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How to correctly use pulse oximeter to measure oxygen?

Pulse oximeters used to assess the oxygen status of patients in various clinical settings have become more and more common monitoring equipment. It provides continuous, non-invasive monitoring of hemoglobin oxygen saturation in arterial blood. Each pulse wave will update its result.


Pulse oximeters do not provide information about hemoglobin concentration, cardiac output, the efficiency of oxygen delivery to tissues, oxygen consumption, oxygenation, or adequacy of ventilation. However, they do provide an opportunity to immediately notice deviations from the patient’s oxygen baseline, as an early warning signal to clinicians to help prevent the consequences of desaturation and to detect hypoxemia before the occurrence of osis.

It has been suggested that increasing the use of pulse oximeters in general wards can make it as common as thermometers. However, it is reported that the staff has limited knowledge of the operation of the equipment, and little knowledge of the working principle of the equipment and the factors that may affect the readings.

Compared with reduced hemoglobin, pulse oximeters can measure the absorption of specific wavelengths of light in oxidized hemoglobin. Arterial oxygenated blood is red due to the mass of oxygenated hemoglobin it contains, which allows it to absorb certain wavelengths of light. The oximeter probe has two light-emitting diodes (LED) on one side of the probe, one red and one infrared. The probe is placed on a suitable part of the body, usually a fingertip or earlobe, and the LED transmits the wavelength of light to the photodetector on the other side of the probe through the pulsating arterial blood. Oxyhemoglobin absorbs infrared light; reduced hemoglobin results in red light. The pulsatile arterial blood in the systole causes oxygenated hemoglobin to flow into the tissue, absorbing more infrared light, and allowing less light to reach the photodetector. The oxygen saturation of the blood determines the degree of light absorption. The result is processed into a digital display of oxygen saturation on the oximeter screen, represented by SpO2.

There are many manufacturers and models of pulse oximeters. Most provide visual digital waveform display, audible arterial beat and heart rate display, and various sensors to suit individuals of age, size or weight. The choice depends on the settings that use it. All personnel who use pulse oximeters must understand its function and correct usage.

Arterial blood gas analysis is more accurate; however, pulse oximetry is considered to be accurate enough for most clinical purposes because of limitations that have been recognized.

Patient condition-To calculate the difference between capillaries and empty capillaries, oximetry measures the light absorption of multiple pulses (usually five). In order to detect pulsating blood flow, sufficient perfusion must be performed in the monitored area. If the patient’s peripheral pulse is weak or absent, the pulse oximeter reading will be inaccurate. The patients most at risk of hypoperfusion are those with hypotension, hypovolemia, and hypothermia, and those in cardiac arrest. People who have a cold but not hypothermia may have vasoconstriction in their fingers and toes and may impair arterial blood flow.

If the probe is fixed too tightly, non-arterial pulsations may be detected, causing venous pulsations in the finger. Venous pulsation is also caused by right-sided heart failure, tricuspid regurgitation, and the tourniquet of the blood pressure cuff above the probe.

Arrhythmia of the heart can cause very inaccurate measurement results, especially if there is a significant apex/bone deficit.

Intravenous dyes used in diagnostics and hemodynamic tests may cause inaccurate estimates of oxygen saturation, usually low. The effects of skin pigmentation, jaundice, or elevated bilirubin levels should also be considered.

Correct use of pulse oximetry measurement involves not only reading the digital display, but also more, because not all patients with the same SpO2 have the same oxygen content in the blood. A saturation of 97% means that 97% of the total hemoglobin in the body is filled with oxygen molecules. Therefore, the oxygen saturation must be explained in the context of the patient’s total hemoglobin level. Another factor that affects oximeter readings is how tightly hemoglobin binds to oxygen, which may vary with changes in various physiological conditions.

Post time: Jan-23-2021