||This article needs attention from an expert in Medicine. (February 2012)
A perfusion pump, c. 1935, an early device for simulating natural perfusion
In physiology, perfusion is the process of delivery of 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 the patient assessment process performed by medical or emergency personnel. The most common methods include evaluating skin color, temperature, condition and capillary refill.
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 heart 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 dissipation.
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.
Perfusion of various tissues can be readily measured in vivo with nuclear medicine methods, mainly positron emission tomography (PET) and single photon emission computed tomography (SPECT). Various radiopharmaceuticals targeted at specific organs are available, some of the most common being
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 labelling (ASL), where arterial blood is magnetically tagged before it enters into the tissue of interest and the amount of labelling is measured and compared to a control recording obtained without spin labelling.
Brain perfusion (more correctly transit times) can be estimated with contrast-enhanced computed tomography.