Pulse oximetry is a noninvasive procedure to monitor the amount of oxygen in a patient’s bloodstream. However, the results of this test are not enough to directly diagnose pulmonary fibrosis (PF). A physician requires the analysis of more detailed tests to correctly diagnose the disease.

What is oxygen saturation?

A pulse oximeter measures the oxygen saturation in the patient’s blood at the time of the test.

When a person breaths in, the air that goes into the lungs then travels, through the blood, to the cells where it is required for normal cellular function.

To travel within the bloodstream, the oxygen molecules need to bind to a protein called hemoglobin. Hemoglobin works as a transporter to carry oxygen from the lungs to the cells.

Oxygen saturation is the measurement of the amount of hemoglobin proteins that are transporting oxygen molecules.

In a healthy person, the oxygen saturation levels in the blood are between 95 and 100 percent, meaning that almost all hemoglobin in the blood transports oxygen. In patients with PF, the amount of oxygen that goes into the lungs while inhaling is less due to the increased difficulty in breathing. Therefore, less oxygen binds to hemoglobin, resulting in oxygen saturation levels of less than 95 percent.

How does a pulse oximeter work?

A pulse oximeter has three main components: LED lights (one red and one infrared), a photodiode, and a processor. The LED lights face the photodiode, which converts light into an electric current. The patient places a fingertip or an earlobe in between these two components. The light transmitted by both LEDs is then partially absorbed by the finger or the ear of the patient, and the photodiode measures how much of the light still gets transmitted.

This test works on the assumption that hemoglobin normally absorbs the red light. When hemoglobin is transporting oxygen, it starts absorbing the infrared light and transmitting the red light.

In a healthy patient, with most of the hemoglobin transporting oxygen, there is more red light being transmitted because more infrared light is absorbed. In patients with PF, because there is more free hemoglobin, the infrared light transmission increases, while the red light decreases.

The final component of the pulse oximeter is the processor, which uses the results obtained by the photodiode and calculates the rate of  transmission between the red and the infrared lights. It creates a rate between the amount of oxygen-transporting hemoglobin and the amount of free hemoglobin. This value is the oxygen saturation result.

Test’s advantages and limitations

Pulse oximetry is an easy, noninvasive method to asses the amount of oxygen in a patient’s blood. But this technique does not provide a complete measurement of a patient’s respiratory sufficiency and on its own is not enough to diagnose a respiratory condition.

This test is not indicated for patients with anemia, since in these patients oxygen does not bind as well to hemoglobin, or for patients with low blood flow.

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