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Capillary refill time (CRT) is defined as the time taken for colour to return to an external capillary bed after pressure is applied to cause blanching.1 It was first described in 19472 and has since become widely adopted as part of the rapid structured circulatory assessment of ill children. Its use has been incorporated into advanced paediatric life-support guidelines, and it is endorsed by many national and international groups. However, despite its ubiquity there is a great deal of variation in how CRT is performed, and little knowledge of factors which may affect its accuracy. In this article, we will give an overview of the use of CRT in children and how its results should be interpreted.
CRT is dependent on the visual inspection of blood returning to the distal capillaries after they have been emptied by the application of external pressure.1 The physiological principles which underpin peripheral perfusion are complex and affected by many different factors.
Capillary blood flow is affected by the driving pressure, arteriolar tone and the constituents of the blood. The driving pressure is influenced by hydrostatic pressure (blood pressure) and, at the level of capillaries, oncotic pressure due to plasma proteins.3 Arteriolar tone depends on a delicate balance between vasoconstrictive (noradrenaline, angiotensin II, vasopressin, endothelin I and thromboxane A) and vasodilatory influences (prostacyclin, nitric oxide and products of local metabolism such as adenosine). The constituents of blood can also impact on capillary blood flow. For example, the size of red blood cells and plasma viscosity can affect flow through narrow capillaries.4 The other main determinant of capillary perfusion is capillary patency which is reflected by the functional capillary density or the number of capillaries in a given area which are filled with flowing red blood cells.1
Vasoconstriction is considered …
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