Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2002-05-28
2004-03-23
Hindenburg, Max F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S323000, C600S338000
Reexamination Certificate
active
06711425
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates generally to pulse oximetry devices and methods. More particularly, the invention is concerned with an improved pulse oximetry system that corrects calibration shifts due to changes in monitoring site characteristics, particularly variations in blood fraction using calibration stabilization. As a result of the calibration stabilization, oxygen saturation monitoring accuracy and availability are improved. The invention is of particular value when applied in fetal pulse oximetry.
Pulse oximeters are employed in patient clinical practice as well as veterinary practice for assessing the level of oxygenation in the blood of a subject, and are well known in the art. Typically, such devices comprise a sensor with light emitting device(s) (emitter(s)) and associated photodetector(s) (detector(s)), attached to a monitoring device performing signal acquisition, analysis, and display/print and/or functions. One particular example of a pulse oximeter is described in U.S. Pat. No. 6,163,715 B1, issued to Larsen et al., which disclosure is incorporated by reference herein.
The signals (oximetry signals) derived from a pulse oximetry sensor are inversely proportional to the net absorption by nearby tissues of the particular wavelengths of light emitted by the sensor. The net absorption at different wavelengths is dependent upon tissue site characteristics, and includes absorption by skin pigmentation, bloodless components such as bone, non-pulsating blood, especially venous, and pulsating blood, predominantly arterial. The signal corresponding to detected light of a given wavelength is thus composed of a baseline or DC component due to non-pulsating absorption, and a pulsating or AC component related primarily to absorption by arterial blood. It is important to note that the light intensity of a given wavelength reaching the detector, and the path the light takes to get there, are determined not only by absorption properties but also by the scattering of light in tissue.
Pulse oximetry sensors typically operate in transmissive mode, wherein light from the emitters passes through one side of a vascularized tissue to reach a detector(s) on the other side of the tissue. This mode is commonly used in neonatal and adult monitoring on fingertips, earlobes, and so forth. Alternatively, the emitters and detector(s) may be placed near each other in a co-planar fashion on the same tissue surface, forming a single active site on the sensor. The light emitted by the sensor enters the tissue proximal to it and, by backscattering, returns in part to the same tissue surface and to the detector(s) of the sensor. Thus an oximetry sensor can operate in a backscattering mode, also known as reflectance mode. The oximetry signals acquired by backscattering are of lower intensity than those obtained in transmissive mode, making this method more susceptible to interference from various sources.
Pulsations occurring in synchrony with the heart rate are apparent in the oximetry signals. These pulsations result from the increased absorption of light occurring during passage of blood through the arterial system. Because the arterial pulsation is the result of systole in the heart, this rapid increase in absorption (decrease in detected light intensity) is referred to herein as the systolic phase of the signal. The period between systolic phases, characterized by a more gradual decrease in absorption, is herein referred to as the diastolic phase. The high pass filtered oximetry signal is commonly inverted when displayed as a photoplethysmographic waveform (i.e., rising with increasing absorption), emphasizing the similarity to an arterial pressure waveform.
The bulk of oxygen transport in the blood takes place bound to the hemoglobin molecule. The oxygenated (HbO
2
) and reduced (Hb) forms of hemoglobin have different optical extinction (absorption) curves, but blood's scattering of light is relatively insensitive to oxygen saturation. By choosing appropriate wavelengths of light, the plurality of oximetry signals can be interpreted to yield the percentage of saturation of the hemoglobin molecules with oxygen (SpO
2
). In the prior art, red and infrared pulsatile amplitudes, scaled by their respective baseline or DC light intensities, are combined in a ratiometric equation based upon the Beer-Lambert model of light absorption by media to yield a ratio R related to SpO
2
. Most commonly, the red wavelength is nominally around 660 nm, and the infrared wavelength is in the range of 880-940 nm.
The relationship of the ratio R to SpO
2
predicted by the Beer-Lambert model is actually a poor fit to empirical data. Therefore, the relationship SpO
2
=ƒ (R) is typically established by calibration of the oximetry system against a standard measurement of arterial oxygen saturation (SaO
2
). The subjects used to perform such calibration must be hypoxic to some degree, either due to a clinical condition or a laboratory procedure, in order to establish SpO
2
accuracy below the typical range of a subject's oxygen saturation. Calibration accommodates the tissue and sensor characteristics, by and large correcting the simplifications resulting from the underlying assumption of a Beer-Lambert model, which disregards the scattering of light by blood and tissue.
One application of this invention is in utero fetal pulse oximetry. The fetal oxygen sensor is inserted in or near the uterus of a mother to noninvasively monitor the condition of a fetus. One particular example of a sensor designed for fetal pulse oximetry is described in U.S. Pat. No. 5,425,362, to Siker et al. incorporated by reference herein. The sensor placement is made through the birth canal to reach a monitoring position on the fetus. This process and its outcome are difficult to satisfactorily visualize, even utilizing intrauterine imaging technologies, such as ultrasound. The fetal oxygen sensor operates in reflectance mode, a method that typically results in lower signal amplitudes, and may be subject to “light shunting”, in which emitted light returns to the detector without traversing the vascularized tissue bed. Thus, fetal pulse oximetry represents a challenging scenario for signal acquisition in medical monitoring.
Even with empirical calibration, oximeter performance differs from the Beer-Lambert prediction when the characteristics of the tissue at the monitoring site vary from the characteristics at the time of calibration. Edema, or the presence of significant extracellular fluid, can result in lowered oxygen saturation readings in neonates, as described by Johnson et al., “The effect of caput seccedaneum on oxygen saturation measurements”. Br. J. Obs. & Gyn. 1990; 97: 493-498. Inaccuracies are particularly evident in conventional pulse oximeters required to operate in low oxygen saturation ranges, e.g., below 75%. In explaining the effects of edema in clinical monitoring of neonates, Johnson et al. (1990) supra also cited changes in photon path length due to increased red absorption as the explanation for lowered saturations. Severinghaus et al., “Effect of anemia on pulse oximeter accuracy at low saturations”. J. Clin Mon. (1990); 6: 85-88, reported that pulse oximetry underestimated the oxygen saturation of anemic patients.
The purpose of fetal pulse oximetry is to reduce the likelihood of fetal morbidity or mortality related to hypoxia, apparent as acidosis at birth. The typical oxygen saturation level to be monitored in the fetus is below 70%, and thus calibration deviation is a concern. The term “calibration deviation” refers to inaccuracies in oxygen saturation determination by a pulse oximeter specifically due to a change in the calibration, or relationship between a ratio of normalized pulse amplitudes, R, and the SpO
2
. Furthermore, Seelbach-Gobel et al., “The prediction of fetal acidosis by means of intrapartum fetal pulse oximetry”. Am. J. Obs. & Gyn. 1999, 180(1): 73-81, present evidence that the level of fetal oxygen saturation correlating to an
Kremer Matthew
OB Scientific, Inc.
Reinhart Boerner Van Duren s.c.
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