Patent ID: 7343186

Claim:
A physiological monitor for determining blood oxygen saturation of a medical patient, comprising: a sensor capable of removable attachment to tissue, wherein said sensor comprises: at least three light emitting diodes, each light emitting diode being proximate said tissue when said sensor is attached to said tissue and adapted to emit light of a different wavelength; and a detector, wherein said detector is adapted to receive light from said at least three light emitting diodes after being attenuated by said tissue, and wherein said detector generates an output signal based at least in part upon said received light; and a signal processor configured to: calculate a ratio (r) based at least in part on at least three normalized photoplethysmographic waveforms derived from said output signal, said ratio represented as: r = ∑ i = 1 n ⁢ α i ⁢ NP RMS , i ∑ i = 1 n ⁢ β i ⁢ NP RMS , i wherein n represents a number of wavelengths of light corresponding to the number of the at least three light emitting diodes, NP RMS,i represents an i th waveform of said at least three normalized plethysmographic waveforms, and α i and β i represent vector coefficients, said ratio configured to have reduced tolerance at lower blood oxygen saturation levels compared to a two wavelength blood oxygen saturation system; generate at least one reference signal based upon said ratio and said at least three normalized photoplethysmographic waveforms; select a data signal corresponding to attenuated light from one of said at least three light emitting diodes; and provide a process estimator output using a multi-variate process estimator based at least in part upon the at least one reference signal and the selected data signal, such that a blood oxygen saturation of a patient may be calculated in response to the process estimator output, wherein said blood oxygen saturation exhibits higher accuracy at lower blood oxygen saturation levels compared to the two wavelength blood oxygen saturation system.