Oxygen Saturation (SpO2) & Pulse Oximeters

Pulse oximeter is a non-invasive device for measuring arterial blood oxygen saturation (SpO2) level and pulse rate. Oximeters are used by people with breathing and heart problems, such as asthma and COPD. A normal healthy person at sea level should be at 96% or above. Most doctors would prefer their patients to maintain a level at above 90%. Medicare would pay for supplemental oxygen if SpO2 falls below 88%.

Pulse oximeter does not measure SpO2 directly. The oximeter uses two LEDs (one red and one infrared) to shine light through a translucent part of the body. It measures the amount of light absorbed by the body and then computes the blood oxygen level based on an algorithm. There are limitations to this approach that will affect its functioning and accuracy. The most accurate way to measure the oxygen saturation level is from a blood sample and would require some elaborate procedures.

The signal (amount of light absorption) has two components - constant and changing. The constant component is due to the presence of nail, skin pigment, flesh, and others. The changing component is the amount of blood flowing through arteries and is caused by heart beats. The SpO2 reading is based on the changing component of the signal. If the constant signal component overwhelms the changing component, the oximeter would fail or give an erroneous reading. This happens when the blood flow is too low, when the skin pigment is too dark, or when there is nail polish on the finger. Movement while taking a measurement would also interfere with the signal.

Hemoglobin is responsible for delivering oxygen to the entire body. A hemoglobin molecule can have up to 4 oxygen molecules. The oxygen saturation level is the ratio of oxygen molecules presented to the maximum number of oxygen molecules possible. For example, if there are 3800 oxygen molecules in 1000 molecules of hemoglobin, then the oxygen saturation is 95% or (3800/4000)*100. Oxyhemoglobin and deoxygenated hemoglobin have different light absorption characteristics. From the ratio of light absorbed during systole and diastole, the oxygen saturation can be derived.

References

What is pulse oximeter

Pulse oximetry