Motion compatible sensor for non-invasive optical blood analysis
A non-invasive optical sensor which uses the motion signal to calculate the physiological characteristic being measured. For pulse oximetry, a least squares or a ratio-of-ratios technique can be applied to the motion signal itself. This is made possible by selecting a site on the patient where variations in motion produce signals of two wavelengths which are sufficiently correlated. In particular, it has been determined that a sensor placed on a nail, in particular a thumbnail, exhibits the characteristics of having the red and infrared signals correlated when used for pulse oximetry, and the resulting signals correlate to arterial oxygen saturation.
1. A photometric processing device for processing detector signals from a radiation detector in a patient sensor also having a radiation emitter, comprising:
a first processing unit configured to determine a blood parameter from a cardiac signal derived plethysmogram from the detector;
a second processing unit configured to determine the blood parameter from a motion artifact waveform from the detector; and
a control unit configured to utilize the first and second processing units responsive to a motion artifact content of the detector signals.
2. The photometric processing device of claim 1 further comprising a processor and a memory, wherein the first and second processing units and the control unit are first, second and third programs stored in the memory.
3. The photometric processing device of claim 1 further comprising a selector configured to select between a reflectance mode and a transmittance mode of the sensor.
4. The photometric processing device of claim 3 wherein the control unit is further configured to switch between the first and second processing units in accordance with a selection of the selector.
5. The photometric processing device of claim 4 wherein the selector is responsive to the motion artifact content of the detector signals.
6. The photometric processing device of claim 1 wherein the radiation emitter emits and the radiation detector detects radiation at two or more wavelengths, wherein the detector signals at the two or more wavelengths are correlated in the presence of motion.
7. The photometric processing device of claim 6 wherein the detector signals produce a closed Lissajous.
8. The photometric processing device of claim 1 wherein the detector signals are generated in response to at least two wavelengths and are correlated in the presence of non-cardiac pulses.
9. The photometric processing device of claim 8 wherein the detector signals include predominately motion-induced variations and the sensor is configured to be placed on a nail on a digit.