Pulse oximetry is a non-invasive medical device that measures the oxygen saturation in a person’s blood, known as SpO2, and their pulse rate. This technology has become widely used in various settings, from homes to hospitals. SpO2 is often considered a “fifth vital sign,” providing valuable insight alongside temperature, blood pressure, and respiration rate.
How Pulse Oximetry Works
Pulse oximeters operate on the principle that oxygenated and deoxygenated hemoglobin absorb light differently. The device emits two specific wavelengths of light: red and infrared. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through, while deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through.
These light beams pass through a translucent part of the body, typically a fingertip or earlobe, where a photodetector measures the amount of light that is transmitted. The device specifically focuses on the pulsating arterial blood, filtering out signals from venous blood and other tissues like skin, bone, and muscle. By analyzing the varying absorption of red and infrared light during each heartbeat, the oximeter calculates the ratio of oxygenated to deoxygenated hemoglobin, which is then displayed as the SpO2 percentage.
Interpreting Your Readings
A pulse oximeter provides two main readings: SpO2, which indicates the percentage of hemoglobin carrying oxygen, and PR, representing your pulse rate in beats per minute. For a healthy individual, a normal SpO2 reading ranges between 95% and 100%. Readings below 90% are considered low and may indicate hypoxemia, a condition where blood oxygen levels are insufficient.
The pulse rate falls between 60 to 100 beats per minute for adults at rest. A lower resting heart rate can signify better cardiovascular fitness. Both SpO2 and pulse rate readings should be considered in the context of an individual’s overall health, activity level, and any pre-existing medical conditions, as these factors can influence what is considered a normal range.
Factors Influencing Accuracy
Several factors can influence the accuracy of pulse oximetry readings. Patient movement can interfere with the device’s ability to accurately detect the pulsating arterial blood flow, resulting in unreliable measurements. Poor perfusion, caused by conditions like cold hands, low blood pressure, or vasoconstriction, reduces blood flow to the extremities. Nail polish, particularly dark colors or thick artificial nails, can absorb or block the red and infrared light emitted by the device, leading to inaccurately low SpO2 readings. Bright ambient light can interfere with the sensor’s ability to detect the transmitted light.
Certain medical conditions and substances can also affect accuracy. Anemia, characterized by low hemoglobin levels, can result in deceptively normal SpO2 readings even though the blood’s overall oxygen-carrying capacity is reduced. Carbon monoxide poisoning is another significant concern; carboxyhemoglobin, formed when carbon monoxide binds to hemoglobin, absorbs light similarly to oxyhemoglobin, leading the pulse oximeter to falsely report a high SpO2, masking dangerously low functional oxygen levels. Very dark skin pigmentation has been reported to sometimes influence the accuracy of readings, with some studies indicating potential overestimation, especially at lower oxygen saturation levels.