There is an ever-growing demand for measuring respiratory variables during a variety of applications, including monitoring in clinical and occupational settings, and during sporting activities and exercise. Special attention is devoted to the monitoring of respiratory rate because it is a vital sign, which responds to a variety of stressors. There are different methods for measuring respiratory rate, which can be classed as contact-based or contactless. The present paper provides an overview of the currently available contact-based methods for measuring respiratory rate. For these methods, the sensing element (or part of the instrument containing it) is attached to the subject’s body. Methods based upon the recording of respiratory airflow, sounds, air temperature, air humidity, air components, chest wall movements, and modulation of the cardiac activity are presented. Working principles, metrological characteristics, and applications in the respiratory monitoring field are presented to explore potential development and applicability for each method.
Respiratory rate is a fundamental vital sign that is sensitive to different pathological conditions (e.g., adverse cardiac events, pneumonia, and clinical deterioration) and stressors, including emotional stress, cognitive load, heat, cold, physical effort, and exercise-induced fatigue. The sensitivity of respiratory rate to these conditions is superior compared to that of most of the other vital signs, and the abundance of suitable technological solutions measuring respiratory rate has important implications for healthcare, occupational settings, and sport. However, respiratory rate is still too often not routinely monitored in these fields of use. This review presents a multidisciplinary approach to respiratory monitoring, with the aim to improve the development and efficacy of respiratory monitoring services. We have identified thirteen monitoring goals where the use of the respiratory rate is invaluable, and for each of them we have described suitable sensors and techniques to monitor respiratory rate in specific measurement scenarios. We have also provided a physiological rationale corroborating the importance of respiratory rate monitoring and an original multidisciplinary framework for the development of respiratory monitoring services. This review is expected to advance the field of respiratory monitoring and favor synergies between different disciplines to accomplish this goal.
The use of wearable sensor technology for athlete training monitoring is growing exponentially, but some important measures and related wearable devices have received little attention so far. Respiratory frequency (fR), for example, is emerging as a valuable measurement for training monitoring. Despite the availability of unobtrusive wearable devices measuring fR with relatively good accuracy, fR is not commonly monitored during training. Yet fR is currently measured as a vital sign by multiparameter wearable devices in the military field, clinical settings, and occupational activities. When these devices have been used during exercise, fR was used for limited applications like the estimation of the ventilatory threshold. However, more information can be gained from fR. Unlike heart rate, trueV˙O2, and blood lactate, fR is strongly associated with perceived exertion during a variety of exercise paradigms, and under several experimental interventions affecting performance like muscle fatigue, glycogen depletion, heat exposure and hypoxia. This suggests that fR is a strong marker of physical effort. Furthermore, unlike other physiological variables, fR responds rapidly to variations in workload during high-intensity interval training (HIIT), with potential important implications for many sporting activities. This Perspective article aims to (i) present scientific evidence supporting the relevance of fR for training monitoring; (ii) critically revise possible methodologies to measure fR and the accuracy of currently available respiratory wearables; (iii) provide preliminary indication on how to analyze fR data. This viewpoint is expected to advance the field of training monitoring and stimulate directions for future development of sports wearables.
What is the central question of this study? By manipulating recovery intensity and exercise duration during high-intensity interval training (HIIT), we tested the hypothesis that fast inputs contribute more than metabolic stimuli to respiratory frequency (f ) regulation. What is the main finding and its importance? Respiratory frequency, but not tidal volume, responded rapidly and in proportion to changes in workload during HIIT, and was dissociated from some markers of metabolic stimuli in response to both experimental manipulations, suggesting that fast inputs contribute more than metabolic stimuli to f regulation. Differentiating between f and tidal volume may help to unravel the mechanisms underlying exercise hyperpnoea. Given that respiratory frequency (f ) has been proposed as a good marker of physical effort, furthering the understanding of how f is regulated during exercise is of great importance. We manipulated recovery intensity and exercise duration during high-intensity interval training (HIIT) to test the hypothesis that fast inputs (including central command) contribute more than metabolic stimuli to f regulation. Seven male cyclists performed an incremental test, a 10 and a 20 min continuous time trial (TT) as preliminary tests. Subsequently, recovery intensity and exercise duration were manipulated during HIIT (30 s work and 30 s active recovery) by performing four 10 min and one 20 min trial (recovery intensities of 85, 70, 55 and 30% of the 10 min TT mean workload; and 85% of the 20 min TT mean workload). The work intensity of the HIIT sessions was self-paced by participants to achieve the best performance possible. When manipulating recovery intensity, f , but not tidal volume (V ), showed a fast response to the alternation of the work and recovery phases, proportional to the extent of workload variations. No association between f and gas exchange responses was observed. When manipulating exercise duration, f and rating of perceived exertion were dissociated from V , carbon dioxide output and oxygen uptake responses. Overall, the rating of perceived exertion was strongly correlated with f (r = 0.87; P < 0.001) but not with V . These findings may reveal a differential control of f and V during HIIT, with fast inputs appearing to contribute more than metabolic stimuli to f regulation. Differentiating between f and V may help to unravel the mechanisms underlying exercise hyperpnoea.
In order to provide further insight into the link between respiratory frequency (fR) and the rating of perceived exertion (RPE), the present study investigated the effect of exercise duration on perceptual and physiological responses during self-paced exercise. Nine well-trained competitive male cyclists (23 ± 3 years) performed a preliminary incremental ramp test and three randomised self-paced time trials (TTs) differing in exercise duration (10, 20 and 30 min). Both RPE and fR increased almost linearly over time, with a less-pronounced rate of increase when absolute exercise duration increased. However, when values were expressed against relative exercise duration, no between-trial differences were found in either RPE or fR. Conversely, between-trial differences were observed for minute ventilation (.VE), .VO2 and heart rate (HR), when values were expressed against relative exercise duration. Unlike the relationship between RPE and both .VE and HR, the relationship between RPE and fR was not affected by exercise duration. In conclusion, fR, but not .VE, HR or [.VO2, shows a strong relationship to RPE and a similar time course, irrespective of exercise duration. These findings indicate that fR is the best correlate of RPE during self-paced exercise, at least among the parameters and for the range of durations herein investigated.
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