Measuring Pulse Oximetry

Updated: Jun 3

By measuring Pulse Oximetry, you can quickly assess the patient's level of oxygen in their blood, also known as oxygen saturation. The oxygen saturation can give a clear indication of the patient's respiratory status and highlight any deterioration in the patient's physiology. The pulse oximetry can also be used continuously in patients that require intensive care or periodically when checking a patient's vital signs.

These notes will explain the measurements taken when using a pulse oximeter and the nursing skills required to perform it. You can find the complete list of notes on clinical skills here.


Measuring Pulse Oximetry: The Device

The pulse oximeter is an electronic device that looks a bit like a clothes peg, and it mostly comes in two primary forms; A finger probe and an earlobe probe. Now to understand how it works, you need to know two things:

  1. Red light is a visible light which is absorbed by deoxygenated blood

  2. Infrared light is an invisible light that is absorbed by oxygenated blood

One of the sides of the pulse oximeter has a built-in light bulb that sends out a beam of red light and a beam of infrared light. The other side of the device has a sensor to receive these beams. When the device is placed on a patient's finger or earlobe, the beams have to go through the patient's capillary system before reaching the sensor.

The sensor will take several readings every second, and it calculates the ratio of red to infrared light, which determines the patient's peripheral oxygen saturation (aka. The SpO2).

If the sensor receives a high amount of red light and a low amount of infrared light, the patient has a lot of oxygenated blood cells in their circulatory system. Meaning that the patient has a good level of oxygen saturation. On the other hand, if the sensor receives a low amount of red light and a high amount of infrared light, it means that the patient has a low oxygen saturation level.

Measuring the Pulse Oximetry: Indications

You'd check your patient's pulse oximetry to assess for early signs of hypoxaemia (low oxygen in the blood). Now hypoxaemia can be suspected for several reasons, such as when patients present with:

  • Respiratory distress

  • Acute deteriorating illness

  • Acute confusion

  • Asthma or COPD Exacerbation

  • Pneumonia

Pulse oximetry is also continuously monitored in patients who are:

  • Under general anaesthesia or receiving sedative medication

  • Evaluating a chronic respiratory condition

  • Being assessed for sleep apnoea or other sleep studies

  • On continuous oxygen therapy

It's important to note that pulse oximetry does not measure the level of oxygen in arterial blood. This means that it does not directly measure the amount of oxygen delivered into the tissues, which is affected by the patient's ventilation and circulation. To obtain a more in-depth look at the patient's oxygen levels and gas exchange, you will need to take an ABG Test.

Measuring the Pulse Oximetry: The Values

Once the device is put in place, it will record a number in percentage. Healthy individuals with no respiratory complications are expected to have a value of 96%-100% saturation without oxygen supplementation (meaning not on an oxygen mask).

Anything lower than 96% can indicate that the patient has a respiratory complication. For example, patients with Chronic Obstructive Pulmonary Disorder are expected to have an SpO2 of 92%-96%, and if their condition is severe, their SpO2 can also be as low as 88%.

However, just because you see a low value, it does not automatically mean that the patient has a problem. In fact, there are several factors listed in the next section that affect the pulse oximeter's effectiveness. In addition, the pulse oximeter is only accurate if the patient's saturations are between 70%-100%, and the values are not reliable if they are lower than that.

Measuring the Pulse Oximetry: Limitations

Even though pulse oximetry is highly effective, it may present false readings if some conditions are not met. For the pulse oximeter to work well, it's essential that the device is cleaned regularly, kept still during the assessment and has no environmental interference. Moreover, the value can be affected if the patient has:

  • Chronic poor circulation

  • Temporarily constricted circulation due to tight clothing or blood pressure cuffs

  • Cardiovascular diseases

  • Cold extremities

  • Anaemia or Skin discolouration

  • Carboxyhaemoglobin or Intravenous dyes

  • Nail polish or synthetic nails

Measuring Pulse Oximetry: The procedure

Start by ensuring that the pulse oximeter device is available and functioning, and read the manual on how to use it. The device is very easy to use in most cases, but its settings might vary from one brand to another. Ensure that the pulse oximeter is set to an adult or paediatric depending on your patient and that the mode is consistent with the approach you'll be taking (i.e. finger or earlobe).

Wipe the device with a suitable alcohol swab, and apply an alcohol hand rub on your hands to follow the infection-control guidelines.

Once you have the pulse oximeter in hand, grab the patient's file and go to their bed. Introduce yourself, and explain why you'll be checking their oxygen level. Go over the selected method and assess the risk factors for obtaining an inaccurate reading, such as cold extremities, nail polish or any other limitation.

Turn on the pulse oximeter and clip the probe onto the patient's finger or earlobe, keeping it steady. Advise the patient not to move until the pulse oximeter screen shows a value.

Unclip the pulse oximeter once you have the reading, wipe it with an alcohol swab, and re-apply the alcohol hand rub. Document the patient's oxygen saturation on their chart and compare it to the previous readings. Explain the value to your patient, and if the patient has an average SpO2 level, no further action will be required.

If your patient was on oxygen therapy, you should check their saturation two times, once with the oxygen mask and once without. Then record both of them and mark down the value that is with supplemental oxygen.

If the reading is abnormal (lower than 96%), you should assess the patient's airway for signs of obstruction and their respiratory system for signs of distress. As we've explained above, chronic respiratory conditions will likely affect the patient's SpO2 levels. If the patient's SpO2 is extremely low and is not in-line with the previous readings, you should immediately inform your senior nurse and contact the patient's medical team.

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