Medical sensor and technique for using the same

[0001] The present disclosure relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.

[0002] In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such physiological characteristics. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.

[0003] One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.

[0004] Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms.

[0005] Pulse oximetry measurement often involves placement of a sensor on a patient's tissue, typically via a lightly adhesive sensor, a clip-style sensor, or a sensor that may be fitted through pressure contact with the tissue. Because these sensors are worn for up to four hours before the sensor is repositioned, pulse oximetry sensors may slightly deform the underlying tissue if the pressure contact is too great. Deformed tissue may be associated with motion artifacts in cases where pressure from the sensor alters the blood flow into the tissue, leading to changes in the pulse oximetry readings.

[0019] One or more embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0020] In accordance with embodiments, sensors for pulse oximetry or other applications utilizing spectrophotometry are provided that apply a sensor to a tissue such that the pressure is sufficient to obtain sensor readings but not so great as to cause patient discomfort or signal artifacts associated with tissue deformation. For example, sensors are provided that include force-sensitive devices adapted to assess the pressure experienced of the sensor against a patient's tissue while in use. Further, sensor assemblies as provided herein may be adapted to alter the pressure that is exerted on a patient's tissue.

[0021] In an embodiment, an oximetry sensor with an integral pressure transducer may be adapted for placement in a hat (for example, a neonatal stocking cap), a headband, or other wearable structure (i.e. a glove, a sock, a wristband) to apply the sensor on the body of the user, FIGS. 1A-1C illustrate an assembly drawing of an embodiment of a sensor assembly 10 including a wearable structure, which may be a hat 11, as shown in FIG. 1A. A reflectance-type pulse oximetry sensor 15, as shown in FIG. 1B, is adapted to be placed or adhered to the inside of a hat 11.

[0022] In an embodiment, the sensor 15 includes a substrate 14 that may be made from any suitable material. In an embodiment, the substrate 14 is a foam or other conformable material. In one embodiment, the substrate 14 is black or dark in color to absorb stray light and minimize any shunting of light between sensor and patient skin. In one embodiment, the substrate 14 may include an adhesive material to secure the sensor directly to the tissue. In one embodiment, the sensor 15 may include an emitter 16 containing emitters for two or more wavelengths of light and a detector 18 spaced apart from the emitter 16. The sensor 15 also includes a pressure transducer 12. The pressure transducer 12 is adapted to provide an indication of the pressure of the sensor 15 against the tissue.

[0023] Also shown in FIG. 1B is a cable 20 for providing drive current to the pressure transducer 12, providing the pressure signal to a downstream medical device, providing drive current to the LED, and providing the detector signal to the medical device, according to an embodiment. In addition to providing the electrical connection to the downstream medical device, the cable may provide shielding to protect the small signals from the detector against external electrical interference. In addition, the sensor 15 may include suitable structures for providing electrical connections to the cable and/or downstream medical device, such as a flex circuit, a Faraday shield, and leads connecting the optical components and the pressure transducer of the sensor 15 to the electrical components.

[0024] In an embodiment, the sensor assembly 10 is shown fully assembled in FIG. 1C. As shown, the sensor 15 is positioned on the interior of the hat 11 such that the emitter 16 and detector 18, as well as the pressure transducer 12, may come into contact with the skin when the sensor assembly 10 is applied to a patient. The sensor 15 may be attached (e.g., adhered or sewn into) to the inside band of a hat. In one embodiment, the hat may include indicators to position the sensor 15 on a particular location on the patient's forehead, for example to position the sensor 15 on the lower forehead region, above the eyebrow, with the sensor optics (emitter 16 and detector 18) located above and predominantly lateral to or centered over the iris. The location of the reflectance sensor 15 in the hat allows appropriate placement of the sensor in the desired forehead location by a user not skilled in sensor placement. FIG. 1C shows that the cable 20 is positioned through a hole in the top of the hat 11. In an embodiment, the cable 20 may be adhered or otherwise constrained in the hat 11 so that the cable generally is positioned away from the sensor 15 to avoid interfering with the patient's eyesight or bothering the patient.

[0025] In some embodiments, it is envisioned that force or pressure data generated from the force-sensitive structures may be further processed by a downstream monitor to generate displays or other information related to the pressure exerted on the tissue by the sensor assembly 10. However, as patients may not be familiar with the medical monitor icons and displays that may be used in conjunction with a sensor assembly 10, in certain embodiments it may be advantageous to provide a sensor assembly 10 with a pressure-sensitive signal that is easily identifiable. FIG. 2A illustrates a sensor assembly 10 that may be applied to a patient's head. The sensor assembly 10 includes a hat 11 and a pulse oximetry sensor 34, including an emitter 16, a detector 18, and a cable 20. The pulse oximetry sensor 34 is placed on the interior of the hat band 26. The sensor assembly 10 also includes a pressure-sensitive film 24 that is adapted to change color upon the application of force. The pressure-sensitive film 24 may be viewed through a viewing window, depicted here as a buttonhole 22 formed in the knit fabric of the hat 11.

[0026] As shown in FIG. 2B, the pressure-sensitive film 24 may be located on the interior of the hat band 26 at approximately the same latitude as the pulse oximetry sensor 34. Such placement may allow the pressure indication of the pressure-sensitive film 24 to approximate the force exerted by the pulse oximetry sensor 34 on the tissue, assuming that the hat band 26 applies force equally around the head. In an embodiment, the pressure-sensitive film 24 may turn from colorless to red as pressure is increased against the tissue. In one embodiment, the pressure-sensitive film 24 may be Pressurex® film, available from Sensor Products Inc. (East Hanover, N.J.), which increases in red color intensity in relation to the amount of force applied. In one embodiment, the sensor assembly 10 may also include a reference color strip 25 that may be compared to the color in the pressure-sensitive film 24. When the color matches or is brighter than the color in the reference color strip 25, a patient or caregiver may adjust the sensor assembly 10 so that less pressure is exerted on the tissue or may choose a hat of a larger size to fit the patient.

[0027] FIG. 3 shows an embodiment of a hat-based sensor assembly 10 for pulse oximetry or other medical monitoring that includes a tightening device, shown here as a strap 32. In certain instances, a patient's head size may fall in between hat sizes. In such cases, it may be preferable to apply a slightly too large hat to the patient to avoid placing a hat on the patient that will be uncomfortably tight. However, if the hat is too large, the emitter 16 and the detector 18 of the sensor 34 may not come into sufficient contact with the forehead tissue for accurate sensor readings. To overcome this problem, a caregiver may tighten the hat-based sensor assembly 10 with a tightening strap 32. The strap may be circled over the band of the hat 11 to tighten the position of the sensor 34 on the skin. The strap 32 may be closed at the appropriate tightness by any suitable means, such as a hook and loop closure 34 depicted here. In certain embodiments, the tightness may be monitored by a pressure transducer 12 connected to a cable 20 by a lead 13. The pressure transducer 12 may provide feedback relating to the tightness of the hat 11 on the tissue. If the hat is too tight or too loose, a signal may be carried by the cable 20 to a downstream monitor, which may display an appropriate warning or indication.

[0028] In an embodiment, the tightness of a hat or headband-based sensor assembly may be adjusted by creating openings in the structure of the hat or headband. FIG. 4 shows a hat-based sensor assembly 10 that includes a sensor 34. The hat includes indicia for slits 40 that may be cut in the fabric of the hat to reduce the tension applied by the hat band 26. If a pressure transducer 12 provides a signal that the hat is too tight, the hat may be adjusted by taking it off the head of the patient and cutting the fabric where indicated by the slits 40. Each slit 40 may be cut one at a time, each time returning the hat to the head of the patient to check the tightness after cutting. In this manner, the hat may have multiple levels of adjustability. For example, a hat that is very tight may be very comfortable after cutting several slits 40, while a hat that is only mildly tight may be rendered comfortable by cutting only one slit 40. In other embodiments, the indicia may indicate other suitable shapes that may be easily cut into the band of the hat. Shown in FIG. 5 is a hat-based sensor assembly 10 that includes indicia for holes 50 that may be punched into the fabric of the hat to reduce the tension applied by the hat band 26. Such an embodiment may be advantageous for settings in which scissors are not readily available and/or desirable. In one embodiment the hat-based sensor assembly 10 may be sold as a kit with a hole-punch that is adapted to punch a hole that matches the size of the hole-shaped indicia 50.

[0029] FIGS. 6A and 6B illustrate an embodiment of a sensor assembly 10 in which the pressure of a medical sensor 34 against the tissue may be adjusted. Illustrated in FIG. 6A is a side view of a headband-based sensor assembly 10 that includes an optical sensor 34 including an emitter 16 and a detector 18. The sensor assembly may include a strap or band 56 that may be fitted around a patient's forehead tissue 50 to contact the sensor 34 with the tissue 50. As shown, in certain embodiments in which the strap 56 is improperly fitted, the sensor 34 is not flush against the tissue 50. In such an embodiment, a pressure transducer 12 may send feedback to a downstream monitor about a lack of contact with the tissue 50. After receiving the feedback, the monitor may provide an indication or warning, after which a healthcare provide may inflate balloon 52 through inflation line 54. As shown in FIG. 6B, after inflation, the sensor 34 and the pressure transducer 12, which are positioned on the assembly 10 so that the inflation of the balloon pushes them towards the tissue 50, may be pressed flush against the forehead of the patient. The pressure sensor 12 may allow a user to adjust the pressure experienced by the patient to a range that is comfortable for the patient, and which does not significantly deform the underlying tissue and allows the sensor 34 to obtain accurate measurements.

[0030] A sensor or sensor assembly including pressure sensing and/or adjusting mechanisms as provided herein and illustrated generically as a sensor assembly 10, may be used in conjunction with a pulse oximetry monitor 100, as illustrated in FIG. 8. It should be appreciated that the cable 20 of the sensor assembly 10 may be coupled to the monitor 100 or it may be coupled to a transmission device (not shown) to facilitate wireless transmission between the sensor assembly 10 and the monitor 100. The monitor 100 may be any suitable pulse oximeter; such as those available from Nellcor Puritan Bennett LLC. Furthermore, to upgrade conventional pulse oximetry provided by the monitor 100 to provide additional functions, the monitor 100 may be coupled to a multi-parameter patient monitor 102 via a cable 104 connected to a sensor input port or via a cable 106 connected to a digital communication port.

[0031] FIG. 9 is a block diagram of an embodiment of a pulse oximeter 100 that may be configured to implement the embodiments of the present disclosure. Light from emitter 16 may pass into a blood perfused tissue, and may be scattered, and then detected by detector 18. A sensor assembly 10 containing an emitter 16 and a detector 18 may also contain an encoder 116 which may be capable of providing signals indicative of the wavelength(s) of light source 16 to allow the oximeter to select appropriate calibration coefficients for calculating oxygen saturation. The encoder 116 may, in an embodiment, be a resistor. In an embodiment, the sensor assembly 10 also includes a pressure sensor/transducer 12 and may be capable of carrying a signal from the pressure sensor 12 to a monitor 100.

[0032] In an embodiment, the sensor assembly 10 may be connected to a pulse oximetry monitor 100. The monitor 100 may include a microprocessor 122 coupled to an internal bus 124. Also connected to the bus may be a RAM memory 126 and a display 128. A time processing unit (TPU) 130 may provide timing control signals to light drive circuitry 132, which controls when the emitter 16 is activated, and if multiple light sources are used, the multiplexed timing for the different light sources. TPU 130 may also control the gating-in of signals from detector 18 through an amplifier 133 and a switching circuit 134. These signals are sampled at the proper time, depending at least in part upon which of multiple light sources is activated, if multiple light sources are used. The received signal from the detector 18 and the pressure sensor 12 may be passed through an amplifier 136, a low pass filter 138, and/or an analog-to-digital converter 140. The digital data may then be stored in a queued serial module (QSM) 142, for later downloading to RAM 126 as QSM 142 fills up. In an embodiment, there may be multiple parallel paths of separate amplifier, filter, and A/D converters for multiple light wavelengths or spectra received, and/or for the pressure data generated by the pressure sensor 12. In one embodiment, the signal from the pressure sensor 12 may be processed in any suitable manner, and may be sent through a different data path than the signal from the detector 18.

[0033] In an embodiment, the monitor 100 may be configured to receive signals from the sensor assembly 10. In certain embodiments, the monitor 100 may receive a signal from the pressure sensor 12 that indicates the magnitude of the pressure against the patient. The signals may be processed by the monitor 100 to indicate a sensor condition such as whether there is sufficient contact or insufficient contact. The monitor 100 may be configured to provide an indication about the sensor condition, such as an audio alarm, visual alarm or a display message, such as indicating the measured pressure. For example, in one embodiment, the pressure transducer may send a signal that the pressure is in the range of 16-40 mm Hg or 20-40 mm. In such an embodiment, the monitor 100 may provide a message “SUFFICIENT CONTACT,” or may simply use a color indication, such as a green light, to indicate the sufficiency of the contact. In one embodiment, when the pressure is less than 16 mm Hg, the monitor 100 may provide a message “CONTACT LOW” or “CHECK SENSOR, or may provide a yellow or red indicator light. Further, the monitor 100 may be configured to receive information about the pressure sensor 12 from a memory chip or other device, such as the encoder 116, which may be on the sensor assembly 10 or the cable 20. In an embodiment, such a device may include a code or other identification parameter that may allow the monitor 100 to select an appropriate software or hardware instruction for processing the signal. In an embodiment, a monitor 100 may run an algorithm or code for processing the signal provided by the pressure sensor 12. In one embodiment, when the encoder 116 indicates that the sensor assembly 10 is configured for neonates, the pressure range for the sensor that is indicative of sufficient contact may be in the range of 16-40 mm Hg. In one embodiment, when the encoder 116 indicates that the sensor assembly 10 is configured for adults, the pressure range for the sensor that is indicative of sufficient contact may be in the range of 16-90 mm Hg, 20-40 mm Hg, or 60-90 mm Hg.

[0034] In an embodiment, the pressure transducer 12 may include one or more force-sensitive structures adapted to provide a signal relating to the pressure of the sensor 15 against the tissue. The pressure sensor 12 may be any appropriate sensor that is capable of converting a force applied to a sensor body into an electrical signal. In certain embodiments, the pressure sensor 12 may take the form of a displacement sensor. In one such embodiment, the pressure or force-sensitive structure may include a strain gauge or other mechanical displacement sensor. In another embodiment, the displacement sensor may include a linear variable differential transformer.

[0035] In other embodiments, a force-sensitive structure may be a resistance-based sensor. The pressure sensor 12 may include an array of electrodes, such as silver electrodes, printed as a matrix of intersecting rows and columns. An additional layer of semiconductive ink may provide an electrical resistance at each intersection on the matrix. Sandwiching these two layers together may create an array sensor. When a force is applied, the change in resistance is measured. Changing the formulation of the ink may produce different sensitivity ranges. Additionally, varying the spacing between rows and columns may yield finer resolution. In certain embodiments, a force-sensitive structure may have a spatial resolution, or sensor electrode spacing, of at least 0.0229 mm². An example of a pressure sensor 12 that is appropriate for use with a sensor 15 according to the present techniques is Flexiforce® film or flexible circuits, available from Tekscan (South Boston, Mass.).

[0036] In an embodiment, the pressure sensor 12 may also include polymers that are force-sensitive resistor materials. Force-sensitive resistor materials, such as those available from Interlink (Carptenteria, Ca.) and Advanced Composites Technology (Boston, Mass.) have a resistance variation under load. A force sensing resistor may be a piezoresistivity conductive polymer, which changes resistance in a predictable manner following application of force to its surface. It is normally supplied as a polymer sheet which has had the sensing film applied by screen printing. The sensing film typically includes both electrically conducting and non-conducting particles suspended in matrix. The particle sizes may be of the order of fraction of microns, and the particles may be formulated to reduce the temperature dependence, improve mechanical properties and increase surface durability. Applying a force to the surface of the sensing film causes particles to touch the conducting electrodes, changing the resistance of the film. Such a polymer-based force-sensitive resistor may be advantageous as it utilizes a relatively simple interface and can operate satisfactorily in moderately hostile environments.

[0037] In some embodiments, the pressure sensor 12 may take the form of a capacitance sensor. In such sensors, the capacitance is inversely proportional to the distance between the electrodes of the sensor. An exemplary capacitance-based sensor, TactArray, is available from Pressure Profile Systems (Los Angeles, Ca.). In certain embodiments, the capacitance sensor may be sensitive to forces or pressures from 1 psi to 200 psi.

[0038] In an embodiment the pressure sensor 12 may also include an elastomeric foam that is sensitive to force. The force-sensitive foam provides measurement of the resistance of a conductive elastomer or foam between two points. The force-sensitive foam may be a carbon doped rubber in which the resistance of the elastomer changes with the application of force, resulting from the deformation of the elastomer altering the particle density.

[0039] In an embodiment, based at least in part upon the received signals corresponding to the light received by detector 18, microprocessor 122 may calculate the oxygen saturation using various algorithms. These algorithms may require coefficients, which may be empirically determined, and may correspond to the wavelengths of light used. The algorithms may be stored in a ROM 146 and accessed and operated according to microprocessor 122 instructions.

[0040] In an embodiment of a two-wavelength system, the particular set of coefficients chosen for any pair of wavelength spectra may be determined by a value indicated by the encoder 116 corresponding to a particular light source in a particular sensor assembly 10. In one embodiment, multiple resistor values may be assigned to select different sets of coefficients. In another embodiment, the same resistors are used to select from among the coefficients appropriate for an infrared source paired with either a near red source or far red source. The selection between whether the near red or far red set will be chosen can be selected with a control input from control inputs 154. Control inputs 154 may be, for instance, a switch on the pulse oximeter, a keyboard, or a port providing instructions from a remote host computer. Furthermore, any number of methods or algorithms may be used to determine a patient's pulse rate, oxygen saturation or any other desired physiological parameter.

[0041] In an embodiment, the sensor assembly 10 includes an emitter 16 and a detector 18 that may be of any suitable type. For example, the emitter 16 may be one or more light emitting diodes adapted to transmit one or more wavelengths of light in the red to infrared range, and the detector 18 may one or more photodetectors selected to receive light in the range or ranges emitted from the emitter 16. Alternatively, an emitter 16 may also be a laser diode or a vertical cavity surface emitting laser (VCSEL). An emitter 16 and detector 18 may also include optical fiber sensing elements. An emitter 16 may include a broadband or “white lights” source, in which case the detector could include any of a variety of elements for selecting specific wavelengths, such as reflective or refractive elements or interferometers. These kinds of emitters and/or detectors would typically be coupled to the rigid or rigidified sensor via fiber optics. Alternatively, a sensor assembly 10 may sense light detected from the tissue is at a different wavelength from the light emitted into the tissue. Such sensors may be adapted to sense fluorescence, phosphorescence, Raman scattering, Rayleigh scattering and multi-photon events or photoacoustic effects. For pulse oximetry applications using either transmission or reflectance type sensors the oxygen saturation of the patient's arterial blood may be determined using two or more wavelengths of light, most commonly red and near infrared wavelengths. Similarly, in other applications, a tissue water fraction (or other body fluid related metric) or a concentration of one or more biochemical components in an aqueous environment may be measured using two or more wavelengths of light, most commonly near infrared wavelengths between about 1,000 nm to about 2,500 nm. It should be understood that, as used herein, the term “light” may refer to one or more of ultrasound, radio, microwave, millimeter wave, infrared, visible, ultraviolet, gamma ray or X-ray electromagnetic radiation, and may also include any wavelength within the radio, microwave, infrared, visible, ultraviolet, or X-ray spectra.

[0042] The emitter 16, the detector 18, and in some embodiments the pressure sensor 12, may be disposed on a sensor body 14, which may be made of any suitable material, such as plastic, foam, woven material, or paper. Alternatively, the emitter 16 and the detector 18 may be remotely located and optically coupled to the sensor assembly 10 using optical fibers. In the depicted embodiments, the sensor assembly 10 is coupled to a cable 20 that is responsible for transmitting signals from the pressure sensor 12 as well as electrical and/or optical signals to and from the emitter 16 and detector 18 of the sensor assembly 10. The cable 20 may be permanently coupled to the sensor assembly 10, or it may be removably coupled to the sensor assembly 10—the latter alternative being more useful and cost efficient in situations where the sensor assembly 10 is disposable.

[0043] In an embodiment, the sensor assembly 10 may include a “transmission type” sensor. Transmission type sensors include an emitter 16 and detector 18 that are typically placed on opposing sides of the sensor site. If the sensor site is a fingertip, for example, the sensor assembly 10 is positioned over the patient's fingertip such that the emitter 16 and detector 18 lie on either side of the patient's nail bed. In other words, the sensor assembly 10 is positioned so that the emitter 16 is located on the patient's fingernail and the detector 18 is located 180° opposite the emitter 16 on the patient's finger pad. During operation, the emitter 16 shines one or more wavelengths of light through the patient's fingertip and the light received by the detector 18 is processed to determine various physiological characteristics of the patient. In each of the embodiments discussed herein, it should be understood that the locations of the emitter 16 and the detector 18 may be exchanged. For example, the detector 18 may be located at the top of the finger and the emitter 16 may be located underneath the finger. In either arrangement, the sensor assembly 10 will perform in substantially the same manner.

[0044] Reflectance type sensors also operate by emitting light into the tissue and detecting the light that is transmitted and scattered by the tissue. However, reflectance type sensors include an emitter 16 and detector 18 that are typically placed on the same side of the sensor site. For example, a reflectance type sensor may be placed on a patient's fingertip or forehead such that the emitter 16 and detector 18 lie side-by-side. Reflectance type sensors detect light photons that are scattered back to the detector 18. A sensor assembly 10 may also be a “transflectance” sensor, such as a sensor that may subtend a portion of a baby's heel.

[0045] While the disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the embodiments provided herein are not intended to be limited to the particular forms disclosed. Indeed, the disclosed embodiments may not only be applied to measurements of blood oxygen saturation, but these techniques may also be utilized for the measurement and/or analysis of other blood constituents. For example, using the same, different, or additional wavelengths, the present techniques may be utilized for the measurement and/or analysis of carboxyhemoglobin, met-hemoglobin, total hemoglobin, fractional hemoglobin, intravascular dyes, and/or water content. Rather, the various embodiments may to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the following appended claims