Non-interfering physiological sensor system

A system includes a light source, a photodetector in optical communication with the light source, and a processor in communication with said photodetector and configured to output a signal representing oxygen saturation independent of an interfering signal from an interfering source. The system may further include an analog-to-digital converter in communication with the processor that is configured to digitize a signal from the photodetector by oversampling and output oversampling data to the processor. The processor may include an averaging filter that averages the oversampling data received from said analog-to-digital converter prior to decimation to generate an oversampling number.

BACKGROUND

Pulse oximeters and tissue oximeters are medical devices that measure absorption of near infrared light to determine blood oxygen saturation. Sometimes, pulse oximeters and tissue oximeters are used simultaneously on a patient's body. Typically, pulse oximeters use a disposable sensor placed on a peripheral site of the body where arterial capillary blood pulsation is high, such as on the finger. The sensor uses one or more light emitting diodes (LEDs) to emit light with two wavelengths and silicon photodetectors to measure light intensity transmitted through or reflected back from the site. To separate the two wavelengths of light, pulse oximeters usually use a single frequency to excite each LED in two different phases shifted by 180 degrees. By analyzing the AC of the reflected light intensities at two different wavelengths separately, pulse oximeters estimate arterial blood oxygen saturation (aSpO2). Because the arterial blood has uniform oxygen saturation across the whole body, a single sensor is usually employed on the patient to measure arterial blood oxygen saturation.

Tissue oximeters, including a cerebral oximeter, analyze the DC components of the reflected light at multiple wavelengths to determine oxygen saturation. Tissue oximeter sensors employs light from one or more LEDs or laser diodes and uses silicon photodiodes, avalanche photodiodes, or similar detectors to measure light absorption. In order to measure absorption of the light that travels deep inside the tissue or organs, tissue oximeters use a large separation between emitters and detectors. The measured light absorption related to the oxygen status of the tissue or organs is called the regional oxygen saturation (rSO2). Because different sites of the body have different values of regional oxygen saturation, multiple sensors on multiple body locations are usually employed to measure patient status. To measure light intensity with a high signal to noise ratio, the tissue oximeter employs a synchronous demodulation technique. To separate light signals from multiple wavelengths and from multiple sensors, tissue oximeters use time division or frequency division multiplexing or similar methods

Because both pulse and tissue oximeters use near infrared light, light generated by one type of oximeter can interfere with the other type of oximeter. This interference can happen regardless of where the sensors are placed on the patient's body. For example, light from one sensor can be detected by the light detectors of another sensor by: (1) propagating directly inside the tissue, (2) exiting from the tissue under the sensor and reflecting from objects surrounding the patient back to the tissue. Interference may be especially common during infant or pediatric monitoring where, for example, a forehead reflectance pulse oximeter sensor and cerebral oximeter sensor are used in close proximity.

Pulse oximeters and tissue oximeters, however, do not use continuous sinusoidal signals for modulation-demodulation. Rather, pulse and tissue oximeters use pulses with spectrums that may include multiple harmonics of a fundamental frequency. The harmonics of the fundamental frequency of one oximeter can fall in a pass band of a demodulator of another oximeter creating interference.

The severity of interference may depend upon differences between the modulation frequencies of the oximeters, a width of the pulses, and the pass band of a filter of the demodulator. The pulse oximeter may be configured to reject multiple harmonics of an AC component of ambient light and/or a main power line. The modulation frequency f1of the pulse oximeter may be related to the frequency of the main power line (e.g., 50 Hz or 60 Hz). For example, the modulation frequency f1may be equal to 1365 Hz so that differences between the modulation frequency f1and the 22nd harmonic of ambient light having a frequency of 60 Hz (i.e., 1365−22*60 Hz=45 Hz) will not fall inside the pass band of the demodulator that usually is equal to the band frequency of plethysmogram (e.g., F=7.5 Hz).

However, light pulses from the pulse oximeter at a frequency (e.g., f1) of, for example, 1365 Hz may be received by and interfere with the tissue oximeter sensor. In particular, the light pulses may interfere with the tissue oximeter sensor light pulses depending on modulation frequency (e.g., f2) of the tissue oximeter pulses.

Because of low light return in the tissue oximeter sensor in comparison to the pulse oximeter sensor, the duration of the light pulses used for the tissue oximeter may be much more than for the pulse oximeter (e.g., f2is less than f1). For example, the modulation frequency f2of the tissue oximeter pulses may be 15 Hz with a pulse duration of 1 millisecond while the frequency f1of the light pulses from the pulse oximeter may be 1365 Hz. The tissue oximeter may use a synchronous demodulator to multiply the signal from the photodiode by +1 or −1 depending on a phase of the light source excitation. Because of this, the synchronous filter may successively demodulate all harmonics of the modulation frequency f2. We can see that the 91stharmonic of the tissue oximeter will interfere with the pulse oximeter (e.g., 15 Hz*91=1365 Hz). Thus, a pulse oximeter signal at 1365 Hz will be demodulated by the tissue oximeter as a DC signal. In practice because the interfering frequency fiis subject of the slight variation from 15 Hz*91 due to low frequency fluctuations of the system clock of the pulse oximeter CPU, the DC surplus from the interfering light will show signs of low frequency variations. In some cases these low frequency variations can be significant and erroneously interpreted as a physiological effect.

Interference may occur with other values of f1and f2, and separation of f1and f2does not assure absence of interference between two oximeters. Accordingly, a new system is needed that eliminates interference.

DETAILED DESCRIPTION

An exemplary system includes a light source in optical communication with at least one photodetector. A processor is configured to output a signal indicative of oxygen saturation. The signal representing oxygen saturation is independent of an interfering signal from an interfering sensor. In one exemplary approach, each light source may be modulated and demodulated with precise sinusoidal signals that have different frequencies relative to the other light sources. Demodulation may be used to extract signals that represent tissue light attenuations from, for example, a tissue oximeter, with high precision. Various implementations of detecting and removing interference are discussed herein. The system may include any number of sensors or light generating devices, including but not limited to, a pulse oximeter sensor or a tissue oximeter sensor.

FIG. 1is a system diagram of an exemplary tissue oximeter100configured to remove interference from an interfering sensor145. The tissue oximeter100may include one or more light sources105and one or more photodetectors110disposed on one or more sensor pads115. The tissue oximeter100may further include at least one trans-impedance amplifier120for each photodetector110, at least one analog-to-digital converter (ADC)125for each photodiode, a current source130, and at least one processor135.

The light sources105may include one or more light emitting diodes (LEDs) configured to transmit light at a predetermined frequency. For example, the light source105may be configured to transmit light in the near-infrared region of the electromagnetic spectrum.

The photodetectors110may each be in optical communication with at least one of the light sources105and configured to receive light with a specific wavelength, such as light in the near-infrared region of the electromagnetic spectrum. Each photodetector110may include, for example, one or more photodiodes.

The trans-impedance amplifiers120are each electrically connected to at least one of the photodetectors110. The trans-impedance amplifier120may include an operational amplifier connected to the photodetector110and a resistor electrically connected to one input of the operational amplifier and to the output of the operational amplifier. The trans-impedance amplifier120may be configured to receive current from the photodetector110and convert the current into a voltage.

The analog-to-digital converters125may each be in communication with one or more trans-impedance amplifiers120. The analog-to-digital converters125are configured to convert the analog voltage generated by the trans-impedance amplifier120into a digital signal. The digital output of the analog-to-digital converter125may represent a magnitude of the voltage output by the trans-impedance amplifier120. The analog-to-digital converter125may be configured to digitize the signal from the photodetector110by oversampling to generate oversampling data.

The current source130may be in communication with the light sources105and provide one or more of the light sources105with current so that the light sources105may generate light. One or more current sources130may be used, and the current output by the current source130may depend upon the light source105.

The processor135is in communication with the analog-to-digital converters125and the current source130and may include a central processing unit (CPU) configured to execute computer programs. The processor135may be configured to receive the digital output from the analog-to-digital converter125(e.g., the oversampling data), as well as control the current source130to adjust the amount of current provided to the light sources105. The processor135may further be configured to demodulate the digital signals received from the analog-to-digital converter125.

In operation, the current source130may excite one or more of the light sources105by transmitting alternating pulses of fixed current to the light source105. The light source105is illuminated (e.g., light source105excitation) when the current pulses exceed a minimum illumination level, and the light source105is turned off when the current pulse falls below the minimum illumination level. Therefore, the light generated by the light sources105pulses at the same frequency as the pulses output by the current source130. The frequency of the pulses may be determined by the processor135or may be set to a predetermined value.

The light generated by the light source105may travel through a patient's body (e.g., tissue) and be received by the photodetector110. Upon receiving the light from the light source105, the photodetector110may output a current that may be converted to voltage by one or more of the trans-impedance amplifiers120. The voltage output of the trans-impedance amplifier120may be continuously sampled by the analog-to-digital converter125and generate a digital output. The processor135may subtract the digital output received from the analog-to-digital converter125during the light source105excitation from the digital output from the analog-to-digital converter125during times when the light sources105are not emitting light to synchronously demodulate the output of the photodetectors110. In order to achieve good signal to noise ratio, the signals from photodiodes may be over-sampled by the analog-to-digital converter125. For example, the sampling frequency may be more than double the highest frequency of the voltage output of the trans-impedance amplifier120. The analog-to-digital converter125may transmit multiple samples to the processor135, and the processor135may average the multiple samples with a moving average filter or a cascade of moving averaging filters. By doing so, the processor135may remove interference from an interfering sensor145, such as a pulse oximeter sensor, or ambient light.

FIG. 2illustrates an exemplary graph200of the voltage output of the trans-impedance amplifier120. The graph includes time represented on the x-axis and voltage represented on the y-axis. The time interval Tiis the pulse duration, and the time interval Tsis the sampling period. As illustrated inFIG. 2, the pulse is oversampled sixteen times with eight sampling periods Ts.

A high voltage signal indicates that the light source105is illuminated and the photodetector110is receiving the light generated by the light source105. A lower voltage signal indicates that the light source105is off. However, the photodetectors110may receive ambient light or other interference when the light source105is off, which is represented by the fluctuating low voltage signal output by the trans-impedance amplifier120.

To establish appropriate frequency response the oversampling data may be averaged by the processor135using a moving averaging filter or cascaded moving averaging filters. If the moving averaging filter is used, the averaging is equivalent to the filtering of the signal. The transfer characteristics (e.g., HN(f)) of the moving average filter may be defined as follows:
HN(f)=(1/N)*(1−zN)/(1−z)=sin(πfN/fs)/Nsin(πf/fs)  (1)
Where fsis the sampling frequency of the analog-to-digital converter125, N is the oversampling number or constant that corresponds to the frequency of the interfering signal as discussed in further detail below, and z=e(−j2πf/fs). The processor135may apply the filter as defined in Equation (1), for example, to demodulate the signal received from the analog-to-digital converter125. As illustrated inFIG. 2, the oversampling number N is sixteen and there are eight sampling periods Ts.

FIG. 3illustrates an exemplary frequency response300of the moving average filter as defined in Equation (1) where the sampling frequency fsis equal to 38,400 Hz. The frequency f is represented on the x-axis and the result of Equation (1) is represented on the y-axis.FIG. 3shows that the filter defined by Equation (1) has multiple notches at the harmonic frequencies of fs. For example, the harmonics occur where the frequency is equal to fs*M/N, where M is an integer representing the number of the harmonic and N is the oversampling number (e.g., 16).

FIG. 4illustrates an exemplary frequency response400where the frequency response of the filter defined by Equation (1) is plotted along with a spectrum of an interfering pulse oximeter signal. As illustrated, the frequency of the first harmonic of the interfering pulse oximeter signal is plotted as a single line at 1365 Hz. The pulse oximeter sensor may use rectangular pulses to excite the light source105so the high harmonics of the interfering signals may have a smaller amplitude than the first harmonic. The harmonics are represented by a series of bars with decreasing amplitude at frequencies defined by M*fi, where M represents the number of the harmonic and fiis the frequency of the light pulses generated by the interfering sensor145.

In Equation (1) above, the sampling frequency fsof the analog-to-digital converter125of the tissue oximeter100may be a fixed value defined by a clock within the processor135. The over-sampling value N may define the efficiency of the demodulator needed to reduce the noise from the analog-to-digital converter125and the trans-impedance amplifier120. The value of N may be varied to some degree without significantly affecting performance of the oximeter. For example, as illustrated inFIG. 4the value of N may be defined as follows in Equation (2):
N=fs*M/fi,(2)
To reduce interference, the interfering frequency fiand harmonics of the interfering frequency (e.g., represented by M*fi) should fall into the notches of the moving average filter defined by Equation (1) as HN(f), reducing the interfering signal from, for example, the interfering pulse oximeter. However, as illustrated inFIG. 4, where the oversampling number N is equal to 16, the frequency fiof the interfering signal falls close to the peaks instead of the notches.

FIG. 5illustrates another exemplary frequency response500where the frequency response of the filter defined by Equation (1) is plotted along with a spectrum of an interfering pulse oximeter signal. In the exemplary approach illustrated inFIG. 5, the oversampling number N is equal to 28 to move the interfering frequency fiand harmonics of the interfering frequency closer to the notches, resulting in the notch of the moving average filter defined by Equation (1) as HN(f) being located at 1371 Hz (e.g., fs/28=1371 Hz), which places the notch close to the interfering frequency fi, which as discussed above, may be 1365 Hz if the interfering sensor145is a pulse oximeter sensor. In this exemplary illustration, the oversampling number N may be adjusted to 29, moving the notches to 1324 Hz (e.g., fs/29=1324 Hz), which surpasses the frequency of the interfering signal fiat 1365 Hz.

To place the notch of the filter at 1365 Hz the processor135may apply a weighted linear combination of two filters as defined in Equation 3 below:
Ha(f)=a*H28(f)+(1−a)*H29(f)  (3)
In Equation (3), the value a is a constant between 0 and 1 (e.g., 0<a<1) representing weight.FIG. 6represents the frequency response600of the exemplary filter defined by Equation (3) as Ha(f) where a is equal to 0.85, which moves the notches of the tissue oximeter sensor100pulses to 1365 Hz to correspond to the frequency of the interfering signal. With the interfering pulse oximeter sensor signal at the notches of the harmonics of the pulses of the tissue oximeter sensor100, the tissue oximeter sensor100is able to eliminate interference from the interfering sensor145. Thus, the interfering pulse oximeter signal is invisible to the tissue oximeter100.

The presented approach to reducing interference from interfering sensors145, such as pulse oximeter sensors, includes selecting the oversampling number of the analog-to-digital converter125before decimation of the frequency of interfering light pulses from, for example, the pulse oximeter. Thus, the processor135of the tissue oximeter100may be configured to automatically determine the frequency of the light pulses of the interfering sensor145.

Instead of the processor135automatically detecting the frequency of interfering light pulses, the tissue oximeter100may include an input device (not shown) that allows a user to manually input the type of interfering sensor145in use based on a brand name or the specifications of the interfering sensor145. The processor135may prompt the user for this information and include a corresponding lookup table. Based on the information provided by the user, the processor135may access the lookup table and configure the tissue oximeter100to filter the interference using the methods previously described. The processor135may have a menu where the user can select the type of interfering sensor145in use by name brand or any other characteristic and automatically set the filter's parameters to remove interference from the interfering sensor145. The filter can be activated upon a user request or automatically upon detection of interference.

In one exemplary approach, the tissue oximeter sensor100may use one or more of the photodetectors110to sample the pulse oximeter light pulses and automatically select the oversampling rate N of the filter so as to place the notch of the filter at the frequency measured by the photodetector110and eliminate interference. This may be done at the beginning of the monitoring period, periodically during monitoring, or may be triggered by the processor135based on detection of a high amplitude aliasing beat frequency caused by interference. It can also be performed on command by the user if or when the user wishes to place a pulse oximeter sensor near the tissue oximeter sensor100.

In another exemplary approach, the tissue oximeter100may use only one of its multiple sensors to automatically select the oversampling rate N of the filter so as to place the notch of the filter exactly at the frequency measured and eliminate interference.

Referring now toFIG. 7, alternatively, the tissue oximeter100may include the light sources105and photodetectors110disposed on the sensor pad115, as well as trans-impedance amplifiers120, analog-to-digital converters125, current source130, and processor135as discussed above with regard toFIG. 1. However, the tissue oximeter sensor100ofFIG. 7may further include a dedicated photodetector140.

The dedicated photodetector140may be configured to detect interference from, for example, the light source of the interfering sensor145. The dedicated photodetector140may include a photodiode and may be located on the same sensor pad115or a different sensor pad as the tissue oximeter sensor100. For example, the dedicated photodetector140may be located near an edge of the sensor pad115as illustrated inFIG. 7, or located on a separate pad so that it may be placed near the pulse oximeter sensor. The dedicated photodetector140may be coupled to the processor135through a separate trans-impedance amplifier120and analog-to-digital converter125. In operation, the dedicated photodetector140may be used to measure the frequency of the interfering pulses from the pulse oximeter and automatically select the oversampling number N of the filter so as to place the notch of the filter at the frequency measured by the dedicated photodetector140to eliminate interference.

Devices or monitors other than the pulse oximeter may also interfere with the tissue oximeter100. For example, the interference may be electromagnetic instead of optical as previously described. For example, impedance pneumography is a technique that measures breathing rate by impressing a high-frequency sinusoidal current across conductive electrodes placed on a patient's chest. While impedance pneumography does not generate significant harmonics, the fundamental frequency may be in a range where it can cause interference with the tissue oximeter100. Likewise, certain electrocardiograph or electroencephalograph monitors may impress a high-frequency sinusoidal current across conductive electrodes placed on the patient to allow measurement of electrode impedance. Additionally, other patient monitors may generate significant electromagnetic interference during operation such as a transcranial Doppler ultrasonic monitor. In each of these cases, the system and techniques described here may be used to reduce or eliminate unwanted interference with the tissue oximeter100.

Instead of using fixed value for the oversampling number N and the constant a, the processor135may be configured to vary the oversampling number N and constant a in a random fashion within predetermined intervals. Random variations the parameters of Ha(f) may introduce a random amplitude modulation of the filter residual. Such modulation will spread the peak frequency of the residual into random noise, producing the random noise instead of the high amplitude alias of the interfering signal from, for example, the pulse oximeter.

Moreover, instead of reducing interference from one or more pulse oximeters, the same techniques may be used to reduce interference from one or more other tissue oximeters on the patient that employ pulses with higher repetition. For example, one tissue oximeter may use multiple channels with different pulse repetition frequencies.

Further, instead of the moving averaging filter or the cascaded moving averaging, a linear combination of the cascaded moving averaging filters may be used.

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.