Optimal pressing strength and time for capillary refilling time

Substantial investigations on the microcirculation have highlighted the importance of the capillary refilling time (CRT), which is widely used in clinical settings [1–3]. However, CRT measurement conditions, including pressing strength and time, are inconsistent (e.g., pressing strength: light, moderate, or firm; pressing time: 3 s, 5 s, or until the capillary bed visually blanches) [2–5]. CRT is conventionally measured visually after manual compression, which limits the accuracy of measurement and precludes the precise determination of how long and strong the nail bed should be pressed. Therefore, we developed a novel device which can be adjusted for pressing strength and time to precisely measure CRT using an electric actuator and strength and color sensors. CRT was measured in ageand sex-matched healthy adults (n = 31) using the developed device under conditions of pressing strength of 1, 3, 5, and 7N, and a pressing time 1, 2, 3, 4, 5, and 6 s (Additional file 1: Methods and supplemental data). There was a significant difference in CRT for pressing strength but not for pressing time (two-way analysis of variance (ANOVA): strength P < 0.001, time P = 0.97) (Fig. 1). There were significant differences in CRT between pressing strengths of 1 and 3 N (P < 0.01) but no significant differences among 3, 5, and 7 N (P = 0.16). Thus, 3–7 N appears to be the optimal strength range for CRT measurement. A strength of 3 N is that which is needed to lift an object weighing 300 g (e.g., two smartphones) using fingers; the examinees would feel a 300-g load on their fingers. To identify sufficient pressing time, we further analyzed the plots of the readings from the color sensor during nail bed compression. The measured values change rapidly in the initial part of compression (rapid phase) until the inflection point, where the slope of color change reduces (slow phase) (Fig. 2a). The longest period of the rapid phase was 1.999 s among all the study subjects (minimum 0.1022 s, 95% 1.149 s) (Fig. 2b). The pressure release during the rapid phase may destabilize this measurement. Thus, a pressing time of 2 s is a good threshold to obtain stable CRT measurements for all subjects. In the study aimed at identifying the standard pressing strength and CRT, pressing the nail bed with 3–7 N for 2 s appears to be optimal. Further development of portable CRT measurement devices fulfilling these conditions may contribute to achieving precise CRT measurements to monitor the microcirculation in clinical settings.

Substantial investigations on the microcirculation have highlighted the importance of the capillary refilling time (CRT), which is widely used in clinical settings [1][2][3]. However, CRT measurement conditions, including pressing strength and time, are inconsistent (e.g., pressing strength: light, moderate, or firm; pressing time: 3 s, 5 s, or until the capillary bed visually blanches) [2][3][4][5]. CRT is conventionally measured visually after manual compression, which limits the accuracy of measurement and precludes the precise determination of how long and strong the nail bed should be pressed. Therefore, we developed a novel device which can be adjusted for pressing strength and time to precisely measure CRT using an electric actuator and strength and color sensors.
CRT was measured in age-and sex-matched healthy adults (n = 31) using the developed device under conditions of pressing strength of 1, 3, 5, and 7 N, and a pressing time 1, 2, 3, 4, 5, and 6 s (Additional file 1: Methods and supplemental data).
There was a significant difference in CRT for pressing strength but not for pressing time (two-way analysis of variance (ANOVA): strength P < 0.001, time P = 0.97) (Fig. 1). There were significant differences in CRT be-tween pressing strengths of 1 and 3 N (P < 0.01) but no significant differences among 3, 5, and 7 N (P = 0.16). Thus, 3-7 N appears to be the optimal strength range for CRT measurement. A strength of 3 N is that which is needed to lift an object weighing 300 g (e.g., two smartphones) using fingers; the examinees would feel a 300-g load on their fingers. To identify sufficient pressing time, we further analyzed the plots of the readings from the color sensor during nail bed compression. The measured values change rapidly in the initial part of compression (rapid phase) until the inflection point, where the slope of color change reduces (slow phase) (Fig. 2a). The longest period of the rapid phase was 1.999 s among all the study subjects (minimum 0.1022 s, 95% 1.149 s) (Fig. 2b). The pressure release during the rapid phase may destabilize this measurement. Thus, a pressing time of 2 s is a good threshold to obtain stable CRT measurements for all subjects.
In the study aimed at identifying the standard pressing strength and CRT, pressing the nail bed with 3-7 N for 2 s appears to be optimal. Further development of portable CRT measurement devices fulfilling these conditions may contribute to achieving precise CRT measurements to monitor the microcirculation in clinical settings.   Figure E1. Picture of developed device. The upper image is the uncovered condition to show the internal structure. The lower image is the covered condition in which we measured CRT of study subjects. 1) Stepper motor. 2) Uni-axial stage. 3) Color sensor, light source and indenter. 5) Industrial camera. 6) Emergency stop switch. 7) Cover for hiding press principle. Size 16 cm × 20 cm × 23 cm. Figure E2. Principle of the device. Figure E3. Definition of CRT . Table E1. Characteristics of subjects and data. (DOCX 29524 kb)

Funding
This work was supported by AMED (#JPhe1502001). The funder had no role in the study design, experiments, collection, analyses, interpretation of data, writing of the manuscript, or decision to submit the manuscript for publication.

Availability of data and materials
The datasets used and/or analyzed during our study are available from the corresponding author upon reasonable request.
Authors' contributions RK and TN: These authors contributed to the study conceptualization and design, acquisition of data, analysis and interpretation of data, statistical analysis, and drafting and critical revision of the manuscript for important intellectual content. MS and TN: These authors contributed to developing the device, interpretation of data, and drafting and critical revision of the manuscript for important intellectual content. TO, HH, and SO: These authors contributed to the study conceptualization and critical revision of the manuscript for important intellectual content. All authors have read and approved the final manuscript.

Ethics approval and consent to participate
The study participants were informed of the experiment and agreements were obtained via consent form before the measurements. All procedures in this study were approved by the Ethical Review Board of the Chiba University Graduate School of Medicine.

Consent for publication
Written informed consent for the publication was obtained.