Limitations of sniff nasal pressure in patients with severe neuromuscular weakness.

BACKGROUND Inspiratory muscle strength in patients with neuromuscular disorders can be assessed using sniff inspiratory nasal pressure (Pn(sn)) and maximum inspiratory mouth pressure (PI(max)). However, the relative merits of Pn(sn) against PI(max) are not known in patients with severe neuromuscular disease. OBJECTIVE To investigate whether severity of disease modifies the relation between Pn(sn) and PI(max). METHODS Vital capacity (VC), Pn(sn), and PI(max) were measured in 258 patients with neuromuscular disorders. RESULTS Data were analysed from 241 patients, 17 being unable to perform PI(max) or Pn(sn) manoeuvres. The correlation between Pn(sn) and PI(max) was +0.94 (p<0.0001), with a mean (SD) difference between Pn(sn) and PI(max) of -4.8 (21.2) cm H(2)O (the limits of agreement were 37.6 and -47.2 cm H(2)O). VC (% predicted) was positively correlated with Pn(sn)/PI(max) (r = +0.86; p<0.0001), with a lower Pn(sn)/PI(max) value in patients with a VC <40% of predicted than in those with a VC >40% (0.80 (0.35) v 1.04 (0.41); p<0.0001). CONCLUSIONS PI(max) is greater than Pn(sn) in patients with a severe restrictive ventilatory defect caused by neuromuscular disease. Pn(sn) may not accurately reflect inspiratory muscle strength in such patients and it is thus advisable to use both tests.