The ability to monitor the physiological parameters of a live subject, and in particular a human patient, is crucial in determining the health status of the patient and the proper medical treatment to be applied to the patient, as well as in understanding the effects of certain variables on physiological processes while conducting research. Several physiological parameters, such as heart rate and brain wave activity, can be monitored by taking advantage of the fact that these processes involve the conduction of electricity within the body and therefore, produce detectable bio-signals. Measurement of these electrical signals has long been accomplished by applying electrically conductive sensors to the surface of the patient's skin or other tissues or by the invasive implantation of the sensor inside the patient's body. Typically, these detected or sensed electrical signals are then relayed to a separate monitoring device whereby the signals are processed and displayed in a useful form.
For example, an electrocardiograph (ECG) system monitors the electrical heart activity of a patient. Conventional ECG systems utilize electrodes or sensors which are placed on a patient's chest in specific locations to detect the electrical impulses generated by the heart during each beat. Usually, these electrical impulses or signals are directly transferred from each electrode or sensor to a nearby, stationary ECG monitor through individual lead wires or cables that are connected to each electrode or sensor. The ECG monitor performs various signal processing and computational operations to convert the raw electrical signals into meaningful information to be displayed or printed out for review by a physician. These systems generally require that the patient be tethered to a stationary monitor and remain sufficiently still in order that the electrodes and wire attachments are not disturbed. Restriction of the patient's movement is oftentimes cumbersome and uncomfortable for a patient and the attending medical staff. It is also ill-suited for emergency situations during which a patient's body must be rapidly moved to a variety of positions and transported to several locations.
In order to alleviate discomfort and to increase a patient's mobility, several portable telemetry systems exist for the monitoring of physiological parameters. Generally, two types of telemetry systems exist for the monitoring of ECG signals. One type of system requires placing conventional ECG electrodes on the skin surface of a patient and connecting the electrodes to a portable, patient-worn telemetry unit by one or more lead wires or cables. The telemetry unit wirelessly transmits the ECG signals to a remote monitoring device. Although the patient has greater mobility, there still exist some disadvantages. Because each electrode or sensor is individually connected to the telemetry unit by an individual lead wire or cable, the several lead wires or cables become confusingly intertwined making it difficult to make the proper corresponding connections between the electrodes and the telemetry unit. Also, because each electrode or sensor is placed separately on the patient, one at a time in a sequence, the chances of acquiring an inaccurate signal due to improper placement on the patient are high. Additionally, the motion of the individual current carrying lead wires in relation to each other causes the generation of electrical artifacts in the transmitted signal.
In order to solve these problems associated with individual electrodes or sensors and their corresponding lead wires, the second type of wireless telemetry system eliminates all wires extending from the electrodes and replaces them with a self-contained strip or patch-like assembly which incorporates the electrodes and the lead wires. The strip or patch assembly is then adhered to the patient's skin or otherwise worn on the body. Generally, the assembly is comprised of a thin and flexible substrate constructed of non-conducting material with the electrodes or conductive areas intended to be in contact with the patient and fully integrated into the surface of the substrate that is in contact with the skin. As such, the electrodes or conductive areas are fixed in set positions thereby greatly reducing the possibility of placing one or more of them improperly on the patient. One or more transmitters or transceivers and the corresponding circuitry are also integrated into the assembly's surface for wirelessly transmitting the detected ECG signals to a remote monitoring location. However, these types of fully-integrated electrode assemblies have several disadvantages stemming from their high cost and lack of flexibility in permitting alternative placement of the electrodes.
For example, most conventional ECG electrodes are relatively inexpensive and detachable from the lead wires so that they may be easily disposed of after each use in case of a failure or defect in the electrode and to maintain a relatively sterile environment that is necessary for medical use. With respect to fully-integrated electrode assemblies, in order to dispose of a failed electrode or sensor element, the entire assembly must impractically be discarded. Also, the entire assembly, if not disposable, must be meticulously cleaned after each use or each patient. Furthermore, most electrodes require the application of aqueous silver chloride gel or hydrogel to the surface of the electrodes to increase their conductivity. These gels will rapidly dry out and lose their conductivity. In order to preserve these gels after being pre-applied to the electrodes, the electrodes must be hermetically sealed when packaged. Thus, when the gels are applied to fully-integrated electrode assemblies, the entire assembly must be hermetically sealed for storage. Once opened, the assembly has no shelf life and must be used immediately or discarded.
Several disadvantages also stem from the use of fully-integrated electrode assemblies. For example, because the electrode or conductive surfaces are generally fixed in one position, the electrode assembly cannot be adapted to varying body sizes. Because of this lack of adaptability, applications that require placing the electrodes at un-conventional positions on the body would require manufacturing several separate configurations of the assembly. Also, because the electrode assembly is fully self-contained, the patient must uncomfortably bear the weight of the entire system, including the telemetry circuitry, on the chest. For neonatal and elderly patients, who tend to be relatively weak, have lower body weight, and generally thinner skin or more sensitive skin than do patients of other ages, this can be very uncomfortable. Furthermore, the power source (e.g. batteries), telemetry circuitry or other electronics may generate external heat that can add to the discomfort or potentially burn or irritate a patient's skin.
To combat the high cost and discomfort associated with a fully-integrated electrode assembly while still maintaining the advantages of an easy to place sensor assembly, disposable chest assemblies that contain a plurality of fixed connections for connecting to separate, conventional electrodes or sensors have been developed. Typically, such chest assemblies consist of a thin and flexible substrate constructed of non-conductive material that spans across the length of the chest. Printed onto or embedded within the substrate are conductive traces that run along its surface extending from the electrode connections to one or more common terminals or trunks. The terminal or trunk connects to a separate monitoring device such as a patient-worn telemetry unit that is attached to a more comfortable weight-bearing location on the body than the chest or is wired to a bedside monitor. Despite these efforts, a long-standing unmet need still exists for a chest assembly of the foregoing type that may be universally connected to a conventional electrode or sensor without significant physical hardship on the patient.
Because the shape and size of conventional electrodes or sensors are not standardized, they are not universally compatible with many of the wires, leads or chest assemblies used in physiological data collection systems. To solve this problem, many wired systems utilize spring-loaded, female-type snap pieces that can adapt to differently-shaped male snap pieces or metal tabs of the electrodes or sensors. These spring-loaded pieces are relatively expensive and cannot be amortized over the life of a disposable chest assembly. Also, because it is important that any fastener fit tightly to the electrode or sensor in order to avoid a conductivity gap, the amount of pressure needed to snap the pieces together can be physically difficult.
With the increasing use of electromagnetic diagnostic imaging devices, including but not limited to x-rays, fluoroscopes, CAT scans and magnetic resonance imaging, there is a further need for lead wires and electrode assemblies that are transparent to these imaging devices. The devices need to be configured and constructed to be sufficiently radiolucent and radiotransparent for medical treatment applications. Conventional wired and wireless ECG systems, particularly those with chest-worn components, significantly interfere with the normal use and viewing of an X-ray film, fluoroscopic or other image that is created by electromagnetic radiation. Due to the desirability of using good conductors, such as metals, in the chest-worn electrodes, sensors and other components to provide a good electrical path for the sensor, these components are oftentimes substantially not radiolucent or radiotransparent and appear as blemishes or shadows or at worse, are completely opaque on the diagnostic image. For an attending physician, it is highly advantageous to be able to monitor an ECG or other vital signs which serve as an indication of a patient's physiological stability while simultaneously viewing the patient's internal organs (e.g. view of the internal blood vessels during cardiac catheterization or angiography); especially during emergency medical procedures in which time is of the essence. Also, because the chest assemblies typically span the entire chest, they greatly impair access for surgical procedures and usually need to be removed during surgery and other medical treatments that require unimpeded access to the chest area.
Efforts have been made to improve the translucency or permeability to X-ray of the electrodes or sensors themselves by lessening the mass or density of the metallic parts used in their construction. For example, a thin layer of metallic foil or conductive paint or ink has been used on the conductive surfaces of heart monitoring and stimulating electrodes. However, to compensate for the decreased thickness of the metallic conductive cross-section, the overall surface area of the foil or painted area must be increased to provide the same amount of conductivity. Because the electrodes or sensors are normally being applied to the non-flat surface of a 3-dimensional object, they often need to be placed at an angle to the viewing plane of the diagnostic device. When placed at these angles, even very thin and flat metallic areas that span a wide surface area appear much thicker on an X-ray film or other image. Also, thin foils or paints which may provide the desired degree of radiolucency are more fragile than solid metallic parts and are more easily worn down or chipped from abrasion.
In another example, the metallic components of an electrode or sensor have been completely eliminated and replaced with one or more layers of thin, carbon or graphite-filled polymers, often in conjunction with conductive adhesives or gels. Lead wires that connect to the electrodes have also been substituted with cables of insulated, carbon fibers. However, these polymers are generally composed of very thin, carbon filaments or particles thereby having the characteristic of high impedance such that a large amount of external heat is generated by a flowing current. As such, carbon based conductors alone will rarely withstand an external defibrillation current applied to the body. To prevent destruction of the conductor due to exposure to higher than normal electrical currents, a thin coating of silver/silver chloride is applied to the surface of the carbon material. Unfortunately the application of the silver/silver chloride negatively affects the electrode's radiolucency and may make it opaque. Furthermore, while carbon or graphite may be more radiolucent than a comparably sized metallic conductor, carbon or graphite materials are less conductive than metals and therefore require a greater quantity of carbon or graphite to conduct the same amount of electricity as a metal counterpart. Thus, the carbon or graphite-filled polymers or cables are relatively thick in comparison making them relatively unwieldy when worn by a patient. Further, the increased thickness of the materials reduces the radiolucency and shows up unsatisfactorily on the resulting images. This is particularly problematic with depth-capturing images such as computed tomography (CT) scans.
Efforts have been made to provide radiolucent current spreading layers within defibrillation or heart-stimulating electrodes. In one example, a conductive mesh backing made of low-resistance, non-corrosive and pliable metal wires is applied to a skin-contacting conductive polymer adhesive matrix or pad such that the open space of the mesh is greater than about 50%. In another example, a pattern of metal or otherwise conductive ink is applied to the surface of a conductive polymer sheet. The objective of the mesh or ink patterns is to provide enough conductive surface so that there is a low amount of electrical resistance for the conduction of high voltage defibrillation pulses without burning or generating a lot of heat, and remaining somewhat radiolucent. Thus, the amount of conductive surface required in these applications is substantially more than is required in an electrode that is simply used for monitoring. Neither of these applications addresses a radiolucent connection to an electrode or sensor that is electrically suitable for the initial acquisition of an electrical signal. Rather, they are only concerned with spreading a large amount of applied current on the backend of the circuit path along the skin surface.
Therefore, it is an object of the present invention to provide a lightweight, disposable and substantially radiolucent chest assembly that universally connects to separate, non-integrated electrodes or sensors for use in a wireless system for monitoring the physiological parameters of a live subject.