Nasal inspiratory pressure as an indirect measurement of respiratory muscle strength measured during SNIP and Psn methods in healthy subjects and subjects with motor neurone disease

Objective To investigate the difference in nasal inspiratory pressure as an indirect measurement of inspiratory muscle strength when the established sniff nasal inspiratory method (SNIP) is compared with a novel method of SNIP application known as Psn.

Design A prospective, correlational design using two different techniques in random order at the same session.

Setting Multidisciplinary clinic in a regional hospital.

Participants Fifty-two normal healthy individuals from hospital staff and family members, as controls, and a group of 25 subjects with motor neurone disease (MND) recruited from a specialised MND clinic.

Interventions In both SNIP and Psn, a measuring nasal bung is placed in one nostril. In SNIP, the contralateral nostril is unoccluded, and in Psn, the contralateral nostril is occluded.

Main outcome measures Inspiratory nasal pressure (cmH2O). Six consecutive measurements of SNIP and Psn were carried out on each subject through both the right and left nostrils, each conducted by the same operator.

Results Data were analysed from 52 control subjects and 25 subjects with MND. Five patients were unable to produce acceptable and reproducible SNIP data, and one control subject was unable to perform Psn. Bland and Altman plots were used to determine the level of agreement between the two sets of measurements. The two methods of nasal inspiratory testing showed a relatively good level of agreement; the inspiratory pressure measured using SNIP was lower than that measured using Psn [mean difference: control subjects −12.3cmH2O (limits of agreement 9.2 to −32.9); subjects with MND −20.3cmH2O (limits of agreement −4.6 to −45.1)]. These results suggest that both methods of application are useful in the detection of respiratory muscle strength. However, Psn yielded significantly higher values.

Conclusion Psn may be used to complement SNIP in the evaluation of respiratory muscle testing of patients, particularly in those patients who have significant bulbar weakness throughout disease progression. Thus, Psn may be more informative in advanced disease. However, the lack of published reference values to date for Psn may limit its utility as a diagnostic or serial measurement tool in the short term.