Capillary Fill: What is it? Uses, Factors Affecting Measurement and Clinical Application of CRT

It was first introduced by Beecher et al.

Capillary refill time (CRT) is defined as the time it takes for a distal capillary bed to regain its color after pressure has been applied to cause scalding.

In 1947 using the categories normal, definite slowdown and very slow. These were correlated with the presence and severity of the shock. In 1980, Champion included the CRT measurement in his trauma score and was subsequently endorsed by the American College of Surgeons.


CRT has been widely used in adults and children and has been incorporated into advanced life support guidelines as part of the rapid and structured cardiopulmonary assessment of critically ill patients.

The upper limit of normal for CRT was defined as 2 seconds, based on the observations of a clinical staff member working with Dr. Champion.

In the last 30 years, this definition, the factors affecting CRT, and the validity of CRT measurements have been debated in the literature.

CRT measurement involves visual inspection of the blood returning to the distal capillaries after they have been emptied by applying pressure. The physiological principles of peripheral perfusion are complex.

The way a distal capillary bed is perfused depends on a number of factors; the main determinants are capillary blood flow and capillary permeability (reflected by functional capillary density, the number of capillaries in a given area that are filled with flowing red blood cells).

The arteriolar tone depends on a fine balance between vasoconstriction (norepinephrine, angiotensin II, vasopressin, endothelin I and thromboxane A  2 ) and influencing vasodilators (prostacyclin and nitric oxide), which together regulate capillary perfusion depending on the metabolic requirements of the tissue cells.

It is hypothesized that alterations in distal capillary bed perfusion will affect CRT measurement by altering the time to refill the distal capillaries with blood. It is important to note that there are no current publications that directly support this theory.

In this article, we focus on the potential use of CRT measurement in anesthesia although the evidence for this specifically does not exist.

Several published studies have determined the factors that affect CRT measurement and these are summarized. In addition, we examine some of the methods for automated CRT measurement.

Factors Affecting CRT Measurement

The nature of clinical CRT measurement makes it susceptible to errors. Several factors can have a significant impact on the results obtained and are rarely considered by healthcare workers.


Age affects CRT measurement. The upper limit of normal for CRT in newborns was found to be 3 seconds, regardless of gender, gestation, weight, size-for-gestational age, nursery containers, or phototherapy.

In children, an upper limit of normal of 2 seconds has been reported. Studies in adults have found a wider variation, with an average increase of 3.3% per decade of age.

One study found a median CRT for the pediatric population (up to 12 years) of 0.8 seconds; for adult men, 1.0 second; adult women, 1.2 seconds; and in those older than 62 years, 1.5 seconds.

This study concluded that if 95% of all normal patients should be within the normal range, then the upper limit of normal for adult women should be increased to 2.9 seconds and for the elderly to 4.5 seconds.


Environment, skin, and core temperature affect the CRT measurement. The CRT of healthy children in a warm environment (mean 25.7 ° C) was <2 seconds, but only 31% had a similar measurement in a cold environment (mean 19.4 ° C).

CRT in newborns is shorter in those breastfed in incubators or under radiant warmers. Similar results have been observed in adults; CRT decreased by 1.2% per degree centigrade increase in ambient temperature.

Local skin temperature affects CRT in both adults and children. In adults, immersion of a hand in cold water at 14 ° C prolongs CRT.

Fingertip temperature varied with room temperature and each 1 ° C reduction in skin temperature was accompanied by a 0.21 second increase in CRT.

Furthermore, a statistically significant relationship was found between CRT and core temperature.

CRT was on average 5% shorter for every 1 ° C increase in tympanic temperature. These relationships also exist for newborns whose CRT decreased as room, skin, and axillary temperatures increased.

Ambient light

Poor lighting conditions make CRT difficult to evaluate.

In daylight conditions (partly cloudy day, approximately 4000 lux), CRT was reported as normal in 94.2% of healthy participants compared to only 31.7% of the same participants in dark conditions (moonlight or lantern, approximately 3 lux).

Pressure application

There is no universal agreement on the optimal duration and amount of pressure or the site used when evaluating CRT. It has been suggested to apply moderate pressure for 3 seconds, 5 seconds, or until the capillary bed turns pale.

Pressure applied for <3 seconds gives a shorter CRT; no differences were found with the pressure applied for 3 to 7 seconds. The application of light pressure (the minimum pressure to cause scalding) resulted in a shorter CRT than moderate pressure and with less variability.

Measuring CRT at different sites on the body will produce different results.

Newborn CRT measurements from the midpoint of the forehead and chest are more consistent than measurements from the heel or palm. The CRT measured at the heel can be significantly longer than at the toe.

In newborns, especially premature babies, it is more difficult to examine the pulp of the finger compared to using the forehead or chest, where movement is less likely to interfere with the test.

The World Health Organization recommends using the thumbnail or big toe; other studies suggest using soft tissue at the patella or forearm level.

A survey of pediatric healthcare workers found that approximately two-thirds perform a CRT on the chest with only one-third using the pulp from the distal phalanx of the finger. This finding is in disagreement with studies, which mainly use the distal phalanx.

Intra and interobserver reliability

Poor interobserver reliability is a major limitation to the use of the test.

The interobserver reliability of CRT measurement (using a standardized method to assess CRT, without a timing device, with a resolution of half a second) in clinically stable adult patients in the ED showed a mean difference in CRT measurements between 0 second doctors.

However, the 95% limits of agreement were -1.7 to +1.9 seconds. More importantly, in only 70% of the subjects studied there was agreement as to whether the CRT is normal or abnormal (using a 2-second upper limit of normal).

In another study, 5 experienced physicians measured CRT in each of 5 patients’ halluces. When evaluating intraobserver reliability, they found a general intraclass coefficient (ICC) of 0.72; however, the overall standard error of measurement was ± 1.94 seconds.

The ICC for interobserver reliability was worse. Two studies standardized the CRT measurement method and used a stopwatch to measure time. The first found that the ICC for interobserver reliability was 0.7, and for intraobserver reliability, 0.96.

The second, a study of neonates, found that the correlation coefficient for CRT measurement in the foot among 3 observers ranged from 0.47 to 0.68 and for the hand, from 0.55 to 0.71. The last 2 results may not be representative of routine clinical practice given the strict method applied for the evaluation of CRT.

A study of children admitted to a district hospital in Kenya evaluated 4 clinical evaluations of CRT in 100 patients. Low-moderate agreement was found (κ = 0.42); however, a better agreement was found for CRT <1 second and> 4 seconds.

In addition to the variations that can occur due to differences in the amount and duration of pressure applied to the finger, the clinician must also decide on the end point of capillary refilling.

The initial rapid partial filling of the capillaries may be followed by a slower full filling. The definition of the end point is subjective and introduces a greater error in the CRT evaluation.

The clinical application of CRT measurement

As mentioned above, there are no publications specifically related to the use of CRT measurement in anesthesia. Its potential use in this field must be inferred from the currently available evidence.


There is a good correlation between CRT measurement and the degree of dehydration in children admitted to hospital with diarrhea.

A CRT of 1.5 to 3 seconds is associated with a fluid deficit of 50 to 100 ml / kg (measured as difference in weight from the time of admission to that after rehydration in infants with diarrhea) and a CRT of> 3 seconds suggests a deficit of> 100 mL / kg.

A prolonged CRT was an important predictor of children who were shown to be> 5% dehydrated based on subsequent weight gain in hospital.

Children with dehydration (≥5% body weight deficit) had a statistically significant longer mean CRT (2.0 ± 1.0 seconds vs. 1.3 ± 0.5 seconds) compared to well-hydrated children.

The presence of fever in these children did not have a clinically important effect on the estimation of CRT.

A more recent review investigating clinical measures to assess dehydration in children found that CRT was the best individual sign for diagnosing children with 5% dehydration.

In children with septic shock in the pediatric intensive care unit, CRT was compared with hemodynamic variables.

The best correlation was between CRT and stroke volume index (  r  = -0.46, 95% confidence interval, -0.67 to 0.18) and lactate (0.47, 0.21 to 0.66), but this was still modest.

CRT showed the best predictive ability to identify a low stroke volume index when it was ≥6 seconds.

It should be noted that most of the patients received inotropic support for their blood pressure, which would have affected CRT but is representative of the pediatric intensive care unit population.

No correlation was found between CRT and other hemodynamic variables in children after cardiac surgery.

Prolonged CRT was independently associated with death in children with severe and complicated malaria in sub-Saharan Africa (a disease that causes 2 million deaths a year).

For children with severe anemia associated with malaria, the risk of dying was twice as high if they had a prolonged CRT. The prolonged CRT (> 3 seconds) is also a component of a prognostic scoring scale developed for African meningococcal epidemics.

A late CRT was identified as one of the strongest warnings of severe infection in developed countries in a recent high-profile review of the clinical features used to confirm or exclude the possibility of severe infection in children attending outpatient settings.

This is consistent with the results published by the World Health Organization for resource-poor countries.


However, the presence of a CRT of> 2 seconds is not predictive of mild to moderate hypovolemia in adults.

CRT was inconsistent when measured before and after rehydration in 32 adult emergency patients with a history suggestive of hypovolemia and hypotension or abnormal orthostatic signs.

An abnormal sign was an increase in heart rate of ≥20 beats per minute or decrease in diastolic blood pressure by> 15 mm Hg when the patient changed from supine to standing position) and in 47 blood donors before and after a 450 ml blood donation.

Using the 2-second upper limit of normal gave a sensitivity of 11% for blood donors, 47% for patients with abnormal orthostatic signs, and 77% for those with hypotension.

CRT measurement with subjective evaluation of peripheral perfusion in resuscitated critically ill adult patients evaluated in the first 24 hours of admission and once they were hemodynamically stable, was able to identify those with more severe organ dysfunction and higher levels of organ dysfunction. lactate.

From the available evidence, it appears that CRT measurement is most useful in evaluating patients with shock states.

In these situations, there may be an alteration in the balance of vasoconstrictor and vasodilator substances and in the crosstalk between endothelial cells, so that the regulation of microvascular blood flow is impaired.

Abnormalities also include arteriovenous shunt, capillaries where flow is intermittent, capillaries “no flow” (capillaries are blocked), failure of capillary recruitment, and increased capillary permeability with interstitial edema.

Capillaries can become clogged due to swelling of endothelial cells, reduced deformability of circulating erythrocytes, leukocyte-platelet-fibrin thrombi, or fluid compression edema, the end result is a reduction in functional capillary density.

This suggests that the CRT may be measuring alterations in the perfusion of the distal capillary bed.

The link between systemic hemodynamics and this peripheral perfusion is relatively loose, so these alterations can be observed even when systemic hemodynamics are within satisfactory targets.

However, if cardiac output and blood pressure are critically altered, then they can affect peripheral perfusion.

If CRT is indeed a simple measure of the state of distal capillary bed perfusion, then it is not surprising that Tibby et al. they did not find an important correlation between systemic hemodynamic variables and CRT measurement.

Findings from other studies that CRT is a good predictor of significant dehydration, severe infection, severe organ dysfunction, and higher lactate levels are related to CRT as a measure of distal perfusion as a whole, rather than being equivalent. to a single hemodynamic variable.

This evidence can be extrapolated to the use of CRT during the preoperative evaluation of patients and in patients undergoing general anesthesia, particularly emergency procedures and those involving significant blood loss and large fluid changes.

More recently, the focus has been on automated CRT measurement methods. These include digital videography (digitally measured capillary fill time [DCRT]) and the use of a photoplethysmographic (PPG) sensor based on a blue light emitter.

The DCRT replaces visual observation by replacing a set of electronic imaging sensors for the human eye.

In a study of 83 children with acute gastroenteritis who were assessed by clinicians to have at least mild dehydration, DCRT was found to be more accurate in determining the presence of significant dehydration (≥5%) than general clinical assessment.

The range of DCRT measurements in well-hydrated children (0.2-0.4 seconds) was substantially less than that of the standard CRT measurement.

The low wavelength light from the blue light PPG sensor only penetrates to the upper capillaries of the skin.

To detect occlusion of skin capillaries and track the refilling process, pressure was applied to the probe until the signal from the PPG sensor disappeared with a subsequent release after 4 to 5 seconds.

Three variables have been suggested to estimate the fill time: time for the signal to reach the original initial level after pressure release; time for the signal to reach its maximum; and time for the signal to return from the maximum to its initial level.

Methods other than CRT are being used to digitally assess peripheral perfusion.

Body temperature gradient measurements, orthogonal polarization spectral imaging, derivation of peripheral perfusion index from pulse oximetry, near infrared spectroscopy, and laser Doppler flowmetry are examples of semi-automated methods.

Each of these methods offers advantages and limitations that have been previously discussed.

Digitized techniques for measuring CRT are not available to the practicing clinician, and current designs that require a computer to process results, especially rebuilt pulse oximeter probes, or a video camera make them impractical for routine use in the clinical environment.

Before automatically measured CRT can replace standard manual testing, techniques must be validated on a wide range of subjects, including studies to assess their robustness under different lighting and temperature conditions.

Digitized CRT measurement offers new techniques for CRT quantification and the opportunity to define a new ‘gold standard’ for non-invasive CRT measurement.

For example, a temperature sensor that measures room temperature or skin temperature could be embedded with the sensor that measures CRT to provide a corrected value for temperature, and a sensor used to measure CRT could be used in combination with other clinical variables. like heart rate.

The pulse oximeter saturation and respiratory rate to produce a diagnostic tool for clinical classification.

In summary, CRT measurement is affected by multiple external factors, but has predictive value in evaluating dehydration and severe infection in children. There is little outcome data to support its use in adults.

In an intensive care unit with good lighting and in a warm room, a CRT of <2 seconds can be reassuring but, as with all tests, clinical decisions should not be based on CRT measurement in isolation, but as an aspect of the clinical picture as a whole.

There is no evidence to justify its use in anesthetized patients. Operating rooms are cold, patients are often covered, limiting access, and because most anesthetics are potent vasodilators, the use of CRT to guide practice is not justified.

The possibility of a false positive or false negative evaluation is simply too great. Digitized and perhaps automated CRT measurements have the potential to overcome some of these limitations.

The use of innovative technology in the evaluation of CRT would be an opportunity to apply more robust and reliable non-invasive methods to evaluate the peripheral circulation.