CN114126483A - Pressure cuff for use in a non-invasive blood pressure measurement device - Google Patents

Abstract

The invention relates to a pressure cuff for use in a non-invasive blood pressure measuring device, comprising: a bladder for applying pressure to a body part; at least one light source for sending light through a body part below the balloon; at least two light detectors, each light detector for detecting light passing through the body part and each light detector for providing a signal in dependence on the amount of light detected; 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 disposed on an inner side of the pressure cuff and spaced apart; wherein the at least two light detectors are configured to detect light emitted from a single light source at a time.

Description

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

Technical Field

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

Background

For many years, it has been known to measure continuous non-invasive blood pressure waveforms, where a pressure cuff is placed around a body extremity (e.g., a finger). For example, EP 0048060 describes that the pressure of the fluid inside the pressure cuff is controlled by a pressure valve based on the signal of the plethysmograph, which pressure valve in turn is controlled by a servo control feedback loop.

The signal of the plethysmograph represents the volume change of the blood inside the blood vessel of the finger below the cuff. The more blood, the more light from the light source of the plethysmograph is absorbed, which results in a lower signal of the plethysmograph (and vice versa). During each heartbeat, blood propagates through blood vessels in the finger. This blood flow also causes the blood volume of the blood vessel to increase, causing the signal of the plethysmograph to decrease.

In the known method, the cuff pressure of the pressure cuff is servo-controlled using a variable impedance proportional valve, so that the signal of the plethysmograph and the blood volume inside the blood vessel below the cuff are kept constant. The pressure exerted on the vessel wall from the inside by the heart beat is continuously counteracted by the pressure exerted by the pressure cuff. When these two pressures are dynamically equal, this results in a constant vessel diameter and vessel decompression. When the vessel wall is relieved of pressure, the back pressure exerted by the pressure cuff is a direct measure of the actual blood pressure inside the vessel and allows continuous non-invasive blood pressure measurements.

Disclosure of Invention

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

The invention provides for this purpose a pressure cuff for use in a non-invasive blood pressure measuring device, the pressure cuff comprising: a bladder for placement around a body part (e.g., a finger) for applying pressure to the body part; at least one light source for sending light through a body part below the balloon; at least two light detectors, each light detector for detecting light passing through the body part and each light detector for providing a signal in dependence on the amount of light detected; wherein the at least two light detectors and the at least one light source are disposed on an inner side of the pressure cuff, and wherein the at least two light detectors are spaced apart; wherein at least two light detectors are configured to detect light emitted from a single light source at a time, and each light detector is configured to detect light of the same wavelength. At least one of the at least one light source and the at least two light detectors may be arranged in pairs, in use substantially on opposite sides of the body part. Each pair comprising a light source and a light detector.

For example, the light detectors are spaced such that they follow an artery in the body part below the balloon. Typically, the body part is a finger and the detector is spaced from a proximal end of the finger towards a distal end (or fingertip) of the finger in a longitudinal direction of the finger.

A balloon wrapped around a body part (e.g., a finger) applies pressure to the body part that counteracts pressure fluctuations of the arteries inside the body part caused by the heart. The counter-acting pressure is applied in dependence on the amount of light received by the light receiver. Balloons that apply pressure to body parts, in particular fingers, usually have a certain pressure transmission profile within the tissue of the body part from the outside-the skin-to the inside, which usually means the outside of the vessel wall within the body part. For example, such a transfer profile has a complete (meaning 1:1) transfer in the middle of the balloon, while the transfer on the outer edges of the balloon is significantly less. Balloons are commonly used to place the arterial wall inside a body part in a pressure-relieved condition, for example, in which the pressure exerted on the inside and outside of the arterial wall is substantially the same. This pressure relief condition requires a certain counter pressure, which may for example be achieved only at a specific location of the balloon, usually in the center or middle of the balloon.

Thus, in the middle of the cuff, the artery of the body part is normally in a pressure relief state, in which the pressure exerted by the balloon is equal to the internal arterial pressure, which is normally high and closes the vein of the same body part in the middle of the cuff. At the distal end of the cuff, the balloon will not normally exert the same pressure, and the veins will remain at least partially open, so that in these veins the pressure may increase due to the entry of blood through the open artery, but the return vein in the middle of the balloon is closed. Second, on the proximal end of the cuff, the arteries may not be fully in a pressure relief state, so that they may pulsate in diameter as the heart beats. Both effects may interfere with the optimal measurement of arterial volume relative to the back pressure applied by the cuff, and thus may interfere with the optimal measurement of blood pressure with the cuff.

When the pressure relief cuff is applied to a body part (typically a finger), the first effect increases over time. Since the vein is closed in the middle below the cuff, the fingertip distal to the cuff will increase its volume and the arterioles, capillaries and venous bed will gradually fill up and increase their pressure from the capillary pressure to the mean arterial pressure of the pressure relieved artery below the cuff.

The third effect is the flow of blood and the red blood cells that absorb light when the artery is depressurized. When relieved, the artery is always open at its relieved diameter and with each heartbeat, blood flows into the fingertips during systole and flows back out of the fingertips during diastole due to the positive and negative pressure gradients during these phases.

In addition, the plethysmograph system may be used to provide input to a servo system to maintain a pressure relief state of an artery inside the body portion below the cuff. For this purpose, light is emitted from a light source, which is influenced, scattered and partially absorbed by 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 amount of blood that absorbs or scatters the emitted light in the blood vessels (arteries and veins) in the body part below the cuff. This scattering occurs, for example, when tissue interactions, such as when light encounters nuclei and mitochondria inside a body part. Scattering in turn increases the distance light must travel, which increases the likelihood of encountering other tissue, thereby increasing the likelihood of being absorbed.

The pressure cuff relies on a plurality of light detectors spaced from one another and preferably on one or more spaced light sources. By applying a plurality of light detectors and/or light sources (e.g. spaced longitudinally above the pressure cuff and/or along an artery of the body part), the absorption of light in the body part below the cuff can be determined by measuring where and how much light emitted from one light source is received by the plurality of detectors. The light detectors may also be arranged in a light detector array to determine where light emitted from the light source is received by the plurality of detectors. The pressure cuff may further comprise at least two light sources, each light source for sending light through a body part below the balloon; wherein the two light sources and the two light detectors are arranged in pairs.

According to the invention, substantially opposite sides of the body part means that, in a cross-sectional view of the body part, each of the elements of a pair is located on two more or less opposite sides of the body part. For example, where one of the pair of elements is located on the left or left half of the cross-sectional view, the other element of the pair will be located on the right or right half of the cross-sectional view.

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

In certain cases, the body part is a finger, and typically an index finger or a middle finger. A light detector and a light source forming a pair are located on opposite sides of the finger. For example, when looking at the index finger in the extended position from above, the light source is on the right side and the light detector is on the left side. In the cross-sectional view of this configuration, the light source is on the left side of the cross-sectional view and the light detector is on the right side of the cross-sectional view, as viewed from the fingertip of the finger (or the distal end of the finger).

The light sources may emit light of substantially the same wavelength. When each light source of the pressure cuff emits light of the same wavelength, scattering or absorption of light passing through the body part can be investigated despite potential scattering and absorption differences between the emitted light of different wavelengths. Each light detector is further arranged to detect light emitted from the light source.

For example, the pressure cuff may contain at least three light sources and at least three light detectors. More light sources and more detectors increase the amount of signal and improve resolution, thereby improving the accuracy of determining the arterial volume. Furthermore, three light sources and three detectors are practical quantities that can still be properly distributed over the pressure cuff for use around the finger.

Each light detector may be arranged to measure the light of each light source that passes through the body part. It is expected that the light detector paired with the emitting light source or closest to the light source receives most of the light passing through the body part, while the other light detectors receive less light from the light source, primarily because the light will take longer to travel to the other light detectors and thus will be subject to more scattering and absorption. In case all light detectors measure substantially the same amount of light, which is emitted from a single light source, it is possible that the light does not pass through the body part or finger in a transmissive mode, but bypasses the body part between the skin and the cuff in a reflective mode. In this light pipe scenario, the signal can be used to determine whether the pressure cuff is wrapped sufficiently snugly around the body part.

The wavelength of the light emitted or emitted by the light source may be located near the isosbestic point of hemoglobin, particularly about 800 nm. According to the invention, about 800nm means a wavelength around 800 nm. The effective attenuation coefficient of the hemoglobin molecule (the main component of the red blood cell) is independent, or at least relatively independent, of its oxygenation, around the 800nm wavelength. Wavelengths falling within the range of about 800nm include, for example, wavelengths between 750nm and 950nm, and particularly wavelengths between 750nm and 850 nm.

The light source may be configured to provide a coherent light beam and, for example, comprise a coherent light emitting diode and/or a laser. Coherent light beams differ from ordinary light beams in that the wavelength range is smaller, in particular photons of a single wavelength are emitted. Furthermore, the emitted light does not diverge, or at least does not diverge much, when using e.g. a laser. Less divergence results in less scattering of light in the body part and therefore should result in less contamination of received light from photodetectors not paired with the emitting light source.

The light detector may be configured for providing a signal depending on the amount of detected light of a certain wavelength, in particular only depending on a certain wavelength range. For example, the wavelength range may coincide with a wavelength range or a single wavelength emitted by the at least one light source.

The invention also relates to a pressure system for use in a non-invasive blood pressure measurement device, the pressure system comprising: a pressure cuff according to the present invention; an actuator (e.g., a pump) for supplying pressurized fluid to the bladder; a variable flow resistance between the actuator and the air bag to vary a pressure supplied to the air bag by the actuator; and a controller arranged to provide a controller signal based on the signal of each light detector. If the actuator is variable, no variable flow resistance is required.

The controller of the pressure system may be arranged to provide a controller signal based on light detected by a pair of light detectors and light sources of the pressure cuff and to correct the signal for light detected by each other light detector. For example, the light detected by each of the other detectors may be multiplied by a constant value as a weighting factor and then subtracted from the light detected by the light detector paired with the light source. For example, the correction includes the step of providing a new signal based on a constant multiplied by the paired detector minus another constant multiplied by the detector signal that does not form 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 signal of the light detector and/or the controller signal.

The controller may be arranged to calculate the set point based on the signal of the light detector. The signal of the photoplethysmograph represents the blood volume inside the blood vessel under the cuff of the finger. The more blood inside the blood vessel, the more light from the light source of the plethysmograph is absorbed, which results in a lower signal on the detector side of the plethysmograph (and vice versa). During each heartbeat, blood is forced through the blood vessels in the finger, causing the blood vessels to dilate and allow more blood to flow through the blood vessels. This also causes the volume of the blood vessel to increase, causing the signal of the plethysmograph to decrease.

The cuff pressure of the pressure cuff is controlled such that the signal of the plethysmograph and the blood volume inside the blood vessel remain constant. The pressure exerted by the heart on the inner vessel wall is constantly counteracted by the pressure exerted by the pressure cuff on the outer vessel wall, which results in a constant diameter of the vessel and a pressure relief of the vessel. When the artery is relieved of pressure, the back pressure exerted by the pressure cuff is a measure of the actual blood pressure inside the blood vessel and allows continuous non-invasive blood pressure measurements.

The control is arranged as a servo feedback system such that at any time the difference between the servo reference level or set point value of the vessel diameter and the actual plethysmograph signal or real value is minimized, ideally zero. The servo reference level in the known method is initially determined automatically and the servo feedback control operates in such a way that: such that the cuff pressure continues to substantially coincide with the instantaneous arterial pressure below the cuff, whether it is a pulsatile or absolute pressure level.

This method requires correcting the reference or set point values over time. This correction is required primarily due to changes in the physiological state of the body part being measured. For example, US4,510,940 describes a method and apparatus for correcting cuff pressure in indirect, non-invasive and continuous measurements of blood pressure in a part of the body by using a plethysmograph, electronic control circuitry and electrically powered pressure valves in a fluid filled pressure cuff. The cuff pressure is controlled by the plethysmograph signal in closed loop operation by means of servo reference levels obtained via a memory circuit. In operation of the device, the servo reference level is adjusted by opening the closed loop of the control circuit for a short period of time, after which in an open loop operation the cuff pressure is adjusted to an intermediate pressure derived from the last measured pressure and the servo reference level is adjusted via the memory circuit.

The set point determination can be improved by limiting the optical receiver signal to the actual pressure relief area of the body part. This region is typically the middle or center of the inflatable cuff. For example, when using one central light source, the light detector directly opposite the light source will receive light information from the center (which center will correspond to the pressure relief artery area), but also from more distal or more proximal areas (corresponding to the artery area that is not fully relieved), for example because the balloon is not large enough to relieve the pressure of the entire area of the body part under the cuff, or because the vein under the cuff is not fully collapsed and contributes to the absorption of light. By correcting the light signal for light received at the proximal and/or distal edge of the cuff, and thus using the light signal from the cuff center primarily, the determination of the set point and the performance of the servo control, and thus the blood pressure determination, can be improved.

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

The correcting or reducing in the method may further comprise the step of processing the signal of the at least one light detector before the correcting or reducing. Typically, one of the light detectors is disposed opposite the at least one light source, with the other light detectors disposed at an angle to the at least one light source. A photodetector opposite the light source provides the primary signal, with an angled photodetector providing the signal to be reduced (or corrected). According to the present invention, the light detector and the light source providing the main signal are considered to form a pair.

The method may further comprise the steps of: repeating the step of transmitting light of one of the light sources through the body part for each light source; detecting light transmitted by a single light source with at least two light detectors, wherein each light detector provides a signal based on the detected light; correcting or reducing signals of the light detectors not paired with the single light source from signals of the light detectors paired with the single light source; and determining a volume of blood flowing through the body part based on the reduced signal.

A cuff or pressure cuff for use in non-invasive blood pressure measurements may comprise an inflatable balloon provided with an air supply channel for inflating the balloon and for deflating the balloon. For example, the air channel may be connected to the bladder using a suitable fitting. The inflatable bladder may comprise a top layer arranged to be in contact with a body part, typically a finger, of the person to be measured. The inflatable bladder may also include a backing layer attached to the flexible printed circuit. A top layer, for example made of Polyurethane (PU), is usually more elastic than a back layer, for example made of polyvinyl chloride (PVC), which may even be inelastic.

The inflatable balloon may also contain at least one or at least two cut-out regions to house or position the light source, and may contain at least two cut-out regions to house or position the light detector. The backing layer may contain corresponding cut-out regions for accommodating the light source and the detector. For example, the printed circuit may be connected to a signalling cable, which may be provided with a suitable connector. For example, the cable for the air supply channel and the air supply source may be located in the housing outside the pressure cuff. For example, the printed circuit may comprise an identification unit and/or a module for processing the signal received from the light detector, which module may be further arranged to perform signal processing steps on the received signal, such as filtering or amplification.

The pressure cuff may also include a fastening device, such as Velcro (Velcro), to wrap the cuff around a body part, typically around a finger. The cuff may also be placed over the ear, or over the temple, in the nostril or in a body cavity of a person.

Drawings

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

fig. 1 schematically shows a device for non-invasive blood pressure measurement according to the prior art;

FIG. 2 schematically illustrates a pressure cuff in accordance with the present invention;

FIG. 3 schematically illustrates a cross-sectional view of a pressure cuff in accordance with the present invention wrapped around a finger;

FIG. 4 schematically illustrates a side view of a pressure cuff in accordance with the present invention wrapped around a finger;

FIG. 5 schematically illustrates the pressure cuff of FIG. 4 with one light source emitting light;

FIG. 6 schematically illustrates the effective attenuation coefficient of hemoglobin in its oxygenated and non-oxygenated forms;

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

FIG. 8 schematically illustrates the effect of light received by a plurality of light detectors;

Detailed Description

Fig. 1 schematically shows a device (1) for non-invasive blood pressure measurement according to the prior art, comprising a pressure cuff (2) which generates a signal (3) based on detected light. This signal (3) representing the blood volume of the artery under the cuff in the finger (4) is compared with a set point (5) by a comparator (6) and the result of the comparison is then transmitted to a controller (7). Based on this information, the controller (7) in turn controls the control valve (8). The valve (8) regulates the pressure supplied by the pump (9) to the pressure cuff (2). 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 measurement, but with an improved pressure cuff.

Fig. 2 schematically shows a pressure cuff (11) for use in non-invasive blood pressure measurement according to the present invention. The cuff (11) comprises an air bag (12) for placing around and applying pressure to a body part. The cuff (11) shown in fig. 2 is a kind of cuff generally used for a finger as a body part. The cuff (11) further comprises three light sources (13, 13 ') and three light detectors (14, 14') each for detecting light and providing a signal in dependence on the amount of light detected. The light source (13, 13 ', 13 ") and the light detector (14, 14', 14") are arranged in pairs and, in use, substantially on opposite sides of the body part or finger.

The light detector (14, 14 ', 14 ") and the light source (13, 13', 13") are arranged on the inner side of the pressure cuff (11) and spaced apart. The inner side of the pressure cuff (11) is the side of the cuff (11) which in use faces a body part or a finger. The light sources (13, 13') emit light of the same wavelength, which is about 800 nm.

The light detector (14, 14 ') and the light source (13, 13') are arranged on a flexible printed circuit (15), and the flexible printed circuit (15) is arranged on an outer wrapper (16). The overwrap (16) is further provided with Velcro (17) for wrapping the overwrap (16) around the body part and securing the overwrap (16). The bladder (12) is disposed on top of or on the inside of the outer wrap (16) and the flexible printed circuit (15). The bladder (12) is also connected to a fluid supply (18) via a connector (19).

Fig. 3 schematically shows a cross-section of a pressure cuff (11) wrapped around a finger (F). The finger (F) is schematically represented by the muscle (M), the skeleton (B) and the two arteries (a). The pressure cuff (11) comprises an air bag (12) wrapped around a finger (F). In this cross-sectional view, the cuff (11) further comprises a light source (13) and a light detector (14), the light detector (14) being for detecting light emitted from the light source (13) and passing through the finger (F) and for providing a signal depending on the amount of light detected. The light source (13) and the light detector (14) are arranged in pairs and are located on opposite sides of the finger (F). In the cross-section shown, 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 the light detector (14) are not in diametrically opposed positions, but on opposite sides. For example, both the light source (13) and the light detector (14) are placed at an angle (α), for example, typically between 0 ° and 45 °, in particular about 20 °, compared to the horizontal (H) or the centre line through the finger (F).

Fig. 4 schematically shows a side view of the pressure cuff (11) wrapped around the finger (F). Fig. 4 shows that the light source (13, 13 ', 13 ") and the light detector (14, 14', 14") are spaced apart, in this case in the longitudinal direction of the finger (F).

Fig. 5 shows the pressure cuff (11) of fig. 4 with one of the light sources (13) emitting light of a single wavelength. The light is transmitted through the finger (F) located inside the cuff (11) and may be scattered or absorbed in the finger (F). Typically, a majority of the emitted light is received by a light detector (14) opposite the light source (13), the light detector (14) forming a pair with the light source (13). Light received by the light detectors (14' ) which do not form a pair with the light source (13) can be ignored or used to correct for scattering and absorption of light from proximal and/or distal regions of the artery that are not fully pressure relieved.

Fig. 6 schematically shows the effective attenuation coefficient of hemoglobin in its oxygenated and non-oxygenated forms. Around the 800nm wavelength, the effective attenuation coefficient of hemoglobin (the blood principal component) is independent, or at least relatively independent, of its oxygenation, since the effective attenuation coefficients of oxyhemoglobin and non-oxyhemoglobin are approximately the same. This is called the isosbestic range. Wavelengths falling within the range of about 800nm include, for example, wavelengths between 750nm and 950nm, and particularly wavelengths between 750nm and 850 nm.

Fig. 7 schematically shows a pressure system (21) for use in a non-invasive blood pressure measurement device, the pressure system (21) comprising: a pressure cuff (11) according to the invention; an actuator (22) (e.g., a pump) for supplying pressurized fluid to the bladder (12); a variable flow resistance (23) located between the actuator (22) and the air bag (12) to vary the pressure supplied by the actuator (22) to the air bag (12); and a controller (24) arranged to provide a set point and a controller signal based on the signal of each light detector (14) of the pressure cuff (11). The controller (24) is further operable to control the variable flow resistance (23) based on a signal of each light detector (14) of the pressure cuff (11).

Fig. 8 schematically shows the effect of light received by a plurality of light detectors (14, 14', 14 ") in a cuff (11) as shown in fig. 4 and 5, where corresponding features have been given the same reference numerals. In the finger (F) arteries (a) and veins (V) are schematically indicated. In the tip of the rightmost finger (F) in fig. 8, arterioles and capillaries are indicated as arterioles (S). The region (B) of the artery (a) intended to be relieved by the cuff (11) is circled (B) in fig. 8. The pressure relief of the artery (a) causes the venules (V) to close or collapse at the same location, such that blood flow from the veins is outward only, while blood flow through the artery is both in and out.

Due to the collapse of the vein (V), the intermediate detector (14) will detect light emitted from the light source (13) and affected by the fully depressurized artery. The left detector (14') will receive light from the light source (13), but will be affected by arteries to the left of the region (B) (not fully relieved) and also by veins that have not fully collapsed to the left of the region (B). The right detector (14 ") will receive light from the light source (13), but will be affected by veins (not fully collapsed) to the right of the area (B), and possibly arteries (not fully depressurized) to the right of the area (B).

For example, the left detector (14') receives a mixed signal from a pressure relief artery, from a portion of a pressurized artery, and possibly from a portion of a collapsed vein. An intermediate detector (14) receives a mixed signal from the pressure-relieved artery, from a portion of the pressurized artery, and possibly from a portion of the open vein. A right side detector (14 ") receives a mixed signal from the pressure relief artery and from a portion of the open vein and possibly from a portion of the pressurized artery.

The left part of the area (B) can be compensated by taking the signal received by the middle detector (14) and subtracting or correcting the signal of the left detector (14'). The right part of the area (B) can be compensated by taking the signal received by the middle detector (14) and subtracting or correcting the signal of the right detector (14 "). For example, the signal used to determine the set point of the system is 2 times the mid signal minus the left signal minus the right signal, wherein it is also contemplated that each signal is provided with its own customized weighting factor.

Figure 10 schematically shows an example of a volume measurement with a cuff according to the invention when the counter-pressure in the cuff follows a ramp-like process from zero to above the systolic pressure, as indicated by 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. The top panel shows the signals from the middle sensor (M), the near side sensor (P) and the far side sensor (D). The light measurements in arbitrary units of measure are shown on the Y-axis. The individual pressure-volume loops are taken at points 1, 2, 3, 4,5 and are shown in fig. 11.

Fig. 11 shows the PV loop as a volume signal for transmural pressure (i.e., intra-arterial pressure minus cuff pressure). The individual heartbeats at times 1, 2, 3, 4,5 in fig. 10 are highlighted in black. They show detailed information for calculating the set point values and also show the typical considerable hysteresis of a human artery.

Fig. 12 shows a typical example of correction information obtained with the cuff according to the present invention. The correction signal includes a differential distal center (a, on top) and a proximal center (B, on bottom).

It is obvious that the invention is not limited to the exemplary embodiments shown and described here, but that many variants are possible within the scope of the appended claims, as will be clear to a person skilled in the art.

Claims (14)

1. A pressure cuff for use in a non-invasive blood pressure measurement apparatus, comprising:

a. an air bag for placing 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 below the balloon;

c. at least two light detectors, each light detector for detecting light passing through the body part and each light detector for providing a signal in dependence on the amount of light detected;

d. wherein the at least two light detectors and the at least one light source are disposed on an inner side 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 each light detector is configured to detect light of the same wavelength.

2. A pressure cuff as recited in claim 1, including at least two light sources, each light source for sending light through the body portion beneath the bladder; wherein the two light sources and the two light detectors are arranged in pairs.

3. The pressure cuff of claim 2, wherein the two light sources emit light of substantially the same wavelength.

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

5. A pressure cuff as recited in any of the preceding claims, wherein each light detector is disposed to measure the light of each light source passing through the body part.

6. The pressure cuff according to any one of the preceding claims, wherein the wavelength of the light emitted by the light source is located in the vicinity of the isosbestic point of hemoglobin, and in particular about 800 nm.

7. The pressure cuff according to any one of the preceding claims, wherein said light source is configured for providing a coherent light beam and for example comprises a coherent light emitting diode and/or a laser.

8. A pressure system for use in a non-invasive blood pressure measurement apparatus, comprising:

a. a pressure cuff as claimed in any one of the preceding claims;

b. an actuator, such as a pump, for supplying pressurized fluid to the bladder;

c. a variable flow resistance between the actuator and the air bag to vary the pressure supplied by the actuator to the air bag;

d. a controller arranged to provide a controller signal based on the signal of each of the light detectors.

9. The pressure system of claim 8, wherein the controller is configured to provide the controller signal based on light emitted and detected by a pair of light detectors and light sources of the pressure cuff, and to reduce or correct light detected by each of the other light detectors.

10. A 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 signal of the light detector and/or the controller signal.

11. A pressure system according to any of claims 8-10, wherein the controller is arranged to calculate a set point based on the signal of the light detector.

12. A method for measuring the volume of blood flowing through a body part, comprising the steps of:

a. placing a pressure cuff according to any one of claims 1-7 around a body part;

b. transmitting light of one of the light sources through the body part;

c. detecting light transmitted 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 emission light source and correcting the signal of at least one other light detector;

e. determining a volume of blood flowing through the body part based on the reduced or corrected signal.

13. The method of claim 12, comprising the steps of:

e. repeating steps b-e for each light source.

14. A method according to claim 12 or 13, wherein step d) comprises the step of processing the signal of the at least one other light detector before reducing the signal.

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