Inspiratory muscle glycemic threshold in COPD: functional and clinical associations in incremental testing

It is known that COPD is multifactorial and reduces the function of several body systems; however, its impact on inspiratory muscle metabolism and its possible clinical implications and for inspiratory muscle training are still unclear. Furthermore, the use of the glycemic threshold (GT) of inspiratory muscles as a viable and clinically useful metabolic measure in COPD is not yet addressed in the literature. This study aimed to verify whether there is an association between the GT of inspiratory muscles with Borg, functional capacity (FC) and cardiovascular autonomic balance in COPD. Included patients with COPD under outpatient follow-up and evaluated them using incremental inspiratory muscle testing (IIMT), six-minute walk test (6MWT), and heart rate variability (HRV). The GT of inspiratory muscles was strongly associated with Borg in the IIMT. The strength endurance and aerobic capacity of inspiratory muscles demonstrated by the exhaustion point (EP) and delta GT/EP showed a strong inverse correlation with the cardiovascular autonomic balance demonstrated by the LF/HF ratio. Delta GT/EP also showed an inverse correlation with dyspnea by the mMRC scale and with the final Borg in the 6MWT. The study concluded that the GT and EP of the inspiratory muscles are strongly associated with Borg in individuals with COPD. Furthermore, the aerobic capacity of the inspiratory muscles was strongly associated with dyspnea on exertion and cardiovascular autonomic balance.