Abstract ASA 2010 San Diego
Capillary Refill Time Assessment Using a Mobile Phone Application (iRefill)
W. Karlen, Ph.D.1, C. Petersen, Ph.D.2, A. Pickard, F.R.C.A.2, G. Dumont, Ph.D.1 and J.
M. Ansermino, F.R.C.P.C.2
1Electrical and Computer Engineering, University of British Columbia and
2Anesthesiology, Pharmacology & Therapeutics, University of British Columbia,
Vancouver, BC, Canada.
Introduction
Identifying capillary refill time (CRT) is an integral part of the clinical assessment of
circulatory status1 and identification of dehydration in children2. However, visual
inspection of the finger to assess CRT has low inter-observer reliability3,4, largely due to
human limitations in estimating short time intervals. To improve precision, we have
developed a mobile phone software application (iRefill) that automatically assesses CRT
using a photo-plethysmogram (PPG) sensor. Commonly used to measure blood oxygen
saturation and heart rate, this sensor can be adapted to replace the human eye to
objectively measure CRT.
Materials & Methods
Prototype Development: The PPG device consisted of a PureLight small soft sensor
connected to an Xpod OEM module (Nonin, Plymouth, USA). The module was
connected to an iPod Touch (Apple, Cupertino, USA) that displayed the PPG waveform
and recorded the data stream. The PPG was recorded with a 16bit resolution at a
sampling rate of 75 Hz to an ASCII file.
Prototype Evaluation: Twelve children between 1 and 5 years who were to undergo
general anesthesia were recruited following ethical board review and written parental
informed consent. While under anesthesia, the PPG sensor was placed on their right
index finger. Pressure was applied to it for 5s using an infant blood pressure cuff inflated
to 20mmHg above the patient's systolic blood pressure. This procedure was repeated
three times on each patient.

Results & Discussion
Prototype Development: We based iRefill on an OpenGL for embedded systems user
interface to maximize portability to mobile platforms; it currently compiles on iPhone OS,
Windows, MAC OS X, and Linux. Operating with 8bit and 16bit Nonin devices, iRefill
automatically detects the correct protocol upon connection. It is also fault tolerant,
allowing the sensor to be inserted and removed during operation. The prototype can
power the sensor for up to five hours (600 CRT measurements) while continuously
recording and displaying data. We are investigating the integration of wireless Bluetooth-
based communication into iRefill.
Prototype Evaluation: The PPG consistently showed a characteristic pattern across 36
normal CRT measurements. The significantly higher spike amplitude generated by the
pressure release compared with the regular PPG pulse amplitude (mean difference 22%
SD 10%) suggests that it is possible to automatically detect the time that pressure on the
sensor has been released.
References
1. PALS Provider Manual Hazinski, H., Zaritsky, A., Nadkarni, V. (AHA, 2002).
2. Mackenzie, A., Barnes, G., Shann, F., Lancet. 2, 605-7 (1989)
3. Otieno, H. et al., Arch Dis Child. 89, 977-979 (2004)
4. Klupp, N., Keenan, A., The Foot. 17, 15-20 (2007)
Funding: University of British Columbia Martha Piper Research Fund
Summary: Prototype development and evaluation of iRefill for assessing capillary refill
time with an iPod Touch.