Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/1455—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters
  • A61B5/14551—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters for measuring blood gases

Definitions

• This invention relates to a method and a system implementation for determining the oxygen saturation (SO 2 ) of blood in a blood vessel or body organ.
• the invention may employ invasive or non- invasive measurement techniques and is suitable for determining blood oxygen saturation in patients in any context, for example, central venous SO 2 monitoring, pulmonary artery SO 2 monitoring, extracorporeal SO 2 monitoring, amputation level assessment, free-flap SO 2 monitoring, etc.
• the standard way to measure blood oxygen saturation in a patient is to direct light into or through the blood, to measure the intensity of the light at either discrete wavelengths or over a substantially continuous spectral range after transmission through or reflection by the blood, and then to calculate SO 2 as a function of the measured intensity values.
• Such devices are described, for example, in International Patent Application No W094/03102.
• Many factors reduce the accuracy of known SO 2 monitors. Beginning with the light source itself, it must be able to produce light at a well-defined wavelength, or over a well-defined wavelength range, and it should do so stably over the life of the measurement instrument - there is no point measuring light absorption at a wavelength that is not produced with enough intensity to allow for a useful range of detection.
• Getting the light to blood is also affected by various irregularities.
• a non-invasive device such as a finger or ear lobe cuff
• inhomogeneities and irregularities in the body tissue between the light- generating device and the blood can influence light transmission in sometimes hard-to-estimate ways, which have nothing to do with the degree of blood oxygen saturation.
• One irregularity that degrades the accuracy of most non- invasive monitors is patient motion, that is, motion artifact, which leads to a change in the path length of the light through the biological tissue and hence to a variation in the intensity of the detected transmitted or reflected light. This problem is in fact so great that it can render these devices inoperative for long periods of time. The problem is particularly severe in critical health care applications, were continuous monitoring is essential.
• WO 94/03102 attempts to address the problem of motion artifact in measuring SaO 2 by transmitting into the blood not only n predetermined wavelengths of light, but also an additional wavelength that makes it possible to cancel the motion artifact.
• WO 94/03102 broadly describes the use of a plurality of wavelengths (including the n+1 motion artifact wavelength) the device exemplified uses three wavelengths.
• the three wavelengths proposed in WO 94/03102 are not sufficient to overcome motion sensitivity.
• the invention provides a method for determining blood oxygen saturation, and a corresponding system implementation, according to which at least two blood absorption reference spectra are compiled, corresponding to two different levels of oxygenation, over a wavelength range.
• Light from a light source is then directed into blood of a subject, for example via one or more optical fibers, either invasively or non-invasively.
• a remitted light absorption spectrum from the blood is then sensed by a detection arrangement.
• computer-executable code in a computation software module then computes an oxygen saturation value as a function of the remitted light absorption spectrum relative to the blood absorption reference spectra.
• the blood absorption reference spectra and the remitted light absorption spectrum are preferably normalized before the oxygen saturation value is computed. Normalization preferably comprises two main procedures: DC-offsetting of the spectra linearly between two isosbestic wavelengths that lie in the wavelength range; and scaling the DC-offsetted blood absorption reference spectra and the remitted light absorption spectrum by a function of the area under each respective DC-offsetted spectrum between the two isosbestic wavelengths.
• the step of computing the oxygen saturation value advantageously comprises computing an optimal value of a cost function that indicates closeness of correspondence between the remitted light absorption spectrum relative to the blood absorption reference spectra.
• the optimal value can be determined by interpolation of the remitted light absorption spectrum relative to at least two of the blood absorption reference spectra.
• the reference spectra at least one minimum blood absorption reference spectrum and one maximum blood absorption reference spectrum are preferably compiled, corresponding to minimum and maximum blood oxygenation values, as well as at least one intermediate blood absorption reference spectrum. Computation of the oxygen saturation value is then done as a function of the remitted light absorption spectrum relative to at least two of the blood absorption reference spectra.
• One way to do this is for the system to determine the two blood absorption reference spectra that are closest to but are respectively greater than and less than the remitted light absorption spectrum; the oxygen saturation value can then be computed by linear interpolation of the remitted light absorption spectrum relative to the closest blood absorption reference spectra.
• Another way comprises computing the oxygen saturation value by non-linear interpolation of the remitted light absorption spectrum relative to at least three of the blood absorption reference spectra.
• the light source preferably generates the light directed into the blood from a white-light LED.
• the spectrum of the white-light LED may then be pre-determined and a representation of the white-light LED spectrum can be stored, for example in a non-volatile medium that can 5 be delivered along with the LED.
• the remitted light absorption spectrum can then be adjusted as a function of the spectrum of the white-light LED.
• Figure 1 illustrates different light absorption spectra of blood at different levels of oxygenation.
• Figure 2 is a block diagram of the main hardware and software components of a system that implements the method according to the invention.
• Figure 3 illustrates a preferred normalization method for absorption spectra. DETAILED DESCRIPTION
• Figure 1 illustrates several characteristics of light absorption by blood over a range of wavelengths.
• a x ( ⁇ ) represents an absorption spectrum of blood with x% oxygenation whereas A y represents an absorption value at wavelength y.
• a 0 ( ⁇ ) and A- ⁇ oo( ⁇ ) representing fully deoxygenated and fully oxygenated blood, respectively, and
• isosbestic wavelengths Five such isosbestic wavelengths are visible in Figure 1 , two of which, at wavelengths 522.7 nm and 586.0 nm, are labeled A 523 and A 58 6 > respectively. Other isosbestic wavelengths are 505.9, 548.6, and 569.7 nm, and there are many more. These standard values are usually rounded, and are reported slightly differently in some literature, depending on the test methodology used.
• this invention involves a method and system implementation that: 1) is invasive (inserted in the body, such as on catheters) or non-invasive (such as sensors placed against the skin, finger cuffs, ear lobe clips, etc.); 2) determines, measures, estimates, etc., blood oxygen saturation; 3) by directing multiple wavelengths of light from a light source, especially over the wavelength region of 500-600 nm; 4) into blood in an artery or any other blood vessel or body tissue; 5) to determine a measured absorption, reflectance, or transmission spectrum; 6) that is matched in any manner (least squares or other metric fits, neural networks, "pattern matching," table comparisons, etc.); 7) against two or more reference spectra representing different predetermined levels of blood oxygenation such that the match yields a measure of actual blood oxygen saturation SO 2 or SaO2.
• Figure 2 illustrates the main hardware and software components of the invention, which are explained below. Shown without further explanation here are one or more processors 340, system memory 345, and system software (such as an operating system), which perform their well-known tasks, in particular, coordinating and controlling the various hardware devices within the monitor 300, as well as executing the processor-executable code that implements the different software modules described below. Other hardware and software components of a conventional computer will of course also be included in the monitor 300 as needed. [0026] In Figure 2, both an invasive and a non-invasive implementation is shown for the sake of simplicity; in practice, only the one or the other will normally be used, but Figure 2 also illustrates the fact that the same monitor 300 according to the invention can be used in either case.
• the source of light 301 is preferably broadband with sufficient spectral energy to allow for adequate discrimination and measurement resolution, at least over the wavelength range that includes the five isosbestic wavelengths that lie in the range of 500-600 nm.
• White light has, by definition, sufficient spectral energy within the visible spectrum in the range of 500-600nm.
• Incandescent, fluorescent and halogen bulbs may be used to approximate white light. Greater thermal stability and longer life can usually be obtained by using white-light LEDs, however, and for that reason these solid-state devices are preferred.
• UV light is generated (long exposure to high intensity UV can produce tissue problems (that is, sunburn);
• IR infrared
• all the power required to produce the spectral content of the LED is usable within the wavelength range of interest and, furthermore, no optical filtering is needed to remove unwanted spectral content; e) they are cheap; and f) the respond fast - since LEDs can be turned on and off very fast, they can be pulsed on and off so as to allow dark signal to be removed without the need for a mechanical shutter.
• the light is led to the blood either directly and invasively, for example, through one or more optic fibers 111 mounted on or in a catheter 110 to a coupler or lens 100 (which may simply be the end of the transmission fiber), or indirectly and non-invasively, for example, by being conveyed from the source through one or more optic fibers 211 and then being directed against the skin of a patient's finger, etc., using a device 200 such as a finger cuff.
• Either dedicated optical fibers 112, 212 may be used to convey the remitted light to the monitor 300, or the transmission fibers 111 , 211 may be used as long as suitable time-multiplexing is arranged.
• any known light-detector 302 may be used to measure the blood's absorption spectrum.
• Some conventional systems use an array of photodetectors, each tuned to the wavelength of a respective one of a plurality of substantially single-wavelength LEDs in the light source 301.
• the preferred light source is a broadband ("white" source). This avoids the need for separate optical transmission fibers (one per wavelength) and also provides sufficient spectral energy over the wavelength region of interest.
• the detector 302 is a conventional spectrometer that generates the measured spectrum using a diffraction grating and an array of photodetectors.
• the signal from the detector 302 must normally be conditioned using known circuitry 304 before being processed digitally. Such conditioning will normally include various forms of filtering, scaling, analog-to-digital conversion, etc. The result of the conditioning will be a conditioned absorption spectrum A CO ⁇ d ( ⁇ ). [0034] As mentioned above, the spectrum of the light source 301 will not be perfectly flat. This will affect the accuracy of the SO 2 (or SaO2) calculations: a "dip" in the measured spectrum might have nothing to do with the blood absorption, for example, but rather with a lower-intensity spectral region in the transmitted light.
• the white and dark reference spectra may be determined using known techniques: Before taking a measurement, the optical sensor (100, 200) is exposed to a standard white reflective surface to give a white reference spectrum. A dark reference spectrum is then also obtained by excluding all excitation light from the optical sensor. [0037]
• An alternative white-balancing method according to the invention takes advantage of the known spectral stability of modern long-life LEDs: Given one or more such LEDs as the light source, in particular, those with silicone encapsulation, the spectrum of the light source can be measured once, in an initial characterization step, and the parameters of this characterization (after normalization, as described below) can be stored in a non-volatile medium 320 such as an EPROM chip.
• This chip or at least the parameters, can be created or determined once, for example by the LED manufacturer as a factory characterization, such that the parameters can be stored with the LED and can be recalled for later use. No further white measurements would then be needed at all.
• the values of A cond ( ⁇ ) can then be adjusted according to any known balancing algorithm to account for variations in the spectrum of the white-light LED and thus to form A meas ( ⁇ ).
• the next step toward estimation of oxygen saturation is normalization of the measured absorption spectrum A me as( ⁇ ). This preferably involves two different procedures: DC-offsetting and area normalization.
• a 0ffS et( ⁇ ) is normalized with respect to its area to give A n0 rm( ⁇ ).
• Well known numerical methods may be used to calculate given Ameas( ⁇ ), A isos1 and A isos2 .
• the normalized measured absorption spectrum A n0 rm( ⁇ ) is compared in a fitting software module 315 with a plurality of reference absorption spectra (stored in numerical form in a memory region or non-volatile storage device 330) to determine a value of SO 2 or S a O 2 , which may be displayed in any known manner by a display device 500.
• a m in( ⁇ ) and A ma ⁇ ( ⁇ ) could be A 0 ( ⁇ ) and A- ⁇ oo( ⁇ ), respectively.
• a m j n ( ⁇ ) and A max ( ⁇ ) are normalized in the same manner as was just described. For example, these spectra may be compiled from whole blood samples (measured in a cuvette), or spectra recorded in skin, or the mean spectra recorded from several individuals.
• a m i n ( ⁇ ) and A max ( ⁇ ) may be chosen to be A 0 ( ⁇ ) and A- ⁇ oo( ⁇ ), respectively.
• the fully oxygenated spectrum A- ⁇ oo( ⁇ ) can be obtained by equilibration of whole blood in a cuvette at 37°C, or in the skin of the forefinger heated to 44°C at maximal reactive hyperemia following release of an inflatable cuff after six minutes of brachial artery occlusion.
• the fully deoxygenated spectrum A 0 ( ⁇ ) can be obtained, for example, by equilibration of whole blood in the cuvette with 95%N, and 5% CO 2 at 37°C or, in skin of the forefinger heated to 44°C at the end of a six minute period of brachial artery occlusion prior to release of the inflatable cuff.
• the reference absorption spectra for a given light source can then be compiled using any known spectrometric technique. Of course, any other known laboratory procedure may be followed to determine A min ( ⁇ ) and A max ( ⁇ ) for any given choice of min and max.
• extrapolation should preferably include at least one intermediate reference spectrum (see below).
• a norm ( ⁇ ) will fall between the two "extreme" absorption profiles, (either the experimentally determined A m i n ( ⁇ ) and A max ( ⁇ ), or A 0 ( ⁇ ) and A t oo( ⁇ ), and in almost all cases, both) as shown in Figure 1 (in non-offset and unnormalized form). The question is then how oxygenated the actual blood is. It is somewhere between min% and max%,
• more than two reference spectra are compiled, that is, not only A min ( ⁇ ) and A max ( ⁇ ), but also at least one intermediate reference spectrum Aj nt er( ⁇ ), whose (preferably normalized) parameters are stored in the component 330 along with the (also preferably normalized) parameters for A m j n ( ⁇ ) and A max ( ⁇ ).
• Such an intermediate spectrum can be determined in vitro in the same way as described above. There are different ways to determine the percentage of oxygenation given at least one intermediate reference spectrum.
• the simplest way is to determine whether A ⁇ 0 r ⁇ ( ⁇ ) lies (wholly or at least mostly) between A m j n ( ⁇ ) and Aj n ter( ⁇ ), or between Ai n ter( ⁇ ) and A max ( ) and then to apply the linear interpolation technique described above, but just within the bracketed range.
• This method of bracketing followed by linear interpolation may be applied quickly even where many intermediate reference spectra are compiled. Note that it is not necessary for the reference spectra to be evenly spaced (in terms of degree of oxygenation). It is thus also not necessary to ensure that the degree of oxygenation of the reference spectra are whole numbers.
• the light source preferably generates white light - for reasons explained - the invention's method of computing the oxygenation value by evaluating a cost function of the remitted absorption spectrum relative to at least two reference spectra could also be used in implementations that transmit discrete wavelengths of light, for example from an array of single-wavelength LEDs, as long as enough wavelengths are included to allow for compilation of a reasonable representation of the remitted spectrum, and at least two of the wavelengths are isosbestic such that they can be used in the spectral normalization procedure.

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