Patent Description:
Obtaining a reliable measure of heart rate after birth in newborn babies is difficult, particularly for those requiring resuscitation or stabilisation. Pulse oximeters typically do not work well in the first ~<NUM> minutes because of poor perfusion to the limbs, where transmission mode pulse oximeter probes are usually placed. Whilst electrocardiograms (ECG) can theoretically work, application of the ECG electrodes is problematic for a few reasons. Firstly, the requirement for a rapid measure of heart rate is generally precluded by the time taken to apply multiple (usually three or four) electrodes to the baby's skin. The emphasis for the clinical team must be primarily on the baby, not on applying ECG electrodes. Secondly, since the baby will be covered in blood and vernix from the birth, adhesion can be problematic. Although the babies can be wiped down, vernix can still remain. This makes application of undivided ECG probes difficult and they can easily fall off. However if greater adhesive strength is used, this can result in skin stripping when the electrodes are removed, particularly in premature babies with weaker or thinner skin. Thirdly, premature babies are typically placed into plastic bags to keep them warm. A long delay, such as that required to reliably apply ECG electrodes, can increase the risk of hypothermia.

<CIT> discloses an electrocardiogram (ECG) electrode patch (<NUM>) for attachment to a neonatal or infant patient. The ECG electrode patch (<NUM>) includes a plurality of at least three electrodes (<NUM>) coupled to a substrate (<NUM>). The plurality of at least three electrodes (<NUM>) includes at least one electrode capable of measuring the electrical activity of the right side of the patient's heart (V4R, V5R, V6R). A plurality of electrical conductors (<NUM>) are coupled to the plurality of electrodes (<NUM>) and to the substrate (<NUM>).

<CIT> discloses. An adherent device to monitor a patient for an extended period comprises a breathable tape. The breathable tape comprises a porous material with an adhesive coating to adhere the breathable tape to a skin of the patient. At least one electrode is affixed to the breathable tape and capable of electrically coupling to a skin of the patient. A printed circuit board is connected to the breathable tape to support the printed circuit board with the breathable tape when the tape is adhered to the patient.

<CIT> discloses a template has a flexible sheet with a fixed dimensional array V1 -V6 positioned in a specific size configuration appropriate for standard electrocardiographic recording.

In accordance with a first aspect of the invention there is provided an electrocardiogram sensor according to claim <NUM>.

The invention allows for an ECG sensor array that can be applied rapidly to provide a reliable measure of heart rate, particularly for newborn babies. The use of a flexible sheet in addition to the substrate for the electrodes allows for an additional area for contacting the skin of the subject, allowing for surface tension between the sheet and the subject to secure the electrodes to the skin without the need for adhesive.

The flexible sheet may comprise holes at positions corresponding to the electrodes, each hole being smaller than a corresponding electrode. This enables the flexible sheet to be provided as a separate unit to the electrode array, with the holes in the flexible sheet being smaller than the electrodes so that the electrodes can be held in place against the subject by the sheet.

The areal extent of the flexible sheet is greater than twice that of the electrode array. The areal extent may for example be between <NUM> and <NUM> times that of the electrode array, thereby allowing for a greater area of contact with the subject without the flexible sheet becoming unwieldy.

The flexible sheet and the electrodes are preferably non-adhesive, as surface tension (or dispersive adhesion) alone is sufficient to maintain contact with the subject (without the need for a tacky or adhesive material).

The substrate may comprise a sheet of a first polymeric material and the flexible sheet may comprise a sheet of a second polymeric material, the second polymeric material having a lower stiffness than the first polymeric material. Selecting a lower stiffness for the second polymeric material allows the flexible sheet to be easier to conform to the shape of the subject's body, while the first polymeric material maintains a required spacing between the electrodes. Alternatively, the flexible sheet may comprise a sheet of paper or any biocompatible substance that provides sufficient adhesion between the subject and flexible sheet.

The first polymeric material may for example have a tensile stiffness of greater than <NUM> GPa while the second polymeric material has a tensile stiffness of less than <NUM> GPa.

The first polymeric material may for example by polyethylene terephthalate or a polyamide. The second polymeric material may be polyethylene or polyvinyl chloride.

The electrodes 101a-d may be wet gel, hydrogel or dry contact type electrodes. For a low impedance connection a saline wet or hydrogel of low resistivity is preferably used when connecting the array <NUM> to a subject.

A flexible sheet <NUM>, an example of which is shown in <FIG>, can be used to secure the connection between the electrodes 101a-d and the skin of the subject by increasing the surface tension between the sensor and the subject. The array <NUM> could hold itself in place with a conductive gel or solution, which may provide enough surface tension to adhere the array to the subject, but there is a risk that the array <NUM> will come loose, particularly if the subject is moved. A flexible sheet <NUM> extending beyond the array <NUM>, i.e. having a greater areal extent than that of the array <NUM>, is beneficial in providing additional surface area for the array to stay in place using surface tension alone. The flexible sheet <NUM> could be integrated with the array <NUM> and substrate <NUM>, or be provided as a separate layer to be assembled prior to use. The areal extent of the flexible sheet <NUM> is greater than twice that of the electrode array <NUM>, and may in some cases be up to three times that of the electrode array <NUM>. The flexible sheet is preferably of a generally rectangular form, having a length that is over <NUM> times that of its width, and in some cases may be up to three times that of its width.

A suitable size for the sheet <NUM> would be such that the length covers at least one side of the torso of the baby and the width is no higher than the abdomen of the baby. Example dimensions of the sheet are between <NUM> and <NUM> in width and at least <NUM> in length. The length may be more variable than the width, and may for example be up to <NUM> or <NUM>, with a longer sheet being able to be wrapped around the torso of the baby for additional security of contact. The sheet <NUM> shown in <FIG> has a number of holes 201a-d equal to the number of electrodes in the array <NUM>. In cases where the sheet <NUM> is provided as a separate component, the holes 201a-d are slightly smaller than that of the corresponding electrodes 101a-d, which allows the electrodes 101a-d to protrude through the holes 201a-d and remain in place when the sheet <NUM> is affixed to the subject. Alternatively, the electrode array <NUM>, flexible sheet <NUM> and flexible substrate may form one integrated unit.

In some embodiments, the electrodes 101a-d may sit proud of the sheet. This improves the connection between the electrodes 101a-d and the skin of the subject.

The dispersive adhesion may arise without the need for an adhesive or tacky material, as a result of wetting of sufficient area of the flexible sheet by fluids on the surface of the subject.

The method may comprise arranging the flexible substrate such that each of the electrodes is aligned with a corresponding hole in the flexible sheet, which is applicable to examples where the flexible sheet is provided as a separate part to the electrode array.

The method may comprise applying a gel, optionally an electrically conductive gel, to a side of the flexible sheet prior to applying the flexible sheet to the subject.

The subject is typically a newborn child. The electrodes and flexible sheet may be applied to the back of the newborn child, or in some cases to the front. The flexible sheet may in some cases be wrapped around the torso of the newborn child.

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which:.

An example electrode array <NUM> for an electrocardiogram sensor is illustrated in <FIG>. The array <NUM>, which is typically for single use, has at least three electrodes arranged in an array, which may for example be a square, triangular, rectangular or circular arrangement. A rectangular arrangement having four electrodes is shown in <FIG>. The electrodes 101a-d are mounted on a common substrate <NUM> that interconnects the spaced apart electrodes 101a-d. The substrate <NUM> may be made from a material such as polyethylene terephthalate (PET), which has a stiffness that is suitable for maintaining the electrodes 101a-d in a fixed relative position while allowing the substrate <NUM> to bend to conform to the shape of the subject's body. The substrate <NUM> contains conductive tracks (not shown) that interface the electrodes 101a-d with a dock <NUM> containing electrical contacts <NUM> for connecting the array <NUM> to a measurement module. As shown in <FIG> the dock <NUM> is provided in a central region of the array <NUM>, although in other arrangements the dock <NUM> may be provided in other positions.

The dock <NUM> may be made from a hard plastic, normally PC/ABS (polycarbonate / acrylonitrile butadiene styrene), and preferably contains a ferrous element to hold a connector module (not shown in <FIG>) in place using a magnetic contact. Sprung contacts on either the module or the dock <NUM> allow a reliable electrical connection to be made to flat contacts on the opposing dock or the module. The dock <NUM> could also be provided at the distal end of a flat flexible extension cable, either as part of the substrate <NUM> or using a different material containing conductive tracks, as shown in the alternative arrangement in <FIG>, which may be more suitable if the array <NUM> is placed on the back of a baby, where a flat comfortable profile for the array is required. Alternatively if an existing wired ECG monitor is available then the array could be used with a cable connecting the electrodes to the existing wired ECG monitor, as shown in <FIG>.

A flexible sheet <NUM>, an example of which is shown in <FIG>, can be used to secure the connection between the electrodes 101a-d and the skin of the subject by increasing the surface tension between the sensor and the subject. The array <NUM> could hold itself in place with a conductive gel or solution, which may provide enough surface tension to adhere the array to the subject, but there is a risk that the array <NUM> will come loose, particularly if the subject is moved. A flexible sheet <NUM> extending beyond the array <NUM>, i.e. having a greater areal extent than that of the array <NUM>, is beneficial in providing additional surface area for the array to stay in place using surface tension alone. The flexible sheet <NUM> could be integrated with the array <NUM> and substrate <NUM>, or be provided as a separate layer to be assembled prior to use. The areal extent of the flexible sheet <NUM> is preferably at least twice that of the electrode array <NUM>, and may in some cases be up to three times that of the electrode array <NUM>. The flexible sheet is preferably of a generally rectangular form, having a length that is over <NUM> times that of its width, and in some cases may be up to three times that of its width.

The electrodes 101a-d face towards the skin of the subject. However, the electrical connections to the dock <NUM> typically point upwards (i.e. away from the subject). In order to secure an electrical connection between the electrodes 101a-d and the dock <NUM>, through holes (not shown) between the flexible sheet <NUM> and the substrate <NUM> may be provided, allowing the conductive tracks (not shown) to pass from the underside of the sheet <NUM> and substrate <NUM> to the top side. Alternatively, the conductive tracks may pass from the electrodes to the edge of the flexible sheet <NUM>, and fold over to the top side. This issue is particularly pertinent where the electrode array <NUM>, flexible sheet <NUM> and flexible substrate form one integrated unit.

A conductive ECG gel smeared over each of the electrodes 101a-d provides the required electrical coupling between the electrodes 101a-d and the skin of the subject and provides an amount of adhesion due to surface tension. Further amounts of gel may be used on the sheet <NUM>, provided this does not short circuit any of the electrodes, to provide additional surface tension between the sheet <NUM> and the skin of the subject. The sheet alone, however, may provide sufficient adhesion, particularly if the subject is still wet and/or if the sheet is sufficiently thin and elastic.

The conductive ECG gel may be a wet gel. The wet gel may have a thickness of approximately <NUM>-<NUM> and be held in an open cell sponge carrier. Further, a moat or wall arrangement may surround the edge of each electrode 101a-d. The moat extends below the base of the electrode, and may form an approximate "U"-shape <NUM>-<NUM> deep (or <NUM>-<NUM> high in the case of a wall) around the edge of each electrode 101a-d.

The moat or wall may be composed of a more rigid material than that of the flexible sheet <NUM>.

This prevents the wet gel moving from each electrode 101a-d over the flexible sheet <NUM> and substrate <NUM>. A micropore cover may also be provided, which can be placed on the surface of a 'wet gel' electrode. This may reduce the irritation caused by the sponge carrier on the skin of the subject, and help to hold the wet gel in place.

Alternatively, hydrogel may be used. This may remove the need for a sponge carrier, moat and micropore cover.

The sheet <NUM> may be composed of a soft polymeric material, for example polyethylene or polyvinyl chloride, or in some cases may be composed of paper or any biocompatible substance that provides sufficient surface tension.

For a wet gel, a <NUM>% to <NUM>% saline solution may be suitable (i.e. <NUM> NaCl or KCl per litre of water to <NUM> NaCl or KCl per litre of water). A conductive gel may have a polyacrylate base, for example an ECG conductive gel available from Dermedics (RTM) International (www. Other ingredients include sodium chloride and water.

A small electronic module may be provided that provides power and wireless transmission of data obtained from the sensor to a processing and display unit. An example processing and display unit <NUM> is illustrated in <FIG>. Two modules 301a, 301b are shown docked to the processing and display unit <NUM>. Docking the modules 301a, 301b to the unit <NUM> allows the modules 301a, 301b to be recharged and stored when not in use. A display <NUM> provides ECG and heart rate traces derived from the electrode array <NUM>. The unit <NUM> is preferably battery powered to allow it to be highly portable.

In order to ensure that the ECG signal can be detected, a difference amplifier with adequate electrode impedance mismatch immunity should be used in the module.

The individual electrode impedance may be measured, to provide notification of any potential operational issues to the user. One method of measuring the impedance may be supplying a low current sinusoidal source to the electrodes, and measuring the voltage generated through each pair of the electrodes, changing one electrode per measurement (i.e. measuring pair AC, BC, BD). Alternatively, impedance can be measured by applying a square wave current and measuring resistance and capacitance of the electrodes sequentially.

For use, one of the modules <NUM> is taken out of the processing and display unit <NUM> and attached to the dock <NUM> in the array <NUM> when required for vital sign monitoring, as illustrated in the electrocardiogram sensor <NUM> in <FIG>. The raw data from the electrodes 101a-d is detected by the module <NUM>. The data can be processed in real time on the module <NUM> to obtain heart rate (and optionally breathing rate) or the raw data can instead be transmitted by the module <NUM> to the processing and display unit <NUM> to process to a heart rate (and optionally breathing rate). Transmitting the data as heart rate rather than raw data reduces the power and bandwidth requirements of the module. As discussed earlier, in alternative examples an existing ECG monitor may be used with a wired connection made from the ECG array directly to the ECG monitor.

An accelerometer/gyroscope and magnetometer in the module <NUM> may provide useful contextual information such as activity, breathing, orientation etc. This can also facilitate noise removal by adaptive filtering.

When a newborn is being intubated, the forced breathing rate is superimposed on both ECG and PPG baselines. This means that, in addition to measuring the heart rate and breathing rate of the subject, the ECG array may be used for confirming the intubation rate.

The module <NUM> can additionally provide optical detection capabilities to pick up a photoplethysmogram in multiple wavelengths (e.g. red, infrared and green) which would facilitate the display of oxygen saturation status. The electrode array <NUM> may comprise additional embedded transmission mode optical sensors for this purpose.

The sheet <NUM> may be either separately packaged with the array <NUM> or be integrated with the substrate <NUM>. If the sheet were not used, the substrate with gel alone would need to have adequate surface tension to prevent movement of the electrodes, which in practice is unlikely to be a reliable method of adhesion.

The array (and sheet if integrated) may be hermetically sealed before use. To prepare the electrocardiogram sensor ready for a recording, the array (and sheet if integrated) is taken out of its packaging. The module may or may not have been placed into the array but it is easier if it is, so that this does not have to be done when the baby is born. This arrangement is shown in <FIG>, with the module <NUM> already in place on the dock <NUM> of the electrode array <NUM>. This also provides the opportunity to check that all connections are made correctly. The processing and display unit <NUM> can be turned on to carry out such a check.

The electrocardiogram sensor, comprising the array <NUM> and sheet <NUM>, is placed on its back, i.e. with the electrical contact surfaces of the electrodes 101a-d facing upwards. If the sheet is to be used and packaged separately, the sheet <NUM> is laid on top of the array <NUM> so that the holes 201a-d on the sheet <NUM> line up with the electrodes 101a-d on the array <NUM>. The sheet <NUM> can be slightly stretched to force the electrodes 101a-d through the holes 201a-d in the sheet <NUM>. This will keep the electrodes 101a-d in place.

Conductive or wet gel is then applied to the electrodes 101a-d and a small amount can be smeared across the sheet <NUM> (away from the electrodes to prevent short circuits). The sheet/array assembly is left upside down in a convenient location and it is now ready to be applied to the baby's skin when the baby is born. It may for example be left on the resuscitation table upside down if the baby is to be placed on top of the electrode array such that the electrode array contacts the back of the baby.

When the baby is born, the baby is normally brought to the resuscitation table and dried using a towel. Although in premature babies (but not to the exclusion of term babies), it may be that only the head is dried and the body is left wet. This gives the opportunity to use the wet skin as additional surface tension for the array/sheet.

Immediately after this, either the baby is placed on top of the array and sheet face up, or alternatively the whole array with electrodes and sheet is lifted and placed onto the baby. The substrate, gel and sheet, together with the fact that the baby's skin can be wet, helps keep the array stuck to the baby's skin, so that no adhesive is required. The baby can then be placed into another plastic bag if required to prevent hypothermia.

<FIG>, <FIG> illustrate alternative examples of electrode arrays to that shown in <FIG> and <FIG>. A triangular array <NUM>, <NUM> is illustrated in <FIG>, with the dock <NUM> positioned within the array in <FIG> and in <FIG> at the distal end of a connector <NUM> extending from the substrate <NUM>. An alternative to this is shown in the array <NUM> illustrated in <FIG>, where a cable <NUM> is used in place of a wireless module attached to a dock. <FIG> shows a triangular array <NUM> provided on a circular substrate <NUM>. Other arrangements are also possible.

<FIG> is an illustration of how an electrocardiogram sensor <NUM> of the type shown in <FIG> may be attached to a newborn baby. The sensor <NUM>, with module <NUM> in place, is positioned on the back <NUM> of the baby <NUM>. The flexible sheet <NUM> is extended across the baby's back <NUM> to provide additional surface tension. In other examples the sheet <NUM> may be extended further around the baby for additional security. In other examples, the module <NUM> may be attached to a dock of the form shown in <FIG>, i.e. at the distal end of a connector extending from the substrate. When attaching the sensor <NUM> to the back of a baby, this arrangement may be preferred since the module will not be covered when the baby is placed in its back, which may affect wireless transmission and cause discomfort for the baby. This problem may, however, be ameliorated to some extent by the module having a low profile.

<FIG> is an illustration of an electrocardiogram sensor <NUM> of the type shown in <FIG> as may be used in combination with a resuscitation table <NUM>. The sensor <NUM>, with module <NUM> in place, is positioned on the chest of the baby <NUM>. The flexible sheet <NUM> is extended across the baby's chest. A recording unit <NUM>, comprising the unit <NUM> of <FIG>, is placed at the end of the bed.

The unit <NUM> may comprise at least one camera and/or one microphone. The at least one camera may be a wide angle camera, and may form part of the unit <NUM> or be removeable. The at least one camera and microphone can be used to capture audio and video of the birth, which is useful for both training purposes and to produce incident reports if necessary. Alternatively, these components may form part of recording unit <NUM>. The at least one camera and/or microphone may be positioned to monitor the display, the newborn, the ancillary equipment or any other feature of the surrounding environment, or any combination of these.

The at least one camera may be a sensitive wide wavelength camera. This camera can be used to assess the Apgar score of a newborn baby (a measure of health, with criteria of Appearance, Pulse, Grimace, Activity, Respiration). The camera can also allow for non-contact photoplethysmogram analysis and blood oxygen saturation level to be measured. These measurements can be taken to complement the ECG heart rate measurement provided by the electrocardiogram sensor <NUM>.

In summary, a solution is proposed herein to overcomes the problems associated with conventional ECG electrodes which require strong adhesive forces and are difficult to apply to newborns through the use of surface tension on an ECG array which can be rapidly and reliably applied to the newborn baby at birth.

Claim 1:
An electrocardiogram sensor (<NUM>) comprising:
an electrode array (<NUM>, <NUM>, <NUM>, <NUM>) comprising a substrate (<NUM>, <NUM>, <NUM>) interconnecting three or more spaced apart electrodes (101a-d); and
a flexible sheet (<NUM>) extending over a greater area than an area over which the electrode array (<NUM>) extends and configured to secure the electrodes (101a-d) to the body of a subject;
the flexible sheet (<NUM>) extends beyond the extent of the electrode array (<NUM>) so as to provide additional surface area for adhering the sensor to the body of the subject by surface tension alone;
wherein the area over which the flexible sheet (<NUM>) extends is greater than twice the area over which the electrode array (<NUM>) extends.