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Coupling of Dyspnea Perception and Occurrence of Tachypnea During Exercise

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Coupling of Dyspnea Perception and Occurrence of Tachypnea During Exercise J Physiol Sci (2017) 67:173–180 DOI 10.1007/s12576-016-0452-5 ORIGINAL PAPER Coupling of dyspnea perception and occurrence of tachypnea during exercise 1,2 1 1 1 Setsuro Tsukada • Yuri Masaoka • Akira Yoshikawa • Keiji Okamoto • 1 1 Ikuo Homma • Masahiko Izumizaki Received: 3 December 2015 / Accepted: 12 April 2016 / Published online: 27 April 2016 Ó The Physiological Society of Japan and Springer Japan 2016 Abstract During exercise, tidal volume initially con- Introduction tributes to ventilatory responses more than respiratory frequency, and respiratory frequency then increases rapidly Respiratory output is primarily regulated by an autonomic while tidal volume stabilizes. Dyspnea intensity is also metabolic control system in the brainstem to maintain known to increase in a threshold-like manner. We tested blood gas homeostasis via reflex feedback pathways from the possibility that the threshold of tachypneic breathing is chemoreceptors. The ventilatory responses to experimental equal to that of dyspnea perception during cycle ergometer challenges such as hypercapnia and exercise have been exercise (n = 27). Dyspnea intensity was scored by a investigated to evaluate the capability of metabolic control visual analog scale. Thresholds were expressed as values of to cope with an increased demand for pulmonary gas pulmonary O2 uptake at each breakpoint. Dyspnea intensity exchange. As a result, participants’ overall ventilation and respiratory frequency started increasing rapidly once increases, depending on an individual’s unique respon- the intensity of stimuli exceeded a threshold level. The siveness to the demand [1–3]. thresholds for dyspnea intensity and for occurrence of The mechanisms that control ventilation seem more tachypnea were significantly correlated. An intraclass complex during exercise than during hypercapnia. Although correlation coefficient of 0.71 and narrow limits of agree- the primary driver of exercise hyperpnea is still controver- ment on the Bland–Altman plot indicated a good agree- sial [4, 5], a feed-forward control mechanism [6] and reflex ment between these thresholds. These results suggest that responses to afferent feedback from the periphery to the the start of tachypneic breathing coincides with the respiratory centers [7] are likely involved. Nevertheless, the threshold for dyspnea intensity during cycle ergometer depth and rhythm of breathing show a similar and stereo- exercise. typical pattern in response to both increases in the level of hypercapnia and exercise intensity [3, 8, 9]. During both Keywords Dyspnea Á Exercise hyperpnea Á Pulmonary O2 hypercapnia and exercise, breathing patterns comprise an uptake Á Respiratory frequency Á Tachypnea initial increase in tidal volume (VT) and a subsequent rapid increase in respiratory frequency (fR) at the VT plateau. This tachypneic breathing occurs once the intensity of stimuli exceed a threshold level [8, 10]. Duffin et al. [3] referred to this as the breathing patterning threshold. This tachypneic pattern may result from reflexes mediated by pulmonary & Masahiko Izumizaki stretch receptors and chest wall receptors [11]. However, [email protected] particularly in humans, mechanisms underlying this tachypneic pattern are still unclear [3, 12]. 1 Department of Physiology, Showa University School of Medicine, 1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142-8555, Exercise and hypercapnia increase overall ventilation Japan with changes in breathing patterns, accompanied by differ- 2 Department of Neurology, Showa University School of ing levels of dyspnea [13, 14]. Dyspnea increases in a Medicine, Tokyo, Japan threshold-like manner against pulmonary O2 uptake (VO2) 123 174 J Physiol Sci (2017) 67:173–180 during exercise and against the end-tidal CO2 fraction dur- monitor (Aeromonitor AE300, Minato Medical Science, ing hypercapnia [15–18]. We have previously reported that Osaka, Japan). This monitor measured flow rate at the the thresholds for dyspnea and fR are significantly corre- mouth with a hot-wire flow meter, and continuously cal- lated, and show a good agreement during gradual hyper- culated minute ventilation (VE, l/min), VT (l), and capnia in healthy human participants [19]. In the previous fR (breaths/min). The monitor also measured (breath-by- paper, the intensity of dyspnea and fR both showed rapid breath) the fractions of O2 and CO2 in expired gas to increases that were virtually simultaneous at a similar level determine VO2 and pulmonary CO2 output (VCO2). The of CO2. That is, changes in the subjective measure of dys- monitor was calibrated before each test. pnea coincide with those in the objective measure of respi- ration, particularly the onset of tachypneic breathing during Measurement of dyspnea intensity hypercapnia. Therefore, to extend our previous investigation [19], here we ask: does this subjective–objective coupling of The intensity of dyspnea was measured using a 100-mm respiratory variables reside in exercise hyperpnea? horizontal visual analog scale (VAS) with the maximum In a series of recent human and animal research under feeling of dyspnea on the right side and the feeling of no conscious conditions, we have attempted to show that dyspnea on the left side of the scale [19]. A piece of paper nonmetabolic factors play a part in alteration in the rhythm displaying the VAS was presented to the participants every of breathing [19–21]. The fR, rather than the VT, is sus- 2 min (Fig. 1). This means that dyspnea intensity was ceptible to the influence of nonmetabolic factors such as measured immediately before the start of exercise (3 min), negative emotions [22]. Because dyspnea causes negative in the middle of each level of exercise (5, 9, and 13 min), emotions [23], the rhythm of breathing is likely to be immediately before the time of change in exercise intensity affected by dyspnea. Therefore, in the present study, we (7 and 11 min), and immediately before the cessation of tested the possibility that the threshold of tachypneic exercise (15 min) (Fig. 1). The participants made a mark breathing is equal to that of dyspnea perception during on the scale at each time point using a pen, and were not cycle ergometer exercise, as previously shown by hyper- allowed to see their previous scores during each test. capnic stimuli [19]. We measured the ventilatory and dyspnea responses to increases in exercise intensity and Exercise compared the thresholds for dyspnea and objective respi- ratory variables. This straightforward approach succeeded The intensity of dyspnea and the breathing pattern were in demonstrating a subjective–objective coupling of respi- measured during cycle ergometer exercise (Fig. 1). Exer- ratory variables during exercise, by seeing the results from cise testing was performed on an electrically braked bicy- a new perspective concerning whether or not nonmetabolic cle ergometer. The participants sat on the seat of the factors contribute to alteration in the rhythm of breathing. ergometer and kept their feet on the pedals for a 3-min rest Methods Participants A total of 27 young adult males (mean age, 21.9 years) (mL/min/kg) participated in this study. Participants gave written 2 . informed consent before participating in the experiments, VO which were approved by the Showa University Committee 150 100 for Human Experimentation. The study conformed to the 50 principles of the Declaration of Helsinki. VAS Of the 27 participants, 22 exercised at least once a week. 073 11 15 We checked before the start of exercise to see whether the Time (min) participant was healthy enough to pedal a cycle ergometer. Fig. 1 Pulmonary O2 uptake (VO2) during exercise in one partici- pant. Exercise testing was performed on an electrically braked bicycle Measurement of pulmonary gas exchange ergometer. After a 3-min rest (filled circles), he started exercise at an and ventilation initial work rate of 50 W, which was increased in a stepwise manner by 50 W every 4 min to 150 W. The VO2 values at each time during exercise (open circles) were predicted by regression analysis between The participants each wore a facemask. Ventilation and time and VO2. The intensity of dyspnea was measured using a pulmonary gas exchange was measured with a respiratory 100-mm horizontal visual analog scale (VAS) at 2-min intervals 123 J Physiol Sci (2017) 67:173–180 175 period, during which we measured baseline values of the the mean within-participant difference is calculated to respiratory parameters. After the 3-min rest period, the detect systematic bias, and linear regression analysis participants started exercise at an initial work rate of 50 W, between the within-participant differences and the mean of which was increased in a stepwise manner by 50 W every both methods is used to show proportional bias (Prism 6; 4 min to 150 W. All participants completed the testing. GraphPad software Inc.). A study exploring the relation- Breath-by-breath VCO2,VE,fR, and VT were plotted ship between the threshold for dyspnea and the ventilatory against VO2 during offline analysis. However, dyspnea threshold during exercise used this plot and showed simi- VAS was not measured in a breath-by-breath way because larities between these thresholds [17]. it was scored at 2-min intervals. Therefore, individual VAS scores were plotted against VO2 at the corresponding time, Statistics which was predicted by regression analysis between time and VO2 in each participant (Fig. 1). The baseline VAS The mean values of the threshold were compared with one- score was plotted against mean VO2 at rest for offline way repeated measures analysis of variance. Post hoc analysis. comparisons were performed using Dunnett’s test (Prism 6; GraphPad software Inc.). For all analyses, significance was Thresholds for dyspnea and respiratory variables recorded at P \ 0.05. All values are presented as mean ± SE. Commercial software was used for analyses (SigmaPlot 12, SYSTAT, Chicago, IL, USA). Each plot was fitted with a segmented linear regression model composed of two seg- Results ments connected by a breakpoint to predict the thresholds for dyspnea (TVAS), VCO2 (TVCO2), VE (TVE), fR (TfR), and Predicted VO2 at the time of dyspnea measurement VT (TVT)[19].
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