Limitations in Using a Pulse Oximeter

Purchase a pulse oximeter is simple over the internet; however you should also understand its limitations and usages to ensure accurate measurements.

Pulse oximeter works by shining two lights though a translucent portion of the patient's body and measuring the variation in light absorption caused by the changes in blood flow. The total light absorbance is the sum of a constant component and a pulsating component, which is almost exclusively the result of arteriolar bed pulsations. Pulse oximetry assumes that arterial blood is the only pulsatile absorber and other sources of fluctuations could contribute to erroneous readings.

Translucency and blood flow at the spot where the measurement is taken can affect the accuracy of a reading. For example, if the area is largely opaque or the blood flow is low or irregular, accurate readings would be difficult. Understanding this is important to interpret a pulse ox reading.

Low atmospheric pressure in high altitude has little effect on a pulse oximeter; however extreme low temperature and poor blood circulation would.

Under various conditions, a pulse oximeter would give inaccurate readings. Keep these factors in mind while taking measurements.

Carbon Monoxide

Carbon monoxide molecules, even in a small amount, can attach to the patient's hemoglobin replacing oxygen molecules. A pulse oximeter cannot distinguish the differences and the reading will show the total saturation level of oxygen and carbon monoxide. If 15% of hemoglobin has carbon monoxide and 80% has oxygen, the reading would be 95%.

This can be dangerous. A pulse oximeter should not be used on people with smoke inhalation, carbon monoxide poisoning, and heavy cigarette smoking.

Hemoglobin Deficiency (Anemia)

Low quantity of hemoglobin may affect the result. The normal values for a person is 11 - 18 g/dl.

Blood Volume Deficiency

Conditions, such as hypovolemia, hypotension, and hypothermia, may have adequate oxygen saturation, but low oxygen carrying capacity. Due to the reduction in blood flow, the sensor may not be able to pick up adequately the pulsatile waveform resulting in no signal or loss of accuracy.

Taking measurement on sick patients with cold hands can be challenging.

Irregular Signals

Irregular signals can post problems for a pulse oximeter. The problem signals can be caused by irregular heartbeats or by patient's movements. If this is the problem, one can tell by looking at the SpO2 waveform which is available on some pulse oximeters.

External Interference

Exposure to strong external light while taking measurement may result in inaccurate readings. Shield the sensors from bright lights.

Strong electro-magnetic fields may also affect readings.

Fingernail Polish and Pressed on Nails

Nail polish and pressed-on nails may interfere with readings. Remove them.

Skin Pigmentation

Dark skin pigmentation can give over-estimated SpO2 readings when it is below 80%. Find a place where the skin color is lighter.

Intravenous Dyes

Intravenous dyes (such as methylene blue, indigo carmine, and indocyanine green) can cause inaccurate readings.

Methemoglobin

Methemoglobin is a form of hemoglobin that does not carry oxygen. It is normal to have 1-2% of haemoglobin in this form. A high level of methaemoglobin would cause a pulse oximeter to have a reading of around 85% regardless of the actual oxygen saturation level. The higher percentage of methaemoglobin can be genetic or caused by exposure to certain chemicals and medications.