A noninvasive method to determine oxygen saturation of the blood. Pulse oximetry is a little-heralded advance in medicine that is probably responsble for saving countless lives. It is in fact referred to as the "fifth vital sign" along with temperature, pulse, respiratory rate, and blood pressure.

Pulse oximetry works on the principle that oxyhemoglobin--oxygenated hemoglobin, what carries oxygen from the lungs to the cells--absorbs light differently than deoxyhemoglobin--hemoglobin that has given up its oxygen. At 100% oxygen saturation, all hemoglobin is oxyhemoglobin. If saturation is less than about 98-100% (at sea level) it suggests difficulty in the cardiopulmonary system which can be from countless causes. Oxygen saturation of less than 90% indicates serious difficulty usually requiring supplemental oxygen. Rapid desaturation can indicate acute respiratory or cardiac distress. The ability to determine saturation in real time and without drawing blood means the ability to identify critical situations and take action before damage is done.

The first pulse oximeter was developed by Hewlett-Packard in 1976. The device used an sensor placed on the ear that shone 8 distinct wavelengths of light through the ear. Digital signal processing was used to determine the differential absoprtion of these wavelengths over time, filtering out the baseline absorption of the tissues, and generated a signal representing the saturation level of the blood, as well as the pulse of the patient. The oximeter was attached to a fiber optic cable going back to the unit with a filtered light source. The device was big and expensive, and the oximeter cable was big and bulky.

In the 1980s and 1990s semiconductor technology allowed for the development of two-wavelength pulse oximetry, with the two wavelengths being made by a red and infrared LED, and a photocell sensor that is usually wrapped around a finger or toe, or in the case of an infant the foot. The infrared wavelength is absorbed at a constant rate relative to hemoglobin saturation, so it is used to determine the constant absorbtion of the tissue (affected by depth, coloring, etc.) The differential analysis of the red wavelength to the infrared wavelength is the signal that is analyzed to determine oxygenation and pulse. Nowadays pulse oximetry sensors are either clip-on reusable sensors, or reusable or disposable wrap-around sensors, of various sizes and shapes for the relevant application.

Pulse oximeters themselves are as small as a fingertip device, or more usually a small computer sized device. Hospital devices are part of standard monitoring equipment that can report to a central station. Portable devices are made that can be carried with a patient, have alarms for when saturation reaches a critical level, and store historical data for analysis and download. These devices have curve-fitting data that is gathered from testing of human volunteers deliberately made to desaturate by depriving oxygen--thus, pulse oximeters are only assumed accurate above 70% saturation, as it would be medically unethical to allow someone to desaturate below that, likely causing organ or brain damage.

Pulse oximeters have about a 2 or 3 per cent accuracy in saturation measurement, although care must be taken to apply the sensor properly and assure its function. Motion of the patient can affect readings, most oximeters can detect some motion and warn that the reading may not be accurate due to motion, and the best can compensate and still provide a reading. The sensor must also not be placed on a nail painted with nail polish, particularly red nail polish, for hopefully obvious reasons.

Modern pulse oximetry means medical professionals have an easy way to get a critical piece of information about patient health that can be used to determine the need for acute care or monitor efficacy of medical treatment. Patients who use oxygen therapy can use pulse oximetry to determine the minimal effective delivery rate of oxygen to reduce complications due to overoxygenation, particularly in premature infants where overoxygenation can cause blindness.

When you hear Dr. Carter on ER say "Sat's 78" it means the patient is at 78% oxygenation, which is cause for concern.


Sources:
http://www.avweb.com/news/aeromed/181936-1.html
http://www.frca.co.uk/article.aspx?articleid=331

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