Twitch Mouth Pressure and Disease Severity in Subjects With COPD

INTRODUCTION: Patients with COPD have impaired respiratory muscle strength. Twitch mouth pressure (P-tw,P-m) in response to magnetic stimulation of the cervical nerve has been suggested to clinically reflect inspiratory muscle strength. However, studies on P-tw,P-m values and their relationship with disease severity are limited. Thus, we tested the P-tw,P-m values of subjects with COPD and investigated the relationship of these values with disease severity. METHODS: We recruited 75 COPD patients and 63 age-matched controls. All participants were tested for P-tw,P-m, sniff nasal pressure (SNIP), and maximum static inspiratory mouth pressure (P-Imax); 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. RESULTS: Subjects with COPD had markedly lower P-tw,P-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 P-tw,P-m values decreased with increasing COPD severity, and similar trends were observed in the SNIP and P-Imax values. Regression correlation analysis showed that P-tw,P-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). CONCLUSIONS: P-tw,P-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 P-tw,P-m values of subjects with COPD decrease with increasing disease severity. P-tw,P-m was better correlated with the BODE index and exereise capacity than SNIP and P-Imax, which suggests that P-tw,P-m more accurately reflects the overall severity and burden of COPD.