About Pulse Oximeters

Principles of Pulse Oximetry Technology

Pulse oximetry is a way to obtain a person's oxygen content that is carried by hemoglobin.

Oxygen saturation in the blood is obtained by measuring light absorption through a translucent site with good blood flow. Normal sites are the fingers, toes, earlobe for adults and foot, palm, big toe, or thumb for infants. Wrist pulse oximeters emit red and infrared light and that light travels through oxygenated and deoxygenated hemoglobin differently. The difference in how the light travels through the bloodstream is due to the difference in color between oxygenated (bright red) and deoxygenated blood (dark red or blue).

On the opposite side of the light, there is a photodetector that measures the light that passes through the bloodstream. When hemoglobin is oxygenated, it absorbs more infrared light and allows more red light to pass through. When blood has a lower oxygen content it absorbs more red light and allows more infrared light to pass through.

Most pulse oximeters send the light by the transmission method. This simply means that the light passes through the bloodstream to the other side. In most cases the light source is passes from one side of a finger to the other. A smaller number of oximeters use reflectance technology. In this type of oximeter the light source and detector are on the same side and the light source bounces from the emitter to the detector across the site.

When the red and infrared lights pass through the bloodstream, the photodetector calculates the red/infrared ratio. This ratio is then converted to the SPO2 percentage. A red/infrared ratio of 0.5 equals approximately 100%, a red/infrared ratio of 1.0 equals approximately 82%, and a ratio of 2.0 equals approximately 0%.

During each heartbeat, there is a surge of arterial blood. During the surge, there is more light absorption. When the heart relaxes, there is less blood flow and there is less light absorption. If the light absorption during the heart relaxing is subtracted from the light absorption at the heart contraction, the overnight pulse oximeter will measure only the arterial blood. Since oximeters measure oxygen levels with each heartbeat the name "Pulse Oximetry" is fitting.

Pulse oximeters have always been unreliable during motion, vasoconstriction, cold, calloused skin, low perfusion, carbon monoxide poisoning, methemoglobinemia, or cyanide poisoning. Overnight pulse oximeters are used to test oximetry levels while the subject sleeps. New technology is improving these readings because they can measure perfusion and strength of the pulse waveform. Overnight pulse oximetry is essential in monitoring oxygenation but it cannot determine the amount of oxygen the patient is using or ventilation. In certain situations it is necessary to measure carbon dioxide levels, pH in the blood, and bicarbonate (HCO3) in addition to oxygen levels by means of capnography (exhaled CO2) or by drawing blood gases. Pulse oximetry only measures oxygen being carried by the hemoglobin. In cases of severe anemia, blood carries less oxygen although the hemoglobin is saturated giving false oximetry readings.

 

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