Oximetry is based on the principle that the color of blood is related to the oxygen saturation level (SaO2) in the blood. For example, as blood deoxygenates, skin loses its pinkish appearance and takes on more of a bluish tint. Current pulse oximeters operate by applying at least one wavelength of light to the patient and measuring the intensity of the light passing therethrough. The pulse oximetry oxygen saturation level (SpO2) is derived from a ratio of relative light intensities. Light absorption through tissue is generally constant for a particular subject, with the exception of the arterial blood, which causes the light absorption to vary with the flow of blood. Thus, the absorption of light through tissue has a pulsatile (AC) component and a constant (DC) component. Because the pulsing is only a function of the fluctuating volume of arterial blood, the AC light intensity level represents the absorption of only the O2Hb and RHb molecules. By measuring only the pulsatile light, pulse oximetry effectively ignores the absorbencies of other tissue material positioned between light source and light detector.
To identify the oxygen saturation level, two wavelengths of light are typically used with different absorption curves such that the ratio of the two absorptions is unique from 0% saturation through 100% saturation. SpO2 can be derived by positioning the tissue between a light source and a detector, passing a light of each of two wavelengths through the tissue, measuring the pulsatile light intensity from each wavelength, determining the ratio of the light intensities, and correlating the ratio to a unique position along a combined absorption curve for the two wavelengths.
To determine a ratio of pulsatile light intensities, the constant component of the light intensity must be factored out. The amplitudes of both the AC and DC components are dependent on the incident light intensity. Dividing the AC level by the DC level gives a corrected AC level that is no longer a function of the incident light intensity. Thus, the ratio R=(AC1/DC1)/(AC2/DC2) is an indicator of arterial SaO2. Conventionally, an empirically derived calibration curve for the relationship between the above ratio and SaO2 provides the pulse oximetry oxygen saturation level SpO2.
Pulse oximeters have gained rapid acceptance in a number of medical applications. Because the light source and detector can be applied to the outside of tissue area, such as to an ear lobe or finger tip, pulse oximeters are a highly non-invasive source of diagnostic information. Pulse oximeters are utilized, for example, in the operating room by anesthesiologists to monitor oxygen saturation levels. Pulse oximeters are also used in doctors' offices to monitor and diagnose respiratory problems such as sleep apnea. More recently the usefulness of pulse oximeters in fetal monitoring has gained considerable attention.
During labor and delivery, it is desirable to know the oxygen saturation level in the baby as a predictor of when emergency procedures, such as cesarean section, might be necessary. However, current pulse oximeters have generally failed to obtain clinical acceptance as a primary indicator of fetal oxygen saturation level. One possible reason is that the non-invasive nature of most pulse oximeters leaves them susceptible to motion artifact problems on the part of the mother and the unborn baby. For example, in a conventional pulse oximeter, a single probe, i.e., containing both the light source and detector is typically positioned against the baby's scalp or cheek and held in place by the pressure of the mother's uterus on the probe. During contractions, the probe could be dislodged or the variation in pressure of the contraction itself might alter the optical path and upset the reading. Obviously, such a probe would become dislodged during the final stages of delivery. A result is that the usable signal time from a conventional pulse oximeter can range anywhere from 20 to 80 percent of the total time the pulse oximeter is monitoring the subject.
Other concerns with pulse oximeters for monitoring fetal oxygen saturation level are poor signal quality because of poor probe contacts, merconium staining (fetal bowel movement), vernix (a cheesy fetal skin covering), hair, and caput formation (swelling in the scalp). Furthermore, certain assumptions are made regarding the consistency of light absorption through tissue. For example, among other things, a variation typically occurs between SpO2 and SaO2, thereby degrading the accuracy of the oximeter.
In addition, the use of pulse oximeters to monitor fetal oxygen saturation level presents other difficulties. For example, the caregiver placing the oximetry sensor on the unborn baby must do so while the baby remains in the mother's womb, thereby limiting the caregiver's view and maneuverability in placing the sensor on the baby. Also, the sensor must remain in place during labor. Furthermore, the sensor provided on the unborn baby can only be used once. Therefore, the cost of the sensor should be kept at a minimum.
Many of the above problems related to the optical measurement of oxygen saturation are also present in other optical measurement applications, such as optical measurement of blood glucose, bilirubin, and hemoglobin.