Twitch mouth pressure reflects COPD disease severity

Researchers from China have shown that a non-invasive method of measuring inspiratory muscle strength accurately reflects the severity of chronic obstructive pulmonary disease (COPD).

Twitch mouth pressure (TwPM) involves magnetically stimulating the cervical nerves to induce mouth pressure changes thought to reflect inspiratory muscle strength.

The research team says that as the test measures a non-volitional outcome, it could provide an objective means for assessing respiratory muscle strength that is less invasive than other methods proposed to-date.

“In contrast to the volitional tests PImax [maximum static inspiratory mouth pressure] and SNIP [sniff nasal inspiratory pressure], TwPM is an objective measurement independent of the instructions of the technologists and the learning curves and cooperation of subjects,” comment Rong-Chang Chen from the First Affiliated Hospital of Ghangzhou Medical University, China.

The study involved 75 patients with COPD, aged an average of 64 years old, and 63 age-matched controls. According to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines, 16 patients had stage II, 32 had stage III, and 27 had stage IV COPD.

The team found that mean TwPM was significantly lower in COPD patients than in controls at 10.0 cmH₂O versus 13.7 cmH₂O in men, and 8.8 cmH₂O versus 11.8 cmH₂O in women.

Among the COPD patients, disease severity significantly correlated with TwPM. For example, men with GOLD stage II disease had a mean TwPM of 11.9 cmH₂O compared with 8.2 cmH₂O in those with stage IV disease.

When the researchers performed PImax and SNIP in these patients, they observed similar correlations with disease severity, and values obtained in COPD patients were also significantly decreased relative to controls.

Furthermore, TwPM showed a moderate inverse correlation with the BODE index, as well as 6-minute walk distance, and forced expiratory volume in 1 second % predicted – a relationship that was stronger than those between SNIP and PImax with these outcomes.

Writing in Respiratory Care, Chen and colleagues say that, while SNIP is less invasive than PImax, it still depends on volitional muscle contraction. By contrast, TwPM is objective and was well-tolerated by the participants in their study.

They conclude therefore that “the TwPM test is sufficiently valid and more suitable than other tests for clinically assessing inspiratory muscle strength.

“Moreover, magnetic stimulation of cervical nerve is simple and easy to apply, and the instrumentation is relatively easy for technologists and for the subjects.”

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