Pulse Oximetry Advantages And Disadvantages

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Pulse oximetry is based around the principle that oxygenated Haemoglobin will absorb more light of certain specific wavelengths of light than deoxygenated haemoglobin and vice versa. Two wavelengths of light are used in pulse oximetry, red light of approximately 600nm, and infrared light with wavelengths of approximately 900nm. These two wavelengths of light are used to differentiate between two different substances, each with their own absorbance characteristics.

When performing pulse oximetry there are various substance that will absorb both wavelengths of light, including the venous blood. To counter this, the readings are taken with respect to the subject’s heart beat and the resulting influx of arterial blood. The blood flow
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With positive temperature coefficient thermistors, resistance increases as the temperature increases. Thermistors measure the temperature of the sensors not the patients themselves. Thermistors require an adhesive cover to attach to skin, and can range from relatively cheap and disposable models, to reusable but more expensive models.
Infrared radiation temperature measurement systems detect the radiation emitted from the tympanic membrane and the surrounding ear canal. The detected energy is proportional to actual temperature of the subject. These sensors are non-contact and relatively sterile. These sensors are also relatively fast, with a response time of approximately 0.1 second.

The light source provides the polychromatic light, producing a variety of wavelengths of light. When a wavelength of light between 360 – 950 nm is required a Tungsten-filament lamp is used. When ultraviolet light is needed, a Deuterium lamp is used.

The filter is used to isolate the required portion of the source spectrum, and reduce and remove some unwanted frequencies.

The Diffraction Grating works separates the wavelengths of the incident light into separate monochromatic beams. On the diffraction grating there are thousands of grooves, that act to reflect polychromatic light at each of the grooves, resulting in interference and the reflected light transmitted in discrete directions.

The diverging wavelengths of light are then reflected by a mirror towards
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