Parameter compensated physiological monitor
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.
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 INVENTIONPulse 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.
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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 ci of an absorbent in solution can be determined by the intensity of light transmitted through the solution, knowing the pathlength dλ, the intensity of the incident light Io,λ, and the extinction coefficient Ei,λ at a particular wavelength λ. In generalized form, the Beer-Lambert law is expressed as:
where μa,λ 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 (HbO2) and reduced hemoglobin (Hb).
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.
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RD/IR=(RedAC/RedDC)/(IRAC/IRDC) (3)
The desired oxygen saturation (SpO2) 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 SpO2, as described with respect to
Conventional pulse oximetry measurements, for example, depend on a predictable, empirical correlation between RD/IR and SpO2. 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.
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.
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.
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
FIGS. 9A-D are graphs of one embodiment of calibration data modification utilizing Bezier curves;
Overview
Parameter compensated physiological monitoring is described below with respect to monitor interface architectures (
The interface architectures according to
Parameter Compensation Architecture
In one embodiment, the primary input 412 is a detector response to at least two emitter wavelengths after transmission through or reflection from a tissue site, from which the physiological monitor 400 may derive at least a conventional physiological measurement, such as an oxygen saturation value, as described with respect to
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The sensor 610 may also have an information element (not shown) that describes information regarding the sensor. In one embodiment, the information element provides the monitor 660 with information regarding available wavelengths for the emitters 620 and/or information regarding the temperature element 630, such as the resistance-temperature characteristics of a thermistor. An information element is described in U.S. Pat. No. 6,011,986 entitled “Manual And Automatic Probe Calibration,” assigned to Masimo Corporation, Irvine, Calif. and incorporated by referenced herein.
Parameter Compensation Signal Processing
FIGS. 8B-C describe one pulse oximeter embodiment of the compensation relationship 800 (
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The responsiveness to blood gas measurements is determined by a sensitivity filter 893 and sensitivity adjustment 809. So as to reduce over-sensitivity of a calibration data to blood gas measurements, calibration data modification may require multiple blood gas input values over a range of saturation values and/or consistency within a tolerance range before calibration data is modified. Also, calibration data modification can be less sensitive to more frequently occurring normal saturation values and more sensitive to the less frequently occurring low saturation values. Hence, the sensitivity filter 893 may have a blood gas input 804 so that responsiveness varies with the range of blood gas sample values. Further, calibration data may be piecewise modified according to ranges of saturation values, so that an entire range of calibration data is not affected by blood gas measurements that are limited to a certain range of saturation values.
TABLE 1, below, illustrates one embodiment of the modification rules 895. Saturation Range is a range of blood gas measurements and a corresponding portion of the calibration curve to be replaced or modified. Number of Samples is the number of blood gas measurements required within the corresponding Saturation Range before a calibration curve modification or replacement is made. Sample Tolerance is the deviation allowed between measured SpO2 and measured SpO2 for a particular blood gas measurement to be considered. For example, the saturation ranges may be in 5% increments, i.e. 100-95%, 95-90%, etc. The number of samples may be, say, 4 for saturation measurements above 75% and 1 for saturation measurements below 75%. The sample tolerance may be SpO2-SpO2=±1%.
Depending on the embodiment, the modification rules 895 may operate on the baseline calibration data to select one of a family of calibration curves, determine the direction and amount of shift in a calibration curve, modify the shape of a calibration curve, rotate a calibration curve around a selected point on the curve, specify one or more points from which a calibration curve may be derived, or a combination of these actions. In this manner a pulse oximeter may be calibrated on site for individual patients, for improved accuracy as compared with total reliance on empirical calibration data derived from many individuals. Calibration curve modification in response to blood gas measurements is described in further detail with respect to FIGS. 9A-D, below.
FIGS. 9A-D illustrate calibration data modification utilizing a Bezier curve. In its most common form, a Bezier curve is a simple cubic equation defined by four points including the end points and two control points, as is well-known in the art. As shown in
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Multiple calibration points may be accommodated by curve-fitting algorithms well-known in the art, such as a least-means-squared computation of the error between the modified calibration curve and the calibration points, as one example. Other polynomials curves may be used to derive modified calibration curves, and two or more sections of Bezier curves or other polynomial curves can be used to represent a modified calibration curve.
In one advantageous embodiment, a blood gas measurement of HbCO and/or MetHb is manually entered into a pulse oximeter and utilized to generate a compensated value of SpO2. As described above, conventional pulse oximetry utilizes two wavelengths, assuming that Hb and HbO2 are the only significant absorbers. However, HbCO and MetHb may also be significant absorbers at RD and IR wavelengths. The presence of significant concentrations of HbCO and MetHb have different effects on a conventional pulse oximeter estimate of oxygen saturation. HbO2 and HbCO have similar extinctions at the RD wavelength, as do MetHb and Hb. At the IR wavelength, HbCO is relatively transparent whereas MetHb has greater extinction than the other hemoglobins. The two wavelength assumption tends to lump HbO2 and HbCO together, i.e. HbCO is counted as an oxygen carrying form of hemoglobin, causing a conventional pulse oximeter to overestimate oxygen saturation. As MetHb increases, RD/IR tends to unity and SpO2 tends to a constant (e.g. 85%) regardless of oxygen saturation. A manually entered value of HbCO and or MetHb is used as a parameter in conjunction with the functions described above with respect to any of
A parameter compensated physiological monitor has been disclosed in detail in connection with various embodiments. These embodiments are disclosed by way of examples only and are not to limit the scope of the claims that follow. One of ordinary skill in the art will appreciate many variations and modifications.
Claims
1. A physiological monitor comprising:
- an optical sensor providing a first sensor signal responsive to a plurality of wavelengths of optical radiation after attenuation by pulsatile blood flowing within a tissue site of a patient;
- a non-optical sensor providing a second sensor signal corresponding to the patient;
- a processor configured to derive a spectral characteristic of the tissue site from the first sensor signal;
- calibration data relating the spectral characteristic to a blood gas parameter of the patient; and
- a correction applied to the calibration data in response to the second sensor signal.
2. The physiological monitor according to claim 1:
- wherein the non-optical sensor has a temperature sensing element, and
- wherein the second sensor signal is responsive to a tissue site temperature.
3. The physiological monitor according to claim 2:
- wherein the calibration data defines a Bezier curve having end points and corresponding control points, and
- wherein the correction repositions the control points so as to generate a modified calibration curve.
4. A physiological monitor comprising:
- a primary input responsive to optical properties of a tissue site of a patient;
- a characteristic of the tissue site derived from the primary input;
- a calibration curve relating the characteristic to a blood gas parameter for the patient;
- a blood gas value derived from the primary input and the calibration curve;
- a secondary input responsive to non-optical properties corresponding to the patient; and
- a compensation applied to at least one of the calibration curve and the blood gas value in response to the secondary input so as to derive a compensated blood gas value that is more accurate than the blood gas value.
5. The physiological monitor according to claim 4 wherein the secondary input provides a measure of the patient's blood pH.
6. The physiological monitor according to claim 5 wherein the secondary input is a blood pH value manually entered through a user interface.
7. The physiological monitor according to claim 5 wherein the secondary input is an output from a blood pH monitor having a sensor in contact with the patient.
8. A physiological monitoring method comprising:
- generating an optical measurement at a tissue site having pulsatile blood flow;
- defining calibration data relating the optical measurement to a blood gas parameter corresponding to the pulsatile blood flow;
- calculating the optical measurement and the calibration data with a processor so as to generate a blood gas value corresponding to the blood gas parameter;
- providing a non-optical measurement corresponding to the tissue site; and
- correcting at least one of the calibration data and the blood gas value according to the non-optical measurement.
9. The physiological monitoring method according to claim 8:
- wherein generating comprises attaching an optical sensor to the tissue site; and
- wherein providing comprises integrating a non-optical sensor element into the optical sensor so as to make the non-optical measurement.
10. The physiological monitoring method according to claim 9 wherein integrating comprises positioning a thermistor within the optical sensor so as to take a temperature measurement of the tissue site.
11. The physiological monitoring method according to claim 8 wherein providing comprises attaching a non-optical sensor to the tissue site.
12. The physiological monitoring method according to claim 11 wherein providing further comprises taking a temperature measurement of the tissue site.
13. The physiological monitoring method according to claim 8 wherein providing comprises manually inputting into the processor via a user interface a non-optical measurement value corresponding to the tissue site.
14. The physiological monitoring method according to claim 13 wherein manually inputting comprises entering a tissue temperature value on a keypad that is in communications with the processor.
15. A physiological monitoring method comprising:
- generating a non-invasive measurement at a tissue site of a patient;
- defining calibration data relating the non-invasive measurement to a blood gas parameter;
- calculating with a processor a blood gas value corresponding to the non-invasive measurement;
- providing an invasive measurement of the patient;
- compensating the blood gas value according to the invasive measurement.
16. The physiological monitoring method according to claim 15 wherein providing comprises measuring a blood pH value for the patient.
17. The physiological monitoring method according to claim 16 wherein measuring comprises manually entering the blood pH value via a user interface into the processor.
18. The physiological monitoring method according to claim 16 wherein measuring comprises outputting the blood pH value from a pH monitor via an instrument interface to the processor.
19. A physiological monitor comprising:
- an optical sensor means for providing a tissue site spectral characteristic for a patient;
- a non-optical input means for providing a compensation parameter for the patient;
- a calibration means for relating the spectral characteristic to an uncompensated blood gas parameter; and
- a correction means responsive to the compensation parameter for generating a compensated blood gas parameter.
20. The physiological monitor according to claim 19 further comprising a processor for applying the correction means to the calibration means.
21. The physiological monitor according to claim 19 further comprising a processor for applying the correction means to the uncompensated blood gas parameter.
22. The physiological monitor according to claim 19 further comprising a user interface means for manually entering a value for the compensation parameter into the processor.
23. The physiological monitor according to claim 19 further comprising an instrument interface means for entering the compensation parameter from an external monitor into the processor.
24. The physiological monitor according to claim 19:
- wherein the calibration means comprises a Bezier curve; and
- wherein the correction means comprises a control means for altering the shape of the Bezier curve.
Type: Application
Filed: Nov 27, 2006
Publication Date: Mar 29, 2007
Inventors: Massi Kiani (Laguna Niguel, CA), Mohamed Diab (Mission Viejo, CA), Ammar Al-Ali (Tustin, CA), Walter Weber (Laguna Hills, CA)
Application Number: 11/604,499
International Classification: A61B 5/00 (20060101);