Coupling of dyspnea perception and occurrence of tachypnea during exercise

Abstract

During exercise, tidal volume initially contributes to ventilatory responses more than respiratory frequency, and respiratory frequency then increases rapidly while tidal volume stabilizes. Dyspnea intensity is also known to increase in a threshold-like manner. We tested the possibility that the threshold of tachypneic breathing is equal to that of dyspnea perception during cycle ergometer exercise (n = 27). Dyspnea intensity was scored by a visual analog scale. Thresholds were expressed as values of pulmonary O₂ uptake at each breakpoint. Dyspnea intensity and respiratory frequency started increasing rapidly once the intensity of stimuli exceeded a threshold level. The thresholds for dyspnea intensity and for occurrence of tachypnea were significantly correlated. An intraclass correlation coefficient of 0.71 and narrow limits of agreement on the Bland-Altman plot indicated a good agreement between these thresholds. These results suggest that the start of tachypneic breathing coincides with the threshold for dyspnea intensity during cycle ergometer exercise.

Keywords: Dyspnea; Exercise hyperpnea; Pulmonary O2 uptake; Respiratory frequency; Tachypnea.

Fig. 1

Pulmonary O₂ uptake (VO₂) during exercise in one participant. Exercise testing was performed on an electrically braked bicycle ergometer. After a 3-min rest (filled circles), he started exercise at an 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 time and VO₂. The intensity of dyspnea was measured using a 100-mm horizontal visual analog scale (VAS) at 2-min intervals

Fig. 2

Typical examples from one participant and pooled data from 22 participants. a The participant is the same person for whom results are shown in Fig. 1. Changes in dyspnea VAS, pulmonary CO₂ output (VCO₂), minute ventilation (Ve), respiratory frequency (fr), and tidal volume (Vt) during exercise are shown. Each plot is fitted with a segmental linear regression model composed of two segments connected by a breakpoint to predict the threshold VO₂ value (T _VAS, T _VCO2, T _VE, T _fR, T _VT). b Pooled data from 22 participants. Values are mean ± SE (n = 22)

Fig. 3

Mean threshold values of 22 participants, with the threshold values of each individual (open circles). The threshold for Vt (T _VT) was significantly higher than that for dyspnea VAS (T _VAS) (P < 0.01, Dunnett’s multiple-comparison post hoc test with T _VAS serving as a control). There were no significant differences between T _VAS and T _VCO2, T _VAS and T _VE, or T _VAS and T _fR. Values are mean ± SE (n = 22)

Fig. 4

Agreement between the thresholds assessed by the intraclass correlation coefficients (ICC) and the Bland–Altman plot. We analyzed the agreement between T _VAS and any of the other thresholds using ICC and the Bland–Altman plot. T _VAS and all other thresholds were significantly correlated (left panels; a–d). The calculated ICC values between T _VAS and any of the other thresholds were also significant. A good agreement of ICC values was given between T _VAS and T _VE (0.65) (b) and between T _VAS and T _fR (0.71) (c). The Bland–Altman plots (right panels; a–d) show that the widths of the 95 % limits of agreement (gray color area) are 18.5 ml/min/kg of VO₂ for T _VCO2 and T _VAS (a), 17.0 ml/min/kg for T _VE and T _VAS (b), 15.5 ml/min/kg for T _fR and T _VAS (c), and 20.4 ml/min/kg for T _VT and T _VAS (d). The Bland–Altman plot suggests better agreement between T _VAS and T _fR than other combinations