Valencell technology: Getting to the heart of the matter
I have recently had an electrocardiogram as part of my physical with 12 leads ‘glued’ to my body and gel everywhere. Well, doctors will be using that method for a while yet, but let’s look at an alternate method of measuring heart rate during exercise---a non-medical activity that does not need American Medical Association (AMA) or FDA approval.
If we look at the report in Reference 1, it examines the accuracy and repeatability of Valencell’s earbud sensor as compared to a 12-lead electrocardiogram method to measure heart rate accurately during exercise. Valencell uses the photoplethysmography (PPG)4 technique to measure heart rate. The PPG method measures blood volume pulse (BVP) via optical plethysmographic methods and provides for heart rate, inter-beat interval (the time intervals between consecutive heart beats that are typically measured via the electrocardiogram) and blood vessel dilation/constriction data.
Let’s take a look at the many external stimuli that make measuring heart rate accurately a very difficult task.
BVP and skin conductance (SC)3
Physiological or biological stress is the human body’s response to an external stimulus from an environmental condition; loud sounds, big crowds, high intensity lighting, and other such stimuli in a challenging situation. The body will respond via the autonomic nervous system (ANS). The ANS is body’s control mechanism that regulates heart rate, perspiration, respiration, pupil diameter and digestion. It is comprised of two main areas: the sympathetic and parasympathetic pathways, which serve opposing functions: the sympathetic nervous system (SNS) prepares the body for action, but the parasympathetic nervous system (PNS) promotes relaxation and digestion.
What is electrodermal activity (EDA)?
EDA is an involuntary change in the electrical properties of the skin. Skin conductance is determined by the SNS. As the SNS is engaged it triggers increased sweating at the skin that can be measured by applying an electrical potential between two points and measuring the resulting current flow between them. Skin conductance is expressed in microsiemens (μS) and increases with the level of arousal. During relaxation, the skin conductance level normally decreases. The EDA signal includes both baseline tonic (skin conductance level, or SCL) and rapid phasic (skin conductance responses, or SCRs) components. The latter occur spontaneously but can also be triggered by external events.
What is cardiovascular response (CR)?
CR is a change in heart rate and is influenced by both the SNS and PNS and can be measured using either an electrocardiogram or photoplethysmographic (PPG) sensor. Heart rate variability (HRV) has been used in medicine and biofeedback regimes to promote health. HRV measures how heart rate varies and researchers have used the frequency components of the inter-beat interval signal to extract low and high frequencies. Low frequencies have sympathetic involvement. High frequencies have sympathetic and parasympathetic involvement, so ratios of these have been used to quantify the relative involvement of the SNS in psychophysiology.
Each heartbeat briefly increases blood volume in the arteries and capillary beds. The amount of blood that gets pumped around the body is an easily accessible measure of affect in terms of heart rate and is, measured by blood volume pulse (BVP) amplitude. Arousal and involvement of the sympathetic nervous system increases blood flow to vital organs and reduces blood flow to extremities such as the fingers. So, sympathetic arousal leads to restriction of the blood vessels which is indexed by reductions in BVP amplitude.
The paper in Reference 3 does a really nice mapping of psychophysiological signal features that are taken from BVP and SC sensors to a number of subject’s emotional state and cognitive load. It has been shown that cognitive tasks have an impact on breathing and heart rate variability (HRV). Reference 3 used the SC and BVP features such as skin conductance response (SCR) and heart rate (HR) to determine the emotional state, and the spectral power in the low frequency (LF) band of the heart rate variability (HRV) spectrum plus SCR helped determine cognitive load.
Valencell has managed to overcome these hurdles and more to determine a repeatable and accurate heart rate during activity--such as motion artifacts.