Non-contact heart rate and heart rate variability measurements: A review

Abstract

The following paper investigates published work on non-contact human physiological parameter measurement, more precisely measurement of the human heart rate (HR) and consequently the heart rate variability (HRV), which is considered to be an important marker of autonomic nervous system activity proven to be predictive of the likelihood of future health related events. The ability to perform measurements of cardiac activity in a non-contact manner could prove to become an important alternative to the conventional methods in the clinical field as well as in the more commercially oriented fields. Some of the published work so far indicates that the measurement of cardiac activity in a non-contact manner is indeed possible and in some cases also very precise, however there are several limitations to the methods which need to be taken into account when performing the measurements. The following paper includes a short description of the two conventional methods, electrocardiogram (ECG) and photoplethysmography (PPG), and later on focuses on the novel methods of non-contact measuring of HR with capacitively coupled ECG, Doppler radar, optical vibrocardiography, thermal imaging, RGB camera and HR from speech. Our study represents a comparative review of these methods while emphasising their advantages and disadvantages.

Introduction

Measuring of human physiological parameters on a regular basis out of the hospitalisation period could become an important feature in health care, affecting healthcare policies and healthcare economics on the one hand and our daily life on the other. During the past few years a lot has been learned about diseases at a genomic level, creating possibilities of an early detection of illness symptoms and improving the treatment process itself. Amongst other findings, numerous studies have shown a significant relationship between the autonomic nervous system (ANS) and cardiovascular mortality. More precisely, perturbations of the ANS and its imbalance were discovered to indicate impending cardiac diseases, which may lead to a sudden cardiac death, one of the leading causes of cardiovascular mortality [1].

The ANS function is necessary for the maintenance of homeostasis. It operates independently of voluntary control through the sympathetic and the parasympathetic nervous systems which often function in an antagonistic manner. The autonomic processes are involved in the control of many bodily functions, such as thermoregulation, blood pressure, regional blood flow, etc. The status of the ANS can therefore be assessed by observing several physiological parameters which can be obtained and processed with different measuring and analytical methods [2]. One of the markers for ANS assessment that has caught the attention of the profession is called the heart rate variability (HRV). Next to the clinical settings (e.g. diabetic neuropathy, myocardial infarction, sudden cardiac death, etc.) this parameter is also used in several other fields, such as sports science and ergonomics [3], [4].

HRV is a measurement of the oscillation between adjacent QRS complex intervals as well as the oscillations between consecutive instantaneous heart rates. Due to the seemingly easy derivation of the parameter, its use has been popularised with many (commercial) measuring devices providing automated HRV measurement [1]. However, the significance and meaning of the parameter analysis is more complex then generally appreciated. Furthermore, incorrect conclusions may lead to excessive extrapolations, which were one of the main reasons for the constitution of the Task Force back in 1996, responsible among other things for defining of measurement standards, result interpretations and identification of areas for future research [1].

The cardiac data used for further HRV analysis are generally obtained with one of the following two methods practised in clinical environment: electrocardiogram (ECG) or photoplethysmography (PPG). Although based on a different concept and measuring different phenomena, both methods provide reliable results when properly executed on the one hand, but are limited by several factors deriving mostly from the need of physical contact with the subject on the other hand. These limitations combined with increasing demands for ubiquitous measuring of human physiological parameters inside and outside of the hospital environment on the one hand and the possibility for use in commercial settings on the other hand has led researchers worldwide to search for a way to optimise the measuring process, freeing it from its limitations. As a result, several promising and innovative methods have been published. They can roughly be grouped into measuring methods using “contact” electrodes, “fixed-in-the-environment” electrodes and “non-contact” electrodes.

The acquisition of cardiac activity parameters in a non-contact manner could become a valuable tool in clinical health care applications as well as in the non-clinical environment. In an ideal measuring setting, the subject would not be aware of the measuring process itself, which would result in a decreased psychological factor of the measurement. Next to eliminating several limitations of the contact sensor based methods, such measurements would therefore also result in more objective readings. The main objective of this paper is to review the published novel and experimental non-contact measuring methods for measuring of heart rate (HR) and HRV. Additionally, we present a comparative review of the discussed novel methods, emphasising their advantages and disadvantages.