What is PI on the Pateint Monitor?

author: Claudia | Engineer of International Sales | UTECH

2024-04-11

1.What is PI?

PI, also known as Perfusion Index, is obtained by transcutaneous blood oxygen saturation monitoring, and is defined as the "light absorption ratio" between pulsating component AC (mainly pulsating arterioles) and non-pulsating component DC (veins, capillaries, and non-pulsating arterial blood and tissue) in the local tissue at 940nm, namely AC905/DC905.

Although data is also obtained from the SpO2 module through optical principles, the main significance of PI does not lie in monitoring oxygen and function. Instead, it is an indicator that reflects hemodynamics and is more closely related to "circulation".

The principle is that the light absorption of the local tissue at the measurement site reaches the maximum during ventricular systole, and the systolic pressure generated causes the peripheral arteries to expand and become hyperemic. At this moment, the arterial blood volume increases under the light source. It is this change that makes it possible to measure the light absorption of the pulsatile component.

2.Clinical significance of PI

In fact, PI reflects the interaction between "central" and "peripheral hemodynamics" and can be simply regarded as a balance between stroke volume and peripheral vascular resistance. However, in more application scenarios, especially in clinical anesthesia management, PI appears as an indicator reflecting vascular tone (ie, vasomotion state).

The basic idea of this application is: if the arterial blood vessels are in a diastolic state, the diameter will be large, unobstructed, and perfusion will be good if the flow rate is large. Therefore, a large PI value indicates that the arterial blood vessels at the measurement site are in a dilated state and have low resistance. On the contrary, peripheral vasoconstriction results in lower PI values.

Due to the numerous and complex influencing factors, PI has a wide range of values, fluctuating between 0.02% and 20%. The PI range (%) of monitors currently on the market is as follows:

Reasons for low perfusion include: low cardiac output, severe anemia, hypothermia, and increased systemic circulation resistance, that is, hypotension, hypovolemia, hypoxia, anemia, and peripheral arterial constriction, etc.

After admission to the ICU, a patient with hemorrhagic shock often has a "weak perfusion state" of less than 0.3 PI, even though oxygen is sufficient, both because of insufficient blood volume and because the peripheral arterioles have contracted under compensatory regulation.

With the elimination of blood transfusion, infusion and etiology, the temporarily sacrificed peripheral artery re-diastole, peripheral perfusion recovery, PI value can gradually increase to more than 1. If only the infusion is sufficient and the blood transfusion is insufficient, the anemia state is not solved, although the blood pressure can rise, but the PI is still low. At this time, it should be considered doing blood gas and continue to take blood.

In patients with elective surgery whose circulation is relatively stable, PI is usually between 1 and 10.

If you want to learn more about PI, please visit the link below:

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