WO2009086452A1

WO2009086452A1 - Pulse oximetry sensor with a pressure sensor

Info

Publication number: WO2009086452A1
Application number: PCT/US2008/088304
Authority: WO
Inventors: Ethan Petersen; Darius Eghbal; Ian N. Mccutcheon; Paul Mannheimer; Don L. Hannula; Carine Hoarau
Original assignee: Nellcor Puritan Bennett Llc
Priority date: 2007-12-26
Filing date: 2008-12-24
Publication date: 2009-07-09

Abstract

Disclosed embodiments may include a pressure sensor comprising a flexible pressure pad and a printed circuit board (PCB) having a plurality of pairs of conductive traces is disclosed. The pressure pad has a conductive surface aligned with the pairs of conductive traces. The conductive surface is configured to electrically couple the pairs of conductive traces. The amount of pressure applied to the pressure pad can be determined by the number of conductive trace pairs capable of conducting an electrical current.

Description

PULSE OXIMETRY SENSOR WITH A PRESSURE SENSOR

BACKGROUND Field

The present disclosure relates generally to medical devices and, more particularly, to sensors for measuring physiological parameters of a patient.

Description of the Related Art

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certain physiological parameters of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such characteristics of a patient. 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. One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built 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, 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 oximetiy refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.

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

Because of the particular physiological parameters that pulse oximeters are capable of determining, the use of pulse oximeters has become common outside the field of medicine also. For example, mountain climbers, hikers, and pilots may desire to know their blood oxygen saturation as they ascend to higher altitudes where the air is thinner. Additionally, people who exercise may be interested in knowing their pulse rate as they work out and, as such, pulse oximeters may be included with cardiovascular exercise equipment.

In some of the above mentioned applications, it may be desirable to have a lightweight and easily portable pulse oximeter capable of making accurate measurements. Among the major considerations when designing a portable or handheld pulse oximetry system is the size and/or weight of the system and length of time that the system can operate on battery power. Typically, a design tradeoff is made between the size and/or weight of the system and the operating time provided by the battery because, generally, the larger the battery, the longer the system can operate.

Ideally, a smaller battery would be implemented while still achieving long operating life.

Generally, there are two types of pulse oximeter sensors: bandage-type and clip-type. The bandage-type sensors may be placed over blood perfused tissue of a patient like a bandage, while the clip-type sensors have a spring and may be configured to fit over a patient's finger, for example. When using a bandage type sensor, the amount of pressure applied to the blood perfused tissue site is determined by how tightly the sensor is manually applied. Alternatively, the amount of pressure applied by a clip-type sensor may be determined by the strength of the spring. Both bandage-type and clip-type sensors may be configured as either reflectance or transmittance sensors. In a reflectance configuration, emitters and detectors are in the same general plane, whereas in a transmittance configuration, the emitters are in a plane generally parallel to the detectors. Measurements made by oximeters may be inaccurate for a number of reasons. One reason may be the application of too much pressure to the tissue site where the oximeter sensor is placed on the patient. When too much pressure is applied, exsanguination may occur causing blood to squeeze out of the tissue site. The exsanguination may reduce pulsatility or even cause pulsatility to disappear at the tissue site where the oximeter sensor is placed. Exsanguination may occur in both types of sensors. In some instances, bandage-type sensors may be more vulnerable to exsanguination than clip-type sensors because pressure is applied manually.

While exsanguination is caused by the application of too much pressure and may lead to inaccurate readings, the application of too little pressure at the tissue site where the sensor is placed on the patient may also result in inaccurate readings. When insufficient pressure is applied, the emitter and detector may not properly couple with the tissue and light shunting may occur. Light shunting results in light being detected that has not passed through tissue, resulting in a higher than normal dc offset. The errors caused by too little pressure may occur in both bandage-type and clip-type sensors.

SUMMARY

Certain aspects commensurate in scope with the disclosure are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms embodiments might take and, these aspects are not intended to Jim it the scope of the disclosure. Indeed, the disclosure may encompass a variety of aspects that may not be set forth below.

hi accordance with an embodiment, there is provided a sensor comprising an electromagnetic radiation source configured to direct electromagnetic radiation into blood-perfused tissue and a photodetector configured to detect the electromagnetic radiation emanating irom the tissue. The sensor includes a pressure sensor configured to indicate whether a proper amount of pressure is applied.

hi accordance with another embodiment, there is provided a method of operation for a pulse oximeter. The method includes sensing the application of pressure to the puise oximeter, and turning the pulse oximeter on when a pressure is applied. The method includes calculating and displaying a physiological parameter when a proper pressure is applied and indicating when an excessive pressure is applied. The method also includes turning the pulse oximeter off when no pressure is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the disclosure may become apparent upon reading the following detailed description and upon reference to the drawings, in which: FIG. 1 illustrates a hand held pulse oximeter in accordance with an embodiment;

FIG. 2A illustrates a pulse oximetry sensor in accordance with an embodiment;

FIG. 2B illustrates the pressure pad of the pulse oximeter sensor of FIG. 2A along the line 2b in accordance with an embodiment;

FIG. 2C illustrates the printed circuit board of the pulse oximeter sensor of

FIG. 2A along the line 2c in accordance with an embodiment;

FIG. 2D illustrates the application of pressure to the pulse oximetry sensor of FIG. 2 A in accordance with an embodiment;

FIG. 2E illustrates the application of excessive pressure to the pulse oximetry sensor of FIG. 2A in accordance with an embodiment;

FIG. 3A illustrates a pulse oximetry sensor in accordance with an embodiment; FIG. 3B illustrates the pressure pad of FIG, 3A in accordance with an embodiment;

FIG. 3C illustrates a cross-sectional view of the pressure pad along the lines 3c of FIG. 3B;

FIG. 3D schematically illustrates the printed circuit board of the sensor of FIG. 3 A in accordance with an embodiment;

FlG. 4A illustrates a pulse oximetry sensor in accordance with another embodiment;

FIG. 4B schematically illustrates the printed circuit board of a pulse oximetiy sensor in accordance with an embodiment;

FIG. 4C illustrates a pulse oximeter sensor in accordance with another embodiment;

FIG. 5A illustrates a pulse oximeter sensor configured to implement various different pressure sensing techniques in accordance with embodiments; FIG. 5B illustrates a push-button pressure sensor for the pulse oximeter sensor of FIG. 5 A, in accordance with an embodiment;

FIG. 5C illustrates a capacitive pressure sensor for the pulse oximeter sensor of FIG. 5A in accordance with an embodiment;

FIG. 5D illustrates a capacitive pressure sensor for the pulse oximeter sensor of FIG. 5 A in accordance with an embodiment;

FIG. 5E illustrates a piezoelectric type pressure sensor for the pulse oximeter sensor of FIG. 5 A in accordance with an embodiment;

FIG. 6A illustrates a pulse oximeter sensor in accordance with embodiments;

FIG. 6B schematically illustrates a capacitive film sensor for use in the sensor of FIG. 6A in accordance with an embodiment;

FIG. 6C schematically illustrates a resistive film sensor for use in the sensor of FIG, 6A in accordance with an embodiment.

FIG. 7 illustrates a pulse oximeter having a remote sensor in accordance with an embodiment; FIG. 8 illustrates a stationary pulse oximeter having an umbilical sensor in accordance with an embodiment;

FIG. 9 illustrates a hand-held pulse oximeter configured with the sensor and the display on opposite sides of the pulse oximeter in accordance with an embodiment;

FIG. 10 illustrates a puise oximeter having the keyboard and display oriented for use by a care giver in accordance with an embodiment;

FIG. 11 illustrates a block diagram of a pulse oximeter in accordance with an embodiment; and

FIG. 12 illustrates a flow chart depicting the operation of an embodiment.

DETAILED DESCRIPTION

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 developer's 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 to those of ordinary skill having the benefit of this disclosure.

In an embodiment, a pressure sensor may be used in conjunction with a pulse oximeter sensor and configured to provide feedback regarding the amount of pressure being applied to blood perfused tissue. The feedback may be capable of indicating an over-pressure condition, which may cause exsanguination to occur, as well as an under-pressure condition, where there may be inadequate coupling of light into the blood perfused tissue. The feedback may, therefore, help ensure that a proper pressure is applied and, consequently, that the pulse oximeter provides relatively more accurate readings.

Turning initially to FIG. 1, an exemplary hand-held pulse oximeter is illustrated in accordance with an embodiment and is generally designated by the reference numeral 10. The housing 12 of the pulse oximeter 10 may be designed to generally fit within the palm of a user's hand, making it easy to carry and convenient to use. Specifically, for example, the pulse oximeter 10 may be 1/2 in. x 1 in. x 2 in. and weigh approximately 0.1 lbs. As such, a user, such as a caregiver or a patient, may carry it around in a pocket or a small bag. Additionally, the pulse oximeter may be used outside of a hospital or health care environment. For example, the small size makes it convenient for use while hiking, flying, or exercising. The handheld pulse oximeter 10 may be a convenient and low-cost way for a user or a patient to self-test for oxygen saturation levels, pulse rate, and other physiological parameters.

In an embodiment, the housing 12 may have a sensor 14, a keypad 16, and a display 18 with the display 18 oriented relative to the sensor 14 to facilitate a user reading the display 18. The sensor 14 may be configured to allow the user to place a finger on the sensor pad or, alternatively, to place the sensor on a forehead. The keypad 16 may be capable of allowing a user to interface with the pulse oximeter 10, to the extent that user interface is desired. For example, the keypad may be configured to allow a user to select a particular mode of operation, In an embodiment (not shown) the keypad 16 may not be provided. The display 18 may be capable of allowing a user to read the various measured parameters of the pulse oximeter, such as oxygen saturation level and/or pulse rate, and it may also be configured to indicate that too much or too little pressure is being applied when applying to the sensor to a particular area.

FIGS. 2A-2E illustrate an embodiment of the sensor of FIG. 1 and is generally designated by the reference numeral 14a. In an embodiment, the pressure sensor 14a may include a pressure pad 20. The pressure pad 20 may have an upper surface capable of contacting a user's finger or forehead, and a lower surface capable of making contact with a printed circuit board 22. The pressure pad 20 may have opaque regions 30a capable of reducing reflections of electromagnetic radiation, and further capable of generally reducing the detection by the detector 26 of electromagnetic radiation originating from sources other than the emitter 24, Additionally, the pressure pad 20 may have transparent areas 30b to allow the emitter 24 and detector 26 to optically couple with a user's skin. In an embodiment, the transparent areas 30b may be positioned directly over where the emitter 24 and detector 26 are mounted on the printed circuit board 22,

The pressure pad 20 may be made of a flexible material, such as silicon rubber, for example, that is capable of deforming by application of pressure, but returns to its original shape once the pressure is removed. It is the durometer and the geometiy of the pressure pad that determines the pressure sensitivity of the pressure pad 20. The durometer and the geometry of the pressure pad 20 may be manipulated during the manufacturing process to create a pressure pad 20 that properly indicates the amount of pressure being applied, as will be described in detail below.

In an embodiment, the pressure pad 20 may have two side tabs 28a-b and a center tab 28c. In an embodiment, the center tab 28c may protrude further outward from the pressure pad 20 than the side tabs 28a-b so that it may make first contact with the printed circuit board 22 when pressure is applied. The two side tabs 28a-b may be configured to make contact with the printed circuit board 22 only when excessive force or pressure is applied to the pressure pad 20. In an embodiment, the pressure pad 20 may be supported on the printed circuit board by tabs 28d-e. Tabs 28d-e may extend generally toward the printed circuit board so that, when no pressure is applied to the pressure pad, the center tab 28c and the side tabs 28a-b do not rest on and/or contact the printed circuit board 22.

In the embodiment shown in FIGS. 2B and 2C, the tabs 28a-c have respective conductive surfaces 3 la-c aligned with pairs of conductive traces 32a-c on the printed circuit board 22. The conductive traces 32a-c on the printed circuit board 22 may be made of, or plated with, electrically conductive material such as gold, for example. The conductive surfaces 3 la-c of the tabs 28a-c may be made of an electrically conductive elastomer or by adding conductive material to the tabs 28a-c. Specifically, the conductive surfaces 3 la-c may be created by impregnating the tabs

28a-c with carbon, silver, or other conductive material. Additionally, the conductive surfaces 3 la-c may be created by painting, gluing or bonding a conductive material or a conductive film on the surface. Furthermore, the conductive surfaces 3 la-c may be created by insert molding of a conductive material to the tabs 28a-c. Accordingly, conductive material may be painted, glued or bonded to the tabs 28a-c, or impregnated into the tabs 28a-c, for example, among other things, to create the conductive surfaces 3 la-c.

When pressure is applied, the pairs of conductive traces 32a-c and the conductive surfaces 3 la-c may make contact. Contact between the surface 31c of the pressure pad 20 and the traces 32c of the printed circuit board 22, as shown in FIG. 2D, may be an indication that sufficient pressure is being applied to the pressure pad 20 by the user. Specifically, that a minimum threshold of pressure has been applied so that the emitter 24 and detector 26 to optically couple with a user's tissue. In accordance with an embodiment, such contact may place the pulse oximeter in an "on" state, whereas otherwise, the oximeter is in an "off state.

If more pressure is applied and the pressure pad 20 is further depressed, the side tabs 28a-b and their conductive surfaces 3 la-b may make contact with the conductive traces 32a-b of the printed circuit board 22, as shown in FIG. 2E. When more than one tab is making contact with the conductive traces 32a-c of the printed circuit board 22, this may be an indication that excessive pressure is being applied or that a maximum pressure threshold has been exceeded. When it is determined that excessive pressure is applied, this may indicate exsanguination is occurring, which may cause errors in the measurements. Consequently, the pulse oximeter may notify the user through the display 18 that too much pressure is being applied or request that the user apply less pressure. In the case that all three conductive surfaces 3 la-c of the pressure pad 20 are in contact with the conductive traces 32a-c of the printed circuit board 22, the pulse oximeter may indicate to the user that too much pressure is being applied or that the user should apply less pressure. Additionally, the sensor may stop taking measurements so that the battery is not consumed while accurate measurements cannot be obtained. Accordingly, the oximeter 10 may be configured to take measurements only when a proper pressure is applied.

In addition to turning on, taking measurements, and turning off in response to pressure being applied to the pressure pad 20, the pulse oximeter 10 may be configured to indicate whether a user should apply more or less pressure. This may be done by displaying a message or an icon indicating that more or less pressure should be applied. Alternatively, a bank of LEDs 19 may be provided that light up according to the amount of pressure that is being applied. In an embodiment, a single LED, such as LED 19a, for example, may be activated to indicate too little pressure is being applied. In an embodiment, three LEDs 19 may be activated to indicate too much pressure is being applied, and two LEDs 19a-b may be activated to indicate an appropriate amount of pressure is being applied. Alternatively, the first LED 19a may indicate too little pressure, the second LED 19b may indicate an appropriate amount of pressure, and the third LED 19c may indicate too much pressure. Thus, a user or caregiver may easily recognize that more or less pressure should be applied,

As illustrated and discussed above in an embodiment, the center tab 28c of the pressure pad 20 may be most proximate the printed circuit board 22, while the side tabs 28a- b may be at a greater distance from the printed circuit board 22. In other embodiments, however, the center tab 28c may be further away from the printed circuit board 22 and the side tabs 28a-b closer to the printed circuit board 22, such that contact of the side tabs 28a-b may indicate that minimum threshold pressure has been achieved, while contact by the center tab 28c with the printed circuit board 22 is indicative of the user applying too much pressure to the pad 20.

The relative height of the tabs 28a-c may affect the differences in pressure required to cause the tabs 28a-c to make contact with the traces 32a-c. Additionally, the material used for the pressure pad 20 and/or the tabs 28a-c may be selected to provide the desired pressure response. Specifically, the durometer of the pressure pad 20 can be selected to provide the desired flexibility to turn "on" the oximeter and to control the amount of pressure indicative of an over-pressure occurrence.

As the conductive surfaces 31a-c on the pressure pad 20 make contact with the conductive traces 32a-c, electrical current is able to flow between pair of traces 32a- c. As discussed above, if none of the conductive traces 32a-c are able to cany a current, this may be indicative of insufficient pressure being applied and the oximeter may prompt a user via the display 18 to apply more pressure. In another embodiment, the pulse oximeter may be off when no pressure is applied.

When only one pair of conductive traces 32a-c, such as the pair of conductive traces 32c, is capable of carrying a current, it may be indicative of a proper pressure is being applied to the pulse oximeter and the oximeter may acquire accurate measurements. Additionally, in an exemplary embodiment, the sensing of pressure may cause the oximeter to turn on.

If, however, more than one pair of conductive traces 32a-c can carry a current, it may be indicative of an over-pressure occurrence and the possibility of inaccurate readings due to exsanguination. Accordingly, the oximeter indicates the over- pressure occurrence to the user.

An embodiment of the pressure sensor 14 is illustrated in FIG. 3A and generally designated by the reference numeral 14b. In an embodiment, the pressure sensor 14b may include a pressure pad 40, which may be made of flexible material, such as silicon rubber, for example, with generally transparent regions 30b over the emitter 24 and detector 26, as described above, to allow for the emitter 26 and detector 26 to optically couple with a user's skin. Additionally, the pressure pad 40 may be supported on the printed circuit board 22 by tabs 28d-e. In an embodiment, the pressure pad 40 has a single protruding tab 42 configured to make contact with the printed circuit board 22 when pressure is applied. The tab 42 may have a convex conductive surface 43, as shown in FIG. 3C. In another embodiment the conductive surface 43 may have a concave curvature (not shown).

In an embodiment, the printed circuit board 22 has pairs of conductive traces 44a-f, as shown in FIG. 3D. The conductive traces 44a-f may be configured so that as the conductive surface 43 of the tab 42 makes contact, an electrical current can be carried between pairs of traces. Depending on how much pressure is applied to the pressure pad 40, the conductive surface 43 may make contact with only a few pairs of conductive traces 44a-c. For example, the conductive surface 43 may contact traces 44a-b. The coupling of only one or a couple of trace pairs may indicate that sufficient pressure is being applied and the oximeter 10 may begin taking more reliable measurements.

As more pressure is applied, however, the conductive surface 43 of the tab 42 may deform and make contact with more pairs of conductive traces 44a-f. The number of conductive traces 44a-f in contact with the conductive surface 43 may be determined and correlated to the amount of pressure being applied. If too many of the pairs of traces 44a-f are electrically coupled together through contact with the conductive surface 43, it may indicate an over-pressure situation. Tn the event that an ovei'-pi'essure situation occurs, the oximeter may indicate to the user to apply less pressure. Additionally, the oximeter may stop taking readings. For example, if the conductive surface 43 comes into contact with all of the traces 44a-f, the oximeter may stop taking measurements.

As mentioned above, the durometer of the pressure pad 40 can be adjusted during manufacture to provide the proper level of rigidity and flexibility. A proper level of rigidity and flexibility is achieved when the amount of pressure it takes for the emitter 24 and detector 26 to optically couple with the user's skin is generally the same pressure required to push the conductive surface 43 into contact with a single pair of traces on the printed circuit board 22 in order to indicate that a minimum pressure threshold has been achieved. Additionally, the proper level of rigidity and flexibility will allow the conductive surface 43 to contact sufficient pairs of traces

44a-f to indicate an over-pressure situation, or that a maximum pressure threshold has been surpassed, when exsangui nation may be occurring.

In another embodiment, multiple tabs of a pressure sensor 14c may be arranged in a concentric fashion between the emitter 24 and detector 26 of the pulse oximeter 10, as illustrated in FIG, 4A, The concentrically arranged tabs 46a-c of the pressure sensor 14c may be at varying heights above the printed circuit board 22. Each of the tabs 46a-c has a conductive surface (not shown) oriented toward the printed circuit board 22. When one of the tabs 46a-c makes contact with the printed circuit board the oximeter may indicate that a proper pressure is being applied. Subsequent contact made by other tabs 46a-c may be indicative of an over-pressure situation causing the oximeter to indicate that too much pressure is being applied. As mentioned earlier, an over-pressure occurrence may cause the oximeter to stop taking measurements, as the measurements may not be accurate.

In an embodiment, the amount of pressure required to cause the oximeter to determine that a proper pressure is being applied, or, alternatively, to indicate to a user to apply more or less pressure, may be dictated by the generally annular geometry and the durometer of the material. Each generally annular tab 46a-c may be coupled to a rolling diaphragm or snap buckling dome 47, which may be capable of allowing differential travel of the tabs 46a-c. As pressure is applied to the snap buckling dome 47, each tab 46a-c may make contact with a pair of conductive traces 48a-c on the printed circuit board aligned with the tabs 46a-c, as illustrated in FIG. 4B.

Similar to the previously discussed embodiments, the tabs 46a-c will make contact with the printed circuit board 22 in order of their presentation. When pressure is applied, for example, tab 46a may be the first tab to make contact with the printed circuit board 22. As the tab 46a makes contact with the printed circuit board

22, the conductive surface (not shown) of the tab 46a electrically couples a pair of conductive traces on the printed circuit board, such as traces 48c, for example. In an embodiment, the coupling of the traces 48c may indicate that sufficient pressure is being applied to the sensor such that the emitter 24 and the detector 26 may be generally optimally optically coupled with the user's tissue. If however, more than a single pair of traces 48a-c are coupled together and capable of carrying an electrically current, it may indicate too much pressure is being applied. When too much pressure is indicated, the oximeter may indicate to the user that less pressure should be applied. The oximeter may also be configured to stop taking measurements due to the possibility of exsanguination causing inaccurate measurements.

A pressure sensor 14d is illustrated in FIG. 4C as an embodiment wherein the centermost tab 49a of annular tabs 49a-c is located most proximate to the printed circuit board 22. In an embodiment, the pressure sensor 14d is configured to be used in conjunction with the traces 48a-c illustrated in FIG, 4B. Each annular tab 49a-c may have a conductive surface (not shown) configured to make contact with the conductive traces 48a-c on the printed circuit board 22. The operation of the pressure sensors 14c and 14d is similar to the aforementioned embodiments. Specifically, when a pair of traces 48a-c are able to conduct electrical current, due to the coupling of the conductive surfaces 49a-c of the annular tabs 49a-c with the traces 48a-c, the oximeter may indicate that a proper pressure is being applied. If, however, several pairs of traces are conducting electrical current, an over-pressure condition may be indicated. Additionally, if none of the traces 48a-c are capable of conducting electrically current, then the oximeter may indicate that more pressure should be applied.

The above described embodiments disclosed the coupling of tabs on the pressure pads with traces on the printed circuit board to indicate the amount of pressure applied. Alternative embodiments of the pulse oximeter sensor may be designed and manufactured to allow the use of a variety of other techniques to measure pressure. FIG. 5A, for example, illustrates a pressure sensor 14e, indicated by a block 50, positioned underneath a pressure pad 52. The pressure sensor 14e may take various forms implementing push-button techniques, capacitive techniques or piezoelectric techniques to measure pressure, as will be discussed in greater detail below.

Tn an embodiment, the pressure sensor 14e includes a push-button switch 50, for example, as illustrated in FIG. 5B. The push-button switch 50a may be mounted on the printed circuit board 22 between the emitter 24 and detector 26. The switch 50a is activated when the pressure pad 52 is compressed enough that the switch 50a is pressed. The pressure sensitivity of the pressure sensor 14e may be set by controlling the durometer of the pressure pad 52 and by adjusting the physical shape of the pad, as discussed above. In this embodiment, only the occurrence of a threshold pressure may be determined. The threshold, for example, may be the minimum pressure threshold necessary for the emitter 24 and detector 26 to optically couple with a user's tissue. The actuation of the switch operating according to this threshold may also be used to turn the oximeter on. Alternatively, the maximum pressure threshold above which exsanguination may occur may be used to indicate that too much pressure is being applied. The actuation of the switch operating according to this threshold may cause the oximeter to request that the used apply less pressure. In another embodiment, multiple switches (not shown) may be implemented to indicate different levels of pressure applied to the pressure pad 52. For example, the pressure sensor may indicate at least three states: insufficient pressure, proper pressure, and/or over-pressure. This embodiment may allow for the oximeter to provide feedback to a user to indicate one of the above mentioned states. Thus, a user would be able to adjust the amount of pressure applied in order to achieve a proper pressure and a proper measurement.

In yet another embodiment, the pressure sensor 14e may be an analog sensor configured to measure a range of pressure. An analog capacitive pressure sensor 50b is illustrated in FIG. 5 C. The capacitive pressure sensor 50b has capacitive plates 54a-b and a diaphragm 56. As those skilled in the art may recognize, the deflection of the diaphragm 56 relative to the capacitive plates 54a-b changes the capacitive coupling of the diaphragm 56 and the plates 54a-b. The changes in capacitive coupling may be measured and correlated to the pressure being applied to the pressure pad 52. The oximeter may be configured to indicate to a user that more pressure is required until a minimum pressure threshold has been reached and that less pressure should be applied when a maximum pressure threshold is exceeded. Additionally, the oximeter may be configured to turn on when a proper pressure is applied and turn off in an under-pressure or over-pressure situation.

In FlG. 5D, an embodiment of a capacitive pressure sensor 50c is illustrated. The capacitive pressure sensor 50c may include a compressible member 60, such as a compressible foam or an elastomeric material, for example, having elastic properties. The compressible member may be located between the pressure pad 52 and the printed circuit board 22. A first conductive layer 62a may be located on a top surface of the compressible member 60 and a second conductive layer may be located on a bottom surface of the compressible member 60. The conductive layers 62a-b may be directly applied to the compressible member 60 or, alternatively, may be foil layers.

The compressible member 60 may have a dielectric constant such that when at rest, there is little or no capacitive interaction between the layers 62a-b. As pressure is applied to the pressure pad 52, the compressible member 60 is compressed and the capacitive interaction between the conductive layer 62a-b increases. This capacitive interaction may be detected and may be correlated to the amount of pressure being applied to the pressure pad 52. As more pressure is applied, the capacitance between the conductive layers 62a-b increases. Alternatively, as pressure is removed from the pressure pad 52, the compressible member 60 may return to a resting state and the capacitive layer 62a-b may again experience little or not capacitive interaction.

Another embodiment may use a piezoelectric pressure sensor 5Od as illustrated in FIG. 5E. The piezoelectric pressure sensor 5Od may include a diaphragm 66 configured to be deflected as pressure is applied to the pressure pad 52. The diaphragm 66 is coupled to a crystal element 68 which generates an electrical signal proportionate to the pressure being applied to the diaphragm 66. The electrical signal produced by the crystal element 68 may be measured and correlated to the amount of pressure being applied. The oximeter may be configured to provide feedback corresponding to exceeding threshold pressure levels, as discussed above. In addition to having a pressure transducer under the pressure pad 52, as was described above with reference to FIGS. 5A-E, the pressure transducer may be located on or inside the pressure pad 70. FIG. 6A illustrates an embodiment of a strain gage pressure sensor 14f for the pulse oximeter 10. The strain gage pressure sensor 14f may be a capacitive strain gage 74 as illustrated in FIG. 6B, The capacitive strain gage 74 may be located inside the pressure pad 70, as shown in FIG. 6A, or, in an alternative embodiment, may be coupled to the surface of the pressure pad 70 using an adhesive. The capacitive strain gage 74 may have traces 76a-b which are interleaved to maximize the capacitive coupling between the traces 76a-b.

The capacitive strain gage 74 may be used to determine the amount of pressure being applied to the pressure pad 50. When pressure is applied to the pressure pad 70 the capacitive characteristics of the conductive traces 76a-b change. The changes can be measured and correlated to the amount of pressure being applied to the pressure pad 50. Until minimum threshold pressure level has been achieved, the oximeter may indicate to the user to apply more pressure. Alternatively, an initial threshold amount of pressure may be necessaiy to place the sensor in an "on" state. The application of excessive pressure, as determined by measuring a pressure level exceeding a maximum pressure threshold, may cause the oximeter to indicate an over-pressure occurrence to the user. When an over-pressure occurs, the oximeter may cease taking measurements, as the measurements may be inaccurate due to exsanguination. In another embodiment, the pressure sensor 14f may be a resistive strain gage 78 as illustrated in FIG. 6C. In this embodiment, the resistive strain gage 78 has a conductive trace 80 configured in a serpentine manner such that when pressure is applied, the resistive characteristics of the trace 80 change. The amount of pressure applied may be determined by correlating the amount of strain detected by the resistive strain gage 78 to the amount of pressure applied to the pressure pad 70, Again, thresholds may be used to allow the oximeter to provide feedback to the user to indicate the amount of pressure being applied,

In yet another embodiment, electrically conductive elastomers (not shown) that change impedance when deformed may be used. Implementation of the electrically conductive elastomers would be similar to the use of strain gages, in that the elastomers may be coupled to the pressure pad 70 or may be inside the pressure pad 70. The changes in the impedance or conductivity of the elastomer due to deformation of the pressure pad 70 may be correlated to the pressure being applied and the oximeter can operate to indicate under-pressure and over-pressure conditions, which may be used to request a user apply more or less pressure, respectively.

As explained above, the use of the pressure sensor in conjunction with the oximeter allows for a user to receive feedback regarding how much pressure is, and should be, applied. Additionally, the pressure sensor can be used to indicate when the oximeter should be turned on and/or off, to extend battery life. Furthermore, the pressure measurements can be used to determine when artifacts in the measurements may be due to movement, as the pressure sensor could determine movement according to changes in pressure.

In order to more effectively provide feedback to a user or clinician, the oximeter may be configured for particular uses. As will be discussed in detail below, depending on the primary use of the pulse oximeter, various configurations other than the handheld version illustrated in FIG. 1 may be desired. For example, FIG. 7 illustrates an exemplary pulse oximeter system with an umbilical sensor 14 which is generally designated by the reference numeral 90. The umbilical sensor 14 may be configured as any one of the previously described pressure sensor embodiments

14a-f. The pulse oximeter 90 has a main unit 92 which may be portable in size. The main unit 92 may house a display 18 and a keypad 16, while the sensor 14g is connected umbilically to the main unit 92 via a cable 94. Alternatively, the sensor 14 may simply transmit information wirelessly to the main unit 92 obviating the need for the cable 94.

In yet another embodiment of an umbilical sensor configuration, the sensor 14h may be connected to a stationaiy pulse oximeter 106, as illustrated in FIG. 8. As above, the sensor 14 is connected umbilically via cables 94 to a pulse oximeter 108 and may be configured as any one of the above described pressure sensor embodiment 14a-f, The pulse oximeter 108, however, may be larger than those previously described and is intended to be placed on a cart or desk, or mounted on a rack. The pulse oximeter 108 is connected to a display 110 that may also be placed on a cart, desk, or a rack. Such a system may provide for additional functionality as more memory and other components may be added without the inherent size and space limitations of the smaller handheld units. For example, the oximeter 106 may be configured to provide more detailed information regarding the amount of pressure being applied to the sensor 14. In one embodiment, the oximeter 106 may be configured to display text 110 indicating to a user to apply more or less pressure to the sensor 14, for example.

If a particular pulse oximeter will be used primarily for spot checks on patients, the system 120 illustrated in FIG. 9 may be desirable. The pulse oximeter system 120 has the display 18 and the keypad 16 on a single face of the pulse oximeter system 120. The sensor 18, however, may be located on an opposite surface of the pulse oximeter system 120 from the display 18 and the keypad 16. Such configuration allows for the pulse oximeter system 120 to be used on the forehead of the patient while the display 18 may be easily readable by a caregiver. The sensor 14 may be configured as any one of the previously described pressure sensor embodiments 14a-f and may indicate whether a proper amount of pressure is applied.

Alternatively, the handheld pulse oximeter may be designed for individual use. As such, it may be useful to provide a pulse oximeter configured to be used in conjunction with a mirror, such as the handheld pulse oximeter 122 illustrated in

FIG. 10. The oximeter 122 has a similar configuration as the oximeter 120, however, it is configured to display a mirror image readout such that a user may view the readout by looking at the display 18 through a mirror. In yet another embodiment, a handheld pulse oximeter 124 with an inverted display and keyboard is illustrated in FIG. 11. The handheld pulse oximeter 124 is configured so that a caregiver may easily read the display 18 and use the keypad 16 while a patient applies pressure with a finger to the sensor 14. In particular, the display 18 is located between the sensor 14 and the keypad 16 and is oriented to facilitate the reading of the display 18 by a caregiver.

Several specific examples have been given above illustrating various embodiments. The main components of the various embodiments may be similar. FIG. 12 illustrates a block diagram of the pulse oximeter in accordance with embodiments and is generally designated by the reference numeral 130. The block diagram 130 is exemplary and an actual implementation may include more or fewer components as needed for a specific application for which the pulse oximeter is to be used. For example, alternative techniques for driving LEDs in an oximeter may be implemented, such as those disclosed in the U.S. Provisional No. 61/009,076, LED

Drive Circuit and Method for Same, by Ethan Petersen, which is incorporated herein by reference.

Radiation detected by the detector 26 is passed from the detector 26 to a pulse oximeter unit 132, The pulse oximeter unit 132 has a microprocessor 134 for calculating various physiological parameters. The microprocessor 134 is coupled to other components such as a RAM 138 and a display 140. A light drive unit 142 controls the emitters 24 and the detected signals are passed through an amplifier 144, a filter 146, and an analog to digital converter 148 before being stored in the RAM 138. The microprocessor uses the digital data stored in the RAM 138 and algorithms stored in the ROM 150 to calculate various parameters. The control inputs 152 allow the user to interface with the pulse oximeter 132.

The techniques implementing each of the embodiments discussed above will be similar. FIG. 13 illustrates an exemplary embodiment and is generally designated by the reference numeral 170. Initially, because no pressure is being applied, the pulse oximeter 10 is off (Block 172). If sufficient pressure is indicated, the pulse oximeter 10 turns on (Blocks 174 and 176). Once the pulse oximeter 10 is on, it continuously monitors the amount of pressure applied to determine whether too much pressure is being applied (Block 178). If too much pressure is being applied, the pulse oximeter 10 indicates that too much pressure is being applied (Block 180). Concurrent with the determination that too much pressure is being applied, the pulse oximeter 10 may stop taking measurements. The system again checks whether sufficient pressure is being applied (Block 174). If it is determined that insufficient pressure is being applied, the system turns off (Block 172). If it is determined that there is sufficient pressure, the pulse oximeter 10 will calculate and display a SpO₂ measurement (Block 182). The technique ends once the user removes pressure from the pressure sensor (Block 184).

While this disclosure may be susceptible to various modifications and alternative forms, 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 invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.

Claims

1. A pressure sensor comprising: a printed circuit board (PCB) comprising a plurality of pairs of conductive traces; and a generally flexible pressure pad, the pressure pad comprising a plurality of tabs capable of moving into closer proximity with the PCB as pressure is applied to the pressure pad, the plurality of tabs comprising conductive surfaces oriented towards the PCB and located at different distances from the PCB, the conductive surfaces generally aligning with the pairs of conductive traces and being capable of electrically coupling the pairs of conductive traces together, wherein the amount of pressure applied to the pressure pad can be indicated by the number of conductive trace pairs capable of conducting an electrical current.

2. The pressure sensor of claim 1, wherein the conductive surfaces comprise conductive material impregnated into the tabs of the pressure pad.

3. The pressure sensor of claim 1 , wherein the conductive surfaces comprise a conductive elastomer,

4. The pressure sensor of claim 1, wherein the conductive surfaces comprise a conductive material painted on to the tabs of the pressure pad.

5. The pressure sensor of claim 1, wherein the conductive surfaces comprise a conductive film bonded to the tabs of the pressure pad.

6. The pressure sensor of claim 1 , wherein the amount of pressure required to cause the conductive surfaces to electrically couple to the pairs of traces is determined at least in part by a durometer and the geometry of the pressure pad.

7. A pressure sensor comprising: a printed circuit board (PCB) comprising a plurality of pairs of conductive traces; and a generally flexible pressure pad capable of moving into closer proximity with the PCB as pressure is applied to the pressure pad, the pressure pad comprising a tab comprising a generally curved conductive surface, the curved conductive surface being generally aligned with the pairs of conductive traces and capable of deforming to electrically couple the pairs of conductive traces as pressure is applied to the pressure pad, and wherein an amount of pressure applied may be indicated by how many conductive trace pairs are conducting an electrical current.

8. The pressure sensor of claim 7, wherein the generally curved conductive surface is generally convex.

9. The pressure sensor of claim 7, wherein the generally curved conduction surface is generally concave.

10. The pressure sensor of claim 7, wherein the generally curved conductive surface comprises a conductive material impregnated into the tab of the pressure pad.

11. The pressure sensor of claim 7, wherein the curved conductive surface comprises a conductive material painted on to the tab of the pressure pad.

12. The pressure sensor of claim 7, wherein the generally curved conductive surface comprises a conductive film bonded to the tab of the pressure pad.

13. The pressure sensor of claim 7, wherein an amount of pressure required to cause the conductive surface to couple conductive trace pairs together is determined at least in part by the durometer and geometry of the pressure pad.

14. A pressure sensor comprising: a printed circuit board (PCB) comprising a plurality of pairs of conductive traces; and a flexible pressure pad capable of moving into closer proximity with the PCB as pressure is applied to the pressure pad, the pressure pad comprising generally annular tabs located at different heights above the PCB, the generally annular tabs comprising conductive surfaces generally aligned with the pairs of conductive traces to electrically couple the pairs of conductive traces as pressure is applied to the pressure pad, an amount of pressure being applied may be indicated by how many conductive trace pairs are capable of conducting electrical current.

15. The pressure sensor of claim 14, wherein the generally annular tabs are coupled to a snap buckling dome.

16. The pressure sensor of claim 14, wherein an amount of pressure required for the conductive surfaces to make contact with the pairs of conductive traces is based at least in part by a durometer of the pressure pad and the generally annular geometry of the snap buckling dome.

17. The pressure sensor of claim 14, wherein the conductive surfaces comprise a conductive material impregnated in the generally annular tabs.

18. The pressure sensor of claim 14, wherein the conductive surfaces comprise a conductive material painted on the generally annular tabs.

19. The pressure sensor of claim 14, wherein the conductive surfaces comprise a conductive material bonded to the generally annular tabs.

20, The pressure sensor of claim 14, wherein the conductive surfaces comprise a conductive elastomeric material.