Twitch mouth pressure and disease severity in subjects with COPD

Patients with COPD have impaired respiratory muscle strength. Twitch mouth pressure (Ptw,m) in response to magnetic stimulation of the cervical nerve has been suggested to clinically reflect inspiratory muscle strength. However, studies on Ptw,m values and their relationship with disease severity are limited. Thus, we tested the Ptw,m values of subjects with COPD and investigated the relationship of these values with disease severity.

We recruited 75 COPD patients and 63 age-matched controls. All participants were tested for Ptw,m, sniff nasal pressure (SNIP), and maximum static inspiratory mouth pressure (PImax ); the BODE (body mass index, air flow obstruction, dyspnea, exercise capacity) index was evaluated for overall severity assessment and the 6-min walk distance (6MWD) was used to determine the exercise capacity of subjects with COPD.

Subjects with COPD had markedly lower Ptw,m values compared with the controls (10.00 ± 2.17 vs 13.66 ± 2.20 cm H2O for males, 8.83 ± 0.89 vs 11.81 ± 1.98 cm H2O for female; each with P < .001). The Ptw,m values decreased with increasing COPD severity, and similar trends were observed in the SNIP and PImax values. Regression correlation analysis showed that Ptw,m values were significantly correlated inversely with the BODE index (R = 0.65, P < .001) but positively correlated with 6MWD (R = 0.59, P < .001) in the COPD group; the SNIP values of subjects with COPD were also correlated inversely with their BODE index (R = 0.49, P < .001) but positively correlated with their 6MWD (R = 0.33, P < .005).

Ptw,m values are 26.8% lower in male subjects with COPD and 25.3% lower in female subjects with COPD compared with the controls. The Ptw,m values of subjects with COPD decrease with increasing disease severity. Ptw,m was better correlated with the BODE index and exercise capacity than SNIP and PImax , which suggests that Ptw,m more accurately reflects the overall severity and burden of COPD.