Capillary Refill Index Measurement with a Temperature Control Procedure

Capillary refill time (CRT) test is a simple and noninvasive method typically used to assess peripheral blood perfusion at the bedside. CRT is defined as the time required for a distal capillary bed (e.g., fingertip) to regain its color after having received enough compression to cause blanching [1]. Because a prolonged CRT suggests a decrease in peripheral perfusion and poor peripheral perfusion is strongly associated with patients' mortality, several studies reported the clinical usefulness of CRT for predicting patients' mortality in the critical care setting [2,3]. However, because the conventional CRT test depends on clinicians' subjective visual assessment, the inter-rater agreement of it has been questioned, and substantial clinical experience is needed to acquire the reliable skill to measure CRT [4, 5, 6, 7]. Therefore, there is great demand for the creation of objective methods to easily assess peripheral blood perfusion.

Previously, the investigators developed a new mechanical device by using pulse oximetry waveforms, which can enable us to measure CRT easily and objectively. The measurement obtained with this device is called Capillary Refill Index (CRI) to differentiate it from traditional visual assessment of CRT [8]. The method calculates the time it takes for blood to return to the fingertip after release from compression by algorithmically analyzing the light intensity waveform of a normal pulse oximeter sensor attached to a patient's fingertip. The investigators' previous study showed that CRI can be an alternative to traditional CRT for assessing peripheral blood perfusion [8, 9, 10].

In the investigators previous study with healthy volunteer subjects [11], the investigators found that lowered fingertip temperature is a critical component that affects measurement values of CRI, which is the same phenomena with the traditional CRT. Since lowered skin temperature causes poor peripheral perfusion, the patients with lower fingertip temperature can show prolonged CRI without reflecting hemodynamic instability, which means lower skin temperature can increase false positives when using CRI to detect patients at high risk for mortality. The investigators hypothesize that the predictive accuracy of CRI for detecting patients at increased risk for mortality may be improved if the patients' skin temperature is warmed up before the measurement.