System and method of determining light source aging

A physiological sensor includes a light source and an age detector circuit in communication with the light source. The age detector circuit is configured to determine an age of the light source based on current-voltage characteristics of said light source. In addition, a method includes measuring an initial I-V characteristic and an actual I-V characteristic of the light source, and comparing the initial I-V characteristic to the actual I-V characteristic since changes in the I-V characteristics indicate aging. Actual I-V characteristics can be compared between light sources when they age at different rates to determine light source aging. Moreover, the method may include updating the memory device with the actual I-V characteristic at predetermined times.

BACKGROUND

Physiological sensors such as pulse and cerebral oximeters use light to measure a variety of physiological characteristics in body tissue. The physiological sensor generally includes a sensor pad assembly with a plurality of light sources in optical communication with at least one light detector. When activated, the light sources transmit light at specific wavelengths through the body tissues to the light detector. The amount of light received by the light detector after attenuation by the body tissue is indicative of the physiological characteristic being tested.

As the light source ages, the output of light by the light source decreases. For example, the output by the light source may decrease by 15% after only 100 hours of operation. In some cases, the decrease in output is not significant. For example, pulse oximeters generally do not depend on light source intensity so long as the intensity is high enough to maintain high signal to noise ratios. However, cerebral oximeters are often affected by light source aging because they employ multiple light sources and at least two light detectors to generate an absorption profile of the brain, which is at least in part based upon multiple weighted differences of the optical densities at different wavelengths from different detector locations.

Until now, light source aging was not a problem for cerebral oximetry because cerebral oximeters were only used in operating rooms for approximately 4-8 hours before being discarded or replaced. Light source aging for such a short period of operation is negligible. Today, however, cerebral oximeters may operate for several days in, for instance, an intensive care unit (ICU). Because individual light sources on the sensor have different aging characteristics, light source aging during long-term monitoring can cause erroneous readings of the cerebral saturation if appropriate corrective measures are not taken. Therefore, it is useful to know the age of the light source to determine if the readings are accurate. Accordingly, a monitoring system employing a method of determining light source aging is needed.

DETAILED DESCRIPTION

A monitoring system executing a method of determining light source aging includes a light source in communication with an age detector circuit that is configured to detect current-voltage (I-V) characteristics of the light source to indicate aging. In one approach, the age detection circuit uploads the I-V characteristics to a memory device, and the monitoring system may also include an interference detection circuit that measures interference caused by external electromagnetic energy sources and only allows the memory device to be updated when substantially no interference is present. Accordingly, the method includes measuring an initial I-V characteristic and an actual I-V characteristic of the light source, and comparing the initial I-V characteristic to the actual I-V characteristic since changes in the I-V characteristics indicate aging. Moreover, the method may include updating the memory device with the actual I-V characteristic at predetermined times to reduce interference.

Referring toFIG. 1, an exemplary monitoring system includes a physiological sensor10that has at least one sensor pad12having one or more light sources14in optical communication with one or more light detectors16. The exemplary physiological sensor10ofFIG. 1illustrates two light sources14and two light detectors16disposed on a single sensor pad12, although the light sources14and light detectors16may be disposed on multiple or different sensor pads12. The physiological sensor10further includes an age detector circuit18configured to determine an age of the light sources14based on I-V characteristics of the light sources14. As discussed in greater detail below, one way to determine the I-V characteristics is to measure a forward voltage of the light sources14with a voltage detector circuit20. The age detector circuit18may use the I-V characteristics to determine or track the total amount of time any of the light sources14has been illuminated. In particular, the age detector circuit18may compare an initial I-V characteristic of each of the light sources14to an actual I-V characteristic of the light sources14measured, the difference of which indicates the age of the light sources14.

The physiological sensor10further includes a memory device22capable of storing information in a tangible computer readable medium, and specifically, the initial and actual I-V characteristics of the light sources14. The memory device22may be any type of non-volatile computer memory, such as electrically erasable programmable read only memory (EEPROM). The memory device22may be part of the age detector circuit18, or as illustrated, merely in communication with the age detector circuit18. In either configuration, the age detector circuit18updates the memory device22with the actual I-V characteristics, and accesses the memory device22to determine the initial I-V characteristic. However, the age detector circuit18may only update the memory device22at predetermined times to reduce corruption by the light sources14or external sources, including RF sources. Specifically, the memory device22is susceptible to corruption by external RF sources, such as electro-surgery equipment, especially during a write cycle. Moreover, writing information to the memory device22interferes with the ability of the physiological sensor10to measure light. To prevent such interference, the physiological sensor10may include an interference detection circuit24in communication with the memory device22and the age detector circuit18. The interference detection circuit24measures interference caused by external RF sources, and allows the age detector circuit18to communicate with the memory device22based on the interference measured, such as, for example, when substantially no RF interference is measured. Furthermore, communication with the memory device22may generate interference with the measurement of light so communication may be limited to times when substantially no light measurements are taking place.

To get the initial I-V characteristic, the age detector circuit18must be calibrated. During calibration, light outputs from light sources14, along with the forward voltage of the light sources14, are stored in and may be retrieved from the memory device22. In order to meet an accuracy goal, the relative change of the light output of the light sources14during operation should be very low—for example, less than 1.0%, and preferably, less than 0.5%. At the same time, light output from each individual light source14should be maintained within 5%.

It will now be explained how forward voltage may be used to determine the age of the light sources14. The forward voltage (Vf) of the light sources14can be described using the following equation:
Vf=(nkT/q)Ln(1+If/I0).  (1)
In Equation (1), Ifis the current, I0is the diode saturation current, ‘q’ is the electron charge, Vfis the forward voltage, T is the junction temperature, ‘k’ is Boltzmann's constant, and ‘n’ is the radiative constant that is related to the recombination rate of each light source14. The radiative constant ‘n’ is equal to 1 for normal radiative recombination and equal to 2 for non-radiative recombination. While ‘n’ varies among light sources14, it increases as the light source14ages.

During long term monitoring, the temperature T and radiative constant ‘n’ are both changing. The radiative constant ‘n’ changes due to aging, and the temperature may change because the physiological sensor10is applied to a patient's body, which may be cooled during surgery. However, in Equation (1), T and ‘n’ are multiplied together, so changes in the forward voltage due to aging and temperature become indistinguishable. Therefore, the following equation may be used to separate the effects that temperature has on aging:
T−T0≈Ta−Ta0.  (2)
In Equation (2), T0and Ta0are the junction temperature and the ambient temperature during calibration, respectively, and Tais the current ambient temperature. Equation (2) states that changes in the temperatures of the light sources14are proportional, if not equal, to changes in the ambient temperature.

According to Equations (1) and (2), the initial forward voltage stored in the memory device22may be adjusted to compensate for the current ambient temperature. This way, the only remaining variable concerning aging of the light sources14is the radiative constant ‘n.’ With temperature no longer a factor, the difference between the initial/adjusted forward voltage and the measured forward voltage is due to aging. It is to be appreciated that changes in the ambient temperature can be measured using the light detector16located on the sensor pad12, and specifically, by monitoring the forward voltage of the light detector16.

To determine aging of multiple light sources14, one of the light sources14may serve as a reference for one or more other light sources14. In particular, the rate of change of the radiative constant ‘n’ depends on the wavelength of each of the light sources14. For instance, the radiative constant for a first light source14A having a wavelength of 810 nm is less than for a second light source14B having a wavelength of 730 nm. However, the first light source14A may be used as a reference to estimate changes in the forward voltage of the second light source14B caused by changes in the ambient temperature, in which case, the difference between the estimated initial/adjusted forward voltage and the measured forward voltage of the second light source14B indicates aging.

The relative change of the forward voltages for the first and second light sources14can be written as:
δVf(810)/Vf0(810)=δn810/n0(810)+δT810/T0(810)(3)
δVf(730)/Vf0(730)=δn730/n0(730)+δT730/T0(730).  (4)
In Equations (3) and (4), Vf0(810), Vf0(730), n0(810), n0(730), T0(810), and T0(730)are the values of the forward voltages, radiative constants, and junction temperatures for the first and second light sources14during calibration. The first and second light sources14may be mounted on the same frame, and changes in the junction temperatures δT810and δT730due to changes in the ambient temperature Ta are approximately the same (i.e., δT810≈δT730). Thus, ignoring δn810, which is approximately equal to 0 since the first light source14A is used as a reference, Equation (5) can be derived from Equations (3) and (4):
δn730/n0(730)=δVf(730)/Vf0(730)−δVf(810)/Vf0(810).  (5)
Using Equation 5, the age of the light source14can be estimated without the effect of the ambient temperature by estimating the changes of the relative difference between the forward voltages of the first and second light sources14.

Equation (5) merely removes the effect of the ambient and/or junction temperature; it does not account for the temperature dependence of the diode saturation current. To do so, the forward voltages of the first and second light sources14must be measured using different currents. Using two different currents results in the following equation that is similar to Equation (1):
ΔVf=(nkT/q)Ln(I1/I2).  (6)
In Equation (6), ΔVfis equal to the difference between the forward voltages of the first and second light sources14. Accordingly, Equation (5) can be rewritten as Equation (7), below:
δn730/n0(730)=δΔVf(730)/ΔVf0(730)−δΔVf(810)/ΔVf0(810).  (7)
Equation (7) provides an accurate prediction of the aging of the light sources14, but also indicates that the forward voltages and excitation currents of the first and second light sources14must be continuously monitored and then stored in the memory device22.

Equations (6) and (7) assume that the currents I1and I2are small, for example, less than 100 uA, and the voltage across each light source14drops due to resistance in the first and second light sources14and/or wiring in the physiological sensor10. Typically, this voltage drop is much less than the forward voltages of the first and second light sources14. For example, the voltage drop is typically much less than 1.5 to 2 volts. However, if this is not the case, the following equation may be used to compensate for voltage drops caused by resistance in the light sources14or from the wiring of the physiological sensor10:
Vf=(nkT/q)Ln(1+I/I0)+(RF*I).  (8)
In Equation (8), RFis additional resistance caused by a cable, connectors, and circuit board, and I is the current contributing to the forward voltage measurement. The value of (nkT/q) can be found if three values of the forward voltage probing currents I1, I2, and I3are used, as shown below in Equation (9):
(nkT/q)=(I3(V2−V1)−I2(V3−V1)+I1(V3−V2))/(I3*ln(I2/I1)−I2*ln(I3/I1)+I1*ln(I3/I2)).   (9)
From Equation (9), nkT/q can be calculated as a linear superposition Vsof the three forward voltages V1, V2, and V3, corresponding to the probing currents I1, I2, and I3. In other words, (nkT/q)=Vs, where
Vs=aV1+bV2+cV3.  (10)
The coefficients a, b, and c correspond to various probing current levels. Accordingly, similar to Equation (5),
δn730/n0(730)=δVs(730)/Vs0(730)−δVs(810)/Vs0(810).  (11)

Using Equations (1)-(11), it is appreciated that spectrometric measurements are sensitive to the differences in the output of the light source14. Large negative differences between δΔVf(730)/ΔVf0(730)and δΔVf(810)/ΔVf0(810)from Equation (7) may be attributed to aging, and the light source14should be treated as expired if such a large negative difference exists.

FIG. 2is a flowchart illustrating an exemplary method100of determining the age of the light source14as previously described. The method100generally includes measuring the initial I-V characteristic of the light source14, measuring the actual I-V characteristic of the light source14, and comparing the initial I-V characteristic of the light source14to the actual I-V characteristic of the light source14.

The difference between the initial I-V characteristic and the actual I-V characteristic, and specifically the initial forward voltage and the actual forward voltage measured, indicates the age of the light source14. Therefore, in one exemplary approach, measuring the initial I-V characteristic may include a step102of measuring an initial forward voltage of the light source14. To determine the initial I-V characteristics, including the initial forward voltage, the method100may include calibrating the physiological sensor10as previously described. Once known, the method100may include a step104of storing the initial I-V characteristic of the light source14in the memory device22, and accordingly, a step106of accessing the memory device22to retrieve the initial I-V characteristic of the light source14.

Likewise, measuring the actual I-V characteristic of the light source14may include a step108of measuring an actual forward voltage of the light source14while the light source14is operating (i.e., emitting light). The actual I-V characteristics are updated to the memory device22, however, to reduce interference, the method100may further include a step110of updating the memory device22with the actual forward voltage, or other I-V characteristic, at a predetermined time. To determine when the memory device22should be updated, the method100may include a step112of measuring interference caused by an external source or the light source14and updating the memory device22when substantially no interference is measured, when substantially no RF interference is measured, when substantially no light is emitted from the light source14, and/or when no forward voltage is passing through the light source14. Moreover, the method100may include changing the polarity of the clock signal of the memory device22when substantially no light is measured from the light source14.

Once the initial and actual I-V characteristics are stored in the memory device22, the age of the light source14may be determined by comparing the initial I-V characteristic to the actual I-V characteristic of the light source14. Specifically, the method100may include a step114of comparing the initial forward voltage to the actual forward voltage of the light source14using Equations (1)-(11). Alternatively, the method100may include a step116of comparing the actual forward voltage of one light source14with the actual forward voltage of another light source14to determine light source aging.

The present embodiments have been particularly shown and described, which are merely illustrative of the best modes. It should be understood by those skilled in the art that various alternatives to the embodiments described herein may be employed in practicing the claims without departing from the spirit and scope as defined in the following claims. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby. This description should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application.