U.S. patent application number 10/531764 was filed with the patent office on 2006-10-19 for cardiac monitoring apparatus and method.
Invention is credited to Nigel Robert Oakley, Gary Steven Ungless.
Application Number | 20060235316 10/531764 |
Document ID | / |
Family ID | 9946164 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060235316 |
Kind Code |
A1 |
Ungless; Gary Steven ; et
al. |
October 19, 2006 |
Cardiac monitoring apparatus and method
Abstract
The present invention pertains to a monitor that includes a
cardiac sensor (36) responsive to a user's heart beat. The monitor
includes a processor coupled to the sensor for generating
heart-rate or other cardiac data. These data can be stored in a
memory. The monitor is physically supported by and receives
electrical signals from a single ECG electrode, and is coupled by
an electrical lead to a second ECG electrode.
Inventors: |
Ungless; Gary Steven;
(Cambridgeshire, GB) ; Oakley; Nigel Robert;
(Cambridgeshire, GB) |
Correspondence
Address: |
SALIWANCHIK LLOYD & SALIWANCHIK;A PROFESSIONAL ASSOCIATION
PO BOX 142950
GAINESVILLE
FL
32614-2950
US
|
Family ID: |
9946164 |
Appl. No.: |
10/531764 |
Filed: |
October 16, 2003 |
PCT Filed: |
October 16, 2003 |
PCT NO: |
PCT/GB03/04487 |
371 Date: |
April 17, 2006 |
Current U.S.
Class: |
600/509 |
Current CPC
Class: |
A61B 5/11 20130101; A61B
5/1118 20130101; A61B 2560/0412 20130101; A61B 5/0006 20130101;
A61B 5/6823 20130101; A61B 2562/0219 20130101; A61B 5/335 20210101;
A61B 5/721 20130101 |
Class at
Publication: |
600/509 |
International
Class: |
A61B 5/04 20060101
A61B005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 18, 2002 |
GB |
0224299.8 |
Claims
1-22. (canceled)
23. A monitor for monitoring a user's heart, comprising: a support
means for securing the monitor in position for sensing the user's
heart beat, the support means being for attachment to a single
adhesive electrocardiogram (ECG) electrode both to support the
monitor and for receiving electrical signals from the ECG
electrode; a means for electrically coupling the monitor to a
second ECG electrode for receiving signals therefrom; a cardiac
sensor for receiving signals from the ECG electrodes; and a
processor coupled to the cardiac sensor for generating cardiac
data.
24. The monitor according to claim 23, in which the coupling means
comprises an electrical lead extending from a housing of the
monitor or a socket in the housing of the monitor for receiving an
electrical lead.
25. The monitor according to claim 23, in which the single adhesive
ECG electrode is a standard ECG electrode.
26. The monitor according to claim 23, in which the maximum lateral
dimension of the ECG electrode is 55 mm or less.
27. The monitor according to claim 23, in which the maximum lateral
dimension of the monitor is 35 mm or less.
28. The monitor according to claim 23, in which the maximum lateral
dimension of the monitor is less than or equal to the maximum
lateral dimension of the ECG electrode.
29. The monitor according to claim 23, in which the monitor, in
use, does not extend beyond an outer edge of the ECG electrode.
30. The monitor according to claim 23, in which the weight of the
monitor is less than 50 grams.
31. The monitor according to claim 23, comprising a memory coupled
to an output of the processor for storing the cardiac data.
32. The monitor according to claim 23, in which the processor
generates inter-beat interval data from signals it receives from
the cardiac sensor.
33. The monitor according to claim 23, further comprising an
accelerometer coupled to the processor, so that the processor can
generate movement data.
34. The monitor according to claim 33, in which the processor
processes signals it receives from the cardiac sensor according to
a predetermined parameter in order to generate the cardiac data and
modifies that parameter in response to signals it receives from the
accelerometer.
35. The monitor according to claim 33, in which a parameter, such
as a gain parameter or a threshold voltage, used in deriving the
cardiac data from an output of the cardiac sensor is variable in
response to an output from the accelerometer or a movement or
activity parameter derived therefrom.
36. The monitor according to claim 23, comprising contacts for
making electrical contact with two ECG electrodes, in which the
same contacts are couplable to an interface for transferring data
from and/or to the monitor, and/or for resetting or reprogramming
the monitor, and/or for recharging a battery for powering the
monitor.
37. The monitor according to claim 33, in which the monitor in use
is secured to the chest or torso of the user so that the
accelerometer is oriented to sense vertical movements of the user's
chest or torso.
38. The monitor according to claim 23, which is of small size and
weight so as to be comfortable for a user to wear for extended data
sampling periods.
39. A method for monitoring a user's heart, comprising the steps
of: sensing the user's heart beat by using a cardiac sensor secured
to the user's body by means of a single ECG electrode, processg
cardiac signals to generate cardiac data; and storing or displaying
the data.
40. The method according to claim 39, comprising the step of
sensing movement of the cardiac senor and processing movement
signals to generate movement data.
Description
[0001] This invention relates to a cardiac monitoring apparatus and
method for monitoring a user's heart rate, or other parameters
derived from heart-beat sensing.
[0002] Heart rate is a physiological parameter that is measured in
a wide variety of situations, for example to determine the health
status and fitness of a person or animal. It can be used, for
example, to give a measure of energy expenditure of an individual
and a number of devices exist for doing this by converting heart
rate to calories used. Many conventional systems comprise a belt
worn around a user's chest and carrying a heart-beat sensor and a
radio transmitter for transmitting measured data to a wrist-worn
display unit.
[0003] Such conventional systems are generally uncomfortable or
impractical to wear for extended periods and also suffer a
significant problem in that correlating heart rate with calories
used may only be effective for exercise rates achieving significant
heart rate increases. Smaller increases in heart rate can be due
to, for example, stress rather than physical exertion and may
therefore be misinterpreted by conventional heart-rate monitoring
systems.
[0004] The invention provides an apparatus and a method for
monitoring a user's heart as defined in the appended independent
claims. Preferred or advantageous features of the invention are set
out in dependent subclaims.
[0005] The invention may thus advantageously provide a monitor for
monitoring a user's heart, comprising a cardiac sensor and a
support means for mounting the monitor only on a single adhesive
pad, which is preferably an electrocardiogram (ECG) electrode. The
monitor is then advantageously both physically supported by the
single electrode and electrically connected to it for receiving
signals for monitoring the user's heart. A lead extends from the
monitor to receive signals from a second ECG electrode, spaced from
the first, to enable cardiac monitoring. Further leads coupling the
monitor to further ECG electrodes may advantageously allow
monitoring of more than one ECG channel. The leads may either
extend from a monitor housing or plug into sockets in the monitor
housing.
[0006] The monitor may comprise a memory for recording data from
the cardiac sensor over a period of time, which may then be
downloaded for analysis. If more than one ECG channel is to be
monitored, it preferably contains sufficient memory to store data
from multiple channels for an extended period, such as 24 hours or
more. Since the monitor is advantageously of light weight and small
size, its housing preferably being of smaller lateral dimension
than an ECG pad, it may advantageously be comfortable to wear
continuously for periods of as long as days or more.
[0007] In a preferred embodiment, the apparatus of the invention
consists of a small, lightweight monitor that may measure not only
heart rate but also inter-beat interval and/or other cardiac
parameters.
[0008] The following description of the invention refers mainly to
a preferred embodiment in which the monitor further comprises an
accelerometer for actigraph measurement. It should be noted,
however. that the skilled person will be readily able to identify
and assess those features and advantages of this embodiment which
similarly apply to the cardiac monitor mounted on a single ECG pad
described above.
[0009] In the preferred embodiment, the invention may
advantageously provide a monitor comprising a heart-beat sensor and
an accelerometer which can be secured in position on a single
adhesive pad for sensing a user's heart-beat and movement, or
activity. The monitor further comprises a processor for receiving
signals from the heart-beat sensor and the accelerometer and for
generating heart-rate or other cardiac data, and movement or
activity data. The monitor preferably comprises a memory in which
the data can be stored.
[0010] The monitoring of both a user's heart rate and movement
addresses the problem outlined above, that heart rate increases are
not necessarily correlated to physical exertion. Thus, a record of
the user's movement can be correlated with heart rate measurements
to improve evaluation of the user's energy expenditure. The range
of uses for such an apparatus or method in the medical field is
widespread. For instance, it can be used in cardiology, sleep
medicine, diabetes, obesity, eating disorders, psychiatric
disorders etc. It can also be used in monitoring the fitness levels
of individuals and as a means for assessing their energy
expenditure. This may be done for a variety of reasons, such as
weight loss, rehabilitation, encouragement to exercise etc.
[0011] The monitor is preferably couplable to a conventional
adhesive electrocardiogram (ECG) electrode or pad attachable to the
user's chest. Two such ECG electrodes are preferably used, as in
conventional ECG measurement. The monitor may then clip directly to
one of the ECG electrodes, achieving both electrical connection and
mechanical support. An electrical lead may then couple the monitor
to the other ECG electrode.
[0012] Many different types of ECG electrode pads are available for
many different applications, for example to cater for different
patient skin types, shapes, and sensitivity, or for long term or
short term applications. Thus, some types of electrode have
adhesive allowing for easy removal after short term use and others
have stronger adhesive allowing for long term skin adhesion. As
illustrated in FIG. 11 some electrode types comprise an electrode
gel 82 beneath a central portion of the pad 84 to improve
electrical connection to the skin, surrounded by an adhesive
portion 86 of the pad which secures the pad to the skin and retains
the gel in position. ECG pads are typically fitted with a standard
4 mm stud 74, couplable to an ECG lead. A monitor that mounts
directly on to an ECG electrode should preferably be compatible
with all these types of pads.
[0013] Preferably, a zero insertion force clip is used to connect
the monitor embodying the invention to a conventional ECG
electrode. If an ECG electrode comprises an electrode gel as
described above, and a large force is applied to connect the
monitor to such a pad, then this gel may be forced out past the
adhesive surrounding it. Advantageously, a zero insertion force
clip, for example a slider clip, may prevent the gel being forced
out. A high application force for attaching a monitor to a clip may
also cause the ECG electrode to deform thereby making attachment of
the monitor difficult or even damaging to the electrode. This
problem may be exacerbated if the user, or patient, has normal or
large amounts of body fat. A zero insertion force clip may,
advantageously, make the monitor easier to mount and prevent
distortion of the ECG electrode during mounting of the monitor.
[0014] Preferably, the unit is securely fixed to the ECG electrode
pad so that it will not rotate on the pad during normal use. Any
rotation of the monitor on the pad may cause a loss of resolution
of the movement (preferably vertical movement) detected by the
accelerometer. Advantageously, a clip with a high clamping force
may reduce rotation on the pad during normal use. A high clamping
force may also, advantageously, reduce contact resistance problems.
These factors may favour a high clamping force and so may further
exacerbate the problems arising if a zero-insertion-force clip is
not used, because otherwise a high clamping force typically
requires a high insertion force. In a preferred embodiment,
therefore, the monitor is attached to the electrode pad by means of
a zero insertion force slider clip, clamping force being provided
by a spring contact and the spring being manually retracted as the
monitor is attached. Being small and of light weight, the monitor
is advantageously unobtrusive and can be worn for long periods by
people of all ages and health or fitness status.
[0015] Data from the heart-beat sensor and the accelerometer are
advantageously stored in a memory within the monitor, which negates
the need for radio or other transmission of data from the monitor.
Data may then be downloaded from the monitor by interfacing it to,
for example, a computer such as a PC. In a preferred embodiment,
the monitor interfaces to the PC through the same contacts as used
for coupling to the ECG electrodes. Particularly advantageously,
the same contacts may also be used for charging a battery within
the monitor.
[0016] By analysing data downloaded to a PC, it may advantageously
be possible to establish whether small but significant changes in
heart rate (usually increases in heart rate) are due to physical
exertion or not, and therefore whether heart rate increases may be
due to, for example, stress. This may improve estimation of energy
expenditure derived from physical activity and its consequences in
terms of heart rate and performance.
[0017] Alternatively, by identifying changes in heart rate which
are not associated with physical activity, conditions such as
stress may be identified and/or monitored.
[0018] In another embodiment, the monitor may interface with an
external device such as a wrist-worn actigraphy device, by means of
radio transmission. In this embodiment, the ECG input lead may be
used as an antenna for RF output. Heart rate and activity data may
then be transmitted to a wrist-worn actigraphy device.
Advantageously, for example this wrist-worn actigraphy device may
sense reduced upper-body activity whilst the user is seated and
engaged in activities such as typing or knitting, which might
otherwise cause a wrist-worn actigraph, on detecting significant
movement, to estimate an inaccurately high level of energy
expenditure. The device may then combine all the data to further
enhance the measurement of physical activity and stress. More
accurate measurements may be made using both chest-worn and
wrist-worn actigraphs in this way, but in alternative embodiments
other systems may be implemented. For example a chest-worn monitor
for cardiac and activity data may transmit data by radio
transmission to a fixed receiver, for example next to an exercise
machine for monitoring the user's heart rate and activity while
exercising.
[0019] Alternatively a chest-worn cardiac monitor (not
incorporating an actigraph) could transmit cardiac data to a
wrist-worn actigraph or to a fixed receiver.
[0020] Normal commercial electrode pad adhesives are designed to
retain normal ECG leads and clips. In many monitoring applications,
longer lead wires are required, and are separately supported to
prevent pulling on the electrode pad. Preferably, the monitor
embodying the invention is of similar weight to a single short ECG
lead wire and clip. This means that the weight of the monitor does
not pull on a normal electrode pad beyond the pad's design load. By
being light weight, the monitor places a low load on the adhesive
pad. Advantageously, this means that the monitor does not require
the separate safety straps or wires that are deemed necessary for
other, heavier, units mounted on multiple pads.
[0021] Preferably, the weight of the monitor is less than 50 g, or
particularly preferably less than 25 g or 15 g. In a preferred
embodiment the monitor weighs about 10 g or less.
[0022] Devices that mount on to multiple adhesive pads, or any
device mounted on a single adhesive pad larger than a conventional
ECG electrode pad, will disadvantageously require complicated
mechanical arrangements, for example articulation or a flexible
housing, to allow for the free movement of the body and resulting
skin expansion and contraction. Advantageously, mounting at a
single point, particularly on a standard ECG electrode pad, reduces
the problems associated with free body movement and skin expansion
and contraction. It is notable that this problem is taken into
account in designing conventional ECG electrodes; these are of
limited size in order to avoid problems arising from skin
stretching and contraction.
[0023] These features of preferred embodiments of the invention may
solve a number of problems in prior art heart-rate monitors. In
prior art systems, for example, the transmission of data from a
chest band using a radio link is subject to a wide range of
interference, such as from electric motors, televisions, telephones
etc., which typically leads to a large number of data points being
lost and classed as "dropouts". Typically 10-20% of data points are
lost in this way per day of monitoring. On-board storage of data
within the apparatus embodying the invention solves this problem,
as well as advantageously eliminating any electromagnetic
transmissions from the apparatus which may interfere with other
apparatus, such as medical apparatus.
[0024] In prior art systems, there is a lack of data storage
facilities to allow for long-term accumulation of data, for example
over periods of more than 24 hours. The memory in the monitor
embodying the invention solves this problem.
[0025] The use of a chest-worn band for supporting a heart-rate
monitor is not suitable for various categories of people, such as
the very young, the very old and the obese, and is not comfortable
for long-term use. The use of ECG electrodes-to support and connect
the monitor of the embodiment solves this problem and makes the
monitor more comfortable to use.
[0026] Prior art heart-beat sensors are typically only used to
measure heart-rate itself and not other important cardiac
parameters such as inter-beat interval. The on-board processor of
the embodiment can be programmed to measure any such parameters,
particularly when combined with the use of ECG electrodes as these
provide a very clear heart-beat signal.
[0027] In a preferred embodiment, when the monitor is supported on
a user's chest or torso, the accelerometer should be oriented to
detect vertical movements of the user's chest or torso. The
inventors have found that this provides the most effective sensing
of user movement, or activity.
[0028] The inventors have also found that the processing of the
heart-beat sensor output to extract heart-rate and other cardiac
information may advantageously be modified in response to the
output from the accelerometer. Thus, for example, the gain and
thresholds for ECG measurement are preferably adjusted based on the
current user activity level measured by the accelerometer. During
periods of activity, noise artefacts tend to be induced in the ECG
signal by variations in skin potentials and using the activity data
to improve the signal-to-noise ratio of the ECG signal helps to
ensure a clean and uninterrupted data stream.
[0029] Although reference has been made to storing movement and
heart-rate measurements in a memory housed within the apparatus of
a preferred embodiment, other possibilities are envisaged within
the scope of the invention. Thus, heart-beat or heart-rate data and
movement data may be downloaded or transmitted to a remote display
unit or data storage unit during use so that these signals may be
monitored by a user, for example during exercise. If a user is
engaged in a repetitive physical exercise such as, for example,
running, output from the movement sensor may not only be valuable
in combination with heart-rate measurement as described above but
may also be used to determine the user's stride rate or the number
of strides performed, for example.
[0030] An apparatus or method embodying the invention may be used
for monitoring human or animal users.
SPECIFIC EMBODIMENTS AND BEST MODE OF THE INVENTION
[0031] Specific embodiments of the invention will now be described
by way of example, with reference to the accompanying drawings, in
which:
[0032] FIG. 1 shows front, side and rear views of a first
embodiment of the invention;
[0033] FIG. 2 is a schematic diagram of the embodiment of FIG. 1
coupled to two ECG electrodes for use;
[0034] FIG. 3 is a schematic diagram of the embodiment of FIG. 1
coupled to two rectangular ECG electrodes for use;
[0035] FIG. 4 illustrates an embodiment of the invention comprising
a monitor and a lead, in which the lead functions as an RF
aerial;
[0036] FIG. 5 is a block diagram of the circuitry of a monitor
embodying the invention;
[0037] FIG. 6 is a more detailed circuit diagram of the circuitry
of FIG. 5;
[0038] FIG. 7 is a block diagram of an interface for coupling the
monitor of FIG. 5 to a PC;
[0039] FIG. 8 is a flow diagram illustrating the operation of the
monitor of FIG. 5;
[0040] FIG. 9 is a state diagram providing an overview of the
operation of the monitor of FIG. 5;
[0041] FIG. 10 is a flow diagram illustrating the functionality of
the monitor of FIG. 5.
[0042] FIG. 11 is a transverse section of an ECG electrode
incorporating an electrode gel;
[0043] FIG. 12 illustrates a zero-insertion-force mounting for a
monitor embodying the invention; and
[0044] FIG. 13 is a side view of a monitor embodying the invention
mounted on an ECG pad.
[0045] FIG. 1 shows the external appearance of a housing of a
monitor 2 according to a first embodiment of the invention, viewed
from the front, side and rear. The monitor is substantially disc
shaped, having a diameter of about 31 mm and a thickness of about
5.5 mm. The rear of the monitor comprises a recessed clip 4 which
is removably attachable to an electrical contact of a conventional
ECG electrode.
[0046] FIG. 2 illustrates the monitor of FIG. 1 in use. Two
conventional ECG electrodes 6, 8 each comprise a circular adhesive
pad which can be stuck to a user's chest. Each also comprises an
electrical contact positioned near a lower edge of the pad and
extending forwards from the pad.
[0047] The clip 4 of the monitor 2 mounts on the electrical contact
of one electrode 6. The monitor comprises an electrical lead 10 for
coupling to the other ECG electrode 8. The lead carries at one end
a plug 12 which is removably insertable into a socket in one side
of the monitor housing, and at the other end a clip 14 which is
removably connectable to the ECG electrode contact.
[0048] For user comfort, the lead 10 should be longer than the
distance between the ECG electrodes, to accommodate user
movement.
[0049] The monitor comprises an accelerometer, as described below,
which is primarily sensitive to movement in a particular direction.
In the embodiment the accelerometer is mounted within the monitor
so as to detect vertical motion of the user's chest, which requires
that the monitor is mounted and retained in the correct orientation
on the ECG pad. The correct orientation for mounting the monitor is
indicated to the user by a marking on the monitor casing. Once
fitted to the ECG electrode, the clip 4 holds the monitor in
position. The lead connecting the monitor to the second ECG pad
also helps to orient the monitor correctly.
[0050] FIG. 3 illustrates a similar monitor 20 mounted on
conventional ECG electrodes 22, 24 of a different type, which are
of generally rectangular shape.
[0051] FIG. 4 illustrates a further embodiment in which the lead 10
coupling the monitor to a second ECG electrode acts as an RF (radio
frequency) aerial, allowing the monitor to transmit such data to an
RF receiver. In this embodiment the lead terminates at a 1 mH
inductor 26 to enhance its functionality as a aerial.
[0052] FIG. 12 illustrates the clip 4, for securing the monitor of
FIG. 1 to an ECG electrode, in more detail. Similar clips may be
used in the embodiments of FIGS. 2 to 4. This zero-insertion-force
clip comprises a slider 70 in which an opening 71 is formed, the
opening having an enlarged portion 72 at one end through which a
conventional 4 mm stud 74 of an ECG electrode may be received. An
anvil 76 extends into the opening and must be manually withdrawn
from the opening (or the slider moved relative to the anvil)
against a spring force to allow the stud to enter the opening. From
the enlarged portion 72 the opening tapers inwardly between two
straight sides 78 at an acute angle to each other. After entry of
the stud into the opening, the anvil (or the slider) is released
and the anvil abuts the stud so that the spring force urges the
stud between the tapering sides 78 of the opening. The spring force
is designed to be high enough and the angle between the tapering
sides small enough to provide a firm grip between the clip and the
stud, to produce good electrical contact and to resist rotation of
the monitor relative to the ECG electrode.
[0053] As shown in FIG. 12 the slider is a plate approximately 1 mm
thick. This allows it to grip a lower neck portion 80 of the
conventional ECG electrode stud (see FIG. 11) and hold the base of
the monitor flush with or close to the upper surface of the ECG
electrode. This clip may advantageously allow zero insertion force
and a much more secure mounting than the spring clips
conventionally used to secure leads to ECG electrodes.
[0054] The monitors illustrated in FIGS. 1, 2, 3 and 4 are of
diameter 31 mm and thickness 5.5 mm. However, a monitor embodying
the invention should advantageously be less than 70 mm, and
particularly preferably less than 50 mm in lateral dimension.
[0055] Preferably, the monitor is less than 35 mm in diameter (or
lateral or vertical dimension). The preferred position for both
actigraphy measurement and ECG electrode pad positioning is on or
near the sternum. In order to fit comfortably between female
breasts, the size of the monitor housing should be similar to or
smaller than that of many normal ECG electrodes. Advantageously, a
diameter of about 30 mm, or a lateral dimension of about 30 mm, is
of this order.
[0056] In addition, the monitor should advantageously be less than
15 mm in thickness and particularly preferably less than 10 mm or
less than 6 mm in thickness. These dimensions aim to ensure user
comfort.
[0057] Advantageously, these thickness dimensions mean that the
monitor housing is of low profile. A low profile may,
advantageously, render the monitor almost invisible when worn under
clothing. The low profile may also reduce interactions between the
monitor and clothing which would otherwise cause movement or
tipping of the monitor, or even rip the monitor from its mounting.
Interaction with clothing may cause undesirable movement of a
monitor of larger diameter or a monitor having a greater thickness.
Devices that hang from an ECG electrode by a wire may also suffer
from the same excessive movement problem.
[0058] When the monitor is clipped to an electrode pad, there may
be a gap between the monitor and the pad. Preferably, this gap is
narrow. A large gap may allow the monitor to tip or allow clothing
to rip the monitor from the pad. Tipping of the monitor is also
undesirable as it may generate spurious movement data or affect the
ability of the accelerometer to detect vertical movement. A narrow
gap, or no gap, between monitor and pad may, advantageously, lessen
these undesirable effects.
[0059] FIG. 13 is a side view of a monitor 20 mounted on an ECG pad
22 as in FIG. 3. The monitor is about 5 mm thick and the support,
or clip, fastening the monitor to the pad is positioned within the
monitor housing so that the base of the monitor housing is flush
with, or spaced by less than 1 mm or 2 mm from, the upper surface
of the ECG electrode to minimise tipping of the monitor relative to
the electrode. The outer rim 26 of the monitor housing is bevelled
to reduce further the risk of the monitor catching on clothing.
[0060] Movements of the monitor, such as twisting or tipping, on
the electrode pad may cause ECG noise artifacts. A small size and
low profile may help to reduce excessive noise artifacts caused by
movement of the monitor. The monitor may then, advantageously, be
used during vigorous exercise without generating excessive noise
artefacts.
[0061] FIG. 5 is a block diagram of a monitor circuit embodying the
invention. FIG. 6 is a more detailed circuit diagram corresponding
to the block diagram of FIG. 5.
[0062] The circuit comprises a microcontroller 30 which receives
inputs from a clock (crystal oscillator) 32, an accelerometer 34,
two ECG electrodes 6, 8 and a communications and power management
module 38. The microcontroller 30 is also coupled to a memory 40
and a battery (re-chargeable coin cell) 42. All of these components
are mounted on a printed circuit board (PCB) which is housed within
a monitor casing or housing such as illustrated in FIGS. 1 to
4.
[0063] The accelerometer is a piezoelectric accelerometer, which is
mounted on the PCB in a predetermined orientation such that it is
most sensitive to motion in a predetermined direction when the PCB
is housed in the monitor casing and the monitor is in use. For
example, in the chest-mounted embodiments of FIGS. 1 to 4, the
accelerometer is oriented to be most sensitive to movement in the
vertical plane (i.e. sensitive to physical movement in the up/down
direction), during use when the user is upright. In this way, a
good approximation of the physical activity of the user may be
deduced. In other applications for sensing other movements of a
human or animal body it may be desirable to mount the accelerometer
in different predetermined orientations within the monitor
casing.
[0064] The signal from the accelerometer is amplified by an
amplifier 44 and filtered by a filter 46 before being input to an
analog input of the microcontroller.
[0065] The ECG electrodes are usually attached to the mid-left
region of the user's chest and the monitor is coupled between them.
The monitor may comprise a small light emitting diode (LED) which
flashes for several beats to indicate when an ECG signal is
initially detected. It may additionally flash whenever a heartbeat
is detected, allowing the user to confirm that the monitor is
picking up an ECG signal.
[0066] Preferably, the LED flash is modulated at high frequency so
that a light receiver can easily detect its light. Advantageously,
this enables remote electrically-isolated short-ranqe readout of
the heart rate in fitness tests such as treadmill tests.
[0067] The signals from the ECG electrodes pass through two monitor
contacts 48 and are amplified in two stages by two amplifiers 50,
52 and filtered by a filter 54 before being input to an analog
input of the microcontroller.
[0068] The ECG signal is processed within the microcontroller to
remove noise artefacts. As the monitor is totally self-contained,
there are no problems with interference from radio frequency
devices or other sources of electromagnetic interference.
[0069] The microcontroller uses a 4.0 MHz internal clock for
instruction timing but uses an external 32.768 kHz oscillator,
shown in FIG. 5 as the clock 32, for real-time clock functions.
[0070] The communications and power management block 38 is coupled
to the monitor contacts 48 and comprises discrete circuitry which
allows various signal levels and frequencies at the contacts to be
discriminated by the microcontroller. This allows the monitor
contacts to be used as monitor inputs or outputs for multiple
functions depending on the device to which the contacts are
coupled. Thus, if the contacts are coupled to ECG electrodes, ECG
signals can be identified and received by the microcontroller. If
the contacts are coupled to an interface unit or reader as
described below, the same contacts can be used by the
microcontroller to download data, re-charge the battery, or other
applications as described below.
[0071] The battery 42 is a surface-mounted manganese lithium
secondary (re-chargeable) coin cell that provides up to 22 days of
continuous operation from a full charge. During operation, the
monitor may continuously record heart rate and physical activity at
one minute intervals. All of the other components are also
surface-mounted on the PCB to provide compact size, simplified
production and increased reliability.
[0072] The circuit is provided with protection from reverse
polarity connection, over-voltage and ESD (electro-static
discharge). The ultra-low power and integrated nature of the
monitor ensures no EMI (electromagnetic) emissions.
[0073] The device is waterproof and can hence be worn continually
to provide an uninterrupted data stream.
Firmware
[0074] In the monitor, certain firmware (embedded software) is
programmed into an internal ROM (read only memory) area of the
microcontroller 30 and controls many of the monitor's functions. In
particular, the firmware enables the sampling of signals from the
accelerometer and the ECG electrodes under timed interrupts, with
movement being sampled at 16 Hz and ECG at 128 Hz or 256 Hz. These
signals are sampled at different rates to reflect the different
rates at which the signals typically vary. The movement data are
integrated over one minute epochs and stored into non-volatile
memory 40. The heart rate data are stored as beats per minute in
the non-volatile memory.
[0075] The microcontroller performs several signal processing
functions and executes internal algorithms on the ECG data. The key
processing functions are as follows.
[0076] Dynamic threshold: the threshold for detection of the ECG
R-wave pulse is dynamically adjusted within a window period to aid
discrimination of true pulse signals during periods of high
noise.
[0077] Variable gain: the gain and dV/dt (rate of change of
voltage) thresholds for ECG measurement are adjusted based on the
current user movement level detected by the accelerometer. During
periods of movement, noise artefacts are induced by variations in
the user's skin potentials. Using the movement data to improve the
signal-to-noise ratio of the ECG signal helps to ensure a clean and
uninterrupted data stream.
[0078] The monitor uses a digitally computed reference level to
determine whether the dV/dt (rate of change of voltage) of the
input signal meets the requirements of an ECG R-wave. This
threshold varies with time and has an absolute minimum level to
prevent spurius noise being seen as an R-wave. During periods of
high activity the minimum threshold level is raised, as described
above, to prevent spurius triggering on movement noise
artifacts.
[0079] IBI Tracking: the inter-beat interval (IBI) is computed and
used to update an internally stored histogram. The histogram
contains discrete time windows and an IBI value falling within a
histogram window causes the histogram to be incremented. An
indication of variation of the inter-beat interval is very useful
in determining certain medical conditions.
[0080] IBI variability logging: normal regular heart-rate data are
stored as beats per minute. If serious variability is detected, the
heart rate is automatically stored at a higher resolution to allow
a more detailed analysis.
[0081] FIG. 9 provides an overview of the firmware operation.
READER
[0082] FIG. 7 is a block diagram of a reader, or interface, for
coupling the monitor to a PC. The reader comprises terminals 60
connectable to the ECG electrode contacts 48 of the monitor. Within
the reader, these are connected to a bi-directional communications
module 62 and a charge/monitor/re-set module 64. Each of these
modules is connected by an RS232 connector 66, or other connector
suitable for interfacing to a PC, such as a USB connector.
[0083] The reader is thus a small module that contains the
electronics necessary for level shifting to and from RS232 (or USB)
in addition to providing control signals for power management of
the monitor. Once the monitor is connected to the reader via the
ECG leads, after simply unclipping the monitor from the ECG
electrodes and connecting the same contacts to the reader,
bi-directional communications may take place between the monitor
and the serial port of the PC. As well as allowing data to be
downloaded from the monitor to the PC, the reader can also charge
the monitor battery, drawing power from the PC serial port or
optionally from a plug-in mains adaptor.
Software
[0084] This software runs on a PC having a serial port to which the
monitor may be coupled via the reader described above. The software
is a 32-bit Windows application written in Visual Basic with an
underlying database used for data management. The software has the
following broad functions.
[0085] Store details of users and test data in structured and
manageable database tables.
[0086] Write user and test parameters to the monitor.
[0087] Read logged data from the monitor.
[0088] Present reports in a user-friendly and flexible manner.
[0089] Provide portable data storage. This means that data can be
exported to other software packages for additional analysis.
[0090] FIG. 8 shows a block diagram of the software structure.
[0091] A core database 100, which is Access compatible, contains
tables for user information such as name, date of birth, height
etc. The database also has tables to contain downloaded heart rate,
or other cardiac data, and movement, or activity, data. The tables
have relational interlinking and the software generates queries to
present users seamlessly with the correct downloaded data.
[0092] When a new user is added 102, their personal details are
stored into the database. A set of test-specific parameters (i.e.
user weight, test start date and time etc.) are also set and stored
106. Alternatively, existing users may be located 108 from the
database using search facilities and the test parameters then set
or selected. Set-up information is then transferred to the monitor
by means of a communications module 104 and a serial link (coupled
through the reader to the monitor).
[0093] The communications module 104 also controls monitor status
management 116, including monitoring the level of charge in the
monitor battery and re-setting the monitor microcontroller where
required.
[0094] In addition, data may be downloaded through the serial link
under the control of the communications module from the monitor to
the database 100 and viewed using a graphical reports module 110.
Graphical reports may be printed 112 or data may be exported 114
directly from the database or via the clipboard from the graphical
reports module.
Functionality of the Monitor Contacts
[0095] As described above, the monitor comprises two electrical
contacts, which can be coupled either to ECG electrodes for
heart-beat sensing or to an external device such as the reader for
various other purposes.
[0096] FIG. 10 illustrates the various functions of the monitor
contacts.
[0097] In total, the two contacts for the ECG electrodes are also
used for five other functions: reading data, writing data, charging
battery, power management and re-setting the CPU of the
microcontroller. This shared functionality of connections allows
greatly reduced size and complexity of the electronics of the
monitor and provides a simplified user interface.
[0098] As shown in FIG. 10, when the monitor contacts are coupled
to ECG electrodes (200), the ECG signals are taken directly from
those electrodes. In a preferred embodiment, the monitor mounts
directly onto one electrode and connects via a cable to the
other.
[0099] When the monitor contacts are coupled to a PC via a serial
interface (202), data can be written by the PC to the monitor. This
allows set-up data to be written to the monitor, including user
identification and any other desired test parameters.
[0100] Similarly, when the monitor contacts are coupled to the
serial interface (204), data can be read from the monitor by the
PC. This allows stored movement (activity) and cardiac data to be
downloaded.
[0101] When the monitor contacts are coupled either to a suitable
serial interface or to a battery charger (206), the same
connections allow the internal re-chargeable battery to be
charged.
[0102] When the monitor contacts are coupled to a suitable
interface, such as the reader described above, battery status can
be monitored and managed (208).
[0103] Finally, when the monitor contacts are coupled to a suitable
interface such as the reader described above, the micro-controller
can be re-set (210) following a total discharge or re-charge of the
battery.
* * * * *