Finger Pulse Oximeter

Finger Pulse Oximeter

Finger pulse oximeters are used to measure the oxygen concentration in blood passing through a fingertip. Since every tissue and organ in our bodies requires oxygen to remain viable, lower than normal blood levels could be a sign that a medical condition exists, one that may require medical attention.

Pulse oximeters work by measuring the amount of oxygen being carried by red blood cells as they pass through the blood vessels below the fingernail.

Red blood cells are red because the hemoglobin protein contains iron. Hemoglobin consists of four heme molecules, each containing an iron ion (an atom missing or having an extra electron). One hemoglobin protein therefore contains four iron ions, and the iron ion confers to hemoglobin the ability to bind four oxygen molecules. As red blood cells pass through the lungs the oxygen concentration is so high from the outside air, that all four iron ions in a hemoglobin protein binds an oxygen molecule. This oxygen saturated blood (arterial) travels into the tissues where the oxygen is exchanged for carbon dioxide and other waste gases. The red blood cells then return to the lungs (venous) to exchange the carbon dioxide for oxygen, and the cycle repeats.

When hemoglobin is fully occupied by oxygen molecules it preferentially absorbs red light (660 nm). After hemoglobin releases oxygen into the tissues surrounding the blood vessels, thus becoming deoxygenated, it preferentially absorbs infrared light (940 nm). The pulse oximeter contains two Light Emitting Diodes (LEDs) that shine red and infrared light through the fingernail. On the other side of the finger, a photodetector in the oximeter senses the amount of light able to pass through the finger. The ratio of pulsatile over total transmitted red light, divided by the same ratio for infrared light, provides the necessary information for the oximeter to calculate the oxygen saturation levels of hemoglobin as it passes through blood vessels in the finger, resulting in an accurate measurement of arterial blood oxygen saturation levels. A pulse oximeter can therefore help monitor how well the ‘lung to tissue’ oxygen exchange system in your body is working.

The amount of oxygen dissolved directly into the blood is around 1.5%. This amount is insignificant by comparison to the amount bound by hemoglobin, which can range between 97 and 99% in a healthy person at rest. Pulse oximeter readings therefore represent the oxygen saturation levels of hemoglobin only.

A pulse oximeter only measures oxygen saturation of hemoglobin in the blood. As a result pulse oximeters can’t detect low hemoglobin concentrations, high carbon dioxide concentrations, or poor circulation. Under these conditions, pulse oximeters will give inaccurate or misleading readings. Other factors that can interfere with accurate readings are irregularities in blood pH levels, blood pressure, finger temperature, and poor circulation.

In spite of these limitations, pulse oximeters can prove to be an important ally, whether you are healthy or suffering from a medical condition. For anyone suffering from specific medical conditions, consult with your doctor first because oximeter readings can be sometimes be misleading.

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