Capillary refill
Capillary refill time (CRT) is defined as the time taken for color to return to an external capillary bed after pressure is applied to cause blanching. It can be measured by holding a hand higher than heart-level and pressing the soft pad of a finger or fingernail until it turns white, then taking note of the time needed for the color to return once pressure is released. In humans, CRT of more than three seconds indicates decreased peripheral perfusion and may indicate cardiovascular or respiratory dysfunction. The most reliable and applicable site for CRT testing is the finger pulp (not at the fingernail), and the cut-off value for the normal CRT should be 3 seconds, not 2 seconds. Assessment

In adults 
CRT can be measured by applying pressure to the pad of a finger or toe for 5–10 seconds. It became popularized in the 1980s when Champion et al. proposed a CRT of less than 2 seconds be deemed normal and included CRT in the Trauma Score. The value of 2 seconds for a normal CRT that was proposed by Dr Champion had been arbitrarily chosen by his nurse, and no evidence supporting that value has subsequently been found. CRT has been shown to be influenced by ambient temperature, age, sex, and the anatomical testing and lighting conditions. The most reliable and applicable site for CRT testing is the finger pulp (not at the fingernail), and the cut-off value for the normal CRT should be 3 seconds, not 2 seconds. To assess shock, central CRT, which is done by assessing capillary refill time at the sternum, rather than finger CRT, is more useful. In infants 
In newborn infants, capillary refill time can be measured by pressing on the sternum for five seconds with a finger or thumb, and noting the time needed for the color to return once the pressure is released (central CRT). The upper normal limit for capillary refill in newborns is 3 seconds. Capillary refill time can also be assessed in animals by pressing on their gums as opposed to the sternum which is generally covered with fur or inaccessible. A prolonged capillary refill time may be a sign of shock and can also indicate dehydration and may be a sign of dengue hemorrhagic fever and decreased peripheral perfusion. Prolonged capillary refill time may also suggest peripheral artery disease. It is generally accepted that the test is affected by many different external factors and therefore should not be relied upon as a universal diagnostic measure. In animals 
Capillary refill time is also used in veterinary medicine. An animal should have a capillary refill time of less than 1 to 1 3/4 of a second. Because animals have fur, the gums are typically used to test capillary refill time. Use 
Capillary refill time is a quick and cheap way to indicate decreased peripheral perfusion and may indicate cardiovascular or respiratory dysfunction. It has also been used to assess or diagnose diseases of the peripheral circulatory system such as Raynaud's syndrome and hand–arm vibration syndrome. There is disagreement as to whether the test is useful, however, and it is poorly standardised. Poor finger or toe capillary refill time is a contraindication for the use of compressive techniques to treat peripheral oedema. Capillary refill time is traditionally used as a clinical indicator of tissue vascularity either after limb injury or after revascularization and the most reliable and applicable site for CRT testing is the finger pulp (not at the fingernail), and the cut-off value for the normal CRT should be 3 seconds, not 2 seconds. In veterinary medicine, poor gum CRT may be an contraindication for use of anaesthesia, as the likelihood of anaesthesia being fatal is higher if there is an underlying heart or lung problem. Gum CRT also used to monitor health during anaesthesia. See also
 Peripheral vascular examination

References

Cardiovascular physiology