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Over the past decade, much effort has been directed at extending the success of oxygen saturation in pulse oximetry (1)(2)(3)(4) to the noninvasive determination of other blood analyte concentrations (5). The tissue transmission window nestled between the strong absorption of hemoglobin in the visible and the high absorption of water in the mid-infrared is a potentially useful wavelength range because of increased tissue penetration. This work reports initial results from a new apparatus that directly compares light transmission through a finger at wavelengths in the pulse oximetry and transmission window regions. Wavelength-dependent changes as a function of applied finger pressure are documented, posing a challenge for instrumental stability and design.The photoplethysmographic method isolates arterial blood effects by analyzing small waveform excursions induced by incremental blood flow in and out of monitored tissues during each heartbeat. Each signal is separated on a pulse-by-pulse basis into a steady or direct current (DC) portion and a pulsatile or alternating current (AC) portion (1). For a signal S , the intent is that this AC/DC fraction, or modulation fraction, approximate d S / S , which is proportional to the absorbance. The ratio of modulation fractions at two wavelengths then approximates the ratio of absorbances at those wavelengths. In pulse oximetry, the saturation is determined from a nonlinear calibration curve of oxygen saturation plotted against this ratio of modulation fractions (1).The present apparatus departs from pulse oximetry practice in using fixed optics that largely eliminate ambient light interference and motion-induced probe-coupling artifacts (4). The hand is placed palm down on a flat surface beneath a light shield, with the examined finger pressed against a 15-mm diameter optical window that is illuminated from below by multiple light-emitting diodes (LEDs) located azimuthally around a 14° half-angle cone. …

Lateral Tissue Inhomogeneity: A Missing Link in Photoplethysmographic Noninvasive Measurement of Arterial Blood Constituents