By Arthur Zenian
The modern-day pulse oximeter is an astonishing photoelectric device that provides a noninvasive means of measuring hemoglobin’s oxygen saturation in the blood or tissue. Knowing that a patient has low oxygen allows healthcare professionals to prevent brain damage, heart failure, and death, which could otherwise result within minutes.
How It Works
The Lambert-Beer Law (known more commonly as Beer’s Law) provides the basis for a pulse oximeter’s functionality. The law illustrates the correlation between light transmission and optical density, relating the concentration of a solute to the intensity of light transmitted through it.
A pulse oximeter sits against the skin and passes light through a person’s body tissues at two different frequencies: 660 nm (red) and 900 nm (infrared). The instrument then reads the amount of each type of light absorbed by the tissues. Hemoglobin that is carrying oxygen (oxyhemoglobin) absorbs one wavelength of the pulse oximeter’s light output, while hemoglobin that is not carrying oxygen (deoxyhemoglobin) absorbs the other wavelength. Backgrounds such as fluid, tissue, and bone are factored out of the measurement. Using the resulting measurement, the oximeter can accurately determine the percentage of oxygen saturation of the arterial blood.
Before pulse oximetry, the only option for measuring a patient’s oxygen saturation was a painful invasive arterial blood gas test and it typically took a minimum of 20 to 30 minutes to obtain the result. This delay is not acceptable as severe brain damage can occur within five minutes of low oxygenation.
According to reports, during this time 2,000 to 10,000 patients died per year because of undetected hypoxemia and there is no estimation of patient morbidity. Now that pulse oximeters are available, it’s important that they be tested for accuracy so they can save lives.
How to Properly Measure SpO2 Accuracy
All pulse oximeter testers currently on the market require the user to select the monitor manufacturer, or groups of manufacturers, in order to accommodate the manufacturer’s R-curves. In addition to saturation and pulse rate, most testers offer user-selectable values of pulse amplitude, tissue transmittance, arrhythmias, motion artifact, and interference from power line frequencies.
Many models also have pre-set combinations of clinically normal and abnormal values, and may allow the user to define custom values. All testers use either an electronic or optical interface with the unit under test. Some models offer both modes.
Electronic testers apply an electrical signal to the monitor through its sensor cable, without inclusion of the sensor. The user-selected electrical signal mimics various values of saturation and other variables. These units offer the option of testing the sensor (presumed to be a finger sensor) by verifying the continuity of the red and infrared LED’s and that of the photodiode. Some models also test to confirm the photodiode’s correct response to the two light signals.
In contrast to the electronic interface, optical testers provide a physical digit or “artificial finger” that includes a mechanical and/or opto-electronic element which allows variable transmission of the two light signals.
This type tests the entire monitor system (sensor, cable, and monitor) at once, which can save time when performance testing many units. As with the electronic types, these enable the user to select a range of values of saturation and other variables and pre-sets, depending on the manufacturer. One vendor offers a set of calibrated fingers, or artificial digits with integrated dyes, to allow transmission of light corresponding to a specific saturation.
Currently marketed SpO2 testers offer a variety of functions, and biomedical engineers must decide how thoroughly to test them. Most SpO2 monitors will be accurate at clinically “normal” values, but HTM professionals must detect when the monitor gives inaccurate values in the abnormal range, whereas clinicians must decide on clinical corrective action.
Arthur Zenian is CEO of enBio Corp. based in Burbank, Calif. Questions and comments can be directed to 24×7 Magazine chief editor Keri Forsythe-Stephens at [email protected].
History of Pulse Oximetry
The following briefly outlines the development of this important device.
- 1864: George Gabriel Stokes discovered that hemoglobin is the oxygen carrier in blood.
- 1935: German physician Karl Matthes developed the first oxygen saturation meter. It used a two-wavelength light source with red and green filters, which was later changed to red and infrared filters.
- 1941: “Oximetry testing” is first used to measure oxygen saturation level with a pulse oximeter.
- 1940s: Glenn Allan Millikan, a British scientist, used a dual light source to create the first practical aviation ear oxygen meter. During World War II, many pilots were saved from under-pressurized cabins via oximetry testing.
- 1964: Hewlett Packard built the first ear oximeter by using eight wavelengths of light. The oximeter was used primarily in sleep laboratories and to monitor pulmonary functions. The unit was large, clumsy to use, and expensive.
- 1972: Takuo Aoyagi, a Japanese bioengineer at Nihon Kohden, developed a pulse oximeter based on the ratio of red to infrared light absorption in blood. He obtained a Japanese patent. Another Japanese company, Minolta, obtained a U.S. patent based on the same concept. Oximetry became clinically feasible.
- 1981: Biox introduced the first commercial pulse oximeter. Initially, it was focused on respiratory care and later expanded into operating rooms. Since then, other manufacturers have entered the market and pulse oximeter technology has improved significantly.
- 1987: Pulse oximetry became part of a standard procedure in administrating general anesthetic in the United States. The use of oximetry quickly spread to other hospital units, such as emergency rooms, recovery rooms, neonatal units, and intensive care units.
- 1995: Fingertip pulse oximeters first appeared on the market.