Pulse oximetry is based on the principle that oxygen is carried in the bloodstream, bound primarily to hemoglobin, according to E. Hill and Stoneham, M.D. One molecule of hemoglobin can carry up to four molecules of oxygen – a 100 percent saturation level. Light is absorbed at two different wavelengths by the hemoglobin. The absorption rate differs, depending on the degree of oxygenation. As the light passes through tissues, it has a pulsatile component. The oximeter measures:
- The oxygen saturation of hemoglobin in arterial blood.
- The pulse rate in beats per minute.
Why Is a Pulse Oximeter Used?
Pulse oximeters provide a rapid indication of a patient’s changing level of oxygenation. Since the level of oxygenation is continuously being monitored, pulse oximeters allow for clinical intervention before significant hypoxia, or low levels of blood oxygen, occurs. This is very important in situations where general anesthesia is being administered, since an anesthetic can deprive the body of the oxygen it needs. That is why many states require the use of a pulse oximeter while administering anesthesia.
A pulse oximeter can also be used to make sure Medicare patients, before beginning home oxygen therapy, meet certain clinical criteria. In addition, they are often used to monitor patients with chronic obstructive pulmonary disease (COPD) and asthma.
How it Works:
A pulse oximeter employs a peripheral probe and a microprocessor unit, which displays a waveform, the oxygen saturation and the pulse rate. Most units have an audible pulse tone. The pitch of the tone is proportional to the oxygen saturation level. The clinician or doctor places the probe on the patient’s finger, ear lobe or nose.
The probes have two light-emitting diodes – one in the visible red spectrum (660 nm) and the other in the infrared spectrum (940 nm) – that pass through the body tissues to a photodetector. As the light beams pass through the tissues, light is absorbed by blood and soft tissue. The amount of light absorbed depends on the degree of oxygenation and hemoglobin within the tissues. Hill and Stoneham offer the following tips for optimal use of pulse oximetry:
- Plug the oximeter into an electrical socket to recharge the batteries.
- Turn on the unit and wait for it to go through calibration and check tests.
- Select the correct probe with regard to size and use.
- Avoid excess force when positioning the probe.
- Allow several seconds for the pulse oximeter to detect the pulse and calculate the oxygen saturation.
- Read off the displayed oxygen saturation and pulse rate.
Advances in Technology:
In the past four years, manufacturers have made strides in oximetry technology, according to a recent report by Anesthesi-ology News. For instance, Nellcor Puritan Bennett of Pleasanton, Calif., now makes a unit reported to provide accurate readings for adults and neonates during motion. Other models are designed to determine signals that could be mistaken for a pulse.
Another example is a model by Masimo Corp. of Irvine, Calif., which is designed to process the red and infrared signals. The company’s Signal Extraction Technology (SET) employs digital signal processing, low-noise hardware and sensor technology designed to decrease the impact of ambient noise and non-arterial physiologic noise produced during motion.
Today, clinicians have more choices when it comes to pulse oximetry. An informed sales rep who understands the different technologies available can help his or her customers get the most accurate results, stay in compliance and increase the quality of care for the patient.