A pulse oximeter measures the amount of oxygen present in the blood by registering pulsations within an arteriolar bed. It is a noninvasive method widely used in the hospital, including for newborns, patients with pulmonary disorders, and patients undergoing pulmonary and cardiac procedures. Oxygen levels can be estimated during exercise, surgery or medical procedures, or while the patient is asleep.
Oximeters are used in such hospital settings as intensive care units, pulmonary units, and in health care centers. Portable hand-held devices are available, and are used for spot-checking patients and for in-home use with a doctor's supervision.
The oximeter consists of a light-emitting diode (LED); a photodetector probe containing a permanent or disposable sensor; alarms for pulse rate and oxygen levels; a display screen; and cables. The device works by emitting beams of red and infrared light that are passed through a pulsating arteriolar bed. Sensors detect the amount of light absorbed by oxyhemoglobin and deoxy-hemoglobin in the red blood cells. The ratio of red to infrared light measured by the photodetector indicates the amount of oxygen present in the blood. The sensor is attached to the body over the arteriolar area in the ear, the finger tip, the big toe, or across the bridge of the nose. Clip sensors can be used on fingers and the ear-lobe.
Several steps can be taken to enhance accurate readings. If possible, the patient should be instructed not to smoke 24 hours prior to pulse oximetry. Fingernail polish should be removed if the oximeter will be attached to the finger. For patients with poor circulation, hands should be slowly warmed with warm towels before attaching the oximeter. Abnormally high or low temperatures, as well as reduced hemoglobin, can influence the amount of oxygen adhering to the hemoglobin within the red blood cells, altering the reading.
Care should be taken with attaching the sensors and selecting the site for optimum reading levels. The sensor should be wrapped securely around the finger to prevent outside light from interfering with the reading and rendering it invalid. An appropriate site is chosen to monitor the oxygen levels by ensuring that there is strong arterial pulsation, and that the capillary bed fills promptly if squeezed.
The device must not be used near flammable anesthetics.
Older devices may be affected by motion. They should be checked regularly to insure proper function.
Health care team roles
Nurses and allied health professionals attach the pulse oximeter and explain to the patient that it is used for monitoring purposes. Staff monitor the site where the sensor has been applied every four hours for clip sensors and every six hours for wrapped sensors. Any loss of pulsation, swelling, or change in color requires a change of site.
Staff should be familiar with the device and the department's protocol to ensure standardization in operating the equipment.
Arteriolar bed—An area in which arterioles cluster between arteries and capillaries.
Arterioles—The smallest branches of arteries.
Capillaries—Tiny blood vessels with a diameter of a red blood cell through which a single layer of cells flows.
Deoxyhemoglobin—Hemoglobin with oxygen removed.
Oxyhemoglobin—Hemoglobin combined with oxygen.
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Margaret A. Stockley, RGN