ABSTRACT
PURPOSE: To investigate whether heterogeneous treatment effects occur for changes in inspiratory muscle strength, perceived dyspnoea, and health-related quality of life (QoL), following eight-weeks unsupervised home-based inspiratory muscle training (IMT) in adults with post-acute COVID-19 syndrome. METHODS: In total, 147 adults with self-reported prior COVID-19 either completed an eight-week home-based IMT intervention (n = 111; 92 females; 48 ± 11 years; 9.3 ± 3.6 months post-acute COVID-19 infection) or acted as "usual care" wait list controls (n = 36; 34 females; 49 ± 12 years; 9.4 ± 3.2 months post-acute COVID-19 infection). RESULTS: Applying a Bayesian framework, we found clear evidence of heterogeneity of treatment response for inspiratory muscle strength: the estimated difference between standard deviations (SDs) of the IMT and control groups was 22.8 cmH2O (75% Credible Interval (CrI): 4.7-37.7) for changes in maximal inspiratory pressure (MIP), and 86.8 pressure time-units (PTUs; 75% CrI: 55.7-116.7) for sustained MIP (SMIP). Conversely, there were minimal differences in the SDs between the IMT and the control group for changes in perceived dyspnoea and health-related QoL, providing no evidence of heterogeneous treatment effects. Higher cumulative power during the IMT intervention was related to changes in MIP (ß = 10.9 [95% CrI: 5.3-16.8] cmH2O per 1SD) and SMIP (ß = 63.7 [32.2-95.3] PTUs per 1SD), clearly indicating an IMT dose response for changes in inspiratory muscle strength. Older age (>50 years), a longer time post-acute COVID-19 (>3 months), and greater severity of dyspnoea at baseline were also associated with smaller improvements in inspiratory muscle strength. CONCLUSIONS: Heterogenous individual responses occurred following an eight-week home-based IMT programme in people with post-acute COVID-19 syndrome. Consistent with standard exercise theory, larger improvements in inspiratory muscle strength are strongly related to a greater cumulative dose of IMT.