Method and apparatus for noninvasively determining hematocrit

Abstract

A method and apparatus for noninvasively determining hematocrit utilizing the frequency dependent electrical impedance characteristics of whole blood by electrically stimulating a patient body portion containing a pulsatile vascular compartment with a constant current source at two selected frequencies, one of which is high enough that the capacitive nature of erythrocytes will significantly affect the magnitude of the impedance of the blood. Sensed voltage across the stimulated body portion at the two frequencies is representative of the corresponding magnitudes, and the method further includes detecting the signal envelopes of the sensed voltages, isolating and converting the pulsatile components of the signal envelopes to the digital domain, normalizing each isolated converted pulsatile component against the voltage baseline of its carrier waveform and creating a ratio of the normalized isolated pulsatile components representative of the patient's hematocrit. The ratio is indexed to a look-up table constructed from clinical studies correlating such ratios at the two selected frequencies to hematocrit, and the hematocrit corresponding to the created ratio is displayed. A plurality of ratios of sets of time-matched pulsatile component segments are created and averaged using a weighted averaging technique which more heavily weights ratios created from more significant segment sets, the weighted ratio average being correlated to hematocrit via the look-up table.