There is an ever-growing demand for measuring respiratory variables during a variety of applications. Respiratory rate, also known as respiratory frequency (f_R), appears to be among the most promising and measured variables because it provides fundamental information. In view of its peculiar neurophysiological regulation, f_R is very sensitive to a variety of physiological, psychological and environmental stressors [1,2,3], and is considered to be one of the most informative vital signs [4,5]. Conversely, tidal volume (the other determinant of minute ventilation) plays a major role in satisfying the metabolic requirements of the human body [1,2,3]. This explains why f_R and tidal volume have been defined as the behavioral and metabolic components of minute ventilation, respectively [1]. The differential control of f_R and tidal volume justifies the special attention devoted to f_R monitoring.

There are different methods for measuring f_R, which complicates the choice of the specific sensor or measurement technique to use. Several factors need to be considered when measuring f_R, including applications, measurement requirements, and user needs. This choice is even more difficult in view of the limited attempts made so far to classify and detail the numerous f_R measuring methods used [6,7,8]. Therefore, we aim to provide an overview of the currently available methods for measuring f_R. Given the extent of the topic, this review focuses on the contact-based methods only, to guarantee that each method is described in sufficient detail to enable the reader to make an informed choice on the f_R method to use.

 

Recently a review paper entitled "Contact-Based Methods for Measuring Respiratory Rate" has been published. The review paper can be access at the following link: https://www.mdpi.com/1424-8220/19/4/908/htm

 

The review paper is structured in nine sections. Section 1 provides a brief description of the importance of monitoring fR in clinical settings, occupational settings, and during sporting activities and exercise. Besides, Section 1 provides a taxonomy of the available techniques for measuring fR, which are categorized according to the measurand. For each of the methods identified in the taxonomy, we have dedicated a separate Section (from Section 2, Section 3, Section 4, Section 5, Section 6, Section 7 and Section 8), which consists of a brief introduction of the measuring principle and a short overview of the most popular sensors that can be used to transduce the physical or chemical quantity into a signal to extract f_R. At the end of each section, a table summarizes metrological properties, sensor characteristics and possible applications, and a short summary describes the strengths and weaknesses of the different sensors. Specifically, we describe methods based on airflow (Section 2), respiratory sounds (Section 3), air temperature (Section 4), air humidity (Section 5), air components (Section 6), respiratory-induced chest wall movements (Section 7), and respiratory modulation of cardiac activity (Section 8). The last section is dedicated to conclusions (Section 9).