A finger mounted pulse oximeter taking measurement through the fingernail.

A wrist mounted remote sensor pulse oximeter with plethysmogram.

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 body, usually a fingertip or earlobe, or in the case of an infant, across a foot. Light of two different wavelengths is passed through the patient to a photodetector. The changing absorbance at each of the wavelengths is measured, allowing determination of the absorbances due to the pulsing arterial blood alone, excluding venous blood, skin, bone, muscle, fat, and (in most cases) nail polish.^[1] With NIRS it is possible to measure both oxygenated and deoxygenated hemoglobin on a peripheral scale (possible on both brain and muscle).

Reflectance pulse oximetry may be used as an alternative to transmissive pulse oximetery described above. This method does not require a thin section of the patient's body and is therefore well suited to more universal application such as the feet, forehead and chest.

History[link]

In 1935 Karl Matthes developed the first 2-wavelength ear O₂ saturation meter with red and green filters, later switched to red and infrared filters. This was the first device to measure O₂ saturation.^[2]

The original oximeter was made by Glenn Allan Millikan in the 1940s.^[3] In 1949 Wood added a pressure capsule to squeeze blood out of ear to obtain zero setting in an effort to obtain absolute O₂ saturation value when blood was readmitted. The concept is similar to today's conventional pulse oximetry but was hard to implement because of unstable photocells and light sources. This method is not used clinically. In 1964 Shaw assembled the first absolute reading ear oximeter by using eight wavelengths of light. Commercialized by Hewlett-Packard, its use was limited to pulmonary functions and sleep laboratories due to cost and size.^{[citation needed]}

Pulse oximetry was developed in 1972, by Takuo Aoyagi and Michio Kishi, bioengineers, at Nihon Kohden using the ratio of red to infrared light absorption of pulsating components at the measuring site. Susumu Nakajima, a surgeon, and his associates first tested the device in patients, reporting it in 1975.^[4] It was commercialized by Biox in 1981 and Nellcor in 1983. Biox was founded in 1979, and introduced the first pulse oximeter to commercial distribution in 1981. Biox initially focused on respiratory care, but when the company discovered that their pulse oximeters were being used in operating rooms to monitor oxygen levels, Biox expanded its marketing resources to focus on operating rooms in late 1982. A competitor, Nellcor (now part of Covidien, Ltd.), began to compete with Biox for the U.S. operating room market in 1983. Prior to its introduction, a patient's oxygenation could only be determined by arterial blood gas, a single-point measurement that takes a few minutes of processing by a laboratory. (In the absence of oxygenation, damage to the brain starts within 5 minutes with brain death ensuing within another 10–15 minutes). In the U.S. alone, approximately $2 billion was spent annually on this measurement. With the introduction of pulse oximetry, a non-invasive, continuous measure of patient's oxygenation was possible, revolutionizing the practice of anesthesia and greatly improving patient safety. Prior to its introduction, studies in anesthesia journals estimated U.S. patient mortality as a consequence of undetected hypoxemia at 2,000 to 10,000 deaths per year, with no known estimate of patient morbidity.^{[citation needed]}

By 1987, the standard of care for the administration of a general anesthetic in the U.S. included pulse oximetry. From the operating room, the use of pulse oximetry rapidly spread throughout the hospital, first to the recovery room, and then into the various intensive care units. Pulse oximetry was of particular value in the neonatal unit where the patients do not thrive with inadequate oxygenation, but also can be blinded with too much oxygen. Furthermore, obtaining an arterial blood gas from a neonatal patient is extremely difficult.^{[citation needed]}

In 1995, Masimo introduced Signal Extraction Technology (SET) that could measure accurately during patient motion and low perfusion. Some have termed newer generation pulse oximetry technologies as high resolution pulse oximetry (HRPO).^[5][6][7] One area of particular interest is the use of pulse oximetry in conducting portable and in-home sleep apnea screening and testing.^[5][8]

In 2009, the world's first Bluetooth-enabled fingertip pulse oximeter was introduced by Nonin Medical, enabling clinicians to remotely monitor patients' pulses and oxygen saturation levels. It also allows patients to monitor their own health through online patient health records and home telemedicine system.^[9]

Function[link]

A blood-oxygen monitor displays the percentage of arterial hemoglobin in the oxyhemoglobin configuration. Acceptable normal ranges for patients without COPD with a hypoxic drive problem are from 95 to 99 percent, those with a hypoxic drive problem would expect values to be between 88 to 94 percent, values of 100 percent can indicate carbon monoxide poisoning. For a patient breathing room air, at not far above sea level, an estimate of arterial pO₂ can be made from the blood-oxygen monitor SpO₂ reading.

Pulse oximetry is a particularly convenient noninvasive measurement method. Typically it utilizes a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form; therefore, the oxy/deoxyhemoglobin ratio can be calculated from the ratio of the absorption of the red and infrared light. The absorbance of oxyhemoglobin and deoxyhemoglobin is the same (isosbestic point) for the wavelengths of 590 and 805 nm; earlier equipment used these wavelengths for correction of hemoglobin concentration.^[10]

The monitored signal bounces in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish, (though black nail polish tends to distort readings)^[11] and discern only the absorption caused by arterial blood. Thus, detecting a pulse is essential to the operation of a pulse oximeter and it will not function if there is none.

Indication[link]

A pulse oximeter (saturometer) is a medical device that indirectly monitors the oxygen saturation of a patient's blood (as opposed to measuring oxygen saturation directly through a blood sample) and changes in blood volume in the skin, producing a photoplethysmogram. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times. Most monitors also display the heart rate. Portable, battery-operated pulse oximeters are also available for home blood-oxygen monitoring.

Advantages[link]

A pulse oximeter is useful in any setting where a patient's oxygenation is unstable, including intensive care, operating, recovery, emergency and hospital ward settings, pilots in unpressurized aircraft, for assessment of any patient's oxygenation, and determining the effectiveness of or need for supplemental oxygen. Assessing 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 a pulse oximeter 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 a pulse oximeter 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.^{[citation needed]}

Because of their simplicity and speed, pulse oximeters are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, especially COPD, or for diagnosis of some sleep disorders such as apnea and hypopnea.^[12] Portable battery-operated pulse oximeters are useful for pilots operating in a non-pressurized aircraft above 10,000 feet (12,500 feet in the U.S.)^[13] where supplemental oxygen is required. Prior to the oximeter's invention, many complicated blood tests needed to be performed. Portable pulse oximeters are also useful for mountain climbers and athletes whose oxygen levels may decrease at high altitudes or with exercise. Some portable pulse oximeters employ software that charts a patient's blood oxygen and pulse, serving as a reminder to check blood oxygen levels.

Limitations[link]

Pulse oximetry measures solely oxygenation, not ventilation and is not a complete measure of respiratory sufficiency. It is not a substitute for blood gases checked in a laboratory, because it gives no indication of base deficit, carbon dioxide levels, blood pH, or bicarbonate HCO3- concentration. The metabolism of oxygen can be readily measured by monitoring expired CO₂, but saturation figures give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin; in severe anemia, the blood will carry less total oxygen, despite the hemoglobin being 100% saturated.

Erroneously low readings may be caused by hypoperfusion of the extremity being used for monitoring (often due to a limb being cold, or from vasoconstriction secondary to the use of vasopressor agents); incorrect sensor application; highly calloused skin; or movement (such as shivering), especially during hypoperfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform.

It is also not a complete measure of circulatory sufficiency. If there is insufficient bloodflow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.

Since pulse oximetry only measures the percentage of bound hemoglobin, a falsely high or falsely low reading will occur when hemoglobin binds to something other than oxygen:

• Hemoglobin has a higher affinity to carbon monoxide than oxygen, and a high reading may occur despite the patient actually being hypoxemic. In cases of carbon monoxide poisoning, this inaccuracy may delay the recognition of hypoxemia (low blood oxygen level).

• Cyanide poisoning gives a high reading, because it reduces oxygen extraction from arterial blood. In this case, the reading is not false, as arterial blood oxygen is indeed high in early cyanide poisoning.

• Methemoglobinemia characteristically causes pulse oximetry readings in the mid-80s.

The only noninvasive method allowing continuous measurement of the dyshemoglobins is a pulse CO-oximeter, invented in 2005 by Masimo. It provides clinicians a way to measure total hemoglobin levels in addition to carboxyhemoglobin, methemoglobin and PVI, which initial clinical studies have shown may provide a new method for automatic, noninvasive assessment of a patient's fluid volume status.^[14][15][16] Appropriate fluid levels are vital to reducing postoperative risks and improving patient outcomes: fluid volumes that are too low (under-hydration) or too high (over-hydration) have been shown to decrease wound healing and increase the risk of infection or cardiac complications.^[17]

Increasing usage[link]

According to a report by Frost & Sullivan entitled U.S. Pulse Oximetry Monitoring Equipment Market, U.S. sales of oximeters were worth $201 million in 2006. The report estimated that oximeter sales in the U.S. would increase to $310 million annually by 2013.^[18]

In 2008, more than half of the major internationally-exporting medical equipment manufacturers in China were producers of pulse oximeters.^[19]

In June, 2009, video game company Nintendo announced an upcoming peripheral for the Wii console, dubbed the "Vitality Sensor", which consists of a pulse oximeter. This marks the onset of the use of this device for non-medical, entertainment purposes.^[20][21]

See also[link]

• Oxygen sensor
• Oxygen saturation
• Capnography, measuring of carbon dioxide (CO₂) in the respiratory gases
• Sleep apnea
• Integrated Pulmonary Index
• Photoplethysmogram
• Respiratory monitoring
• Arterial blood gas
• Medical equipment
• Mechanical ventilation

References[link]

External links[link]

• PhoneOximeter: Pulse oximetry on a cell phone for the developing world
• Principles of pulse oximetry explained using diagrams