WO2017211866A1

WO2017211866A1 - Method and system for measuring aortic pulse wave velocity

Info

Publication number: WO2017211866A1
Application number: PCT/EP2017/063780
Authority: WO
Inventors: Diana KJÆR THING RIKNAGEL; Johannes Jan Struijk
Original assignee: Aalborg Universitet
Priority date: 2016-06-07
Filing date: 2017-06-07
Publication date: 2017-12-14

Abstract

The present invention relates to a method and system for determining the aortic pulse wave velocity. The method comprises receiving an indication of a measurement of a length of a human aorta to provide an aortic length, arranging a first and second sound transducer outside the body of the patient, and identifying the opening of the aortic valve with measurement from the first sound transducer as a first time, and the pulse wave arrival time at the aortic bifurcation with measurements from the second sound transducer as a second time. The aortic transit time is determined based on the difference between the second time and the first time, and the aortic pulse wave velocity is calculated based on the aortic length and the aortic transit time.

Description

METHOD AND SYSTEM FOR MEASURING AORTIC PULSE WAVE

VELOCITY

Technical field

The present invention relates to a method for determining the aortic pulse wave velocity, and a system for determining the aortic pulse wave velocity.

Background art

Arterial Pulse Wave Velocity (PWV) is the velocity with which the heartbeat-initiated pressure pulse travels along an artery. The clinical importance of Arterial PWV is derived from its correlation with vessel wall stiffness, which is the elasticity (compliance) of the arteries, describing the capability of an artery to expand and to contract in response to pressure changes. Arterial PWV is thus a valuable measure of peripheral arterial condition and is widely accepted as the "gold standard" measure of arterial stiffness.

However, Aortic Pulse Wave Velocity (AoPWV) is even more interesting, since it is directly related to the stiffness of the aorta. AoPWV is considered a marker of cardiovascular risks in patients with hypertension, diabetes and renal disease.

Despite the serious interest in the scientific community for the AoPWV, the large majority of studies does not include the AoPWV as a direct measure, but uses surrogate measures instead. This is due to the fact that commercially available instruments for the measurement of arterial PWV do not support a direct measure of AoPWV. The commercially available equipment uses a surrogate measure of the carotid-femoral PWV instead. Currently, only the use of echocardiography, magnetic resonance imaging (MRI), and invasive pressure measurements enable AoPWV measurements. The cost and complexity of the equipment required is thus prohibitive for a widespread clinical use of AoPWV. In summary, AoPWV is a reliable measure of arterial stiffness, which in turn is prognostic or diagnostic for diseases such as cardiovascular disease, hypertension, diabetes mellitus, and renal disease. Hence, a measurement of a patient showing an AoPWV e.g. outside of standard deviations, may thus prompt further diagnostic studies or examinations. The AoPWV, if attainable, is thus an interesting intermediate finding of diagnostic relevance.

Hence, there is a desire to provide a method and system which can provide an accurate measurement of AoPWV with a relatively low cost and complexity.

Summary of the invention

It is an object of the present invention to improve the current state of the art, to at least partly solve the above problems, and to provide a noninvasive measurement of the Aortic Pulse Wave Velocity (AoPWV). These and other objects are achieved by a method for determining the aortic pulse wave velocity, and a system for determining the aortic pulse wave velocity.

According to a first aspect of the present invention, a method for determining the aortic pulse wave velocity is provided. The method

comprises:

- receiving an indication of a measurement of a length of a human aorta outside the body, to provide an aortic length;

- arranging a first sound transducer outside the body of the patient for measuring the opening of the aortic valve;

- arranging a second sound transducer outside the body of the patient for measuring the pulse wave arrival time at the aortic bifurcation;

- identifying the opening of the aortic valve with measurement from the first sound transducer as a first time, and the pulse wave arrival time at the aortic bifurcation with measurements from the second sound transducer as a second time;

- determining the aortic transit time based on the difference between the second time and the first time; and - calculating the aortic pulse wave velocity based on the aortic length and the aortic transit time.

According to at least a second aspect of the present invention, a system for determining aortic pulse wave velocity is provided. The system comprises:

- a first sound transducer adapted to measure the opening of the aortic valve;

- a second sound transducer adapted to measure the pulse wave arrival time at the aortic bifurcation;

- a control unit connected to the first sound transducer and to the second sound transducer; and

wherein the control unit is configured to receive measurement signals from the first sound transducer to identify the opening of the aortic valve as a first time, and to receive measurements signals from the second sound transducer to identify the pulse wave arrival time at the aortic bifurcation as a second time, and wherein the control unit is further configured to determine the aortic transit time based on the difference between the second time and the first time and calculate the aortic pulse wave velocity based on an aortic length measurement and the aortic transit time.

The present invention is based on the realization that a precise noninvasive measure of AoPWV may be provided by placing two sound transducers at two different positions outside the body of the patient and measuring different characteristics of sounds originating from the aorta. On the basis of the sounds thus recorded, it is possible to calculate the pulse wave velocity through the aorta. Hence, the present invention enables a convenient, simple and cost-effective measurement of the AoPWV. Further, the system is easy and quick to handle and little expertise is needed to operate the system.

Apart from research and clinical applications, the system may also be used in pharmaceutical trials. Significant changes of aortic pulse wave velocity have been seen in various studies with drugs. Hence, it should be noted that the system may also be used for further development of applications within many clinical disciplines and for the pharmaceutical industry as well. With such a system a complete platform of human body sound recording may be developed, with which AoPWV, cfPWV, peripheral artery PWV, heart sounds (including murmurs), murmurs from arteries (eg., renal or carotid arteries), lung sounds, etc. can be recorded and interpreted. Opportunities for applications of the inventive system thus exist in almost all clinical disciplines and in the pharmaceutical industry.

In at least one exemplary embodiment, the method is repeated to provide an average aortic pulse wave velocity from a plurality of

measurements by the sound transducers. In at least one exemplary

embodiment, the control unit is configured to calculate an average aortic pulse wave velocity from a plurality of measurements by the sound

transducers. By using a plurality of measurements, the reliability of calculated average aortic pulse wave velocity may be more robust.

The length of the human aorta may be approximated as the distance from the position of the hyoid bone to the position to the umbilicus. This distance may be measured using any known means, for example by using a tape measure or ruler and may be provided as input by a clinician who performs the measurement. Alternatively, the aortic length may be

approximated as the distance between the sternal notch and the symphysis. It should of course be noted that the total aortic length may also be

approximated by other means, such as the height of the patient in question and using standardized tables. Further, the patient records may contain prior measurements or approximations made by MRI, X-ray or the like.

The first sound transducer is preferably arranged at the aortic valve auscultation region for measuring aortic valve opening time. The aortic auscultation region is located between the second and third rib on a human patient. The second sound transducer is preferably placed at the umbilicus for measuring pulse wave arrival time at the aortic bifurcation. In other words, the measurements are preferably performed from the aortic root, also known as the aortic beginning, to the aortic bifurcation. The first and second sound transducers may of course be placed at other areas of the body of the patient to provide recorded signals from which the aortic valve opening time and the pulse wave arrival time at the aortic bifurcations can be detected. For example, the second sound transducer may be placed in the vicinity of the umbilicus. The vicinity hereby refers to the second sound transducer being placed in a radius of 10 cm from the umbilicus.

The first and second sound transducers may be releasably fastened to a patient utilizing an adhesive, or an adhesive tape. The sound transducers may alternatively be known as microphones. As an alternative, the sound transducers may be accelerometers, which are able to convert vibrations caused by the sounds from aortic valve opening and pulse wave arrival, respectively, to a recorded signal.

The first and second sound transducers may be synchronized.

Thereby, the aortic transit time may be calculated as the difference between the first and second time. Stated differently, the aortic transit time is calculated by the first time being subtracted from the second time. The first and second sound transducers may be synchronized by any known means, for example the measurement from the first and second sound transducers may be time-stamped. Alternatively, the control unit connected to both the first and second sound transducer may synchronize the received

measurement signals.

The sound transducers may be wired or wirelessly connected to the control unit. Alternatively, the sound transducers may be connected in series or parallel to each other. The sound transducers may be wirelessly connected via a Bluetooth piconet or through other wireless protocols and standards such as WiFi. Thus making handling the system very easy and quick and with little expertise needed.

In at least one exemplary embodiment, the opening of the aortic valve is identified by sounds having a frequency in the range of 25 Hz to 300 Hz. In at least one exemplary embodiment, the opening of the aortic valve is identified by sounds having a frequency in the range of 1 Hz to 300 Hz. In at least one exemplary embodiment, the opening of the aortic valve is identified by sounds having a frequency in the range of 25 Hz to 200 Hz. In at least one exemplary embodiment, the opening of the aortic valve is identified by sounds having a frequency in the range of 1 Hz to 25 Hz. The opening of the aortic valve is preferably identified by a low frequency heart sound which occurs simultaneously as the S1 -heart sound. The low frequency heart sound comprises sounds waves in the frequency range of 1 Hz to 25 Hz. The S1 - heart sound typically comprises sound in the frequencies of 25 Hz to 200 Hz. Therefore the identification of the low frequency heart sound may be facilitated in the frequency range of 1 Hz to 25 Hz. However, using a larger frequency range may also provide more information, such that the low frequency heart sound may be more easily distinguished from the S1 heart sound.

According to at least one exemplary embodiment, the first sound transducer comprises a low pass filter having a cutoff frequency of 300 Hz. According to at least one exemplary embodiment, the first sound transducer comprises a band pass filter having a lower cutoff frequency at 25 Hz and a higher cutoff frequency at 200 Hz. According to at least one exemplary embodiment, the first sound transducer comprises a low pass filter having a cutoff frequency of 25 Hz. Alternatively, a low pass filter or band pass filter with the above noted cutoff frequencies may arranged in the control unit.

In at least one exemplary embodiment, the pulse wave arrival time at the aortic bifurcation is identified as the start of the pulse wave. The start of the pulse wave may be identified by reaching a threshold value. The threshold value may be a portion of the peak amplitude value of the pulse wave. The threshold value may be 10% of the peak amplitude value of the pulse wave. Alternatively, the threshold value may be 15%, or 20%, or 25% or 30% of the peak amplitude value of the pulse wave. The pulse wave comprises sound in the frequency range of 1 Hz to 20 Hz. Hence, the frequency components of the pulse wave may also be used to identify the pulse wave. For example, a frequency filter may be used in the second sound transducer, or in the control unit. Alternatively, the control unit may use a frequency separation, in software or hardware, to distinguish the pulse wave. In at least one exemplary embodiment, the identifying of the opening of the aortic valve with measurement from the first sound transducer as a first time, and the pulse wave arrival time at the aortic bifurcation with

measurements from the second sound transducer as a second time is performed automatically by a control unit comprising a processor. The opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation may be identified with a computer based interpretation of the signals.

For example, a database which comprises a large number of validation measurements of the sounds signals in healthy subjects may be used. Then, the processor may use curve fitting, least squares approximation or machine learning to identify the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation as measurement signals from the first and second sound transducer which are similar and comparable to the

measurement from healthy subjects.

As an alternative, an algorithm for processing the received signal may be used, wherein the algorithm is adapted to detect and identify the points in time.

In at least one exemplary embodiment, the system further comprises a display and user input means. The display may be any known display in the art such as an LCD, LED or OLED display. The user input means may be any known user input means in the art such as a touch-interface, e.g. on the display, a keypad, a keyboard, a mouse or other pointer input means.

In at least one exemplary embodiment, the method further comprises: - displaying the measurements from the first sound transducer and the second sound transducer on a display;

- receiving user input to identify the first time and the second time; and

- correcting the first time and the second time based on the user input. In at least one exemplary embodiment, the control unit is configured to display the measurements from the first sound transducer and the second sound transducer on the display, and to receive user input to identify the first time and the second time, wherein the control unit is configured to correct the first time and the second time based on the user input, wherein the corrected first and second time are used to determine the aortic transit time and calculate the aortic pulse wave velocity.

Stated differently, a user may provide input such that the first and second time is corrected or verified. Human beings have an enormous potential for finding patterns, and it may be useful to verify or correct the identification of the first time and the second time. The user input may advantageously be used to further teach the control unit, e.g. using machine learning, by adding the correct first time and second time to a database.

In at least one exemplary embodiment, the identifying of the opening of the aortic valve with measurement from the first sound transducer as a first time, and the pulse wave arrival time at the aortic bifurcation with

measurements from the second sound transducer as a second time further comprises:

- displaying the measurements from the first sound transducer and the second sound transducer on a display;

- receiving user input to identify the first time and the second time. Stated differently, the measurements are presented to a user on a display and the user provides input to identify the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation as the first and second time.

According to at least a third aspect of the present invention, a system for determining aortic pulse wave velocity is provided. The system comprises:

- a first sound transducer adapted to measure the opening of the aortic valve;

- a display;

- user input means;

- a control unit connected to the first sound transducer, the second sound transducer, the display and the user input means; and wherein the control unit is configured to receive measurement signals from the first sound transducer and the second sound transducer, and display the measurements on the display, and to receive user input via the user input means to identify the opening of the aortic valve as a first time and the pulse wave arrival time at the aortic bifurcation as a second time, and to receive an aortic length measurement,

wherein the control unit is further configured to determine the aortic transit time based on the difference between the second time and the first time and calculate the aortic pulse wave velocity based on an aortic length measurement and the aortic transit time.

Effects and features of this third aspect of the present invention are largely analogous to those described above in connection with the first and second aspect of the inventive concept. Embodiments mentioned in relation to the first and second aspect of the present invention are largely compatible with the third aspect of the invention.

In at least one exemplary embodiment, the control unit is configured to calculate an average aortic pulse wave velocity from a plurality of

measurements by the sound transducers. By using a plurality of

measurements, the reliability of calculated average aortic pulse wave velocity may be more robust.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc.]" are to be interpreted openly as referring to at least one instance of the element, device, component, means, step, etc., unless explicitly stated otherwise.

Brief description of the drawings

The above objects, as well as additional objects, features and advantages of the present invention, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, when taken in conjunction with the accompanying drawings, wherein:

Figure 1 is schematic view of a system according to at least one example embodiment of the invention.

Figure 2 is schematic view of a system according to at least one example embodiment of the invention.

Figure 3 is a flowchart schematically illustrating at least one example embodiment of the invention. Detailed description of preferred embodiments of the invention

In the present detailed description, embodiments of a method and system are discussed. It should be noted that this by no means limits the scope of the invention, which is also applicable in other circumstances for instance with other types or variants of systems than the ones shown in the appended drawings. Further, that specific components are mentioned in connection to an embodiment of the invention does not mean that those components cannot be used to an advantage together with other

embodiments of the invention.

The present invention encompasses methods and system for determining the aortic pulse wave velocity by way of recorded signals from two sound transducers. An abnormal aortic pulse wave velocity may be prognostic or diagnostic for diseases such as cardiovascular disease, hypertension, diabetes mellitus, and renal disease, and is thus a finding of intermediate diagnostic relevance which may prompt further examinations of the patient in question.

Figure 1 is a schematic view of a system for determining the aortic pulse wave velocity of a patient 101 . The system comprises a first sound transducer 103, a second sound transducer 104, and a control unit 102.

In various embodiments, the system 102 may further comprise a display 105 and user input means 106. The display 106 may be any known display in the art such as an LCD, LED or OLED display. The user input means 106 may be any known user input means in the art such as a touch- interface, e.g. on the display 105, a keypad, a keyboard, a mouse or other pointer input means.

Each sound transducer 103, 104 is, during use, preferably releasably fastened to the patient 101 utilizing an adhesive, or an adhesive tape. The sound transducers 103, 104 may alternatively be known as microphones. As an alternative, the sound transducers may be accelerometers, which are able to convert vibrations caused by the sounds from aortic valve opening and pulse wave arrival, respectively, to a recorded signal.

The first sound transducer 103 may be adapted to measure the opening of the aortic valve of the patient 101 . To this end, the first sound transducer 103 may comprise a low pass filter having a cutoff frequency of 300 Hz, or 25 Hz. Alternatively, such a low pass filter may be arranged in the control unit 102 whereby the first sound transducer 103 is configured to send the recorded sound signal to the control unit 102.

The second sound transducer 104 may be adapted to measure the pulse wave arrival time at the aortic bifurcation of the patient 101 . For example, the second sound transducer 104 may be configured to send the recorded sound signal to the control unit 102.

The first sound transducer 103 and the second sound transducer 104 are connected to the control unit 102 as indicated by the lines in figure 1 . The connection is preferably wireless but may of course be wired. In figure 1 a parallel connection is shown, alternatively the first and second sound transducer 103, 104 may be connected in series to the control unit 102.

Optionally, the first and second sound transducer 103, 104 may be connected in parallel to each other, whereby the recorded signal from each sound transducer is multiplexed on the connection to the control unit 102. The wireless connection may be a Bluetooth piconet or through another wireless protocol and standard such as WiFi.

The control unit 102 comprises a processor and a non-transitory memory for storing instructions. The processor may be a general purpose processor or a processor specifically adapted to the functions of the present invention. The functionality of the present invention may be embodied wholly or partially in the control unit as various electronic modules, for example comprising DAC or AD converters as well as amplifiers and filters.

Alternatively, the functionality of the present invention may be embodied wholly or partially in the control unit as various software modules. Hence, the skilled person realizes that the present invention is not limited to the restricted number of examples provided herein, but rather many different embodiments are to be construed encompassed by the present invention. For example, some of the functionality may be provided on an "on-demand" basis from a remote server, typically known as a "cloud-service".

In use, the first sound transducer 103 is arranged outside the body of the patient and releasably fastened preferably at the aortic valve auscultation region on the patient 101 as shown in figure 2. It is of course possible to arrange the first sound transducer 103 at other regions to detect the opening of the aortic valve of the patient 101 . The second sound transducer 104 is arranged outside the body of the patient 101 and releasably fastened preferably at the umbilicus on the patient 101 as shown in figure 2. It is of course possible to arrange the second sound transducer at other regions to detect the pulse wave arrival time at the aortic bifurcation. For example, the second sound transducer may be placed in the vicinity of the umbilicus. The vicinity hereby refers to the second sound transducer being placed in a radius of 10 cm from the umbilicus.

The first and second sound transducers 103, 104 are thus understood to be placed in acoustic contact with the body of the patient 101 such that the sound transducers 103, 104 can record sounds or vibrations originating from within the body of the patient 101 .

The control unit 102 is configured to receive measurement signals, also called recorded signals, from the first sound transducer 103 to identify the opening of the aortic valve as a first time ta, and to receive measurements signals, also called recorded signals, from the second sound transducer 104 to identify the pulse wave arrival time at the aortic bifurcation as a second time tb. The control unit 102 is further configured to determine the aortic transit time T_a based on the difference between the second time tb and the first time ta and calculate the aortic pulse wave velocity based on an aortic length measurement l__a and the aortic transit time T_a.

In order to calculate the aortic pulse wave velocity the control unit 102 may be configured to perform repeated measurements and calculations based on the recorded signals from one or more data points from the first and second sound transducers 103, 104.

The first and second sound transducers 103, 104 may be

synchronized. Alternatively, the signals from the first and second sound transducers 103, 104 may be recorded simultaneously. Thereby, the aortic transit time may be calculated as the difference between the first and second time. Stated differently, the aortic transit time is calculated by the first time being subtracted from the second time. The first and second sound

transducers may be synchronized by any known means, for example the measurement from the first and second sound transducers may be time- stamped. Alternatively, the control unit 102 may synchronize the received measurement signals.

In more detail, and with reference to Figure 2 which shows a schematic cross-section of the patient 101 , the aortic pulse wave velocity may be defined as the total aortic length l__a divided by the aortic transit time T_a:

AoPWV =—

The total aortic length L_a may be approximated as the distance from the position of the hyoid bone to the position to the umbilicus. This distance may be measured using any known means, for example by using a tape measure or ruler and may be provided as input by a clinician who performs the measurement to the control unit, e.g. via the user input means 106.

Alternatively, the aortic length may be approximated as the distance between the sternal notch and the symphysis. It should of course be noted that the total aortic length may also be approximated by other means, such as the height of the patient in question and using standardized tables or the like. Further, there may be no need to actually measure the patient and input data via the user input means 106, as the patient records may contain prior measurements or approximations made by MRI, X-ray or the like.

The aortic transit time T_a is determined by the difference between the opening of the aortic valve as a first time ta and the pulse wave arrival time at the aortic bifurcation as a second time tt>. The aortic transit time T_a is thus calculated as:

Ta ^{~ ~} ta The opening of the aortic valve, and the first time ta, is preferably identified by a low frequency heart sound which occurs simultaneously as the S1 -heart sound. The low frequency heart sound comprises sound waves in the frequency range of 1 Hz to 25 Hz. The S1 -heart sound typically comprises sound in the frequencies of 25 Hz to 200 Hz. The opening of the aortic valve, and thus the first time ta may be identified by sounds having a frequency in the range of 1 Hz to 300 Hz. However, using a smaller frequency range, such as 1 Hz to 25 Hz may facilitate the identification of the low frequency heart sound.

The pulse wave arrival time at the aortic bifurcation, and thus the second time tt>, may be identified as the start of the pulse wave. The start of the pulse wave may be identified by reaching a threshold value. The threshold value may be a portion of the peak amplitude value of the pulse wave. The threshold value may be 10% of the peak amplitude value of the pulse wave. Alternatively, the threshold value may be 15%, or 20%, or 25% or 30% of the peak amplitude value of the pulse wave. The pulse wave comprises sound in the frequency range of 1 Hz to 20 Hz. Hence, the frequency components of the pulse wave may also be used to identify the pulse wave. For example, a frequency filter may be used in the second sound transducer, or in the control unit. Alternatively, the control unit may use a frequency separation, in software or hardware, to distinguish the pulse wave.

There are of course several different variations and embodiments possible of the technology described in the foregoing; the control unit 102 may be configured to automatically identify the first time ta and second time tb from the recorded signals from the first and second sound transducers 103, 104 and then provide the aortic pulse wave velocity. Alternatively, the control unit 102 may be further configured to display the measurements from the first sound transducer 103 and the second sound transducer 104 on a display 105 with the determined first time ta and second time tb, respectively. Then user input may be received via user input means 106 to identify the first time and the second time; whereby the first and second time may be corrected based on the user input. The corrected first and second time are used for

determining the aortic transit time T_a and calculating the aortic pulse wave velocity AoPWV by the control unit 102.

To automatically identify the first and second time from the recorded signals, the control unit 102 may comprise a database which comprises a large number of validation measurements of the sounds signals in healthy subjects. Then, the processor of the control unit 102 may use curve fitting, least squares approximation of machine learning to identify the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation as measurement signals from the first and second sound transducer which are similar and comparable to the measurement from healthy subjects. As an alternative, an algorithm for processing the received signal may be used, wherein the algorithm is adapted to detect and identify the points in time.

In another embodiment, the control unit 102 may be configured to display the recorded signals from the first sound transducer 103 and the second sound transducer 104 on a display 105. The control unit 102 is further configured to receive user input via the user input means 106 to identify the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation, and to receive an aortic length measurement l__a via the user input means. The control unit 102 then determines the aortic transit time T_a based on the difference between the second time and the first time and calculates the aortic pulse wave velocity based on an aortic length measurement and the aortic transit time as described in the foregoing. It should of course be noted that the control unit 102 may be

configured to calculate an average aortic pulse wave velocity from a plurality of measurements by the first and second sound transducers 103, 104. The reliability and robustness of the calculated average aortic pulse wave velocity may thus be improved.

Figure 3 is a flowchart schematically illustrating a method for determining aortic pulse wave velocity according to at least one embodiment of the invention. It is noted that these method steps correspond to the functions of the system 100 described in the foregoing, however, the method may of course also be performed with another system.

The first step 301 comprises receiving an indication of a measurement of a length of a human aorta to provide an aortic length l__a. The total aortic length l__a may be approximated as the distance from the position of the hyoid bone to the position to the umbilicus. This distance may be measured using any known means, for example by using a tape measure or ruler and may be provided as input by a clinician who performs the measurement to the control unit, e.g. via the user input means. Alternatively, the aortic length may be approximated as the distance between the sternal notch and the symphysis. It should of course be noted that the total aortic length may also be

approximated by other means, such as the height of the patient in question and using standardized tables or the like. Further, there may be no need to actually measure the patient and input data via the user input means 106, as the patient records may contain prior measurements or approximations made by MRI, X-ray or the like.

The next step 302 comprises arranging a first sound transducer 103 on the patient 101 . The first sound transducer 103 is preferably releasably fastened at the aortic valve auscultation region on the patient 101 as shown in figure 2.

The next step 303 comprises arranging a second sound transducer 104 on the patient. The second sound transducer 104 is preferably releasably fastened at the umbilicus on the patient on the patient 101 as shown in figure 2. The first sound transducer 103 and the second sound transducer 104 may alternatively be arranged at other regions of the body of the patient 101 in question to record the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation, respectively.

It should be noted that the steps 301 , 302 and 303 may be performed in any order as they are not dependent upon each other.

The next step 304 comprises identifying the opening of the aortic valve with measurement from the first sound transducer 103 as a first time ta, and the pulse wave arrival time at the aortic bifurcation with measurements from the second sound transducer 104 as a second time tt>.

The step 304 may be performed either automatically by a control unit 102 and processor. Alternatively, the identification may also be corrected or verified by user input, or the identification may be performed manually by user input. These alternative embodiments are outlined above in connection with figure 1 .

The next step 305 comprises determining the aortic transit time T_a based on the difference between the second time tb and the first time ta. The first time ta is subtracted from the second time tb to provide the aortic transit time Ta. Therefore, the first and second sound transducers may be

synchronized.

The next step 306 comprises calculating the aortic pulse wave velocity AoPWV based on the aortic length l__a and the aortic transit time T_a. The aortic pulse wave velocity AoPWV is preferably calculated as the total aortic length La divided by the aortic transit time T_a acquired in the previous step 305.

The method may be repeated to provide an average aortic pulse wave velocity from a plurality of measurements by the sound transducers. The reliability and robustness of the calculated average aortic pulse wave velocity may thus be improved.

Variations and alternative embodiments of the method outlined above and in connection with figure 3 should be understood to correspond to the embodiments elucidated in connection with the system 100 explained in conjunction with figure 1 and 2. The skilled person also realizes that there are further variations and embodiments which are possible and within the scope of the invention. For example, more than two sound transducers may be used to enhance or replace the detection of the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation. Further, the identification of the opening of the aortic valve and the pulse wave arrival time at the aortic bifurcation may be wholly or partially automatic by algorithms or curve fittings as described in the foregoing. Furthermore, the detection may be enhanced by restricting which frequencies are comprised in the recorded signals, either by electrical filters comprised in the sound transducers or the control unit, or by software filters comprised in the control unit.

Although the figures may show a specific order of method steps, the order of the steps may differ from what is depicted. Also two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps. Additionally, even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Furthermore, 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.

The embodiments of the present disclosure may be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwired system.

Claims

1 . Method for determining aortic pulse wave velocity, the method comprising:

- receiving an indication of a measurement of a length of a human aorta outside the body to provide an aortic length;

- determining the aortic transit time based on the difference between the second time and the first time; and

- calculating the aortic pulse wave velocity based on the aortic length and the aortic transit time.

2. The method according to claim 1 , wherein the opening of the aortic valve is identified by sounds having a frequency in the range of 25 Hz to 300 Hz.

3. The method according to claim 2, wherein the opening of the aortic valve is identified by sounds having a frequency in the range of 1 Hz to

25 Hz.

4. The method according to anyone of the preceding claims, wherein the pulse wave arrival time at the aortic bifurcation is identified as the start of the pulse wave.

5. The method according to anyone of the preceding claims, wherein the identifying of the opening of the aortic valve with measurement from the first sound transducer as a first time, and the pulse wave arrival time at the aortic bifurcation with measurements from the second sound

transducer as a second time is performed automatically by a control unit comprising a processor.

6. The method according to claim 5, wherein the method further comprises:

- receiving user input to identify the first time and the second time; and

- correcting the first time and the second time based on the user input, wherein the corrected first and second time are used for determining the aortic transit time and calculating the aortic pulse wave velocity.

7. The method according to claim 1 , wherein the identifying of the opening of the aortic valve with measurement from the first sound transducer as a first time, and the pulse wave arrival time at the aortic bifurcation with measurements from the second sound transducer as a second time further comprises:

- receiving user input to identify the first time and the second time.

8. The method according to any one of the preceding claims, wherein the method is repeated to provide an average aortic pulse wave velocity from a plurality of measurements by the sound transducers.

9. A system for determining aortic pulse wave velocity, the system comprising: - a first sound transducer adapted to measure the opening of the aortic valve;

10. The system according to claim 9, wherein the first sound transducer and the second sound transducer are synchronized.

1 1 . The system according to claim 9 or 10, further comprising a display and user input means.

12. The system according to claim 1 1 , wherein the control unit is configured to display the measurements from the first sound transducer and the second sound transducer on the display, and to receive user input to identify the first time and the second time, wherein the control unit is configured to correct the first time and the second time based on the user input, wherein the corrected first and second time are used to determine the aortic transit time and calculate the aortic pulse wave velocity.

13. The system according to any one of claims 9 to 12, wherein the first sound transducer and/or the control unit comprises a low pass filter having a cutoff frequency of 300 Hz.

14. The system according to any one of claims 9 to 12, wherein the first sound transducer and/or the control unit comprises a low pass filter having a cutoff frequency of 25 Hz.

15. A system for determining aortic pulse wave velocity, the system comprising:

- a first sound transducer adapted to measure the opening of the aortic valve;

- a display;

- user input means;

- a control unit connected to the first sound transducer, the second sound transducer, the display and the user input means; and

wherein the control unit is configured to receive measurement signals from the first sound transducer and the second sound transducer, and display the measurements on the display, and to receive user input via the user input means to identify the opening of the aortic valve as a first time and the pulse wave arrival time at the aortic bifurcation as a second time, and to receive an aortic length measurement,