WO2011132117A1

WO2011132117A1 - Method and system for monitoring or assessing the cardiovascular system of a subject

Info

Publication number: WO2011132117A1
Application number: PCT/IB2011/051591
Authority: WO
Inventors: Jens Muehlsteff; Elke Naujokat
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2010-04-21
Filing date: 2011-04-13
Publication date: 2011-10-27

Abstract

There is provided a method for assessing the health of a cardiovascular system of a subject, the method comprising obtaining a plurality of measurements of the heart rate and pulse arrival time of the subject during a test procedure in which the subject performs exercises and rests; comparing the measurements of the heart rate to the measurements of the pulse arrival time obtained during at least part of the test procedure; and determining one or more parameters relating to the health status of the cardiovascular system of the subject from the comparison.

Description

Method and system for monitoring or assessing the cardiovascular system of a subject

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method and system for assessing or monitoring the health of the cardiovascular system of a subject, and in particular to a method and system for assessing or monitoring the health of the cardiovascular system of a subject that can provide an indication and assessment of the mechanical heart pumping function and/or the status of the vasculature of the subject

BACKGROUND TO THE INVENTION

Coronary artery disease (CAD) is caused by an accumulation of plaques within the walls of the arteries that supply the myocardium with oxygen and nutrients. After years or decades of progression, some of these plaques may rupture and, along with the activation of the blood clotting system, limit blood flow to the myocardium, resulting in an acute coronary syndrome (ACS). This may be either a heart attack (myocardial infarction - meaning that muscle cell necrosis in the affected regions of the myocardium has occurred) or unstable angina (meaning that the patient has persistent or recurrent chest pain at rest but without evidence of myocardial necrosis). Risk factors comprise age, sex, family history but also lifestyle-related aspects such as smoking habits, physical inactivity, overweight/obesity etc. CAD is the single most common cause of death in the EU, accounting alone for approximately 22% of all deaths and resulting in direct health costs of€23 billion. Among patients who have survived a first heart attack, most will go on to have further episodes of ACS in the following years or will develop heart failure (or both). Preventing patients from having a second heart attack by improving adherence to lifestyle and medication regimens has a significant cost saving potential.

After myocardial infarction (MI), patients receive acute treatment in hospital. Typically, this includes antithrombotic and antiplatelet therapy and/or a percutaneous coronary intervention (PCI). The latter often includes stent implantation which helps to reduce the threat of abrupt closure or re-stenosis. After discharge, ideally a cardiac rehabilitation programme should start as soon as possible. However, the availability and realization of cardiac rehabilitation programmes differs largely between the healthcare systems of the different countries. For example in Spain, cardiac rehabilitation after an MI is only available for about 10% of all patients. In Germany, patients are usually offered a 3- week cardiac rehabilitation programme. Most often, these programmes are carried out in specialized rehabilitation clinics. To a lesser extent, cardiac rehabilitation programmes are also available in outpatient clinics. In the UK, 6-12-week cardiac rehabilitation programmes with nurse support are available. Typically, the patient will follow a programme described in a manual while living his normal daily life at home. Every 1-2 weeks a trained rehabilitation nurse visits them at home and evaluates their progress, discusses problems, motivates them and supports them with further therapy measures.

Devices are commercially available that measure a user's ECG signal either with a belt around the chest or with sensors integrated into a shirt and provide real-time feedback on heart rate via a dedicated wristwatch which is wirelessly connected to the sensor. Some devices allow 'target zones' for heart rate to be defined and for respective visual and/or audible alarms to be set. Advanced models include software that provides a more detailed analysis, for example on heart rate variability or performance development. Software packages are also available for creating individual training plans. However, these kinds of products only target healthy users.

Post-MI patients sometimes use these kinds of devices for the self- management of their condition, although this is usually on their own initiative. Manufacturers of these devices do not generally support this group of users, for example by providing appropriate training plans or recommendations for target heart rate zones.

Usually, feedback on the effectiveness of training from a medical point of view is only available from encounters with health professionals, for example through cardiopulmonary exercise testing (clinical performance examination - CPX) which is a procedure that is only available at a clinic or at a cardiologist's office and requires dedicated equipment (ergometer bike, apparatuses for testing lung function at rest and during exercise, a 12-lead ECG, etc.), and takes about 40 minutes. CPX is currently not used for progress monitoring during rehabilitation programmes, but rather for medical check-ups of cardio patients (for example once a year).

Therefore, there is a need for a method and system that can be used to monitor and/or assess the cardiovascular system of a subject, and in particular that can provide an indication of the mechanical heart pumping function and/or the status of the vasculature of the subject. In addition, the method and system should be suitable for use by subjects that are undergoing rehabilitation following an MI. SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method for assessing the health of a cardiovascular system of a subject, the method comprising obtaining a plurality of measurements of the heart rate and pulse arrival time of the subject during a test procedure in which the subject performs exercises and rests; comparing the measurements of the heart rate to the measurements of the pulse arrival time obtained during at least part of the test procedure; and determining one or more parameters relating to the health status of the cardiovascular system of the subject from the comparison.

In a preferred embodiment, the step of comparing comprises plotting the plurality of heart rate measurements against the plurality of pulse arrival time measurements. The resulting plot allows a number of useful parameters to be obtained and allows the health status and changes in the health status of the subject to be easily visualized by the subject and healthcare providers.

To allow the determined parameters to be easily compared to previously obtained parameters for the subject or another subject, the plurality of measurements of the heart rate and the pulse arrival time are normalized prior to the step of comparing, and preferably the plurality of measurements of the heart rate and the pulse arrival time are normalized with respect to a respective resting heart rate measurement and pulse arrival time measurement obtained at the start of the test procedure before the subject performs exercises.

Preferably, the step of determining comprises determining one or more parameters relating to the health of the cardiovascular system of the subject from the plot of the plurality of heart rate measurements against the plurality of pulse arrival time

measurements.

Embodiments of the invention provide that the one or more parameters include determining the area covered by the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements for part or all of the test procedure;

determining the slope of the line formed by the plot of the plurality of heart rate

measurements against the plurality of pulse arrival time measurements during different stages of the test procedure, including an initial or intermediate part of the exercise, a final part of the exercise, an initial or intermediate part of a recovery period and a final part of the recovery period; and/or determining the vector defining the difference between the points on the plot representing the start and end of the test procedure respectively. In an embodiment, the method further comprises the step of displaying the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements obtained during the test procedure. This allows the subject or healthcare professional to easily visualize the health status of the subject's car dio -vascular system.

In a preferred embodiment, the method further comprises repeating, at least once, the steps of obtaining, comparing and determining for the subject and comparing the values for the one or more parameters obtained during each iteration of the step of determining to provide an indication of the change in the health of the cardiovascular system of the subject over time.

According to a second aspect of the invention there is provided a system for use in assessing the health of a cardiovascular system of a subject that comprises a processor configured to receive a plurality of measurements of the heart rate and pulse arrival time of the subject obtained during a well-defined test procedure in which the subject performs exercises and rests, compare the measurements of the heart rate to the measurements of the pulse arrival time in the context of the test procedures and determine one or more parameters relating to the health status of the cardiovascular system of the subject from the comparison.

According to a third aspect of the invention there is provided a computer program product comprising computer readable code that is configured to cause a computer or processor to perform the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will now be described with reference to the following drawings, in which:

Fig. 1 is a block diagram illustrating the components of the system according to an embodiment of the invention

Fig. 2 is a drawing illustrating a subject wearing the system according to an embodiment of the invention;

Fig. 3 is a flow chart illustrating the operation of the system according to an embodiment of the invention;

Fig. 4 is a set of graphs illustrating changes in different cardiovascular system parameters over a number of activity and resting phases for a fit and healthy subject;

Fig. 5 is a set of graphs illustrating changes in different cardiovascular system parameters over a number of activity and resting phases for a healthy but unfit subject; Fig. 6 is a graph showing a plot of heart rate versus pulse arrival time for a fit subject;

Fig. 7 is a graph showing a plot of heart rate versus pulse arrival time for an unfit subject;

Fig. 8 is a graph showing the (normalized) plots of heart rate versus pulse arrival time for two subjects;

Fig. 9 is a flow chart illustrating the steps performed in step 103 of Figure 3 in a specific embodiment of the invention;

Fig. 10 is a graph illustrating a parameter that can be determined from the plot of heart rate versus pulse arrival time according to the invention;

Fig. 11 is a graph illustrating another parameter that can be determined from the plot of heart rate versus pulse arrival time according to the invention;

Fig. 12 is a graph illustrating yet another parameter that can be determined from the plot of heart rate versus pulse arrival time according to the invention;

Fig. 13 is a graph illustrating yet another parameter that can be determined from the plot of heart rate versus pulse arrival time according to the invention;

Fig. 14 is a graph illustrating yet another parameter that can be determined from the plot of heart rate versus pulse arrival time according to the invention; and

Fig. 15 is a graph illustrating yet another parameter that can be determined from the plot of heart rate versus pulse arrival time according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system 2 according to an embodiment of the invention is illustrated in

Figure 1. In order to monitor or assess the cardiovascular system of a subject, the system 2 comprises one or more sensors 4, 6 that are worn or carried on the body of the subject and that provide measurements of the heart rate (HR) and pulse arrival time (PAT) during physical activities of the subject over time. The measurements from the one or more sensors

4, 6 are provided to a processor 8 where they are analyzed and used to determine one or more parameters relating to the cardiovascular system of the subject.

In particular, in this illustrated embodiment, the system 2 comprises a heart rate sensor 4 and a sensor 6 for use in providing measurements of the pulse arrival time.

Heart rate sensors 4 and pulse arrival time sensors 6 are known in the art and will not be described in detail herein. Nevertheless, in a preferred embodiment of the invention, the heart rate sensor 4 comprises an ECG sensor having one or more electrodes that are used to measure the electrical activity of the heart muscle, and the sensor 6 comprises, for example, a

photoplethysmogram (PPG) sensor that measures arterial blood volume changes, or a piezo- electric sensor that measures vibrations due to passing pulse pressure waves.

As known, the pulse arrival time can be determined by the processor 8 as the time interval between the R-peak in measurements from the ECG sensor 4 and a passing pulse in an artery at a certain body position (for example the arm or leg) as measured by the sensor 6. The pulse arrival time is sensitive to stroke volume (cardiac output), peripheral resistance and blood pressure changes.

After determining the parameter or parameters that relate to the health and training status of the cardiovascular system of the subject from the sensor measurements, the processor 8 may provide the result or other feedback derived from the result to the subject on a display screen of the system 2 (not shown in Figure 1), store the result in a memory (also not shown in Figure 1) for later review by the subject or a healthcare professional and/or transmit the result to a remote monitoring station via wired or wireless means (also not shown in Figure 1) for review by a healthcare professional.

The feedback provided to the subject or healthcare professional can include the parameters obtained from the measurements of the heart rate and pulse arrival time, an indication or alarm that a threshold heart rate and/or pulse arrival time has been achieved or exceeded, instructions relating to the exercise or activity to be completed by the subject, etc. The feedback can be provided visually, audibly and/or through tactile means (such as vibration).

Figure 2 shows a subject 10 wearing the system according to an embodiment of the invention. Thus, the heart rate sensor 4, which may comprise an ECG sensor, is placed on the chest of the subject 10 and is held in position by a strap or band 12, and the sensor 6, which may comprise a PPG sensor or piezo-electric sensor is attached to the wrist of the subject 10 by a strap or band 14.

It will be appreciated by those skilled in the art that either or both of the heart rate sensor 4 and/or sensor 6 may be integrated directly into clothing to be worn by the subject 10 rather than being held in place by respective straps or bands.

The processor 8 may be incorporated into the same unit or housing as either the heart rate sensor 4 or sensor 6 and receive the measurements from the other sensor via a wired or wireless connection. Alternatively, the processor 8 may not necessarily be carried by the subject 10 themselves, but could be located in another unit in the subject's home (for example in a PDA, general purpose computer, a base unit or docking station for the sensors 4, 6, etc.). In this case, the sensors 4, 6 can provide the measurements to the processor 8 via wireless or wired means for real-time or near real-time processing of the measurements, or the measurements can be uploaded to the processor 8 at various intervals through the base unit or docking station.

Figure 3 illustrates a method of operating the system according to the invention. In step 101, a plurality of measurements of the heart rate and pulse arrival time are obtained for a subject during the course of a test procedure.

As described below in more detail, the parameters determined according to the invention are indicative of how the subject's cardiovascular system reacts to and/or recovers from periods of increased physical activity.

Therefore, the test procedure preferably involves at least one period in which the subject performs a specified exercise and at least one period in which the subject rests after completing the exercise, although it will be appreciated from the discussion below that, depending on the specific parameters to be obtained from the heart rate and pulse arrival time measurements, the test procedure (i.e. the time frame over which measurements of the heart rate and pulse arrival time are collected) can comprise multiple periods of exercise and rest, or could include just a single period of exercise without a rest period. Preferably (although this is not essential), the subject starts the test procedure (or first exercise period in the test procedure) with their heart rate at or near their resting heart rate and their pulse arrival time at or near their resting pulse arrival time rate.

The specified exercise can comprise any suitable exercise that will result in the heart rate of the subject increasing, so for example walking, riding an exercise bike or running on a treadmill would be suitable exercises.

Once sufficient measurements of the heart rate and pulse arrival time have been obtained (the number of measurements required being dependent on the particular parameters to be determined), the measurements of heart rate and pulse arrival time are compared (step 103) to determine one or more parameters relating to the mechanical heart pumping function and the status of the vasculature of the subject 10.

The comparison and parameter extraction are described in more detail below. A set of graphs illustrating the changes in different standard cardiovascular system parameters over a number of activity and resting phases for a fit and healthy 25 year old subject are shown in Figure 4. In particular, the graphs show the variations of heart rate, pulse arrival time (PAT), pre-ejection period (PEP), and systolic and diastolic blood pressure (SBP, DBP) for the subject during five phases of a test procedure which (as shown by the activity graph) starts with a resting period, followed by physical exercise at 75 W, another resting period, physical exercise at 175 W and a final resting period. It will be appreciated that these measurements will be obtained by a device 2 that contains additional sensors to the heart rate sensor 4 and pulse arrival time sensor 6.

From Figure 4, it can be seen that at the beginning of the test procedure, the subject has a PAT of 220 ms and a PEP of 90 ms. During the test procedure, significant changes of the systolic blood pressure are induced, whereas the diastolic blood pressure remains almost constant compared to the blood pressure measurement accuracy. The maximal heart rate during the test procedure is 134 beats/min. For this specific subject, it can be seen that during the first activity phase and the following resting period the PEP changes jointly with the PAT (5PAT_max = -55 ms) and almost no PTT variation is observed. The PAT and PEP reach their base levels again after a first recovery period of 4 minutes. However, in the second activity phase, the maximum PEP variation is roughly the same of the first phase, but the PAT is smaller indicating a significant contribution by δΡΤΤ of about -20 ms (in the PAT/PEP graph, the upper line in the second activity phase is that representing the PEP while the lower line is that representing the PAT). This δΡΤΤ disappears quickly in the second recovery phase. PEP and PAT then evolve in parallel reaching their base levels within 5 minutes.

Figure 5 shows a corresponding set of graphs illustrating the changes in these standard cardiovascular system parameters over the same activity and resting phases for a 34 year old subject who does not regularly exercise. In this case, during both activity phases and the following resting phases, there are contributions of δΡΕΡ as well as δΡΤΤ (again, in the PAT/PEP graph, the upper line shows the PEP while the lower line shows the PAT). For the first activity period there is a δΡΤΤ = -15 ms and during the second activity period there is a δΡΤΤ = -30 ms. The baseline levels of PAT and PEP from the beginning of the test procedure are not reached again in the allowed recovery phases, which indicates that the subject requires a longer recovery or recuperation time for PEP and PAT compared to heart rate.

Thus, it can be seen that comparing the evolution of the heart rate to that of the pulse arrival time over a test procedure can provide an indication of the health and training status of the cardiovascular system of the subject, and in particular an indication of the mechanical heart pumping function and the status of the vasculature of the subject. A plot of heart rate versus pulse arrival time for a fit and healthy subject is shown in Figure 6. The plot has been generated from the measurements for heart rate and pulse arrival time for the first approximately 500 seconds of an exercise procedure as shown in Figure 4, including a resting period, exercise period and recovery period. Thus, it can be seen that, prior to the start of the exercise period (marked as region 50 in Figure 6), the subject's heart rate and pulse arrival time are relatively constant around the values 72 bpm and 220 ms respectively. However, once the exercise is commenced (indicated by arrow 55), the heart rate increases and the pulse arrival time decreases with an almost linear correlation. Once the exercise is completed and the subject starts to recover (indicated by arrow 60), there is a relatively fast decrease in the heart rate compared to the corresponding increase in the pulse arrival time. It can be seen that the subject's heart rate and pulse arrival time have returned to their pre-exercise values (approximately) by the end of the recovery period. Hysteresis behavior can clearly be seen in Figure 6.

A corresponding plot of heart rate versus pulse arrival time for the unfit subject is shown in Figure 7. Again, the plot has been generated from the measurements for heart rate and pulse arrival time during the first 500 seconds (approximately) of the test procedure shown in Figure 5. As with the fit subject, it can be seen that, prior to the start of the exercise period (marked as region 70 in Figure 7), the subject's heart rate and pulse arrival time are relatively constant around particular values, although they are 80 bpm and 200 ms respectively for this subject. As with the fit subject, the heart rate increases and the pulse arrival time decreases with an almost linear correlation during the exercise period (marked with arrow 75). However, although the subject's heart rate approximately returned to its pre-exercise/resting level by the end of the recovery period (that period being indicated by arrow 80), it can be seen that the pulse arrival time did not return to its pre-exercise value within the same or allowed time frame. Therefore, the graph clearly indicates that this subject's cardiovascular system is still recovering from the exercise period, even when the heart rate has returned to its pre-exercise or resting level. As with Figure 6, hysteresis behavior can also clearly be seen in Figure 7.

Therefore, because of the different response times of heart rate and pulse arrival time, in particular during a recovery period, the measurement and analysis of pulse arrival time alongside heart rate can provide additional information that is useful for the personalized and optimized training of post-MI subjects.

Further useful information can be obtained from a comparison of the two plots in Figures 6 and 7 for the fit and unfit subjects respectively. Figure 8 contains the plots for both the fit and unfit subjects with the measurements of heart rate and pulse arrival time for each subject being normalized with respect to that subject's resting or pre-exercise heart rate and pulse arrival time. Thus, prior to the exercise, both plots start around the point (1, 1). It can be seen from Figure 8 that there are significant differences between the reactions of the subjects' cardiovascular systems to the exercise and recovery periods.

The graph shown in Figure 8 can be used to compare a subject's performance to that of a model "fit" subject (with the "fit" subject being appropriate for the particular subject under test - so a fit 60 year old subject for a 60 year subject that has suffered an MI, etc.). Alternatively, the results of multiple test procedures completed by a subject can be plotted on the same graph to illustrate how the subject's cardiovascular system is changing over time (i.e. over the course of a rehabilitation programme or a training programme).

In view of the above, the flow chart in Figure 9 sets out a preferred implementation of step 103 from Figure 3. In particular, step 103 preferably comprises normalizing the plurality of heart rate and pulse arrival time measurements with respect to a base line or resting heart rate and pulse arrival time measurement that is obtained prior to the start of an exercise period or prior to the start of a test procedure (step 1031); plotting the normalized measurements of heart rate against the normalized measurements of pulse arrival time (step 1033); and analyzing the plot to determine the one or more parameters relating to the cardiovascular system of the subject (step 1035).

Some exemplary parameters that can be extracted from the heart rate versus pulse arrival time plot are described below and illustrated in Figures 10 to 15. Each of Figures 10 to 15 is based on the plot of normalized heart rate measurements versus normalized pulse arrival time measurements for the fit subject (and so correspond to the normalized version of Figure 6).

One type of parameter that can be extracted by the processor 8 from the plot of heart rate versus pulse arrival time is shown in Figure 10 as the area 90 generally

encompassed by the plot for the complete test procedure or for one or more exercise periods followed by their respective recovery periods. Methods and algorithms for determining the area enclosed by a plot or graph are well known in the art, and will not be described in further detail herein. If this parameter is monitored over time (i.e. at various points during a rehabilitation or exercise programme), then an increase in the area of the graph indicates a positive progression of the training status of the subject.

Another type of parameter that can be extracted by the processor 8 from the plot of heart rate versus pulse arrival time is the gradient or slope of part of the line formed by the plot of the measurements obtained during the test procedure or a part of the test procedure (for example a single exercise period followed by a single rest period if the test procedure comprises multiple exercise and/or rest periods). Methods and algorithms for determining the slope of part of a plot or graph from the plot itself or from the measurements making up the plot are well known in the art and will not be described in further detail herein.

A first slope or gradient measurement can be made for an initial or intermediate part of the exercise period as shown by line 91 in Figure 11. This gradient or slope characterizes the generally linear correlation between the changes in the heart rate and the changes in the pulse arrival time from the start of the exercise period which is discussed above. It will be appreciated that the relevant initial or intermediate part of the exercise period can comprise most of the plot corresponding to the exercise period (as in Figure 11) or just a small section of the plot. If this parameter is monitored over time (i.e. at various points during a rehabilitation or exercise programme), then an increase in the slope indicates typically an improving performance of the car dio -vascular system of the patient and a positive impact of a training program.

Another slope or gradient measurement can be taken from a final part of the exercise period as shown by line 92 in Figure 12. This gradient or slope characterizes the linear correlation between the changes in the heart rate and the changes in the pulse arrival time at the end of the exercise period, which is generally different to the linear correlation of the heart rate and pulse arrival time that occurs during an initial and intermediate part of the exercise period. If this parameter is monitored over time (i.e. at various points during a rehabilitation or exercise programme), then an increase in the slope indicates an improvement in the car dio -vascular status of the subject.

Yet another slope or gradient measurement can be made for an initial or intermediate part of a recovery period following the exercise period as shown by line 93 in Figure 13. This gradient or slope characterizes the generally linear correlation between the changes in the heart rate and the changes in the pulse arrival time at the start of the recovery period. If this parameter is monitored over time (i.e. at various points during a rehabilitation or exercise programme), then an increase in the slope indicates an improvement in the cardio- vascular status of the subject.

Yet another slope or gradient measurement can be made for a final part of a recovery period following the exercise period as shown by line 94 in Figure 14. This gradient or slope characterizes the generally linear correlation between the changes in the heart rate and the changes in the pulse arrival time at the end of the recovery period and typically the slope should increase with improvements in the training status of the car dio -vascular system of the subject.

It should be emphasized that responses of different subjects can show a large variability due to different co-morbidities and related therapy programmes (e.g. medications). A physician supervising the cardio -vascular status of a patient during a rehabilitation program will therefore define personalized parameters taking into account the specific situation of the patient.

Another type of parameter that can be extracted by the processor 8 from the plot of heart rate versus pulse arrival time is shown in Figure 15 as the vector 95 between the initial state of the subject's cardiovascular system and the state of the subject's

cardiovascular system following the recovery period. As shown in Figure 15, prior to the exercise period, the subject's cardiovascular system is in a resting state with the plot being generally centered around point (1, 1) which corresponds to the subject's resting heart rate and resting pulse arrival time. Ideally, the subject's heart rate and pulse arrival time will return to, or quite near to, these resting values by the end of the allowed recovery period

(which would result in a vector having a zero or a nearly zero magnitude). However, in many cases (particularly for unfit and/or post-MI subjects), the heart rate and/or pulse arrival time will not have returned to their resting values by the end of the recovery period, so the magnitude and/or direction of the vector between point (1, 1) and the location of the plot at the end of the recovery period will provide an indication of the health of the cardiovascular system of the subject.

After extraction of one or more of these parameters, the processor 8 can compare the extracted values to values obtained from previous test procedures performed by the subject or by a 'model' subject to determine whether the subject is improving over time. The result of this comparison can be provided to the subject or their healthcare professional so that action can be taken to set an appropriate rehabilitation programme (which includes specifying types and lengths of exercises, the length of the recovery period, the maximum recommended heart rate and/or pulse arrival time, etc.) or to re-evaluate an existing rehabilitation programme.

In addition to the feedback described above, the system 2 according to the invention can display the graph of heart rate versus pulse arrival time to the subject or healthcare professional as the subject is performing the test procedure, after the subject has completed the test procedure, or it can be stored in the system 2 for later review by the subject or a healthcare professional. Although not shown in Figure 1 , the system 2 can comprise additional sensors for measuring other standard parameters of the subject's cardiovascular system. For example, the system 2 can additionally comprise any or all of the following sensors:

a phonocardiography (PCG) sensor for measuring the heart sounds associated with the opening and closing of the heart valves;

a (or a further) piezo-electric sensor positioned on the thorax for measuring the vibrations caused by the heart contraction;

a bio-impedance sensor for measuring the impedance changes associated with the pulsatile changes in blood volume in the addressed body part;

- an accelerometer for measuring thorax movements associated with breathing activity and/or for measuring the posture and/or movement of the subject;

an automated event-marker based on automated detection of a fall using an accelerometer.

In these embodiments, the system 2 can provide additional measurements of the cardiovascular system and general health of the subject 10 that can be of use to the subject 10 and/or healthcare professional. For example, provided the appropriate sensors are present in the system 2, any or all of the following measurements can be obtained:

Pulse Transit Time (PTT) which is the time interval from the closure of the aortic valve until the onset of an arriving pulse (e.g. PPG);

- Left Ventricular Ejection Time (LVET) which can be estimated from PPG pulse contour analysis signals or by analysis of heart sounds;

Pre-Ejection Period (PEP) which is measured as part of PAT or by analysis of heart sounds;

Quantified activity level which is derived from accelerometer signals; and/or - Classification of activities which is also derived from accelerometer signals e.g. to discriminate resting phases, exercises and recovery periods. An optional activity sensor 9 is shown in Figure 1 that is connected to the processor 8 which can be used for the classification of activities.

As described above, the system 2 is particularly intended for use by post-MI subjects for improving the evaluation of the effectiveness of their rehabilitation programmes, and it can be used in systems that remotely monitor post-MI subjects in their home. However, the system 2 can also be used by other subjects to provide an indication of the health of their cardiovascular system as they exercise, particularly the status of their mechanical heart pumping function and their vasculature. 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.

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. 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 processor 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 measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method for assessing the health of a cardiovascular system of a subject, the method comprising:

obtaining a plurality of measurements of the heart rate and pulse arrival time of the subject during a test procedure in which the subject performs exercises and rests (101);

comparing the measurements of the heart rate to the measurements of the pulse arrival time obtained during at least part of the test procedure (103); and

determining one or more parameters relating to the health status of the cardiovascular system of the subject from the comparison (103).

2. A method as claimed in claim 1, wherein the step of comparing (103) comprises plotting the plurality of heart rate measurements against the plurality of pulse arrival time measurements (1033).

3. A method as claimed in claim 2, wherein the plurality of measurements of the heart rate and the pulse arrival time are normalized prior to the step of comparing (1033).

4. A method as claimed in claim 3, wherein the plurality of measurements of the heart rate and the pulse arrival time are normalized with respect to a respective resting heart rate measurement and pulse arrival time measurement obtained at the start of the test procedure before the subject performs exercises.

5. A method as claimed in any of claims 2, 3 or 4, wherein the step of determining (103) comprises determining one or more parameters relating to the health of the cardiovascular system of the subject from the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements (1035).

6. A method as claimed in claim 5, wherein the step of determining one or more parameters from the plot (1035) comprises determining the area covered by the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements for part or all of the test procedure.

7. A method as claimed in claim 5 or 6, wherein the step of determining one or more parameters from the plot (1035) comprises determining the slope of the line formed by the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements that were obtained during an initial or intermediate part of the exercise.

8. A method as claimed in claim 5, 6 or 7, wherein the step of determining one or more parameters from the plot (1035) comprises determining the slope of the line formed by the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements that were obtained during a period at the end of the exercise.

9. A method as claimed in claim 5, 6, 7 or 8, wherein the step of determining one or more parameters from the plot (1035) comprises determining the slope of the line formed by the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements that were obtained during an initial part of a rest period after completion of the exercise.

10. A method as claimed in claim 5, 6, 7, 8 or 9, wherein the step of determining one or more parameters from the plot (1035) comprises determining the slope of the line formed by the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements that were obtained during a later part of the rest period.

11. A method as claimed in claim 5, 6, 7, 8, 9 or 10, wherein the step of determining one or more parameters from the plot (1035) comprises determining the vector defining the difference between the points on the plot of the heart rate measurements against the pulse arrival time measurements representing the start and end of the test procedure respectively.

12. A method as claimed in any one of claims 2 to 11, further comprising the step of:

displaying the plot of the plurality of heart rate measurements against the plurality of pulse arrival time measurements obtained during the test procedure.

13. A method as claimed in any preceding claim, wherein the method further comprises:

repeating, at least once, the steps of obtaining (101), comparing (103) and determining (103) for the subject; and

comparing the values for the one or more parameters obtained during each iteration of the step of determining to provide an indication of the change in the health of the cardiovascular system of the subject over time.

14. A system (2) for use in assessing the health of a cardiovascular system of a subject, the system (2) comprising:

a processor (8) configured to:

receive a plurality of measurements of the heart rate and pulse arrival time of the subject obtained during a test procedure in which the subject performs exercises and rests;

compare the measurements of the heart rate to the measurements of the pulse arrival time; and

determine one or more parameters relating to the health status of the cardiovascular system of the subject from the comparison.

15. A computer program product comprising computer readable code that is configured to cause a computer or processor to perform the method claimed in any of claims 1 to 13.