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Oxygen Saturation
By Robin H. Steinhorn, MD

Robin H. Steinhorn, MDQ. What is oxygen saturation? How is it measured, and when do doctors become concerned when measuring it?

A. Most of the oxygen in our bloodstream is transported by hemoglobin, the red-colored protein in our red blood cells. Hemoglobin does the work of picking up oxygen from the lungs and delivering it to the rest of the body. Oxygen saturation tells us how much oxygen is bound to hemoglobin: when blood leaves the lungs it should be well-saturated with oxygen to deliver to the body.

We can directly measure the oxygen saturation in any blood sample. However, we usually continuously measure oxygen saturation using a device called a pulse oximeter. This is a terrific advance, because a pulse oximeter works without the need for a blood sample.

The pulse oximeter uses a probe attached to your baby's wrist, finger or toe which is linked to a small computerized unit. While it's pretty technical, the basic idea is a red light comes from the probe, and the computer measures the pattern of light absorbed by hemoglobin in the artery. The computer is then able to calculate the oxygen saturation of the hemoglobin in the artery.

In general, the oxygen saturation in a baby with no lung or heart problems is high (93-100%). When it falls much below 90%, it can be a sign that something is wrong with the lungs or heart, and that oxygen or other therapies are needed. However, the ideal oxygen saturation is not the same for every baby, so be sure to ask your doctors and nurses about what the goal is for your own child.

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