Method and apparatus for demodulating signals in a pulse oximetry system

Abstract

A method and an apparatus measure blood oxygenation in a subject. A light source is activated to cause a first emission at a first wavelength and a second emission at a second wavelength. A detector detects a composite signal indicative of an attenuation of the first and second wavelengths by tissue of a patient. The composite signal is demodulated into a first intensity signal and a second intensity signal. Blood oxygenation in the subject is determined from the first and second intensity signals.

What is claimed is: 
     
         1 . A system which measures one or more physiological characteristics of a living subject non-invasively by measuring pulsing light of two or more wavelengths after attenuation by body tissue, the system comprising:
 at least one emitter configured to emit light of at least two or more different wavelengths using a modulation scheme that cycles the at least two or more different wavelengths, the at least one emitter positioned to emit light in the direction of a living subject;   at least one detector configured to detect the light of the at least two or more different wavelengths emitted from the at least one detector after the light has been attenuated by tissue of the living subject, the detector further configured to generate a detector signal representative of the emitted light;   a demodulation system configured to demodulate the detector signal, the demodulation system configured to adjust the demodulation of the detector signal to reduce crosstalk based, at least in part, on previously demodulated signal values.   
     
     
         2 . The system of  claim 1 , the demodulation system is further configured to adjust the demodulation based on previously demodulated signal values during a period where one or more of the at least two different wavelengths is turned off. 
     
     
         3 . The system of  claim 1 , wherein the demodulation is adjusted during an initial calibration period. 
     
     
         4 . The system of  claim 3 , wherein the demodulation is adjusted during periodic calibration periods during measurement. 
     
     
         5 . The system of  claim 1 , wherein the demodulation system comprises a plurality of demodulation coefficients used to demodulate the detector signal. 
     
     
         6 . The system of  claim 5 , wherein at least one demodulation coefficient is used for each of the at least two or more different wavelengths. 
     
     
         7 . The system of  claim 1 , wherein the previously demodulated signal values comprises a plurality of demodulated signal values corresponding respectively to the at least two or more different wavelengths. 
     
     
         8 . The system of  claim 7 , wherein each of the plurality of demodulated signal values is determined using a demodulation coefficient. 
     
     
         9 . The system of  claim 1 , wherein the demodulation has been diagonalized. 
     
     
         10 . A method of demodulating a composite signal generated by applying at least first and second periodic pulses of electromagnetic energy to a system having a physiological parameter to be measured and by non-invasively receiving signals responsive to said electromagnetic energy after attenuation by body tissue of a living subject, said signals received as a composite signal having at least first and second components responsive to said at least first and second pulses respectively, said method comprising:
 receiving, at one or more signal processors, a composite signal responsive to electromagnetic energy of at least first and second periodic pulses of electromagnetic energy after attenuation by body tissue;   applying, using the one or more signal processors, a first demodulation scheme to said composite signal to generate at least a first demodulated signal; and   automatically adjusting, using the one or more signal processors, the first demodulation scheme based at least in part on the at least a first demodulated signals.   
     
     
         11 . The method of  claim 10 , wherein the adjustment reduces crosstalk. 
     
     
         12 . The method of  claim 10 , further comprising applying a second demodulation scheme to said composite signal to generate a second demodulated signal and adjusting the second demodulation scheme based at least in part on the second demodulated signal. 
     
     
         13 . The apparatus of  claim 12 , wherein said signal processor is further configured to adjust at least one of the first and second demodulation schemes in the absence of at least one of the first and second periodic pulses of electromagnetic energy. 
     
     
         14 . The apparatus of  claim 12 , wherein said signal processor is further configured to adjust at least one of the first and second demodulation schemes based on the demodulated signals in order to further reduce crosstalk interspersed with measurements. 
     
     
         15 . The method of  claim 10 , wherein adjusting occurs continuously. 
     
     
         16 . The method of  claim 10 , wherein adjusting occurs interspersed with measurements. 
     
     
         17 . A system for demodulating a composite signal generated by applying at least first and second periodic pulses of electromagnetic energy to a system having a physiological parameter to be measured and by non-invasively receiving signals responsive to said electromagnetic energy after attenuation by body tissue of a living subject, said signals received as a composite signal having at least first and second components responsive to said at least first and second pulses respectively, said method comprising:
 means for receiving a composite signal responsive to electromagnetic energy of at least first and second periodic pulses of electromagnetic energy after attenuation by body tissue;   means for applying a first demodulation scheme to said composite signal to generate at least a first demodulated signal; and   means for automatically adjusting the first demodulation scheme based at least in part on the at least a first demodulated signals.   
     
     
         18 . The method of  claim 17 , wherein the adjustment reduces crosstalk. 
     
     
         19 . The method of  claim 17 , further comprising applying a second demodulation scheme to said composite signal to generate a second demodulated signal and adjusting the second demodulation scheme based at least in part on the second demodulated signal. 
     
     
         20 . The apparatus of  claim 19 , wherein said signal processor is further configured to adjust at least one of the first and second demodulation schemes in the absence of at least one of the first and second periodic pulses of electromagnetic energy.