the effects of other tissues are corrected for, generating a continuous signal for pulsatile arterial blood. The ratio of the red light measurement to the infrared light measurement is then calculated by the processor (which represents the ratio of oxygenated hemoglobin to deoxygenated hemoglobin), and this ratio is then converted to SpO2 by the processor via a lookup table based on the Beer–Lambert law. The signal separation also serves other purposes: a plethysmograph waveform ("pleth wave") representing the pulsatile signal is usually displayed for a visual indication of the pulses as well as signal quality, and a numeric ratio between the pulsatile and baseline absorbance ("perfusion index") can be used to evaluate perfusion. where HbO2 is oxygenated hemoglobin (oxyhemoglobin) and Hb is deoxygenated hemoglobin. Derived measurements Due to changes in blood volumes in the skin, a plethysmographic variation can be seen in the light signal received (transmittance) by the sensor on an oximeter. The variation can be described as a periodic function, which in turn can be split into a DC component (the peak value) and an AC component (peak minus trough). The ratio of the AC component to the DC component, expressed as a percentage, is known as the (peripheral) perfusion index (Pi) for a pulse, and typically has a range of 0.02% to 20%. An earlier measurement called the pulse oximetry plethysmographic (POP) only measures the "AC" component, and is derived manually from monitor pixels. Pleth variability index (PVI) is a measure of the variability of the perfusion index, which occurs during breathing cycles. Mathematically it is calculated as , where the maximum and minimum Pi values are from one or many breathing cycles. It has been shown to be a useful, noninvasive indicator of continuous fluid responsiveness for patients undergoing fluid management. Pulse oximetry plethysmographic waveform amplitude (ΔPOP) is an analogous earlier technique for use on the manually-derived POP, calculated as . History In 1935, German physician Karl Matthes (1905–1962) developed the first two-wavelength ear O2 saturation meter with red and green filters (later red and infrared filters). It was the first device to measure O2 saturation. The original oximeter was made by Glenn Allan Millikan in the 1940s. In 1943 and as published in 1949, Earl Wood added a pressure capsule to squeeze blood out of the ear so as to obtain an absolute O2 saturation value when blood was readmitted. The concept is similar to