Device and method for non-invasive assessment of maximum arterial compliance

Abstract

The present invention relates to a device (10) for determining an arterial compliance of a subject (12). The device (10) comprises an inflatable cuff (14), a pressure sensor (18) which is configured to sense a pressure signal (52) that is indicative of a pressure within the cuff (14), a second sensor (20) which is at least partly integrated in the cuff (14) and configured to sense a second signal (56) that is responsive to expansions and contractions of the cuff (14) caused by a pulsating blood flow in an artery (50) of the subject (12), and a processing unit (22). The processing unit is configured to determine based on said part of the pressure signal (52) a pulse pressure of the subject (12), to determine based on said part of the second signal (56) an arterial volume change of the artery (50) of the subject (12) during at least one cardiac cycle, and to determine the arterial compliance of the subject (12) based on pulse pressure and the arterial volume change.

Claims

1. A device for determining an arterial compliance of a subject, comprising: an inflatable cuff which is attachable to a body part of the subject; a pump adapted to inflate and a valve adapted to deflate the inflatable cuff; a pressure sensor which is configured to sense a pressure signal that is indicative of a pressure within the inflatable cuff; an optical sensor, which is at least partly integrated in the inflatable cuff, and is configured to sense a second signal that is responsive to expansions and contractions of the inflatable cuff caused by a pulsating blood flow in an artery of the subject; and a processing unit which is configured: to evaluate at least parts of the pressure signal and the second signal that are recorded while the inflatable cuff is attached to the body part of the subject and being deflated from above systolic pressure to below diastolic pressure over a plurality of cardiac cycles, to determine based on said part of the pressure signal a pulse pressure (ΔP) of the subject, to determine based on said part of the second signal an arterial volume change (ΔV.sub.art) of the artery of the subject for each cardiac cycle of the plurality of cardiac cycles recorded while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure; and to determine the arterial compliance (C) of the subject either by determining a maximum arterial volume change (ΔV.sub.art max) of the arterial volume changes (ΔV.sub.art) determined for each cardiac cycle of the plurality of cardiac cycles and dividing the maximum arterial volume change (ΔV.sub.art max) by the pulse pressure (ΔP), or by dividing each of the determined arterial volume changes (ΔV.sub.art) of the plurality of cardiac cycles by the pulse pressure (ΔP) to receive a plurality of arterial compliance values, and then determining a maximum of the plurality of arterial compliance values.

2. The device according to claim 1, wherein the processing unit is configured to determine the pulse pressure of the subject based on the pressure signal by an oscillometric evaluation.

3. The device according to claim 2, wherein the processing unit is configured to determine the pulse pressure of the subject not only based on the pressure signal, but also based on the second signal.

4. The device according to claim 1, wherein the optical sensor further comprises an optical fiber.

5. The device according to claim 4, wherein the optical fiber is integrated into the inflatable cuff, the optical sensor further comprising a light source, which is configured to produce light that is coupled into the optical fiber at a first end of the optical fiber, and a light detector, which is configured to detect the light at a second end of the optical fiber opposite the first end.

6. The device according to claim 5, wherein the light detector is configured to transform the detected light into an electrical signal that is proportional to an intensity of the detected light and that represents the second signal.

7. The device according to claim 5, wherein the inflatable cuff comprises an inflatable part and a stretchable carrier material connected to the inflatable part, wherein the optical fiber is integrated in the stretchable carrier material.

8. The device according to claim 7, wherein the stretchable carrier material is arranged at a side of the inflatable cuff that is configured to contact the skin of the subject.

9. The device according to claim 5, wherein the optical fiber is arranged in the inflatable cuff as a folded winding.

10. The device according to claim 9, wherein the optical fiber comprises at least two folded windings.

11. The device according to claim 1, wherein the optical sensor further comprises a strain gauge.

12. A method for determining an arterial compliance of a subject, the method comprising: attaching an inflatable cuff to a body part of the subject; inflating the inflatable cuff to a first pressure above systolic pressure; deflating the inflatable cuff to a second pressure below diastolic pressure; receiving a pressure signal from a pressure sensor that is indicative of a pressure within the inflatable cuff while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure; receiving a second signal from an optical sensor that is responsive to expansions and contractions of the inflatable cuff caused by a pulsating blood flow of the subject while the inflatable cuff is deflated from above systolic pressure to below diastolic pressure over a plurality of cardiac cycles; determining a pulse pressure (ΔP) of the subject based on the pressure signal; determining, based on the second signal, an arterial volume change (ΔV.sub.art) of an artery of the subject for each cardiac cycle of the plurality of cardiac cycles recorded while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure; and determining the arterial compliance of the subject as a maximum arterial compliance value associated with one cardiac cycle of the plurality of cardiac cycles, wherein the maximum arterial compliance value is determined by: determining a maximum arterial volume change (ΔV.sub.art max) of the arterial volume changes (ΔV.sub.art) determined for each cardiac cycle of the plurality of cardiac cycles and dividing the maximum arterial volume change (ΔV.sub.art max) by the pulse pressure (ΔP) to obtain the maximum arterial compliance value, or dividing each of the determined arterial volume changes (ΔV.sub.art) of the plurality of cardiac cycles by the pulse pressure (ΔP) to receive a plurality of arterial compliance values, and then determining a maximum of the plurality of arterial compliance values to obtain the maximum arterial compliance value.

13. The method according to claim 12, wherein the pulse pressure of the subject is determined by an oscillometric evaluation either based on the pressure signal or based on the pressure signal and the second signal.

14. A method implemented by a processing unit for determining an arterial compliance of a subject, the method comprising: receiving a pressure signal from a pressure sensor that is indicative of a pressure within an inflatable cuff attached to a body part of the subject while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure over a plurality of cardiac cycles; receiving a second signal from an optical sensor that is responsive to expansions and contractions of the inflatable cuff caused by a pulsating blood flow of the subject while the inflatable cuff is deflated from above systolic pressure to below diastolic pressure over the plurality of cardiac cycles; determining a pulse pressure (ΔP) of the subject, based on the pressure signal; determining, based on the second signal, an arterial volume change (ΔV.sub.art) of an artery of the subject for each cardiac cycle of the plurality of cardiac cycles recorded while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure; and determining the arterial compliance of the subject as a maximum arterial compliance value associated with one cardiac cycle of the plurality of cardiac cycles, wherein the maximum arterial compliance value is determined by: determining a maximum arterial volume change (ΔV.sub.art max) of the arterial volume changes (ΔV.sub.art) determined for each cardiac cycle of the plurality of cardiac cycles and dividing the maximum arterial volume change (ΔV.sub.art max) by the pulse pressure (ΔP) to obtain the maximum arterial compliance value, or dividing each of the determined arterial volume changes (ΔV.sub.art) of the plurality of cardiac cycles by the pulse pressure (ΔP) to receive a plurality of arterial compliance values, and then determining a maximum of the plurality of arterial compliance values to obtain the maximum arterial compliance value.

15. A device for determining an arterial compliance of a subject, comprising a processing unit that is configured: to receive a pressure signal from a pressure sensor indicative of pressure in an inflatable cuff attached to a body part of the subject and a second signal from an optical senor indicative of intensity of light in an optical fiber in the inflatable cuff; to evaluate at least parts of the pressure signal and the second signal that are recorded while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure over a plurality of cardiac cycles; to determine based on said part of the pressure signal a pulse pressure (ΔP) of the subject; to determine based on said part of the second signal an arterial volume change (ΔV.sub.art) of an artery of the subject for each cardiac cycle of the plurality of cardiac cycles recorded while the inflatable cuff is being deflated from above systolic pressure to below diastolic pressure; and to determine the arterial compliance (C) of the subject as a maximum arterial compliance value associated with one cardiac cycle of the plurality of cardiac cycles, wherein the maximum arterial compliance value is determined by: determining a maximum arterial volume change (ΔV.sub.art max) of the arterial volume changes (ΔV.sub.art) determined for each cardiac cycle of the plurality of cardiac cycles and dividing the maximum arterial volume change (ΔV.sub.art max) by the pulse pressure (ΔP) to obtain the maximum arterial compliance value, or dividing each of the determined arterial volume changes (ΔV.sub.art) of the plurality of cardiac cycles by the pulse pressure (ΔP) to receive a plurality of arterial compliance values, and then determining a maximum of the plurality of arterial compliance values to obtain the maximum arterial compliance value.

16. The device according to claim 15, wherein the processing unit is further configured to determine the pulse pressure of the subject based on the pressure signal by an oscillometric evaluation.

17. The device according to claim 16, wherein the processing unit is configured to determine the pulse pressure of the subject further based on the second signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings

(2) FIG. 1 shows a first embodiment of a device according to the present invention;

(3) FIG. 2 shows a block diagram illustrating a control model for controlling the device according to the embodiment shown in FIG. 1;

(4) FIG. 3 shows an exemplary embodiment of an inflatable cuff that may be used as part of the device according to the present invention;

(5) FIG. 4 shows a schematic view of a cross section of the cuff shown in FIG. 3 while being positioned at an upper arm;

(6) FIG. 5 illustrates an exemplary pressure signal monitored by means of a pressure sensor that may be part of the device according to the present invention; and

(7) FIG. 6 shows a schematic block diagram illustrating the method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(8) FIG. 1 shows an exemplary embodiment of a device for determining an arterial compliance of a subject. The device is therein denoted in its entirety with reference numeral 10. The subject, i.e. a human, is denoted in FIG. 1 with reference numeral 12.

(9) The device 10 comprises an inflatable cuff 14, means 16 for inflating and deflating the inflatable cuff 14, a pressure sensor 18, a second sensor 20, and a processing unit 22. The inflation and deflation means 16, the pressure sensor 18, the second sensor 20 and the processing unit 22 are according to the exemplary example illustrated in FIG. 1 installed in an external housing 24 that is arranged locally remote from the inflatable cuff 14. The inflatable cuff 14 is in the example shown in FIG. 1 attached to an upper arm of the subject 12. The inflatable cuff 14 is connected to the pressure sensor 18 via a hose 26. This hose 26 is also connected to the inflation/deflation means 16. The inflatable cuff 14 is furthermore connected to the second sensor 20 via one or more data connections 28.

(10) It shall be noted that only parts of the second sensor 20 are according to the example given in FIG. 1 installed in the external housing 24, while at least one part 30 is integrated into the inflatable cuff 14. According to the embodiment illustrated in FIG. 1, this part 30 of the second sensor 20 includes an optical fiber, as will be discussed in detail further below. The optical fiber 30 is preferably also used as data connection 28 between the part of the optical fiber 30 that is integrated into the inflatable cuff 14 and a light source 32 and a light detector 34 which are in this example installed in the external housing 24.

(11) It shall be noted that FIG. 1 illustrates an embodiment of the device 10 according to the present invention wherein the fewest possible parts of the device 10 are integrated into the inflatable cuff 14, namely only parts of the second sensor 20 (i.e. the optical fiber 30). However, also the other parts of device 10, i.e. the inflation/deflation means 16, the pressure sensor 18, the processing unit 22, the light source 32 and/or the light detector 34, may generally each be integrated into the inflatable cuff 14 as well, either partly or as a whole. For example, all of these components could be integrated into the inflatable cuff 14. In another example, all components except the processing unit 22 may be integrated into the inflatable cuff 14, while the processing unit 22 is arranged remote from the inflatable cuff 14 and connected to the pressure sensor 18 and the second sensor 20 by means of a wireless data connection. In yet another example of the device 10, the components 16-22 may be distributed such that they are arranged in a plurality of different external housings.

(12) The device 10 according to the present invention may optionally further comprise a display 36. This display 36 may also be either arranged remote from the inflatable cuff 14 (e.g. installed in the external housing 24) or it may be integrated into the inflatable cuff 14.

(13) FIG. 3 shows an exemplary example of the inflatable cuff 14 in an enlarged, transparent view. According to the therein illustrated example, the inflatable cuff 14 comprises an inflatable part 38. This inflatable part 38 is connected via the hose 26 to the inflating/deflating means 16. The inflating/deflating means 16 may comprise a pump for inflating the inflatable part 38, and a valve for deflating the inflatable part 38. The inflatable part 38 preferably comprises one or more sealed chambers which may be filled up with air.

(14) The inflatable cuff 14 furthermore comprises a fixation means 40 that may e.g. comprise a hook- and -loop fastener (Velcro® fastener). The inflatable cuff 14 may be therefore easily attached to a suitable body part of the subject 12, e.g. onto an upper arm. According to the embodiment shown in FIG. 3, the inflatable cuff 14 further comprises the optical fiber 30. As it is shown in FIG. 3, the optical fiber 30 is preferably not fully wound around the inflatable cuff 14 as a direct loop, but arranged as a folded winding. The optical fiber 30 is preferably integrated into a stretchable carrier material 42. This stretchable carrier material 42 preferably covers one side of the inflatable part 38 that contacts the skin of the subject 12 when the inflatable cuff 14 is attached to the upper arm of the subject 12. The same may be seen in the cross section illustrated in FIG. 4, where reference numeral 44 indicates the skin of the subject 12, reference numeral 46 indicates the muscle of the subject 12, reference numeral 48 indicates a bone of the subject 12 and reference numeral 50 indicates an artery of the subject 12.

(15) The mode of operation of the device 10 may be explained best with reference to the schematic control model illustrated in FIG. 2. The pressure sensor 18 senses a pressure signal 52 that is indicative of a pressure within the inflatable cuff 14, i.e. within the inflatable part 38 of the inflatable cuff 14. This pressure signal 52 is transmitted to the processing unit 22. An example of such a signal is schematically illustrated in FIG. 5.

(16) FIG. 5 shows the pressure within the inflatable cuff 14 over time. The pressure signal 52 is also denoted as inflatable cuff deflation curve. It may be seen that the inflatable cuff 14 is initially inflated to a pressure that is above systolic pressure. Hence, the brachial artery 50 is fully occluded obstructing the blood flow through the artery. The pressure within the inflatable cuff 14 is then slowly decreased by deflating the inflatable cuff 14 to below diastolic pressure. While deflating the inflatable cuff 14, the pressure inside the inflatable cuff 14 is measured with the pressure sensor 18 leading to the pressure signal 52.

(17) The pressure signal 52 is not only influenced by the actively reduced pressure within the inflatable part 38 of the inflatable cuff 14, but also contains signal parts that are due to the pulsating blood flow within the brachial artery 50 of the subject 12. These signal parts are oscillating signal parts that occur shortly before the pressure within the inflatable cuff 14 is reduced to the systolic pressure and remain in the pressure signal 52 even after the pressure is reduced below diastolic pressure, i.e. almost until the pressure within the inflatable cuff 14 is completely reduced to atmospheric pressure.

(18) The oscillating signal parts of the pressure signal 52 that are induced by the pulsating blood flow may be filtered out from the pressure signal 52. The oscillometric waveform 54 that results from said filtering step is shown in the lower part of FIG. 5.

(19) The processing unit 22 is configured to analyze the pressure signal 52, to derive the oscillometric waveform 54 therefrom, and to then determine the blood pressure of the subject 12 based on an inflatable cuff deflation curve 55 and the oscillometric waveform 54. This may be done by means of an oscillometric evaluation. By means of appropriate oscillometric algorithms one can determine from the oscillometric waveform 54 the systolic pressure, the mean pressure and the diastolic pressure. Usually, the mean pressure is determined first, since the mean arterial pressure is approximately equal to the mean cuff pressure at the point where the maximum oscillations occur within the oscillometric waveform 54. The mean arterial pressure is therefore comparatively easy to determine (106 mmHg in the example shown in FIG. 5). The systolic pressure and the diastolic pressure (systolic pressure and diastolic pressure are in the example given in FIG. 5 at 157 mmHg and 92 mmHg) may be determined by the processing unit 22 based on the mean arterial pressure that has been determined beforehand. Algorithms for such an oscillometric evaluation are well-known in the art.

(20) Simultaneously to receiving the pressure signal 52, the processing unit 22 also receives a second signal 56 from the second sensor 20. The second signal 56 is generated as follows: A signal source 58 generates a control signal 60 that is input to the light source 32 and the processing unit 22. Based on this control signal 60 the light source 32 produces light that is coupled into the optical fiber 30 at a first end of the optical fiber 30. The light is then guided through the optical fiber 30, as the light is kept in the core by total internal reflection. However, the pulsating blood flow in the brachial artery 50 of the subject 12 causes a modulation of the light within the optical fiber 30. This may be detected by the light detector 34 as variations in the light intensity of the light received at the light detector 34. The light detector 34 detects the light at a second end of the optical fiber 30 opposite the first end to which the light source 32 is coupled. The change of the light intensity detected at the light detector 34 is directly proportional to the perturbation of the optical fiber 30. The second signal 56 generated by the light detector 34 is thus responsive to expansions and contractions of the inflatable cuff 14 caused by the pulsating blood flow in the brachial artery 50 of the subject 12. The second signal 56 may of course also include signal parts that relate from shape changes within the inflatable cuff 14 that result from the deflation of the inflatable part 38. However, since the optical fiber 30 is integrated into the stretchable carrier material 42 that is positioned on the inside of the inflatable part 38 close to or in contact with the skin 44 of the subject 12 (see FIG. 4) those signal parts are insignificant or may be at least filtered out. The remaining signal parts of the second signal 56 are indicative of the movements of the inflatable cuff 14 caused by the pulsating blood flow. These signal parts are thus proportional to the changes of the cross section of the brachial artery 50 over time, and therefore also proportional to the arterial volume change.

(21) Thus, the arterial volume oscillations during heartbeats may be monitored with the aid of the observed optical transmission loss and amplitude oscillations of the light which reflect the change in arterial volume.

(22) In order to extract the arterial amplitude oscillations from the second signal 56, the processing unit 22 may be configured to correlate the control signal 60 and the second signal 56. Based on this, an arterial volume change of the brachial artery 50 of the subject 12 may be calculated or at least estimated by the processing unit 22.

(23) The processing unit 22 may be furthermore configured to finally determine/estimate the arterial compliance of the subject. The arterial compliance of the subject is preferably calculated based on the pulse pressure ΔP being the difference between the systolic pressure P.sub.s and the diastolic pressure P.sub.d, and based on the arterial volume change ΔV.sub.art as follows: C.sub.art=ΔV.sub.art/ΔP. The processing unit 22 may either store the calculated arterial compliance C.sub.art in a memory for later analysis or it may display the calculated value on the display 36. The processing unit 22 may also be configured to transmit the calculated data via an antenna to an external computing, display and/or storage device.

(24) The arterial compliance C.sub.art is preferably calculated within the processing unit 22 as follows: The processing unit 22 derives from the pressure signal 52 the oscillometric waveform 54 and estimates the systolic pressure P.sub.s and diastolic pressure P.sub.d based on oscillometric waveform 54. The processing unit 22 then calculates the pulse pressure ΔP by computing ΔP=P.sub.s−P.sub.d. Next, the arterial volume amplitude V.sup.i.sub.art is determined for each i.sup.th cardiac cycle during the cuff deflation period. For all N-cardiac cycles during the cuff deflation a slope s.sup.i is computed according to s.sup.i=V.sup.i.sub.art/ΔP, giving a set S={s.sup.i, . . . s.sup.n}. The local slope s could be expressed in units of mm.sup.2/mmHg. It shall be noted that V.sub.art and ΔV.sub.art may be indicated in units of mm.sup.2 representing a cross-sectional area of the brachial artery 50 that is proportional to the arterial volume. Finally, the processing unit 22 may calculate the arterial compliance C.sub.art as C.sub.art=max ({s.sup.i, . . . , s.sup.n}). In other words, the subject's arterial compliance C.sub.art is associated to the maximum arterial compliance that is estimated during the cuff deflation period.

(25) FIG. 6 summarizes the method according to the present invention in a schematic block diagram. In a first step S10, the inflatable cuff 14 is attached to a body part of the subject 12. The inflatable cuff 14 is then inflated in step S12 to a first pressure that is above systolic pressure P.sub.s. Next, the inflatable cuff 14 is deflated in step S14 to a second pressure below diastolic pressure P.sub.d. During the deflation of the inflatable cuff 14, the pressure within the inflatable cuff 14 is measured (step S16). Simultaneously in step S18, the second signal is sensed by the second sensor 20. The processing unit 22 then calculates the pulse pressure of the subject based on the pressure signal 52 (step S20). The processing unit 22 also determines an arterial volume change of the brachial artery 50 of the subject during at least one cardiac cycle based on the second signal 56 provided by the second sensor 20 (step S22). Finally, the arterial compliance of the subject 12 is determined in step S24 based on the determined pulse pressure and the arterial volume change.

(26) It shall be noted that the above-illustrated embodiment of the device 10 may also be modified in that a strain gauge is integrated into the inflatable cuff 14 and used instead of the optical fiber 30. Such an electro-chemical sensor would also sense a signal that is responsive to the expansions and contractions of the inflatable cuff 14 caused by the pulsating blood flow in the brachial artery 50 of the subject 12. Such a signal could thus also be used to determine the arterial volume change of the subject.

(27) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

(28) In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

(29) Any reference signs in the claims should not be construed as limiting the scope.