Perfusion

In physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. The word is derived from the French verb "perfuse" meaning to "pour over or through."

Tests of adequate perfusion are a part of patient triage performed by medical or emergency personnel in a mass casualty incident.

Calculation
Perfusion ("F") can be measured with the following formula, where Pa is mean arterial pressure, Pv is mean venous pressure, and R is vascular resistance:

$$F = \frac{ P_A - P_V }{ R} $$

The term "Pa - Pv" is sometimes presented as "ΔP", for the change in pressure.

The terms "perfusion" and "perfusion pressure" are sometimes used interchangeably, but the equation should make clear that resistance can have an effect on the perfusion, but not on the perfusion pressure.

Overperfusion and underperfusion
The terms "overperfusion" and "underperfusion" are measured relative to the average level of perfusion across all tissues in an individual body, and the terms should not be confused with hypoperfusion and "hyperperfusion", which measure the perfusion level to the tissue's current need.

Tissues like the skin are considered overperfused and receive more blood than would be expected to meet the metabolic needs of the tissue. In the case of the skin, extra blood flow is used for thermoregulation. In addition to delivering oxygen, the blood helps dissipate heat by redirecting warm blood close to the surface where it can cool the body through the sweating and thermal radiation.

Measurement using fMRI
Two main categories of functional magnetic resonance imaging (fMRI) techniques can be used to measure tissue perfusion in vivo.
 * The first is based on the use of injected contrast agent that changes the magnetic susceptibility of blood and thereby the MR signal which is repeatedly measured during bolus passage.
 * The other category is based on Arterial spin labeling (ASL), where arterial blood is magnetically tagged before it enters into the tissue of interest and the amount of labeling is measured and compared to a control recording obtained without spin labeling.