# Perfusion: Wikis

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# Encyclopedia

A perfusion pump, c. 1935, an early device for simulating natural 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 "perfuser" 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 calculated with the following formula, where Pa is mean arterial pressure, Pv is mean venous pressure, and R is vascular resistance: [1]

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

The term "Pa - Pv" is sometimes presented as "ΔP", for the change in pressure.[2]

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 relative 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 sweating and thermal radiation.

## Measurement using MRI

Two main categories of magnetic resonance imaging (MRI) 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.

## Discovery

In 1920 August Krogh was awarded the Nobel Prize in Physiology or Medicine for the discovery of the mechanism of regulation of the capillaries in skeletal muscle. Krogh was first to describe the adaptation of blood perfusion in muscle and other organs according to demands through opening and closing the arterioles and capillaries.

## References

1. ^ Richard E. Klabunde. "Cardiovascular Physiology Concepts". Retrieved 9/12/06.
2. ^ Physiology at MCG 7/7ch04/7ch04p26 - "Renal Perfusion Pressure and Vascular Resistance"