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What Can Affect a Pulse Oximeter Reading?

By Contributing Writer ; Updated July 27, 2017

The Device

The pulse oximeter measures the oxygen saturation in the patient's blood instead of directly through a blood sample and differences of blood volume in the skin. The process creates a photoplethysmograph (PPG) that gives a volumetric measurement of an organ optically. A medical monitor shows the patient's oxygenation and/or heart rate. This displays the percentage of atrial hemoglobin in the oxyhemoglobin configuration. Normal levels range from 95 to 100 percent, but readings below 90 are common as well.

What Affects the Reading?

The pulse oximeter is not a comprehensive measurement for respiratory sufficiency since there are factors that can affect the readings. For example, someone afflicted with hypoventilation who receives 100% oxygen may show normal levels even though he may be suffering from respiratory acidosis. Insufficient blood flow can cause tissues to read high-oxygen saturation despite having a case of hypoxia. High levels of methemoglobin also cause the pulse oximeter to read closer to 85 percent, regardless of the actual oxygen saturation in the blood. A patient who is shivering or suffering from seizures or tremors can affect the oximeter's reading. And If the sensor is too loose or too tight, the reading will be impaired.

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How the Device Functions

The pulse oximeter device is applied to the finger and over the fingernail. The light source and detector on the end starts to read the oxygen saturation. The red and infared light reflect through the skin then transmit the information to the device. The program calculates the data and determines a reading on the screen. The measurements for oxygen saturation shows as SpO2 for hemoglobin.

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