Pulse Oximeters

2 January, 2011 | Anesthesiology Equipment, Pulse Oximeters

Pulse Oximeter

Pulse oxymetry is a non-invasive technique of measuring oxygen saturation in arterial blood. (SpO2).
This process is based on two factors:

  • A pulsatile signal is generated by the heart in arterial blood, which is not present in venous blood and other tissues.
  • Oxyhemoglobin and reduced hemoglobin have different absorption spectra. Also, it is important to note that both spectra are within the optical window of water (and the soft tissue).

Pulse oximeters measure oxygen saturation by means of a sensor attached to the patient’s finger, toe, nose, earlobe or forehead. Typically, the sensor uses two light-emitting diodes (LEDs) at wavelengths of 660nm and 940 nm (infrared) and a photodetector placed opposite them. The photodetector measure the amount of red and infrared light that passes through the tissue to determine the quantity of light absorbed by the oxyhemoglobin and hemoglobin. As the proportion of oxyhemoglobin increases in the blood, the absorbance of the red wavelength decreases, while the absorption of infrared increases. SpO2 is determined by calculating the ratio of red-to-infrared light absorbencies and comparing it with values in a look-up table or calibration curve, which is a standardized curve developed empirically by simultaneous measurement of SaO2 and light absorbencies.

SpO2 is physiologically related to arterial oxygen tension (PaO2) according to the O2Hb dissociation curve. Because the O2Hb dissociation curve has a sigmoid shape, oximetry is relatively insensitive in the detection of developing hypoxemia in patients with high baseline PaO2.

SpO2 measurements made by a pulse oximeter are defined as being accurate if the root-mean-square (RMS) difference is less than or equal to 4.0% SpO2 over the arterial oxygen saturation (SaO2) range of 70% to 100%, SpO2 accuracy should be determined by clinical study of healthy or sick subjects, whereby SpO2 measurements are compared with SaO2 measurements.

Pulse oximeters can also measure pulse rate. The standard states that pulse rate accuracy should be defined as the RMS difference between paired pulse rate data recorded with the pulse oximeter and a reference method.

There are several limitations of pulse oximetry:

  • skin pigmentation
  • ambient light
  • intravenous dyes
  • low perfusion
  • motion artifact

As pulse oximetry technology has advanced, manufacturers have attempted to reduce the effect of some of the limitations mentioned above. Particular improvements have been made in the ability of oximeters to deal with low signal-to-noise conditions observed during periods of motion or low perfusion.

Regular functional checks should be carried out on equipment to ensure it is safe to use. This should include visual checks, e.g. checking for signs of damage. Functionality of an oximeter can be checked using a pulse oximeter tester or simulator. These simulate the properties of a finger and its pulsatile blood flow. Their purpose is allowing to test a pulse oximeter and the continuity of probes. They cannot be used to validate the accuracy of a pulse oximeter.