A probe is placed on the finger, toe, ear lobe or nose. Two light-emitting diodes produce beams at red and infrared frequencies (660 nm and 940 nm, respectively). There is a photo detector on the other side. The diodes flash at approximately 30 times per second. The diodes are switched on in sequence, with a pause with both diodes off. This allows compensation for ambient light. The microprocessor analyses the changes in light absorption during the arterial pulsatile flow and ignores the non-pulsatile component of the signal (which results from the tissues and venous blood).
The oxygen saturation is estimated by measuring the transmission of light through the pulsatile tissue bed. This is based on the Beer-Lambert law:
This is a combination of two laws describing absorption of monochromatic light by a transparent substance through which it passes:
Beer’s law: the intensity of transmitted light decreases exponentially as the concentration of the substance increases. August Beer, German Physicist (1825-1863)
Beer's Law is given by:
A is the absorbance - how much light is absorbed while passing through the filter
I is the intensity of light transmitted
Io is the original intensity of light before passing through the filter
Lambert’s law: the intensity of transmitted light decreases exponentially as the distance travelled through the substance increases. Johann Lambert, German Physicist (1728-1777).
The light absorbed by non-pulsatile tissues is constant (DC). The non-constant absorption (AC) is the result of pulsatile blood pulsations. The photo detector generates a voltage proportional to the transmitted light. The AC component of the wave accounts for between 1-5% of the total signal. The high frequency of the diodes allows the absorption to be calculated many times per second. This reduces movement effects on the signal.
The microprocessor analyses both the DC and AC components at 660 nm and 940 nm. The absorption of oxyhaemoglobin and deoxyhaemoglobin at these two wavelengths is very different. Hence, these two wavelengths provide good sensitivity.
This is the point at which two substances absorb a certain wavelength of light to the same extent. In oximetry, the isobestic points of oxyhaemoglobin and deoxyhaemoglobin occur at 590 nm and 805 nm. These points may be used as reference points where light absorption is independent of the degree of saturation. Some earlier oximeters corrected for haemoglobin concentration using the wavelength at the isobestic points.
Thus comparison of absorbencies at different wavelengths allows estimation of the relative concentrations of HbO and Hb (i.e. saturation). Modern pulse oximeters may use two or more wavelengths, not necessarily including an isobestic point.