Knowledge Base

The Vital Signs Module

Main Topics

• Introduction to the Vital Signs Module
• Analyze heart rate, heart rate variability and breathing rate
• Analysis in phases
• Visualize vital signs

Introduction To The Vital Signs Module

Vital Signs

The Vital Signs Module allows you to measure heart rate, heart rate variability and breathing rate. Heart rate and heart rate variability are measured by means of remote photoplethysmography (RPPG). Breathing rate is measured by detecting upper-body movement associated with respiration.

Remote PPG

Photoplethysmography is a simple and low-cost optical technique that can be used to detect blood volume changes in the tissue under the skin. It is based on the principle that changes in the blood volume result in changes in the light reflectance of the skin.

With each cardiac cycle the heart pumps blood to the periphery. Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue.

PPG is often used non-invasively to make measurements at the skin surface. In remote PPG (RPPG), FaceReader can detect the change in blood volume caused by the pressure pulse when the face is properly illuminated. The amount of light reflected is then measured. When reflectance is plotted against time, each cardiac cycle appears as a peak. This information can be converted to heart rate average and variability.

For an overview of the accuracy of the estimated heart rate, see Gudi et al. (2020).

References

Photoplethysmography

Reisner, A. T., Shaltis, P. A., McCombie, D. and Asada, H. H. (2008). Utility of the Photoplethysmogram in Circulatory Monitoring. Anesthesiology, 108, 950-958.

http://en.wikipedia.org/wiki/Photoplethysmogram

Remote PPG And FaceReader

Gudi, A.; Bittner, M.; Van Gemert, J. (2020) Real-time webcam heart rate and variability estimation with clean ground truth for evaluation. Applied Sciences 10(23): 8630.

Tasli H. E., Gudi, A., and den Uyl, M. (2014). Integrating Remote PPG in Facial Expression Analysis Framework. Proceedings of the 16th International Conference on Multimodal Interaction, Istanbul, November 12-16, 2014, 74-75.

http://www.vicarvision.nl/pub/RPPG_Facial_Expression_Analysis.pdf

Please cite the original work in your studies when based on FaceReader's remote PPG.

Patent Number

The European Patent Office has granted patent EP2960862A1 to Vicarious Perception Technologies for the method for stabilizing vital sign measurements using parametric facial appearance models via remote sensors (https://patents.google.com/patent/EP2960862A1/en).

Vital Signs Module

With FaceReader's Vital Signs Module, you can analyze heart rate, heart rate variability (HRV), and breathing rate of the test participant without additional hardware other than a suitable webcam.

Heart rate variability can be derived from the single heart beats that are used to calculate the heart rate. HRV measures the variability between heart beats, which can give important insights into the physiology of a person as it reflects changes in activation of the autonomic nervous system. With FaceReader, you can obtain the timing of individual heart beats (which can be visualized in FaceReader's heart beat log), as well as HRV measures like RMSSD and SDNN. In addition, FaceReader can measure a test participant's respiration rate.

RMSSD

One of the most commonly used measures of heart rate variability is the root mean square of successive differences (RMSSD).

The differences in time between single heart beats are called RR intervals or interbeat intervals.

RR_n = beat_n+1 - beat_n

Differences between the RR intervals are called successive differences (SD).

SD_n = RR_n+1 - RR_n

The RMSSD is the root mean square of two successive RR intervals. The unit of RMSSD is the same as the time unit chosen in equation 1 (usually it is milliseconds).

RMSSD = √( 1/(n-1) ∑_i=1ⁿ (SD_n)² )

SDNN

SDNN is the standard deviation of normal to normal R-R intervals. "Normal" means that abnormal beats, like ectopic beats (heart beats that originate outside the right atrium's sinoatrial node), have been removed.

Source: FaceReader 10 Reference Manual (Help), Noldus Information Technology