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|United States Patent Application
Ungless; Gary Steven
;   et al.
October 19, 2006
Cardiac monitoring apparatus and method
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.
Ungless; Gary Steven; (Cambridgeshire, GB)
; Oakley; Nigel Robert; (Cambridgeshire, GB)
SALIWANCHIK LLOYD & SALIWANCHIK;A PROFESSIONAL ASSOCIATION
PO BOX 142950
October 16, 2003|
October 16, 2003|
April 17, 2006|
|Current U.S. Class:
|Class at Publication:
||A61B 5/04 20060101 A61B005/04|
Foreign Application Data
|Oct 18, 2002||GB||0224299.8|
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
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
33. The monitor according to claim 23, further comprising an accelerometer
coupled to the processor, so that the processor can generate movement
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
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
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
 This invention relates to a cardiac monitoring apparatus and method
for monitoring a user's heart rate, or other parameters derived from
 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.
 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.
 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
 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.
 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
 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.
 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.
 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.
 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.
 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
 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.
 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.
 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.
 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.
 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
 Alternatively, by identifying changes in heart rate which are not
associated with physical activity, conditions such as stress may be
identified and/or monitored.
 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.
 Alternatively a chest-worn cardiac monitor (not incorporating an
actigraph) could transmit cardiac data to a wrist-worn actigraph or to a
 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.
 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.
 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.
 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.
 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.
 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
 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
 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
 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
 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.
 An apparatus or method embodying the invention may be used for
monitoring human or animal users.
SPECIFIC EMBODIMENTS AND BEST MODE OF THE INVENTION
 Specific embodiments of the invention will now be described by way
of example, with reference to the accompanying drawings, in which:
 FIG. 1 shows front, side and rear views of a first embodiment of
 FIG. 2 is a schematic diagram of the embodiment of FIG. 1 coupled
to two ECG electrodes for use;
 FIG. 3 is a schematic diagram of the embodiment of FIG. 1 coupled
to two rectangular ECG electrodes for use;
 FIG. 4 illustrates an embodiment of the invention comprising a
monitor and a lead, in which the lead functions as an RF aerial;
 FIG. 5 is a block diagram of the circuitry of a monitor embodying
 FIG. 6 is a more detailed circuit diagram of the circuitry of FIG.
 FIG. 7 is a block diagram of an interface for coupling the monitor
of FIG. 5 to a PC;
 FIG. 8 is a flow diagram illustrating the operation of the monitor
of FIG. 5;
 FIG. 9 is a state diagram providing an overview of the operation of
the monitor of FIG. 5;
 FIG. 10 is a flow diagram illustrating the functionality of the
monitor of FIG. 5.
 FIG. 11 is a transverse section of an ECG electrode incorporating
an electrode gel;
 FIG. 12 illustrates a zero-insertion-force mounting for a monitor
embodying the invention; and
 FIG. 13 is a side view of a monitor embodying the invention mounted
on an ECG pad.
 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.
 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
 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
 For user comfort, the lead 10 should be longer than the distance
between the ECG electrodes, to accommodate user movement.
 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
 FIG. 3 illustrates a similar monitor 20 mounted on conventional ECG
electrodes 22, 24 of a different type, which are of generally rectangular
 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.
 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.
 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
 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.
 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.
 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.
 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.
 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
 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.
 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.
 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.
 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.
 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
 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
 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.
 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.
 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
 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.
 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.
 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.
 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
 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
 The device is waterproof and can hence be worn continually to
provide an uninterrupted data stream.
 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.
 The microcontroller performs several signal processing functions
and executes internal algorithms on the ECG data. The key processing
functions are as follows.
 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.
 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.
 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
 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
 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
 FIG. 9 provides an overview of the firmware operation.
 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.
 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.
 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
 Store details of users and test data in structured and manageable
 Write user and test parameters to the monitor.
 Read logged data from the monitor.
 Present reports in a user-friendly and flexible manner.
 Provide portable data storage. This means that data can be exported
to other software packages for additional analysis.
 FIG. 8 shows a block diagram of the software structure.
 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.
 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).
 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.
 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
 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.
 FIG. 10 illustrates the various functions of the monitor contacts.
 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
 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.
 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.
 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.
 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.
 When the monitor contacts are coupled to a suitable interface, such
as the reader described above, battery status can be monitored and
 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.
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