What is Perfusion Index (PI)

Perfusion Index or PI is the ratio of the pulsatile blood flow to the non-pulsatile static blood flow in a patient's peripheral tissue, such as finger tip, toe, or ear lobe. Perfusion index is an indication of the pulse strength at the sensor site. The PI's values range from 0.02% for very weak pulse to 20% for extremely strong pulse. The perfusion index varies depending on patients, physiological conditions, and monitoring sites. Because of this variability, each patient should establish his own "normal" perfusion index for a given location and use this for monitoring purposes.

Perfusion index is normally monitored with pulse oximeters. PI is also a good indicator of the reliability of the pulse oximeter reading. For most pulse oximeters for general use, the reading is unreliable or unavailable if PI is at or below 0.4%. There are oximeters, such as those from Masimo, designed for extreme low PI. Most people that use an oximeter at home would not need a perfusion index indicator because they are considered to be in general good health. A perfusion index adds a lot of sensitivity to the oximeter sensor thus adding to the cost of the oximeter.

The pleth (Plethysmograph), available in many pulse oximeters, is a graphical representation of the perfusion index.

In a hospital, perfusion index, along with many other parameters, is used to monitor critically ill patients. Studies have shown that PI has a high correlation with capillary refill time and central-to-toe temperature difference. In neonatal acute care, a low PI is an objective and accurate measure of acute illness. It is superior to qualitative approach such as foot warmth.

Perfusion index is also used as an early warning of anesthetic failure. Studies have shown that an increase in PI is an early indicator that general or epidural anesthesia has initiated peripheral blood vessel dilation, which typically occurs before the onset of anesthesia. Lacking the spike would indicate the lack of anesthetic effect.

Other uses of the perfusion index can be found throughout various literature. As we learn more about PI, more clinical applications are being discovered.