Background: Assessment of the critically ill is traditionally based on vital signs (blood pressure, pulse, respiratory rate, temperature and level of consciousness). Altered vital signs are, however, late indicators of deranged hemodynamics pointing to a need for additional, more sensitive markers of circulatory compromise. In the beginning of the 20th century, the capillary refill (CR) time evolved as a possible, non-invasive adjunct to early prediction of the outcome in the critically ill. The manoeuvre entails application of blanching pressure on the skin of the finger pulp or sternum for 5 seconds. After release of the pressure, the observer estimates time in seconds for the skin to return to original colour. This time is hypothesized to reflect the dynamics of the microcirculation and its possible connection with hemodynamics. In the 1980s the “normal capillary refill time” was set to < 2 seconds and later extended to 3 seconds, without a clear scientific foundation. Naked-eye estimations of CR time met increasing scepticism in the 1990s due to subjectivity and poor prognostic value for shock or death. Several basic traits, such as age and sex, as well as ambient temperature, were also shown to independently influence the CR time. Various methods have evolved with the capability to measure CR time quantitatively, one of which is Polarisation Spectroscopy Imaging (PSI). PSI measures the Red Blood Cell (RBC) concentration in tissue (e.g. the skin) and can be used to measure CR time.

Objectives: The purpose of this study was to establish basic characteristics for quantified CR (qCR), identify possible influencing factors in healthy subjects and to investigate how this relates to current practice. We also sought to identify technical demands for transfer of the technique into clinical studies. In paper I we analysed the (qCR) time characteristics at 5 different skin sites (forehead, sternum, volar forearm, finger pulp and dorsum finger). The objective of paper II was to investigate the inter- and intra-observer variability of naked eye CR assessments of different professions, nurses, doctors and secretaries (representing laymen). In paper III we observed the effect of low ambient temperature on the qCR time in different skin sites. In paper IV, we transferred the equipment from a laboratory to a clinical setting in the Emergency Department (ED) for application on potentially critically ill patients. In this study we evaluated the most important factors determining a reliable data collection and influencing the amount of data possible to analyse.

Methods: qCR time was measured in a total of 38 volunteers and 10 patients in different skin sites (2-5 skin sites) at different ambient temperatures. PSI (TiVi 600 and 700, WheelsBridge AB, Linköping, Sweden) was used to determine the rapid temporal changes in RBC concentration in skin during the CR manoeuvre. Films using a range of the first measurements from paper I were shown for assessment to 48 observers working in the ED.

Results: In paper I we could delineate qCR curves and suggest 2 possible equivalents to the naked-eye observed CR time which we named Time to Return to Baseline 1 (tRtB1) and Time to Peak (t_pk). We demonstrated differences in qCR-curves depending on skin site and possibly due to skin temperature. In paper II we showed a poor inter- and intra-observer reproducibility in visually estimating the CR time regardless of profession (clinicians or laymen). Paper III demonstrated a rapid effect of ambient temperature on qCR time in peripheral skin sites such as finger pulp. The forehead, regarded as a more central skin site was the most temperature stable site and showed least variability in qCR time as determined using tRtB1. Paper IV, a study on patients in an ED setting, yielded assayable data in 80% of the measurements. We identified critical performance parameters to address in the further development of a more robust, easy-to-use device for future validation of the possible relevance of qCR in patient triage and monitoring.

Conclusions: CR time can be quantified using PSI. Quantified CR time demonstrated a large variability between different skin sites, specifically, skin temperature was shown to be an important factor influencing qCR time, particularly at the fingertip. Naked-eye estimates of CR time were highly variable, both within and between observers. Agreement between quantified CR time and naked-eye estimates was poor. The prototypic PSI technique was feasible in a clinical setting and, with further improvements, clinical evaluation of qCR in relation to relevant patient outcomes will be possible.