constant times the detector signal(s) not forming a pair.

The controller of the pressure system may be arranged to continuously control the variable flow resistance and/or the actuator, based on the signals of the light detector and/or the controller signal.

The controller may be arranged to calculate a set-point based on the signals of the light detectors. The signal of an optical plethysmograph is representing the volume of blood inside the blood vessels under the cuff of the finger. The more blood inside the vessels, the more light from a light source of the plethysmograph is absorbed, which results in a lower signal of the detector side of the plethysmograph (and vice versa). During every heartbeat, blood is forced through the blood vessels in the finger, causing the vessels to expand and allow more blood to flow through the vessels. This also causes a volume increase of the vessels, and thus a signal decrease of the plethysmograph.

The cuff pressure of the pressure cuff is controlled such that the signal of the plethysmograph, and thus the volume of blood inside the blood vessels, is kept constant. The pressure exerted by the heart on the internal blood vessel walls is continuously counteracted by a pressure exerted by the pressure cuff on the external blood vessel walls, which results in a constant diameter of the blood vessels and an unloading of the vessels. When the artery is unloaded, the counter pressure exerted by the pressure cuff is a measure for the actual blood pressure inside the blood vessel, and allows for a continuous non-invasive blood pressure measurement.

This control is arranged as a servo feedback system such that at any moment the difference between a servo reference level or setpoint value for the diameter of the blood vessels and the actual plethysmograph signal or real value is minimized, ideally to zero. The servo-reference level in the known method is initially determined automatically and the servo feedback control is operated in a way such that the cuff pressure continuously corresponds substantially with the momentary arterial pressure under the cuff both for pulsations and for absolute pressure level.

The method requires correction of the reference or set point value over time. This correction is required mainly due to changes in the physiological status of the measured body part. US 4,510,940 for instance describes a method and a device for correcting the cuff pressure in the indirect, non-invasive and continuous measurement of the blood pressure in a part of the body by using a plethysmograph in a fluid-filled pressure cuff, an electronic control circuit, and an electric pressure valve. The cuff pressure is controlled by the plethysmograph signal in closed-loop operation with the aid of a servo- reference level obtained via a memory circuit. The servo-reference level, in operation of the device, is adjusted by opening the closed loop of the control circuit for a short interval, after which, in open-loop operation the cuff pressure is adjusted at an intermediate pressure derived from the pressure last measured and the servo-reference level is adjusted via the memory circuit.

The set point determination may be improved by restricting the light receiver signals to the areas of the body part that are actually unloaded. This area is typically the middle or centre of the inflatable cuff. When for instance one central light source is used, the light detector directly opposite this light source would receive light information from the center, which would correspond to the area of the artery which is unloaded, but also from areas more distal or proximal, corresponding to areas of the artery which are not fully unloaded, for instance because the bladder is not large enough to unload the full area of the body part under the cuff or because the veins under the cuff are not fully collapsed and contribute to the absorption of light. By correcting the light signal for light received at the proximal and/or distal edges of the cuff, and thus using light signals predominantly from the centre of the cuff, one may improve the determination of the set point and the performance of the servo control, and thus improve the blood pressure determination.

The present invention further relates to a method for measuring a volume of blood flowing through a body part, comprising the steps of: placing a pressure cuff according to the invention around a body part; sending light of one of the light sources through the body part; detecting the light, sent by the single light source, with at least two light detectors, wherein each of the light detectors provides a signal based on the detected light; providing the signal of the light detector closest to the emitting light source, and correcting for, or subtracting, the signal of at least one of the other light detectors; and determining a volume of blood flowing through the body part based on the corrected or subtracted signal.

Correction or subtraction in the method may also comprise the step of processing the signals of the at least one of the light detectors before correction or subtracting. Typically one of the light detectors is arranged opposite to the at least one light source, wherein the other light detectors are arranged at an angle from the at least one light source. The light detector opposite to the light source provides the main signal, wherein the light detectors at an angle provide the signal which is to be subtracted (or corrected for). The light detector and light source providing the main signal are considered to form a pair according to the invention.

The method may further comprise the step of repeating, for each light source, the steps of sending light of one of the light sources through the body part; detecting the light, sent by the single light source, with at least two light detectors, wherein each of the light detectors provides a signal based on the detected light; correcting for, or subtracting the signal of the light detectors, which do not form a pair with the single light source from the signal of the light detector which does form a pair with the single light source; and determining a volume of blood flowing through the body part based on the subtracted signal. A cuff, or pressure cuff, used in non-invasive blood pressure measurements may comprise an inflatable bladder provided with an air supply channel for inflating the bladder and for evacuating the bladder. The air channel may for instance be connected to the bladder using a suitable fitting. The inflatable bladder may comprise a top layer arranged to be brought into contact with the body part of the person to be measured, typically a finger. The inflatable bladder may further comprise a back layer attached to a flexible printed circuit. The top layer, for instance made of polyurethane (PU), it typically more elastic than the back layer, for instance made of polyvinyl chloride (PVC), which could even be non-elastic.

The inflatable bladder may further comprise at least one, or at least two cut-away areas, to accommodate or position light sources, and at least two cut-away areas to accommodate or position light detectors. The back layer may comprise corresponding cut-away areas for accommodating the light sources and detectors. The printed circuit may for instance be connected to a signalling cable, which may be provided with a suitable connector. The cable and an air supply for the air supply channel may for instance be located in a housing outside the pressure cuff. The printed circuit may for instance comprise an identification unit and/or a module for processing signal received from the light detectors, which module may further be arranged to perform signal processing steps, such as filtering or amplification, on the received signals.

The pressure cuff may further comprise fastening means, such as Velcro, to wrap the cuff around a body part, typically around a finger. The cuff may also be placed on an ear, or on the temples of a person, in a nostril or in a body cavity.

The invention will be explained by means of the non-limiting working examples depicted in the following figures. Specifically:

- figure 1 schematically shows a device for non-invasive blood pressure measurements according to the prior art;

- figure 2 schematically shows a pressure cuff according to the present invention;

- figure 3 schematically shows a cross sectional view of a pressure cuff according to the present invention wrapped around a finger;

- figure 4 schematically shows a side view on a pressure cuff according to the present invention wrapped around a finger; - figure 5 schematically shows the pressure cuff of figure 4, with one light source emitting light;

- figure 6 schematically shows the effective attenuation coefficient of haemoglobin, in its oxygenated and unoxygenated form;

- figure 7 schematically shows a pressure system for use in a non-invasive blood pressure measurement device according to the present invention; and

- figure 8 schematically shows the effects of light received by multiple light detectors;

Figure 1 schematically shows a device (1) for non-invasive blood pressure measurements according to the prior art, comprising a pressure cuff (2), which generates a signal (3) based on the detected light. This signal (3), representative for the volume of blood of the arteries under the cuff in the finger (4) is compared to a set-point (5) by a comparator (6), which comparison is then communicated to a controller (7). Based on the information, the controller (7) in turn controls a control valve (8). The valve (8) regulates the pressure supplied to the pressure cuff (2) by a pump (9). The pressure supplied to the pressure cuff (2) is measured by a transducer (10).

The present invention uses a similar device for non-invasive blood pressure measurements, but with an improved pressure cuff.

Figure 2 schematically shows a pressure cuff (11) according to the present invention for use in a non-invasive blood pressure measurements. The cuff (11) comprises a bladder (12), for placement around a body part and applying pressure to the body part. The cuff (11 ) shown in figure 2 is one typically used for a finger as the body part. The cuff (11 ) further comprises three light sources (13, 13’, 13”) as well as three light detectors (14, 14’, 14”), each for detecting light and providing a signal in dependence of the amount of detected light. The light sources (13, 13’, 13”) and light detectors (14, 14’, 14”) are arranged in pairs, and in use are substantially on opposite sides of the body part, or finger.

The light detectors (14, 14’, 14”) and the light sources (13, 13’, 13”) are arranged on the inside of the pressure cuff (11), and are spaced apart. The inside of the pressure cuff (11 ) is the side of the cuff (11 ) which, in use, is facing towards the body part, or finger. The light sources (13, 13’, 13”) emit light of the same wavelength, which wavelength is about 800nm.

The light detectors (14, 14’, 14”) and the light sources (13, 13’, 13”) are arranged on a flexible printed circuit (15), which in turn is arranged on an outer wrap (16). This outer wrap (16) is further provided with Velcro (17), for wrapping the outer wrap (16) around the body part, and securing the outer wrap (16). The bladder (12) is arranged on top, or on the inside of, the outer wrap (16) and the flexible printed circuit (15). The bladder (12) is further connected to a fluid supply (18) via a connector (19).

Figure 3 schematically shows a cross section of a pressure cuff (11) wrapped around a finger (F). The finger (F) is schematically represented by muscle (M), bone (B) and two arteries (A). The pressure cuff (11) comprises a bladder (12), wrapped around the finger (F). In the cross sectional view, the cuff (11) further comprises a light sources (13) as well as a light detector (14) for detecting light emitted from the light source (13) and passed through the finger (F), and for providing a signal in dependence of the amount of detected light. The light source (13) and light detector (14) are arranged in a pair, and are located on opposite sides of the finger (F). In the shown cross section, the light source (13) is located on the right side, and the light detector (14) is located on the left side. The light source (13) and light detector (14) are not on exact opposite locations, but are on opposite sides. Compared to a horizontal (H) for instance, or a centre line through the finger (F), the light source (13) and light detector (14) are for instance both placed at an angle (a), typically between 0 and 45 degrees, in particular about 20 degrees.

Figure 4 schematically shows a side view on a pressure cuff (11) wrapped around a finger (F). Figure 4 shows that the light sources (13, 13’, 13”) and light detectors (14,

14’, 14”) are spaced apart, in this case in longitudinal direction of the finger (F).

Figure 5 shows the pressure cuff (11 ) of figure 4, wherein one of the light sources (13) emits light, of a single wavelength. This light is sent through the finger (F) located inside the cuff (11), and may be scattered or absorbed in the finger (F). Typically, the majority of the emitted light is received by the light detector (14) opposite from the light source (13), which forms a pair with the light source (13). Light which is received by the light detectors (14’, 14”) not forming a pair with the light source (13), may be disregarded, or used to correct for scattering and absorption of light coming from proximal and/or distal regions of not fully unloaded arteries.

Figure 6 schematically shows the effective attenuation coefficient of haemoglobin, in its oxygenated and unoxygenated form. Around 800nm wavelength the effective attenuation coefficient of haemoglobin, a main constituent of blood, is independent, or at least relatively independent, on the oxygenation thereof, since the effective attenuation coefficient of oxygenated and unoxygenated haemoglobin is approximately the same. This is called the isosbestic range. Wavelengths falling within the scope of about 800nm include for instance wavelengths between 750 and 950nm, in particular between 750 and 850nm.

Figure 7 schematically shows a pressure system (21) for use in a non-invasive blood pressure measurement device, comprising: a pressure cuff (11) according to the present invention; an actuator (22), such as a pump, for supplying pressurized fluid to the bladder (12); a variable flow resistance (23), located between the actuator (22) and the bladder (12), to vary the pressure supplied to the bladder (12) by the actuator (22); and a controller (24), arranged to provide a setpoint and a controller signal based on the signals of each of the light detectors (14) of the pressure cuff (11). The controller (24) may further be used to control the variable flow resistance (23), based on the signals of each of the light detectors (14) of the pressure cuff (11).

Figure 8 schematically shows the effects of light received by multiple light detectors (14, 14’, 14”), in a cuff (11) as shown in figures 4 and 5, wherein corresponding features have been given the same reference numerals. In the finger (F) schematically the artery (A) and vein (V) are indicated. In the tip of the finger (F), most right in figure 8, small arterioles and capillaries are indicated as small vessels (S). The area (B) of the artery (A) that is intended to be unloaded by the cuff (11) is encircled (B) in figure 8. The unloading of the artery (A) causes the smaller vein (V) to close or collapse at the same location, such that the flow of blood from the veins is only outwardly, and flow of blood through the artery is both in and out.

The middle detector (14) will, due to the collapse of the vein (V), detect light emitted from the light source (13) and influenced by the fully unloaded artery. The left detector (14’) will receive light from the light source (13), but will be influenced by the artery (not fully unloaded) on the left of area (B) ans also be influenced by the vein not fully collapsed to the left of area (B). The right detector (14”) will receive light from the light source (13), but will be influenced by the vein (not fully collapsed) on the right of area (B), and likely be influenced by the artery (not fully unloaded) on the right of area (B) as well.

The left detector (14’) for instance receives a mixture of signals from the unloaded artery, a part of the loaded artery and possibly a part of the collapsed vein. The middle detector (14) receives a mixture of signals from the unloaded artery, a part of the loaded artery and possibly a part of the open vein. The right detector (14”) receives a mixture of signals from the unloaded artery and a part of the open vein and possibly a part of the loaded artery.

By taking the signal received by the middle detector (14), and subtracting or correcting for the signal of the left detector (14’) a compensation for the left part of area (B) can be made. By taking the signal received by the middle detector (14), and subtracting or correcting the signal of the right detector (14”) a compensation for the right part of area (B) can be made. The signal used, for instance for determining the set point of the system, for instance be 2 times the middle signal minus the left signal minus the right signal, wherein it is also envisioned that each of the signals is provided with their own, tailored, weighing factor.

Figure 10 schematically shows an example of the volume measurements made with a cuff according to the invention, when the counter pressure in de cuff follows a ramp-like procedure from zero to above systolic pressure, as shown as the straight line in the bottom graph. Figure 10 employs a system with a single emitter (in the middle of the cuff) and 3 sensors. Top panel shows signals from the middle sensor (M), the proximal sensor (P), the distal sensor (D). On the Y-axis light measurements in arbitrary measures are shown. Individual pressure-volume loops are taken at points 1 ,2, 3, 4, 5 and shown in figure 11.

Figure 11 shows the PV loops as the volume signal against transmural pressure (which is intra-arterial pressure minus cuff pressure). The individual heart beats at moments 1 ,2, 3, 4, 5 in figure 10 are highlighted in black. They show the detailed information used to calculate the setpoint value, and also show the considerable hysteresis which is typical for human arteries. Figure 12 shows a typical example of the correction information obtained with a cuff according to this invention. Correction signals include differential distal-center (A, on top) and proximal-center (B, on bottom).

It will be apparent that the invention is not limited to the exemplary embodiments shown and described here, but that within the scope of the appended claims numerous variants are possible which will be self-evident to the skilled person in this field.

Claims

1 . A pressure cuff for use in a non-invasive blood pressure measurement device, comprising: a. a bladder, for placement around a body part such as a finger, for applying pressure to the body part; b. at least one light source, for sending light through the body part under the bladder; c. at least two light detectors, each for detecting light passed through the body part and each for providing a signal in dependence of the amount of detected light; d. wherein the at least two light detectors and the at least one light source are arranged on the inside of the pressure cuff, and wherein the at least two light detectors are spaced apart; e. wherein the at least two light detectors are configured to detect light emitted from a single light source at a time, and are each configured to detect light of the same wavelength.

2. Pressure cuff according to claim 1 , comprising at least two light sources, each for sending light through the body part under the bladder; wherein the two light sources and the two light detectors are arranged in pairs.

3. Pressure cuff according to claim 2, wherein the two light sources emit light of substantially the same wavelength.

4. Pressure cuff according to any of the preceding claims, comprising at least three light sources and at least three light detectors.

5. Pressure cuff according to any of the preceding claims, wherein each light detector is arranged to measure the light passed through the body part of each of the light sources.

6. Pressure cuff according to any of the preceding claims, wherein the wavelength of the light sent by the light sources lies around the isosbestic point of haemoglobin, and in particular is about 800nm.

7. Pressure cuff according to any of the preceding claims, wherein the light sources are configured for providing a coherent bundle of light, and for instance comprise coherent light emitting diodes and/or lasers.

8. A pressure system for use in a non-invasive blood pressure measurement device, comprising: a. a pressure cuff according to any of the preceding claims; b. an actuator, such as a pump, for supplying pressurized fluid to the bladder; c. a variable flow resistance, located between the actuator and the bladder, to vary the pressure supplied to the bladder by the actuator; d. a controller, arranged to provide a controller signal based on the signals of each of the light detectors.

9. Pressure system according to claim 8, wherein the controller is arranged to provide the controller signal based on the light emitted and detected by one pair of light detector and light source of the pressure cuff, and subtract or correct for the light detected by each of the other light detectors.

10. Pressure system according to claim 8 or 9, wherein the controller is arranged to continuously control the variable flow resistance and/or the actuator, based on the signals of the light detector and/or the controller signal.

11. Pressure system according to any of claims 8-10, wherein the controller is arranged to calculate a set-point based on the signals of the light detectors.

12. Method for measuring a volume of blood flowing through a body part, comprising the steps of: a. placing a pressure cuff according to any of claims 1-7 around a body part; b. sending light of one of the light sources through the body part; c. detecting the light, sent by the single light source, with at least two light detectors, wherein each of the light detectors provides a signal based on the detected light; d. providing the signal of the light detector closest to the emitting light source, and correcting for the signal of at least one of the other light detectors; e. determining a volume of blood flowing through the body part based on the subtracted or corrected signal.

13. Method according to claim 12, comprising the step of: e. repeating steps b-e for each light source 14. Method according to claim 12 or 13, wherein step d) comprises the step of processing the signal of the at least one of the other light detectors before subtracting the signal.