Optical Fibre Sensor for Capillary Refill Time and Contact Pressure Measurements under the Foot

Abstract

Capillary refill time (CRT) refers to the time taken for body tissue to regain its colour after an applied blanching pressure is released. Usually, pressure is manually applied and not measured. Upon release of pressure, simple mental counting is typically used to estimate how long it takes for the skin to regain its colour. However, this method is subjective and can provide inaccurate readings due to human error. CRT is often used to assess shock and hydration but also has the potential to assess peripheral arterial disease which can result in tissue breakdown, foot ulcers and ultimately amputation, especially in people with diabetes. The aim of this study was to design an optical fibre sensor to simultaneously detect blood volume changes and the contact pressure applied to the foot. The CRT probe combines two sensors: a plastic optical fibre (POF) based on photoplethysmography (PPG) to measure blood volume changes and a fibre Bragg grating to measure skin contact pressure. The results from 10 healthy volunteers demonstrate that the blanching pressure on the subject's first metatarsal head of the foot was 100.8 ± 4.8 kPa (mean and standard deviation), the average CRT was 1.37 ± 0.46 s and the time to achieve a stable blood volume was 4.77 ± 1.57 s. For individual volunteers, the fastest CRT measured was 0.82 ± 0.11 and the slowest 1.94 ± 0.49 s. The combined sensor and curve fitting process has the potential to provide increased reliability and accuracy for CRT measurement of the foot in diabetic foot ulcer clinics and in the community.

Keywords: blood volume changes; capillary refill time; contact pressure; fibre Bragg grating (FBG); optical fibre; peripheral arterial disease; photoplethysmography (PPG); plastic optical fibre.

Figure A1

Applied pressure (blue trace), reflected light intensity changes (orange trace) and underfoot temperature (black trace) of ten participants (P1–P10).

Figure A1

Applied pressure (blue trace), reflected light intensity changes (orange trace) and underfoot temperature (black trace) of ten participants (P1–P10).

Figure A2

The 10 curves fitting each participant (P1–P10), with the maximum (0.9) and minimum (0.1) thresholds in black dashed lines.

Figure A2

The 10 curves fitting each participant (P1–P10), with the maximum (0.9) and minimum (0.1) thresholds in black dashed lines.

Figure 1

(a) Contact pressure sensor schematic, and (b) photograph of the final constructed sensor.

Figure 2

(a) PPG sensor for measuring changes in blood volume. The POF brown (receives light) and POF red (transmits light) channels are only used in this application. (b) Side view of PPG sensor. (c) 3D mould for embedding PPG sensor in an epoxy layer. (d) Photograph of the PPG sensor integrated within the epoxy layer used for CRT measurements.

Figure 3

(a) Side view schematic of the combined sensor of the CRT measurements (PPG and contact pressure) (LED—light emitting diode, POF—plastic optical fibre, FBG—fibre Bragg grating, PD—photodiode) (b) Photograph of the sensor.

Figure 4

(a) Pressure calibration system comprising a stepper motor and scale to apply a known force to the FBGs. (b) Photograph of the force measurement plate (RS Scan Footscan) with the combined sensor on top of it. The active area of the foot scan plate was 488 mm × 325 mm with 4096 sensors (arranged in a 64 × 64 matrix). The dimension of each foot scan plate sensor (pixel) was 7.62 mm × 5.08 mm. (c) Pressure map under the foot. The location of the combined optical fibre sensor underfoot is highlighted in the white frame.

Figure 5

Schematic (a) and photograph (b) of the experimental setup of the CRT and contact pressure measurements; light source (LED, λ = 530 nm), photodetector (PD + TIA) PDA36A, interrogator (SmartScope), DAQ–data acquisition card. The sensor sits under the foot of the volunteer.

Figure 6

Recording of the changes of the PPG (orange trace) and the skin contact pressure (blue trace) simultaneously. The normal stage was approximately 0 to 10 s; the applying pressure stage was 10 to 21 s, and the refilling stage started after 21 s.

Figure 7

Normalised refilling signals (orange trace) with the polynomial curve fitting (blue trace). Estimating the CRT used the maximum and minimum thresholds method. The maximum threshold is 0.9 (black dashed line), and the minimum threshold is 0.1 (black dashed line). The CRT is the calculated time of the refilling signal between 0.9 and 0.1. A1 is the amplitude of the blanching intensity relative to the starting intensity, and A2 is the amplitude of the minimum intensity to the next local maximum.

Figure 8

The temperature responses of FBG1 (a) and FBG2 (c) sensors within the epoxy layer; the green traces show the temperature changes measured with a thermocouple and the blue trace presents the wavelength shift peak of FBGs. (b) and (d) are the hysteresis diagrams of the wavelength shift of FBG1 (b) and FBG2 (d) against variable temperatures.

Figure 9

The wavelength shift of FBG1 against the wavelength shift of FBG2 with increase and decrease of temperature.

Figure 10

(a) The loaded and unloaded pressure steps of FBG1 sensor after compensating for temperature. (b) FBG1 wavelength shift against loaded and unloaded pressure.

Figure 11

Bland–Altman plot comparing simultaneous response of optical fibre pressure with commercially available plantar pressure measurement device (Footscan). Three volunteers repeatedly stepped on the optical fibre sensor and Footscan. (LOA—limit of agreement).

Figure 12

Applied pressure (blue trace), reflected light intensity changes (orange trace) and underfoot temperature (black trace) of one participant. The data were recorded from the contact pressure sensor, the PPG (light intensity) and data logger, respectively. Application and release of pressure was repeated ten times.

Figure 13

(a–j) Ten normalised refilling curves (orange) and curve fit (blue) for a single subject. (k) Shows the 10 refilling fitted curves for a single subject along with the maximum (0.9) and minimum (0.1) thresholds (black dashed lines).

Figure 13

(a–j) Ten normalised refilling curves (orange) and curve fit (blue) for a single subject. (k) Shows the 10 refilling fitted curves for a single subject along with the maximum (0.9) and minimum (0.1) thresholds (black dashed lines).

Figure 14

Example of a refilling signal that reached a stable blood volume after (a) and before (b) passing the first max blood volume to max recovery (A2). The stabilise estimation is based on the stability of the PI of the PPG signals.

Figure 15

CRT against coefficient C₀ of the polynomial fitting for all ten volunteers.