Perfusion

Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue,[1] usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is delivered to tissue,[2] or volume of blood per unit time (blood flow) per unit tissue mass. The SI unit is m3/(s·kg), although for human organs perfusion is typically reported in ml/min/g.[3] The word is derived from the French verb "perfuser" meaning to "pour over or through".[4] All animal tissues require an adequate blood supply for health and life. Poor perfusion (malperfusion), that is, ischemia, causes health problems, as seen in cardiovascular disease, including coronary artery disease, cerebrovascular disease, peripheral artery disease, and many other conditions.

A Lindbergh perfusion pump, c. 1935, an early device for simulating natural perfusion

Tests verifying that adequate perfusion exists are a part of a patient's assessment process that are performed by medical or emergency personnel. The most common methods include evaluating a body's skin color, temperature, condition (dry/soft/firm/swollen/sunken/etc), and capillary refill.

During major surgery, especially cardiothoracic surgery, perfusion must be maintained and managed by the health professionals involved, rather than left to the body's homeostasis alone. As the lead surgeons are often too busy to handle all hemodynamic control by themselves, specialists called perfusionists manage this aspect. There are more than one hundred thousand perfusion procedures annually.[5]

Discovery

In 1920, August Krogh was awarded the Nobel Prize in Physiology or Medicine for his discovering the mechanism of regulation of capillaries in skeletal muscle.[6][7] Krogh was the first to describe the adaptation of blood perfusion in muscle and other organs according to demands through the opening and closing of arterioles and capillaries.

Malperfusion

Malperfusion can refer to any type of incorrect perfusion though it usually refers to hypoperfusion. The meaning of the terms "overperfusion" and "underperfusion" is relative to the average level of perfusion that exists across all the tissues in an individual body. Perfusion levels also differ from person to person depending on metabolic demand.

Examples follow:

  • Heart tissues are considered overperfused because they normally are receiving more blood than the rest of tissues in the organism; they need this blood because they are constantly working.
  • In the case of skin cells, extra blood flow in them is used for thermoregulation of a body. In addition to delivering oxygen, blood flow helps to dissipate heat in a physical body by redirecting warm blood closer to its surface where it can help to cool a body through sweating and thermal dissipation.
  • Many types of tumors, and especially certain types, have been described as "hot and bloody" because of their overperfusion relative to the body overall.

Overperfusion and underperfusion should not be confused with hypoperfusion and hyperperfusion, which relate to the perfusion level relative to a tissue's current need to meet its metabolic needs. For example, hypoperfusion can be caused when an artery or arteriole that supplies blood to a volume of tissue becomes blocked by an embolus, causing either no blood or at least not enough blood to reach the tissue. Hyperperfusion can be caused by inflammation, producing hyperemia of a body part. Malperfusion, also called poor perfusion, is any type of incorrect perfusion. There is no official or formal dividing line between hypoperfusion and ischemia; sometimes the latter term refers to zero perfusion, but often it refers to any hypoperfusion that is bad enough to cause necrosis.

Measurement

In equations, the symbol Q is sometimes used to represent perfusion when referring to cardiac output. However, this terminology can be a source of confusion since both cardiac output and the symbol Q refer to flow (volume per unit time, for example, L/min), whereas perfusion is measured as flow per unit tissue mass (mL/(min·g)).

Microspheres

Microspheres that are labeled with radioactive isotopes have been widely used since the 1960s. Radioactively labeled particles are injected into the test subject and a radiation detector measures radioactivity in tissues of interest.[8] Application of this process is used to develop radionuclide angiography, a method of diagnosing heart problems.

In the 1990s, methods for using fluorescent microspheres became a common substitute for radioactive particles.[9]

Nuclear medicine

Perfusion of various tissues can be readily measured in vivo with nuclear medicine methods which are mainly positron emission tomography (PET) and single photon emission computed tomography (SPECT). Various radiopharmaceuticals targeted at specific organs are also available, some of the most common are:

  • 99mTc labelled HMPAO and ECD for brain perfusion (rCBF) studied with SPECT
  • 99mTc labelled Tetrofosmin and Sestamibi for myocardial perfusion imaging with SPECT
  • 133Xe-gas for absolute quantification of brain perfusion (rCBF) with SPECT
  • 15O-labeled water for brain perfusion (rCBF) with PET (absolute quantification is possible when measuring arterial radioactivity concentration)
  • 82Rb-chloride for measuring myocardial perfusion with PET (absolute quantification is possible)

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 an injected contrast agent that changes the magnetic susceptibility of blood and thereby the MR signal which is repeatedly measured during bolus passage.[10]
  • The other category is based on arterial spin labelling (ASL), where arterial blood is magnetically tagged before it enters into the tissue being examined and the amount of labelling that is measured and compared to a control recording obtained without spin labelling.[11]

CT

Brain perfusion (more correctly transit times) can be estimated with contrast-enhanced computed tomography.[12]

Thermal diffusion

Perfusion can be determined by measuring the total thermal diffusion and then separating it into thermal conductivity and perfusion components.[13] rCBF is usually measured continuously in time. It is necessary to stop the measurement periodically to cool down and reassess the thermal conductivity.

See also
  • Reperfusion injury – Tissue damage after return of blood supply following ischemia or hypoxia
  • Machine perfusion
  • Perfusionist
  • Myocardial perfusion imaging – Nuclear medicine imaging method
  • rCBF – Brain blood supply
  • Cerebral edema – Excess accumulation of fluid (edema) in the intracellular or extracellular spaces of the brain
  • Pressure ulcer — continuous external pressure impairs perfusion

References
  1. American Psychological Association (APA): perfusion. (n.d.). Dictionary.com Unabridged (v 1.1). Retrieved March 20, 2008, from Dictionary.com website: http://dictionary.reference.com/browse/perfusion
  2. Thomas DL, Lythgoe MF, Pell GS, Calamante F, Ordidge RJ (2000). "The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging". Phys Med Biol. 45 (8): R97-138. doi:10.1088/0031-9155/45/8/201. PMID 10958179.
  3. Engblom H, Xue H, Akil S, Carlsson M, Hindorf C, Oddstig J, Hedeer F, Hansen MS, Aletras AH, Kellman P, Arheden H (2017). "Fully quantitative cardiovascular magnetic resonance myocardial perfusion ready for clinical use: a comparison between cardiovascular magnetic resonance imaging and positron emission tomography". J Cardiovasc Magn Reson. 19 (1): 78. doi:10.1186/s12968-017-0388-9. PMC 5648469. PMID 29047385.
  4. "Perfusion > What is Perfusion?". Cardiovascular Perfusion Forum.
  5. "Perfusion > Perfusion Services". Specialty Care Services Group.
  6. Larsen, E. H. (2007). "August Krogh (1874–1949): 1920 Nobel Prize". Ugeskrift for Laeger. 169 (35): 2878. PMID 17877986.
  7. Sulek, K. (1967). "Nobel prize for August Krogh in 1920 for his discovery of regulative mechanism in the capillaries". Wiadomosci Lekarskie. 20 (19): 1829. PMID 4870667.
  8. Studies of the Circulation with Radioactive Microspheres., Wagner et al, Invest. Radiol., 1969. 4(6): p. 374-386.
  9. "Fluorescent Microspheres" (PDF). Fluorescent Microsphere Resource Center. Archived from the original (PDF) on 2012-10-02.
  10. Huettel, S. A.; Song, A. W.; McCarthy, G. (2009), Functional Magnetic Resonance Imaging (2 ed.), Massachusetts: Sinauer, ISBN 978-0-87893-286-3
  11. Detre, John A.; Rao, Hengyi; Wang, Danny J. J.; Chen, Yu Fen; Wang, Ze (2012-05-01). "Applications of arterial spin labeled MRI in the brain". Journal of Magnetic Resonance Imaging. 35 (5): 1026–1037. doi:10.1002/jmri.23581. ISSN 1522-2586. PMC 3326188. PMID 22246782.
  12. L. Axel. Cerebral blood flow determination by rapid-sequence computed-tomography: theoretical analysis. Radiology 137: 679–686, December 1980
  13. Vajkoczy P, Roth H, Horn P, et al. (August 2000). "Continuous monitoring of regional cerebral blood flow: experimental and clinical validation of a novel thermal diffusion microprobe". Journal of Neurosurgery. 93 (2): 265–74. doi:10.3171/jns.2000.93.2.0265. PMID 10930012. S2CID 30375395.
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