| Cardiac monitoring apparatus and method -> Monitor Keywords |
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  Cardiac monitoring apparatus and methodRelated Patent Categories: Surgery, Diagnostic Testing, Cardiovascular, Heart, Detecting Heartbeat Electric SignalThe Patent Description & Claims data below is from USPTO Patent Application 20060235316. Brief Patent Description - Full Patent Description - Patent Application Claims
[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. Continue reading... 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