The common pulse oximeter has a simple mission: to monitor the percentage of oxygen-saturated hemoglobin in a patient’s blood. Yet, the fact that its accuracy is often debatable complicates its use. So how do you monitor the monitors? Is it the probe — again? Try these approaches and test devices to assure that your oximeters check out just fine, every time.
The common pulse oximeter has a simple mission: to monitor the percentage of oxygen-saturated hemoglobin in a patient’s blood. Yet, its accuracy is often debatable. What’s the story? Let’s take a look at pulse oximeters and the test devices used to assess their condition.
What is it? What does it do?
A pulse oximeter provides a noninvasive means of monitoring pulse rate, pulse volume and blood oxygen-saturation level. Its probe is a sensor containing red and infrared LEDs, a photodiode receiver, and a cable. The cable attaches the sensor to the oximeter’s engine, also called the “box” or “monitor,” where microprocessors compute and analyze input data and display information for the user. Depending on its oxygen-saturation level, hemoglobin absorbs red and infrared light differently, so the sensor shines both red and infrared light through a part of the anatomy where pulsating arterial blood is near the skin’s surface, such as an adult’s finger or a baby’s foot. Light not absorbed after passing through the capillary bed generates a current, which the box receives through the cable and uses to calculate oxygen saturation or “sat value.”
To differentiate between arterial blood and other tissues, the oximeter compares the ratio of light absorption at different points in the cardiac pulse cycle. The resulting SpO2 value is an estimation of arterial oxygen saturation, or SaO2, that would result from a blood gas analysis. Most current oximeters are manufactured for +/-2 percent accuracy within the SaO2 range of 70 percent to 100 percent, with 95 percent-plus considered a normal SaO2 level. It is common knowledge that oximeters tend to be less accurate at lower levels of oxygen saturation.
“Pulse oximeters are an easy-to-use, noninvasive clinical device for rapidly assessing patient condition; they are not intended to replace the more comprehensive and invasive laboratory tests requiring arterial blood gas analysis,” says Mike Bryles, senior product manager, Fluke Biomedical, the former DNI Nevada (Carson City, Nev.).
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