Must Read | Demystifying Finger Pulse Oximeter

What is a pulse oximeter?

The pulse oximeter is used to measure arterial oxygen saturation (SpO2). The finger oxygen we usually use is a kind of pulse oximeter.

SpO2 = actual arterial oxygen content / total oxygen saturation ×100%


In addition to HbO2 and Hb, there are actually many other proteins in the body that can bind to oxygen, but the proportion in normal people is relatively small, and it is not closely related to a tissue oxygen supply, so most oximeters use SpO2 The calculation of is simplified to calculate only these two parameters.

What is The Princple of Pulse Oximeter

The oximeter has a luminous source, which emits two wavelengths of light, and HbO2 and Hb have different absorbance for these two wavelengths.

When the artery is pulsating, the blood volume at the measurement site will change to form a certain waveform. The oxygen meter calculates SpO2 based on the waveform and absorption rate.

In addition to arteries, veins and surrounding soft tissues will also absorb the light waves emitted by the light emitters, which is “background noise” for arterial blood oxygen.

However, the pulsation of the vein is very weak, and the absorption of light intensity by the vein and surrounding soft tissues can be regarded as a constant value.

Arteries have obvious pulsatility. The light intensity received by the opposite receiver will periodically change with the pulse of the artery. The oximeter can determine which light intensity is absorbed by the Hb and HbO2 in the artery. Thus, SpO2 is calculated by the formula.

When measuring, try to choose parts with thinner soft tissues and richer arterial blood flow, such as hands, toes, and earlobes.

What does Perfusion index PI Mean?

Perfusion index PI

The blood perfusion index is calculated based on the pulsation waveform in the figure above and reflects the blood perfusion of the monitored person at the measurement point. The higher the PI measured at the measuring point, the more accurate the SpO2 measured.

What Factors May Affect Accuracy?

Internal factors

Venous pulsation: The peripheral blood vessels of patients with septic shock are dilated, forming a certain degree of the arteriovenous shunt, and the venous pulsation is enhanced, which interferes with SpO2.

Low perfusion: The arterial pulsation signal is very weak in the low perfusion state, the signal-to-noise ratio is very low, and the error is large, resulting in low readings. When the systolic blood pressure is less than 80mmHg, the accuracy of SpO2 is significantly reduced, and the reading may be much lower than the actual oxygen and condition of the patient. At this time, placing the probe on the forehead may be more accurate than the extremities.

Carboxyhemoglobin: The absorption power of carboxyhemoglobin for 660nm wavelength light is almost equal to HbO2. Therefore, even if the patient is severely hypoxic when CO poisoning, SpO2 readings can be normal, so be alert.

Methemoglobin: Methemoglobin can absorb light at 660 and 940nm. When the methemoglobin content is less than 20%, the SpO2 drop is about 1/2 of the methemoglobin content, but when the methemoglobin content is higher, the SpO2 close to 85% will no longer reduce.

Severe anaemia: haemoglobin is less than 50g/L, and SpO2 reading is lower than normal.

Dopamine and norepinephrine: the long-term application will cause peripheral vasoconstriction, affect PI waveform, and affect accuracy.

Others: skin is too thick, skin pigmentation, nail polish (blue nail polish has the greatest impact, followed by green and black, red nail polish does not affect), onychomycosis, etc.

External factor

Motion interference (finger shaking, etc.), ambient light, optical detection circuits, etc.

So wait a few seconds for the pulse waveform to stabilize before the reading is more accurate.

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