Evaluation of accuracy of capillary refill index with pneumatic fingertip compression

Capillary refill time (CRT) is a method of measuring a patient's peripheral perfusion status through a visual assessment performed by a clinician. We developed a new method of measuring CRT using standard pulse oximetry sensor, which was designated capillary refill index (CRI). We evaluated the accuracy of CRI in comparison to CRT image analysis. Thirty healthy adult volunteers were recruited for a derivation study and 30 patients in the emergency department (ED) were for validation. Our high fidelity mechanical device compresses and releases the fingertip to measure changes in blood volume using infrared-light (940 nm). CRT was calculated by image analysis software using recorded fingertip videos. CRI and CRT were measured at: room temperature (ROOM TEMP), 15 °C cold water (COLD), and 38 °C warm water (REWARM). Intra-rater reliability, Bland-Altman plots, and correlation coefficients were used to evaluate the accuracy of the novel CRI method. CRI (4.9 [95% CI 4.5-5.3] s) and CRT (4.0 [3.6-4.3]) in the COLD group were higher than the ROOM TEMP and REWARM groups. High intra-rater reliability was observed in both measurements (0.97 [0.95-0.98] and 0.98 [0.97-0.99], respectively). The Bland-Altman plots suggested a systematic bias: CRI was consistently higher than CRT (difference: + 1.01 s). There was a strong correlation between CRI and CRT (r = 0.89, p < 0.001). ED patients had higher CRI (3.91 [5.05-2.75]) and CRT (2.21 [3.19-1.23]) than those of healthy volunteers at room temperature. The same difference and correlation patterns were verified in the ED setting. CRI was as reliable as CRT by image analysis. The values of CRI was approximately 1 s higher than CRT.