Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2000-09-29
2002-12-17
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06494576
ABSTRACT:
BRIEF STATEMENT OF THE INVENTION
This invention relates to the use of spectrophotometric absorption for non-invasively measuring the chemistry of the blood in the eye.
BACKGROUND OF THE INVENTION
Photographic retinal oximetry, like pulse oximetry, computes arterial O
2
saturation by using variations in the absorption of light in the red and IR (infrared) wavelengths caused by the pulsation of arterial blood. Arterial oxygen saturation of the blood as determined by a photographic retinal oximeter is designated SpO
2
(and is shown in FIG.
3
). Increased arterial blood flow during systole expands tissue beds by delivering additional blood with each pulse. When a pulse of light is shone onto a blood-perfused tissue bed, each of the arterial pulsations alters the amount of light transmitted through and reflected back from the tissue bed. By working only with the variations in the light caused by the pulsing of the tissue bed, a photographic retinal oximeter can be operated in a fashion to ignore light absorption by nonpulsatile elements in the light transmission pathway (e.g., choroid, retina, lens and cornea). This type of analysis is called transmission (or reflectance) oximetry and it uses light at wavelengths of 660 nm (red light, primarily absorbed by reduced hemoglobin) and 910 or 940 nm (infra-red light, primarily absorbed by oxyhemoglobin).
More recent developments in techniques for non-invasive analysis of patients by using light have enabled the use of reflectance oximetry (as opposed to the transmission oximetry just discussed). This newer technique monitors SpO
2
(partial pressure of oxygen in the blood) by measuring light reflected from perfused tissues. This approach could be dangerous for additional monitoring capabilities and comparison with the values obtained by transmission oximetry.
Oximetry describes various spectrophotometric techniques that determine the HbO
2
saturation (i.e., saturation of hemoglobin with oxygen). If blood exposed to light of a particular wavelength and intensity, measurement of the light absorbed by the oxygenated hemoglobin moiety (whether partially or fully oxygenated) is proportional to the relative amount of HbO
2
present. This relationship can be expressed mathematically by A=alc (Equation 1), where A is the amount of light absorbed, a is the absorption of HbO
2
at a given wavelength, 1 is the length of the light path, and c is the concentration of HbO
2
. Rearranging Equation 1 gives the following mathematic relationship for absorption: a=A/lc (Equation 2). A calibration constant can be derived by comparison of absorption between two substances with identical absorption at a given wavelength (e.g., a standard (st) and an unknown (u) from the equality: (A/lc)
st
=(A/lc)
u
(Equation 3). If the light path length is held constant, the concentration of the unknown substance is determined by the relationship: c
u
=A
u
×c
st
/A
st
(Equation 4).
Application of these principles to patient monitoring assumes that the measured change in absorption is a function having as its predominant parameter the different forms of hemoglobin present in the blood. The presence of other substances with spectral activity in the light wavelengths used for analyzing biological fluids and molecules will likely result in erroneous measurements (to some degree). Two applications of these principles are routinely used in the clinical management of anesthetized and critically ill patients.
Pulse oximeteres are dual-wavelength spectrophotometers that use a light-emitting diode as a light source and a photodiode as a light detector. The source and detector are usually incorporated into a digital clip that is applied “clothes-pin” fashion to the end of a finger. When the light source and detector are separated by the pulsating arterial vascular bed at the end of the finger, the degree of change in the transmitted light (light emitted minus light absorbed) is proportional to the size of the arterial pulse, the wavelengths of light, and the HbO
2
concentration. If the pulse is considered to be entirely due to the passage of arterial blood and the appropriate light wavelengths (e.g., 660 nm and 940 nm) are used, the SpO
2
can be continuously measured. The clinical accuracy of pulse oximeters is excellent for HbO
2
saturations≧80% when compared with laboratory co-oximeters. At lower oxyhemoglobin concentrations, agreement between the pulse oximeter and the co-oximeter is diminished. Nevertheless, the pulse oximeter still reliably trends the changes in HbO
2
saturation.
Fiberoptic techniques allow flow-directed pulmonary artery catheters to measure continuously the HbO
2
concentration of mixed venous blood in the pulmonary artery. Mixed venous oximetry is an application of reflectance spectrophotometry, in which light of appropriate wavelengths is flashed down a fiberoptic path; the resultant reflected light from the hemoglobin passes back up the fiberoptic path. The ratio of reflected light between (or among) the different wavelengths is proportional to the mixed venous HbO
2
saturation (SvO
2
). The fiberoptic catheter must be calibrated for reading during use for this technique to provide accurate results. Stability of the calibration is unaffected by temperature variations or by hemoglobin concentration, provided the subject's hematocrit is at least 40%. Another source of error, calibration curves can shifted by 1% for every 0.1 change in the pH of the subject's blood. Thus, calibration against (i) a standard sample of known HbO
2
, (ii) saturation before insertion, or (iii) a measured SvO
2
obtained from a blood sample taken after catheter placement, is feasible and reliable (and desirable). Mixed venous fiberoptic oximetry results correlate well with co-oximetric measurement of SvO
2
. Clinically acceptable accuracy of these techniques is unaffected by body temperature, hemoglobin concentration, cardiac index, or method of calibration.
Noninvasive oximeters typically measure red and infrared light transmitted through and/or reflected by a tissue bed. Accurate estimation of SaO
2
(arterial oxygen saturation) using this method encounters several technical problems. First, there are many light absorbers in the path of transmitted light other than arterial hemoglobin (e.g., cornea, lens, and vitreous and venous and capillary blood). The photographic retinal oximeter takes into account the effect of absorption of light by these tissues and venous blood by assuming that only arterial blood pulsates.
FIG. 4
illustrates schematically the series of absorbers in a typical sample of living tissue. At the top of
FIG. 4
is the “ac” (pulsatile) component, which represents absorption of light by the pulsating arterial blood in the choroid and retina. The “dc” (baseline) component represents absorption of light by the tissue bed, including venous, capillary, and nonpulsatile arterial blood. The pulsatile expansion of the arteriolar bed increases the path length, thereby increases absorbance. Pulse oximeters use only two wavelengths of light:; 660 nm (red light) and 940 nm (near-infrared light). The photographic retinal oximeter first determines the ac component of absorbance at each wavelength and then divides this value by the corresponding dc component to obtain a “pulse-added” absorbance that is independent of the intensity of incident light, both ac and dc values determined photographically at the peak or crest (ac) and trough (dc) of the arterial pulse. The oximeter then calculates the ratio R of these pulse-added absorbance, which is empirically related to SaO
2
by the formula R (ac
660
/dc
660
) (ac
940
/dc
940
) (Eq. 5).
It was a fortuitous coincidence of technology and physiology that allowed the development of solid-state pulse oximeter sensors. Light-emitting diodes are available over a relatively narrow range of the electromagnetic spectrum. Among the available wavelengths are some that not only pass through the skin but also are absorbed by both oxyhemoglobin and reduced hemoglobin. For best sensitivit
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