Differences between sniff mouth pressures and static maximal inspiratory mouth pressures

Measurements of static maximal inspiratory mouth pressure (PImax) and dynamic sniff mouth pressure (Psniff) are frequently used to assess inspiratory muscle strength. The aim of the present study was to examine, in 42 healthy subjects, the equivalence of PImax and Psniff, the influence of body posture on PImax and Psniff, and the effects of nasal patency and repeated manoeuvres on Psniff. PImax was significantly higher than Psniff, 11.05 +/- 0.42 versus 8.53 +/- 0.31 kPa. Because of the low agreement between PImax and Psniff, the two measurement methods were not interchangeable. The limits of agreement were 2.56 +/- 3.92 kPa. Sitting PImax was 11.05 +/- 0.42 kPa, and supine PImax was 9.36 +/- 0.41 kPa. Sitting and supine Psniff were 8.53 +/- 0.31 kPa and 7.52 +/- 0.33 kPa, respectively. Psniff performed with one nostril open instead of two, was higher: 9.67 +/- 0.32 kPa left and 9.62 +/- 0.32 kPa right versus 8.53 +/- 0.31 kPa when both nostrils were open. After 20-28 sniff manoeuvres, Psniff fell from 8.43 +/- 0.31 kPa to 7.83 +/- 0.30 kPa, suggesting some mechanism of inspiratory muscle fatigue. We conclude that measurement of sitting PImax yields the highest values for inspiratory muscle strength. The two measurement methods, Psniff and PImax, are not interchangeable. Measurement of Psniff is affected by nasal patency.