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  Parameter compensated physiological monitorUSPTO Application #: 20070073127Title: Parameter compensated physiological monitor Abstract: A monitor has an optical input from which a spectral characteristic can be derived. The monitor also has a non-optical input from which a parameter can be determined. A compensation relationship is determined for the spectral characteristic based on the parameter. A physiological measurement is determined based on the spectral characteristic and the compensation relationship. (end of abstract) Agent: Knobbe Martens Olson & Bear LLP - Irvine, CA, US Inventors: Massi E. Kiani, Mohamed Diab, Ammar Al-Ali, Walter M. Weber USPTO Applicaton #: 20070073127 - Class: 600331000 (USPTO) Related Patent Categories: Surgery, Diagnostic Testing, Measuring Or Detecting Nonradioactive Constituent Of Body Liquid By Means Placed Against Or In Body Throughout Test, Infrared, Visible Light, Or Ultraviolet Radiation Directed On Or Through Body Or Constituent Released Therefrom, Determining Blood Constituent, Oxygen Saturation, E.g., Oximeter, Calibrated The Patent Description & Claims data below is from USPTO Patent Application 20070073127. Brief Patent Description - Full Patent Description - Patent Application Claims
REFERENCE TO RELATED APPLICATIONS [0001] The present application is a continuation of U.S. patent application Ser. No. 10/714,526, filed on Nov. 14, 2003 entitled "Parameter Compensated Pulse Oximeter"; which claims the benefit U.S. Provisional Application No. 60/426,638, filed Nov. 16, 2002, entitled "Parameter Compensated Physiological Monitor," and is a continuation-in-part of U.S. patent application Ser. No. 10/671,179, filed Sep. 25, 2003, entitled "Parameter Compensated Pulse Oximeter," which claims the benefit of U.S. Provisional Application No. 60/413,494, filed Sep. 25, 2002, entitled "Parameter Compensated Pulse Oximeter." The present application incorporates the disclosures of the foregoing applications herein by reference. BACKGROUND OF THE INVENTION [0002] Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for measuring the oxygen saturation level of arterial blood. Pulse oximeters perform a spectral analysis of the pulsatile component of arterial blood in order to determine the relative concentration of oxygenated hemoglobin, the major oxygen carrying constituent of blood, and reduced hemoglobin. These instruments have gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care by providing early detection of decreases in the arterial oxygen supply, which reduces the risk of accidental death and injury. [0003] FIG. 1 illustrates a pulse oximetry system 100 having a sensor 110 and a monitor 150. The sensor 110 has emitters 120 and a detector 130. The emitters 120 typically consist of a red light emitting diode (LED) and an infrared LED that project light through blood vessels and capillaries underneath a tissue site, such as a fingernail bed. The detector 130 is typically a photodiode positioned opposite the LEDs so as to detect the emitted light as it emerges from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled "Low Noise Optical Probe," which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. [0004] Also shown in FIG. 1, the monitor 150 has drivers 152, a sensor front-end 154, a signal processor 155, a display driver 157, a display 158 and a controller 159. The drivers 152 alternately activate the emitters 120 as determined by the controller 159. The front-end 154 conditions and digitizes the resulting current generated by the detector 130, which is proportional to the intensity of the detected light. The signal processor 155 inputs the conditioned detector signal and determines oxygen saturation based upon the differential absorption by arterial blood of the two wavelengths projected by the emitters 120. Specifically, a ratio of detected red and infrared intensities is calculated by the signal processor 155, and an arterial oxygen saturation value is empirically determined based on the ratio obtained, as described with respect to FIGS. 2-3, below. The display driver 157 and associated display 158 indicate a patient's oxygen saturation along with pulse rate. [0005] The Beer-Lambert law provides a simple model that describes a tissue site response to pulse oximetry measurements. The Beer-Lambert law states that the concentration c.sub.i of an absorbent in solution can be determined by the intensity of light transmitted through the solution, knowing the pathlength d.sub..lamda., the intensity of the incident light I.sub.o,.lamda., and the extinction coefficient E.sub.i,.lamda. at a particular wavelength .lamda.. In generalized form, the Beer-Lambert law is expressed as: I .lamda. = I 0 , .lamda. .times. e - d .lamda. .mu. a , .lamda. ( 1 ) .mu. a , .lamda. = i = 1 n .times. i , .lamda. c i ( 2 ) where .mu..sub.a,.lamda. is the bulk absorption coefficient and represents the probability of absorption per unit length. The Beer-Lambert law assumes photon scattering in the solution is negligible. The minimum number of discrete wavelengths that are required to solve EQS. 1-2 are the number of significant absorbers that are present in the solution. For pulse oximetry, it is assumed that wavelengths are chosen such that there are only two significant absorbers, which are oxygenated hemoglobin (HbO.sub.2) and reduced hemoglobin (Hb). [0006] FIG. 2 illustrates top-level computation functions for the signal processor 155 (FIG. 1), described above. In particular, pulse oximetry measurements are conventionally made at a red wavelength corresponding to 660 nm and an infrared wavelength corresponding to 940 nm. At these wavelengths, reduced hemoglobin absorbs more red light than oxygenated hemoglobin, and, conversely, oxygenated hemoglobin absorbs more infrared light than reduced hemoglobin. [0007] In addition to the differential absorption of hemoglobin derivatives, pulse oximetry relies on the pulsatile nature of arterial blood to differentiate hemoglobin absorption from absorption of other constituents in the surrounding tissues. Light absorption between systole and diastole varies due to the blood volume change from the inflow and outflow of arterial blood at a peripheral tissue site. This tissue site also comprises skin, muscle, bone, venous blood, fat, pigment, etc., each of which absorbs light. It is assumed that the background absorption due to these surrounding tissues is invariant and can be ignored. That is, the sensor signal generated by the pulse-added arterial blood is isolated from the signal generated by other layers including tissue, venous blood and baseline arterial blood. [0008] As shown in FIG. 2, to isolate the pulsatile arterial blood, the signal processor 155 (FIG. 1) computes ratios 215, 265 of the AC portions 212, 262 of the detected red (RD) 201 and infrared (IR) 206 signals with respect to the DC portions 214, 264 of the detected signals 201, 206. Computations of these AC/DC ratios 215, 265 provide relative absorption measures that compensate for variations in both incident light intensity and background absorption and, hence, are responsive only to the hemoglobin in the arterial blood. Further, a ratio of the normalized absorption at the red wavelength over the normalized absorption at the infrared wavelength is computed:RD/IR=(Red.sub.AC/Red.sub.DC)/(IR.sub.AC/IR.sub.DC) (3) The desired oxygen saturation (SpO.sub.2) 282 is then computed empirically from this "red-over-infrared, ratio-of-ratios" (RD/IR) 272. That is, the RD/IR output 272 is input to a look-up table 280 containing empirical data 290 relating RD/IR to SpO.sub.2, as described with respect to FIG. 3, below. [0009] FIG. 3 shows a graph 300 depicting the relationship between RD/IR and SpO.sub.2. This relationship can be approximated from Beer-Lambert's Law, described above. However, it is most accurately determined by statistical regression of experimental measurements obtained from human volunteers and calibrated measurements of oxygen saturation. The result can be depicted as a curve 310, with measured values of RD/IR shown on a x-axis 302 and corresponding saturation values shown on an y-axis 301. In a pulse oximeter device, this empirical relationship can be stored in a read-only memory (ROM) for use as a look-up table 280 (FIG. 2) so that SpO.sub.2 can be directly read-out from an input RD/IR measurement. For example, an RD/IR value of 1.0 corresponding to a point 312 on the calibration curve 310 indicates a resulting SpO.sub.2 value of approximately 85%. SUMMARY OF THE INVENTION [0010] Conventional pulse oximetry measurements, for example, depend on a predictable, empirical correlation between RD/IR and SpO.sub.2. The relationship between oxygen saturation and tissue spectral characteristics, such as RD absorbance as compared with IR absorbance, however, vary with other parameters such as site temperature, pH and total hematocrit (Hct), to name just a few, that are not accounted for in the conventional photon absorbance model. A parameter compensated physiological monitor advantageously utilizes one or more parameters that are not considered in conventional physiological monitoring in order to derive a more accurate physiological measurement. Parameters may be input from various sources, such as multiple parameter sensors, additional sensors, external instrumentation and manual input devices. A compensated physiological measurement accounts for these parameters by various mechanisms including modification of calibration data, correction of uncompensated physiological measurements, multidimensional calibration data, sensor wavelength modification in conjunction with wavelength-dependent calibration data, and modification of physiological measurement algorithms. [0011] One aspect of a parameter compensated physiological monitor has a primary input from which a spectral characteristic of a tissue site can be derived. The monitor also has a secondary input from which at least one parameter can be determined. A compensation relationship of the spectral characteristic, the parameter and a compensated physiological measurement is determined. A processor is configured to output the compensated physiological measurement in response to the primary input and the secondary input utilizing the compensation relationship. [0012] A parameter compensated physiological monitoring method includes the steps of inputting a sensor signal responsive to a spectral characteristic of a tissue site and deriving a physiological measurement from the characteristic. Other steps include obtaining a parameter, wherein the physiological measurement has a dependency on the parameter and determining a relationship between the spectral characteristic and the parameter that accounts for the dependency. A further step is compensating the physiological measurement for the parameter utilizing the relationship. [0013] Another aspect of a parameter compensated physiological monitor has a primary input for determining a spectral characteristic associated with a tissue site. The monitor also has a secondary input means for determining a parameter that is relevant to measuring oxygen saturation at the tissue site and a compensation relationship means for relating the spectral characteristic, the parameter and an oxygen saturation measurement. BRIEF DESCRIPTION OF THE DRAWINGS [0014] FIG. 1 is a block diagram of a prior art pulse oximeter; [0015] FIG. 2 is a top-level functional diagram of conventional pulse oximetry signal processing; [0016] FIG. 3 is an exemplar graph of a conventional calibration curve; [0017] FIG. 4 is a top-level block diagram of a parameter compensated physiological monitor having sensor, external instrument and manual parameter inputs; [0018] FIG. 5 is a block diagram of a parameter compensated pulse oximeter having a manual parameter input; [0019] FIG. 6 is a block diagram of a parameter compensated pulse oximeter having a multi-wavelength sensor along with sensor site temperature and external instrument pH parameter inputs; [0020] FIG. 7 is a top-level functional block diagram of a compensation relationship having spectral characteristic and parameter inputs and a compensated physiological measurement output; Continue reading... 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