GB2367896A

GB2367896A - Heart rate monitor

Info

Publication number: GB2367896A
Application number: GB0013668A
Authority: GB
Inventors: Ross B Haffenden; Richard Bayford
Original assignee: Seven of Nine Ltd
Current assignee: Seven of Nine Ltd
Priority date: 2000-06-05
Filing date: 2000-06-05
Publication date: 2002-04-17
Also published as: GB0013668D0

Abstract

A wrist mounted heart rate monitoring device (1, 2) which operates by monitoring changes in blood impedance due the heart pumping cycle. The device comprises signal generating means (5, 3) for applying a signal to a subject whose heart rate is to be monitored; detecting means (3, 6-12) for detecting an output signal resulting from said signal having passed through a portion of the subject; output signal variation monitoring means (6-12) for monitoring changes in the output signal caused by impedance changes in the signal path due to blood flow through said portion; and means (6-12) for determining heart rate from the output signal variation.

Description

Heart Rate Monitor This invention relates to heart rate monitors.

Physical exercise for the purpose of maintaining or increasing fitness or controlling weight is extremely popular. There is an increasing recognition that monitoring your heart rate during such exercise is important. There are existing systems for monitoring heart rate during physical exercise but these suffer from a number of disadvantages. In particular some systems are relatively complex, expensive, uncomfortable or otherwise unsuitable for use by at least some exercisers. It would be desirable to provide a heart rate monitor which does not suffer from these difficulties.

A particular set of existing heart rate monitors for use whilst exercising rely on the electrical signals generated by a subject's heart as it pumps blood around the body. In such systems, because the electrical signals generated are very small, it is necessary to have relatively widely spaced contact points on a user's body. In one particular implementation the contact points are provided by a device which is mounted using a chest strap. However,

such devices can be uncomfortable and are not liked by all users. Other devices rely on the two contact points being provided by separate parts of the body, for example by two hands grasping independent electrodes or wrist mounted devices where one electrode contact is provided on the wrist and the other is provided by touching a finger of the opposite hand to the device instantaneously to obtain a reading. These latter devices suffer from the disadvantage that they will only work when the user's hands are in particular positions.

It is an object of the present invention to provide a heart rate monitor which alleviates at least some of the problems associated with the prior art.

According to the present invention there is provided a heart rate monitoring device comprising: signal generating means for applying a signal to a subject whose heart rate is to be monitored; detecting means for detecting an output signal resulting from said signal having passed through a portion of the subject; output signal variation monitoring means for monitoring changes in the output signal caused by

impedance changes in the signal path due to blood flow through said portion; and means for determining heart rate from the output signal variation.

Such a device does not rely on the electrical signals output by the heart and therefore does not necessarily call for widely spaced contact positions on the subject.

Blood comprises a variety of different cells suspended in a solution known as plasma. Plasma has a relatively high salt content and as such is a reasonably good electrical conductor. On the other hand, the cells suspended in the plasma, and particularly the lipid membranes of the cells, are poor conductors. As blood is pumped around the body, the relative proportions of cells and plasma in any given portion of an artery change with time in a pattern which is indicative of the subject's heart beat. When the plasma is densely populated with cells the conduction path through the blood is more labyrinthine and hence the impedance of the blood increases. It is this phenomenon which enables heart rate to be monitored in the present system.

The signal generating means may be arranged to act as a constant current source. The signal generating means may be arranged to apply an alternating current signal the subject. Preferably the signal generating means is arranged to act as an alternating current constant current source. It should be noted that in this context, a constant current source is a source arranged such that the current which it delivers is substantially unaffected by the impedance of the current path into which the current is injected. Thus in the case of an alternating current constant current source, although the current delivered varies with time in accordance with a driving oscillating signal, the current delivered is constant in the sense that, the current output at a given point in one cycle is substantially identical to the current output at the same point in any other cycle. The use of a constant current source is advantageous for a number of reasons, in particular it minimizes the problems associated with contact impedance. The use of alternating current is preferred for a number of reasons, in particular it is considered safer than direct current.

The detecting means may be arranged for detecting

potential difference between two points on a subject caused by the applied signal.

The means for determining heart rate may comprise deciding means for deciding when the output signal passes a selected threshold to facilitate identification of heart beat peaks.

The means for determining heart rate may comprise timer means. The means for determining heart rate may comprise processing means.

The processing means may be arranged under the control of appropriate software to perform a comparison between a model heart beat signal and the output signal in order to identify preselected portions of the subject's heat beat. The comparison may be carried out as a least squares fit.

The signal generating means may comprise two electrodes arranged for contacting a subject. The detecting means may comprise two electrodes for contacting a subject. Preferably the device uses a four electrode system, with separate electrodes for the generating means and the detecting means. This has advantages because it reduces or eliminates the

problems caused by varying contact resistance which is likely to occur as a wearer moves. If a two electrode system is used, ie common electrodes for the signal generating and detecting means, then any measurements made are likely to be dominated by changes in contact impedance.

The heart rate monitor may comprise a main housing and a securing strap. One or more electrodes may be mounted on the securing strap. Preferably four electrodes are provided on the strap. Preferably the heart rate monitor is arranged to be worn on the wrist of a user.

Preferably the electrodes are arranged to maximize the probability of contact with a subject's skin being maintained. Preferably the electrodes are elongate.

Where the electrodes are mounted on a securing strap, the electrodes are preferably disposed at positions spaced in a longitudinal direction along the strap.

The or each electrode is preferably oriented with a long edge running in a transverse direction.

According to another aspect of the present invention there is provided a method of monitoring heart rate

comprising the steps of : applying a signal to a subject whose heart rate is to be monitored; detecting an output signal resulting from said signal having passed through a portion of the subject; monitoring changes in the output signal caused by impedance changes in the signal path due to blood flow through said portion; and determining heart rate from the output signal variation.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 schematically shows a heart rate monitor device arranged for mounting on a user's wrist.

Figure 2 shows part of a wrist strap of the device shown in Figure 1 ; Figure 3 schematically shows internal components of the device shown in Figure 1, and Figures 4A to 4D are charts schematically showing the

signal at different points in a monitoring process carried out by the monitoring device shown in Figures 1 and 3.

Referring particularly to Figures 1 and 2, a heart rate monitoring device arranged for use whilst exercising generally comprises a main housing portion 1 which is mounted on a adjustable wrist strap 2. The adjustable wrist strap 2 has four electrodes 3 mounted thereon. The electrodes 3 are connected via leads 4 to the central housing 1. The wrist band includes hook and loop fastening means (not shown) between two end portions 2a and 2b such that the strap is adjustable in length.

Each of the electrodes 3 is of stainless steel and has a generally rectangular elongate shape. Each of the electrodes 3 is arranged with its long edge substantially perpendicular to the longitudinal length of the strap 2. The electrodes 3 are spaced from one another in the longitudinal direction. The spacings in the longitudinal direction between the electrodes 3 may be of the order of 1 to 2 cms and the width of the electrodes in the longitudinal direction may be in the order of 0. 5cm.

The dimensions of the electrodes and spacings therebetween are chosen to enhance the signals measurable by the system as far as possible. The elongate shape and orientation of the electrodes 3 helps to ensure that the electrodes 3 stay in contact with the skin of the wearer as the wearer moves. The strap 2 and electrodes 3 are arranged such that the metallic surface of each of the electrodes 3 directly contacts with the skin of the user. In some circumstances it may be preferable to provide a gel or other substance on the electrodes to improve contact with the skin. The central housing 1 comprises a display la for displaying a monitored heart rate and, optionally, other information, a number of user operable buttons (not shown) and various elements of circuitry as described in more detail below with respect to Figures 3 and 4.

Figure 3 schematically shows circuitry of the heart rate monitor. An alternating constant current source 5 is connected between a first pair of the electrodes 3. This current source 5 is arranged to supply a generally sinusoidal current to the body of a subject whose heart rate is to be monitored. The current source 5 is preferably arranged to generate and apply

a signal having a frequency in the range of 10 to 50 kHz and an amplitude of approximately 1 mA. The frequency and magnitude of the signals are chosen so as to not harm the user and to ensure that the cells suspended in the blood act as insulators. A second pair of electrodes 3 are connected to a differential amplifier 6 which generates an output representing the difference in potential between the second pair of electrodes 3. The output of the differential amplifier 6 is fed into a circuit 7 for removing any DC bias in the signal. The output of the bias removing circuit 7 is connected to a demodulator 8 which is arranged to subtract the carrier signal generated by the constant current source 5 from the signal picked up across the second pair of electrodes 3. An output of the demodulator 8 is connected to an anti-aliasing low pass filter 9 (with a roll off at approximately 45 Hz), the output of which is fed into a sample and hold circuit 10. The output of the sample and hold circuit 10 is connected to an analogue digital converter 11 which in turn is connected to a microprocessor 12, comprising timer means amongst other things. The microprocessor 12 is also connected to the sample and hold circuit 10 so that the microprocessor 12 can issue control signals to control

the timing of the sampling.

As the signal passes from the constant current source 5 through the first pair of electrodes 3 and the body of the subject whose heart is to be monitored, the signal tends to undergo a phase shift. The demodulator 9 is arranged to take account of any such phase shift when demodulating its received signal. An oscillator (not shown) used to drive the constant current source 5 is also used as a reference signal by the demodulator 8.

In use, the user must firstly properly locate the monitoring device on his or her wrist. At this point some care should be taken regarding the positioning of the electrodes 3 on the wrist. In particular it is preferable that the electrodes 3 are located across the undersurface of the wrist so that they are close to the main artery.

Once the monitoring device has been located and appropriately activated, for example by pressing an "on"switch, the constant current source 5 outputs an appropriate signal via the first pair of electrodes 3.

This signal passes through the skin of the user and

through the surrounding part of the user's body and in particular through the main artery in the wrist. At the same time, the potential difference between the second pair of electrodes 3 is monitored by the device.

As discussed in the introduction, this potential difference will vary with time as the heart pumps blood around the body and through the main artery in the wrist. This changing potential difference is monitored by the circuitry of the heart rate monitoring device. The potential difference is detected by the differential amplifier 6, any DC bias is then removed by the bias removing circuit 7 before the resulting signal is demodulated to remove the oscillating driving signal. After demodulation, the resulting signal is representative of the impedance changes occurring due to changing blood flow in the region of the electrodes. After passing through the low pass filter 9, the signal is sampled at a rate chosen in accordance with the accuracy of heart rate monitoring which is required. The sampling rate might be in the order of 500 times a second. The signal is then encoded to digital in the analogue to digital converter 11 and fed into the microprocessor 12 for

further processing.

In the microprocessor 12 the first step is to operate on the signals such that relative changes in the signal are considered rather than its absolute value.

This can be termed a normalising process and is achieved by using the magnitude Vtl of the signal at a time tl as a reference and then calculating a resulting pulse signal at later times t2 using the following equation: (1) (Vtl-V)/Vtl = Pulse signal at time t2, where Vtz equals the magnitude of the signal input to the microprocessor 12 at time t2.

This operation also has the effect of inverting the signal which in turn causes the resulting pulse signal at time t2 to have the same sense as the original heart beat since the potential difference detected between the second pair of electrodes 3 varies inversely with heart beat.

Figures 4A to 4D schematically show the type of signal present at various points of the monitoring process.

Figure 4A shows the signal as input to the demodulator 8. Figure 4B shows the signal input to the sample

hold circuit 10 which is the analogue equivalent or envelope of the signal input into the microprocessor 12. Figure 4C shows the pulse signal at time t2 as calculated in accordance with equation 1 above (as well as the analogue equivalent or envelope of the pulse signal).

After calculating the pulse signal at time t2 in accordance with equation 1, the microprocessor 12 performs a comparison between the pulse signal and a model heart beat signal. This comparison is performed to identify the timing of the peak in amplitude of the pulse signal for that heart beat pulse. It will be appreciated that once timing signals for two or more heart beat pulses have been determined it is possible to determine the heart rate of the user. The number of heart beat pulses which are detected before determining the heart rate of the user is a matter of design choice.

In the present embodiment the comparison between the pulse signal obtained in equation 1 and a model heart beat signal is performed using least squares fit analysis. Figure 4D schematically shows the pulse signal P and a model heart beat signal M. A number of

sampling points, in this case seven (Ap to Gp) are used in a least square fit with corresponding points (Am to Gm) of the model heart beat signal M. The value L which is to be minimised in the least squares fit procedure is calculated using the equation given below:

(2) L = i.. (Ap-AJ'+.... (Gp-GJ' The timing of the model heart beat signal M is shifted until L is minimized. Once this is achieved the timing of the respective peak of the model heart beat signal M can be taken as the timing of the respective peak of the pulse signal P.

In alternatives a number of sample points other than 7 may be used in calculating the least squares fit. Also rather than concentrating on the major peak of the pulse signal, the least squares fit may be used to identify another characteristic part of a heart beat signal.

In other alternatives, the heart beat peak (or other characteristic part) of each heart beat pulse may be identified by a technique other than performing a least squares fit. For example, it may be determined that a peak has occurred when the detected signal exceeds a certain threshold or the rate of change in the detected

signal satisfies certain values.

It should be noted that different processing circuitry may be provided, for example, a digital demodulator may be used and the anti-aliasing filter dispensed with.

In some versions of the device the constant current source may be driven for only part of the time that the device is in operation to conserve energy.

Claims

1. A heart rate monitoring device comprising : signal generating means for applying a signal to a subject whose heart rate is to be monitored; detecting means for detecting an output signal resulting from said signal having passed through a portion of the subject; output signal variation monitoring means for monitoring changes in the output signal caused by impedance changes in the signal path due to blood flow through said portion; and means for determining heart rate from the output signal variation.

2. A heart rate monitoring device according to claim 1 in which the signal generating means is arranged to act as an alternating current constant current source.

3. A heart rate monitoring device according to claim 1 or claim 2 in which the detecting means is arranged for detecting potential difference between two points on a subject caused by the applied signal.

4. A heart rate monitoring device according to any preceding claim in

which the means for determining heart rate comprises deciding means for deciding when the output signal passes a selected threshold to facilitate identification of heart beat peaks.

5. A heart rate monitoring device according to any preceding claim in which the means for determining heart rate comprises processing means arranged under the control of appropriate software to perform a comparison between a model heart beat signal and the output signal in order to identify preselected portions of the subject's heat beat.

6. A heart rate monitoring device according to any preceding claim which uses a four electrode system, with separate electrodes for the generating means and the detecting means.

7. A heart rate monitoring device according to any preceding claim which comprises a main housing and a securing strap with one or more electrodes.

8. A heart rate monitoring device according to claim 6 or claim 7 in which the electrodes are arranged to maximize the probability of contact with a subject's skin being maintained.

9. A heart rate monitoring device according to any one of claims 6 to 8 in which the electrodes are elongate.

10. A heart rate monitoring device according to claim 7 or either of claims 8 and 9 when dependent on claim 7 in which there are a plurality of electrodes disposed at positions spaced in a longitudinal direction along the strap, each electrode being oriented with a long edge running in a transverse direction.

11. A method of monitoring heart rate comprising the steps of: applying a signal to a subject whose heart rate is to be monitored; detecting an output signal resulting from said signal having passed through a portion of the subject; monitoring changes in the output signal caused by impedance changes in the signal path due to blood flow through said portion; and determining heart rate from the output signal variation.