Vital signs - Oxygen saturation (SpO2): Nursing skills

As a healthcare professional, you will help provide safe care to patients who have or are at risk for hypoxemia, or low levels of oxygen in the blood. These patients require pulse oximetry, or pulse-ox for short, which is a non-invasive, easy, and pain-free method of measuring the percentage of hemoglobin that’s saturated with oxygen. This is known as the saturation of peripheral oxygen or SpO2 for short. A normal SpO2 usually indicates that the amount of oxygen travelling through the bloodstream to the tissues is enough to meet the needs of the body.

Now, a pulse oximeter consists of a probe, which is attached by a cable to a pulse oximeter. The probe has a light source on one side and a photodetector, or sensor, on the other side.

So, when it gets clipped onto a body part, two different wavelengths of light shine through the tissues on one side, and on the other side, the sensor detects how much of each wavelength has been absorbed by the arterial blood in the tissues. The principle is that, when hemoglobin is bound to oxygen, it absorbs a different wavelength of light than when it is not bound to oxygen, so the percentage of hemoglobin bound to oxygen can be calculated by the device. LO1, LO2

Now, there are several types of probes, depending on the site where they can be placed. The most commonly and easily used ones are digit probes, which can fit onto a finger or a toe. There are also earlobe probes, which attach to the patient’s ear. Less commonly, if the digits or earlobes are inaccessible, a pulse oximetry probe can be applied across the forehead and secured with a headband. Both earlobe and forehead probes tend to be more accurate than digit probes in cases when blood flow to the extremities is compromised or if the patient moves their hands or feet frequently, creating motion artifacts.

There are also sensor pads that can be used on several different sites, including an adult's nose bridge and a newborn's palms or soles.

Now, pulse oximeter probes can be disposable or reusable, and they also come in two types: adhesive and clip sensors. In general, adhesive sensors are better for younger children or if you need to place the sensor on an earlobe, nose bridge, or toe. Clip sensors are good for spot-checking a patient’s oxygen saturation but are not the best for continuous monitoring, especially in children because they are very easy to take off. LO3

Now, a SpO2 of 95% or greater is typically considered normal.
Anything below that is usually considered low for most patients. But if it falls below 90% it can get really dangerous for the patient. This may occur in patients with various lung conditions that interfere with gas exchange, meaning movement of oxygen and carbon dioxide between the blood, lungs, and tissues.

In other cases, though, readings can be low despite adequate systemic oxygenation because of poor blood flow to the extremities, which makes it hard for the pulse oximeter to pick up a proper signal. Now, this could be due to peripheral vascular disease, which happens when one of the arteries supplying the extremities becomes narrowed; as well as hypotension, or low blood pressure; hypothermia, or decreased body temperature; certain medications; as well as peripheral edema, or accumulation of fluid and swelling of the limbs.

Another important situation to consider when interpreting a pulse-ox reading is that patients with carbon monoxide poisoning can have a normal pulse-ox reading but still be deficient in oxygen. This is because the pulse oximeter can’t distinguish between oxygenated hemoglobin and carboxyhemoglobin, which is hemoglobin bound to carbon monoxide. Therefore, in carbon monoxide poisoning, the pulse oximeter value can appear normal, but the true saturation of oxygen is much lower.

Readings can also be inaccurate meaning that false signals are generated when they shouldn’t be due to motion artifacts, like when the patient is shivering or restless, if the patient is wearing nail polish, or if bright light, such as sunlight, is shining on the probe, making it harder to measure the oxygen saturation.

First, here are some common care tips to keep in mind. In general, depending on the condition of the patient, pulse oximetry can be used intermittently, meaning that you go and check the patient’s SpO2 at certain times, or continuously, where the patient is continuously connected to the pulse oximeter.

In the latter case, an alarm can notify you if the SpO2 falls below a certain level or if the probe falls off. If the patient is a child, remember that they tend to move around a lot, so it might be best to place the probe on their toe; otherwise, you may get inaccurate readings. Also, avoid using forehead or nose probes with these patients.

Children, especially neonates, have delicate skin and can be easily injured, so remember to protect their skin, check under the probe regularly for skin breakdown, and routinely place the probe in a different spot.

In all cases, make sure to use the appropriate probe for those sites.

If the patient is restless or moving too much, encourage them to stay still, try to hold their limb steady, or simply place the probe in a different spot.

Before clipping on a finger probe, check for the presence of nail polish or artificial fingernails and remove them, if possible.