Background: Photoplethysmography (PPG) sensors, typically found in wrist-worn devices, can continuously monitor heart rate (HR) in large populations in real-world settings. Resting heart rate (RHR) is an important biomarker of morbidities and mortality, but no universally accepted definition nor measurement criteria exist. In this study, we provide a working definition of RHR and describe a method for accurate measurement of this biomarker, recorded using PPG derived from wristband measurement across the 24-hour cycle. Methods: 433 healthy subjects wore a wrist device that measured activity and HR for up to 3 months. HR during inactivity was recorded and the duration of inactivity needed for HR to stabilise was ascertained. We identified the lowest HR during each 24-hour cycle (true RHR) and examined the time of day or night this occurred. The variation of HR during inactivity through the 24-hour cycle was also assessed. The sample was also subdivided according to daily activity levels for subset analysis. Findings: Adequate data was obtained for 19,242 days and 18,520 nights. HR stabilised in most subjects after 4 minutes of inactivity. Mean (SD) RHR for the sample was 54.5 (8.0) bpm (day) and 50.5 (7.6) bpm (night). RHR values were highest in the least active group (lowest MET quartile). A circadian variation of HR during inactivity was confirmed, with the lowest values being between 0300 and 0700 hours for most subjects. Interpretation: RHR measured using a PPG-based wrist-worn device is significantly lower at night than in the day, and a circadian rhythm of HR during inactivity was confirmed. Since RHR is such an important health metric, clarity on the definition and measurement methodology used is important. For most subjects, a minimum rest time of 4 minutes provides a reliable measurement of HR during inactivity and true RHR in a 24-hour cycle is best measured between 0300 and 0700 hours. Funding: This study was funded by Google.

The Journal of Biomedical Optics (JBO) publishes peer-reviewed papers on the use of novel optical systems and techniques for improved health care and biomedical research.

Completely noninvasive monitoring of hemoglobin concentration has not yet been fully realized in the clinical setting. This study investigates the viability of measuring hemoglobin concentration noninvasively by evaluating the performance of two types of sensor using a tissue phantom perfused with a blood substitute. An electrical sensor designed to measure blood volume changes during the cardiac cycle was used together with an infrared optical sensor for detection of erythrocyte-bound hemoglobin. Both sensors demonstrated sensitivity to changes in pulse volume (plethysmography). The electrical sensor produced a signal referred to as capacitance plethysmograph (CPG) a quantity which was invariant to the concentration of an infrared absorbing dye present in the blood substitute. The optical sensor signal (photoplethysmograph) increased in amplitude with increasing absorber concentration. The ratio PPG:CPG is invariant to pulse pressure. This quantity is discussed as a possible index of in vivo hemoglobin concentration.

A simple laboratory method is presented for producing optical fibers with tips polished at various angles. Angled optical fiber tips are used in applications such as optical sensing and remote laser surgery, where they can be used to control the angle of light leaving the fiber or direct it to the side. This allows for greater control and allows areas to be reached that otherwise could not. Optical fibers were produced with tip angles of 45 deg using a Perspex mounting block with an aluminum base plate. The dispersion of light leaving the tip was tested using a blue (470 nm) LED. The angle imposed an angular shift on the light diffracting out of the tip of approximately 30 deg. Additionally, some light reflected from the tip surface to diffract at 90 deg through the side of the fiber. These observations are consistent with theory and those seen by other studies, validating the method. The method was simple to perform and does not require advanced manufacturing tools. The method is suitable for producing small quantities of angle-tipped optical fibers for research applications.

Animal models are widely used to investigate the pathological mechanisms of spinal cord injury (SCI), most commonly in rats. It is well known that compromised blood flow caused by mechanical disruption of the vasculature can produce irreversible damage and cell death in hypoperfused tissue regions and spinal cord tissue is particularly susceptible to such damage. A fiberoptic photoplethysmography (PPG) probe and instrumentation system were used to investigate the practical considerations of making measurements from rat spinal cord and to assess its suitability for use in SCI models. Experiments to assess the regional perfusion of exposed spinal cord in anesthetized adult rats using both PPG and laser Doppler flowmetry (LDF) were performed. It was found that signals could be obtained reliably from all subjects, although considerable intersite and intersubject variability was seen in the PPG signal amplitude compared to LDF. We present results from 30 measurements in five subjects, the two methods are compared, and practical application to SCI animal models is discussed.