Perfusion Index (PI) is a measurement obtained from a pulse oximeter that represents the ratio of pulsatile blood flow to non-pulsatile static blood flow in a patients peripheral tissue, such as the finger tip, toe, or ear lobe. It is an indication of the pulse strength at the sensor site, and its values range from 0.02% for very weak pulse to 20% for extremely strong pulse. The PI varies depending on patients, physiological conditions, and monitoring sites. A higher PI (closer to 20%) represents dilation of the arteries or a very strong blood flow, while a lower PI (closer to 0.02%) represents constriction or weak blood flow. The PI is useful for quickly evaluating the appropriateness of a sensor application site for pulse oximetry, and a site with a high PI number generally indicates an optimal monitoring site. PI is a relative number and varies depending on each patients "normal" PI. In the neonatal acute care setting, a low PI has been shown to be an objective indicator of severe illness. PI is also a good indicator of the reliability of the pulse oximeter reading, and for most pulse oximeters for general use, the reading is unreliable or unavailable if PI is at or below 0.4%.