Introduction: Respiratory effort perception is a complex phenomenon, essential to maintenance of homeostasis, and is influenced by a number of factors. Anxiety is associated with greater breathlessness in clinical populations. Anxiety and vigilance towards respiratory sensations have previously been shown to be inter-related but have not yet been explored in a controlled laboratory setting. Perceived breathing effort is often viewed in the context of load-capacity balance, but the influence of the absolute magnitude of pressure generation is under-explored.
Methods: Maximum inspiratory pressure (PImax) was measured with a differential pressure transducer during a sustained maximal inspiratory effort against an occlusion and calculated as the greatest one-second mean pressure. Breathing vigilance and anxiety were assessed prior to loading using the Breathing Vigilance Questionnaire (BVQ) and State-Trait Anxiety Inventory (STAI).
Inspiratory threshold loading was applied at 30%, 50% and 80% of each participant’s PImax using an inspiratory muscle training device (POWERbreathe Plus IMT). A “sham” load at 4cmH2O was delivered using an inverted Philips Threshold PEP device.
Each load was applied for twelve breaths, with five minutes of tidal breathing between loads. Order was randomised. Participants rated breathing difficulty using a 100mm visual analogue scale (VAS-D) immediately after each load. Friedman’s ANOVA with Dunn’s post hoc test using Bonferroni correction for multiple comparisons was used to examine differences in VAS-D at each load. Linear mixed effects modelling (LMM) was used to quantify the relationship between load and VAS-D, and the influence of BVQ score, STAI score and PImax.
Results: Thirty healthy adults (eighteen female) were studied (median (IQR) age 32.0 (24.3 – 44.5) years, mean (SD) PImax 119 (48)cmH2O. Mean (SD) BVQ score was 10 (4), STAI-state was 24 (7), STAI-trait was 32 (6). Only three and seven participants respectively scored above the accepted threshold of 37 for “no or low anxiety”.
Median (IQR) VAS-D varied with IMT dose (Baseline: 4 (0 – 10)mm, sham 11 (3 – 18)mm, 30% 29 (12 – 54)mm, 50% 41 (28 – 66)mm, 80% 73 (47 – 94mm), p<0.001). Individual values are shown in Table 1. On post hoc testing, all VAS-D values differed significantly from one another (p values 0.048 to <0.001), with the exception of baseline versus sham and 30% versus 50% (p=1.00 and p=0.604 respectively).
LMM showed a significant (p<0.001) relationship between VAS-D and load: slope (95% confidence interval) 0.72 (0.61 – 0.83)mm/%PImax. Neither BVQ nor STAI score influenced this relationship significantly. PImax significantly influenced the load-perception relationship: slope (95% CI) of load versus VAS-D in the combined model 0.37 (0.09 – 0.65)mm/%PImax, p=0.01), additional influence of baseline PImax 0.003 (0.001 – 0.005)mm/%PImax/cmH2O, p=0.009.
Conclusions: Perceived difficulty of breathing increases with applied threshold load. In a population with low state and trait anxiety and low levels of breathing vigilance, this relationship is not modulated by anxiety or breathing vigilance scores. Underlying respiratory muscle strength does however exert a significant relationship on load perception, suggesting that absolute magnitude of imposed load in addition to the fraction of the individual’s capacity determines response to sensory feedback from the respiratory system.