Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-01-12
2003-07-15
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S323000, C600S326000
Reexamination Certificate
active
06594513
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to spectrophotometric methods and apparatus for quantitatively determining the degree of oxygen saturation of the hemoglobin in the blood within a body part or organ, and in particular pertains to determination of the percent oxygen saturation of intra-cerebral blood.
2. Description of the Related Art
Background
It is generally known that metabolism and more particularly oxygen sufficiency and adequacy of utilization are parameters of fundamental importance in assessing the function of any body organ. This is made self-evident when one considers that the energy provision for tissue function is underwritten for better than 90 percent by oxidative reactions involving the reduction of O
2
to H
2
O. In the absence of sufficient oxygen, this process becomes impaired with a corresponding impairment in organ function. Also recognized is the fact that an excess of oxygen also impairs organ function. For ease of explanation, the description to follow is based primarily on considering the effect of an insufficiency rather than an excess of oxygen.
In instances of extensive oxygen deprivation, over a period of time the organ loses viability and as a result the individual often has the same fate, especially if that organ is the brain. Although all organs are adversely affected by oxygen insufficiency, perhaps the problem is most acute in the case of the brain because of its complete dependence on oxidative metabolism for proper function and viability. For example, an absence of oxygen in the brain for more than a dozen seconds produces dysfunction and an absence for longer than a few minutes spells irreversible damage. A less acute impairment of oxygen availability leads to a gradual loss in brain function, especially with respect to the higher centers of the cerebral cortex. An excess of oxygen will also adversely affect the brain. For example, a neo-natal patient may well go blind by reason of excess oxygen.
Because of the vital role that oxygen sufficiency plays in human physiology, intensive efforts have been made over the years to measure this parameter in various organs and most particularly in connection with the assessment of brain and heart function. Numerous spectrophotometric methods exist for non-invasive, continuous, monitoring of metabolism in a body organ both by use of transillumination and reflectance. By “transillumination” is meant the practice of passing light through a body organ and by “reflectance” is meant the practice of diffusely reflecting light from a body organ. These methods are used primarily as instruments to determine the fraction of total hemoglobin in blood that carries oxygen. By common agreement, oxygenated hemoglobin is designated HbO
2
whereas Hb refers to the deoxygenated form and total hemoglobin is abbreviated Hb
t
.
For about half a century, physicians have relied on measurements of the percentage of the hemoglobin in the blood that is saturated with oxygen (the “% O
2
Sat”) in attempts to assess the oxygenation of tissues. The introduction of this parameter, measured in blood samples drawn from the patient, was a great step forward over the visual observation of the color of various tissues (skin, gums, fingernail beds, etc.)
By common practice, arterial blood samples are used for the assessment of the % O
2
Sat. Venous blood samples vary unpredictably depending on the metabolic activity of the organ(s) drained by the donor vein. Even in a given organ blood can be channeled through bypass vessels that can and do vary in their dilation or constriction thus changing the fractions of the blood in the nutritive and in the bypass circulations. Mixed central venous blood as taken from the right side of the heart is difficult to obtain and is therefore not routinely available. It also lacks the specificity required for the assessment of oxygenation status of individual organs.
Although arterial blood has the identical % O
2
Sat throughout the body, the parameter does not predict adequacy of organ oxygenation. Matters are complicated by such unknowns as the rate of blood flow, the pH and the CO
2
content and by the amount of hemoglobin in the blood which affects the total amount of O
2
that can be transported. Instrumentation has been devised to measure these complicating factors in the blood samples and the use of radio-active molecules has been introduced to measure blood flow in certain organs, especially the brain.
In addition for the spectrophotometric measurements of blood in intact tissues, it should be recognized that, depending on the oxygen supply, the oxidative metabolic enzyme cytochrome c oxidase has an absorption spectrum in the spectral region used (750-950 nm). Corrections for this problem have not been introduced in the present art.
Starting in the 1930's, many attempts were made to determine oxygenation status non-invasively by optical means, i.e. to substitute instrumentation for the physicians eye. None of these instruments was truly successful or affordable in the routine clinical setting until introduction of the so-called pulse oximeter in the early 1980's which provides arterial % O
2
Sat values in a non-invasive manner. The technique depends on measuring the color of new blood entering the field of observation with each heartbeat. Typically the finger tip, earlobe or toe is transilluminated with two wavelengths, one in the visible (VIS) and one in the near infrared (NIR) range of the spectrum. Alternatively, reflectance optics may be used as described in U.S. Pat. No. 5.692,503. The stable background light signal is subtracted from the measurement made during the pulse of new blood arriving after the heartbeat. Several U.S. patents teach the pulsatile approach by either transillumination or reflectance (see for example U.S. Pat. No. 5,337,745) for either hemoglobin determinations or concentrations of other blood components such as glucose. Thus this oxygenation measurement again reports on the % O
2
Sat (or on concentrations of other components) in the arterial blood. The measurement therefore reports only on the quality of pulmonary function in oxygenating the blood. No information is present about oxygen sufficiency in the tissue(s).
With pulse oximetry, all the drawbacks and limitations of the arterial sampling techniques are still present and in fact are exacerbated by the absence of a sample in which hemoglobin and CO
2
content and the pH can be measured. Also, the technique requires firm pulsatile flow which is often not present in seriously ill patients and is frequently not measurable in deeper tissue such as the brain.
It can thus be seen that the field of physiological monitoring of patient oxygenation status requires a fundamentally different approach that provides information on the steady state of tissue oxygenation rather than being limited to the efficacy of pulmonary function.
A further aspect of the prior art to be appreciated is the application of the so-called Beer-Lambert law for determining concentrations of light absorbing molecules by measuring circuit parameters from two conditions, for instance of the light being transmitted in the absence of these molecules and in their presence or alternatively the light intensity measured directly without passing through the test subject as compared to the light being transmitted through the test subject. Various literature sources discuss how this law is applied, one such source being U.S. Pat. No. 3,923,403 and another being the above-mentioned U.S. Pat. No. 5.692,503.
An appreciation of how various combinations of measuring and reference wavelengths have been applied in the prior art for physiological measurements is also deemed useful to an appreciation of the present invention. Typically a single “reference” wavelength is used for samples that have a tendency to scatter light. The data from a measuring wavelength is then compared to those from the reference. The former is commonly chosen to be that of the absorption peak whereas the reference is chosen at a more neut
Jobsis Frans F.
Jobsis Paul D.
Kremer Matthew
Olive & Olive P.A.
Winakur Eric F.
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