Pulse Oximetry
What is pulse oximetry?
[According to Wikipedia...]
Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.
A sensor is placed on a thin part of the patient's anatomy, usually a fingertip ... and a light containing both red and infrared wavelengths is passed from one side to the other. Changing absorbance of each of the two wavelengths is measured, allowing determination of the absorbances due to the pulsing arteria blood alone... Based upon the ratio of changing absorbance of the red and infrared light caused by the difference in color between oxygen-bound (bright red) and oxygen unbound (dark red or blue, in severe cases) blood hemoglobin, a measure of oxygenation (the per cent of hemoglobin molecules bound with oxygen molecules) can be made.
[Pulseox.info adds...]
Oxygen is carried in the blood attached to haemoglobin molecules. Oxygen saturation is a measure of how much oxygen the blood is carrying as a percentage of the maximum it could carry.
One haemoglobin molecule can carry a maximum of four molecules of oxygen, if a haemoglobin molecule is carrying three molecules of oxygen then it is carrying 3/4 or 75% of the maximum amount of oxygen it could carry.
One hundred haemoglobin molecules could together carry a maximum of 400 (100 x 4) oxygen molecules, if these 100 haemoglobin molecules were carrying 380 oxygen molecules they would be carrying (380 / 400) x 100 = 95% of the maximum number of oxygen molecules that could carry and so together would be 95% saturated.
Oxygen saturation is also refered to as SpO2.
Limitations include: Poor signal, Carbon dioxide, Delays, Carbon monoxide, Methaemoglobin, Skin pigmentation, and Other issues.
[Further information regarding accuracy form NIH.GOV...]
The accuracy of commercially available oximeters differ widely,probably because of the different algorithms employed in signal processing[8]. These algorithms are limited by the range ofsaturations that can be safely obtained in volunteers, and also the accuracy ofthe measurement standard (CO-oximeter) [11]. Comparisonof pulse oximetry with direct CO-oximeter measurements should be reported interms of the mean difference between the two techniques (bias) and the standarddeviation of the differences (precision).
In healthy volunteers, oximeters commonly have a mean difference(bias) of < 2% and a standard deviation (precision) of < 3% whenSaO2 is 90% or above. Comparable results have also been obtained in critically ill patients with good arterial perfusion. Accuracy of pulseoximetersdeteriorates when SaO2 falls to 80% or less.
[On Methods of Light emitting in oximeters via OXIMETER.ORG...]
There are two methods of sending light through the measuring site: transmission and reflectance. In the transmission method, as shown in the figure on the previous page, the emitter and photodetector are opposite of each other with the measuring site in-between. The light can then pass through the site. In the reflectance method, the emitter and photodetector are next to each other on top the measuring site. The light bounces from the emitter to the detector across the site. The transmission method is the most common type used and for this discussion the transmission method will be implied.
[When to Use a pulse oximeter by Wikipedia...]
Pulse oximetry data is necessary whenever a patient's oxygenation is unstable, including intensive care, critical care, and emergency department areas of a hospital. Data can also be obtained from pilots in unpressurized aircraft,[2] and for assessment of any patient's oxygenation in primary care. A patient's need for oxygen is the most essential element to life; no human life thrives in the absence of oxygen (cellular or gross). Although pulse oximetry is used to monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount of oxygen being used by a patient. For this purpose, it is necessary to also measure carbon dioxide (CO2) levels. It is possible that it can also be used to detect abnormalities in ventilation. However, the use of pulse oximetry to detect hypoventilation is impaired with the use of supplemental oxygen, as it is only when patients breathe room air that abnormalities in respiratory function can be detected reliably with its use. Therefore, the routine administration of supplemental oxygen may be unwarranted if the patient is able to maintain adequate oxygenation in room air, since it can result in hypoventilation going undetected.